Treatment of anaerobic digestion effluent using membrane distillation: Effects of feed acidification on pollutant removal, nutrient concentration and membrane fouling

Anaerobic digestion effluent contains large amounts of ammonia and phosphates. When discharged, this causes an unacceptable environmental pollution; however, it provides an opportunity for nutrient recovery. In this study, membrane distillation (MD) was applied to treat anaerobic digestion effluent and feed acidification was utilized to enhance pollutant removal and nutrient concentration for potential recovery. The results show that feed acidification substantially increased the rejection of ammonia (from 66% to 99%) and was thus beneficial for ammonia recovery. In addition, an excellent rejection of DOC (> 99.5%) and sulfonamides (100%) was achieved, regardless of the pH changes. The fluorescent intensity of organic compounds in the permeate was generally < 3 parts per thousand of that in the feed water under all pH conditions, indicating the absence of membrane wetting. Flux decline was reduced by 20% as the feed pH decreased from 8.0 to 5.0 and the reversibility of the membrane flux increased by 10%. EDS characterization showed that membrane scaling, likely due to calcium, magnesium and silicon deposition, was the primary culprit of membrane fouling and flux decline; feed acidification substantially reduced membrane scaling. Feed acidification suppressed membrane fouling and thus reduced the energy demand of the anaerobic digestion effluent treatment using MD.