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FLUID WASTE DISPOSAL

KAY W. CANTON EDITOR

Nova Science Publishers, Inc. New York

Copyright © 2010 by Nova Science Publishers, Inc. All rights reserved. No part of this book may be reproduced, stored in a retrieval system or transmitted in any form or by any means: electronic, electrostatic, magnetic, tape, mechanical photocopying, recording or otherwise without the written permission of the Publisher. For permission to use material from this book please contact us: Telephone 631-231-7269; Fax 631-231-8175 Web Site: http://www.novapublishers.com NOTICE TO THE READER The Publisher has taken reasonable care in the preparation of this book, but makes no expressed or implied warranty of any kind and assumes no responsibility for any errors or omissions. No liability is assumed for incidental or consequential damages in connection with or arising out of information contained in this book. The Publisher shall not be liable for any special, consequential, or exemplary damages resulting, in whole or in part, from the readers‘ use of, or reliance upon, this material. Independent verification should be sought for any data, advice or recommendations contained in this book. In addition, no responsibility is assumed by the publisher for any injury and/or damage to persons or property arising from any methods, products, instructions, ideas or otherwise contained in this publication. This publication is designed to provide accurate and authoritative information with regard to the subject matter covered herein. It is sold with the clear understanding that the Publisher is not engaged in rendering legal or any other professional services. If legal or any other expert assistance is required, the services of a competent person should be sought. FROM A DECLARATION OF PARTICIPANTS JOINTLY ADOPTED BY A COMMITTEE OF THE AMERICAN BAR ASSOCIATION AND A COMMITTEE OF PUBLISHERS. LIBRARY OF CONGRESS CATALOGING-IN-PUBLICATION DATA Fluid waste disposal / editor, Kay W. Canton. p. cm. Includes index. ISBN 978-1-61122-590-7 (eBook) 1. Sewage disposal. I. Canton, Kay W. TD741.F55 2009 628.3--dc22

Published by Nova Science Publishers, Inc.  New York

2009037445

CONTENTS Preface Chapter 1

Chapter 2

Chapter 3

Chapter 4

Chapter 5

Chapter 6

Chapter 7

Chapter 8

ix Treatment of Wastewater by Electrocoagulation Method and the Effect of Low Cost Supporting Electrolytes Lazare Etiégni, K. Senelwa, B. K. Balozi, K. Ofosu-Asiedu, A. Yitambé, D. O. Oricho and B. O. Orori

1

Application of Sulphate-Reducing Bacteria in Biological Treatment Wastewaters Dorota Wolicka

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Utilization of Water and Wastewater Sludge for Production of Lightweight-Stabilized Ceramsite Zou Jinlong, Yu Xiujuan, Dai Ying and Xu Guoren

83

Modelling and Observation of Produced Formation Water (PFW) at Sea D. Cianelli, L. Manfra, E. Zambianchi, C. Maggi and A. M. Cicero Disposal of Sulfur Dioxide Generated in Industries Using Eco-Friendly Biotechnological Process – A Review A. Gangagni Rao and P.N. Sarma Novel Biological Nitrogen-Removal Processes: Applications and Perspectives J.L. Campos, J.R. Vázquez-Padín, M. Figueroa, C. Fajardo, A. Mosquera-Corral and R. Méndez Application of Microbial Melanoidin-Decomposing Activity (MDA) for Treatment of Molasses Wastewater Suntud Sirianuntapiboon and Sadahiro Ohmomo Wastewaters from Olive Oil Industry: Characterization and Treatment L. Nieto Martínez, Gassan Hodaifa,M. Eugenia Martínez and Sebastián Sánchez

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137

155

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viii Chapter 9

Chapter 10

Chapter 11

Contents Usability of Boron Doped Diamond Electrodes in the Field of Waste Water Treatment and Tap Water Disinfection Hannes Menapace, Stefan Weiß, Markus Fellerer, Martin Treschnitzer and Josef Adam Utilization of Biosolids as Fertilization Agents on Agricultural Land: Do the Obvious Benefits of Recycling Organic Matter and Nutrients Outweigh the Potential Risks? Veronica Arthurson Integrated Approach for Domestic Wastewater Treatment in Decentralized Sectors Rani Devi and R. P. Dahiya

Chapter 12

Biodegradation Characteristics of Wastewaters Fatos Germirli Babuna and Derin Orhon

Chapter 13

Batch Treatment of a Coffee Factory Effluent for Colour Removal Using a Combination of Electro-Coagulation and Different Supporting Electrolytes L. Etiégni, D. O. Oricho, K. Senelwa B. O. Orori, B. K. Balozi, K. Ofosu-Asiedu and A. Yitambé

Chapter 14

Water as a Scarce Resource: Potential for Future Conflicts M. A. Babu

Chapter 15

Recycling Wastewater After Hemodialysis: An Environmental and Cost Benefits Analysis for Alternative Water Sources in Arid Regions Faissal Tarrass, Meryem Benjelloun and Omar Benjelloun

Chapter 16

Chapter 17

Index

Pb (II) Ions Removal by Dried Rhizopus Oligosporus Biomass Produced from Food Processing Wastewater H. Duygu Ozsoy and J. Hans van Leeuwen Control of Plasticizers in Drinking Water, Effluents and Surface Waters Rosa Mosteo, Judith Sarasa, M. Peña Ormad and Jose Luis Ovelleiro

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249 265

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PREFACE Wastewater is any water that has been adversely affected in quality by anthropogenic influence. It comprises liquid waste discharged by domestic residences, commercial properties, industry, and/or agriculture and can encompass a wide range of potential contaminants and concentrations. In the most common usage, it refers to the municipal wastewater that contains a broad spectrum of contaminants resulting from the mixing of wastewaters from different sources. With the dwindling available water resources in the world coupled with high population growth, pressure is being exerted on water and wastewater plant managers the world over to find cost-effective methods to treat a wide range of wastewater pollutants in a diverse range of situations. This new and important book gathers the latest research from around the globe on fluid waste disposal with a focus on such topics as: wastewaters from the olive industry, application of sulphate-reducing bacteria in biological treatment wastewaters, electrocoagulation treatment method, usability of boron doped diamond electrodes in wastewater treatment and others. Chapter 1 - Coagulation and flocculation are traditional methods of treating of polluted water. Electrocoagulation (EC) presents a robust novel and innovative alternative in which a sacrificial metal anode doses water electrochemically. This has the major advantage of providing active cations required for coagulation, without necessarily increasing the salinity of the water. Electrocoagulation is a complex process with a multitude of mechanisms operating synergistically to remove pollutants from water. A wide variety of opinions exist in the literature for key mechanisms and reactor configurations. A lack of a systematic approach has resulted in a myriad of designs for electrocoagulation reactors without due consideration of the complexity of the system. A systematic, holistic approach is required to understand electrocoagulation and its controlling parameters (pH, temperature, conductivity, current density). This will enable a priori prediction of the treatment of various pollutant types. Electrocoagulation involves applying a current across electrodes in water. This results in the dissolution of the anode (either aluminum or iron). These ions then form hydroxides which complex with and/or absorb contaminants and precipitate out. The precipitate with the contaminants can then be removed from the water by settling and decantation or filtration. EC has the potential to be applied in many other areas besides the textile and semiconductor industry. It has been successfully tested in the pulp and paper industry, as well as tea and coffee processing. However over electrical potential within electrodes during electrocoagulation normally causes extra voltage, which wastes energy. There have been attempts to reduce this extra voltage which, in these days of World energy crisis, will render

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the electrocoagulation process uneconomical. The inclusion of supporting electrolyte such as NaCl achieves this. One of the methods pioneered by researchers at Moi University in Kenya is the use of wood ash leachate as supporting electrolyte which in some cases could reduce energy consumption by as much as 80%. Other supporting electrolytes tested are ash from bagasse and from coffee husks. These supporting electrolytes are relatively inexpensive, but they all generally result in large amount of coagulated sludge. Other supporting electrolytes such phosphate rock are less effective than wood ash, but they yield almost 50% less sludge after electrocoagulation. Most of the supporting electrolytes have an added advantage of reducing other wastewater pollution parameters such as BOD, COD, TSS, TS, turbidity, pH and color. Because of the inherent benefits of these low cost supporting electrolytes, electrochemical methods could be a credible alternative to more traditional wastewater treatment approaches. Chapter 3 - Disposal of wastewater treatment sludge (WWTS) and drinking-water treatment sludge (DWTS) is one of the most important environmental issues nowadays. Traditional options for WWTS and DWTS management, such as landfilling, incineration, etc., are no longer acceptable because they can cause many environmental problems. Conversion of WWTS and DWTS into useful resources or materials is of great interest and must be intensely investigated. To attain this goal, WWTS and DWTS were used as components for making ceramsite. Part I: SiO2 and Al2O3 were the major acidic oxides in WWTS and DWTS, so their effect on characteristics of ceramsite was investigated. Results show that WWTS and DWTS can be utilized as resources for producing ceramsite with optimal contents of SiO2 and Al2O3 ranging from 14–26% and 22.5–45%, respectively. Bloating and crystallization in ceramsite above 900 ℃ are caused by the oxidation and volatilization of inorganic substances. Higher strength ceramsite with less Na-Ca feldspars and amorphous silica and more densified surfaces can be obtained at 18%≤Al2O3≤26% and 30%≤SiO2≤45%. Part II: Fe2O3 and CaO were the major basic oxides, so their effect on characteristics of ceramsite was also investigated. The optimal contents of Fe2O3 and CaO are in the range of 5%–8% and 2.75%–7%, respectively. Higher strength ceramsite with more complex crystalline phases and fewer pores can be obtained at 6%≤Fe2O3≤8%. Lower strength ceramsite with more pores and amorphous phases can be obtained at 5%≤CaO≤7%, which implies that excessive Ca2+ exceeds the needed ions for producing electrical neutrality of silicate networks. Part III: To investigate stabilization of heavy metals in ceramsite, leaching tests were conducted to find out the effect of sintering temperature, pH, and oxidative condition. Results show that sintering exhibits good binding capacity for Cd, Cr, Cu, and Pb and leaching contents of heavy metals will not change above 1000 ℃. Main compounds of heavy metals are crocoite, chrome oxide, cadmium silicate, and copper oxide, which prove that stronger chemical bonds are formed between these heavy metals and the components. Leaching contents of heavy metals decrease as pH increases and increase as H2O2 concentration increases. Leaching results indicate that even subjected to rigorous leaching conditions, the crystalline structures still exhibit good chemically binding capacity for heavy metals and it is environmentally safe to use ceramsite in civil and construction fields. It is concluded from the 3 parts that utilization of WWTS and DWTS can produce high performance ceramsite, in accordance with the concept of sustainable development. Chapter 4 - Through the last decades, the ever increasing energetic demands have been accomplished by exploiting new natural reservoirs, including offshore oil and gas deposits located in marine coastal areas. During the extraction and production phases, large amounts of

