Operation and maintenance
Operation of the Biogas Plant
If operational short-comings are often reported, the set up of the system is not appropriate to the farmer. The main task of a biogas engineer is to design and construct a user-friendly biogas unit. Operation and necessary maintenance must be logical to the user and should not be a burden to the ones attending the plant. A well designed biogas unit is easy to maintain. The ease of maintenance ensures constant attention by the farmer. Nevertheless, even with a perfect design, a minimum of daily care is needed to receive a proper service from the unit.
The clay sealing of the lid must stay moist. Therefore, the lid must be covered with water all the time. In order to reduce evaporation and prevent mosquito breeding, machine oil can be added on the surface of the water. Mineral oil pollutes ground water and should only be used in small quantities and with care. From time to time the water above the lid must be checked and refilled when necessary. A small control opening in the topmost cover makes control much easier and therefore more likely. When water is controlled, possible leakages at the lid are also detected.
Once in a while the expansion chamber should be cleaned In order to avoid solids assembling in the corners and thus, reducing the gas storage capacity.
The plant must be fed regularly in order to achieve regular gas production. The substrate should be free of stalks and other impurities in order to avoid scum formation and blockage of the inlet and outlet pipes. After removing straw and waste fodder from the dung, it should be mixed sufficiently with urine or water to avoid separation of solid and liquid material inside the digester. Every day the liquid from the urine chamber must be transferred into the mixing chamber. How much liquid, i.e. how often the urine chamber needs to be emptied per day, depends on the amount of dung. As a rule of thumb 1 kg of dung requires 1 l of liquid. The user is to be advised by the BES-staff.
Chopping of the fodder into pieces of 3-5 cm length saves fodder grass and reduces the amount of stalk mixing with the dung on the floor.
In dry environments the amount of urine might not be enough to obtain the required mixing rate. Water must then be added. However, It is better to give more water to the cows instead of adding water to the plant. Beside producing more urine the cows will be in better health and produce more milk.
The overflowing slurry should move away from the outlet. Otherwise It can block the overflow and the gas pressure might increase until it escapes through the inlet pipe or blows off the water trap. Therefore, the outlet and the slurry canal must be cleaned. This must be part of the daily routine of cleaning the stable and feeding the plant. The problem becomes less, if a proper slope is maintained and the slurry canal is shaded off from direct sunshine.
The slurry distribution system must be cleaned and slurry directed towards the plants for fertilization. If this is not done, the biogas plant does not suffer, but the farmer will waste valuable manure.
Fig.40: Cleaning the overflow point at the expansion chamber
Maintenance of Toilets
Well designed toilets do not require any maintenance except for cleaning the inlet and the floor of the cubical. Once a toilet is soiled and starts smelling badly, It is very difficult to again achieve cleanliness. Clean water should be used without disinfectants so as not to kill the methane bacteria in the plant. Soap water, from time to time, can be tolerated.
Regular Maintenance of Appliances
Psychologically, the stove should be regarded as a kitchen ware and not as a fire place. There is no maintenance needed besides keeping it clean like other kitchen vessels and utensils.
The lamp needs cleaning of the glass screen in order to have bright light all the time. Cleaning should be done only if necessary to avoid shocks on the lamp that can destroy the gas mantle. Gas mantles of lamps only have a certain life time and ought to be replaced frequently. They are fixed with a string to the nozzle and the replacement is easy and does not require any skill. Used mantles are radioactive. Therefore, dust or pieces of the broken mantle should not come in contact with foodstuff. Children should be protected from inhaling the dust when they are around. Hands and working place should be cleaned with water after replacement of the mantle.
Disturbance of the System
Trouble shooting becomes necessary if the customer, this is the owner or the user of the gasplant, complains about insufficient service or any nuisance caused by the plant. There are three sources for possible complains:
-Insufficient gasplant performance
-inadequate amount or kind of feeding material
-too high expectations on the service of a biogas unit
The latter is very difficult to deal with, because the fault was done when persuading the farmer with wrong promises. Serious information about possibilities and limitations of a biogas plant are the only way to have content customers. On the other hand, the farmer might have exaggerated the amount of feed material available to him. This can be the case when the animals are taken out for grazing when zero-grazing was expected. The actual amount of dung must be checked in order to distinguish between short-comings in gasplant performance or underfeeding of the gasplant. A well functioning plant produces between 35 to 40 litres of biogas per kg of fresh cattle dung, depending on fodder, temperature and retention time. In rainy or colder seasons the gas production may drop to 60-70% of the normal rate.
Interruption of Gas Production
"There is not enough gas" is by far the most common problem mentioned by the user. When the cause is found, normally the remedy is easy, except for scum problems. The following steps will help to find the reason for the gas shortage quickly:
Ask the user if gas supply is only less or if it stopped completely. Check the information given by the user concerning the appliances complained about. Ask if the problem occurred suddenly or gradual and when it was noticed first. If the problem occurred suddenly, a technical fault is very likely. If gas supply dropped gradually, one may guess that there is something wrong with the performance of the plant. This might be caused either by unsuitable properties of the feeding material or by inadequate feeding practices.
