DESIGN OF A BIOGAS PLANT THAT USES HUMAN WASTE AS FEEDSTOCK

DESIGN OF A BIOGAS PLANT THAT USES HUMAN WASTE AS FEEDSTOCK

A

GROUP WORK TERM PAPER

IN

BIOFUELS TECHNOLOGY (CHEN 820)

PRESENTED TO Dr. KABIRU MU’AZU DEPARTMENT OF CHEMICAL ENGINEERING FACULTY OF ENGINEERING AHMADU BELLO UNIVERSITY, ZARIA. MARCH, 2015.

TABLE OF CONTENTS TABLE OF CONTENTS....................................................................................... ii LIST OF FIGURES............................................................................................. iv LIST OF TABLES............................................................................................... v NOMENCLATURE............................................................................................ vi ABSTRACT..................................................................................................... vii CHAPTER 1: INTRODUCTION............................................................................. 1 CHAPTER 2: LITERATURE REVIEW....................................................................3 CASE STUDY................................................................................................ 3 BRIEF HISTORY OF BIOGAS........................................................................... 3 BIOGAS IN NIGERIA...................................................................................... 4 BIOGAS PRODUCTION................................................................................... 4 Pre-treatment of feedstock...............................................................................4 Anaerobic digestion process............................................................................. 6 FACTORS THAT AFFECT BIOGAS PRODUCTION...............................................8 Waste composition/volatile solids......................................................................8 Alkalinity and pH.......................................................................................... 9 Volatile fatty acids concentration.......................................................................9 Temperature................................................................................................. 9 C/N ratio................................................................................................... 10 Retention Time (RT).................................................................................... 10 Organic Loading Rate (OLR)..........................................................................11 Mixing..................................................................................................... 11 DIGESTER TECHNOLOGY............................................................................ 11 Floating Drum Plant..................................................................................... 12 Fixed Dome Plant........................................................................................ 12 BIOGAS CLEANING..................................................................................... 13 Removal of H2S.......................................................................................... 13 Removal of CO2.......................................................................................... 13 Digestate................................................................................................... 14 CHAPTER 3: DESIGN CONSIDERATIONS...........................................................15 LAYOUT FOR COLLECTION AND STORAGE OF THE HUMAN WASTE................15 PLANT LAYOUT.......................................................................................... 16 2

Site Consideration....................................................................................... 16 Proposed plant layout:.................................................................................. 17 SIZING OF DIGESTER................................................................................... 18 Amount of feedstock.................................................................................... 18 Volume of digester occupied by slurry..............................................................20 Biogas production....................................................................................... 20 Total volume of biodigester............................................................................21 Number of people expected to use the gas on daily basis........................................22 CHAPTER 4: PROCESS MANUAL FOR OPERATION OF PLANT..............................23 SAFETY AND ENVIRONMENTAL CONTROL...................................................23 FIRE PROTECTION SECTORS........................................................................24 HARMFUL EXHAUST GASES........................................................................25 GERMS....................................................................................................... 25 EMISSIONS OF SMELLS...............................................................................26 NOISE PROTECTION.................................................................................... 26 CHAPTER 5: UTILIZATION OF BY-PRODUCT SLUDGE.........................................28 POST TREATMENT OF DIGESTATE................................................................28 CHAPTER 6: PROCESS INSTRUMENTATION AND CONTROL................................29 THE NEED FOR CONTROL SYSTEM...............................................................29 TYPES OF CONTROL SYSTEM.......................................................................30 DESIGN OF A PROCESS CONTROL SYSTEM....................................................30 CHAPTER 7: CONCLUSION.............................................................................. 32 REFERENCES................................................................................................. 33

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LIST OF FIGURES Figure 2.1: Classification of the anaerobic digestion system............................................8 Figure 3.1: Layout for human waste collection from the residential area...........................15 Figure 3.2: Map of Maitama District showing proposed sewage lines and proposed biogas plant location (Map source: Google Maps)......................................................................16 Figure 3.3: Proposed plant layout...........................................................................18

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LIST OF TABLES Table 1: Keys of equipment shown in plant layout......................................................17 Table 2: Biogas yield for the different feedstock types (From Mukumba et. al., 2013, pg. 17). 21 Table 3: Composition of poisonous substance present in biogas......................................23

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NOMENCLATURE AD

Anaerobic Digestion

ECN

Energy Commission of Nigeria

HS

High Solids

OLR

Organic Loading Rate

RT

Retention Time

TS

Total Solids

VFA

Volatile Fatty Acid

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ABSTRACT This term paper assignment was carried out as group work to design a biogas production plant from human waste generated from the Maitama District of Abuja with a population of 1 million people. The gas generated is to be used by the residents of the same Maitama District. The fixed dome plant was employed and the volume of the digester was estimated to be 123,703m3; this volume includes volume of slurry and volume of gas generated since it is a fixed dome plant. The gas production was obtained to be 49,957 m 3 per day. With a biogas requirement of 0.6 m3 per person, it was estimated that about 83,262 of the 1 million people will be able to use the gas produced.