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water are brought to the surface along with the hydrocarbons. These waters include the ‗formation water‘, that lies underneath the hydrocarbon layer, and ‗additional water‘ usually injected into the reservoirs to help force the oil to the surface. Both formation and injected waters, named ―produced formation waters‖ (PFWs), are separated from the hydrocarbons onboard offshore platforms and then disposed into the marine environment through ocean diffusers. PFWs contain several contaminants and represent one of the main sources of marine environment pollution associated with oil and gas production. This makes the study of PFW fate of paramount importance for a proper management of environmental resources as well as for planning and optimizing the discharge and monitoring procedures. In the first part of this chapter we provide a detailed description of the chemical characteristics of PFWs and their potential toxic effects and review the mixing processes governing their dispersion in the marine environment. In the second part of the work we briefly review past efforts in observing and modelling PFW spreading in the ocean. Finally, we propose a multidisciplinary approach, integrating in situ observations and numerical modelling, to assess dispersion of PFWs in space and time. As a case study we will refer to the results of a previous study conducted in the Northern Adriatic Sea, a sub-basin of the Mediterranean Sea, where a number of offshore natural gas (CH4) extraction platforms are currently active. Chapter 5 - Sulfur dioxide (SO2) is a known pollutant and responsible for various ill effects on living and non-living organisms. SO2 emissions can be reduced by using nonconventional energy sources or using conventional fuels containing less sulfur. However, under the present circumstances SO2 emissions cannot be completely avoided due to the reasons of rapid industrialization. Various technologies are available for the removal of SO2 from flue and waste gases. Most of these technologies fall under the category of physical, chemical or thermal. All these technologies generate secondary pollutants ending up in disposal problems and also cost prohibitive. Biotechnology offers relatively cheaper solutions for the conventional problems. Due to this reason, biotechnology is making in roads into the conventional treatment processes in all the fields. Over the last decade, efforts have been made to develop biotechnological alternatives to conventional physico- chemical processes for the removal of SO2 from flue gases known as Biological flue gas desulphurization (BIOFGD).SO2 from flue gas can be absorbed in a suitable organic media. In the aqueous phase SO2 would be converted to sulfite and some part may again be converted to sulfate due to the presence of dissolved oxygen. Therefore, the aqueous phase will be having both sulfate and sulfite, which can be reduced to sulfide using Sulfate Reducing Bacteria (SRB) under anaerobic conditions. The sulfide formed in the anaerobic reactor could be converted to elemental sulfur using Sulfur Oxidizing Bacteria (SOB) under partial microbial aerobic conditions. The elemental sulfur can be used either as a soil conditioner or raw material for industrial applications. Therefore, BIO-FGD process could be an environmentally benign and economically viable alternative for the disposal of SO2 emitted from the industries especially from power plants and refineries. The present article reviews the state of art of BIO-FGD process. Chapter 6 - Since the requirement for nutrient removal is becoming increasingly stringent, a high efficiency of nitrogen removal is necessary to achieve a low total nitrogen concentration in the effluent. Biological nitrification and denitrification processes are generally employed to remove nitrogen from wastewater. Unfortunately, these processes are

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not suitable to treat wastewater with a low COD/N ratio because it involves the addition of an external organic carbon source and, therefore, an increase of the operational costs. Several alternative processes for nitrogen removal can be applied in order to reduce partially (―nitrite route‖) or totally (anammox, autotrophic denitrification) the organic matter required. Such processes suppose not only an economical way to treat these wastewaters but they are also more environmentally friendly technologies (lower production of CO2, N2O and sludge; lower energy consumption). Up to now, they were basically applied to the return sludge line of municipal wastewater treatment plants (WWTPs). However, these processes could even be implemented in the actual WWTPs in order to achieve more compact and energy efficient systems. Their potential advantages can make them also feasible technologies to treat polluted ground water or to remove nitrogen compounds from recirculating aquaculture systems. Chapter 7 - This review will discuss the melanoidin-decomposing activity (MDA) among microorganisms. The focus will be on the potential use of the microbial-MDA to treat the wastewater discharged from factories using molasses as the raw material (molasses wastewater: MWW) because molasses is one of the most useful raw materials in various types of industries, such as the fermentation and animal feed industries. However, the wastewater discharged from factories using molasses contains a large amount of dark brown pigment, melanoidin pigment: MP, which is poorly decomposed and/or decolorized by normal biological treatment processes, such as the activated sludge or anaerobic treatment systems (anaerobic pond or anaerobic contact digester), because, the microorganisms in those wastewater treatment systems showed very poor MDA. The distribution of MDA among microorganisms and the mechanism of decomposing activities, in particular, were reviewed. Also, the application of the isolated strains having the MDA to treat molasses wastewater in the wastewater treatment plant was tested. Chapter 8 - Countries in the Mediterranean basin are among the main producers of olive oil. The elaboration of olive-oil is typically carried out by small companies in small facilities. The olive-oil plants produce high and variable amounts of residual waters of olives and oliveoil washing (OMW) that has a great impact in the environment. According to the procedure used different types of OMW with different chemical oxygen demand can be generated, the OMW from the three phase process (COD = 150 g O2 L-1) and the OMW from olives washing (COD = 0.8-4.5 g O2 L-1) and olive oil washing (COD = 1.1- 6 g O2 L-1) in the two-phase process. The uncontrolled disposal of OMW is a serious environmental problem, due to its high organic load, and because of its high content of microbial growth-inhibiting compounds, such as phenolic compounds. The improper disposal of OMW to the environment or to domestic wastewater treatment plants is prohibited due to its toxicity to microorganisms, and also because of its potential threat to surface and groundwater. These waters normally are stored in great rafts of accumulation for their evaporation during the summer. This solution among others until the moment dose not represent a definitive solution for this problem, especially as the administrations more and more demanding the preparation of this spill and the constructive quality of the rafts. Today, effective technologies have been proposed such as the chemical oxidation process using ferric chloride catalyst for the activation of H2O2 as a treatment of OMW produced from two-phase process. In the previous works the authors have described the experimental results on laboratory-scale. These results have been taken to pilotindustrial scale, making the chemical oxidation in the optimum conditions of operations: [H2O2] = 5% (w/v), using a ferric chloride catalyst with a relation of [FeCl3]/[H2O2] = 0.25

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(w/w), at OMW pH and environmental temperature. The final average value of COD obtained next to 370 mg L-1 (%CODremoval = 86.2%), and the water obtained can be destined to irrigation or disposed directly to the municipal wastewater system for their tertiary treatment. OMW from three-phase process does not allow direct chemical and biological purification for its content in phenolic compounds and generally used natural and forced evaporation process. Another way of using is the application of OMW nutrients to the growth of microorganisms such as microalgae. Chapter 9 - Over the past few years one main focus on the research efforts at the Institute for Sustainable Waste Management and Technology (IAE) has been on possible applications for reactors with boron doped diamond electrodes (BDD) in the field of (waste) water treatment. This article deals with the technical construction of the electrodes used (continuous reactor with a different number of plate electrodes), which were produced by a spin-off of the institute. The electrodes consist of conductible industrial diamond particles (< 250 µm), which are mechanically implanted on a fluoride plastic substrate. These electrodes showed a high mechanical and chemical stability in different test runs. At the institute, treatment methods for micro pollutants (e.g. pharmaceuticals and complexing agents) were developed with electrochemical oxidation by BDD. In this case test runs were made on laboratory scale and technical scale treatment units and elimination rates up to 99 % were achieved. In this project the analytic is partly provided by the ―Umweltbundesamt GmbH‖ (UBA), one of the project partners. This agency has been a project partner in different studies about pharmaceuticals in the ecosystem. These techniques could also be used for the waste water treatment of alpine cabins. Pilot projects have been set up. On the basis of these results a follow-up project was launched last October, in which an alternative treatment process for oilin-water emulsions and mixtures was developed by the usage of electrochemical oxidation with BDD. A third possible application is the disinfection of drinking water from contaminated ground and spring water. In this process oxidation agents like ozone or OH radicals produced in situ by the BDD reactor from the treated water are used to eliminate bacterial contaminants (for example e. coli) in the water. Chapter 10 - Treatment of wastewater, commonly performed at municipal sewage plants, generates sanitized water and sewage sludge. Anaerobic degradation of sewage sludge results in the production of different gases, including the economically valuable methane, and digested residue (biosolids) with potential value as a crop fertilizer. Traditionally, digested sewage sludge is disposed either into water, onto or into the earth or into the air. However, alternative exploitation of digested sewage sludge in agriculture has several advantages over commercial fertilizers, including environmental aspects benefiting agricultural sustainability and increased crop yield. Additionally, residue utilization is nearly always a cheaper option than disposal costs. Biosolids obtained from the treatment of municipal sewage sludge consist of a mixture of organic and mineral compounds that significantly affect soil microbial communities and their biogeochemical activities when applied as a crop fertilizer. The microorganisms influence soil quality through nutrient cycling, decomposition of organic matter and maintenance of soil structure, in turn, affecting agricultural and environmental quality, and subsequently, plant and animal health. Moreover, both soil and residue normally contain considerable quantities of microorganisms, including both beneficial and potentially human pathogenic species that may be supported by the new conditions in the soil. Thus, soil amended with biosolids may

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present a modified microbial community composition after some time and, hence, a modified ecosystem function. At the end of the present chapter, we discuss whether the potential risks of recycling biosolids to agricultural cropland are acceptable for consumers, producers and scientific expertise, in view of the resulting alterations in soil microbial diversity, activity and accompanying functions. Furthermore, optimal ways of managing the recycling process to achieve the most favourable balance of benefits and risks for the community are highlighted. Chapter 11 - The purpose of the present study was to design an integrated wastewater treatment system for a nalla (riverlet) flowing through Indian Institute of Technology Delhi (IITD), India, besides its cost estimation and comparison with the conventional wastewater treatment system. The design parameters for integrated aeration-cum-adsorption tank were worked out for 240 m3 / d flow rate of the wastewater. The important parameters used for the design included initial COD and BOD concentration in the influent, treatment time, adsorbent dose, pH, adsorbent particle size and the desired COD and BOD in the effluent after treatment as prescribed by Central Pollution Control Board, (CPCB) Delhi, India. All the design parameters of this system were similar to those of conventional system except for the replacement of aeration tank in conventional system by the aeration-cum-adsorption tank. The concentration of COD and BOD of the treated effluent by the integrated system were well within the permissible limits of CPCB standards (for COD it is 100 ppm and for BOD of 30 ppm) to discharge in the canal for irrigation purpose. It was worth mentioning here that the adsorbents used in the present study were based on discarded materials which were available free of cost. Of course, the cost of their transportation and processing should have been taken into account. The total cost estimated for the conventional system and the adsorption based system would be Rs. 198,312 and Rs. 141,275 respectively (including civil work, machinery, labour, adsorbent and miscellaneous). The cost difference for the two systems would be approximately Rs 57,037. This design of integrated system has resulted into saving of cost by 28 % over the conventional system. Thus, it is a good approach for saving of conventional energy in addition to saving the cost of treatment and can be applicable for any country for decentralized sector. Moreover, it is an open ended research and we can recommend more research by changing the adsorbents types and operating parameters to improve the model. Chapter 12 - The objective of this chapter is to put forward an overview of biodegradation characteristics of wastewaters by emphasizing the significance of COD fractionation. Recalcitrant COD fractions of effluents can be used as a tool to evaluate whether discharge standards can be met with a prescribed biological treatment. Moreover, the appropriate type of biological treatment applicable to the wastewater under investigation can be addressed and the performance of an existing biological treatment system can be appraised with reference to inert COD fractions. Besides recalcitrant COD fractions of segregated industrial effluent streams can be regarded as an essential input of a sound industrial wastewater management strategy adopting minimization at source philosophy. Last but not least, data on COD fractions can be used as a solid source of information for modelling studies that define the design and performance of biological treatment systems. In this context, COD fractionation data on a wide spectrum of activities ranging from various industrial sectors to hotels is presented. Segregated industrial wastewater streams together

