The influence of pH and alkalinity on the process performance of simultaneous anaerobic sulfide and nitrate removal was studied in Upflow Anaerobic Sludge Blanket (UASB) reactor.When the influent pH was controlled between 7.5 and 8.0

the maximum sulfide and nitrate removal loading rate were 2.96 kg·(m3·d)-1 and 0.47 kg·(m3·d)-1

respectively.The basifying reaction and remnant sulfide led to increasing pH values that were finally beyond the bacteria toleration and caused process instability at high loading rates.When the reaction pH was controlled between 6.9 and 7.1

the maximum sulfide and nitrate removal loading rates were 4.78 kg·(m3·d)-1 and 0.99 kg·(m3·d)-1

respectively.The alkalinity should be adjusted at （454.1±40.5）mg ·L-1 to keep a neutral condition.The alkalinity change in the process can indicate the style and extent of the dominant reaction.When S/N=5∶2 (with sulfur as main product)

S/N=5∶8 (with sulfate as main product) and S/N=5∶5 (with the mixture of sulfur and sulfate as main product)

the ratios of removed sulfide to decreased alkalinity were 2.27

2.00 and 5.00.