Biogas is produced by bacteria during digestion or fermentation of organic matter under airless condition (anaerobic process). The gas consists mainly of CH4 and CO2. This mixture of gases is combustible if the methane content is more than 50%. Biogas from animal dung contains approx. 60% methane.
Fig. 2: The big-chemical process of anaerobic digestion The different groups of bacteria responsible for fermentation live in an interacting eco-system. Each type of bacteria depends on others. The fermentation time is shortest when populations of different bacteria are adequately balanced.
In practice, the term slurry is used for the digester content or the digested substrate flowing out of the plant. In digesters observed by CAMARTEC, slurry is found in different conditions inside the digester:
- a light and rather solid fraction, mainly straw or fibrous particles, which float to the top forming the scum
- a very liquid, watery fraction remaining in the middle layer of the digester
- a viscous fraction below which is the real slurry or sludge
- heavy solids, mainly sand and soil particles which rest at the bottom.
Slurry separates less if the feed material is homogeneous and the TS-content is high.
Fig. 3: Slurry condition inside the CAMARTEC digester (1) Settlement of sand and soil. (2) Viscous slurry or sludge, having a TS-content of 6-7%. (3) Liquid slurry fraction, having a TS-content of 12%. (4) Floating scum, having a TS-content between 15 and 50 %. (5) Biogas.
Biogas Technology includes everything which is needed to produce and utilize the products of anaerobic digestion which are biogas and manure. Beside energy and fertilizer. other benefits of biogas technology are improved sanitation and environmental protection. The conditions to produce biogas are:
- digestable substrate, i.e. organic matter plus water
- a vessel where the substrate is not in contact with air
- a digestion temperature between 15ºC and 35°C
- a retention time longer than 30 days to allow the bacteria to produce the biogas. (The retention time is considerably reduced in industrial high-tech plants).
If methane producing bacteria are already present in the substrate (e.g. in dung from ruminants), biogas production begins within 3 to 5 days. At the farm site. biogas plants are filled slowly and gas production is used only after the plant has been filled completely. If there are problems with certain substrate starting the gas production, 20% of cattle dung should be mixed in the first filling as a starter.
The gas production potential of a certain substrate is high when organic matter content is high and the C/N ratio ranges from 20: 1 to 40: 1. The speed of the gas production depends further on the physical properties of the substrate and the temperature (optimum at 35°C). Dry and fibrous material takes longer to digest than fine-structured and wet substrate. Favoured total solid (TS) contents of the undigested substrate are between 7% and 11% which is approximately reached if dung is mixed with an equal volume of water or urine. A healthy digestion process shows a pH of 7.0 (neutral stage of substrate).
A biogas plant consists of the digester and the gas storage space. A continuous gas plant is charged and discharged regularly, e.g. every day. A batch-plant is filled once and emptied only after the material has been digested. A normal farmers biogas plant is a continuous plant with automatic discharge at the overflow.
Fig. 4: Relation of gas production and retention time
The daily gas production (8p) is measured in litre of biogas produced by l kg of total solids (TS) added per day. The total solids content of fresh cattle dung is 15-25 %. The retention time (RT) is the calculated period of days the substrate remains in the biogas plant before it reaches the overflow. The gas production per day depends on the slurry temperature.
Curve (1) is taken from different sources at 30°c, mainly from India. Curve (2) shows results from field research by UNDARP/BORDA in India on floating drum plants at a temperature of 27°C. Curve (3) is the average gas production with CAMARTEC fixed dome plants at 24°C average digester temperature. he points (4) show some selected samples of CAMARTEC plants of average performance, recorded during the BORDA Biogas Survey 1988. Performance is defined as daily gas production per square root of the total solid content of the daily fed substrate times the active digester volume (8p·(TS·VD)^(-0,5)).
Biochemical problems are rare, even in simple gasplants. Technical problems may occur with immature designs and unsuitable, i.e. scum forming, feed material. There are three well performing and mature designs available which are suitable for farm households:
- The fixed dome plant
- the floating drum plant
- the plastic covered ditch.
In most large scale extension programmes fixed dome plants have been chosen for dissemination because they are long lasting and cheaper than the floating drum plant. Fixed dome plants need the least maintenance of all other types. But building them requires great care in design and workmanship. Once they are constructed well, they are robust and of reliable performance.
The size of the digester depends on the required digester volume (VD) which is found by multiplying the wanted retention time (RT) with the volume of daily fed substrate (VS). In fixed dome plants, the active digester volume is defined by the digester volume below the zero-line, minus half the expansion chamber volume below the overflow line.
The gasholder volume (VG) depends on the daily gas production and the pattern in which the biogas is used. If gas consumption is regular and equally distributed over day and night and from day to day, gas storage space can be small. Irregular and rather concentrated gas consumption demands larger gas holder.
