Increased carboxylate production in high-rate activated A-sludge by forward osmosis thickening

Abstract

Domestic wastewater represents a considerable feedstock for organics but the high dilution makes their recovery typically unsuccessful. Here we investigated three routes to 10-fold concentrate the organics using forward osmosis (FO) (Draw solution (DS) 2.2M MgCl2): directly on domestic wastewater, A-sludge, or secondary sludge, with the end goal of increasing volatile fatty acid (VFA) yield from subsequent 9-day fermentation tests. Forward osmosis concentrated the total COD by a factor of 8.2±1.2, 10.1±2.4 and 4.8±0.2 with respect to the raw streams of wastewater, secondary sludge and A-sludge. The soluble fraction of the COD was concentrated up to 3.5 times in the A-sludge and 2.1 times in the secondary sludge; the result of a combined effect of the chemical action of Mg2+ (diffused from the DS) on sludge disaggregation and cell lysis, and the physical action of recirculation and air-scouring of the A-sludge in the FO-unit.The FO-concentrated A-sludge produced 445±22mgCOD-VFAg−1 CODfed, which was 4.4 times higher than for the untreated A-sludge. No VFA were produced from untreated secondary sludge, but after FO-concentration 71±5mgCOD-VFAg−1 CODfed could be reached. Due to the low organics in wastewater even after FO-concentration (1.08±0.08gCODL−1), no notable VFA production occurred. The combination of A-stage technology and membrane technology for dewatering and COD concentration could be a key advancement to increase VFA production from domestic wastewater, whereby at least 45% of the COD can be recovered as valuable VFA.