biogas

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Bio gas:

By Dr.Parthasarathi Thota Bio gas

Biogas :

Biogas is about 20 percent lighter than air and has an ignition temperature in the range of 650° to 750° C It is an odourless and colourless gas that burns with clear blue flame similar to that of LPG gas ( Sathianathan , 1975). Its calorific value is 20 Mega Joules (MJ) per m3 and burns with 60 percent efficiency in a conventional biogas stove. This is the mixture of gas produced by methano genic bacteria while acting upon biodegradable materials in an anaerobic condition. Biogas is mainly composed of 50 to 70 percent methane, 30 to 40 percent carbon dioxide (CO2) and low amount of other gases Biogas

Biodigester :

Biodigester Simple biogas plants. Floating-drum plant (A), fixed-dome plant (B), fixed-dome plant with separate gas holder (C), balloon plant (D), channel- typedigester with plastic sheeting and sunshade (E). The biodigester is a physical structure, commonly known as the biogas plant. Since various chemical and microbiological reactions take place in the biodigester , it is also known as bio-reactor or anaerobic reactor. The main function of this structure is to provide anaerobic condition within it. As a chamber, it should be air and water tight. It can be made of various construction materials and in different shape and size. Construction of this structure forms a major part of the investment cost. Some of the commonly used designs are discussed below.

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Floating Drum Digester: Experiment on biogas technology in India began in 1937. In 1956. Jashu Bhai J Patel developed a design of floating drum biogas plant popularly known as Gobar Gas plant. In 1962, Patcl's design was approved by the Khadi and Village Industries Commission (KVIC) of India and this design soon became popular in India and the world. In this design, the digester chamber is made of brick masonry in cement mortar. A mild steel drum is placed on top of the digester to collect the biogas produced from the digester. Thus, there are two separate structures for gas production and collection. With the introduction of fixed dome Chinese model plant, the floating drum plants became obsolete because of comparatively high investment and maintenance cost along with other design weaknesses.

Floating-drum plants :

Floating-drum plants Advantages: It is simple, easily understood operation - the volume of stored gas is directly visible. The gas pressure is constant, determined by the weight of the gas holder. The construction is relatively easy, construction mistakes do not lead to major problems in operation and gas yield. Disadvantages: High material costs of the steel drum, the susceptibility of steel parts to corrosion. Because of this, floating drum plants have a shorter life span than fixed-dome plants and regular maintenance costs for the painting of the drum.

Floating gas holder type Biogas Plant (KVIC Model):

Floating gas holder type Biogas Plant (KVIC Model) Composite unit of a masonry digester and a metallic dome.Maintenance of constant pressure by upward and downward movement

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Fixed Dome Digester: Fixed dome Chinese model biogas plant (also called drumless digester) was built in China as early as 1936. It consists of an underground brick masonry compartment (fermentation chamber) with a dome on the top for gas storage. In this design, the fermentation chamber and gas holder are combined as one unit. This design eliminates the use of costlier mild steel gas holder which is susceptible to corrosion. The life of fixed dome type plant is longer (from 20 to 50 years) compared to KVIC plant. Based on the principles of fixed dome model from China, Gobar Gas and Agricultural Equipment Development Company (GGC). The concrete dome is the main characteristic of GGC design

Fixed-dome plants :

Fixed-dome plants Advantages: are the relatively low construction costs, the absence of moving parts and rusting steel parts. If well constructed, fixed dome plants have a long life span. The underground construction saves space and protects the digester from temperature changes. The construction provides opportunities for skilled local employment. Disadvantages: are mainly the frequent problems with the gas-tightness of the brickwork gasholder (a small crack in the upper brickwork can cause heavy losses of biogas). Fixed-dome plants are, therefore, recommended only where construction can be supervised by experienced biogas technicians. The gas pressure fluctuates substantially depending on the volume of the stored gas. Even though the underground construction buffers temperature extremes, digester temperatures are generally low.

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Deenbandhu Model: In an effort to further bring down the investment cost, Deenbandhu model was put forth in 1984 by the Action for Food Production (AFPRO), New Delhi. In India, this model proved 30 percent cheaper than Janata Model (also developed in India) which is the first fixed dome plant based on Chinese technology. It also proved to be about 45 percent cheaper than a KVIC plant of comparable size. Deenbandhu plants are made entirely of brick masonry work with a spherical shaped gas holder at the top and a concave bottom. A typical design of Deenbandhu plant is shown in Figure 1.3 (Singh. Myles and Dhussa , 1987). The Soudi Asian Partnership/Nepal (SAP/N), an INGO working in Nepal, has introduced Deenbandhu model plants in Bardiya district of Nepal. About 100 plants were constructed by SAP/N in the villages of Bardiya district in 1994. Preliminary studies carried out by BSP did not find any significant difference in the investment costs of GGC and the Deenbandhu design plants.

Deenbandhu Model:

Deenbandhu Model

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Bag Digester: This design was developed in 1960s in Taiwan. It consists of a long cylinder made of PVC or red mud plastic (Figure 1.4). The bag digester was developed to solve the problems experienced with brick and metal digesters. A PVC bag digester was also tested in Nepal by GGC at Butwal from April to June 1986. The study concluded that the plastic bag biodigester could be successful only if PVC bag is easily available, pressure inside the digester is increased and welding facilities are easily available (Biogas Newsletter, No. 23, 1986). Such conditions are difficult to meet in most of the rural areas in developing countries.

Balloon plants :

Balloon plants The balloon plant consists of a digester bag (e.g. PVC) in the upper part of which the gas is stored. The inlet and outlet are attached directly to the plastic skin of the balloon. The gas pressure is achieved through the elasticity of the balloon and by added weights placed on the balloon. Advantages are low cost, ease of transportation, low construction sophistication, high digester temperatures, uncomplicated cleaning, emptying and maintenance. Disadvantages can be the relatively short life span, high susceptibility to damage, little creation of local employment and, therefore, limited self-help potential. A variation of the balloon plant is the channel-type digester, which is usually covered with plasticsheeting and a sunshade ( FigureE ). Balloon plants can be recommended wherever the balloon skin is not likely to be damaged and where the temperature is even and high.

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Plus Flow Digester: The plug flow digester is similar to the bag digester. It consists of a trench (trench length has to be considerably greater than the width and depth) lined with, concrete or an impermeable membrane. The reactor is covered with either a flexible cover gas holder anchored to the ground, concrete orgalvanized iron (GI) top. The first documented use of this type of design was in South Africa in 1957. Figure 1.5 shows a sketch of such a reactor ( Gunnerson and Stuckey, 1986). This design has not been constructed at the field level in Nepal.

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Anaerobic Filter: This type of digester was developed in the 1950's to use relatively dilute and soluble waste water with low level of suspended solids. It is one of the earliest and simplest type of design developed to reduce the reactor volume. It consists of a column filled with a packing medium. I a great variety of non-biodegradable materials have been used as packing media for anaerobic filter reactors such as stones, plastic, coral, mussel shells, reeds, and bamboo rings. The methane forming bacteria form a film on the large surface of the packing medium and are not earned out of the digester with the effluent. For this reason, these reactors are also known as "fixed film" or "retained film" digesters ( Bioenergy Systems Report. 1984). Figure 1.6 presents a sketch of the anaerobic filter ( Gunnerson and Stuckey. 1986). This design is best suited for treating industrial, chemical and brewery wastes.

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Upflow Anaerobic Sludge Blanket: This UASB design was developed in 1980 in the Netherlands. It is similar to the anaerobic filter in that it involves a high concentration of immobilized bacteria in the reactor. However, the UASB reactors contain no packing medium, instead, the methane forming bacteria are concentrated in the dense granules of sludge blanket which covers the lower part of the reactor. The feed liquid enters from the bottom of the reactor and biogas is produced while liquid flows up through the sludge blanket (Figure 1.7). Many full-scale UASB plants are in operation in Europe using waste water from sugar beet processing and other dilute wastes that contain mainly soluble carbohydrates ( Bioenergy Systems Report, 1984).

Common and popular biogas plant types in India (Swaminathan and Rajashekaran,1987; Shivastava et al,1993):

Common and popular biogas plant types in India (Swaminathan and Rajashekaran,1987; Shivastava et al,1993) Name of plant Developed at Capacity (m3 gas/day) Kaccha-pucca plant Punjab agricultural university,Ludhiana 3 Capsule plant Himachal pradesh agricultural university.Palampur 2 Luknow type Janata plant Plannig,reaserch and action division (PRAD), Lucknow 1.7 Astra plant Indian institute of science, Bangaluru 4.25 TNAU plant College of agricultural engineering, coimbtore 2.5 Kalinga plant Govt.implements factory, Bhubneshwar 3.0 Janata plant Govt.implements factory, Bhubneshwar 3.0 IARI plant Divn . Of Soil Sci & Agri. Chemi.,I.A.R.I.,New Delhi 3.0

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Name of plant Developed at Capacity (m3 gas/day) Bharat plant Gujarat energy development agency,Vadodara Between 1-10 Ganesh plant Gobar gas reaserch and development centre,Rampur Various models giving 2-140 Pragati plant 2-6 Deenabandhu plant Action for food production (AFFPRO), Lucknow 2.0 Low cost PE plant AMM Murugappa chettiar reaserch centre, Chennai 2.0 Sultanpur plant m/ sVikas engineering corporation,sultanpur Kamadhenu plant Mr.Madasur ramchand

TNAU SAKTHI MODEL BIOGAS PLANT :

TNAU SAKTHI MODEL BIOGAS PLANT 3. General Information : TNAU sakthi model plant is made of brick, cement, sand and jelly. The only skill required is arch (dome) construction which can be done by masonry work.  The new model not only eliminates the centering, false roof and false pillar for the dome construction but also separates outlet tank. Hence the cost of construction of this digester is lesser than other types of biogas plants. 4. Cost of the unit : Rs. 7000 (2 m 3 plant) 5. Cost of operation : Rs. 4/- per day 6. Salient features : 15 to 20 per cent cost saving when compared to deenbhandu biogas model. 1. Function : Cooking, lighting and engine running 2. Specification : i. Components : Digester, inlet pipes and effluent collector ii. Feed material : Cow dung, pig manure, poultry droppings etc. iii. Shape of the plant : Spherical

JANATA PLANT :

JANATA PLANT 1. Function : Household cooking, lighting and running engines 2. Specification : i . Components : Digester, and inlet and outlet tanks ii. Feed material : Cow dung, pig manure, poultry droppings etc. iii. Shape of the plant : Cylindrical 3. General Information : This is a semi-continuous flow plant for producing biogas from cattle waste in domestic level. This is a fixed dome model. Main feature of janata design is that the digester and gas holder are part of a composite unit made of bricks and cement masonry. It requires centering for making the dome shaped roof and skilled and trained mason is a must for the construction. Based on the requirement and availability of feed material the size of the plant may be fixed suitably. 4. Cost of the unit : Rs. 10000 (2 m 3 plant)Rs. 12000 (3 m 3 plant) 5. Cost of operation : Rs. 5/day 6. Salient features : 20 � 30% costs saving than KVIC floating drum type plant

COMMUNITY BIOGAS PLANT :

COMMUNITY BIOGAS PLANT 1. Function : Cooking, lighting and running engines 2. Specification : i . Gas volume : 35 m 3 ii. Gas holder height : 1.0 m iii. Inlet/outlet opening : 2.0 x 1.2 m iv. Initial dung required : 35 to 40 tonnes of cowdung v. Daily loading rate : 600 to 700 kg vi. No. of cattle required : 60 to 70 animals General Information : The community level biogas plant will be constructed in a common place, the feed material will be collected from a group of households and the produced biogas will be distributed to all the beneficiaries. The size and cost of the plant may vary based on the availability of feed material, requirement of biogas and initial investment. 4. Cost of the unit : Rs. l,00,000/- 5. Cost of operation : Rs. l 00/day 6. Salient features : Rate of biogas production  :  1.5 m 3 /h No. of hours 5 hp dual fuel engine can run: 14 h Electricity production potential :50 kWh No. of beneficiaries for Cooking gas : 40 - 50 families

Pragati Model :

Pragati Model Combination of Deenbandhu and KVIC designs Lower part of the digester is semi spherical with conical bottom Floating drum acts as a gas storage

TERI’s Mark-4 System :

TERI’s Mark-4 System Features  Completely Spherical in shape  Reinforced dome with layers of Ferro-cement and tile bricks  Slurry Inlet box to avoid short circuits Stirrer to have a homogenous mixture of slurry.  60% gas storage

Sanitary latrine with biogas plant :

Sanitary latrine with biogas plant  Toilet linked biogas plants for conversion of night soil into biogas  Popular in rural areas of some western districts  Serves the purpose of sanitation and conversion of night soil into manure