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CHAPTER 1: INTRODUCTION It is an indisputable fact that energy plays a very important role in the socio-economic development of any country. The amount of energy produced and consumed by a given country is a direct indicator of the level of development of the country; the more energy a country is able to produce and consume, the more developed the country tends to be. It is reported that energy consumption in Africa is less than 5% of the global consumption even though 13% of the world’s population lives there and 10% of the world’s crude oil reserves are located there (Bugaje and Mohammed, 2008). This is a rather pitiable position and as such there is the need for efficient harnessing of the abundant resources that Africa is blessed with. One of such ways is the development of biofuels of which biogas is one. Biogas is a methane-rich gas produced from the anaerobic digestion of organic materials. It can be produced domestically using human and animal wastes and can serve as a cheap source of energy. Apart from serving as a cheap source of energy, its production also serves as a waste management technique because anaerobic treatment eliminates the harmful micro-organisms. After biogas has been produced, the byproduct sludge (digested slurry) is a good fertilizer while the treated effluent from the anaerobic digestion is a good animal feed when treated and mixed with molasses and grains. Biogas production from human wastes (faeces) has already been developed and is used in some parts of the world like China and India. In Nigeria, the technology of biogas is at an infant stage even though efforts are continually being made to promote and develop it. This report presents a study about the design of a biogas plant using human 1

waste generated from the residents of the Maitama District in Abuja. The gas generated is to be sent back to the residents to serve as fuel for heating and cooking.

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CHAPTER 2: LITERATURE REVIEW CASE STUDY The geographical area under study is the Maitama District in The Federal Capital Territory Abuja. The population of the area is put at 1 million people. The biogas plant is to produce biogas from the human faeces and urine generated from the entire population of the area. The amount of waste generated per person is 1kg of urine and 150 g of faeces per day. This puts the total amount of urine generated at 1 million kg and 150, 000 kg of faeces per day. The biogas to be generated is to be used by the people of the district. It is assumed that each person requires 0.6m 3 of biogas for lighting and cooking per day.

BRIEF HISTORY OF BIOGAS The use of biogas as a fuel is believed to have started as long ago as the 10 th century B.C. by the Assyrians who used it for heating bath water; there are also suggestions that anaerobic digestion of solid waste may well have been applied in ancient China (Bond and Templeton, 2011). Well documented attempts to harness the anaerobic digestion of biomass by humans date from the mid-nineteenth century, when digesters were constructed in New Zealand and India, with a sewage sludge digester built in Exeter, UK to fuel street lamps in the 1890s. In Guangdong Province, China, commercial use of biogas has been attributed to Guorui Luo. In 1921, he constructed an 8 m 3 biogas tank fed with household waste and later that decade founded a company to popularise the technology. The first German sewage treatment plant to feed biogas into the public gas supply began to do so in 1920, while in the same country the first large agricultural biogas plant began operating in 3

1950. The spread of biogas technology gained momentum in the 1970s, when high oil prices motivated research into alternative energy sources.

BIOGAS IN NIGERIA Despite the numerous advantages of using biogas technology as a source of energy and a source of nitrogen-rich fertilizer, it has made only little impact in Africa and Latin America. There are however, some efforts by the Government of Nigeria to encourage its use. The two Energy Commission of Nigeria (ECN) Renewable Energy Centres established in Sokoto and Nsukka in 1982 by the Federal Government of Nigeria pioneered research work in biogas in Nigeria. Today, many tertiary institutions are involved in biogas research. The success story of the use of biogas in Nigeria is illustrated by its use in Kwachiri community where a community of forty (40) have been using it for their daily cooking needs since 2003 (Energy Commission of Nigeria, no date, pg. 35)

BIOGAS PRODUCTION The process of biogas production can be divided into four stages: pre-treatment, digestion, gas upgrading and digestate treatment. The level of pre-treatment depends on the type of feedstock; e.g. manure need to be mixed, whereas municipal solid wastes are sorted and shredded.

Pre-treatment of feedstock The pre-treatment of feedstock consists in separating the recyclable or non-digestible wastes from the municipal solid wastes. Source separation has a significant effect upon the quality of the digestate. Mechanical pre-treatment leads to a lower quality digestate.

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The removal of all contaminants is not possible especially for the smaller fraction such as heavy metals. The resultant fraction is thus more contaminated. There are a variety of pre-treatment processes that are chosen based on the characteristics of the incoming waste and the effects they have on digestion. Separation technologies for metals, glass and plastic are usually necessary. The pre-treatment of feedstock for AD involves: 

Providing a uniform small particle size feedstock for efficient digestion



Removing the non-biodegradable materials



Protecting the downstream plant from components that may cause physical damage



Removing materials which may decrease the quality of the digestate.

