ADVERTISEMENTS:
Effluent coming out from a biogas plant has high water content. For its more effective utilisation, it is generally desirable that it be dried first. Drying improves its properties and makes the transportation easier. Separated water can be re-used for slurry-preparation which leads to higher gas yield than fresh water. Due to reduction in volume following water-separation, storage needs are also somewhat reduced.
Although there are distinct advantages of water separation from effluent, nevertheless it adds to the costs and poses some practical difficulties. For instance, bringing water to the upper surface of the sludge due to its being widely scattered in all layers poses practical difficulties.
Some of the possible methods of water separation are:
1. Water Absorption by Using Agricultural Crop Residues:
ADVERTISEMENTS:
In this method, agricultural crop residues are spread over digested slurry (sludge) which help to absorb its water content. This resulting waste is somewhat acidic in nature which helps to conserve ammonical nitrogen which during sun drying is substantially lost. This method however has the limitation that separated water is permanently lost which cannot be reused in slurry preparation.
2. Water Separation by Chemical Sedimentation:
This is another practical method but it suffers from added costs following use of several chemicals. For making the process inexpensive, it is necessary that as far as possible locally available chemicals should be used in the sedimentation process.
3. Water Separation by Centrifugation:
Water separation by centrifugation is another effective method but it also suffers from high operation costs. Based on laboratory studies it is possible to separate out water by rotating the slurry at a speed of 3000 rpm. In centrifuge-separation a basket type of bucket is normally employed. Operating difficulties are faced at times when centrifuge gets clogged with the thick rotating effluent.
G.L. Patankar carried out several experiments at the Gobar Gas Development Centre, Borivli, and Mumbai in centrifuge-separation. For this purpose 100 g of effluent was made to rotate centrifugally for 10 minutes which separated 52 g of water leaving 48 g of dry sludge.
Separated water consisted of 0.8973 per cent total solids, 0.0367 per cent total nitrogen and 0.0005 per cent ammonical nitrogen. Dried sludge contained 13.92 per cent total solids, 0.357 per cent total nitrogen and 0.011 per cent ammonical nitrogen. As for the bacterial count, nutrient and thioglycollic agar were counted as 1.9 x 208 and 109 in separated water and dry sludge, respectively.
ADVERTISEMENTS:
4. Water Separation by Filtration:
In sewage purification plants, filter beds are mostly used for separating water and solids in effluents. The process originally developed for sewage plants can also be effectively used for separating water and total solids from biogas plant sludge.
However, use of filter beds makes the process expensive which is one of its limitations. The process also requires large space. In place of filter beds, it is possible to use GI mesh for filtration. While using wire mesh it is necessary that it be kept cleared of accumulated solids possibly with the help of a stirrer or rubber wiper.
G.L. Patankar carried out several experiments for separating out water from effluent by filtration process. The filtration was carried out both without and with agitation of effluent. In the first case, the effluent was allowed to filter through 1 sq. ft. nylon mesh. Two kg of effluent was made to filter for a period of 24 hours which separated 0.75 kg of water leaving 1.24 kg of dry sludge. The separated water contained 1.2 per cent total solids, 0.031 per cent total nitrogen, and 0.0078 percent ammonical nitrogen. Separated sludge contained 11.25 per cent total solids, 0.32 per cent total nitrogen and 0.013 per cent ammonical nitrogen.
ADVERTISEMENTS:
In second case, i.e., with agitation 2 kg of effluent was made to filter through the same mesh and for the same period which separated 1.047 kg of water leaving 0.953 kg as dry sludge. Separated water contained 2.36 per cent total solids, 0.097 per cent nitrogen and 0.011 per cent ammonical nitrogen. Separated sludge consisted of 11.85 per cent total solids, 0.30 per cent total nitrogen and 0.0083 per cent ammonical nitrogen.
From these experiments several conclusions can be drawn. Due to presence of anaerobic bacteria in separated water, its re-use in slurry preparation can lead to higher gas yield. However further experiments need be carried out for determining yield in precise terms under varying purity levels of separated water.
Efficiencies of water separation by filtration with agitation and by centrifugation were found to be sufficiently close. In case of filtration with agitation, amount of total solids in separated water was found to be 2.5 times more than when it was done by centrifugation.
In India many simple methods have been tried to remove water such as those involving use of grass beds, pebble beds and sand beds. However they get choked after a few days resulting in complete loss of slurry water rich in nitrogen and other nutrients. R.K. Malik described the use of a simple basket of about 5 ft dia made of mulberry plant sticks to separate water from the plant effluent. These baskets require no special techniques for their production and are already made by local craftsmen in villages which are used for handling dung manure and fodder.
Biogas plant slurry is a rich manure which can be effectively used as a partial substitute for chemical fertiliser for increasing soil productivity by following proper techniques for its application. From a small biogas plant of capacity range 5-8 m3, following high slurry discharge rate of 80-120 litres/day and with high water content of the order 93-94 per cent, filtration of plant slurry for dewatering poses practical difficulties particularly if the plant is located in low temperature zones or hilly areas.
It is neither possible to store the slurry in liquid form in large ‘kacha’ pits for months till it is applied in agricultural fields nor it can be easily dried due to low ambient temperatures. Solution to this problem lies in using a properly designed dewatering pit. Dewatering pit not only helps in speedily preparing biogas plant manure but also ensures availability of separated water for reuse in slurry preparation which is rich in anaerobic flora and thus conducive for added biogas yield.
A.K. Kalia and S.S. Kanwar developed the design of biogas plant slurry dewatering pit which can be easily constructed in rural areas. They carried out series of experiments to analyse the impact of different filtration materials on dewatering efficiency. They tested various filters made of twigs and plant matter.
The volatile solid content of the slurry could be increased after 5th day to 11.5 per cent in the drying bed using a filter made of willow twigs. Depending upon the capacity of biogas plant, two to three dewatering filtration pits can be constructed one after another to enable continuous dewatering of plant effluent.