...and lo, what is being wasted can be utilised by rural communities for larger benefits
BIOGAS, an asset to rural India for fuel can also provide water and manure from the plant's slurry. The biogas plant utilises animal dung and water to produce high quality gas and slurry, which can serve as an excellent manure. But since this slurry is very dilute (94-95 per cent moisture) its storage, usage and handling becomes difficult. Therefore, the slurry requires to be processed to separate and recover both water and manure from the slurry for easy handling (Changing Villages, Vol 15, No 1-2).
Filter beds were designed by Indian Agricultural Research Institute and the Centre of Science of Villages, Wardha, but this experiment was unsuccessful since it resulted in free flow of slurry rather than separating water from it. This dewatering study was carried out on a gobar bio-gas plant at Anand, Gujarat. Various types of vegetation such as matured rice straw, fresh neem leaves and green leaves in different amounts were tried. These experiments suggested that some critical factors, which need to be identified, were involved in its separation.
It was decided that a filter bed at least four cm thick should be prepared using only succulents, that is, soft leafy water storing plants and small leaves instead of sticks and twigs to make the mixed filter bed into a relatively dense strainer.
Two masonary tanks of 100 50 50 cm with 12 cm wide brick walls and five cm high edge near the floor were constructed adjacent to the slurry outlet. The tank was filled with effluents for 10 days to collect its water. Round gravel stones are also placed, making a 15 cm thick bed. A stout wire netting of 98 48 cm size was fixed on the edge of the tank. Fresh green weeds were then spread on the gravel bed to about 3-4 cm thickness. Slurry was then uniformly spread on the dense bed all over the leaves to preclude puncturing and leaking. Then the slurry was allowed to flow naturally out of the digester to fill on this bed.
The filtered water is collected and placed in a drainage pit. The amount of total solids in the filtered water and the moisture in the semi-solid residue can then be determined in samples dried in oven at 105c. After one tank gets filled with the effluent in five days it is directed to pass into the second tank. Thus water filtration takes about 10 days in each tank. The digester is filled daily with 50 kg dung and 50 litres water in slurry form. The dewatered slurry, manure, can be removed along with the grassy screen and stored in a pit. The manure can also be used directly in the field soil. The highest proportion of water (87.6 per cent) can be recovered from the spent slurry in just five days duration.
The success of this dewatering technique entirely depends on the proper preparation of the grass bed and uniform spreading of slurry on the bed surface for effective filtration. This stone gravel filter bed was suggested by C L Gupta of Pondicherry Ashram and financed as a research project by the government of Gujarat.
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