reactorHRT and OLR are important factors that reflect a supply-demandrelationship between microorganisms and the substrate. It shouldbe step-feed to keep balance between VFA accumulation andmethane production, especially for the treatment of wastewatercontaining toxic and refractory compounds. If the HRT is too shortor the OLR is too high, this may cause serious problems of sludgewashout, superacidulation and low COD removal efficiency [24].In this study, HRT shortened stepwise from 45 to 18 h with OLRincreasing from 0.6 to 3.44 kg COD/m3· d in fluctuation. Fig. 2 showsthe process performance of the UASB reactor in terms of COD, pH,ORP, total oil concentration, polar organics, biogas production rateand composition during the experiment.