Co-digestion strategies and pre-treatments to enhance biogas production from animal manure

Abstract

One of the common trends in livestock production in developed countries is to intensify livestock production and increase the size of farms, which is usually accompanied by the production of locally surplus animal manure that may lead to environmental problems. Although biogas production from manure by anaerobic digestion offers a promising option for generating renewable energy, a biogas plant can barely achieve economic sustainability if the feedstock is manure alone. Co-digestion strategies and/or pre-treatments of the feedstock are required to increase the biogas yield. In co-digestion, manures provide a buffering capacity as well as a wide range of nutrients, while the addition of other carbon-rich substrates balances the carbon to nitrogen (C/N) ratio, thereby decreasing the risk of ammonia inhibition. A wide variety of substrates can be used, such as fruit and vegetable waste, food waste, the organic fraction of municipal solid waste, maize, crop residue or raw glycerin from biodiesel production. The addition of small amounts of raw glycerin from biodiesel plants may increase the biogas production of cattle manure to 900 litres per kg of volatile solids. Applying ultrasound as a pre-treatment may produce a two-fold increase in biogas yield. The use of high organic loading rate reactors, such as a induced bed reactors (IBR), afforded much better results than continuously stirred tank reactors (CSTR), obtaining very high organic matter removal (up to 90%) and almost doubling biogas production. These findings suggest the convenience of applying innovative strategies in manure anaerobic digestion facilities to achieve economic and environmental sustainability.