Removal of trace organics from water using a pumped bed-membrane bioreactor with powdered activated carbon

Abstract

A pumped bed-membrane bioreactor (PB-MBR) combining powdered activated carbon (PAC) adsorption with bacterial degradation was studied for the purification of tap water, spiked with 2 mg/l of phenanthrene, to sub-ppm levels. The feedwater was supplemented with nitrogen and phosphorous to reach a COD:N:P ratio of 100:10:1. PAC concentration was maintained at 5 g/l. Polyacrylonitrile (PAN) flat-sheet membranes of 250 and 1500 kDa MWCO were tested. The overall efficiency during long term operation was tested at different hydraulic conditions with intermittent biosolids wastage and high frequency-chemical cleaning (every 2–3 days) or without chemical cleaning and without sludge removal. With intermittent PAC dosage-high frequency chemical cleaning steady fluxes of 150–200 l/m 2 h were maintained. With no chemical cleaning-no solids wastage, steady fluxes of 22.5±3.8 and 28.8±4.8 l/m 2 h were achieved for the 250 and 1500 kDa membranes, respectively. An almost complete removal of phenanthrene was observed in all cases, regardless the type of membrane, operating protocols or hydraulic conditions applied (permeate concentration was consistently below 5 μg/l). The relatively high permeate fluxes and constant filterability observed are attributed to the turbulent flow regime and high PAC-slurry recirculation ratio generated.