Most digestion systems require pre-treatment of waste to obtain homogeneous feedstock. The pre-processing involves separation of non-digestible materials and shredding. The waste received by AD digester is usually source separated or mechanically sorted. The separation ensures removal of undesirable or recyclable materials such as glass, metals, stones etc. In source separation, recyclables are removed from the organic wastes at the source. Mechanical separation can be employed if source separation is not available and the resultant fraction is then more contaminated leading to lower compost quality. The waste is shredded before it is fed into the digester in order to enhance the digestion rate. Chemical pre-treatment changes the composition of waste by reducing particulate organic matter to soluble form i.e. proteins, fats, carbohydrates or lower molecular weight compounds. Alkalis are added to increase the pH to 8-11 during this process. Thermal and chemical pre-treatments do improve hydrolysis and promote solubilisation. Ultrasonic pre-treatment also has been researched to reduce retention time. 5

Anaerobic digestion process The anaerobic digestion of organic matter is a complex process which can be categorised into three steps. Hydrolysis An important step of the anaerobic biodegradation process is the hydrolysis of the complex organic matter. During the anaerobic digestion of complex organic matter, the hydrolysis is the first and often the rate-limiting step. In this process hydrolytic organisms called acidogens hydrolyse complex organic matter such as proteins, poly carbonates, lipids, etc. to simple organic compounds (formate, acetate, propionate, butyrate and other fatty acids, etc.) An approximate chemical formula for the mixture of organic waste is C6H10O4. A hydrolysis reaction where organic waste is broken down into a simple sugar (glucose) can be represented by the Eq. 2.1.

C6H10O4 + 2H2O

C6H12O6 + 2H2 .........................2.1

Acidification In this stage, the hydrolysed compounds are fermented into volatile fatty acids (acetic, propionic, butyric, valeric acids etc.), neutral compounds (ethanol, methanol), ammonia, and the pH falls as the levels of these compounds increases. Carbon dioxide and hydrogen are also evolved as a result of the catabolism of carbohydrates. The group of microorganisms responsible for this biological conversion is obligate anaerobes and facultative bacteria, called acidogens .Typical reactions in the acid-forming stages are shown below in Eq. 2.2, glucose is converted to ethanol and Eq. 2.3 shows glucose is transformed to propionate.

C6H12O6

2CH3CH2OH + 2CO2 ……. 2.2 6

C6H12O6 + 2H2

2CH3CH2COOH + 2 H2O ………… 2.3

Methanogenesis Methanogenesis is the last stage of anaerobic digestion which involves the production of methane from the raw materials produced in the previous stage. Methanogens which carry out the terminal reaction in the anaerobic process are the most important in anaerobic digester systems. The methane is produced from a number of simple substances: acetic acid, methanol or carbon dioxide and hydrogen. The reactions that occur during this step are:

2 CH3CH3OH+ CO2

2CH3COOH + CH4 ………2.4

CH3COOH

CH4 + CO2 ……………………2.5

CH3OH + H2

CH4 + H2O ……………………2.6

CO2 + 4H2

CH4 + 2H2O ……………….2.7

Types of anaerobic digesters A wide variety of systems have been developed to anaerobically treat municipal solid waste. They can be split into different categories as following:  Continuous versus batch process  Mesophilic versus thermopilic digestion  Single stage versus multi-stage digestion Figure 1 depicts the classification of the anaerobic digestion system based on the operating criteria.

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Figure 2.1: Classification of the anaerobic digestion system

FACTORS THAT AFFECT BIOGAS PRODUCTION The rate at which the microorganisms grow is of vital importance in the anaerobic digestion process. The operating parameters of the digester must be controlled so as to enhance the microbial activity and thus increase the anaerobic degradation efficiency of the system. Some of these parameters are discussed in the following section.

Waste composition/volatile solids The wastes treated by anaerobic digestion may comprise a biodegradable organic fraction, a combustible and an inert fraction. The biodegradable organic fraction includes kitchen scraps, food residue, and grass and tree cuttings. The combustible fraction includes slowly degrading lignocellulosic organic matter containing coarser wood, paper, and cardboard. As these lignocellulosic organic materials do not readily degrade under anaerobic conditions, they are better suited for waste-to energy plants. Finally, the inert fraction contains stones, glass, sand, metal, etc. This fraction ideally 8

should be removed, recycled or used as landfill. The removal of inert fraction prior to digestion is important as otherwise it increases digester volume and wear of equipment.

Alkalinity and pH Sufficient alkalinity is essential for pH control. Alkalinity serves as a buffer that prevents rapid change in pH. The alkalinity is the result of the release of amino groups and production of ammonia as the proteinaeceous wastes are degraded. Anaerobic bacteria, specially the methanogens, are sensitive to the acid concentration within the digester and their growth can be inhibited by acidic conditions. It has been determined that an optimum pH value for anaerobic digestion lies between 5.5 and 8.5. During digestion, the two processes of acidification and methanogenesis require different pH levels for optimal process control. The retention time of digestate affects the pH value.

