Operation and Maintenance Manual of 6m3 Fixed Dome Biogas Plant
1. Introduction Biogas is produced by the degradation of biological matter under anaerobic reaction in absence of free oxygen in a digester. A domestic biogas unit is a digesting chamber where organic materials such as manure and vegetable waste ferments to provide biogas, through the release of methane. Biogas is seen as a clean fuel, and provides a feasible alternative to cooking gas. Biogas is a mixture of gases produced by methanogenic (methane producing) 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. These gases include oxygen (O 2), hydrogen sulfide (H 2S), ammonia (NH3) and hydrogen (H2). Biogas is colourless gas that burns with clear blue flame similar to that of LPG. Its calorific value is 20 Mega Joules (MJ) per m 3 and burns with 60 percent efficiency in a conventional biogas stove. In regions where both hot and cold climates are prevalent, the KVIC fixed model is ideal because the digestion chamber is underground providing good insulation against the cold. We use a design optimized for a longer 45 day cycle so that the mixture gets more time for digestion. During winter months, in colder climates, the production of gas may be reduced still the plants function very well giving biogas every day. We have found that the critical factors for success with biogas plants are ensuring quality during construction, and training user groups in correct use and maintenance of the plants. Ignorance of these critical issues often leads to the failure of biogas plants. Nowadays the use of biogas plants has spread from use in the treatment of animal excrement to the treatment of biodegradable municipal solid waste also. It is expected that biogas will be a significant source of energy in the future to preserve the environment, solve the pollution problem and to promote better health to agriculture and community.
2. Components of a biogas plant 2.1 Foundation The foundation of the plant is bowl shaped with a collar around the circumference. The digester with dome, which is the heart of the biogas plant, is constructed on this collar. 2.2 Digester The digester is main part of the plant where the mixture of organic material and water decomposes to produce biogas by bacterial activity.
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2.3 Dome The dome functions as a gas storage space which is constructed on the digester. 2.4 Gas outlet pipe A nipple is fitted on the top of the dome, which is connected to a GI/PVC pipe through a pressure gauge. The gas reaches the kitchen through this pipe. 2.5 Inlet The pipe through which organic waste and water enters the digester is called the inlet pipe and is connected to a small tank for mixing waste and water. 2.6 Outlet The portion of the plant where the slurry accumulates after coming out of the digester is called outlet tank. It is small and rectangular in shape, which is connected to the dome opening. An opening for slurry discharge is provided in the outlet tank. 2.7 Balancing tank The volume of the dome up to the opening of the outlet tank into the balancing tank is equivalent to the gas storage volume. Only the amount of gas equivalent to this volume can be used in a burner. The main function of this part is to provide pressure for release of gas from the dome. When the gas storage is full the level of slurry rises up to the outlet of the balancing tank (reference point in the balancing tank) and is discharged to the outside.
Fig 1 Fixed dome Biogas plant design
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3. Factors affecting biogas production 3.1 Organic Waste Daily quantity of waste added must be sufficient, if too much or too little is added, very little or no gas will be produced as the bacteria does not have sufficient time to break down the waste. 3.2 Time Suitable fermenting and breaking down time of waste is between 40-60 days. 3.3 Chemicals Chemicals such as antibiotic, pesticide, chemical fertilizer or other chemical products may damage bacteria that break down the organic materials in the chamber. The bacteria may stop working and gas will not be produced. Therefore chemical substances should not be released into biogas plants. 3.4 Temperature Optimum temperature for bacterial growth is 37o C. If the temperature is higher or lower than the optimum, bacteria will not develop, decreasing gas production. 3.5 pH Balance pH between 7- 8.5 is optimal. Below pH 6.5, methane producing bacteria will not survive.