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with domestic sewage and end-of-pipe industrial effluents are evaluated in terms of their biodegradation characteristics. Chapter 13 - In the present study, two types of colour removal systems were tested on effluent samples collected from a coffee pulping factory which discharged on average 15 m3 of wastewater daily with a colour index of about 2500 OH that was too high for direct discharge into a river in Kenya. The two colour removal systems used were: (i) electrolysis combined with wood ash or coffee husks leachate and (ii) electrolysis combined with phosphate rock solutions at a rate of 0.5 g/l to 4g/l. Phosphate rock is often used as agricultural liming agent. The surface area of the electrodes was set at close to 75 m2/m3 of effluent with a current density of 1,200 mA/m2. The experiments were laid out in a stratified random sampling design and the data were analysed using the Statistical Package for Social Scientists (SPSS) computer programme version 10.0. Electrolysis combined with phosphate rock (ELPHOS) proved to be the best process in terms of power consumption (68% reduction) compared with the 57% reduction by electrolysis combined with wood ash (ELCAS) and the 58% reduction by electrolysis combined with coffee husks ash (ELCHAS). Besides the 100% colour removal, ELPHOS also reduced other effluent physico-chemical parameters such as BOD, COD, TSS and TS by 79%, 80%, 69%, and 88% respectively. The analysis of ELPHOS treated wastewater showed that the mill could discharge an effluent that meets local discharge standards for colour requirements. It is recommended that recycling of the treated water by ELPHOS back to the factory for cleaning and washing purposes be considered since the quality meets the requirement for uses of fresh water for cleaning purposes. Furthermore, calculation of power consumption based on a scale-up batch reactor of 15 m3 proved less expensive to treat the factory effluent than a set of 12 one 100-L reactors similar to the one used in the field. Chapter 14 - The major aim of this paper is to review the major problems of water resources in the developing countries. It is based on problems related to population growth and pollution and how these are more likely to lead to future conflicts. We know that fresh water is only 3 % of the total global water and 78% of this is in glaciers. This makes it a scarce and precious resource which must be sustainably managed. The paper also analyses some of the already existing and potential conflicts based on water resources. It reviews the potential threats to Ugandan water resources and problems which are most likely to occur as a result of these threats. Factors hindering treatment of wastewater as a remedy to pollution in developing countries have also been discussed. The methodology used in this paper is based on literature review of the most current issues that affect water resources world-wide. The review is limited to scientific facts and no political factors affecting water resources have been included. It has been found that although Uganda is endowed with 66km2/year of renewable water resources, population increase, deforestation, degradation of wetlands and pollution are major threats to its water resources. Problems associated with water quality and quantities are more likely to result into internal conflicts which are bound to spread beyond Ugandan borders. Chapter 15 - Water is a vital aspect of hemodialysis. During the procedure, large volumes of water are used to prepare dialysate and to clean and reprocess machines. This paper evaluates the technical and economical feasibility of recycling hemodialysis wastewater for irrigation uses, such as watering gardens and landscape plantings. Water characteristics, possible recycling methods, and the production costs of treated water are discussed in terms of the quality of the generated wastewater. A cost-benefit analysis is also performed through

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comparison of intended cost with that of seawater desalination, which is widely used in irrigation. Chapter 16 - Heavy metal pollution is a serious problem in many developed and developing countries. Lead had been recognized as a particularly toxic metal and comes into water bodies mainly from metallurgical, battery, metal plating, mining and alloy industries. In order to minimize the impacts of this metal on human health, animals and the environment, lead-contaminated water and wastewater need to be treated before discharge to water bodies. This chapter concerns an investigation of potential usage of corn-processing wastewater as a new alternative low-cost substrate to produce biosorbent and evaluate this biosorbent to remove Pb(II) ions from aqueous solutions. For this aim, Rhizopus oligosporus cultivated on corn-processing wastewater and dried biomass of these fungi was used as an adsorbent. The adsorption experiments were conducted in a batch process and the effects of contact time (148 hours), initial pH (2-7), initial metal ion concentration (20-100 mg L-1) and adsorbent dosage (0.5-5 g L-1) on the adsorption were investigated. Pb (II) ion concentrations before and after adsorption were measured using Inductively Coupled Plasma-Mass Spectrometry. Maximum adsorption capacity was achieved at pH 5.0. The isothermal data of dried fungal biomass could be described well by the Langmuir equation and monolayer capacity had a mean value of 59.88 mg g-1. The pseudo-second order reaction model provided the best description of the data with a correlation coefficient 0.99 for different initial metal concentrations. This result indicates that chemical sorption might be the basic mechanism for this adsorption process and Fourier Transform Infrared Spectroscopy analyses showed that amide I and hydroxyl groups play an important role in binding Pb (II). Because of the high activation capacity of adsorbent and low cost of process dried R. oligosporus biomass presents a good potential as an alternative material for removal of Pb (II) ions from the aqueous solutions. Chapter 17 - The main objective of this research work is to determine the presence of di(2-ethylhexyl) phthalate, di(2-ethylhexyl) adipate and diisodecyl phthalate, in different water samples (drinking waters, effluents and surface waters). Different analytical methods were studied in order to know the best methodology for the quantification of these compounds. Solid-liquid and liquid-liquid extraction were investigated and finally the liquidliquid extraction and analysis by gas chromatography followed by mass spectroscopy was chosen because of offering the highest recovery rate. In the whole of this research study, the control of background pollution by reagents and material was extremely important. The problem of background pollution is more serious in the trace analysis of phthalates and adipates as a consequence of their presence in almost all equipment and reagents used in the laboratory. Respect to the control of the selected plasticizers in the different water samples, bis (2ethylhexyl) phthalate and bis (2-ethylhexyl) adipate were detected in drinking water, effluents and surface waters. On the other hand, diisodecyl phthalate was not detected in any sample.

In: Fluid Waste Disposal Editor: Kay W. Canton, pp. 1-48

ISBN: 978-1-60741-915-0 © 2010 Nova Science Publishers, Inc.

Chapter 1

TREATMENT OF WASTEWATER BY ELECTROCOAGULATION METHOD AND THE EFFECT OF LOW COST SUPPORTING ELECTROLYTES Lazare Etiégni1*, K. Senelwa1, B. K. Balozi1, K. Ofosu-Asiedu2, A. Yitambé3, D. O. Oricho1, and B. O. Orori1 1

Moi University, Department of Forestry & Wood Science, P. O. Box 1125 Eldoret, Kenya. 2 J.I.C., Dept. of Chem. Eng. Box 10099, Jubail Industrial City-31961, Kingdom of Saudi Arabia. 3 Kenyatta University, Department of Public Health P.O Box 43844-00100 Nairobi, Kenya

ABSTRACT Coagulation and flocculation are traditional methods of treating of polluted water. Electrocoagulation (EC) presents a robust novel and innovative alternative in which a sacrificial metal anode doses water electrochemically. This has the major advantage of providing active cations required for coagulation, without necessarily increasing the salinity of the water. Electrocoagulation is a complex process with a multitude of mechanisms operating synergistically to remove pollutants from water. A wide variety of opinions exist in the literature for key mechanisms and reactor configurations. A lack of a systematic approach has resulted in a myriad of designs for electrocoagulation reactors without due consideration of the complexity of the system. A systematic, holistic approach is required to understand electrocoagulation and its controlling parameters (pH, temperature, conductivity, current density). This will enable a priori prediction of the treatment of various pollutant types. Electrocoagulation involves applying a current across electrodes in water. This results in the dissolution of the anode (either aluminum or iron). These ions then form hydroxides which complex with and/or absorb contaminants and precipitate out. The precipitate with the contaminants can then be *

Corresponding author: E-mail: [email protected]

2

Lazare Etiégni, K. Senelwa, B. K. Balozi et al. removed from the water by settling and decantation or filtration. EC has the potential to be applied in many other areas besides the textile and semiconductor industry. It has been successfully tested in the pulp and paper industry, as well as tea and coffee processing. However over electrical potential within electrodes during electrocoagulation normally causes extra voltage, which wastes energy. There have been attempts to reduce this extra voltage which, in these days of World energy crisis, will render the electrocoagulation process uneconomical. The inclusion of supporting electrolyte such as NaCl achieves this. One of the methods pioneered by researchers at Moi University in Kenya is the use of wood ash leachate as supporting electrolyte which in some cases could reduce energy consumption by as much as 80%. Other supporting electrolytes tested are ash from bagasse and from coffee husks. These supporting electrolytes are relatively inexpensive, but they all generally result in large amount of coagulated sludge. Other supporting electrolytes such phosphate rock are less effective than wood ash, but they yield almost 50% less sludge after electrocoagulation. Most of the supporting electrolytes have an added advantage of reducing other wastewater pollution parameters such as BOD, COD, TSS, TS, turbidity, pH and color. Because of the inherent benefits of these low cost supporting electrolytes, electro-chemical methods could be a credible alternative to more traditional wastewater treatment approaches.

INTRODUCTION With the dwindling availability of water resources in the World coupled with high population growth, pressure is being exerted on water and wastewater plant managers the world over to find cost-effective methods to treat a wide range of wastewater pollutants in a diverse range of situations. Traditionally coagulation, flocculation and lagooning have been used as chemical and biological processes with varying degrees of success to treat polluted waters. However a more cost-effective and proven method to clean an ever widening range of water pollutants, on-site, and with minimum additives, is required for sustainable water and wastewater management. Electrocoagulation treatment of water seems to fit this description. Colloidal dispersions in water or wastewater often referred to as sols consist of discrete particles held in suspension by their extreme small size (1-200 nm), state of hydration (chemical combination with water), and surface electric charge. The chemistry of coagulation and flocculation is primarily based on the electrical properties of the particles. Like charges repel each other while opposite charges attract. Particles finer than 0.1 µm (1x10-7 m) in water or wastewater remain continuously in motion due to electrostatic charges (often negative) which cause them to repel each other. There are two types of colloids - hydrophilic and hydrophobic. Hydrophilic are readily dispersed in water and their stability depends on the affinity for water rather than the slight negative charge they possess. Hydrophobic colloids on the other hand have no affinity for water and their stability depends on the charge they possess, usually positive. The electrostatic repulsion between the colloidal particles leads to a stable sol. The surface or primary charge of colloidal particles comes from charged groups within the particles or the adsorption of charged particles. The sign and magnitude of the surface charge depends on the character of colloids, the pH (the lower the pH the more positive the charge becomes), the ionic strength and the characteristics of the water or wastewater. The surface of the colloid has a certain δ-potential (zeta potential) which is the magnitude of the charge at the surface of shear. The δ-potential is derived from the diffused double-layer

Treatment of Wastewater by Electrocoagulation Method…

3

theory applied to hydrophobic colloids (Figure 1), and can be estimated using Smoluchowski‘s (1872-1917) electrokinetic mobility equation:

δε μ=

--------------------------------- (1) (1)

ε where μ = the electrophoretic mobility δ = zeta potential ε = the electric permittivity ε = the viscosity of the water or wastewater

Zeta potential can also be calculated using the following relationship for electrostatic force

4πqd δ=

--------------------------- (2) D

(2)

q = charge per unit area d = thickness of the layer surrounding the shear surface through which the charge is effective π = pi (= 3.142857) D = dielectric constant of the liquid + + + + +

+

+

+ +

Stern layer

+ +

+

+

+ +

Surface shear

+

+

+ +

+

+ +

Particle + +

+

+

+

Bulk of solution n

+ + + +

+

+

+

+ +

+ + +

+ +

Zeta potential +

Fixed layer of ions

Figure 1. Diffused double layer.

Electric potential surrounding particle Diffusion layer of counterions

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Lazare Etiégni, K. Senelwa, B. K. Balozi et al.

The diffused double layer (Figure 1) consists of two parts: an inner region, also referred to as Stern layer, which includes ions bound relatively strongly to the surface (including specifically adsorbed ions) and an outer, diffuse or movable region in which the ion distribution is determined by a balance of electrostatic forces and random thermal motion. The potential in this region, therefore, decays as the distance from the surface increases until, at sufficient distance, it reaches the bulk solution value, conventionally taken to be zero. The repulsive force of the charged double layer scatters particles thus preventing agglomeration. Particles with high zeta potential have a very stable sol. The zeta potential is the overall charge a particle acquires in a specific medium. In other words, it is a measure of the magnitude of electrical charge surrounding the colloidal particles. The magnitude of the zeta potential gives an indication of the potential stability of the colloidal system. Zeta potential can be equated to the amount of repulsive force which keeps the particles in suspension. If the zeta potential is large, then more coagulants will be needed to destabilize colloidal particles. If all the particles have a large negative or positive zeta potential they will repel each other and there is dispersion stability. When particles have low zeta potential values, there is no force to prevent the particles coming together and there is dispersion instability. A dividing line between stable and unsable aqueous dispersions is generally taken at either +30 or -30mV.