Volatile fatty acids concentration Volatile fatty acids (VFA’s) are important intermediate compounds in the metabolic pathway of methane fermentation and cause microbial stress if present in high concentrations. The intermediates produced during the anaerobic bio-degradation of an organic compound are mainly acetic acid, propionic acid, butyric acid, and valeric acid. Amongst these, acetic and propionic acids are the major VFAs present during anaerobic bio-degradation and their concentrations provide a useful measure of digester performance. Acetate yield is increased slightly with increasing pH, whereas butyrate yield is increased with decreasing pH.

Temperature Due to the strong dependence of temperature on digestion rate, temperature is the most critical parameter to maintain in a desired range. There are two temperature ranges that provide optimum digestion conditions for the production of methane i.e. the mesophilic 9

and thermophilic ranges. The optimum temperature for mesophilic digestion is 35°C and a digester must be maintained between 30°C and 35°C for most favourable functioning. The thermophilic temperature range is between 40°C-65°C. A thermophilic temperature reduces the required retention time. The microbial growth, digestion capacity and biogas production could be enhanced by thermophilic digestion, since the specific growth rate of thermophilic bacteria is higher than that of mesophilic bacteria.

C/N ratio The relationship between the amount of carbon and nitrogen present in organic materials is represented by the C/N ratio. Microorganisms need nitrogen for the production of new cell mass. A nutrient ratio of the elements C: N: P: S at 600:15:5:3 is sufficient for methanisation. Optimum C/N ratios in anaerobic digesters should be between 20–30 in order to ensure sufficient nitrogen supply for cell production and the degradation of the carbon present in the wastes. As the reduced nitrogen compounds are not eliminated in the process, the C/N ratio in the feed material plays a crucial role.

Retention Time (RT) The required retention time for completion of the anaerobic digestion reactions varies with differing technologies, process temperature, and waste composition. The retention time for wastes treated in mesophilic digester range from 10 to 40 days. Lower retention times are required in digesters operated in the thermophilic range. The RT is the ratio of the digester volume to the influent substrate flow rate. The Eq. 2.8 gives the time of substrate to be inside the digester.

RT=

V Q

……………………………………………………….2.8

Where V = digester volume (m3) 10

Q = flow rate (m3/d) RT = retention time (d)

Organic Loading Rate (OLR) Low solids anaerobic digestion systems contain less than 10 % Total Solids (TS) and High Solids (HS) processes range about 20% or higher TS. An increase in TS in the reactor results in a corresponding decrease in reactor volume. The OLR is a measure of the biological conversion capacity of the anaerobic digestion system. Feeding the system above its sustainable OLR results in low biogas yield due to accumulation of inhibiting substances such as fatty acids in the digester slurry. In such a case, the feeding rate to the system must be reduced. OLR is a particularly important control parameter in continuous systems.

Mixing The purpose of mixing inside the digester is to homogenize the material. Furthermore, mixing prevents scum formation and avoids temperature gradients within the digester. However excessive mixing can disrupt the microbes so slow mixing is preferred. The kind of mixing equipment and amount of mixing varies with the type of reactor and the solids content in the digester.

DIGESTER TECHNOLOGY The floating – drum and fixed – drum are the two basic types of tested biogas plants that have gained widespread acceptance. The floating drum plant has a metal gasholder that floats on the digester while in the fixed – drum plant, gas storage is by the displacement principle. Biogas is produced from digesters, which consist of two basic parts: a tank, which holds the slurry (and a gas cap drum seal on the tank, which captures the gas released from the slurry. 11

Floating Drum Plant The floating drum plant consists of a digester and a moving gasholder that floats either directly on the fermentation slurry or in a water jacket of its own. The drum rises when gas is collected in it and falls when gas is drawn off from it. The gas drum is prevented from tilting by a guide frame called the drum holder. This type of pant is simple to operate and construct, and provides a constant pressure. The disadvantage is its high construction cost and corrosion of the floating drum resulting in its short life of less than 5 years in tropical coastal regions and the regular maintenance cost due to painting of the drum. The floating drum can be replaced by balloon above the digester. This reduces construction costs, although this type is still under test for practical conditions. Floating drums can be made from glass-fibre reinforced plastic and high-density polyethylene with the attendant increase in construction cost. Floating-drums can also be made from wire-mesh-reinforced concrete, although this is liable to hairline cracking.

Fixed Dome Plant This type of plant is made up of a digester with a fixed, non-movable gas space at upper part of the digester. When gas is produced it displaces slurry in the compensating tank. The gas pressure increases with the volume of gas stored. When gas production is low, there will be little gas in the holder and the gas pressure will be low. Constantly, the gas pressure becomes too high if the gas production is high. This type of plant is not suitable for such applications as in engines where the gas is required to be delivered at a constant pressure. For such application, a gas pressure regulator or a floating gasholder is required. The advantage for this type of plant is its low construction cost ad it has no 12

moving part hence longer life (20 years or more). A disadvantage, however, is that it’s not often gaslight because of the occurrence of cracks. Also, high fluctuation in gas pressure occurs with low digester temperature. Furthermore, this type of digester is recommended only where construction can be supervised by experienced biogas technicians.