4. OPERATION AND MAINTENANCE 4.1 Hydraulic testing The digester of the plant on completion of construction is checked for water leakages before commissioning through hydraulic testing. This is done by filling the digester with water and marking the level. Thereafter, after a period of 7 days the water level is rechecked. In case of leakages the water level will go down and then soap solution test for detecting gas leakages will be done. 4.2 Soap solution test Soap solution is poured/sprayed on the top of dome, the digester and thereafter all vents of the digester are closed and checked for leakage. Any section of the dome emitting bubbles is identified and will be sealed. 4.3 Digester start�up Though different methods exist for initial commissioning of the digester, the proven method is to fill the digester to half the required level with a mixture of animal dung (cow or buffallo) and water, in a ratio of 1 kg of dung with 5�10 litres of water. This should be left for approx 2 weeks for the process to start, after which daily feeding of organic waste can start. Alternatively, the digester can be made functional by filling it up with water to half the required level and feed it with the standard daily mixture, i.e., 300 kg dung together with 300 kg water. This method is considered safer but also takes more time for digestion and
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gas production. It is recommended to use only dung for start‐up, and to wait for adding other organic waste until the digester is producing a stable amount of gas. Even then, it is recommended to slowly increase the amount of co‐substrate (organic waste), adding 20% of the eventual amount for at least one week, and then increasing it to 40% in the week after that, until the full 100% is reached. The gas produced in the initial period is mainly CO 2 and hence will not burn in the initial weeks. Hence it is necessary to (partly) deflate the gas storage a number of times. The methane content of the gas inside the storage will then increase much faster. When the digester has been fed with half the amount of water and/or dung, it will take approximately one month to fill to the required level. At this time, sufficient gas will have been built up to pressurize the gas storage. 4.4 Stage 1. Adding of seed (cow /buffalo dung) Steps 1. Fill the digester with water to half the required level 2. Open the gas release valve on top of the dome to release the air until the pressure gauge reading is at 0 3. Close/ plug the opening of the inlet pipe 4. Add cow or buffalo dung daily as shown in Table 1 5. Weigh out the required amount of dung and water and add into the inlet tank 6. Mix thoroughly to form a mass of homogenous slurry 7. Remove any unwanted items like straw, logs of wood 8. Keep the content for about 30 minutes in the tank 9. Let the content to flow into the digester by opening the inlet 10. Push/poke with a rod through the inlet and outlet 11. Continue addition of dung for seven days following steps 3 to 10 12. Clean mixing chamber after every addition of the waste into the digester chamber
Table 1 Quantity of dung and water for daily addition Sizes of Bio-gas plants 4-6M3
8M3
12M3
Cow/buffalo dung (litres)
300/day
600 /day
800/day
Water (litres)
300/day
600/day
800/day
Type of Materials
•
This addition will start producing gas within 2-3 days.
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•
Don’t use the gas; wait for complete filling of the gas storage (when the balancing tank overflows, gas storage is full)
•
Deflate (partly) the gas storage a number of times by opening the valve
•
Once the gas is full, release the gas from the tank 3 times before actual use.
(Note: Avoid excessive feeding, which will have negative effect as decomposition is likely to be slow). 4.5 Stage 2 - Adding of Organic Waste After 7 days of the first addition of dung, add the organic materials regularly on daily basis as shown in Table 2 Table 2 Quantity of organic waste addition Sizes of Bio-gas plants Type of Materials Organic Waste (litres per day) Water (litres per day )
4-6 M3
8M3
12M3
50 50
70 70
100 100
1. Close/ plug the opening of the inlet pipe 2. Add kitchen waste daily as shown in Table 2 3. Chop the waste as fine as possible 4. Remove any unwanted items by segregation 5. Weigh out the required amount of waste and water and add into the inlet tank 6. Mix thoroughly to form a mass of homogenous slurry 7. Remove any non-digestible hostile materials still present 8. Keep the content for about 30 minutes in the tank 9. Let the content to flow into the digester by opening the inlet 10. Push/poke with a rod through the inlet and outlet 11. Continue addition of waste thereafter following steps 1 to 10 12. In the first week add 10 kg/ litr of waste and 10 litre water 13. In the second week add 20 kg/ litr of waste and 20 litre water 14. In the third week add 40 kg/ litre of waste and 40 litre water 15. In the fourth week add 50 kg/ litre of waste and 50 litre water 16. Clean mixing chamber and gate after every addition of the waste into the digester chamber
4.6 Operation & Maintenance of the plant after commissioning Typical daily operations activities include preparing and adding the daily mixture, daily checkup of the system, and managing the digester effluent. The day-to- day operation of a biogas unit requires high level
