ADVERTISEMENTS:
In this article we will discuss about:- 1. Meaning of Biogas 2. Uses of Biogas 3. Purification Process 4. Microorganisms Involved in Biogas Production 5. Process of Production 6. Factors Affecting Biogas Yields 7. Advantages 8. Disadvantages.
Meaning of Biogas:
Biogas is the mixture of gases (about 60% methane) produced by anaerobic bacterial digestion of organic matter. Biogas consists mainly of methane (CH4; ca. 60%) and CO2 (ca. 40%) plus traces of hydrogen (H2) and several other gases. It is produced by anaerobic degradation of a variety of organic materials, ranging from simple sugars to polymers like cellulose and nucleic acids by a community of organisms.
ADVERTISEMENTS:
Anaerobic digestion occurs in nature in the sediments of lakes and ponds, and in rumen of cattle; it is carried out by man in a variety of reactors called digesters and even in land-fill sites under non-septic conditions. A land-fill site is a location at which a pit was filled with, usually, domestic refuse, covered with soil and, generally, landscaped.
Uses of Biogas:
Biogas may be used in the following three ways:
(i) Direct combustion,
(ii) Generation of electricity and
ADVERTISEMENTS:
(iii) Purification and export as compressed gas or in liquid form.
Use of gas for direct combustion close to the site of production is the most economic. It may be used to heat green-houses, boilers, furnaces, fire brick or cement kilns, for household work like cooking etc.
Alternatively, electricity may be generated by using modified internal combustion engines or gas turbines. The main problems arise due to the contaminating gases present in traces which are corrosive to the metal parts of engines; this problem can be partially overcome by cleaning of the biogas and by a suitable modification of the engines and engine oils. Biogas is used for electricity generation in USA and UK by utilizing the gas generated at landfill sites.
Lastly, the gas may be purified (pure CH4) and exported via a pipeline as compressed gas or liquid. The purification process is complex and expensive but is being used mainly in USA (landfill sites) and in one project in Chile; in Chile the purified biogas is used to supplement the existing gas distribution system of Santiego.
Purification Process of Biogas:
1. The Substrate:
The substrate usually employed for biogas generation is a waste product of industrial, agricultural, animal husbandry, or domestic and municipal origin. Therefore, the waste would contain a variable proportion of non-biodegradable matter in form of plastics, inorganic materials, lignin etc. Lignin is virtually non-degradable under anaerobic conditions.
Generally, the wastes are divided on the basis of their organic dry matter or total solids (TS) content into the following four categories:
(i) Low (0.2-1% organic dry matter),
(ii) Medium (1-5%),
ADVERTISEMENTS:
(iii) High (5-12%) strength and
(iv) Solid (20-40%) wastes.
The decision of digestor design will depend mainly on whether the waste is completely soluble or it has particulate matter as well.
The wastes may originate mainly from the following three sources:
ADVERTISEMENTS:
(i) Industrial and Food Processing Wastes:
These arise from sugar (e.g., beet washing water), potato, vegetable and fruit processing, brewery and distillery wastes and as whey from cheese production. These are usually non-particulate medium to high strength wastes.
(ii) Animal Excreta and Agricultural Wastes:
These are solid wastes rich in cellulose and lignocellulose. Excreta from ruminant animals arc richer in lignocellulose and less favourable for anaerobic digestion than that of non-ruminants.
ADVERTISEMENTS:
Agricultural biomass like straw, slower, bagasse etc. show poor digestibility and often high C : N ratio (carbon: nitrogen ratio); these may have better alternative uses. In India, cowdung is widely used to produce biogas, locally called ‘gobar gas’.
(iii) Domestic and Municipal Wastes:
These are in the form of solid wastes and sewage, respectively. In many cases, domestic solid wastes are used to fill large pits (landfill sites). The sewage sludge is separated by settling down and is digested anaerobically; the biogas so obtained is used to generate electricity for operating the sewage treatment plants. Ordinarily, the electricity generated is enough for this purpose with only marginal surpluses for other uses.