BIOGAS CLEANING Removal of H2S Hydrogen sulphide in the fermentation gas impairs the lifetime of pipework and all installations for the utilization of biogas. It is toxic and strongly corrosive to many kinds of steel. The most common methods for hydrogen sulphide removal are: -

air/oxygen dosing to digester biogas iron chloride dosing to digester slurry iron oxide activated carbon water scrubbing NaOH scrubbing Biological desulphurisation of biogas can be performed by micro-organism.

Removal of CO2 Removal of carbon dioxide enhances the energy of the gas either to reach vehicle fuel standard or natural quality gas. At the present time, four different methods are used commercially to achieve it: -

Water scrubbing Polyethylene glycol scrubbing Carbon molecular sieves Membrane separation

On a small scale, CO2 can be removed by bubbling the gas through lime.

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Digestate Anaerobic digestion can be seen as a method to treat the organic waste but, in order to extract the maximum recovery value from these wastes, the digestate should have a useful purpose and benefit should be derived from its production. Its main advantage is that it has a high nutrient content. Its quality should be acceptable for purpose such as soil amendment or landscaping. The chemical aspects of quality management of digestate are related to the presence of: - Heavy metals and other inorganic contaminants - Persistent organic contaminants - Nutrients (NPK) Anaerobic digestion draws carbon, hydrogen and oxygen from the feedstock. Essential plant nutrients (N, P, and K) remain largely in the digestate. Therefore the digestate can be use as fertilizer or for soil amendment in agriculture, landscaping. Such use permit the creation of a nutrient cycle and maintains or improve soil structure due to the application of organic manure.

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CHAPTER 3: DESIGN CONSIDERATIONS LAYOUT FOR COLLECTION AND STORAGE OF THE HUMAN WASTE The sewage from each house will be channelled through underground lateral connection pipes that run from the house to an underground sanitary sewer line. Sanitary sewer lines from different streets will be joined together and all the waste will be channelled to the waste storage of the biogas production plant. Figure 3.1 shows the proposed layout. Lines L1 to L6 shown are the different lateral lines that collect human waste from each house; they run to meet line S which is the sanitary sewer that collects the human wastes from each street and channels the waste to the biogas production plant. HOUSE 1 L1

HOUSE 2 L2

HOUSE 3 L3

S L4 HOUSE 4

L5 HOUSE 5

L6 HOUSE 6

Figure 3.2: Layout for human waste collection from the residential area.

Material of construction for the sewage lines is concrete as is usually the case with sewage lines.

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Figure 3.3: Map of Maitama District showing proposed sewage lines and proposed biogas plant location (Map source: Google Maps)

PLANT LAYOUT Site Consideration The biogas plant is generally environmentally friendly and technically safe to operate within the metropolis since the by-products are not toxic and its product can be used 16

directly for domestic purposes and for electricity generation. Plant site location considerations include: 

The plant should be located close to its source of raw material to minimise distance



of transportation The plant should be located close to the point of consumption of its product

 

(residential area that will use the gas) The plant should be located close to farmlands to dispose the digestate for use. The plant should be located close to a water treatment plant for the treatment of the



wastewater generated The digester pit should not be dug within 13 metres of a drinking water well or



spring. If the water table is reached during digging, it will be necessary to cement the inside

  

of the digester pit although this increases cost. It must be ensured that there is enough space to build the digester Provision for slurry storage needs to be made A site open and exposed to the sun is necessary

Proposed plant layout: Table 1: Keys of equipment shown in plant layout

Keys E1 E2 E3 E4 E5 E6 E7 E8 E9 E10 E11 E12 E13

Equipment Biomass storage tank Magnetic separator Crusher Pulper Screen Water storage tank Mixer Digester Carbon dioxide scrubber Hydrogen sulphide scrubber Biogas storage tank De-watering tank Digestate storage tank

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Figure 3.4: Proposed plant layout

SIZING OF DIGESTER Amount of feedstock The available feedstock is from 1 million people. Each person produces 1 kg of urine and 150g of faeces everyday. Consortium on rural technology (1986, pg. 11) gives an estimate that: 

Human faeces is made up of about 66 – 80% water and the rest solids and 18



Urine is made up of 93 – 96% water and the rest solids.

