Contents 1 Introduction 1 1.1 gen 1

Chapter 1
GENERAL BACKGROUND

The process by which organic waste materials are converted into biogas and carbon dioxide is referred to as anaerobic digestion (AD). It involves the breakdown of organic matter by the concerted actions of a wide range of microorganisms in the absence of oxygen. The process consists of a complex series of reactions. The sum of these being a fermentation which converts a wide array of substrate materials, having carbon atoms at various oxidation/reduction states, to molecules containing one carbon in its most oxidized (CO2) and the most reduced (CH4) state. Minor quantities of nitrogen, hydrogen, ammonia and hydrogen sulphide (usually less than 1% of the total gas volume) are also generated.
Anaerobic conversions are among the oldest biological technologies utilised by mankind, initially for food and beverage production. They have been applied and developed over centuries, although the most dramatic advances have been achieved in the last few decades with the introduction of various form of high‐rate treatment processes, particularly for industrial wastewater. There are many ways to treat municipal solid waste (MSW), industrial wastewater, sewage sludge or waste materials from food production industry including biological operations. High organic loading rates and low sludge production are among the many advantages anaerobic processes exhibit over other biological unit operations. Initially AD was looked upon as only a method of waste elimination but recently has shown the potential as a source of energy also. The main characteristic of anaerobic process is biogas produced which can replace fossil fuel sources and therefore has a direct positive effect on greenhouse gas reduction. Among other advantages, energy recovery from renewables can help to reduce GHG emissions since - unlike combustion of natural gas, liquefied gas, oil and coal - energy generation from biogas is an almost carbon neutral way to produce energy from regional available raw materials. Carbon dioxide emissions emerging from the biogas combustion process are part of the natural carbon cycle and get absorbed and consumed by plants while growth.
The by-product of anaerobic digestion of organic materials is commonly referred to as biogas because of the biological nature of gas production. Bio gas technology refers to the production of a combustible gas and a value added fertilizer (called slurry or sludge) by the anaerobic fermentation of organic materials under certain controlled conditions of temperature, pH, C/N ratio etc. Of all the renewable technologies in use today bio gas technology has the lowest financial inputs per KWh of output. In addition bio gas has the potential to alleviate some of the more pressing problems in developing countries. India having a rural population of around 60% and being an agricultural based economy and having the largest cattle in the world has huge potential of bio gas energy. Apart from being a source of energy, biogas technology has the added advantage that it provides nutrient rich fertilizer, a means of disposal of organic wastes safely without harming the environment and helping maintain hygiene. Bio gas technology is extremely appropriate to the ecological and economic demands of the future. It is progressive sustainable and contributes much to the social advancement. The economic benefit of a bio gas plant is greater than that of its most competing candidates. Bio gas technology must not be looked upon as only a source of energy. It has far greater positive implications. The rich organic manure it produces, the sanitation it results in improves the quality of human life.

Fig 1:

When a non-ligneous bio mass is kept in a closed chamber for a few days, it ferment and produces an inflammable gas which consists mainly of methane and carbon di oxide. This gas is known as bio gas with methane compromising about 60 % of it. This process is called anaerobic fermentation or biomethanation or bio gasification. The plant consists of a bio gas reactor or digester and a gas holder which is usually placed directly above a digester. Inlet tank and pipe are provided to feed the raw materials mixed with water to the reactor. An outlet tank and pipe are provided to remove the digested sludge or fermentation residue to be used as bio manure. Decaying biomass and animal wastes are broken down to elementary nutrients and soil humus by decomposer organisms, fungi and bacteria. The process are favoured by wet, warm and dark conditions. The final stages are accomplished by many different species of bacteria classified as either aerobic or anaerobic. In closed conditions with no oxygen available from the environment, anaerobic bacteria are able to exist by breaking down general carbohydrate material. The carbon may be ultimately divided between fully oxidized CO2 and fully reduced CH4. Nutrients like soluble nitrogen compounds remain available in solution, so providing excellent fertilizer and humus. Being accomplished by micro-organisms, the reactions are also classified as fermentations but in anaerobic conditions yielding methane, the term digestion is often preferred.
ADVANTAGES
1. Utilization of locally available, renewable resources. 2. It is the cheapest of the renewable energy technologies. 3. It is an almost carbon-neutral energy supply. Environmentally friendly recirculation of organic waste from industries and households. 4. Apart from the energy point of view, looked upon as an effective method of waste management. 5. Acts as a source of local energy supply – no overland lines required. 6. Most suitable technology for rural areas and developing countries. Anaerobic digestion technology can be readily integrated with rural development. It reduces deforestation and saves fuel wood in context of rural areas. Results in smoke free, less pollutant fuel for cooking, lighting and running agro-machinery. Additionally produces value added fertilizer resulting in increased agricultural productivity with nutrient recycling. It has been observed that the use of digested slurry as a fertilizer improves soil fertility increases crop yield by 10-20%.[1] Also now a days organic farming has become a big trend and also fetches the farmer higher prices also. Positive effects on the health of rural people as disease carrying pathogens and parasites are destroyed. Protection of subsoil water-improved nitrogen exploitation which reduces leaching and thus protects the drinking water.