In any case, the waste must contain adequate quantities of other nutrients to support the microorganism growth. If needed, phosphorus, nitrogen and trace elements should he added to the substrate. The C : N ratio should be below 40: 1 and the pH should be around 7; many animal excreta have neutral pH and adequate buffering.
ADVERTISEMENTS:
2. The Digester:
The digester design will mainly depend on the type of waste to be handled and the level of operation, i.e., small rural or large industrial operations. The various types of digesters used for anaerobic digestion are briefly described below. The digesters may be run in a batch or a continuous mode, the latter more desirable for large scale operations.
(i) Low Technology Digesters:
These digesters are simple in design and consist of the following provisions:
(i) A digester,
(ii) A lank for preparation of the substrate for feeding the digester,
ADVERTISEMENTS:
(iii) An effluent transfer tank and
(iv) A biogas storage tank.
Various designs of low cost digesters have been developed and often the biogas storage tank is combined with the digester (Fig. 40.10). An example of such a digester is the ‘gobar gas plant’ designed to produce biogas from cowdung.
The gobar gas plant consists of an underground circular pit lined with cemented brick wall which serves as the digester. The pit is covered with an inverted and vertically movable metal lank (closed on top, open on the bottom) which serves as biogas storage tank.
A tank is made from bricks on a raised platform above the ground; this serves as cowdung mixing tank to feed the digester through an inlet pipe that delivers the cowdung slurry near the bottom of the digester.
The spent cowdung slurry is removed to a drying bed (using sun energy for drying) and is ultimately used as a manure. The gas flows out through an outlet pipe which has a provision of drainage of the excess moisture (Fig. 40.10).
(ii) Batch Digesters:
These are suited for digestion of solid wastes, where hydrolysis occurs in a solid bed. and the leachate is circulated through a methanogenic digester and back over the solid waste bed. A series of such reactors can be used for large scale operations. Landfill sites are a type of low cost batch digesters. These sites are pits filled with solid domestic refuse and covered with soil.
After some time, anaerobic digestion of the organic matter present in the refuse generates methane which can be recovered by boring a number of gas wells into the top of the landfill (Fig. 40.11). The wells may be vertical, short-horizontal, short-vertical and bell-shaped; their walls are made up of perforated or slotted pipes.
The wells are interconnected by pipes and the gas is taken to a gas pump or compressor. The gas is chilled to remove the moisture from it; the gas is then filtered and analysed before use. Landfill sites are being used for biogas recovery on commercial scale in UK, USA and some other developed countries.
(iii) Continuous Flow Stirred Tank Digesters:
These digesters are continuously fed as well as stirred to achieve a very high mixing; the spent substrate is also continuously removed. They are suited to treat medium to high strength (2-10% solids) wastes. Mixing is achieved either mechanically or by recirculation of the biogas.
Gas may be collected in a separate tank or the digester itself may have a floating roof. These are the most common type of digesters used for sewage and animal excreta treatment; some of the digesters used for sewage treatment are of 12,000,000 litre.
(iv) Plug Flow Type of Digesters:
These are tubular digesters without any stirring device. Mixing however does occur due to the gas formation. Such digestors suffer from scum formation, especially in digesters having large diameters. Full size tubular digesters have been used in South Africa and USA.
(v) Up-flow Anaerobic Sludge Blanket Digesters:
In such digesters, the liquid waste flows through a blanket of sludge consisting of the bacteria settled as granules of about 4 mm in diameter. Such digesters are suited to treat low strength (ca. 1% solids), soluble wastes, e.g., sugar beet washings. These are not suited to treat wastes having particulate materials.
(vi) Film Reactors:
These reactors contain a support for attachment of the bacterial cells to avoid their washout due to higher waste flow rates. The support may be in the form of a stationery material, e.g., inert solids like gravel, PVC supports, glass beads etc., which forms an anaerobic filter through which the waste passes and is degraded.