FACTORS AFFECTING ZETA POTENTIAL There are several factors that can affect zeta potential

1. pH In aqueous media, the pH of a sample is one of the most important factors that affect its zeta potential. A zeta potential value on its own without defining the solution conditions is virtually meaningless. A zeta potential versus pH curve will be higher or positive at low pH and lower or negative at high pH. There may be a point where the plot passes through zero zeta potential. This point is called the isoelectric point and is very important from a practical consideration. It is normally the point where the colloidal system is least stable.

2. Conductivity The thickness of the double layer (κ-1) depends upon the concentration of ions in solution and can be calculated from the ionic strength of the medium. The higher the ionic strength, the more compressed the double layer becomes. The valence of the ions will also influence double layer thickness. A trivalent ion such as Al3+ will compress the double layer to a greater extent in comparison to a monovalent ion such as Na+. Inorganic ions can interact with charged surfaces in one of two distinct ways (i) non-specific ion adsorption where they have no effect on the isoelectric point and (ii) specific ion adsorption, which will lead to a

Treatment of Wastewater by Electrocoagulation Method…

5

change in the value of the isoelectric point. The specific adsorption of ions onto a particle surface, even at low concentrations, can have a dramatic effect on the zeta potential of the particle dispersion. In some cases, specific ion adsorption can lead to charge reversal of the surface.

3. Concentration of a Formulation Component The effect of the concentration of a formulation component on the zeta potential can give information to assist in formulating a product to give maximum stability. The influence of known contaminants on the zeta potential of a sample can be a powerful tool in formulating the product to resist flocculation for example.

COAGULATION Schulze, in 1882, first showed that colloidal systems could be destabilized by the addition of ions having a charge opposite to that of the colloid (Benefield et al., 1982). Coagulation in water or wastewater chemistry is a process in which a chemical referred to as a coagulant is added to destabilize dispersed colloidal particles so that they agglomerate. Coagulation experiments using natural products such as Moringa oleifera have also been tried with varying degrees of success (Kasser et al., 1990; Ogutveren et al., 1994; Ndabigengesere et al., 1995; Mohammed, 2001; Bhuptawat and Chaudhari, 2003). The objectives of coagulation are to (i) destabilize suspended and colloidal particles to enhance their removal through sedimentation and filtration and (ii) to precipitate dissolved maters i.e. PO43-, color, natural organic matter (NOM). Coagulation process may require several reaction steps: (i) hydrolysis of multivalent metal ions; (ii) adsorption of hydrolysis species at the solid-solution interface for the destabilization of colloidal particle (reduction of zeta potential); (iii) aggregation of destabilized particles by interparticle bridging; (iv) aggregation of destabilized particles by particle transport and van der Waals‘ forces; (v) ―aging‖ of flocs formed in the process; and (vi) precipitation of metal hydroxides (Stumm and O‘Melia, 1968).

ELECTROCOAGULATION Electrocoagulation is a process that applies a current across electrodes through a liquid, using a variety of anode and cathode geometries, including plates, balls fluidized bed spheres, wire mesh, plates (either aluminum or iron), rods, and tubes. This results in the dissolution of the anode (Equation 3 & 12). These ions then form hydroxides which complex with and/or absorb contaminants and precipitate from water or wastewater. They are subsequently removed by surface complexation and electrostatic attraction according to the following equations:

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Lazare Etiégni, K. Senelwa, B. K. Balozi et al.

WITH IRON ELECTRODES In acidic medium, Fe(s) → Fe2+(aq) + 2e-

(Equation 3)

4Fe2+(aq) + 10 H2O(l) + O2 (g) → 4 Fe (OH)3(s) + 8H+ (aq)

(Equation 4)

Cathode:

2H+ (aq) + 2e- → H2 (g)

(Equation 5)

Overall:

4 Fe(s) + 10 H2O(l) + O2 (g) → 4Fe(OH)2(s) + 4H2 (g)

(Equation 6)

Anode:

In alkaline medium, Anode:

Fe(s)

Fe2+ (aq) + 2e-

Fe2+(aq) + 2OH-(aq) 2H2O(l) + 2e-

Cathode: Overall:

Fe (s) + 2H2O(l) Floc + H2 (g)

(Equation 7)

Fe(OH)2(s)

(Equation 8)

H2(g) + 2OH-(aq)

(Equation 9)

Fe(OH)2(s) + H2(g) Floats

(Equation 10) (Equation 11)

WITH ALUMINUM ELECTRODES In acidic medium Anode:

Al (s) 2Al3+ (aq) + 4H2O(l) + O2 (g)

Cathode:

2H+ (aq) + 2eOverall: 2Al(s) + 4H2O(l) + O2 (g)

Al3+ + 3e-

(Equation 12)

2Al(OH)3(s) + 2H+ (aq) (Equation 13) H2 (g)

(Equation 14)

2Al(OH)3(s) + 2H2(g)(Equation 15)

In alkaline medium Anode:

Al (s) Al3+ (aq) + 3OH- (aq)

Cathode: Overall:

2H2O(l) + 2e2Al(s) + 3H2O(l)

Al3+ + 3e-

(Equation 16)

Al(OH)3(s)

(Equation 17)

H2(g) + 2OH-(aq)

(Equation 18)

Al(OH)3 + 3/2H2 (g)

(Equation 19)

Treatment of Wastewater by Electrocoagulation Method…

7

The cation hydrolyses in water to form a hydroxide. The following equations (20 to 23) are an illustration of this phenomenon in the case of aluminum: pH

Al3+ + H2O AlOH2+ + H2O Al(OH)2+ + H2O Al(OH)30 + H2O

AlOH2+ + H+ Al(OH)2+ + H+ Al(OH)30 + H+ Al(OH)4- + H+

(Equation 20) (Equation 21) (Equation 22) (Equation 23)

ELECTROCOAGULATION MECHANISMS The electrocoagulation overall mechanism is a combination of mechanisms that operate concurrently or in series but synergistically. The main mechanism may vary throughout the dynamic process as the reaction progresses, and will almost certainly shift with changes in operating and environmental parameters and pollutant types. Highly charged cations destabilize any colloidal particles by the formation of polyvalent polyhydroxide complexes. These complexes have high adsorption properties, forming aggregates with pollutants. The pollutants presumably act as a ligand to bind with iron or aluminum ions resulting in the formation of amorphous polymeric complexes (hydroxo-complexes). These compounds with a large specific surface area are very active and able to coagulate and adsorb pollutants soon after their in situ generation (Rajeshwar and Ibanez 1997; Scott, 2001). Besides the generation of polyvalent cations described above, electrocoagulation includes also the production of electrolysis gases that are hydrogen and oxygen (Equation 5, 6, 9, 10, 14, 15 & 19). Evolution of hydrogen gas aids in mixing and flocculation. Once the floc is generated, the electrolytic gas binds to and creates a buoyant force on the floc leading to its flotation and ultimately to the removal of the pollutant as a floc - foam layer at the liquid surface (Equation 11). Other flocs that are heavier settle at the bottom of the reactor. There are many ways in which species can interact in solution: 1. Migration to an oppositely charged electrode (electrophoresis) and aggregation due to charge neutralization. 2. The cation or hydroxyl ion (OH-) forms a precipitate with the pollutant. 3. The metallic cation interacts with OH- to form a hydroxide, which has high adsorption properties thus bonding to the pollutant (bridge coagulation). 4. The hydroxides form larger lattice-like structures that sweep through the water (sweep coagulation). 5. Oxidation of pollutants to less toxic species. 6. Removal by electroflotation and adhesion to bubbles (Figure 2). Electrocoagulation process has been around for some time. The process was proposed before the turn of the last century with Vik et al. (1984) describing a treatment plant in London built in 1889 (for the treatment of sewage by mixing with seawater and electrolyzing). In 1909, Harries (cited in Vik et al., 1984) in the United States, received a patent for wastewater treatment by electrolysis with sacrificial aluminum and iron anodes. Matteson and Dobson (1995) described a device of the 1940‘s, the ―Electronic Coagulator‖

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Lazare Etiégni, K. Senelwa, B. K. Balozi et al.

which electrochemically dissolved aluminum (from the anode) into solution, reacting this with the hydroxyl ion (from the cathode) to form aluminum hydroxide. The hydroxide flocculates and coagulates the suspended solids purifying the water. A similar process was used in Britain in 1956 for which iron electrodes were used to treat river water (Matteson and Dobson, 1995). Because of its capability to remove several types of water pollutants, the recent thirty years have seen an explosion of journal article reports on electrocoagulation methods probably due to new and more stringent environmental regulations on a wide range of water and wastewater pollutants. This has further translated into a number of electrocoagulation devices, designed to purify water or wastewater, being put on the market.

REACTIONS WITHIN THE ELECTROCOAGULATION REACTOR Several distinct electrochemical reactions are produced independently within the electrocoagulation reactor. They are as follows: Emulsion breaking, resulting from the oxygen and hydrogen ions that bond into the water receptor sites creating a water insoluble complex that separate water from pollutants. Seeding, resulting from the anode reduction of metal ions that become new centers for larger, stable, insoluble complexes that precipitate as complex metal ions. Bleaching by the oxygen ions produced in the reaction chamber oxidizing pollutants such as dyes, cyanides, biohazards, chlorolignins from pulp and paper mill effluent. DC power supply

Coagulation & Flocculation

Cathode

Anode

Flocs

H2

Sediments Figure 2. Electrocoagulation process interactions (Hydrogen discharge at the cathode generates gas micro-bubbles that cause the floatation of flocs and the increase of pH).

Treatment of Wastewater by Electrocoagulation Method…

9

Electron flooding of the water that eliminates the polar effect of the water complex, allowing colloidal materials to precipitate. The increase of electrons creates an osmotic pressure that ruptures bacteria, cysts, and viruses. Oxidation reduction reactions that are forced to their natural end point within the reactor which speeds up the natural process. Electrocoagulation induced pH swings toward neutral although this will not always be the case and will depend on the type of electrolyte used.

TYPE OF ELECTRODES Electrode material can subtancially affect the performance of an electrocoagulation reactor. The heart of EC is the dimensionally stable oxygen evolution anode which is usually expensive. The anode material determines the cation introduced into solution. Several researchers have studied the choice of electrode material with a variety of theories as to the preference of a particular material. The most common electrodes were aluminum or iron plates as described by Vik et al. (1984) and Novikova and Shkorbatova (1982). Do and Chen (1994) have compared the performance of iron and aluminum electrodes for removing color from dye-containing solutions. Their conclusion was that the optimal electrocoagulation conditions varied with the choice of iron or aluminum electrodes, which in turn was determined by initial pollutant concentration and pollutant type.

STIRRING RATE Bazrafshan et al. (2008), while comparing chromium removal efficiency with iron and aluminum electrodes, showed that removal efficiency of chromium with aluminum electrodes was lower than chromium removal efficiency with iron electrodes. Metal consumption equally was much lower with aluminum than with iron electrodes. Conversely, power consumption was lower with aluminum than with iron electrodes for the same concentration of pollutant. However, as the chromium concentration in the solution increased to 500.0 mg/L , the consumption of the electrode reduced, but efficient chromium removal occurred due to the large amount of flocs formation that helped sweep away chromium. For example, iron electrode consumption for the initial concentration of 5.0 mg/l and voltage of 40 V was 9.01 g while for an initial concentration of 500.0 mg/L it was 7.70 g (Bazrafshan et al., 2008). The highest efficiency of chromium removal (for both iron and aluminum electrodes) was measured in acidic medium (pH = 3) for an initial chromium concentration of 500.0 mg/L and at lower concentrations, the removal efficiency was almost complete at all pH values. At high chromium concentration, however, the complete removal would have required longer time i.e. higher power consumption. Some researchers have investigated the relationship between ―size‖ of the cation introduced and removal efficiency of organic waste (Baklan and Kolesnikova, 1996; Vlyssides et al., 1997). The size of the cation produced (10-30μm for Fe3+ compared to 0.05-1 μm for Al3+) was suggested to contribute to the higher efficiency of iron electrodes. Their conclusion was based on a single experiment, however, using chemical absorption of oxygen

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Lazare Etiégni, K. Senelwa, B. K. Balozi et al.

as the only measure. Hulser et al. (1996) observed that electrocoagulation is strongly enhanced at aluminum surfaces in comparison to steel. This is attributed to a higher efficiency due to the in situ formation of dispersed aluminum-hydroxide complexes through hydrolysis of the aluminate ion, which does not occur with steel electrodes. Tsai et al. (1997) employed Fe and Al anodes to simultaneously utilize electrocoagulation, responsible for removal of high molecules, and oxidation during treatment of a raw leachate. Iron anodes provided better COD removal at low applied voltages than did aluminum (Englehardt et al., 2006). As a general statement the efficiency of aluminum or iron electrodes will depend on the specific type of pollutant and also on the different set of operating parameters (Kobya et al., 2003).

