Methane Production from a Pilot-Scale Fixed-Film Anaerobic Digester and Plug-Flow Digester Loaded with High-Solids Dairy Manure

Three pilot-scale anaerobic digesters (two of a unique fixed-film design, of which one was fed micronutrients, one not, and an identically sized plug-flow digester, which served as the control) were monitored for methane production as a comparative indication of performance. All of these pilot-scale digesters were loaded daily with high-solids dairy manure, passed through a 1.5" x 3" mesh screen, directly from The Spring Valley Dairy (which uses sand bedding). Testing took place over a two and half month period first at 13 day and then 5 day Hydraulic Retention Time (HRT) loading rates. Neither fixed-film digester experienced any plugging difficulties. Average methane production at 5 day HRT of the fixed-film digester fed micronutrients was greater than the control by 17.65%; and the fixed-film digester not fed micronutrients averaged 14.37% more methane than the control. Average production of methane per volume of digester per day increased from .51 volumes of methane per volume of manure at 13 day HRT to .95 at 5 day HRT, for fixed-film with micronutrients and .46 at 13 day HRT to .91 at 5 day HRT, for the fixedfilm without micronutrients. Informal observation by five individuals confirmed a large decrease in offense odors from effluent at 5 day HRT and 13 day HRT, for all digesters, as compared to five days old untreated manure. Equivalent methane production/vol. manure/day at 5 day HRT was approximately twothirds of the production at 13 day HRT for the fixed-film digesters, which could be sufficient to meet gas needed for electrical production on many farms, since many farm digesters produce an excess of gas. In addition, it was observed that effluent from the digesters (treating less than 1% of the total waste stream) which were emptied into the manure storage pit for the farm, actually activated the pit to produce large amounts of biogas, reduce crusting, solids and for the first time, also largely reduced the odors from the pit stored manure when spread. The fixed-film digester without micronutrients was later drained at the end of the testing period and reloaded to test the recovery time of the fixed-film digester. Restart to previous production levels was accomplished within two days.