The main factors that influence the selection of a particular design or model of a biogas plant are : :

The main factors that influence the selection of a particular design or model of a biogas plant are : Economic : An ideal plant should be as low-cost as possible (in terms of the production cost per unit volume of biogas0 both to the user as well as to the society. At present, with subsidy, the cost of a plant to the society is higher than to an individual user. Utilization of Local Materials: Use of easily available local materials should be emphasized in the construction of a biogas plant. Durability: Construction of a biogas plant requires certain degree of specialized skill which may not be easily available. A plant of short life could also be cost effective but such a plant may not reconstructed once its useful life ends. Especially in situation where people are yet to be motivated for the adoption of this technology and the necessary skill and materials are not readily available, it is necessary to construct plants that are more durable although this may require a higher initial investment. Suitable for the Type of Inputs: The design should be compatible with the type of inputs that would be used. If plant materials such as rice straw, maize straw or similar agricultural wastes are to be used then the batch feeding design or discontinuous system should be used instead of a design for continuous or semicontinuous feeding. Frequency of Using inputs and Outputs: Selection of a particular design and size of its various components also depend on how frequently the user can feed the system and utilize the gas.

Inputs and Their Characteristics :

Inputs and Their Characteristics Any biodegradabel organic material can be used as inputs for processing inside the biodigester . However, for economic and technical reasons, some materials are more preferred as inputs than others. If the inputs are costly or have to be purchased, then the economic benefits of outputs such as gas and slurry will become low. Also, if easily available biodegradable wastes are used as inputs, then that benefits could be of two folds: (a) economic value of biogas and its slurry; and (b) environmental cost avoided in dealing with the biodegradable waste in some other ways such as disposal in landfill One of the main attractions of biogas technology is its ability to generate biogas out of organic wastes that are abundant and freely available. addition to the animal and human wastes, plant materials can also be used to produce biogas and biomanure . For example, one kg of pre-treated crop waste and water hyacinth have the potential of producing 0.037 and 0.045 m3of biogas, respectively. Since different organic materials have different bio-chemical characteristics, their potential for gas production also varies. Two or more of such materials can be used together provided that some basic requirements for gas production or for normal growth of methanogens are met. Some characteristics of these inputs which have significant impact on the level of gas production are described below.

C/N Ratio ::

C/N Ratio : The relationship between the amount of carbon and nitrogen present in organic materials is expressed in terms of the Carbon/Nitrogen (C/N) ratio. A C/N ratio ranging from 20 to 30 is considered optimum for anaerobic digestion . If the C/N ratio is very high, the nitrogen will be consumed rapidly by methanogens for meeting their protein requirements and will no longer react on the left over carbon content of the material. As a result, gas production will be low. On the other hand, if the C/N ratio is very low, nitrogen will be liberated and accumulated in the form of ammonia (NH4), NH4 will increase the pH value of the content in the digester. A pH higher than 8.5 will start showing toxic effect on methanogen population. Animal waste, particularly cattle dung, has an average C/N ratio of about 24. The plant materials such as straw and sawdust contain a higher percentage of carbon. The human excreta has a C/N ratio as low as 8.Materials with high C/N ratio could be mixed with those of low C/N ratio to bring the average ratio of the composite input to a desirable level. In China, as a means to balance C/N ratio, it is customary' to load nee straw at the bottom of the digester upon which latrine waste is discharged.

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Dilution and Consistency of Inputs: Before feeding the digester, the excreta, especially fresh cattle dung, has to be mixed with water at the ratio of 1:1 on a unit volume basis (i.e. same volume of water for a given volume of dung) However, if the dung is in dry form, the quantity of water has to be increased accordingly to arrive at the desired consistency of the inputs (e.g. ratio could vary from 1:1.25 to even 1:2). The dilution should be made to maintain the total solids from 7 to 10 percent. If the dung is too diluted, the solid particles will settle down into the digester and if it is too thick, the particles impede the flow of gas formed at the lower part of digester. In both cases, gas production will be less than optimum. A survey made by BSP reveals that the farmers often over dilute the slum'. For thorough mixing of the cow dung and water (slurry), GGC has devised a Slurry Mixture Machine that can be fitted in the inlet of a digester. The specifications of the Slurry Mixture Machine which presently costs Rs 625 is given in Figure 1.8. It is also necessary to remove inert materials such as stones from the inlet before feeding the slurry into the digester Otherwise, the effective volume of the digester will decrease. Volatile Solids : The weight of organic solids burned off when heated to about 538" C is defined as volatile solids. The biogas production potential of different organic materials, given in Table 1.2, can also be calculated on the basis of their volatile solid content. The higher the volatile solid content in a unit volume of fresh dung, the higher the gas production For example, a kg of volatile solids in cow dung would yield about 0.25 m3 biogas ( Sathianathan . 1975).

Digestion :

Digestion Digestion refers to various reactions and interactions that take place among the methanogens . Nonmethanogens and substrates fed into the digester as inputs. This is a complex physio -chemical and biological process involving different factors and stages of change. This process of digestion ( methanization ) is summarized below in its simple form. The three-stage anaerobic fermentation of biomass from: Production and Utilization of Biogas in Rural Areas of Industrialized and Developing Countries, Schriftenreihe der gtz , No. 97, p. 54; after: Märkl , H.: Mikrobielle Methangewinnung; in: Fortschritte der Verfahrenstechnik, Vol. 18, p. 509, Düsseldorf,

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Hydrolysis: The waste materials of plant and animal origins consist mainly of carbohydrates, lipids, proteins and inorganic materials. Large molecular complex substances are solubilized into simpler ones with the help of extracellular enzyme released by the bacteria. This stage is also known as polymer breakdown stage. For example, the cellulose consisting of polymerized glucose is broken down to dimeric, and then to monomeric sugar molecules (glucose) by cellulolytic bacteria. Acidification: The monomer such as glucose which is produced in Stage 1 is fermented under anaerobic condition into various acids with the help of enzymes produced by the acid forming bacteria. At this stage, the acid-forming bacteria break down molecules of six atoms of carbon (glucose) into molecules of less atoms of carbon (acids) which are in a more reduced state than glucose. The principal acids produced in this process are acetic acid, propionic acid, butyric acid and ethanol.

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Methanization : The principle acids produced in Stage 2 are processed by methanogenic bacteria to produce methane. The reactions that takes place in the process of methane production is called Methanization and is expressed by the following equations ( Karki and Dixit. 1984). CH3COOH …………………….. CH4 + CO2 Acetic acid Methane Carbon dioxide 2CH3CH2OH + CO2 ………………CH4 + 2CH3COOH Ethanol Carbon dioxide Methane Acetic acid CO2 + 4H2……………………………… CH4 + 2H2O Carbon dioxide Hydrogen Methane Water The above equations show that many products, by-products and intermediate products are produced in the process of digestion of inputs in an anaerobic condition before the final product (methane) is produced. Obviously, there are many facilitating and inhibiting factors that play their role in the process. Some of these factors are discussed below.

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There are many species of methanogens and their characteristics vary. The different methane forming bacteria have many physiological properties in common, but they are heterogeneous in cellular morphology. Some are rods, some cocci , while others occur in clusters of cocci known as sarcine . The family of methanogens ( Methanobacteriacea ) is divided into following four generaon the basis of cytological differences (Alexander, 1961). A. Rod-shaped Bacteria (a) Non- sporulating , Methanobacterium (b) Sporulating . Methanobacillus B. Spherical (a) Sarcinae , Methanosarcina (b) Not in sarcinal groups, Methanococcus A considerable level of scientific knowledge and skill is required to isolate methanogenic bacteria in pure culture and maintain them in a laboratory. Methanogenic bacteria develop slowly and are sensitive to a sudden change in physical and chemical conditions. For example, a sudden fall in the slurry temperature by even T C may significantly affect their growth and gas production rate (Lagrange, 1979). Various types of methanogenic bacteria.The spherically shaped bacteria are of themethanosarcina genus; the long, tubular ones are methanothrix bacteria, and the short, curved rodsare bacteria that catabolize furfural and sulfates.The total length of the broken bar at top left,which serves as a size reference, corresponds to1 micron.

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pH value : The optimum biogas production is achieved when the pH value of input mixture in the digester is between 6 and 7. The pH in a biogas digester is also a function of the retention time. In the initial period of fermentation, as large amounts of organic acids are produced by acid forming bacteria, the pH inside the digester can decrease to below 5. This inhibits or even stops the digestion or fermentation process. Methanogenic bacteria are very sensitive to pH and do not thrive below a value of 6.5. Later, as the digestion process continues, concentration of NH4 increases due to digestion of nitrogen which can increase the pH value to above 8. When the methane production level is stabilized, the pH range remains buffered between 7.2 to 8.2. Temperature: The methanogens are inactive in extreme high and low temperatures. The optimum temperature is 35° C. When the ambient temperature goes down to 10" C, gas production virtually stops. Satisfactory gas production takes place in the mesophilic range, between 25º to 30° C. Proper nsulation of digester helps to increase gas production in the cold season. When the ambient temperature is 30° C or less, the average temperature withinthe dome remains about 4º C above the ambient temperature (Lund, Andersen and Tony-Smith, 1996).

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Loading Rate : Loading rate is the amount of raw materials fed per unit volume of digester capacity per day. In Nepalese conditions, about 6 kg of dung per m3 volume of digester is recommended incase of a cow dung plant (BSP, 1992). If the plant is overfed, acids will accumulate and methane production will be inhibited. Similarly, if the plant is underfed, the gas production will also be low. Retention Time : Retention time (also known as detention time) is the average period that a given quantity of input remains in the digester to be acted upon by the methanogens . hi a cow dung plant, the retention time is calculated by dividing the total volume of the digester by the volume of inputs added daily. Considering the climatic conditions of Nepal, a retention time of 50 to 60 days seems desirable. Thus, a digester should have a volume of 50 to 60 times the slurry added daily. But for a night soil biogas digester, a longer retention time (70-80 days) is needed so that the pathogens present in human faeces are destroyed. The retention time is also dependent on the temperature and upto 35° C, the higher the temperature, the lower the retention time (Lagrange, 1979).

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Toxicity: Mineral ions, heavy metals and the detergents are some of the toxic materials that inhibit thenormal growth of pathogens in the digester. Small quantity of mineral ions (e.g. sodium, potassium, calcium, magnesium, ammonium and sulphur ) also stimulates the growth of bacteria, while very heavy concentration of these ions will have toxic effect. For example, presence of NH4 from 50 to 200 mg/1 stimulates the growth of microbes, whereas its concentration above 1,500 mg/1 produces toxicity. Similarly, heavy metals such as copper, nickel, chromium, zinc, lead, etc. in small quantities are essential for the growth of bacteria but their higher concentration has toxic effects. Likewise, detergents including soap, antibiotics, organic solvents, etc. inhibit the activities of methane producing bacteria and addition of these substances in the digester should be avoided. Although there is a long list of the substances that produce toxicity on bacterial growth, the inhibiting levels of some of the major ones are given in Table 1.4.

Slurry :

Slurry This is the residue of inputs that comes out from the outlet after the substrate is acted upon by the methonogenic bacteria in an anaerobic condition inside the digester. After extraction of biogas (energy), the slurry (also known as effluent) comes out of digester as by-product of the anaerobic digestion system. It is an almost pathogen-free stabilized manure that can be used to maintain soil fertility and enhance crop production. Slurry is found in different forms inside the digester as mentioned below: a light rather solid fraction, mainly fibrous material, which float on the top forming the scum; a very liquid and watery fraction remaining in the middle layer of the digester; a viscous fraction below which is the real slurry or sludge; andheavy solids, mainly sand and soils that deposit at the bottom. There is less separation in the slurry if the feed materials are homogenous. Appropriate ratio of urine, water and excrement and intensive mixing before feeding the digester leads to homogeneous slurry.

Reference::

Reference: http://www.indiaenergyportal.org/overview_detail.php http://www.terienvis.nic.in/ppt/maithani.pdf http://agritech.tnau.ac.in/org_farm/orgfarm_manure.html http://ejournal.icrisat.org/agroecosystem/v2i1/v2i1vermi.pdf www.inseda.org/.../CD%20.../52.../ Farmyard %20 Manure -063.doc http://www.bioenergyindia.org/types.aspx http://www.sciencedirect.com/science/article/pii/S1537511002002453 FAO 1996 Biogas Technology A Training Manual for Extension A system approach to biogas technology from "Biogas technology: a training manual for extension" (FAO/CMS, 1996) A.K.Agarwal and S.V.Jogdand Energy from Biogas, Indian Dairyman,april2010pp36-41 http:// www.google.co.in/images

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