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of discipline in routine activities like feeding (including avoiding non-digestible hostile materials), poking, keeping water level in balancing tank, to ensure optimum gas production and long life-span of the biogas unit. Many problems in the performance of biogas plants occur due to user mistakes or operational neglect. Often, these problems can be reduced with a little bit of attention and care. A digester of this type is virtually free from major maintenance and has a life of approximately 25 years. 4.6.1 Daily Feeding Process The digester should be fed at least once per day. 1. Close/ plug the opening of the inlet pipe 2. Add kitchen waste daily as shown in Table 2 3. Chop the waste as fine as possible 4. Remove any unwanted items by segregation 5. Weigh out the required amount of waste and water and add into the inlet tank 6. Mix thoroughly to form a mass of homogenous slurry 7. Remove any non-digestible hostile materials still present 8. Keep the content for about 30 minutes in the tank 9. Let the content to flow into the digester by opening the gate 10. Push/poke with a rod through the inlet and outlet 11. Continue addition of waste thereafter following items 1 to 10 12. Clean mixing chamber and gate after every addition of the waste into the digester chamber Make sure that hostile materials do not enter the inlet or the digester. If any such material is found, it should be removed at the inlet tank. The amounts of substrate fed into the digester may be recorded on the log sheet (Appendix-1) to monitor the performance of the biogas plant. 4.6.2 Importance of Agitation Poking with sticks through the inlet and outlet is recommended. The stick should be strong, long enough but not too heavy. It should have a plate fixed at the end (small enough to fit in the inlet/outlet pipes) to produce a movement of the slurry. Regular poking also ensures that the inlet/outlet pipes do not clog up. A special problem of small scale fixed dome plants is the clogging up of the overflow point. This can lead to slurry flow back into the digester and consequent over-pressure and clogging of the gas outlet. Poking ensures higher gas production. Experience shows that stirring and poking is hardly ever done as frequently as it should be. 4.6.3 Managing digester effluent While adding the daily mixture, a similar amount of digester effluent will flow out of the digester outlet. The effluent is a potent fertilizer, containing all the nutrients present in the original inputs, and can be used in fields and gardens.
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4.6.4 Daily check‐up Each day, the whole biogas system should be checked for irregularities (tears or cracks in dome or hoses, tightness of hose clamps). The water level inside the pressure relief system should be checked (hose to be emerged in 15 cm water). The pressure should be checked regularly. Check the level of water in the condense trap, and empty the bottle if necessary. Clean the system regularly. 4.6.5 Weekly / monthly checks •
The leakage of the dome in the plant should be checked and fixed up weakly
•
Flexible pipes above ground should be checked for leakage weakly
•
Clean the balancing tank monthly
•
Every three months, all of the connections in the system should be checked for gas tightness. Use a spray‐bottle with soap and water to spray all connections.
4.6.6 Annual maintenance work •
Remove the slurry (about 60% ) from the digester and add water to make up the level and then start operation of the plant as referred in 4.2.1 Stage 2 - Adding of Organic Waste
•
The whole plant and digester should be exposed to a pressure test once a year to detect lesser leakages
•
Check the plant in respect of corrosion and, if necessary, renew protective coating material
5. General Maintenance 5.1 Mixing chamber maintenance All non-organic materials must be removed from the waste when mixed with water, before releasing it into the mixing chamber. Non-organic materials will shallow the level of the digester chamber and cause blockage in pipes. This will include 1. All kinds of plastics and metals 2. Aluminum foils 3. Egg shells 4. Onion peels (floats on the top of the digester being light weight and will block the gas pipe) 5.2 Gas Pipe Maintenance •
Gas pipe should be installed against the wall or post and secured tightly. The pipe will be broken easily if installed independently or loosely
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Gas pipe should be covered safely to prevent the damage caused by animals, humans or vehicles
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Rubber hose should be checked every month for any water condensed inside. Valve should be closed and hose emptied if there is water trapped inside. Hose clamp must be tightened after replacing the hose
5.3 Pressure Gauge A pressure gauge is an important instrument in biogas system. It indicates any malfunction in biogas system •
Check for the pressure normally it should be 30-40 psi
•
Check for any rust on the gas pipe
5.4 Biogas Equipment Maintenance •
Valve must be closed for safety before cleaning any gas equipment
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It will be necessary to use special burners for cooking and heating. It is best to use a burner with an adjustable air inlet control. The addition or subtraction of air to the gas creates a hotter flame with better use of available gas.
5.4.1 Burner Always clean the burner by removing the head of the burner and pushing through the holes with a sharp object like a iron needle, wire or nail so that gas will flow out easily. Use a wire brush to get rid of sediment. Later scrub the rust or dirt out. Flame should come out from each hole of the burner head evenly, pale blue flame indicates clean burner but red flame indicates presence of sediment. After using stove, the gas valve must be closed first and then the stove must be turned off. If only valve is closed it may cause rust on the switch. 6. Caution in Biogas Use In a new chamber, gas produced after animal excrement has been first added must be vented as it cannot be used. It should be vented 2-3 times or until gas becomes flammable. Use pressure gauge to check the pressure. Pressure should be at 30-40 psi • Light should be lit closely at the head of burner before opening the valve. If the valve is opened first, gas may come out exceedingly and is dangerous. • Do not use igniter to lighten the stove because biogas is a slowly flammable passive gas. • When valve is left open, excess gas will result in a bad smell. Close valve immediately and open windows and doors for ventilation. Do not light the burner until there is no more gas leaking.