These digesters have been used to treat vegetable processing wastes of medium to high strength (1-10% solids) and animal wastes. They have the problem of clogging and channelling in the filter. Alternatively, the particles to which the bacteria are attached become suspended and remain in constant motion in the flowing waste, but they are not washed out with the outflow of treated waste.
These are akin to fluidized bed reactors and are so called, the particles used are sand, PVC particles and carbon granules. These reactors do not suffer from clogging and channelling, are very efficient and can treat particulate wastes.
Microorganisms Involved in Biogas Production:
Several hundred species of microorganisms are involved in the anaerobic digestion and biogas production.
These bacteria can be divided into the following four trophic groups:
(i) Hydrolytic and fermentative bacteria,
(ii) Syntrophic H2 producing bacteria,
(iii) Methanogenic bacteria and
(iv) Acetogenic bacteria.
(i) Hydrolytic and Fermentative Bacteria:
This group includes both obligate and facultative anaerobes, and may occur upto 108-109 cells/ml of sewage sludge digesters. They remove the small amounts of O2 present and create anaerobic conditions. These bacteria hydrolyze and ferment the organic materials, e.g., cellulose, starch, proteins, sugars, lipids etc., and produce organic acids, CO2 and H2. (Fig. 40.12).
Digestion of complex polysaccharides is rate limiting, and the lignin associated with cellulose often shields the latter from enzyme action. Therefore, usually only 50% of the polysaccharides present in the waste may be digested.
(ii) Syntrophic H2 Producing Bacteria:
This group is also called obligate H2 producing or obligate proton reducing bacteria since they oxidise NADH by reducing H+ to H2, and thereby produce hydrogen. These bacteria breakdown organic acids having greater than 2 carbon atoms in their chain to produce acetate, CO2 and H2.
However, they are able to grow freely and produce H2 only under low H2 partial pressure which is maintained by methanogens. Sewage sludge digesters have about 4 x 106 cells/ml of this group. Examples of these bacteria are Syntrophomonas wolfei, and S. wolinii.
(iii) Methanogenic Bacteria:
This group of bacteria converts acetate, and CO2 + H2 into methane. Thus methanogens remove the H2 produced by obligate H2-producing bacteria, thereby lowering the H2 partial pressure and enabling the latter to continue producing H2.
Methanogenic bacteria are the strictest possible anaerobes known. They may occur upto 106-108 cell/ml of the slurry in digesters. These belong to Archaebacteria and oxidise H2 by reducing CO2 to obtain energy. Examples of methanogenic bacteria are Methanosarcina barkeri, Methanobacterium omelianskii, etc.
(iv) Acetogenic Bacteria:
These bacteria oxidise H2 by reducing CO2 to acetic acid which is then used up by methanogens to generate methane, CO2 and H2. Thus acetogenic bacteria also remove H2 and enable the obligate H2 producing bacteria to continue their function.
Process of Biogas Production:
The process of biogas production is explained using ‘gobar gas’ as an example. ‘Gobar gas’ plants are based on excreta of cattle and other farm animals, which contains about 20% inorganic particles or ‘rajkans’ (meaning dust particles).
The level of dust particles is reduced to about 10% by mixing the dung with water in 1: 1 ratio. The feeding rate of a typical dung based biogas plant is at the rate of 3,500 kg dung/day.
Generally, spent slurry at about 2% (v/v) of the fresh dung slurry is added back to maintain the microbial population. Calcium ammonium nitrate at the rate of 1% (w/w) of the dung is added to the slurry. In addition to cowdung, human excreta (upto 3% of slurry) and kitchen waste can also be used. Addition of human excreta markedly increases biogas output, perhaps due to its higher nitrogen content which supports microbial growth.
The optimal temperatures for biogas production are between 35-38°C. Lower temperatures lead to lower gas yields, and at 15°C biogas production may come to a halt.
Therefore, biogas production during winters and in colder regions requires thermal insulation and/or heating of the digesters. The pH of slurry should be around 7, which is not a problem when cowdung is used as substrate. Under favourable conditions, the biogas yield may be upto 60 1 /kg of dung.
