Separation of the liquefaction-acidification and gasification phases of anaerobic digestion was studied in a simple, plug-flow reactor (PFR) charged with 3 to 10 wt% solids-content feed slurries. The unmixed PFR had zigzagging parallel channels with floors sloping towards the outlet. Steady-state runs were conducted at hydraulic retention times of 32 to 13 days and organic loading rates (OLRs) of 0.84 to 6.84 kg-VS/m3-d. Phase separation was first observed when the OLR was 2.05 kg-VS/m3-d. Longitudinal pH and volatile fatty acid (VFA) profiles at this OLR showed a lowest pH of 6.10 and a highest VFA accumulation of 1500 mg/l. The acidogenic phase predominated within the first 50 cm from the inlet, and the methanogenic phase prevailed in the remainder of the PFR. At an OLR of 6.84 kg-VS/m3-d the pH dropped to 4.4, and the VFA accumulated to 9600 mg/l with the acidogenic phase extending to 83 cm from the PFR inlet. Unlike a CSTR digester, the PFR always recovered from the initial sour digestion and afforded high VS reductions of 78 to 63% corresponding to methane yields of 0.36 to 0.28 m3/kg-VS added.

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