For this study, the values of water content that will be employed are: 75% water content in faeces and 95% water content in urine. Thus the amount and total solids concentration of the feedstock is calculated:

Total amount of faeces per day=

Total solids∈faeces=

(

)

25 ×150,000 kg=37,500 kg 100

Total amount of urine per day =1

Total solids∈urine=

150 kg ×1,000,000 ( persons ) =150,000 kg 1000 person

kg ( person ) ×1,000,000 ( persons ) =1,000,000 kg

5 × 1,000,000 kg=50,000 kg 100

Thus , total amount of solid ∈ feedstock=37,500 kg +50,000 kg=87,500 kg Total weight percent solids∈ feed=

Total weight of solids 87,500 87,500 ×100 = = =7. Total weight of feed 150,000+1,000,000 1,150,000

The problem stated that the feed slurry should have a total solid concentration of 20%. Since the human waste does not give the required amount, animal waste (such as cow dung, bird droppings, etc.) can be added to make up the remaining 12.39% of the weight. The assumed animal waste to be used is cow dung. Assuming that the supplementary cow dung added is completely dry, the amount in weight that will be required is calculated thus:

Total amount of dry cow dung required=12.39 of total weight of slurry =

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12.39 × 1,150,000 kg=142,48 100

This puts the new total weight of the slurry at: Total weight of slurry after addition of cow dung=1,150,000+142,485=1,292,485 kg

The rule of thumb that will be used is: 1000kg = 1m3. Thus,

Volume of slurry =

1,292,485 3 × 1=1,292.5 m per day 1000

Volume of digester occupied by slurry The volume of the digester occupied by slurry Vd is calculated from: V d =V s × Rt

Where Vs = Volume of substrate = 1,292.5m3 Rt = Retention time = 30 days Thus, volume of digester occupied by slurry, V d =1,292.5× 30=38,775 m

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Biogas production Rate of production of biogas per day G, is calculated from: G=W s × G y

Where Ws = Weight of feedstock produced per day Gy = Gas yield of substrate The gas yield for the different feedstock are given in table 2.

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Table 2: Biogas yield for the different feedstock types (From Mukumba et. al., 2013, pg. 17)

Gas yield (m3/kg of wet biomass)

Feedstock type Cattle dung

0.036

Human waste

0.07

Gas production due ¿ humanwaste =150,000× 0 .07=10,500 m3

Since we assumed dry weight of cow dung was added and the data in table 2 is given for wet cattle dung, we need to calculate the equivalent wet weight of the cattle dung added in order to know its gas yield. Spuhler (no date, slide 21) gives the average total solids concentration of cattle dung to be 13%. Thus,

Equivalent wet weight of cattle dung used=

142,485 =1,096,038 kg 0.13

Gas production due ¿ cow dung=1,096,038 ×0 .036=39,457 m3

Total gas production=10,500+39,457=49,957 m3 per day

Total volume of biodigester The type of digester that will be used is the fixed dome type. The volume of the biodigester will hold both the feedstock and the gas produced. The amount of space reserved for the gas is obtained based on a rule of thumb given by Energy Commission of Nigeria (no date, pg. 42) to be 60-70% of the total daily gas production. Thus, using 70% of total gas production, Amount of space ¿ hold the gas produced=1.7 × 49,957=84,928 m

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3

Total volume of digester =( Volume of by slurry ) + ( Volume occupied by gas )=38,775m 3 +84,928 m 3=123

Number of people expected to use the gas on daily basis The estimated production rate of biogas is 49,957 m3. Assuming each person requires 0.6m3 of gas for lighting and cooking per day, we can estimate the number of people that are expected to use the gas produced daily using:

Number of people that will use the gas=

Total volume of gas produced 49,957 = ≈ 83,262 people Total amount of gas used per person 0.6

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CHAPTER 4: PROCESS MANUAL FOR OPERATION OF PLANT SAFETY AND ENVIRONMENTAL CONTROL The regulations and laws guiding the design, construction and operation of biogas plant are aimed at the elimination or minimization of all safety hazards affecting both people and the environment. Danger to life and health in a biogas plant can be caused by suffocation and toxification in tanks, silos, and pits filled with gases like H 2S, CH4, and CO2. Some of these gases are heavier than air and tend to remain in the vessels. The following gases are example of dangerous gas even in low concentration associated to biogas. Table 3: Composition of poisonous substance present in biogas

Gases

Composition (ppm)

Hydrogen sulphide (H2S) Carbon monoxide (CO) Chlorine (Cl2) Carbon dioxide (CO2)

10ppm 50ppm 0.5ppm 5000ppm

Also, below are some hazards associated to a biogas to the plant and environment • Emission of pollutants into air, groundwater and surface water, especially when disposing residues. • Danger of explosions caused by flammable gas/air mixtures. All flammable gases, vapours, and dust are able to form explosive mixtures. • Injuries caused by machinery and plant parts, e.g., rotating machinery, electromagnetic forces, electrostatic charging, or burning by hot surface.

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• Freezing of gas or substrate pipes, e.g., by condensation of water in the pipes leading to blocking and finally to bursting of the pipe • Corrosion caused by aggressive substances like ammonium nitrate, H2S, and many others Plugging of tubes by solid components • Noise, especially in the areas of the CHP. In biogas plants, the formation of explosive gas mixtures can occur. Therefore, a system for plant security must exist relating to installation and operation of electrical devices in areas with danger of explosions. These areas are enclosed spaces which serve as gasholders or where gases are produced, or other enclosed spaces which are connected, e.g., by pipes, to these spaces Channels and storage tanks Spaces inside machinery, tanks, and pipes, which contain biogas Spaces around machinery where gas is discharged, e.g., overpressure security valves at the bioreactor or the gasholder. Buildings which are used for gas consumption (heaters or engines) and are not connected to bioreactors or gasholders are excluded from the above – mentioned prescriptions. In these buildings, the installation of gas pipes and electrical wires should be easy to keep under surveillance. Ideally, the gas pipes and the electrical wires should be installed on different walls.

