5. The agricultural biogas unit
General
Most faults in biogas units are caused by planning mistakes. Siting of the biogas plant and layout of the biogas unit is as Important as the construction Itself. A good biogas plant at the wrong place Is a useless installation. Similarly, filling a plant with unsuitable material will result In an unproductive unit. Careless planning of the site may require unnecessarily additional structures or cause further labour input. Thorough inspection and assessment of site are preconditions for a profitably functioning biogas unit. This is especially true when using standardized structural elements;
Survey of Site
BES staff first check the proposed feed material for its suitability. After observing the overall environment of the farm, the master plan of the biogas unit is made on the spot In cooperation with the persons having decision making power at the farm. The technician must check if the space Is sufficient and must take the levels of the proposed structures. Planning the utilization of slurry is probably the most Important point to be discussed at the first site meeting with the farmer. The cost of the slurry distribution system should be calculated and made known to the farmer before starting the construction.
To allow swift construction work, access for transport and place for storage of material and excavated soil must be clear before starting. The farmer must be informed about providing approx. 500 1 of water per day during the building period for masonry and concrete construction. There must also be agreement as to which building materials are to be provided by the farmer and which quality requirements are to be observed.
The BES staff writes a report about the findings and assessments and gives reasons for decisions made at site. This is required to keep colleagues at the BES headquarter informed. Such records are also helpful In case of customers trying to save money by constantly complaining about the plant's performance.
Tools and equipment
There are three essential tools which are given to the farmer because they are part of the biogas unit:
-The dipper to scoop urine and water from the urine chamber into the mixing chamber and to take out and pour slurry in case of compost preparation. Several designs have been tested. The most durable solution was found to be a dipper made from a Ø 6" plastic pipe and a 1,30 m long wooden handle. The handle passes through both rims of the pipe and is fixed with a nail to the upper rim. Dippers from metal proved to corrode quickly and handles fixed on a shaft broke within a short time.
-The squeegee is used to clean the stable floor with only a little water, pushing the urine into the urine chamber and the solids into the mixing chamber.
-The chopping block is needed to chaff the fodder with a panga (machete). Chaffed grass Is eaten completely by the animals without leaving the stems or allowing them to be tossed out of the trough where they would mix with the dung and might block the biogas plant. The chopping block can be a standing solid log of wood but it is better to use the wood across the fibre to avoid the knife getting stuck. Truck tires have proofed to be an elegant solution Instead of wooden blocks because the knife Jumps up by itself when chaffing the grass. Mechanical chaff-cutters are, of course, an even better solution.
As farmers are not aquatint with this kind of equipment, It is best to provide these in order to stress the importance for adequate operation of the biogas unit.
Fig.12: Necessary equipment to operate a biogas unit
(1) Dipper made of a piece of plastic pipe with a wooden handle coming through. (2) Squeegee made from wooden boards with tyre rubber clamped between and wooden handle coming through both the boards and the rubber lip. (3) Chopping block or (4) used truck tyre for chaffing of fodder grass. The tyre leans against the trough and may be rolled to the place of use.
Principles of Layout
A biogas unit is a considerable investment. It should not be looked at as a temporary structure.
The agricultural biogas plant belongs to the stable. Without any exception. The distance to the kitchen is of secondary importance. With fixed dome plants, there is no practical limitation to the length of gas pipes, except for the cost. As a matter of principle, sustainability has first priority over cost reduction. This means that everything must be arranged in such way that it is less work to feed the plant than not.
On sloping ground, the stable lies higher than the biogas plant. On flat grounds the floor of the stable might be elevated in order to allow dung and urine to enter the plant by gravity. Handling of slurry demands high labour input and can be avoided by proper planning. The outlet of the biogas plant Is directed towards, or drains into, the fields. Overflowing slurry should never be allowed to accumulate on neighbour's or public ground. The biogas unit must be functional even when attendance and maintenance is poor. The owner has the final decision, but he often can not oversee the consequences of a decision. Beware of false compromises!
Fig.13: Position of gasplant to stable floor The ideal situation is a sloping ground, falling from the stable via the gasplant to the crop plantation (1). On horizontal ground (2), it might be necessary to lift the floor of the stable (H).
