Selective concentration of sewage by forward osmosis for methane fermentation

Anaerobic fermentation has emerged as an effective method for carbon recovery in wastewater treatment, especially for sewage sludge. However, the first supernatant from primary settling tanks, despite containing more organic matter than sludge, is rarely used due to its low total organic carbon (TOC) concentration—typically only several tens of mg/L. For efficient methane fermentation, the TOC must be concentrated to at least 600 mg/L. Conventional concentration methods involving phase transitions, such as distillation, are energy-intensive. Forward osmosis (FO), a low-energy process that avoids phase changes, offers an alternative; however, conventional FO membranes concentrate not only organic matter but also inhibitory ionic species, such as Na + , K + , and SO₄ 2− . This study proposes the use of a loose FO membrane and a high–molecular weight draw solute (Pluronic 17R-4, Mw = 2700) to selectively concentrate organic matter while allowing ion leakage. Loose FO membranes cannot effectively reject small ions, such as Na + and Cl − , thus requiring draw solutes with higher molecular weights. Pluronic 17R-4, a thermoresponsive poloxamer, was employed as a high-molecular-weight draw solute. Poloxamers exhibit thermal phase separation, facilitating regeneration using waste heat a promising energy-efficient FO approach. The TOC of the first supernatant was successfully concentrated over 100-fold, reaching more than 2000 mg/L. Methane fermentation of the concentrated solution resulted in successful biogas production, whereas a conventional FO membrane led to inhibition due to ion accumulation. This approach demonstrates the feasibility of FO with loose membranes for low-energy, high-efficiency carbon recovery from dilute wastewater streams. • Loose FO membrane concentrated TOC over 100-fold from sewage supernatant. • Ion leakage through loose FO membrane reduced fermentation inhibition. • Pluronic 17R-4 enabled efficient draw solute recovery via phase separation. • Methane was successfully produced from FO-concentrated sewage supernatant. • Loose FO membrane outperformed normal FO in flux and fermentation compatibility.