DISADVANTAGES 1. High initial investment costs and regular staff expenses. 2. Possibility of unpleasant odour. 3. Requirement of sufficient land area for storage and spreading of digestate. 4. Exclusive cultivation of energy crops may cause ecological problems (monocultures, intensive farming). 5. Culturally deep seated prejudices against the use of some wastes, compounded by superstition and resistance to change. 6. Excessive water requirement especially for dry areas.

CONFIGURATIONS OF BIOGAS SYSTEMS
Generally, for the production of biogas by anaerobic digestion processes, residues from livestock farming, food processing industries, waste water treatment sludge, and other organic wastes can be utilised. Anaerobic digesters can be designed and engineered to operate using a number of different variants and process configurations. An overview on classification methods is shown below.

Anaerobic digestion processes can be classified according to the total solids (TS) content of the slurry in the digester. When looking at the total solids content (TS) of the feedstock to the digesters, biogas reactors can either be designed to operate at a high solids content (TS > ~20%), or at a low solids concentration. Plants treating substrates with high solids content are referred to as dry fermentation reactors, those with low solids content are called wet fermentation systems. Low-solids digesters can transport material through the system using standard pumps with a significantly lower energy input but require more volume and area due to an increased liquid-feedstock ratio. The dry fermentation process utilises solid, stackable biomass and organic waste, which cannot be pumped. It is mainly based on a batch wise operation with a high TS content ranging from 20 to 50% at mesophilic temperatures. Digesters which solely work on the dry system with very little or no additional liquid are inoculated with digested substrate and thus, inoculants and fresh material have to be mixed in suitable ratios beforehand. In wet fermentation systems the substrates don’t need to be mixed or inoculated as bacteria rich percolation liquid re-circulated from the digester effluent takes over the role of the bacterial inoculation and process starting.
Anaerobic digesters can be categorized further on the basis of number of reactors used, into single stage and multi stage. In a multi stage reactor the different biochemical reactions take place in different reactors with the more favourable conditions maintained in each reactor for the specific reaction. Temperature phased and acid-gas phased digesters come under the multi reactor category.
Regarding the flow pattern of anaerobic digesters two basic types can be distinguished: batch and continuous. In continuous flow reactors which normally are completely mixed the processes involved in anaerobic digestion proceed spatially as well as temporally in parallel steps whereas batch reactors exhibit temporally staggered sequences. The operation of batch-type digesters consists of loading the digester with organic materials and allowing it to digest. Once the digestion is complete, the effluent is removed and the process is repeated. For example, covered lagoons and anaerobic sequencing batch reactors (SBR) are operated in batch mode. A covered lagoon consists of a pond containing the organic wastes which is fitted with an impermeable cover that collects the biogas. The cover can be placed over the entire lagoon or over the part that produces the most methane. The substrate enters at one end of the lagoon and the effluent is removed at the other. Cover lagoons are not heated and operate at ambient temperatures which implies seasonal variations in reaction and conversion rates. The advantage of anaerobic lagoons are relatively low costs which are partly offset by lower energy yields and poor effluent quality. Anaerobic sequencing batch reactors (SBR) are discontinuously operated in a fill and draw mode. Filling of the tank is followed by a reaction period yielding biogas. During this stage the substrate is allowed to settle to the bottom of the tank and the solids separate from the effluent liquor. After that the supernatant and the digested substrate are withdrawn except a small portion which is retained in the tank in order to inoculate the incoming feed with active microorganisms.
In a continuous or quasi-continuous digester, organic material is constantly or regularly fed into the digester where it is moved forward either mechanically or by the force of the new feed pushing out digested material. Unlike batch-type digesters, continuous digesters produce biogas without the interruption of loading material and unloading effluent. Continuous digesters include plug-flow systems, continuous stirred tank reactors (CSTR), and high-rate biofilm systems such as upflow anaerobic sludge blanket reactors (UASB). In most cases, a plug-flow digester comprises a stirred and heated horizontal tank which is fed at one end and the emptied at the other. By continuous feeding a ‘plug’ of substrate is slowly moved through the tank towards the effluent. This mode of operation has various advantages including the prevention of premature removal of fresh substrate through hydraulic short-circuiting and a high sanitising potential. Since the plug flow digester is a growth based system where the biomass is not conserved, it is less efficient than a retained biomass system (e.g. UASB) and inoculation may be required.
Basically, a continuous stirred tank reactor (CSTR) consists of a closed vessel equipped with stirring devices providing mixing of the content. The reactor is continuously fed with substrate and due to the mixing it can be assumed that the concentrations of the compounds inside the vessel equal those at the effluent. Also, there is no liquid-solid separation or stratification and, hence, the solids retention time (SRT) is the same as the hydraulic retention time (HRT). Since the biomass is suspended in the main liquid and will be removed together with the effluent, relatively long HRTs are required to avoid an outwash of the slow-growing methanogens. Up-flow anaerobic sludge blanket reactors (UASB) belong to the group of so-called high-rate anaerobic reactors. The term “high-rate” refers to reactor configurations that provide significant retention of active biomass, resulting in large differences between the SRT and the HRT, and operation at relatively short HRTs, often on the order of two days or less (Grady et al., 1999). In an UASB digester the influent is introduced into the bottom of the vessel with a relatively uniform flow across the reactor cross section and distributed such that an upward flow is created. In the upper portion of the tank a cone shape with a widening cross section is introduced reducing the flow as it rises. As a consequence, combined with the flow rising upward from the bottom, gradually descending sludge will be held in equilibrium forming a blanket which suspends in the tank. Small sludge granules begin to form whose surface area is covered with aggregations of bacteria. Finally the aggregates form into dense compact biofilms referred to as "granules". Substrate flows upwards through the blanket and is degraded and converted to biogas by the anaerobic microorganisms. Treated effluent exits the granular zone and flows upward into the gas-liquids-solids separator. There, the gas is collected in a hood and the supernatant liquid is discharged while separated solids settle back to the reaction zone. The combined effects of influent distribution and gas production result in mixing of the influent with the granules. Some variants of biofilm reactors use up-flow reactors provided with an internal packing to improve sludge blanket stability. The media have a high specific surface and allow for the growth of attached biomass.
Depending upon the temperature maintained in the digester we have three types of digestion possible, which will be discussed in detail later in the report.
The constant volume semi-batch or the fixed dome type, constant pressure semi batch or the floating drum type and the plug flow reactor semi-batch type or the polythene tubular anaerobic digester are the three most used configuration in the developing world due to the simplicity in construction and operation.

