Techno-Economic Assessment of Anaerobic Co-digestion of Cattle Manure and Wheat Straw (Raw and Pre-treated) at Small to Medium Dairy Cattle Farms

Abstract

Anaerobic digestion of dairy cattle manure is a common practice; however, the low biogas yield of manure can hamper the profitability of anaerobic digestion systems in small to medium dairy cattle farms . To make this technology more attractive to farmers, an increase in biogas yield per cubic meter of reactor could be achieved by co-digesting animal manure with co-substrates such as straw (in its raw form or pre-treated). A Techno-economic assessment of anaerobic co-digestion of cattle manure and raw and briquetted straw has been carried out. The results obtained from the techno-economic assessment showed that for a farm of 250 adult cattle heads the revenues generated in an anaerobic mono-digestion process are not able to offset the initial required investment. However, the co-digestion of manure with raw or briquetted straw showed better economic performance and positive returns (net present values > 0, internal rate of return > 9% and a RETURN of the investment in 11 years) for farmers willing to implement anaerobic digestion. Electricity selling price, and the price of the straw are the key parameters to determine the profitability of the system. Moreover, pre-treatments to increase the straw biogas production have been assessed and evaluated from a technic and economic perspective. Alkali and microwave-alkali straw pre-treatments showed the best results with an increase in biogas production of 156% and 92% compared to raw straw.