Many modern anaerobic digesters in developed countries consist of a digestion process in which solids are reduced to biogas, followed by mechanical separation that removes the majority of the remaining solids from the effluent. Experience has shown that such systems are often plagued with plugging due to excessive solids in the digester influent. Moreover, the mechanical separation equipment is prematurely degraded due to the elevated temperatures and corrosive compounds in the digester effluent. Reversing the order of separation and digestion offers a proven method of eliminating these problems, but at the expense of lower biogas production. The work presented in this paper quantifies this difference in biogas production by comparing the biogas yields of dairy wastewater feedstocks with and without prior mechanical solids separation through a 0.75-mm screen. Laboratory-scale batch digesters were operated up to 40 days at 35–40 °C and monitored for mass of volatile solids consumed and biogas production. Although the initially separated influent contained only half as much volatile solids, the ultimate biogas yield was only 25% less than that obtained with non-separated influent, demonstrating some tradeoff between higher energy production and system reliability.

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