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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