The economics of the treatment of dairy cow slurry by anaerobic digestion

Abstract

The cost of treating dairy cow slurry by anaerobic digestion on 50, 100 and 200 cow farms is calculated using the value of the energy produced to offset the running costs. The impact of technical changes is evaluated, including: four retention times, pre-treatment separation, reduction in water entering the slurry, use of an effluent-influent heat exchanger, three operating temperatures and improved gas yield. The effects of changes in energy prices and component costs are considered. The results are presented as average annual costs for operating the systems over a ten-year period at given levels of energy utilization and price. The annual cost of anaerobic digestion with present technology ranges from £9/cow to £70/cow making it uneconomic when considered solely as a source of energy. Lower costs may be achieved by combining several technical and operational improvements. The importance of keeping the digester in operation for the full year is shown. The best methods of improving the economics of anaerobic digestion are good waste management to maintain a high slurry dry matter and increasing the gas yield at short retention times.