The Cost Benefit of Refinery Effluent Pretreatment Upstream of Membrane Bioreactors

The established classical method of treating oil refinery effluent is flotation followed by biological treatment. Membrane bioreactors (MBRs) offer more advanced treatment, producing a clarified and potentially reusable treated effluent, but demand robust pretreatment to remove oil and grease (O&G) down to consistent, reliably low levels. An analysis of a full-scale conventional oil refinery ETP (effluent treatment plant) based on flotation alone, coupled with projected performance, energy consumption and costs associated with a downstream MBR, have demonstrated satisfactory performance of flotation-based pretreatment. The flotation processes, comprising an API (American Petroleum Institute) separator followed by dissolved air flotation (DAF), provided similar to 90% removal of both total suspended solids (TSS) and O&G coupled with 75% COD (chemical oxygen demand) removal. The relative energy consumption and cost of the pretreatment, normalised against both the volume treated and COD removed, was considerably less for the API-DAF sequence compared to the MBR. The combined flotation specific energy consumption in kWh was found to be almost an order of magnitude lower than for the MBR (0.091 vs. 0.86 kWh per m(3) effluent treated), and the total cost (in terms of the net present value) around one sixth that of the MBR. However, the nature of the respective waste streams generated and the end disposal of waste solids differ significantly between the pretreatment and MBR stages.