Evaluation of the performance of a 550 cow plug-flow anaerobic digester under steady-state conditions

The objective of this study was to characterize the performance of a full-scale, mesophilic plugflow anaerobic digester for 550 dairy cattle manure based on the degree of waste stabilization, fecal coliform and Mycobacterium avium paratuberculosis density reductions, and biogas production and utilization. M. avium paratuberculosis is a pathogen responsible for paratuberculosis (Johne's disease) in dairy cattle and other ruminants. Under steady-state conditions, average reductions in total solids, total volatile solids, and chemical oxygen demand were 25.1, 29.7, and 41.9 percent, respectively, with no loss of nitrogen or phosphorus. Reductions in fecal coliform and M. avium paratuberculosis densities wereapproximately 99.9 and 99 percent respectively. Biogas composition averaged 59.1 percent methane at an average production rate of 1,214 m(3) (42,868 ft(3)) per day. This translates to 2.27 m(3) (80 ft(3)) per cow-day and 0.34 m(3) per kg (5.46 ft(3) per lb) of chemical oxygen demand destroyed. The annual income derived from the use of the biogas produced to generate electricity is estimated to be $39,474 per year. Based on this income estimate, the simple payback period for the capital invested is approximately 5.6 years.