FIRE PROTECTION SECTORS In order to reduce the fire risk, the plant is to be divided into fire protection sectors, e.g., the bioreactor and gasholder, the gas consumption equipment, and the gas compressor. Certain distances must be maintained between the fire protection sectors. Depending upon how much space is available in between, the material of the external walls of buildings containing equipment or of protecting walls has to be chosen.

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All gas holders should be fire protected with non-flammable material and all safety doors within the process plant should be open in case of fire outbreak. In the plant, adequate space should be provided for brigade vehicles for easy access in case of fire outbreak. Finally, at least 12 portable unit of suitable extinguishing agent should be available per plant or per plant or fire protection sector.

HARMFUL EXHAUST GASES From biogas plants, climate - relevant gases and gases harmful to humans can escape, such as ammonia, methane, nitrous oxide, and others. The amount of leakage depends on the applied technology and the substrates. To avoid the emission of these gases to the environment, it is necessary to design the gas holder in such a way that there will not be leakage.

GERMS The microorganisms taking part in the fermentation process are mainly known to be harmful to human health. Therefore special protection devices are necessary, substrates and also residues often contain organisms which do not participate in the process, like viruses and parasites, which can have severe effects on the health of humans as well as on the environment Therefore certain measures for protection are necessary. For all who work in biogas plants, the danger of an increased ingestion of endotoxins exists. Endotoxins are metabolic products of microorganisms. They are taken up from humans via skin cracks. At low concentrations endotoxins can induce fever, and at high concentrations a stimulation of the mucous membrane; respiratory diseases up to chronic inflammations of the respiratory system can be evoked. Increased endotoxins concentrations could be found with increased aerosol formation, e.g., at the exhaust of a capsulated surface blower of an activated sludge tank, when cleaning a chamber filter press, when cleaning pump pits, etc. People living in the neighbourhood are not 25

endangered in general, since the germ concentration in the air is low and independent on the weather conditions.

EMISSIONS OF SMELLS Smell - intensive materials in biogas plants include particularly ammonia, organic acids, phenol, and hydrogen sulphide (H2S). Smells are often causes of annoyance and/or complaints about biogas plants. The door emissions depend particularly on the composition of the materials used (liquid manure, co - ferments). The composition of the liquid manure varies depending upon animal species, stable technology, feeding, and water requirement for the cleaning of the stable (dilution). 

Hydrogen sulphide: Hydrogen sulphide, a mostly inevitable component of biogas,



not only smells, but can have a toxic effect in higher concentrations. Ammonia: Ammonia (NH3) has a strong smell, induces eutrophy, and is indirectly relevant to the climatic situation, since ammonia is partly converted to nitrous oxide (N2O) in the soil. The degradation of organic substances results in nitrogen in the form of ammonium in the substrate. Since during the degradation the pH value increases about one unit due to the decomposition of acids and the temperature in the residue storage tank is high due to the high processing temperature in the bioreactor, a lot of volatile ammonia is liberated.

To solve the problems associated with the emission of this smell due to emission of these gases, proper design should be done on the gas cleaning to remove the gases.

NOISE PROTECTION Noise is defined to be disturbing sound. Any location where sound does not cause disturbance, even it is very loud, does not incur restrictions. The area in a biogas plant where noise is most intense is near the gas engine. Near CHP plants, the limiting value 26

for workplaces is far exceeded. The noise radiates through the exhaust pipe and the ventilation openings of the plant area. Transportation of the co - ferments, pumps, compressors, and emergency cooling systems also causes noise. To prevent this sound absorbers have to be installed in exhaust gas pipes and/or in openings for ventilating.

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CHAPTER 5: UTILIZATION OF BY-PRODUCT SLUDGE The by-product sludge that is produced from the anaerobic digestion process will be used as a fertilizer. This is because it is a better fertilizer and soil conditioner than either compost or fresh manure. This is because: 

The liquid effluent contains may elements essential to plant life. It contains nitrogen, phosphorus, potassium and small amounts of metallic salts that are



indispensable for plant growth. When the sludge is applied on the soil as fertilizer, its nitrogen is converted to ammonium ions (NH4+), which fix themselves to the negative charged clay particles of the soil, thereby making nitrogen available to the plants.

But before using the sludge as fertilizer, it is treated

POST TREATMENT OF DIGESTATE The digestate is dewatered through gravity extraction and pressing. Part of the extracted liquid is used to dilute incoming waste and the rest is discharged in sewage. The solid cake, with a Total Solid content of about 40%, is treated aerobically for about two weeks to completely stabilize it. Inert material is separated from the compost through a rotary screen.

