Anaerobic Digestion of Dairy Farm Effluents With Particular Reference to the Microbiological Aspects.

The treatability of dairy cow faeces obtained over a period of 12 mouths from 12 farms using different feed types was investigated. Raeces/water slurries (1:10) were anaerobically digested in the laboratory and examined for changes in dewatering time, suspended solids concentration, permanganate value and pH. The results were used in designing a pilot digester which was constructed at Merrist Wood Agricultural College. Anaerobic digestion was shown to decrease the dawatering time and suspended solids concentrations of slurries and, when combined with a crude oxidation process, a decrease in permanganate value was also obtained. Bacteria isolated from fresh faeces were shown to produce a variety of effects when inoculated into sterilised diluted faecal material and it was concluded that a highly mixed population would be required to produce improvements in the handling properties and pollution load as well as the acid and gas production which result together during the natural anaerobic digestion process. The hypothesis that anaerobic treatment followed by aerobic treatment could be more effective for use with, bovine slurries than the usual aerobic-anaerobic order followed in conventional sewage treatment works was tested in field trial carried out on a Hampshire farm. An excavated pit full of dairy cow effluent with a crust of straw and other floating solids 16 to 36 inches thick acted as an anaerobic digester eliminating the normally prohibitive coat of a manufactured digester. A useful reduction in permanganate value was found after subsequent aeration of the digested material. It was considered that the anaerobic-aerobic treatment system would be generally effective for use with dairy farm effluents provided the individual circumstances were taken into account in devising the form, of the system