The objective of this Study was to compare the performance of anaerobic baffled reactor (ABR) treating concentrated rubber latex wastewater under different pH adjustment substances and recycling ratios (R). Two ABRs, one received wastewater pretreated with NaOH and the other with ash, were operated at 35 degrees C under identical HRTs from 10 to 1.25 d. Results show that both ABRs had highest COD and sulfate removal efficiencies at HRT 10 d (averaged 82.71% and 96.16% of ABR-NaOH, and 80.77% and 96.60% of ABR-Ash, respectively), where majority of the influent COD and sulfate were removed by the first compartment of the ABR at all conditions tested. Increasing R (0, 0.3 and 0.5) raised the hydraulic loading on the system and resulted in a drop of organic removal efficiency and methane yield. Translocation of sulfate reducing bacteria and methanogens in the ABRs caused by increased organic loading and effluent recycle is discussed. The results show great potential of parawood ash as a pH adjustment substance for acidic wastewaters. (C) 2008 Elsevier Ltd. All rights reserved.