Generally, choice of reactor type is determined by waste characteristics, especially particulate solid contents. Consequently, the process must be able to convert solids to gas without clogging the anaerobic reactor. Solids and slurry waste are mainly treated in continuous flow stirred tank reactors (CSTR), while soluble organic wastes are treated using high-rate biofilm systems such as UASB reactors (Boe, 2006). MICROBIAL ASPECTS OF THE ANAEROBIC PROCESS
From a microbiological viewpoint, the anaerobic degradation of complex organic matter into methane and certain by-products is a complex multi-step process of metabolic interactions performed by a well-organised community of microbial populations. Accordingly, a variety of microorganisms coexist in anaerobic digesters even when a single substrate is utilized and their concerted activity is necessary for the complete bioconversion of organic materials to methane, carbon dioxide as well as trace gases such as hydrogen sulphide and hydrogen. Maintaining a healthy bacterial population heavily depends on the microbial status and suitable operating conditions.

Digestion of particulate composites can be roughly subdivided into four phases, termed hydrolysis/liquefaction, acidogenesis, acetogenesis and methanogenesis. These phases are a series of interlinked reactions proceeding spatially as well as temporally in consecutive and parallel steps and hence, influence one another.

Hydrolysis is a process where complex macromolecular organic matter comprising carbohydrates, proteins and fats is subject to enzymatic degradation and transformed to monosaccharides, amino acids and long chain fatty acids (LCFA). Further anaerobic digestion finally leads from acidogenesis, acetogenesis and methanogenesis via intermediates and by-products to biogas production (CH4, CO2).

