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

•Forward osmosis thickening increases VFA production from sludge fermentation.•Concentration through FO favorites production of longer chain fatty acids.•Forward osmosis combined physical and chemical pretreatment increasing the soluble/total COD ratio.

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) 2.2 M 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 ± 22 mg COD-VFA g−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 ± 5 mg COD-VFA g−1 CODfed could be reached. Due to the low organics in wastewater even after FO-concentration (1.08 ± 0.08 g COD L−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.

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