ELECTRODE PASSIVATION One of the greatest operational issues with electrocoagulation is electrode passivation. The passivation of electrodes is of concern for the longevity of the process. Passivation of aluminum electrodes has been widely reported in the literature (Nikolaev et al., 1982; Osipenko and Pogorelyi, 1977). The latter also observed that during electrocoagulation with iron electrodes, deposits of calcium carbonate and magnesium hydroxide were formed at the cathode and an oxide layer was formed at the anode. Nikolaev et al. (1982) investigated various methods of preventing electrode passivation and suggested the following options for its control: Changing polarity of the electrode; Hydromechanical cleaning; Introducing inhibiting agents; Mechanical cleaning of the electrodes. According to these researchers, the most efficient and reliable method of electrode maintenance was to periodically mechanically clean the electrodes or wash the electrodes with 8% sulfuric acid between runs in batch which for large-scale, continuous processes is a challenging issue. Corrosion promoters such as Cl - ions have been found to induce thinning of passive layer, enhance dissolution and promote depassivation (spontaneous depassivation). Other types of electrodes with a wide range of materials have been tested for electrocoagulation process. These materials include: Graphite, Platinum oxide, Iridium oxide, lead oxide, tin oxide, boron doped diamond (BDD). Graphite electrodes are deemed to be cheap but unstable and for most part ineffective (Barisa et al., 2009). They become easily fouled during the electrocoagulation process and this reduces their effectiveness. Platinum and Iridium oxide electrodes are too expensive and ineffective. Electrodes made of lead oxide (PbO 2) and tin oxide (SnO 2) are easy to manufacture but they are highly unstable. Boron doped diamonds are materials suitable for use as anodes in the electrocoagulation of organic compounds. Due to their very high resistance to deactivation via fouling and extreme electrochemical stability they show no significant corrosion even under high current densities. They have good chemical, mechanical and

Treatment of Wastewater by Electrocoagulation Method…

11

thermal resistance and a wide electrochemical potential window in aqueous solutions. Above all they can provide very high current efficiencies. Diamond coated electrodes have been investigated worldwide over the past number of years with notable results (Fryda et al., 2003). It is possible to vary electrical properties of diamond from semiconductor (very wide band gap) to close to metallically conductive by varying the boron doping level (1019-1021 cm-3). The most important electrochemical properties of BDD electrodes are their very high corrosion stability in electrochemical applications and their extremely high overvoltage for water electrolysis (Fryda et al., 1999). This large working potential window in aqueous electrolytes provides the possibility of producing strong oxidizing solutions with extremely high efficiency. As reported by Michaud and Comninellis (2000), compared to other electrode materials, BDD electrodes produce hydroxyl radicals on their surface with higher current efficiency. These hydroxyl radicals completely mineralize organic impurities in water or wastewater, such as oil, cooling fluid, toxic compounds (Tennakone et al., 1995). As diamond electrodes are both stable as anodes and cathodes, it is possible to reverse polarity in order to prevent calcium build-up on the electrode surface. Through the use of diamond electrodes, it is possible to obtain an electrochemical process which, without the addition of further chemicals, results in an environmentally friendly and relatively maintenance-free method for the treatment of waste water. Nonetheless, despite the promising results with respect to effectiveness and energy efficiency which have been demonstrated for wastewater treatment, electrosynthesis and electroplating, BDD electrodes remain extremely expensive. A new anode coated IrOx−Sb2O5−SnO2 onto titanium has also been proposed (Xueming et al., 2002). Accelerated life test showed that the electrochemical stability of the Ti/IrOx−Sb2O5−SnO2 anode containing only 2.5 mol % of IrO x nominally in the activated coating was even higher than that of the conventional Ti/IrO x anode. Its service life for electroflotation application is predicted to be about 20 years. Voltametric investigation demonstrated that the Ti/IrO x−Sb2O5−SnO2 anode could provide fast electron transfer. The present anode had a fork-like design and arranged in an interlocking manner with the cathode with a similar shape. Such an innovation in electrode configuration and arrangement is claimed to allow bubbles produced at both electrodes to be dispersed into wastewater flow quickly and, therefore, enhances the effective contact between bubbles and particles, favorable for high flotation efficiency. In addition, the novel electrode system reduces the interelectrode gap to 2 mm, a spacing that is technically difficult for a conventional electrode system (Xueming et al., 2002). This small gap results in a significant energy saving. Easy maintenance is another advantage of this novel electrode system.

AREAS OF APPLICATION OF ELECTROCOAGULATION According to Can et al. (2006), electrocoagulation has been proposed in recent years as an effective method to treat various wastewaters such as: landfill leachate, restaurant wastewater, saline wastewater, tar sand, paper mill effluent, coffee factory effluent, tea factory effluent, oil shale wastewater, urban wastewater, laundry wastewater, nitrate and

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Lazare Etiégni, K. Senelwa, B. K. Balozi et al.

arsenic bearing wastewater, and chemical mechanical polishing wastewater. The electrocoagulation process can successfully remove a wide range of pollutants in a much shorter time than conventional treatment methods (Ogutveren et al., 1994; Kongsricharoern and Polprasert, 1995, ). They include: removal of metals, oil, BOD, TSS, TDS, FOG, color etc., from wastewater before final disposal, thus reducing or eliminating discharge surcharges; reconditioning antifreeze by removing oil, dirt, and metals; reconditioning brine chiller water by removing bacteria and fat; pretreatment before membrane technologies such as reverse osmosis, ultrafiltration and nanofiltration; preconditioning boiler makeup water by removing silica, hardness, TSS; reconditioning boiler blow down by removing dissolved solids eliminating the need for boiler chemical treatment; recycling water, allowing closed loop systems; harvesting protein, fat and fiber from food processor waste streams; de-watering sewage sludge and stabilizing heavy metals in sewage, lowering freight and allowing sludge to be land applied; conditioning and polishing drinking water; removing chlorine and bacteria before water discharge or reuse (Cenkin and Belevstev ,1985; Biswas and Lazarescu,1991; Browning,1996; Adhoum et al., 2004).

COLOR Color is found mostly in surface waters, although some groundwater inside deep wells may also contain color that is noticeable (APHA-AWWA, 1992; AWWA, 1999). Many domestic and industrial wastewaters are rarely colorless and the color levels depend on the industrial process and the age of the wastewater i.e. the travel time in the collection and treatment system (Kim et al., 2005). The pulping and bleaching of wood for example generally produce large amounts of wastewaters that contain lignin derivatives and other dissolved wood by-products. Lignin derivatives which are usually brownish in color remain resistant to biological degradation during wastewater treatment. The brownish color of a pulp and paper mill effluent is mainly attributed to products of lignin polymerization formed during pulping and bleaching operations. These chromophoric groups are mainly quinonic types with conjugated double bonds originating from pulping processes (Luner et al., 1970). When disposed of into natural watercourses, they add color which persists for great distance. Additionally, colored effluents from pulp and paper mills for example result in reduced photosynthetic activity, increased long term BOD, increased water treatment cost for users downstream, and increased toxicity (Springer et al., 1995). Several studies have been carried out to determine the effectiveness of EC in color removal. In general, the findings indicate that EC is more cost effective than normal or conventional coagulation. Moreover, other wastewater pollution parameters are reduced (Orori et al., 2005; Kashefialasl et al., 2006, Oricho et al., 2008). Electrocoagulation combined with wood ash or bagasse ash has also been applied on tea factory effluent. In one study by Maghanga (2008) on tea factory effluent, the treated effluent COD, BOD and electrical conductivity were reduced by 96.6%, 42.4%, and 20.9% respectively. Supporting electrolytes from wood ash, phosphate rock and bagasse ash further reduced power consumption by between 64% and 16%, confirming the effectiveness of this process.

Treatment of Wastewater by Electrocoagulation Method…

13

ADVANTAGES OF ELECTROCOAGULATION (EC) Electrocoagulation has several advantages that are as follows: EC produces effluent with less total dissolved solids (TDS) content compared to chemical treatments. If this water is reused, the low TDS level contributes to a lower water recovery cost. EC requires simple equipment and is easy to operate with sufficient operational latitude to handle most problems encountered during its running. Wastewater treated by EC can give palatable, clear, colorless and odorless water. Sludge formed by EC tends to be readily settable and easy to de-water, because it is composed of mainly metallic oxides/hydroxides. Flocs formed by EC are similar to chemical floc, except that EC floc tends to be much larger, contains less bound water, is acid-resistant and more stable, and therefore, can be separated faster by filtration. The EC process can remove the smallest colloidal particles, because the applied electric field sets them in faster motion, thereby facilitating their agglomeration and subsequent coagulation. The EC process often avoids uses of chemicals and so there may be no problem of neutralizing excess chemicals and no possibility of secondary pollution caused by chemical substances added at high concentration as when chemical coagulation of wastewater is used alone. The gas bubbles produced during electrolysis can carry the pollutant to the top of the solution where it can be more easily concentrated, collected and removed. The electrolytic processes in the EC cell are controlled electrically and with no moving parts, thus requiring less maintenance.

DISADVANTAGES OF ELECTROCOAGULATION (EC) High capital cost has often been cited as one of the major disadvantages of EC although labour requirement may also be high when running an EC batch reactor. Higher voltages and thus high specific energy consumption are also seen as a big disadvantage of the system. The final deficiency of this process relates to the fact that an EC reactor is an electrochemical cell whose performance is directly related to the operational state of its electrodes. As mentioned earlier, they vary widely in design and mode of operation- from simple vertical plate arrangements to packed-bed style reactors containing various metallic packings, and in material used (Ogutveren et al., 1992; Barkley et al., 1993). Potential for electrode passivation, thus slow reaction rates is another draw back because passivation impedes dissolution which normally provides the coagulants in situ. Electrode passivation, specifically of aluminum electrodes, has been widely observed and acknowledged as detrimental to reactor performance (Osipenko and Pogorelyi, 1977; Novikova and Shkorbatova, 1982). This formation of an inhibiting layer, usually an oxide, on the electrode surface prevents metal dissolution and electron transfer, thereby limiting coagulant addition to the solution. Over time, the thickness of this layer increases, reducing the efficacy of the electrocoagulation

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Lazare Etiégni, K. Senelwa, B. K. Balozi et al.

process as a whole. The use of new materials, different electrode types and arrangements (Pretorius et al., 1991; Mameri et al., 1998) and more sophisticated reactor operational strategies (such as periodic polarity reversal of the electrodes mentioned above) have led to significant reductions in the impact of passivation. The issue, however, is still seen as a serious potential limitation for applications where a low-cost, low maintenance water treatment facility is required.

DESIGN OF ELECTROCOAGULATION UNITS The inherent complexity of the electrocoagulation reactions makes it difficult to model and control this process. Adequate scale-up parameters, a systematic approach to the optimization and a priori prediction for the performance of the electrocoagulation reactor are yet to be established. A literature survey reveals that previously each ―new‖ system has been considered separately on an individual basis. The key driver for the development of any particular application of this process has generally been the removal of a specific pollutant i.e. color, heavy metal, COD, tannin etc. There has been little or no attempt to provide a holistic approach to electrocoagulation. Consequently, despite more than a century‘s worth of applications, many of them deemed successful, the science and engineering behind EC reactor design is still largely empirical and heuristic. It has failed to take full advantage of the potential success and incorporation in the understanding behind the science of electrocoagulation. A literature review indicates that EC reactors can be configured as batch or continuous and that the majority of reactors reported so far fall in the latter category with continuous feed and outflow operating under pseudo-steady state. Electrocoagulation systems require amperage to treat the water. The amount of amperage drawn is dependent upon the conductivity of the water or wastewater. If the water is not conductive then no amperage will be used. The system should be designed with adequate wiring and electrical capacity to deliver adequate amperage if needed by a particular water stream.

PHYSICAL DESIGN ISSUES There has been a range of laboratory, pilot and industrial scale electrocoagulation units produced. The designs range from fully integrated units to ‗stand alone‘ reactors. The electrocoagulation process has been combined with many units including microfiltration, dissolved air flotation (DAF), sand filtration and electroflotation. Obviously, pre- and postwater treatment impacts significantly on the performance of the electrocoagulation reactor. The design of the electrocoagulation process influences its operation and efficiency (Holt et al., 2005). The design phase should consider the following physical factors: Continuous versus batch operation Reactor geometry Reactor scale-up Current density

Treatment of Wastewater by Electrocoagulation Method…

15

GEOMETRY Geometry of the reactor affects operational parameters including bubble path, flotation effectiveness, floc formation, fluid flow regime and mixing/settling characteristics. From the literature, the most common approach involves plate electrodes (aluminum or iron) and continuous operation. Water is dosed with dissolved metal ions as it passes through the electrocoagulation cell. A downstream unit is often required to separate pollutant and water.

SCALE-UP ISSUES One of the cornerstones of chemical engineering is to establish key scale-up parameters to define the relationships between laboratory and full-scale equipment. The surface area to volume ratio (S/V) is a significant scale-up parameter. Electrode area influences current density, position and rate of cation dosing, as well as bubble production and bubble path length. Mameri et al. (1998) reported that as the S/V ratio increases the optimal current density decreases. However, the S/V ratio was not widely reported. Some of the values reported are listed in Table 1 below: The values reported here seem empirical with no specific criteria for their choice. A more rigorous and consistent approach is clearly required to establish a set of design characteristics for Electrocoagulation reactors. The prime differentiator between pollutant removal by settling or flotation would seem to be the current density employed in the reactor. A low current produces a low bubble density, leading to a low upward momentum flux—conditions that encourage sedimentation over flotation (Holt et al., 2002). As the current is increased, so does the bubble density resulting in a greater upwards momentum flux and thus more likely removal by flotation. Other researchers such as Zolotukhin (1989) scaled up an electrocoagulation-flotation system from laboratory to industrial scale. The following dimensionless scale-up parameters have been chosen to ensure correct sizing and proportioning of the reactors: Reynolds number – indication of the fluid flow regime; Froude number – indication of buoyancy; Weber criteria – indication of the surface tension; Gas saturation similarity; Geometric similarity. Table 1. S/V values reported in the literature. Reference (Author) Amosov et al. Osipenko and Pogorelyi Novikova and Shkorbatova Orori et al. Oricho et al. Maghanga

Year 1976 1977 1982 2005 2008 2008

S/V (m2/m3) 30.8 18.8 42.5 80.0 75.5 18.2

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Lazare Etiégni, K. Senelwa, B. K. Balozi et al.

Horizontal Flow

Vertical flow

Figure 3. Types of electrodes set-up during EC.

Electrodes during EC can be set up as parallel vertical or horizontal sheets as can be seen in Figure 3. The turbulence generated by the gases at the anode and cathode can be used in both types of flow. However, vertical flow allows for more improved separation by electroflotation as compared with horizontal flow.

FACTORS AFFECTING ELECTROCOAGULATION PROCESSES Several studies have shown that electrocoagulation is quite complex and may be affected by several operating parameters such as pollutant concentrations, initial pH, electrical potential voltage, COD, turbidity, pollutant type and concentration, bubble size and position, floc stability and agglomerate, size and type of supporting electrolyte. The complexity and number of possible interactions are highlighted in Figure 2.

EFFECT OF PH ON ELECTROCOAGULATION Optimal pH reported for electrocoagulation reactions varies significantly. These discrepancies probably derive from the complex and variability of wastewater composition, and the different operating conditions used in the electrocoagulation studies. It has been established that pH has a considerable effect on the efficiency of the electrocoagulation process (Springer et al., 1995, Chen et al., 2000, Li et al. 2001). The wastewater pH determines the speciation of metal ions and influences the state of other species in solution and the solubility of products formed. The pH of the medium also changes during electrocoagulation process, as observed by other investigators. This change depends on the type of electrode material and initial pH and alkalinity. In a study by Bazrafshan et al. (2008) on the removal of Chromium VI from synthetic chromium solutions by electrocoagulation

Treatment of Wastewater by Electrocoagulation Method…

17

using aluminum electrode, it was observed that there was an increased in the solution pH for an initial pH of less than 7. The increase was ascribed to hydrogen evolution at the cathodes contrary to Chen et al. (2000) assertion that the pH increase is due to the release of CO2 from wastewater owing to H2 bubble disturbance. At low pH, wastewater is over saturated with CO2 which can be released during H2 evolution, causing a pH increase. In addition, if the initial pH is acidic, reactions would shift towards a pH increase (Bazrafshan et al. 2008). During the same experiment, in alkaline medium (pH > 8), the final pH did not vary considerably but a slight drop was recorded. This result concurs with previous published works and suggests that electrocoagulation can act as a pH buffer (Gao et al.,2005). In the same study of chromium removal by electrocoagulation carried out over a wide range of Cr concentrations, it was also observed that the influent pH did not significantly affect the removal efficiencies of Cr VI. This means that for practical applications, pH adjustment before treatment is not required. In another study by Springer et al. (1995) on the effect of pH on the color removal reaction by electrocoagulation, it was found that higher pH slowed the electrocoagulation reaction, thereby increasing power consumption. In a separate study on color removal from a pulp and paper mill effluent, Orori (2003) found that decreasing the original effluent pH led to a significant reduction in power consumption during electrocoagulation combined with wood ash leachate. Lowering pH from 12.0 to pH 10.0 significantly (P 0.05) reduced power consumption by between 20 to 21% during electrochemical removal of a paper mill effluent color. It was postulated that decreasing the original effluent pH increased ionisation of wastewater, which increased the rate of iron (II) ions production at the anode and hydrogen at the cathode. Consequently, decreasing pH led to increased production of positively charged iron (II) ions, which attracted the negatively colored flocci (Springer et al., 1995). Thus increased production rate of these ions led to an increase in the rate of color removal at lower pH than at higher pH. Therefore lower pH facilitated color removal and lowered electrical power consumption. Li et al. (2001) reported that COD removal was at least 20% higher at pH 4.0 than at pH 8.0 after a 4-hour electrolysis. Vlyssides et al. (2003) found that pH was the most significant operational parameter in electrolyzing leachate, compared to Clconcentration, temperature, applied voltage, SO42- concentration and leachate input rate. Lower pH favored COD removal and saved energy consumption within the range pH 5.5 – 7.5. The disagreement in these investigations suggests further work, perhaps in terms of the mechanisms by which pH affects COD removal in leachate electrolysis. Theoretically, it can be stated that acidic conditions decrease the concentrations of CO32and HCO3- , both well-known scavengers of OH radical generated at the anodes (Li et al., 2001), while alkaline conditions promote the Cl-→Cl2→ClO-→Cl- redox cycle. Therefore, low pH may enhance direct oxidation, while high pH may enhance indirect oxidation (Wang et al., 2001). Thus, solution pH influences the overall efficiency and effectiveness of electrocoagulation. An optimal pH seems to exist for a given pollutant, with optimal pH values ranging from 6.5 to 7.5 (Holt et al.,2002). Kashefialasl et al. (2006) showed that the maximum efficiency of color removal during the treatment of dye solution containing colored index acid yellow 36 by electrocoagulation using iron electrodes was observed at pH range 7–9 as expected considering the nature of the reaction between ferrous and hydroxide ions. When the pH of solution was lower than 6, Fe(OH)3 was in soluble form (Fe+3) and when it was higher than 9, Fe(OH)3 was in soluble form {Fe(OH)4-} and because Fe(OH)3 played a major role in removing color, when pH of

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Lazare Etiégni, K. Senelwa, B. K. Balozi et al.

solution was 8, color removal was the highest. The dye solution with different initial concentrations in the range of 20-60 mg/l was treated by EC at optimized current density and time. In contrast, other investigators have found that pH variation does not considerably alter COD removal in leachate electrolysis. Chiang et al. (1995a) have reported that the pH effect on chlorine/hypochlorite production efficiency was insignificant over the range pH 4-10 during an electrolysis experiment in saline water conducted to help elucidate the mechanism of electrolyzing leachate. Cossu et al. (1998) found that a pseudo-first-order rate constant for COD reduction in real leachate increased only slightly at pH 3, compared with pH 8.3. Also, Wang et al. (2001) have reported that at pH 8.9 and 10, COD removal was approximately 4% higher than at pH 7.5, not a very significant effect.

EFFECT OF CURRENT DENSITY ON ELECTROCOAGULATION Current density (i) is the current delivered to the electrode divided by the active area of the electrode. Varying the current easily controls this parameter. Current density determines both the rate of electrochemical metal dosing to the water and the electrolytic bubble density production. Current densities ranging from 10 to 2000 A/m2 have been reported (Holt et al., 2005). The majority of the sources used for this write-up report a current density in the range 10 – 150 A/m2. Different current densities are desirable in different situations. Current densities reported for electrochemical oxidation of leachate ranged from 5 to 540 mA/cm2 (Englehardt et al., 2006). It is reported that at least 5 mA/cm2 is required to achieve effective oxidation of organics in leachate. Table 2. Percent of chromium removal during electrocoagulation process using aluminum electrodes (Initial concentration = 50 mg l−1). T = 60 min 98.62 98.74 98.88 98.40 98.44 98.72 92.00 97.64 98.34

T = 40 min 94.78 95.64 95.80 89.76 91.72 95.72 90.80 92.18 92.58

Source: Bazrafshan et al. 2008, with permission

T = 20 min 83.76 85.64 88.98 82.76 83.14 83.46 64.60 77.00 81.80

Voltage (V) 20 30 40 20 30 40 20 30 40

pH 3

7

10

High current densities are desirable for separation processes involving flotation cells or large settling tanks, while small current densities are appropriate for electro-coagulators that are integrated with conventional sand and coal filters. A systematic analysis will be required to define and refine the relationship between current density and desired separation effects. Current density (current per unit area of electrode) in an electrochemical process indicates gross reaction rate. For example under weaker oxidative conditions, leachate may darken and

Treatment of Wastewater by Electrocoagulation Method…

19

brown precipitates may form at the anode (Cossu et al., 1998; Li et al., 2001). Increasing current density improves COD and NH3-N treatment efficiencies at the same charge loading. Bazrafshan et al. (2008) showed that increasing electrocoagulation voltage increased the removal efficiency of Chromium, which was also helped by higher pHs as can be seen in Tables 1 and 2. Chiang et al. (1995b) reported that during electrolytic treatment of leachate, COD removal at 25 mA/cm2 was approximately 50% higher than that observed at 6.25 mA/cm2, for the same charge loading (1.178 x 105 Coulombs/L). This is probably due to the fact that increasing current density during electrolysis enhances chlorine generation, which may have been responsible for subsequent removal of pollutants (Costaz et al., 1983; Chiang et al., 1995a). Li et al., (2001) have shown that the effect of current density on treatment was not evident between 30 and 120 mA/cm2 at a low Cl- concentration (1650 mg/L), but became noticeable when Cl- concentration reached the 5000 mg/L level. Table 3. Percent of chromium removal during electrocoagulation process using aluminum Electrodes (Initial concentration = 500 mg l−1). T = 60 min 25.60 35.80 83.00 20.40 24.60 80.80 23.00 26.80 52.00

T = 40 min 22.0 27.00 71.20 19.60 20.40 64.60 13.80 22.00 41.20

Source: Bazrafshan et al. 2008, with permission.