HYDROLYSIS

As complex organic polymeric materials cannot be utilized by microorganisms unless they are broken down to soluble compounds, anaerobic degradation starts with the hydrolysis step in which the organic polymers get solubilised into simpler and more soluble intermediates which can then pass the cell membrane. Once inside the cell, these simple molecules are used to provide energy and to synthesize cellular components. This phase is also termed liquefaction as the degradation processes involve the dissociation of water.
Hydrolytic reactions which comprise two phases are propelled by extracellular enzymes secreted by bacteria which are obligate or facultative anaerobes. In the first phase a bacterial colonization takes place where the hydrolytic bacteria cover the surface of solids. Bacteria on the particle surface release enzymes and produce the monomers which can be utilized by the hydrolytic bacteria themselves, as well as by the other bacteria. In the second phase the particle surface will be degraded by the bacteria at a constant depth per unit of time (Vavilin et al., 1996). Released enzymes include cellulase, cellobiase, xylanase and amylase for degrading carbohydrates into simple sugars (monosaccharides), protease for degrading protein into amino acids and lipase for degrading lipids into glycerol and LCFA. The overall hydrolysis rate depends on organic material size, shape, surface area, biomass concentration, enzyme production and adsorption (Parawira et al., 2005; Grady et al., 1999; Boe, 2006). It is commonly found that hydrolysis is the rate-limiting step for digestion when the substrate is in particulate form (e.g. swine waste, cattle manure and sewage sludge) while methanogenesis is the rate-limiting step for readily degradable substrate (Vavilin et al., 1996; Vavilin et al., 1997).

ACIDOGENESIS

The step subsequent to hydrolysis is referred to as acidogenesis (also termed fermentation) which is generally defined as an anaerobic acid-producing microbial process without an additional electron acceptor or donor (Gujer and Zehnder, 1983). The monosaccharides and amino acids resulting from hydrolysis are degraded to a number of simpler products such as volatile fatty acids (VFA) including propionic acid (CH3CH2COOH) and butyric acid (CH3CH2CH2COOH) as well as acetic acid (CH3COOH). However, the organisms oxidising LCFA are required to utilise an external electron acceptor such as hydrogen ions or CO2 to produce H2 or formate (Batstone et al., 2002a).
The degradation of monosaccharides (e.g. glucose) can manifest in different pathways which leads to the emergence of different products such as VFA, lactate, and ethanol with different yields of energy. The dominant pathway depends on several factors such as substrate concentration, pH and dissolved hydrogen concentrations. For example, under very high organic loads, lactic acid production becomes significant. At higher pH (>5) the production of VFA is increased, whereas at low pH (<5) more ethanol is produced. At even lower pH (<4) all processes may cease.

However, hydrogen partial pressure has been reported to have most influence on the fermentation pathway. At low partial pressures of hydrogen the fermentation pathway to acetate and hydrogen is favoured rather than ethanol or butyrate formation. Thus, in a system where the hydrogen-utilising organisms (such as methanogens) maintain low partial pressure of hydrogen, the fermentation pathway to acetate and hydrogen contributes the main carbon flow from carbohydrates to methane formation. However, higher VFA and alcohols are still produced continuously by the degradation of lipids and amino acids (Schink, 1997; Boe, 2006). These products cannot be utilised directly by the methanogens and must be degraded further in a subsequent process that is referred to as acetogenesis (Björnsson, 2000).
Acidogenesis is often the quickest step in the anaerobic conversion of complex organic matter in liquid phase digestions. So, process-failure in the anaerobic digestion of complex organic matter due to the influence of various toxic or inhibitory components leads to a halt of methane production and an accumulation of long- and short-chain fatty acids (Vavilin et al., 1996).

ACETOGENESIS

As already mentioned before, the degradation of higher organic acids formed in acidogenesis is an oxidation step with no internal electron acceptor. Thus, the oxidising organisms (normally bacteria) require an additional electron acceptor such as hydrogen ions or CO2 for the conversion to acetate, carbon dioxide and hydrogen (Batstone et al., 2002a). This intermediate conversion is crucial for the successful production of biogas, as these compounds cannot be utilised directly by methanogens. Since acetogens are obligate hydrogen producers and in the same time depend on a low partial pressure of hydrogen, they maintain a syntrophic (mutually beneficial) relationship with hydrogen-consuming methanogenic archaea. This interspecies hydrogen transfer where the methanogens serve as a hydrogen sink allows the fermentation reactions to proceed. Syntrophy means, literally, “eating together” and is a special case of symbiotic cooperation between two metabolically different types of microbial organisms which depend on each other for degradation of a certain substrate, typically for energetic reasons (Schink, 1997; Björnsson, 2000).

