The cost and performance of an MF-RO/NF plant for trace metal removal

Abstract

The performance of a membrane system based on microfiltration (MF) and reverse osmosis/nanofiltration (RO/NF) has been examined with reference to the removal of trace metals from classically-treated municipal wastewater, and associated costs determined. Metals fate was assessed both at full and pilot scale. Metals rejection was found to be 94.5–99% on average; permeate metal concentrations were between 0.01 and 0.7μg/L for Cu and Ni and 0.7–5.7μg/L for Zn, and largely unaffected both by feed concentration and membrane type. The operational expenditure (OPEX) was calculated from information from the full-scale plant, primarily comprised energy demand (47%) and membrane replacement (37%) and was largely independent of plant size for flow rates from 1 to 100megalitres per day (MLD). Results confirmed the membrane-based process to reliably remove metals down to levels below 6μg/L, but only at a cost (~18p/m3 OPEX) which makes the process untenable for this duty exclusively. Moreover, a further cost would be incurred from the management of the concentrate stream. However, the reliably high levels of removal permit possibility of employing RO or NF technology for treating only a fraction of the wastewater and blending with the untreated stream, depending on the required discharged wastewater quality.