EFFECT OF SULFATE ON ANAEROBIC DEGRADATION OF BENZOATE IN UASB REACTORS

Wastewaters containing benzoate and sulfate were treated in two upflow anaerobic sludge blanket (UASB) reactors at 34-37 degrees C for 320 d . The sulfate concentration was increased stepwise in Reactor-A up to 7,500 mg/L, and was kept mostly constant at 3,000 mg/L in Reactor-B. B oth reactors removed over 98% of organic chemical-oxygen demand (COD) for sulfate up to 6,000 mg/L, despite the fact that the mixed liquor c ontained up to 769 mg S/L of total sulfides and up to 234 mg S/L of di ssolved H2S. Sulfate-reducing efficiency decreased with the increase i n sulfate concentration, but increased with time at each sulfate conce ntration. Reactor-B consistently reduced 89% of sulfate. However, both organic COD removal and sulfate-reducing efficiencies of Reactor-A dr opped drastically at 7,500 mg SO42-/L, and showed no sign of recovery after 50 d. The system failure was likely due to the increased sulfate , instead of sulfide, toxicity. From the COD balance, 93.4% of COD rem oved was converted to methane instead of sulfides, with a net sludge y ield of 0.047 g volatile suspended solids (VSS)/g COD. The sulfur bala nce was over 97%.