BIOGAS FROM CROP WASTES

Nearly all organic substrates have potential of significant energy generation via the process of anaerobic fermentation. There are several factors which must be taken into consideration to operate the digester based on alternate feed materials effectively. The factors which effects the biogas production in cattle dung holds good for this material also. So to make use of alternate material for biogas generation it is essential to control the environmental and operational factors. A wide variety of plant wastes as well as crop residues in the farm, terrestrial and aquatic species have been studied for their potential for biogas generation. The characteristics of the plant wastes and cattle dung are quite different therefore, anaerobic digestion of plant wastes need additional requirements for maintaining environmental and operational parameters. Some of basic requirements for crop wastes for biogas production are summarized as below.
 The C/N ratio of wastes varies widely from waste to waste. Maturity and type of species greatly effect the C/N ratio. It is also reported that fresh crop wastes has low C/N ratio, while after some time it increases. These is a necessity to bring C/N ratio to the optimum level of 30:1.
 Pre-processing of crop wastes is also essential in order to increase it's density and feed required quantity in the digestion chamber, this also accelerates the anaerobic reaction. This process includes chopping, cutting, mixing with other feed, steaming (if material is hard such as wood) for bringing the required C/N ratio and the concentration of solids to 7-9 % and reduce retention time.
 The density of crop wastes is less. Therefore, if it is used as such, it may form scum on the top of slurry in the digesters, thus, inhibiting methane production process.
 Pre-digested crop wastes have low hydraulic retention time, and it settles at the bottom, which require a perfect stirring mechanism, either mechanically or through gas recirculation. The system should be more reliable and effective for anaerobic digestion of the crop wastes.
The paddy straw, obtained as spoiled and waste material during collection, storage and use of paddy can be converted into biogas and valuable manure. It has been observed that wheat straw can yield biogas at a rate of 36 l/kg of dry matters, where as from the paddy straw the gas production is 260 l/kg of dry matter fed. Plant materials generate considerably higher biogas yield per kilogram of total solids than several animal wastes. Vegetable matter from young plants generates more gas than from old plants, and dry vegetable matter generates more gas than green vegetable matter. The studies carried out for anaerobic fermentation of crops and organic wastes at loading concentrations of 3-10 % TS.,biogas yield decreased with increasing loading concentrations. Above 5 % total solids, digestion of crops with high soluble carbohydrate content needs continual addition of alkali to maintain a pH of nearly 7. The composition of biogas varied with the material from 50 % methane from newsprint to 68 % from cattle manure. The balance was carbon dioxide in all cases with less than 0.001 % hydrogen sulphide. The potential of Gliricidia leaves is established as feed for biogas production and the use of digested effluent as a rich fertiliser which is superior to fresh glilricidia leaves. Mirabilis leaves produced nearly 400 l of gas/kg of dry matter which is double the amount produced by cattle dung. Methane content in biogas obtained from Mirabilis leaves was 69 % as against 62 % normally obtained from cattle dung.
The stalks of maize and sweet sorghum collected before and after the juice extraction, dried and finely powdered to pass a 40-60 mesh sieve, mixed with cow dung in the ratio of 1:1 on dry weight basis produced highest amount of biogas.This is obtained from a mixture of cow dung and fresh stalks of sweet sorghum followed by maize.
Rabbit droppings slurry at 9 % TS produced biogas at 0.24 m3/Kg as compared to 0.12 m3 gas/Kg of cow dung at the same concentration. In 6 m3 digester, 5 to 5.5 m3 of biogas was produced continuously after reaching steady state. The methane content of biogas from rabbit droppings was 68 to 70 % compared to 50 to 60 % from cow dung.