Co-Digestion of Agricultural Wastes: An Introduction

Agricultural waste is the most prolific feedstock for anaerobic digestion plants worldwide. A wide range of crop wastes can be used for biogas production, including straw, husk, bagasse, corn cob, wood wastes etc. The key factors driving the anaerobic digestion of agricultural wastes are physico-chemical properties, pretreatment process, C:N ratio, moisture content, and co-digestion potential.

Anaerobic Digestion of Agricultural Wastes

Agriculture residues are made up of cellulose, hemicellulose and lignin. The biodegradation of cellulose and hemicellulose is relatively easy, but anaerobic digestion of lignin is quite difficult due to its complex nature. Pre-treatment of crop wastes is essential to prepare agricultural biomass for the anaerobic digestion process.

The pretreatment crop wastes enhances the biodegradability of lignocellulosic biomass substances by augmenting the contents of carbohydrates leading to increase in biogas production. The main pre-treatment methods include size reduction, electron irradiation, thermal treatment, enzymatic action. The reduction in particle size is helpful in lactic-acid bacteria fermentation, thereby avoiding significant loss of organic matter.

The high C:N ratio is the main problem with anaerobic digestion of agricultural wastes, such as rice husk. To overcome this problem, a substrate having low C:N ration can be mixed with that having a higher C:N ratio in order to create an optimized condition for microbial propagation.

For example, agricultural residues, such as wheat straw, have a high C:N ratio (>100) while cattle dung has a low C:N ratio (< 25). For optimizing the C:N ratio of crop wastes, co-digestion with animal manure, sewage sludge, food waste or poultry litter is the recommended option.

What is Co-Digestion

Co-digestion (or anaerobic co-digestion) is a well-established and well-proven anaerobic digestion technology for simultaneous biodegradation of more than one organic substrates in a single bioreactor. The process efficiency of co-digestion is better than mono-digestion, primarily due to improved digestibility of organic waste.

Co-digestion is an attractive opportunity for farmers to treat their own agricultural waste. Farmers can efficiently manage agricultural biomass, besides generating additional incomes by treating organic waste from other sources and by the sale of heat, electricity and biofertilizer from the biogas plant.

Co-digestion of Crop Wastes and Animal Manure

In the co-digestion of crop wastes and animal manure, animal manure provides buffering capacity besides several important nutrients. On the other hand, the high carbon content of agricultural biomass optimizes the C:N ratio of the substrate, thus reducing the risk of ammonia inhibition in the digester.

In mono-digestion, accumulation of volatile fatty acids (VFAs) may take place leading to methanogenesis inhibition and drop in pH. To offset this inhibition, animal waste is a good co-substrate for agricultural waste on account of its high alkalinity.