Membrane bioreactors for municipal wastewater treatment - a viable option to reduce the amount of polar pollutants discharged into surface waters?

The potential of a lab-scale membrane bioreactor (MBR) to remove polar pollutants from municipal wastewater was studied for industrial and household chemicals over a period of 22 months parallel to a conventional activated sludge (CAS) treatment. For half of the compounds, such as benzotriazole, 5-tolyltriazole (5-TTri), benzothiazole-2-sulfonate and 1,6-naphthalene disulfonate (1,6-NDSA), removal by MBR was significantly better than in CAS, while no improvement was recorded for the other half (1,5-NDSA, 1,3-NDSA, 4-TTri and naphthalene-1-sulfonate). The influence of operational conditions on trace pollutant removal by MBR was studied but no significant effects were found for variation of hydraulic retention time (7h-14h) and sludge retention time (26d-102d), suggesting that the lowest values selected have already been high enough for good removal. It is shown that the seemingly inconsistent results reported here and in previous studies regarding the comparison of trace pollutant removal in MBR and CAS are highly consistent. MBR is neither superior for well degradable compounds that are already extensively degraded in CAS treatment nor for recalcitrant compounds that are not amenable to biodegradation. For most compounds of intermediate removal in CAS treatment (15-80%), among them pharmaceuticals, personal care products and industrial chemicals, the MBR is clearly superior and reduces the effluent concentration by 20-50%. Despite of this clear benefit of MBR, the effect is not pronounced enough to serve as a sole argument for employing MBR in municipal wastewater treatment.