As shown in Figure 2-4, low H2 partial pressure is essential for acetogenic reactions to be thermodynamically favourable (ΔG’ < 0), whereas hydrogen consuming methanogenesis becomes more favourable at higher pressures. Thus, these reactions can only occur simultaneously within a narrow range of very low PH2. The shaded area shows the theoretical operating region for syntrophic acetogenesis from propionate. An example of the free energy yield for the conversion of butyrate to acetate and methane is shown in Table 2-2. The degradation of butyrate to acetate is not energetically feasible because it carries out a reaction which is endergonic under standard conditions, but is dependent on co-culture with a hydrogen-scavenging partner organism (hydrogenotrophic methanogens). The second reaction in Table 2-2 provides a yield of energy which is partly transferred by the methanogens back to the acetogens. Thus, the overall syntrophic reaction is thermodynamically favourable with a small energy yield (ΔG’ < 0). The low energy yield makes the organisms very slow-growing and sensitive to changes in organic load and flow rate. Acetogens are sensitive to environmental changes, and long periods are likely to be required for these bacteria to adjust to new environmental conditions (Björnsson, 2000).

Acetogenic bacteria not only profit from hydrogenotrophic methanogens, but also aceticlastic methanogens, as acetate removal has an influence on the energetics of VFA oxidizing reactions, especially in iso-valerate degradation, where three molecules of acetate and only one molecule of H2 are formed. Moreover, acetate accumulation may have a biochemical inhibitory effect on acetogenesis (Boe, 2006).

METHANOGENESIS

During methanogenesis, the fermentation products such as acetate and H2/CO2 are converted to CH4 and CO2 by methanogenic archaea which are strict obligate anaerobes. Other methanogens are able to grow on one-carbon compounds such as formate, methanol and methylamine. Generally, methanogens are specialists in substrate utilisation, as some of them can use only one substrate.
The archaea relevant for anaerobic digestion are commonly divided into two groups: one group, termed aceticlastic methanogens, split acetate into methane and carbon dioxide. The second group, termed hydrogenotrophic methanogens use hydrogen as the electron donor and CO2 as the electron acceptor to produce methane. Nearly all known methanogenic species are able to produce methane from H2/CO2, whereas only a few species of methanogens are believed to be capable of utilising acetate as a substrate. However, it has been estimated from stoichiometric relations that about 70% of the methane formed in anaerobic digesters is derived via the acetate pathway. The hydrogen pathway is more energy yielding than the acetate pathway, and is normally not rate limiting. It is, however, of fundamental importance due to its ability to keep the hydrogen pressure low in the system (Klass, 1984; Pavlostathis and Giraldo-Gomez, 1991;Björnsson, 2000).
Moreover, apart from methanogenic reactions, the inter-conversion between hydrogen and acetate catalysed by so-called homoacetogenic bacteria also plays an important role in the methane formation pathway. Depending on the external hydrogen concentration, homoacetogens can either oxidize or synthesize acetate which allows for contention with several different microbes, including methanogens. As can be seen from Table 2-3, the H2 consumption by hydrogenotrophic methanogenesis is thermodynamically more favourable than homoacetogenesis (ΔG°’<0). Regarding acetate consumption, aceticlastic methanogenesis is also more favourable than acetate oxidation. As already mentioned, hydrogenotrophic methanogenesis works better at high hydrogen partial pressure (Figure 2-4), while aceticlastic methanogenesis is independent from hydrogen partial pressure. At higher temperatures (> 30°C) the acetate oxidation pathway becomes more favourable (Boe, 2006).

Hydrogenotrophic methanogenesis has been found to be a major controlling process in the overall scheme of anaerobic digestion. Its failure will strongly affect the syntrophic acetogenic bacteria and the fermentation process as a whole (Schink, 1997). The accumulation of reduced fermentation products in anaerobic digester is mainly due to inadequate removal of hydrogen and acetate due to several reasons. For example, high organic load increases hydrogen and VFA production beyond the capacity of methanogens resulting in accumulation of VFA, or the decreasing in capacity of methanogens due to inhibition by toxic compounds or pH drop (<6) (Boe, 2006).
The hydrogen-consuming methanogens are among the fastest growing organisms in the anaerobic digestion process as their minimum doubling time has been estimated to be six hours, compared with 2.6 days for the slow-growing acetoclastic methanogens. Also, hydrogenotrophic methanogens have been found to be less sensitive to environmental changes than acetoclastic methanogens. Hence, methanogenesis from acetate tends to be rate limiting in the anaerobic treatment of easily hydrolysable substrates (Björnsson, 2000).

FACTORS AFFECTING ANAEROBIC DIGESTION
The performance of an anaerobic process is affected by many factors. These range from process factors such as the solid retention time, organic and hydraulic loading rates to environmental factors such as temperature, pH, nutrient supply, and the presence of toxics to operational factors such as mixing and the characteristics of the waste being treated. Optimum values of these parameters must be maintained in the digester. The most important factors are as follows: