TEXTILE INDUSTRY report.pdf
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STUDY OF WASTEWTER TREATMENT OF TEXTILE INDUSTRY PREPARED BY AMIL MEMON (150080717001) AJAY ANDODARIYA (150080717002) JAIMIN PATEL (150080717009) KAUSHAL PATEL (150080717011) ENVIRONMENTAL ENGINEERIG In CIVIL ENGINEERING DEPARTMENT
BIRLA VISHVAKARMA MAHAVIDHYALA, V.V.NAGAR AUTONOMOUS INSTITUTE
TEXTILE INDUSTRY The textile industry is concerned with the design and production of yarn, cloth, clothing, and their distribution. The raw material may be natural or synthetic using products of the chemical industry. The textile industry is generally having three main categories:
Cellulose fibers (cotton, rayon, linen, ramie, hemp and lyocell), Protein fibers (wool, angora, mohair, cashmere and silk) and Synthetic fibers (polyester, nylon, spandex, acetate, acrylic, ingeo and polypropylene).
WATER USES AND WASTE WATER PRODUCED The various processes involved in the textile industries produce large amounts of gas, liquid and solid wastes. The textile industry uses a various types of chemicals and a large amount of water for all of its manufacturing processes. About 200 L of water are used to produce 1 kg of textile. Water is mainly used for: (a) the application of chemical onto textiles and (b) rinsing the manufactured textiles. The quantity of water required varies from industry to industry depending on the dyeing process and the type of fabrics produced. Wet processes usually use a lot of chemicals and water. About 80-150 m3 of water are used to produce 1 kg of fabrics. It is estimated that about 1,000-3,000 m3 of waste water is reproduced after processing about 12-20 tonnes of textiles per day. The water coming out after the production of textiles contains a large amount of dyes and other chemicals which are harmful to the environment. The different industries have the level of toxicity or harmfulness of the textile effluents. In the textile dyeing process, there is always a portion of unfixed dye which gets washed away along with water. The textile wastewater is found to be high in the unfixed dyes.
CHARACTERISTICS OF WASTEWATER FROM INDUSTRY The characteristics of textile effluents vary and depend on the type of textile manufactured and the chemicals used. The textile wastewater effluent contains high amounts of agents causing damage to the environment and human health including suspended and dissolved solids, biological oxygen
demand (BOD), chemical oxygen demand (COD), chemicals, odor and color. Most of the BOD/COD ratios are indicating the presence of non-biodegradable substances. The textile effluents contain trace metals like Cr, As, Cu and Zn, which are capable of harming the environment. Dyes in water give out a bad color and can cause diseases like hemorrhage, nausea, severe irritation of skin and dermatitis. The suspended solid concentrations in the effluents play an important role in affecting the environment as they combine with oily scum and interfere with oxygen transfer mechanism in the air-water interface. Inorganic substances in the textile effluents make the water unsuitable for use due to the presence of excess concentration of soluble salts. These substances even in a lower quantity are found to be toxic to aquatic life.
TREATMENT METHODS OF WASTEWATER Textile industry effluents discharged undergo various physico-chemical treatments such as flocculation, coagulation and ozonation and biological treatment for the removal of nitrogen, organics, phosphorus and metal removal. Treatment of textile effluents involves three treatment processes: 1. Primary treatment 2. Secondary treatment and 3. Tertiary/ Advanced Methods of treatment.
1) PRIMARY TREATMENT It is the first process in treatment of wastewater or effluent from textile industries which removes suspended solids, excessive quantities of oil and grease and gritty materials. The following diagram shows the steps of primary treatment of effluent:
DIAGRAM 1: PRIMARY TREATMENT PROCESSES
FIGURE 1: PRIMARY TREATMENT PROCESSES
The effluent is first screened for coarse suspended materials such as yarns, lint, pieces of fabrics, fibers and rags using bar and fine screens. The screened effluent then undergoes settling for the removal of the suspended particles. The floating particles are removed by mechanical scraping systems. The first step of screening (coarse screening), the effluent is carried out to prevent damages from plastics, metals paper and rags. Coarse screens have an opening of 6 mm or larger. Coarse screening is followed by fine screening (1.5-6 mm opening) and very fine screening (0.2-1.5mm opening). Fine screening helps in the reduction of suspended solids in the effluent. Sedimentation takes place after screening which makes use of gravity to settle the suspended particles such as clay or silts present in the effluent. Simple sedimentation was not found to be effective because it does not remove colloidal particles in the effluents. Another disadvantage of the process is the large space it occupies. Equalization ensures that the effluent have uniform characteristics in terms of pollution load, pH and temperature. Neutralization is done to reduce the acidic contents of the effluents. Sulphuric acid and boiler flue gas are the most commonly used chemicals to alter the pH. A pH value of 5-9 is considered ideal for the treatment process. Coagulation carried out settling. Colloidal particles in the effluent carry charges on their surfaces and addition of chemicals to the effluent changes the surface property of the colloids hence causing them to clump together and settle. Ferrous sulphate, lime, alum, ferric sulphate and ferric chloride are some of the most commonly used chemicals in the coagulation step. The settled particles are collected as sludge. Disposal of sludge is one of the biggest challenges of treatment plants. Mechanical flocculation is a physical process which involves slow mixing of the effluent with paddles bringing the small particles together to form heavier particles that can be settled and removed as sludge. Some of the disadvantages with flocculation system are: (a) they are in a risk of getting short-circuited and (b) the floc formation in the system is difficult to control. Care should be taken that the sludge disposed from the bottom of the system would not suspend the solids into the system again.
2) SECONDARY TREATMENT The Secondary treatment process is mainly carried to reduce BOD, phenol and oil contents in the wastewater and to control its color. This can be biologically done with the help of microorganisms under aerobic or anaerobic conditions. Aerobic Bacteria use organic matter as a
source of energy and nutrients. They oxidize dissolved organic matter to CO2 and water and degrade nitrogenous organic matter into ammonia. Aerated lagoons, trickling filter and activated sludge systems are among the aerobic system used in the secondary treatment. Anaerobic treatment is mainly used to stabilize the generated sludge. The following diagram shows the processes involved in secondary treatment:
DIAGRAM 2: SECONDARY TREATMENT PROCESSES
FIGURE 2: SECONDARY TREATMENT PROCESSES Aerated lagoons are one of the commonly used biological treatment processes. This consists of a large holding tank lined with rubber or polythene and the effluent from the primary treatment is aerated for about 2-6 days and the formed sludge is removed. The BOD removal efficiency is up to 99% and the phosphorous removal is 15-25%. The nitrification of ammonia is also found to occur in aerated lagoons. Additional TSS removal can be achieved by the presence of algae in the lagoon. The major disadvantage of this technique is the large amount of space it occupies and the risk of bacterial contamination in the lagoons.
Trickling filters are another common method of secondary treatment that mostly operates under aerobic conditions. The effluent for the primary treatment is trickled or sprayed over the filter. The filter usually consists of a rectangular or circular bed of coal, gravel, Poly Vinyl Chloride (PVC), broken stones or synthetic resins. A gelatinous film, made up of microorganisms, is formed on the surface of the filter medium. These organisms help in the oxidation of organic matter in the effluent to carbon dioxide and water. Aerobic activated sludge processes are commonly used. It involves a regular aeration of the effluent inside a tank allowing the aerobic bacteria to metabolize the soluble and suspended organic matters. A part of the organic matter is oxidized into CO2 and the rest are synthesized into new microbial cells. The effluent and the sludge generated from this process are separated using sedimentation; some of the sludge is returned to the tank as a source of microbes. A BOD removal efficiency of 90-95% can be achieved from this process, but is time consuming. Sludge’s formed as a result of primary and secondary treatment processes pose a major disposal problem. They cause environmental problems when released untreated as they consist of microbes and organic substances. Treatment of sludge is carried out both, aerobically and anaerobically by bacteria. Aerobic treatment involves the presence of air and aerobic bacteria which convert the sludge into carbon dioxide biomass and water. Anaerobic treatment involves the absence of air and the presence of anaerobic bacteria, which degrade the sludge into biomass, methane and carbon dioxide.
3) TERTIARY TREATMENT / ADVANCED METHODS OF TREATMENT Textile effluents may require tertiary or advance treatment methods to remove particular contaminant or to prepare the treated effluent for reuse. Some common tertiary operations are removal of residual organic color compounds by adsorption and removal of dissolved solids by membrane filtration. The wastewater is also treated with ozone or other oxidizing agent to destroy many contaminants.
The adsorption process is used to removes color and other soluble organic pollutants from effluent. The process also removes toxic chemicals such as pesticides, phenols, cyanides and organic dyes that cannot be treated by conventional treatment methods. Dissolved organics are adsorbed on surface as waste water containing these is made to pass through adsorbent. Most commonly used adsorbent for treatment is activated carbon. It is manufactured from carbonaceous material such as wood, coal, petroleum products etc. A char is made by burning the material in the absence of air. The char is then oxidized at higher temperatures to create a porous solid mass which has large surface area per unit mass. The pores need to be large enough for soluble organic compounds to diffuse in order to reach the abundant surface area.
There are some other materials such as activated clay, silica, fly ash, etc are also known to be promising adsorbents.
Ion exchange process is normally used for the removal of inorganic salts and some specific organic anionic components such as phenol. All salts are composed of a positive ion of a base and a negative ion of an acid. Ion exchange materials are capable of exchanging soluble ions and cations with electrolyte solutions. For example, a cation exchanger in the sodium form when contacted with a solution of calcium chloride will scavenge the calcium ions from the solution and replace them with sodium ions. This provides a convenient method for removing the hardness from water or effluent. Ion exchange resin is available in several types starting from natural zeolite to synthetics which may be phenolic , sulphonic styrenes and other complex compounds. The divalent ions such as calcium and magnesium in general have high affinity for the ion exchange resins and as such can be removed with high efficiencies. In the ion exchange process the impurities from the effluent streams is transformed into another one of relatively more concentrated with increased quantity of impurities because of the addition of regeneration chemicals. The process cannot be used for removal of non-ionic compounds.
MEMBRANE FILTRATION 1. REVERSE OSMOSIS :-
The process of reverse osmosis is based on the ability of cellulose acetate or nylon to pass pure water at fairly high rates and to reject salts. To achieve this, water or waste water stream is passed at high pressures through the membrane. The applied pressures has to be high enough to overcome the osmotic pressure of the stream, and to provide a pressure driving force for water to flow from the reject compartment through the membrane into the clear water compartment. In a typical reverse osmosis system, the feed water is pumped through a pretreatment section which removes suspended solids and if necessary, ions such as iron and magnesium which may foul the system. The feed water is then pressurized and sent through the reverse osmosis modules. Clear water permeates through the membrane under the pressure driving force, emerging at atmospheric pressure. The pressure of reject stream is reduced by a power recovery, which helps drive the high pressure pump and then is discharged. Reverse osmosis can be used as end-of-pipe treatment and recycling system for effluent. After primary, secondary and/or tertiary treatment, further purification by removal of organics and dissolved salts is possible by use of reverse osmosis. RO membranes are susceptible to fouling due to organics, colloids and microorganism. Scale causing constituents like hardness, carbonate. Silica, heavy metals, oil etc has to be removed from the feed. As the membranes are sensitive to oxidizing agents like chlorine or ozone, they should also be absent.
2. ULTRA FILTRATION This process is similar to reverse osmosis. The difference between reverse osmosis and ultra filtration is primarily the retention properties of the membranes. Reverse osmosis membranes retain all solutes including salts, while ultra filtration membranes retain only macro molecules and suspended solids. Thus salts, solvents and low molecular weight organic solutes pass through ultra filtration membrane with the permeate water. Since salts are not retained by the membrane, the osmotic pressure differences across ultra filtration membrane are negligible. Flux rates through the membranes are fairly high, and hence lower pressures can be used. Ultra filtration membranes may be made from cellulose acetate, polyelectrolyte complexes, nylon and inert polymers. Hence, acidic or caustic streams may also be processed and the process is not usually limited by chemical attack of the membranes. 3.
Nano filtration can be positioned between reverse osmosis and ultra filtration. Nano filtration is essentially a lower pressure version membrane where the purity of permeate water less important. This process is used where the high salt rejection of reverse osmosis is not necessary. The nano filtration is capable of removing hardness elements such as calcium or magnesium together with bacteria, viruses, and color. Nano filtration operated on lower pressure than reverse osmosis and as such treatment cost is lower than reverse osmosis treatment. Nano filtration is preferred when permeate with TDS but without color, COD and hardness is acceptable. Feed water to nano filtration should be of similar qualities as in case of reverse osmosis. Turbidity and colloids should be low. Disinfection of feed may also necessary to remove micro-organism. 4. OZONATION Ozone is one of the strongest oxidizers commercially available and popular for disinfection of potable water. Besides this it has multiple applications. Large, complex organic molecules, detergents, phenols etc. can be broken into simpler compounds by ozonation. Among the industrial applications, oxidation of organic and inorganic, deodorization, and decolorization are the main usages. Ozone is an unstable gas at temperature and pressure encountered in water and waste water treatment plants. For most industrial applications ozone has to be produced at the site. Although, there are several methods by which ozone can generated, the corona discharge method is widely used procedure. An ozone generation unit incorporates a series of electrodes fitted with cooling arrangements mounted in a gas tight container. When the source gas (air or oxygen) is passed through narrow gap separating electrodes, the oxygen gets converted into ozone.
Ozone is applied to waste water by means of diffuser tubes or turbine mixers. Ozone doses in level of 2 mg/l have been reported to result in virtually complete removal of color and hard pollutants such as detergents. The treated water after sand filtration becomes clean and sparkling.
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