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• When operating a biogas plant special attention has to be paid to the following dangers: • Breathing in biogas in a high concentration and over longer periods of time can cause poisoning and death from suffocation. The hydrogen sulfide contents of biogas are highly poisonous. Therefore, all areas with biogas operating appliances should be well ventilated. Gas pipes and fittings should be checked regularly for their gas-tightness and be protected from damage. • After emptying biogas plants for repair, they have to be sufficiently ventilated before being entered. Here the danger of fire and explosion is very big (gas/air mixture!). • The so-called chicken test (a chicken in a basket enters the plant before the person) guarantees sufficient ventilation. • Biogas in form of a gas-air mixture with a share of 5 to 12 % biogas and a source of ignition of 600°C or more can easily explode. Smoking and open fire must therefore be prohibited in and around the biogas plant. 7. Gas Saving Tips •
Food ingredients and condiments must be prepared before starting stove.
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Cooking container should be placed 1 inch above the head of the stove to save gas and avoid
loss of heat energy while cooking. •
Lid should be used regularly to cover cooking container when boiling or steaming to save energy
and time for cooking 8. Certain Problems and Solutions Table 3 Problems and Solutions Control Gas pressure
Indications Gas pressure
Problems Pressure relief valve
Solutions it should be cleaned or renewed
too high
Malfunctions Gas pipe is blocked
Unclog the pipe
Outlet of balancing tank is clogged Leakage in gas conducting
Use soap solution test to check for
too low
Parts
leakage of valves and joints, pipes
Gas production
Change of pH value Leakage in digester or piping system
Gas
Gas production
clearly
pressure
under
normal levels
Blocked gas pipes due to water or
Enough pressure
Strong
alien elements Plant is overloaded or
but gas
odor
sludge
and hoses and seal the part Identify the problem and
act
accordingly
Stop addition of waste temporarily
fermenting conditions are suboptimal
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have bad smell
pH factor is too low
and is
Correct pH-value by adding sodium bicarbonate (Na2CO3)
nonflammable
9.
Monitoring
Monitoring subsumes all activities of data collection regarding an individual biogas unit or biogas programs. Collecting data on the performance of biogas units is necessary to detect problems in the unit’s performance. Data necessary for the optimization of the performance of the existing biogas unit should be recorded (See appendix-1, appendix-2 and appendix-3). The records should include the following data: •
Amount and type of waste, including the amount of mixing water
•
Amount of gas production
•
pH of the digester
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Records on breakdowns and their causes – by means of previously recorded breakdowns it is easier to compare the breakdowns and detect the reasons for failure.
10. Repair The most frequently occurring disturbance which require repair is insufficient gas production which can have a variety of different reasons. Repair measures are being taken in case of acute disturbances or during routine maintenance work. Repair measures which go beyond routine maintenance work have to be carried out by specialists, since the biogas plant owner in most cases does not have the required tools and the necessary technical know-how. In any case, annual maintenance service should be carried out by a skilled biogas technician.
Operators Operators of of aa biogas biogas plant plant must must keep keep in in mind mind that that this this is is aa living living system. system. Introduction Introduction of of any any toxic toxic or or bactericidal bactericidal material material into into the the biogas biogas plant plant would would kill kill the the methanogens methanogens and and the the system system would would stop stop producing producing methane. methane. Overfeeding Overfeeding should should also also be be avoided. avoided. A A biogas biogas plant plant harbours harbours several several other other micro-organisms micro-organisms besides besides the the methanogens. methanogens. If If the the biogas biogas plant plant receives receives more more feed feed than than the the digestive digestive capacity capacity of of the the methanogens, methanogens, the the undigested undigested food food serves serves the the non-methanogenic non-methanogenic organisms organisms as as nutrition, nutrition, causing causing the the latter latter to to increase increase their their numbers. numbers. Increase Increase in in the the population population density density of of nonmethanogenic nonmethanogenic organisms organisms causes causes reduction reduction in in that that of of the the methanogens, methanogens, inhibiting inhibiting the the formation formation of of methane. methane.
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