The digesters in various biogas production schemes may be operated either under mesophilic (20-25°C to 40-45°C) or thermophilic (50-55°C to 60-65°C) conditions; each involves different bacterial species, but mesophilic operation is safer and more stable. However, thermophilic operation is more likely to inactivate pathogens and animal parasites.
Factors Affecting Biogas Yields:
Biogas yield [measured as m3 gas/kg volatile solids (volatile solids estimated at 500°C)] depends on the type of waste, temperature during digester operation, the retention time (the period of time a given sample of waste/substrate stays in the digester/fermenter before it flows out) and the presence of inhibitors.
The maximum possible gas yield on complete digestion of carbohydrates (starch, cellulose, glucose) would be 0.8 m3/kg; for fatty acids this value is about 1.5 m3/kg, and for proteins is about 0.9 m3/kg. But the organic matter conversion is almost always incomplete. Typical gas yields would be 0.6 m3/kg volatile solids (VS) for sewage sludge, 0.4 m3/kg VS of pig excreta and 0.2 m3/kg VS of cowdung.
Generally, mesophilic fermentation at about 35°C gives the maximum gas yields, while thermophilic bacteria give best yields around 55°C. For sewage sludge, the gas yield at 20°C may be only 80% of that at 35°C. Gas yields increase with retention time since a greater proportion of the organic matter will be digested.
But increased residence time increases the cost of operation since an increase in retention time reduces the quantity of wastes treated/day. Increasing the biodegradable solids content of the waste would enhance gas production (gross) but the solids content should not exceed 10-12% since pumps cannot operate with higher solids content.
Toxic components may include ammonia, SO42-, antibiotics etc. Agricultural wastes, especially, pig and chicken manure, generate high levels of ammonia which may inhibit biogas production. Most of the N content of the waste is however retained during anaerobic digestion so that the spent slurry is as good a source of nitrogen as the substrate itself.
Some wastes, e.g., from paper industry, may be rich in sulphate (SO42-). SO42- competes with CO2 for H2, thereby reducing methane yields. In addition, antibiotics may be used in animal feed, and detergents/disinfectants etc. may be present in the waste; these inhibit biogas production to varying degrees.
Anaerobic digestion is mainly used for pollution control, but its use as an energy source is also important. The estimation of net energy yields is rather complex in view of the factors affecting biogas yields. The total world production of biogas is only a tiny fraction of the total energy requirement. It is thought that the biogas technology will develop as a greater emphasis is placed on organic pollution control.
Advantages of Biogas:
ADVERTISEMENTS:
1. The technology is cheaper and much simpler than those for other biofuels, and it is ideal for small scale applications.
2. Recovery of the product (methane) is spontaneous as the gas automatically separates from the substrates.
3. Dilute waste material (2-10% solids) can be used as substrate.
4. Organic pollutants are removed from the environment and used to generate useful biogas; this helps clean up the environment.
5. Aseptic conditions are not needed for operation.
6. Any biodegradable matter can be used as substrate.
7. Biogas is suitable for heating boilers, firing brick and cement kilns, and for running suitably modified internal combustion engines (to generate electricity).
8. There is much reduced risk of explosion as compared to pure methane.
9. Anaerobic digestion inactivates pathogens and parasites, and is quite effective in reducing the incidence of water-borne diseases.
Disadvantages of Biogas:
1. The product (biogas) value is rather low; this makes it an unattractive commercial activity.
2. The biogas yields are lower due to the dilute nature of substrates used.
3. The process is not very attractive economically (as compared to other biofuels) on a large industrial scale
4. Recombinant DNA technology and even strain improvement techniques can not be used to enhance the efficiently of the process since the conditions of digestion are non-aseptic and exert their own selection pressure.
5. The only improvement in the process, therefore, can be brought about by optimising the environmental conditions of the anaerobic digestion.
6. Biogas contains some gases as impurities which are corrosive to the metal parts of internal combustion engines.
