There are, however, the following disadvantages: (i) Total gas available is less (ii) Retention time is more. Preliminary batch digestion experiment was not successful in reducing BOD.
Sen and Bhaskar experienced that in seeded undiluted waste, the volatile acid rose aiming to a higher value of the order of 1000 mg/l and process failed, indicating the necessity for dilution, different dilutions and different detention time with sludge as a seed did not show any significant improvement increasing the temperature, i.e., from mesophilic (10- 40°C) to rang.
The digested cow dung sludge was observed sludge for distillery waste, (iii) In case of continuous digestion, the digester is charged once or twice and the digested material is removed. The volume remains constant.
The advantages are constant rate of gas efficient -decomposition and less retention time. But material solidifies and floats to form a crust on the liquid system, laid a special device are necessary to break the crust formed.
Parameters for Maximization Production:
The rate of methane production depends upon the presence of methanogenic bacteria. The methanogenic bacteria need the following requirements to survive:
Temperature A majority of bacteria proliferates in the mesophilic range to the carbon dioxide absorber by the heating unit. The carbon dioxide reservoir is a simple arrangement for the collection of carbon dioxide, produced by the digester, and separated by the carbon dioxide absorber and the heating unit, with an inlet from the heating, and an outlet to the pump, connected to the switch.
The digester is first seeded with a 20% solution of fresh or semi digested cow dung in tap water at 37 + 1°C temperature. The diluted spent-wash is, then, added to acclimatize the seed and some amount is withdrawn to check the pH, BOD, COD and solids.
The pH is maintained between 6.5 and 8 by the addition of lime. The concentration of spent-wash is slowly increased. The gas produced, containing methane and carbon dioxide 40% is sent into the carbon dioxide absorber where the carbon dioxide produced is absorbed in calcium carbonate forming calcium bicarbonate, which decomposes into carbon dioxide and calcium carbonate in the copper tube.
Calcium carbonate is returned to the carbon dioxide absorber after being condensed in the condenser. The carbon dioxide is returned to the carbon dioxide reservoir which uplifts the movable drum and connects the system with the pump to complete this circuit.
In the absence of carbon dioxide, the drum of the carbon dioxide reservoir goes down and the circuit to the pump is disconnected. Thus, the continuous separation and recirculation of carbon dioxide remains automatic.
Concentration-incineration as a pollution control measure is receiving more attention, especially because it aims at zero pollution. In distillery effluents, the solid content is about 10%. This effluent is concentrated in evaporators, at different stages, to about 60% solid content.
This concentrated material is burnt in the boiler. Even though the basic principle is very simple and obvious, the equipment required, for concentration in the evaporators as well as combustion, is fairly expensive because of the material used in their construction.
However, sufficient amounts of energy, steam and electricity are produced to meet the requirements of the distillery, the water coming out from the condenser can be reused in the fermenters and, thus, reduce the total water requirement; and potash can be used as fertilizer.
As can be seen from this approximately 4.5 crores will be required to increase concentration-incineration unit for 60 per day capacity distillery. This is much more than the amount required constructing a new distillery. For traditional batch fermentation of this capacity, the total capital requirement will be about Rs. 2 crores, and even a continuous process with all the facilities for process control will not cost more than Rs. 2.4 crores.