Experimental biogas plants for schools can be made out of 4 kg paint-tins (0 17,5 cm) and 2 kg milk powder tins (0 15 cm). The gas valve of such a floating drum model is made by a U-pipe filled with water. For gas release, the water is drained off and must be re-filled for closing the valve again.
Fixed Dome Plant
In fixed dome plants the gas is stored in the upper part of the rigid digester structure. Fixed dome plants are sometimes called "Chinese" or "hydraulic" digesters. The accumulating gas needs room and pushes part of the substrate into an expansion chamber, from where the slurry flows back into the digester as soon as gas is released. The volume of the expansion chamber is equal to the volume of gas storage. Gas pressure is created by the difference of slurry levels between the inside of the digester and the expansion chamber. The main building material is plastered brickwork.
Fig. 5: Small-scale biogas plants for rural areas in tropical countries
(A) Fixed dome plant. The gas collects in the upper part of the
digester(1) and displaces the slurry into the expansion chamber(2).
(B) Floating drum plant. The gas collects in a floating steel gas holder (3) which rises according to the volume of gas production.
(C) Plastic covered biogas plant. The gas is collected under an inflating plastic cover (4). A wooden roof (5) protects the plastic against sunlight and increases the gas pressure by its weight.
Fig. 6: System of the fixed dome plant
The digester (1) is filled via the inlet pipe (2) up to the bottom level of the expansion chamber (3). The level of original filling is called the zero line. The gasplant is closed by a gas-tight lid (4). Under the airless (anaerobic) condition, biogas is produced. When the gas valve (5) is closed, biogas collects in the upper part of the digester, called the gas storage part (6). The accumulating gas displaces part of the slurry into the expansion chamber. When the expansion chamber is full, slurry overflows into the slurry drain for use as manure. When the main valve (9) is opened, the gas escapes off the gas storage part until the slurry levels inside the digester and inside the expansion chamber balances. The gas pressure "p" depends on the prevailing difference of the slurry levels ( 10).
The substrate is filled daily so that slurry flows out daily at the time when large amount of gas is stored. Regular gas consumption requires smaller gas storage space. Consequently, the zero-line will rise. While daily feeding of the plant continues, gas is released before the slurry reaches the overflow level. The slurry level rises also when there is gas leakage. The level in the expansion chamber at zero gas pressure indicates the level of the zero line. The volume of slurry above the zero line inside the expansion chamber is equal to the gas storage space.
Fig. 7: Different models of fixed dome plants Fixed dome plants originate from China and were built already before 1960. Several variations with or without a removable cover at the top have been developed. (1) Biogas plant from Chengdu/China; (2) Janata Plant from India; (3) Dheenbandhu Plant of AFPRO from India; (4) Modified BORDA plant from Cankuso in Burundi..
The terminus "biogas unit" should underline the importance of integrated planning when applying biogas technology. The biogas unit describes the total package offered to the farmer in connection with biogas extension work. The main components are: The biogas plant itself, the stable, the toilet, the slurry storage pit, the slurry distribution canals, the gas piping system, the appliances and the tools to handle the substrate. In individual cases other components could as well be part of the biogas unit, for example, rain water tanks, fish ponds. compost pits, demonstration fields, gas generators or engines with their attachments, etc., etc. one may distinguish between agricultural biogas units and sanitary biogas units.
The big-latrine is the centre part of a sanitary biogas unit. The septic tanks of big-latrines are designed as integrated fixed dome biogas plants. Sanitary aspects, i.e. rather maintenance-free but clean toilets, are more important than a high gas production.
Fig. 8: Principal lay-out.(A) Agricultural Unit; (B) Sanitary unit
(1) Biogas plant; (2) Cattle stable; (3) Toilet; (4) Slurry distribution system; (5) Fodder grass or vegetable plantation; (6) Shrub or tree plantation; (7) Hedge between public area and slurry area; (8) Gas pipe; (9) Place of gas consumption; (10) Dung and urine collection chamber; (11) Fodder trough; (12) Chaffing block; (13) Urine drain (14) Sleeping boxes; (15) Milking stand; (16) Calves' box; (17) Exercising area, separated for cows and heifers.
Biogas Appliances are pieces of equipment for utilizing the energy of the gas. Either special biogas appliances are used or LPG equipment is adapted. Biogas is mainly used in stoves for cooking and in gas lamps for lighting. Frequently, refrigerators and incubators, coffee roasters, baking ovens and water heaters, chicken or piglet heaters, Power engines for milling or generating electricity are fuelled with biogas.
Biogas Extension Service
The biogas extension service (BES) comprises of the organization, the staff and the logistic needed to work for the extension of biogas technology. The BES might be a governmental body, a non-governmental voluntary or commercial organisation or a development project of international cooperation. Normally, the costs for the superstructure of the extension work are not included in the price the gasplant owner has to pay. Because of the benefits for the society as a whole it is justifiable to cover the cost of the superstructure from public funds.