This paper aims to expand the knowledge about methods for improving the rate of solids hydrolysis, volatile acid formation and methane gas production. It studies the effects of phase separation and operational modes on the performance of the process. Two systems were investigated. The two-phase system consisted of a CSTR acid-phase digester and an upflow methane-phase digester, the one-phase system comprised of an upflow anaerobic digester. In order to provide optimal contact conditions and recycle liberated indigenous enzymes or cell biomass fractions, continuous recycling of the effluent sludges to the sludge bed of the reactor was used.

In general, the nonbiodegradable portion of the substrates contained in the mixed sludges was found to be about 45% on the basis of TVS. The TVS reduction of the two-phase system was 53%, this is higher than in the one-phase system. Methane production rate and methane content were 0.351 l/g TVS added and 66.6%, respectively.