10. Use of slurry
General Properties of Digested Organic Matter
Digested substrate has almost no smell and is more liquid than undigested dung. These facts are the result of the same process which leads to the production of biogas, the transformation of long carbon chains (cellulose, alcohol and organic acids) into short carbon molecules such as CH4 and CO.. As part of the total carbon content of the substrate is transferred into biogas, the carbon/nitrogen ratio becomes more narrow. Only if the C/N ratio of a manure is narrower than that of the soil, one may talk of nitrogen supply by the manure.
Nitrogen is a major nutrient required for the plant growth. Nitrogen from organic manure has to be extracted by bacteria from large organic molecules and transformed into smaller inorganic water soluble compounds before plants can use it. This transformation is called mineralisation. During the digestion process in a biogas plant, part of the organic nitrogen is mineralized to ammonium (NH.+) and nitrate (NO3-) and thus, may be taken up by the plants immediately. The short term fertilizer value of the dung is doubled while the long term fertilizing effects are cut by half. Under tropical conditions the short term value is of greater importance because rapid biological activities degrade even the slow degrading manure fraction in relatively short time.
If ammonia is not dissolved in water it may escape as gas into the air. Therefore, digested slurry has to be kept moist or covered by soil to preserve its fertilizer value. The best way is to bring it immediately in liquid form to the roots of the plants. An other possibility is to compost the slurry together with other organic material. During composting ammonia is bound again in organic form by bacteria and does not evaporate.
Many chemical processes take place at the same time which need different attention. But in general, two rules must be followed for preserving the plant nutrients of both the undigested dung and the digested slurry:
-avoid long storage times and
-keep manure moist, cool and covered.
To avoid long storage times it is better to clean the stable twice a day instead of every second day. It is also better to use the slurry directly on permanent crops like fodder grass, trees or vegetables than on annual crops like maize or millet.
The Slurry Disposal
Clever and realistic planning of the site of slurry utilization is the key to an economical biogas unit. Insufficient slurry disposal leads to blockage of the outlet and rising gas pressure Inside the fixed dome plant. The volume-of digested slurry is about twice as much as that of fresh dung. Slurry manure must reach the crops without loosing too much of its fertilizer value. Whenever possible, slurry should be distributed directly to the crop by gravity. The best way Is using irrigation channels for slurry distribution. To prevent loss of nitrogen, the manure pits and slurry distribution channels should be covered or placed under shade of plants. A compromise must be found between shading the manure and not disturbing the flow of the slurry by roots growing into the canal. Shading by fodder grass Itself might be less troublesome than by trees.
If fertilizing follows the cropping pattern, space for a liquid storage tank of sufficient size must be provided beside the point of overflow to bridge the time of no fertilizer use. Similar space would be needed if composting of slurry is envisaged.
The point of overflow can be extended nearer to the field to allow sufficient slope when the expansion chamber is shaped like a canal. There is also the possibility of arranging the compost heap parallel to this expansion canal. Slurry might then be taken out at convenient spots for pouring over the compost.
Use of Liquid Slurry
It is more Important to use the slurry instead of propagating complicated and labour intensive systems of optimum manure utilization. Any use of slurry is a good use. Therefore, whenever possible, slurry should be used in liquid form immediately after leaving the overflow of the biogas plant. A minimum slope of 2,5% is required for short distance distribution. Slope is to be increased for longer distances and in dry areas. Distribution of liquid slurry needs management Uncontrolled distribution may create swamps or thick layers of dried slurry sealing off the roots of crops or trees from necessary oxygen supply.
The most labour saving slurry utilization is for fodder grass. It should be encouraged when controlled fertilizing is unlikely. The fodder is planted near the stable where it is used. The gasplant is near to keep slurry distribution channels short.
Distribution channels need a minimum slope of 2,5% (1), in dry areas 5X may be required. The slurry is dumped amongst the fodder grass when cleaning the channels (2). Slurry flows mainly to recently harvested areas (3). Slurry is spread on vegetable filelds higher than the outlet and near the plant by buckets (4), The stable in the background fulfils the minimum requirements.
Fodder grass should be cut when it is only 80 cm high. Slurry is led always to the freshly cut area. Per cow 500 m² of fodder grass are a guiding figure. Fodder grass is a permanent crop which makes the installation of a permanent distribution system advisable. The main distributor could be constructed even in concrete or laid out with concrete slabs. Branch canals every 2 m would allow equal fertilizer supply. Covered channels would be the best.
Where gravity distribution is not possible, liquid slurry must be carried to the plants by buckets or on specially adapted wheelbarrows. Wheelbarrow transport needs passable pathways.
Use of Slurry for Compost
The preparation of compost is best if distribution by gravity is not possible. Investment and labour input are reasonable and the nutrition value of the manure is preserved. Composting is a form of storing the slurry over some time without loosing too much nitrogen. Compost is also a method of increasing the amount of organic manure which stabilizes the soil structure. Compost is superior to liquid slurry for long-term improvement of soil fertility. Compost releases its nutrients slowly and therefore, is applied in few but larger doses over the year.
In principal, compost is, prepared in alternate layers of liquid slurry and fibrous agricultural residues. Compost should be made in heaps instead of pits because air is required to promote the rotting process. Compost should be kept in shade and should never dry out. A less optimal but utilized compost is better than propagating the ideal method which will not be applied by the farmer. The compost dam which is regularly poured over with slurry and sometimes turned over, is a reasonable compromise.
When compost is prepared, it is advisable to shape the expansion chamber of the biogas plant like a canal running parallel to the compost hear.
A wooden wheelbarrow ( 1 ) developed by CAMARTEC is modified to serve as a slurry transport cart. It is heavy but stout and has almost no metal parts which could corrode. The lid (2) prevents the slurry from spilling over. At the front it is held by a slot (3) at the longitudinal slat. The slurry is either dumped into a distribution system or taken out from the wheelbarrow by buckets.
Compost consists of slurry and fibrous organic residues, like grass, leaves, and straw. It has a total solids content of 50%. The compost heap must be turned over several times during the 6 months ripening period, The inside temperature of a good compost heap is 60-70°C. The rotting process demands air. Therefore, the compost heap should be narrow and above the ground. The ideal compost heap is roofed and set in alternating layers of residues and slurry ("Indore" method) (1). In connection with an expansion canal of a biogas plant, the compost dam (2) is an appropriate compromise. Dumping 01° residues starts at the far end. The compost dam "grows" to the front and is poured over regularly with slurry (3). After some weeks it is turned over to the side (4).
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