T = 20 min 21.80 24.80 51.80 13.60 17.80 41.00 8.80 12.80 32.00

Voltage, (V) 20 30 40 20 30 40 20 30 40

pH 3

7

10

Figure 4. Effect of current density on the efficiency of color removal from a solution with concentration of the dye = 50 ppm (Source: Kashefialasl, et al., 2006 with permission).

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Lazare Etiégni, K. Senelwa, B. K. Balozi et al.

This result corroborates the importance of indirect oxidation during the electrolytic treatment of leachate. In addition, Moraes et al. (2005) reported that color removal from leachate strongly depended upon current density. Color removal efficiency at 116 mA/cm2 was five times higher than that at 13 mA/cm2, after 180 minutes of electrochemical treatment. In the treatment of dye solution containing colored index acid yellow 36 by electrocoagulation using iron electrodes Kashefialasl et al. (2006) showed that as current density increased so did color removal from the dye solution up to a certain maximum as shown in Figure 4. During electrocoagulation, electrical current not only determines the coagulant dosage rate but also the bubble production rate and size and the floc growth, which can influence the treatment efficiency by electrocoagulation (Letterman et al., 1999; Holt et al., 2002). This is ascribed to the fact that at higher voltage the amount of anode material oxidized increases, resulting in a greater amount of precipitate for the removal of pollutants. In addition, it has been demonstrated that bubble density increases and their size decreases with increasing current density resulting in a greater upwards flux and a faster removal of pollutants and sludge flotation (Khosla et al., 1991). As the current decreased, the time needed to achieve similar efficiencies increases. This expected behavior is explained by the fact that the treatment efficiency is mainly affected by charge loading (Q = It), as reported by Chen et al. (2000). However, the cost of the process is determined by the consumption of the sacrificial electrode and the electrical energy. It has also been established that for a given time, the removal efficiency increased significantly with increase of current density. The highest electrical potential normally produces the quickest treatment.

EFFECT OF THE CONCENTRATION OF POLLUTANTS Several investigations have shown that the initial concentration of pollutants has a bearing on the efficiency of the electrocoagulation process (Orori, 2003, Etiegni et al., 2007, Mahvi and Bazrafshan, 2007). A set of experiments was performed with different initial concentrations of chromium to determine the time required for its removal under various conditions of electrocoagulation process (Bazrafshan et al. 2008). SPP1

Power Consumption (MWh))

45

SPP2

40 SPP3

35

SPP4

30

SPP5

25 20 15 10 5 0 15

20

25

30

35

40

45

o

Temperature ( C)

Figure 5. Effect of temperature on power consumption by ELCAS at five sampling points along a pulp and paper Mill effluent treatment system (Source: Orori, 2003 with permission).

Treatment of Wastewater by Electrocoagulation Method…

21

The results obtained at different electrical potentials showed that initial concentration of chromium may have an effect on the efficiency of its removal and for higher concentration of chromium, higher electrical potential or more reaction time is needed. On the other hand, if the initial concentration increases, the time required should increase too. It is clear from Tables 1 & 2 that at higher concentrations, longer time is needed for removal of chromium, but higher initial concentrations of chromium were reduced significantly in relatively less time compared to lower concentrations. The time taken for its reduction thus increases with the increase in concentration. This can be explained by the theory of dilute solution. In dilute solution, formation of the diffusion layer at the vicinity of the electrode slows the reaction rate, but in concentrated solution the diffusion layer has no effect on the rate of diffusion or migration of metal ions to the electrode surface (Chaudhary et al., 2003). Chromium removal with respect to time by electrocoagulation process at different pH levels is shown in Tables 1 & 2.

EFFECT OF DISTANCE BETWEEN THE ELECTRODES

Metal removal (%)

Numerous research work have been conducted on the effect of electrode distance on the removal of wastewater contaminants by EC (Springer et al., 1995; Ecobar et al.,2006). For some researchers, the electrode gap did not seem to have an impact on the electrocoagulation process although at the narrowest of gap, the clogging of electrodes appeared to reduce the rate of reaction (Springer et al., 1995).

Distance between the electrodes (cm) Source: Escobar et al., 2006, with permission Figure 6. Effect of electrode gap on the removal of (A) Cu, (B) Pb and (C) Cd Current density=36 Am2, Electrolysis time =10 min, Conductivity = 900 mS/cm.

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Lazare Etiégni, K. Senelwa, B. K. Balozi et al.

EFFECT OF GAP BETWEEN ELECTRODES Escobar et al. (2006) while studying the optimization of the electrocoagulation process for the removal of copper, lead and cadmium in natural waters and simulated wastewater found that increasing the gap between the electrodes reduced metal removal due to a decrease in current flow and coagulant generation (Figure 6). An optimal distance of 2.0 cm for removal of lead and 2.5 cm for removal of cadmium and copper was identified for subsequent experiments. It will appear that each solution has an optimum electrode gap that should be determined experimentally before the EC can be optimized. In general, small gap results in a significant energy saving but makes it difficult to clean the electrode surface in conventional EC

EFFECT OF TURBIDITY In order to study the effect of turbidity (10, 50 and 200 NTU) on removal efficiency of cadmium a set of experiments was performed with different initial concentrations of cadmium (5, 50 and 500 mg l-1). The results obtained at optimum condition (pH=10, time reaction = 60 min and voltage = 40 V) showed that the removal efficiency for various concentrations of cadmium was fairly unchanged and hence electrocoagulation process can be applied efficiently for cadmium removal in the presence of turbidity (Mahvi and Bazrafshan, 2007).

EFFECT OF TEMPERATURE Raising temperature during electrocoagulation increases the rate of reaction (Shenz et al., 2006). Springer et al. (1995) showed that the time required for color removal reaction through electrocoagulation to reach 0.2 Absorbance Units (AU) was cut approximately by half by increasing temperatures from 23oC to 80oC (12 min vs 7 min). Orori (2003) studied the effect of temperature on color removal by electrocoagulation combined with wood ash leachate (ELCAS) and the results are shown in Figure 5. It was found that at 40oC color removal consumed less than 50% the electric power used at 20oC by ELCAS treatment. This was attributed to fast movement of electrons at higher temperatures compared to low temperatures.

EFFECT OF SUPPORTING ELECTROLYTES The underlying principle of EC (Figure 2) is the generation of cations by the dissolution of sacrificial anodes that induce flocculation of the dispersed pollutants contained by the zeta potential reduction system (Calvo et al., 2003; Mollah et al., 2004). During EC processes, high energy can be consumed leading to longer and slower reaction rates. For the EC to be effective, various types of electrodes and configurations have been tested. Several studies have also been conducted to determine the impact of certain additives such as supporting electrolytes during EC (Eichhorn et al.,1996). A supporting electrolyte (SE) is used to increase conductivity in the majority of all electroanalytical or electrosynthetic experiments

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23

such as electrocoagulation in aqueous and non-aqueous solutions (Lund et al., 1991; Fry, 1996). While a large number of experiments have been performed with electrodes under conditions where no SE was deliberately added, it is increasingly common practice to operate an EC in the presence of a certain amount of ions such as chloride or ammonium or salts such as NaCl, Na2SO4 and NaNO3 (Lopes et al., 2004; Orori et al., 2005; Shenz et al., 2006; Englehardt et al., 2006; Uğurlu, 2006; Oricho et al., 2008; Yildiz et al. 2008). Some of the electrolytes used in past experiments are shown in Table 4 and their respective effects on effluent color removal. Hu et al., (2003) carried out an experiment on defluoridation by EC and studied the effect of coexisting anions. The results showed that the type of dominant anion had a direct impact on the EC defluoridation reaction. Defluoridation efficiency was nearly 100% and most of the fluoride removal reaction occurred on the surface of the anode in the solution without the coexisting anions, due to the electro-condensation effect. In the solutions with co-existing anions, most of the defluoridation took place in the bulk solution. The residual fluoride concentration was a function of the total mass of Al3+ liberated. It was found that sulfate ions inhibited the localized corrosion of aluminum electrodes, leading to lower defluoridation process because of lower current efficiency. However the presence of chloride or nitrate ions prevented the inhibition of sulfate ions, and the chloride ions were more efficient. Different corrosion types occurred in different anion-containing solutions and the form of corrosion affected the kinetic over-potential of the EC reaction. When the concentration of NaCl salt or any other supporting electrolyte in solution increases, solution conductivity increases. Consequently, with respect to the solution voltage if any SE is added: V = EC - EA- δA- δC - IRcell - IRcircuit

(24)

where: EC EA δA δC IRcell IRcircuit

= Electrical potential difference at the cathode = Electrical potential difference at the anode = Zeta potential at the anode = Zeta potential at the cathode = voltage-drop across the cell = voltage-drop across the circuit

the necessary voltage for access to a certain current density will reduce, and the consumed electrical energy will be decreased (Kashefialasl et al., 2006). Excess SE affects the current in the bulk of the solution, which is maintained mostly by the ions of the SE, and migration effects on charged substrates can be neglected. The SE can also have some affects on the double layer reducing the Zeta potential of the substrate ions and helping their agglomeration or coagulation. Orori et al. (2005) showed that when the volume of wood ash leachate increased during color removal from a pulp and paper mill effluent, the power consumption reduced considerably by almost 80%. Similar results were also obtained by Etiégni et al. (2007) and Oricho et al. (2008).

24

Lazare Etiégni, K. Senelwa, B. K. Balozi et al. Table 4. Effect of the electrolyte concentration on the efficiency of color removal. Type of Electrolyte *NaCl *BaCl2 *KCl *NaBr *KI *Na2SO3 *Na2CO3 **Wood ash leachate ***Phosphate rock ****Bagasse ash leachate *****NH4NO3 *****Na2SO4 **Alum **Ca(OH)2

Applied voltage (v) 2.9 5.1 2.7 4.2 5.3 3.8 4.2 23 23 24 3.9 3.5 23 23

Conductivity (μS/cm) 19.13 9.67 18.75 11.77 9.18 13.8 15.8 4823.12 1150-1730 310

Color removal (%) 83 77 80 80 79 76 76 100 100 100

3456.23 3358.34

100 94 100 100

*Source: Kashefialasl et al., 2006: (Current density =127.8A/m2, Time of electrolyses =6min) **Source: Orori , 2003 (Current density= 250 A/m2, Time of electrolysis= 150 s) ***Source: Etiégni et al., 2007: (Current density = 222.2 A/m2, Time of electrolysis = 55 s) **** Source: Maghanga, 2008: (Current density = 55 A/m2, Time of electrolysis = 4 min) ***** Source: Lopes et al., 2004: (Current density = 2 mA/cm2, Time of electrolysis = 70-96 hrs)

30

Power (Whr)

25

OSA

NSA

OSC

NSC

OSR

NSR

20 15 10 5

0 0

1000

2000

3000

4000

5000

Electrolyte Dosage (g/m3 )

Source: Etiégni et al., 2009 Figure 7. Effect of electrolyte volume on the power consumption.

