High rate anaerobic digestion technologyWhen compared to cattle dung poultry litter has a high nitrogen anf phosphorous and hence offers an immence potential for biomethanation in aneorobic digesters with energy recovery in the form of gas besides organic manure. For the gas production conventional digesters are used for biomethanation of organic wastes including cattle dung. For this purpose many types of digesters have been developed. The high rate reactor is one of them. The knowledge base of high rate anaerobic digesters is rather limited in most of the developing countries except for few sewage treatment plants, where commercial in-house or open sale utilization of biogas was practiced. The American Society of Civil Engineers classify the digesters as low rate, high rate, anaerobic contact and phase separation
variations. A low rate digester operates with a very low volatile solids (VS) loading rate of 0.6 to 1.6 kg / m3 / day, a feed solids concentration of about 2- 5%, and high hydraulic residence time of 30 to 60 days. Being open intermittent feed, unless environmental conditions are controlled, these systems are unstable. High Rate Digestion operates at higher feed solids concentration, in general has supplemental heating for either the mesophilic (30 - 38° C) or thermophilic (50- 60° C) ranges and uniform feeding rates. As it is operated at high solids concentration, it would result in reduced tank volumes. The anaerobic contact process involves thickening of digested sludge and recycling to the inlet with a similarity to
contact stabilization in activated sludge. This is mainly meant for high strength soluble wastes only as it has hydrolysis and acid methane formation phase separated. Though a sound process theoretically, this has practical difficulties in keeping the phase separation in the two separate tanks and often requires process readjustments. In general, a vast majority of successful digesters are of the high rate mesophilic type, but the problems arises from either equipment plugging. Line problem incidental to each type accounted for almost 65- 85% of the problems encountered with such systems. Current technologies of digestion The information made available from various digester manufacturers reveals that various technology providers claim differing energy recovery potentials for the same feed and operating temperatures. Theoretically, given a unit weight of feed, and all other conditions remaining unchanged, the conversion to energy and new cells is a fixed percentage and cannot be influenced unduly by any single factor. However, the one variable, which may have some
influence, is mixing, which would either result in a complex mix or stratified regime or subsequently result in higher or lower gas yields.
variations. A low rate digester operates with a very low volatile solids (VS) loading rate of 0.6 to 1.6 kg / m3 / day, a feed solids concentration of about 2- 5%, and high hydraulic residence time of 30 to 60 days. Being open intermittent feed, unless environmental conditions are controlled, these systems are unstable. High Rate Digestion operates at higher feed solids concentration, in general has supplemental heating for either the mesophilic (30 - 38° C) or thermophilic (50- 60° C) ranges and uniform feeding rates. As it is operated at high solids concentration, it would result in reduced tank volumes. The anaerobic contact process involves thickening of digested sludge and recycling to the inlet with a similarity to
contact stabilization in activated sludge. This is mainly meant for high strength soluble wastes only as it has hydrolysis and acid methane formation phase separated. Though a sound process theoretically, this has practical difficulties in keeping the phase separation in the two separate tanks and often requires process readjustments. In general, a vast majority of successful digesters are of the high rate mesophilic type, but the problems arises from either equipment plugging. Line problem incidental to each type accounted for almost 65- 85% of the problems encountered with such systems. Current technologies of digestion The information made available from various digester manufacturers reveals that various technology providers claim differing energy recovery potentials for the same feed and operating temperatures. Theoretically, given a unit weight of feed, and all other conditions remaining unchanged, the conversion to energy and new cells is a fixed percentage and cannot be influenced unduly by any single factor. However, the one variable, which may have some
influence, is mixing, which would either result in a complex mix or stratified regime or subsequently result in higher or lower gas yields.
Underground masonry structure type poultry litter waste biomethanation plant
A typical poultry litter waste biomethanation plant of high rate reactor system which can successfully produce gas is described below.
i) Reactor: A drum type. partially underground masonry structure can be constructed having approximately 5.5 m diameter and 10 m depth. The bottom of the digester is truncated to 1 m dia. The drum diameter is nearly 7 m and height 1.5 m with the dome rise of 0.5 m. Recirculation arrangement can be made to draw the slurry from top layer as well as middle layer and distribute from the bottom. The reactor can also be installed with immobilization arrangements for microbes. The floating drum can be allowed to travel with in a limited height by suitable locking arrangements. The drum can have a water seal at its bottom and fitted with a gas pipe of 75 mm diameter at the top.
ii) Water remover: A cylindrical drum fitted with overflow arrangement can be used as water remover from the moisture laden gas. This can be fitted at the immediate outlet of the gas line. An overflowing arrangement can be made to continuously drain out water during operation. A removable lid can be provided for easy cleaning and refitting.
iii) Water shower: The water can be arranged to be sprayed to remove hydrogen sulphide and CO2 to some extent if present in the gas using a cylindrical drum with counter current flow of gas and water shower.
iii) Water shower: The water can be arranged to be sprayed to remove hydrogen sulphide and CO2 to some extent if present in the gas using a cylindrical drum with counter current flow of gas and water shower.
iv) Activated carbon bed: To remove carbon dioxide from the biogas produced from the gas plant so as to make it rich in methane for operating the engine, an arrangement fitted with activated carbon bed to three fourth of its height in a cylindrical container made of iron can be made.
v) Filter bed for moisture filtering and gas storage: In order to remove gas-laden moisture these two to three beds can be provided as the last components of gas cleaning chain before admitting the gas to compressor. A single or two-stage compressor can be fitted in the line to suck and compress the gas for admitting to a cylindrical storage drum. The outlet of the compressor may be fitted with a non-return valve. The storage cylinder should be capable of withstanding at least 100-psi pressure, also fitted with a non-return valve. The gas stored in the cylinder can used as fuel for running the generator sets to produce electric power.
Operation of the plant: If the poultry shed is a layered type arrangement the poultry litter is not mixed with other bed materials. The feedstock is collected by a tractor and transported to the biomethanation site. The feedstock is unloaded into a shredder where it can be reduced in size and conveyed in to a tank where water is mixed with the shredded feedstock in the appropriate ratio and delivered to the reactor. The gas produced from the reactor is stored in a cylindrical container and used to operate generator sets. Part of the power produced is used to run the shredder, compressor, recirculation pump and other accessories.
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