Experience has shown that effective oil/water/solids separation and wasteload equalization are essential for the successful operation of refinery biological wastewater treatment systems. The performance of these upstream operations critically affects the quality of the final effluent from activated sludge units, especially when nitrification is a treatment objective. Upstream treatment also influences the final effluent quality that can be obtained by rotating biological contactors (RBCs) and trickling filters, both of which have short hydraulic retention times and tend to lose efficiency as free oil accumulates in the biomass.