Check If there is gas in the plant. If the slurry level In the outlet chamber is high and slurry at the overflow is fresh, there is gas production. If the gas pressure is high but no gas reaches the point of use, there must be a blockage somewhere.
If there was no discharge of slurry because of not enough pressure inside the plant, there might be a leakage. Ask or observe if there is smell of gas in the kitchen. Check the lid for bubbles. Check the valves and than the Joints for leakages by applying soaped water to it. If no leakage is found, close the main valve and wait one day for gas pressure building up. If gas is produced, which can be seen if bubbles come up at the outlet or inlet pipe, but pressure does not build up, there must be either a leakage which opens up by increased pressure or a crack in the dome below a certain slurry level. A crack in the dome is the worst of all cases. The plant must be emptied and cracks must be repaired.
If there is no gas production at the plant, observe the smell of the slurry. If it smells sour, the fermentation process has been disturbed. Wait some time (maximum 4 weeks) without feeding the plant or feed with material from an other stable. If gas production does not start by itself again, the plant must be emptied and refilled with fresh material. Such a break down of fermentation is very rare and rarely happens with cattle dung, except in case of animal diseases treated with high doses of antibiotics. In most of the cases, gas is produced but can not arrive at the place of consumption.
If gas is produced but not available at only some of the stoves, lamps or other appliances, there is a blockage in the piping system or the Jet. If Jets are clean, there might be a water blockage in the piping system just before the appliances. Ask the customer, if gas was flickering before it finally went off. In this case, check If there is a water trap at the lowest point of the piping system. If not, change the pipe line or place a water trap. If there is once water in the pipe, there will be always water in the pipe. Reconstruction is the only solution.
The Problem of Scum
If there is heavy gas release from the inlet but not enough gas available for use, scum is most likely the reason. Often the gas pressure does not build up because of the continuous release through the inlet. Slurry does not overflow for weeks. There is the danger of blocking the gas pipe by rising scum because of daily feeding without equivalent discharge. The lid must be opened and scum is to be taken out by hand.
Fig.41: Scum formation in a fixed dome plant
The scum may prevent the gas from reaching the gas outlet pipe (1). Instead, the gas will form large bubbles below the scum (2) from where the gas escapes through the inlet pipe (3). If the gas cannot escape, it might also burst the brickwork structure. Therefore, unsuitable dung should not enter the plant. Suitable dung should be filled in fresh. Stalks and other fibrous material should be sorted out and be stored directly on the compost heap. Dried dung should be thoroughly mixed with urine or water before entering the plant.
Straw, grass, stalks and even already dried dung tends to float to the surface. Solid and mineral material tends to sink to the bottom and, in the course of time, may block the outlet pipe or reduce the active digester volume. In proper mixed substrate there is no such separation because of sufficient friction within the paste-like substance.
With pure and fresh cattle dung there is no scum problem. Floating layers will become a problem when husks are part of the fodder. This is often the case in pig breeding. Before installing a gasplant at a piggery, the kind of fodder and consequently the kind of dung, must be checked to ensure If it is suitable for a biogas plant. It might be necessary to grind the fodder into fine powder. The user must be aware of this and the occurring costs before deciding on a biogas unit. The problem is even bigger with poultry droppings. The kind of fodder, the sand the chicken pick up, and the feathers falling to the ground make poultry dung the most difficult substrate. In case of doubt, no gasplant should be build.
Scum can be avoided by stirring, but....
Stirring must be done for 5 minutes every hour, throughout day and night to avoid scum formation. This can only be assured by automatic mechanisms and should not be expected from workers attending the stable. For simple gasplants, stirring is not a viable solution against scum formation.
Scum can be broken by stirrers, but....
Scum is not brittle but very filthy and tough. Scum can become so solid after only a short time, that It needs heavy equipment to break it. It remains at the surface after being broken up. To destroy it by fermentation, it must be kept wet. Either the scum must be watered from the top or pushed down into the liquid. Both operations demand costly apparatus. For simple gasplants, stirring is not a viable solution for breaking the scum.
The only solution for simple biogas plants to avoid scum is by selecting suitable feed material and by sufficient mixing of the dung with liquid before entering the plant.
Trouble with Feeding the Plant
When there is a problem of charging dung and urine to the plant, there is a blockage at the inlet pipe. The problem might be caused by straw or grass and could be solved by thorough poking. If this happens more often, there might as well be scum blocking the inlet pipe from below. By entering a stick or a pipe into the inlet one may find out where and of which nature the blockage will be. Don't be surprised to find stones or other trash in the pipe which had been placed there by playing children. If it feels sandy, there is a heavy accumulation of soil below the inlet pipe. In this case, open the lid and scrap the sand off With a dipper or steel shovel. Only in serious cases the plant needs to be emptied.
Faults at Appliances
It is surprising to see, how much complaints arise because equipment was not kept clean. Often food has dropped into the- burner head but sometimes dust or cinders block the Jets of burners or lamps. Normally, blockage of the jet is removed with help of a fine wire or needle. Only when blockage occurs in short intervals, the jet must be dismantled and cleaned. The rubber tube is to be disconnected and freed from dust by blowing through.
If a lamp starts to loose its brightness, it is very likely that dust particles have blocked the nozzle. Again, the nozzle must be unscrewed and cleaned.
