Immersed membrane bioreactors for nitrate removal from drinking water: Cost and feasibility

Brine disposal remains a key consideration when employing ion exchange (IEX) for nitrate removal. To circumvent this issue, research was conducted into developing an immersed membrane bioreactor (MBR) as a replacement for ion exchange. This paper compares the economics of the two process options whilst also considering MBR for brine treatment. At a blend flow rate of 3000 m3.d–1, total life cycle costs were £0.095, 0.128 and 0.064.m–3 for IEX, MBR and IEX with brine recycle MBR respectively. Capital and O&M costs associated with MBR were extremely sensitive to membrane price variation. For blend flows >700 m3.d–1, IEX with brine recycle MBR proved the most cost effective option (total life cycle cost £0.044–0.087.m–3, 1500–30000 m3.d–1) when assuming similar hydrodynamic efficiencies to the IEX replacement MBR. Further cost reductions (≤17%) can be found by operating Brine recycle MBR at higher salinity (~100 g NaCl.L–1), provided an appropriate halo-tolerant community can be established.

MBR designed for IEX replacement exerts a very low specific energy demand compared to conventional MBR designs and a low specific O&M relative to conventional IEX. However, membrane cost renders the process uncompetitive vs. IEX with brine disposal at flows >700 m3.d–1. IEX with brine recycle MBR is cost competitive with total life cycle cost savings of £0.038– 0.059.m–3 over IEX with brine disposal. The process is less sensitive to membrane costs and a further total life cycle cost reduction of ≤17% is possible by operating at considerably higher salinities. To validate process viability, research is required to assess the impact of varying saline concentrations on nitrate removal. Furthermore, although the influence of salinity shocking on fouling propensity in municipal wastewater MBR has been documented [19], fouling for a permanently high saline environment is less well understood which also needs to be addressed.