Anaerobic digestion of agricultural wastes and weeds for biogas production.
An experiment was conducted to study the biogas production capacity of some agricultural wastes and common weeds of upland soils of north India. The following substrates were placed in separate bottles with top outlet: T1, banana stem; T2, maize stover; T3, rice straw; T4, wheat straw; T5, sugarcane bagasse; T6, water hyacinth [Eichhornia crassipes]; T7, Cannabis sativa; T8, Croton sparsiflorus; and T9, Parthenium hysterophorus. Approximately 200 ml of screened slurry of an active biogas plant running on cow dung was added to inoculate the bottles. Gas outlets were provided on the bottle tops, and the bottles were sealed and incubated at room temperature (30-35°C) for eight weeks. The loss in carbon and apparent gain in nitrogen lowered the carbon:nitrogen (C:N) ratio at 60 days of anaerobic digestion. The total loss in carbon of substrate after 60 days of digestion varied from 13.1 to 18.7%. The production of carbon dioxide and methane resulted in organic carbon loss. The loss of carbon increased the percentage of nitrogen content. The hydrolysed nitrogen remained either soluble in slurry water or it interacted with the hydrolysed substrates. The decrease in carbon content and increase in nitrogen content of the substrate with the lapse of time decreased the C:N ratio at 60 days of digestion. The highest gas production (118.65 litres/kg total solid (TS)) was recorded in T1. Gas production in other treatments varied from 90.26 to 100.07 litres/kg TS. The optimum C:N ratio was 38.2 to 22.1 throughout the anaerobic digestion.