Anaerobic digestion of equine waste:

The goals of this project were to determine the methane production potential of horse manure during anaerobic digestion; to examine the effect of softwood chip bedding, pelleted Woody Pet® softwood bedding, and straw on the methane production potential of equine stall waste; and to investigate the feasibility of co-digestion of waste food and equine waste under thermophilic conditions. Initial results suggested that softwood bedding may have inhibited methane production in 15 L semi-continuous digesters. However, further extensive investigation in batch and continuous flow digesters determined that softwood bedding did not inhibit methane production and, on the contrary, contributed to methane production. The methane production potential for horse manure at 35°C averaged 139 ± 65 L/ kg VS (average ± standard deviation) and 29 ± 15 L/ kg wet weight, corresponding to 9.2 ± 4.8 x 105 kJ / metric ton wet weight. The energy production potential of stall waste with softwood chip bedding ranged from 4.0 ± 0.4 x 105 kJ / metric ton wet weight to 6.6 ± 0.8 x 105 kJ / metric ton wet weight, depending upon the relative amount of bedding present. Co-digestion of equine waste and food waste under thermophilic conditions was performed at the 20 L and 6.3 m3 scale. The 20 L thermophilic digesters were fed a variety of food wastes in addition to stall waste containing softwood bedding. The methane production from these digesters was 356 ± 61 L/kg VS-d. The large-scale (6.3 m3) digester was operated in excess of one year primarily on waste food and horse manure (no bedding). The loading rate increased over time to 1.7 kg VS/m3-d. The methane content of the biogas was 55.7 ± 5.2 %. Total ammonia nitrogen approached 5 g/L, suggesting a higher C:N ratio feed stock mixture than that afforded by the waste food and horse manure mixture might be necessary for future applications.M.S.Includes bibliographical references (p. 115-122)by Brian A. Wartel