It appears that leachates from wood ash contain a wide range of ions or supporting electrolytes that may be helping or assisting the electrocoagulation reaction (Figure 7). Chou et al. (2009) studied the effect of NaCl concentration on the removal efficiency of indium

Treatment of Wastewater by Electrocoagulation Method…

25

(III). They showed that there was an increase removal efficiency up to 83% when NaCl (used as supporting electrolyte) concentration was 8 g/l. The concentration of supporting electrolyte was adjusted to the desired levels by adding a suitable amount of NaCl to the synthetic wastewater. Increasing the concentration of the supporting electrolyte from 0 to 200ppm led to an increase in indium (III) ion removal efficiency, whereas with the concentration of the supporting electrolyte increasing, the specific energy consumption decreased by almost 80%. When the concentration of the supporting electrolyte increased, the solution ohmic resistance decreased, so the current required to reach the optimum applied voltage diminished, decreasing the consumed energy (Chou et al. (2009). Although some SEs are available commercially, they can be extracted from material otherwise considered as waste. Several research papers have been recently published on the use of leachate from ash emanating from wood, coffee husk or bagasse as supporting electrolyte (Orori et al., 2005; Etiégni et al., 2007, Oricho et al., 2008).

OPERATING COST ANALYSIS OF ELECTROCOAGULATION PROCESSES Several studies have been carried out on the operating cost of electrochemically treated wastewater (Bayramoglu et al., 2004; Can et al., 2006; Bayramoglu et al., 2007). In a study by Bayramoglu et al. (2004) for the treatment of textile wastewater by EC using aluminum and iron electrode materials, the effect of wastewater characteristics and operational variables on the technical performances of COD and turbidity removal efficiencies as well as on the EC operating cost were determined.. Only direct costs such as material (electrodes and chemical reagents) and energy costs were considered for the calculation of the operating cost. Other cost items such as labor, maintenance and solid/liquid separation costs, depreciation of fixed investment such as rectifier and electro-coagulators were not taken into account. This simplified cost equation was used to evaluate the effect of various process variables on the operating cost: Operating cost + aCenergy + bCelectrode

(25)

where Cenergy and Celectrode, were consumption of energy and electrode material per kg of COD removed, which are normally obtained experimentally. Unit prices, a and b, determined for a specific market are as follows: a= electrical energy price and b= electrode material price for aluminum or for iron. Using equation 6, Bayramoglu et al. (2004) found that for iron electrode, the operating cost decreased initially with pH until pH = 5, where it remained constant up to pH=7, beyond which it increased (Figure 8). For aluminum electrodes, the EC cost increased with initial pH.

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Lazare Etiégni, K. Senelwa, B. K. Balozi et al.

Operating cost $/kg of COD

Figure 8 shows the effect of initial pH on the EC operating cost.

Initial pH Source: Bayramoglu et al. (2004) with permission Figure 8. Effect of initial pH on EC cost.

EFFECT OF CONNECTION MODES ON EC OPERATING COST When cells are set-up in an electrocoagulation process, one can choose from different modes or connections depending on the required voltage, the expected output and the overall efficiency of the EC system. Kobya et al. (2007) studied the effect of wastewater pH, current density and operating parameters for two sacrificial electrode materials, Fe and Al, and three electrode connection modes - namely monopolar-parallel (MP-P), monopolar-serial (MP-S) and bipolar-serial (BP-S) on the EC operating cost. The highest consumption of electrode material occurred with bipolar series mode (BP-S); approximately 0.27 kgm−3 for Fe electrode and between 0.18–0.23 kgm−3 for Al electrode. Monopolar parallel (MP-P) mode showed the lowest electrode consumption for both electrode Fe and Al materials: 0.12 kgm−3 for Al electrode and 0.16 kgm−3 for Fe electrode (Kobya et al., 2007). When the consumption of energy was compared for the three modes, as seen in Figure 10, only a minor change was observed with pH for all of the systems using Fe or Al electrodes. MP-S and BP-S modes exhibited high consumptions of energy because of the serial connection that required higher potential. When MP-P mode was used, it consumed the lowest energy or approximately 0.63 kWhm−3 and 0.7 kWhm−3 for Fe and Al electrode respectively. The effect of the initial pH on amount of sludge production is depicted in Figure 12. Sludge amounts vary from 0.65 to 1.0 kgm−3 for Fe electrode and from 0.9 to 1.3 kgm−3 for Al electrode (Kobya et al., 2007). In general, more sludge was produced with BP-S mode than with MP-P mode because of high electrode material consumed leading to high power consumption and higher electro-coagulant produced in situ. MP-P mode for both

Treatment of Wastewater by Electrocoagulation Method…

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electrode materials was therefore economically more feasible owing to its low electrical energy consumptions and amount of sludge produced (Figure 12).

EFFECT OF ELECTRICAL CONDUCTIVITY OF EC COST Bayramoglu et al. (2004) studied the impact of electrical conductivity on the operating cost of a dye wastewater treatment system using two sets of electrodes (Figure 11). For both electrode materials, operating cost decreased with increasing conductivity and the decrease was almost similar for iron and aluminum electrodes with only a slight difference at 3500 μS/cm for aluminum electrode. For aluminum, the percentage of the electrode consumption cost with respect to the total cost was nearly constant as 76%. For iron, on the other hand, this ratio increases from 33 to 58%, with increasing conductivity from 1000 to 4000 μS /cm. The decrease in operating cost was probably due to a decrease in solution ohmic resistance. As SE increased, lower current required to reach the optimum applied voltage leading to the overall decreased of consumed energy (Chou et al. (2009).

Figure 9. Different types of electrode connection modes: a-Monopolar parallel (MP-P), b-Monopolar Serial (MP-S), c-Bipolar parallel (BP-P) modes. (Source : Kobya et al., 2007, with permission).

EFFECT OF RETENTION TIME ON EC COST Kobya et al. (2006) studied the effect of detention time during the treatment of potato chips manufacturing wastewater by electrocoagulation. They found that both energy and electrode consumption increased with retention time. Retention time is therefore likely to affect the operating cost of EC.

EFFECT OF CURRENT DENSITY ON EC COST In a study aimed at determining the impact of current density on operating cost of EC Kobya et al. (2007) showed that, in the case of iron, current density did not have a substantive impact on the performances of MP-P and MP-S modes; COD removal reached a maximum of

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Lazare Etiégni, K. Senelwa, B. K. Balozi et al.

67% with MP-P mode for a current density of 50Am−2. However, for aluminum electrode, the effect of the current density was more pronounced on COD removal, especially for MP-P mode and that lower current densities were more favourable. For example a 30Am−2 was preferred with MP-S mode.

Source: Kobya et al. (2003) with permission Figure 10. Effect of initial pH on energy consumption.

EFFECT OF POLYELECTROLYTE AND SE ON THE EC OPERATING COST As a general rule, EC operating cost has been found to reduce with the addition of polyelectrolyte up to an optimum concentration beyond which it usually rises, although this will also depend on the type of polyelectrolyte. Can et al. (2006) showed alum and polyaluminum chloride (PAC) increased operating EC operating cost when their concentration increased (Figure 13). However, Orori et al. (2005) found that increasing the concentration of wood ash leachate reduced power consumption and reduced operating cost, although the cost of electrode replacement and sludge removal was not included in the overall operating cost calculations.

Operating cost $/kg COD

Treatment of Wastewater by Electrocoagulation Method…

Conductivity, μS/cm Source: Bayramoglu et al. (2004) with permission. Figure 11. Effect of wastewater conductivity on EC cost.

Source: Bayramoglu et al. (2007) with permission

Figure 12. Effect of initial pH on sludge formation

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Lazare Etiégni, K. Senelwa, B. K. Balozi et al.

Operating cost ($/kg COD)

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Polyelectrolyte dosage (kg/m3) Source: Can et al. (2006) with permission Figure 13. Effect of polyelectrolyte addition on EC operating cost.

WOOD ASH LEACHATE USED AS A SUPPORTING ELECTROLYTE What Is Wood Ash? Wood ash is the residue powder left after the combustion of wood. The main producers of wood ash are wood industries, power plants, homesteads especially in Third World countries. Large amount of this residue are produced every day. Typically 6-10 percent of burned wood results in ash. Wood ash is commonly disposed of in landfills or agricultural lands, but with rising disposal costs ecologically friendly alternatives are becoming more attractive. These alternatives will be based on the ash composition. It has been demonstrated that wood ash composition is a function of the wood combustion temperature as can be seen in Table 5 (Etiégni and Campbell, 1991). Wood combustion produces a highly alkaline ash that can be used to neutralize acidic effluent. As can be seen in Table 5 below Ca, K, Mg, Mn, Fe and Na are important elements found in wood ash. Misra et al., (1993) analyzed samples of wood ash using Inductively Coupled Plasma Emission Spectrometer (ICPES) and X-ray diffraction (XRD) to identify the minerals present in wood ash. A list of the compounds identified in ash is shown below in Table 6. The low temperature ash at 600oC showed strong peaks corresponding to calcium carbonate. Pine and aspen ash contained relatively higher amounts of potassium compared to poplar ash and showed strong peaks corresponding to K2Ca(CO3)2. Pine ash contained calcium manganese oxide, Aspen ash had sulfates of calcium and potassium, and poplar ash, silicates of K, Mg, and Ca. At higher temperatures (1000oC) where most industrial wood-fired boilers operate, with the dissociation of carbonates, XRD patterns showed predominant presence of calcium and magnesium oxides. In addition, pine ash which contains more

Treatment of Wastewater by Electrocoagulation Method…

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manganese showed the presence of calcium manganese oxide and manganese oxide. Similarly, poplar, being richer in sodium, displayed weak peaks corresponding to sodium calcium silicate. It appears that when the ash is left standing in air, calcium oxide reacts with atmospheric water vapor to form calcium hydroxide. However calcium hydroxide is unstable at temperatures over 600oC. Table 6 also indicates that small amounts of potassium may be present as K2SO4 as the peaks corresponding to this compound become distinct at higher temperatures. Low temperature ash produced from the wood waste appears to contain predominantly calcium carbonate while at high temperatures the content changes to predominantly calcium oxide. What this Table shows is the close relationship of ash composition with combustion temperature. Many of these elements, when in solution, will behave as counter-ions. Wood ash leachate added to wastewater does the following: it hydrolyzes hydroxo-metallic positively charged ions are added to the wastewater medium the solution ionic strength is increased the solution electrical conductivity increases the positive hydroxo-metallic ions are adsorbed on the negative charge of the colloids surface, reducing the zeta potential to destabilization point the electrostatic distance between colloid particles is reduced and the energy barrier is overcome to allow agglomeration the presence of non-hydrolyzing counter-ions (Na+, Ca2+, Mg 2+) leads to the compression of the double layer (Figure 1) which leads to the reduction of the Zeta potential to van der Waals levels. with wood leachate, Al3+ and Fe2+ are also added and help neutralize the solution charge. They form precipitates that catch colloids in the flocs. these destabilized colloids and hydroxo-metallic complex by van der Waals forces lead to adsorption and flocculation.

Important Consideration of Wood Ash Leachate as Supporting Electrolyte One of the most important factors that need to be considered when using wood ash leachate as supporting electrolyte is the time required to allow leaching to take place and the ash to water ratio for leaching. In an experiment conducted on wood ash leaching, Etiégni and Campbell (1991) found that the total dissolved solids (TDS), K, Na, and Mg concentration increased linearly as the ash to water ratio increased (Figure 14). However, the percentage of ash dissolving did not change significantly, as approximately 10% of the ash dissolved at 50 g/L and 9% at 390 g/L.

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Lazare Etiégni, K. Senelwa, B. K. Balozi et al. Table 5. Chemical composition of wood ash samples produced at different combustion temperatures given as the concentration in μg/g. (Source: Etiégni & Campbell, 1991). Element Aluminum Antimony Arsenic Barium Beryllium Bismuth Cadmium Calcium Cerium Chromium Cobalt Copper Iron Lanthanum Lead Lithium Magnesium Manganese Molybdenum Nickel Phosphorus Potassium Selenium Silicon Sodium Titanium Vanadium Ytterbium Zinc Zirconium Carbonate

Temperature (oC) 538 649 10,415 12,825 264 142