A laboratory investigation of anaerobic sludge digestion for dairy manure

An increasing trend toward total confinement of dairy herds as well as livestock being fed for marketing has resulted in farmers having large volumes of manure produced in small areas. The encroachment of suburbia into farming areas has added new problems to the burden of manure disposal. Efficient, economical methods of stabilizing large volumes of manure and rendering it free from odor while reducing its potential as a pollutant are necessary. Anaerobic sludge digestion is one possible method of stabilizing manure and eliminating odor with a minimum of labor and with possible valuable by products to help offset its cost. The purpose of this study was to design and build laboratory equipment with which to investigate anaerobic sludge digestion of dairy manure, and to investigate the effects of sludge mixing on reduction of total volume, total solids, volatile solids and on gas production. The laboratory equipment was built and was operated continuously for three months. The data collected indicated that continuously mixed digesters were considerably more effective than unmixed or partially mixed digesters. Increasing the speed of mixing had little effect on total or volatile solids content of the settled sludge. Increased mixing speed significantly increased the total and volatile solids content of the supernatant. An optimum mixing speed for maximum gas production was observed, above or below which gas production decreased, The author concluded that anaerobic sludge digesters may play an important role in agricultural waste disposal and that gas mixing should be preferred to mechanical mixing.