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CHAPTER 6: PROCESS INSTRUMENTATION AND CONTROL In the design of a biogas plant and industrial plants generally, the methods of controlling the systems are important factors in choosing the type of process and setting the labour requirements. Processes are controlled precisely to give more uniform and higher quality products by the application of automated systems, often leading to higher profits. Therefore any process with an input and output which may be flow, temperature, pressure, concentration or any other inventory, environmental or quality variable that is to be held at a desired value must have some measures of control. Changes in process output may occur as a result of the following:      

Changes in weather conditions or raw material quality and quantity Deviations from operating temperatures or ambient temperature Deviations from operating pressure Deviations from operating flow Manually, when operators change production rate And cyclically as a result of oscillations in other control loops

Variations in any of the factors stated above will drive the output (controlled variable) further away from the set point (desired value) thus requiring a corresponding change in another variable called manipulated variable.

THE NEED FOR CONTROL SYSTEM The rational arrangement of equipment and human interventions are not enough reasons to introduce a control system, the following bring about need for a control system. 1. To ensure the stability of an inherently dynamic chemical process 2. To optimize the performance of a chemical process

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3. To suppress the interference of external disturbance or influences.

TYPES OF CONTROL SYSTEM A. Feedback Control System B. Feedforward Control System C. Cascade Control System D. Feedforward+Feedback Control System Of all the control systems listed above a feedback control systems is the best for the plant under consideration in this study as they can improve control system performance over feed forward control.

DESIGN OF A PROCESS CONTROL SYSTEM The various (flow rates, temperatures, pressures, concentrations etc.) associated with a chemical process plant are divided into two groups; a. Input variables: which denote the effect of the surroundings on the chemical process b. Output variables: which denote the effect of the process on the surroundings. The input variables can be further classified into the following categories 1. Manipulated variable: if their values can be adjusted freely by the human operator or a control mechanism. 2. Disturbances: if their values are not the result of adjustment by an operator or a control system The output variables are also classified into the following categories. 1. Measure output variables: if their values are known by directly measuring them 30

2. Unmeasured output: if they are not o cannot be measured The aims of control system are as follows: 1. To monitor process output variable by measurements. 2. To make rational decision regarding what corrective action is needed on the basis of information about the current and desired state of the process 3. To effectively implement this decisions on the process.

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CHAPTER 7: CONCLUSION From this study, it was determined that an average population of 1 million people can be able to generate enough waste that could be used to produce enough biogas that can serve as many as more than 80,000 people. Apart from serving as a source of energy, biogas production serves as a technique of controlling and treating wastes. The byproduct produced can be used as a high nutrient fertilizer. Even though the design problem addressed in this study describes a relatively large plant, it was understood during the course of the study that the technology can be applied to much smaller scales of production. One that is as small as could digest the waste from a single household and produce gas for the household. It is thus sensible to conclude that biogas production from human and animal wastes is surely a win-win venture. If properly harnessed, it holds the potential to provide a continuous supply of energy that is cheap, easy to produce and can be produced domestically. This also implies that a decentralised energy supply can be obtained while facilitating self sufficiency in a sustainable way.

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REFERENCES Bond, Tom and Templeton, Michael R. (2011). "History and future of domestic biogas plants in the developing world" in Energy for Sustainable Development Volume 15, Issue 4 Available at: www.sciencedirect.com Bugaje, I. M. and Mohammed I. A. (2008). Biofuels Production Technology. Zaria: Science and Technology Forum Consortium On Rural Technology (1986). Biogas from Human waste., Delhi: Consortium On Rural Technology David, Abutu (2010). DESIGN OF A PILOT PLANT FOR THE PRODUCTION OF 5000m3 OF BIOGAS PER ANNUM FROM BIOMASS (MUNICIPAL SOLID WASTE) (Unpublished final year project). Ahmadu Bello University, Zaria. Energy Commission of Nigeria (no date) Biofuels Training Manual Mahe, Haliru Jimoh (2010). DESIGN OF A PROCESS PLANT FOR THE PRODUCTION OF 5000m3 PER ANNUM OF BIOGAS FROM BIOMASS. (Unpublished final year project). Ahmadu Bello University, Zaria. Mukumba, Patrick, Makaka, Golden, Mamphweli, Sampson and Misi, Sherpherd (2013) "A possible design and justification for a biogas plant at Nyazura Adventist High School, Rusape, Zimbabwe" in Journal of Energy in Southern Africa Vol 24 No 4. Oladipo, Adewale (2010). DESIGN OF A PILOT PLANT FOR THE PRODUCTION OF 5000m3 OF BIOGAS PER ANNUM FROM BIOMASS (MUNICIPAL SOLID WASTE) (Unpublished final year project). Ahmadu Bello University, Zaria. Spuhler, Dorothee (no date) Anaerobic Digestion (Small-scale) [PowerPoint slides]. Available at: www.sswm.info

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