The high levels of pollution associated with mining activities necessitate more efficient methods of treating mining effluent before it is released into the environment. Phosphate -mining wastewater contains high concentrations of sulfate that can be removed and recovered as elemental sulfur (S0), which is a useful resource. The Membrane Biofilm Reactor (MBfR) uses gas-transfer membranes for the delivery of gases to microorganisms that carry out oxidation-reduction reactions that lead to the breakdown of contaminants. The two main microorganisms involved in the treatment of sulfate wastewater using the MBfR are sulfate-reducing bacteria (SRB) for the reduction of sulfate into sulfide and sulfur-oxidizing bacteria (SOB) for the oxidation of sulfide into S0. In this work, the kinetic processes involved in sulfate reduction and sulfide oxidation for SRB and SOB were modeled using the steady-state biofilm model and mass balances on a completely mixed biofilm reactor. The model results identified trends of substrate removal, biofilm accumulation, and hydraulic retention time (HRT) for the design of the sulfate-treatment system. The HRT required for 97.5% sulfate removal was about 0.1 d and that for 97.5% sulfide removal about 0.2 d. Higher levels of biofilm accumulation occurred with sulfide oxidation due to the larger biomass yield of the SOB. The needed delivery of H2 gas required for sulfate reduction and O2 gas for sulfide oxidation, as well as the alkalinity changes, also were determined based on the removal levels.