Recovery of organic and ammonia resources from thermally hydrolyzed sludge liquor by fermentation and contactless membrane distillation

Abstract

A large amount of waste sludge is generated every day from wastewater treatment in large cities. Waste sludge contains rich contents of organics and nutrients that deserve to be recovered. This research utilized thermal hydrolysis (TH) as a pretreatment method to hydrolyze organics in sludge and improve its biodegradability. The hydrolyzed sludge was then processed with oleaginous yeast fermentation to produce bio-lipids for energy recovery or with anaerobic acidogenic fermentation to produce short-chain fatty acids (SCFAs) for organics recovery. Considering the high concentration of ammonia in the fermented sludge liquor, a new contactless membrane distillation (MD) system was developed to recover ammonia nitrogen from the sludge. TH released 36.8% of organics from the sludge into the supernatant, which was more effective than other sludge pretreatments via acid (2.4%), ultrasound (12.0%), and alkaline (27.0%) treatments. The inoculated oleaginous yeast Trichosporon dermatis made use of 45.2% of soluble organics in the thermally hydrolyzed sludge liquor (THSL) to produce most of the intracellular lipids within 1 d. The total organics conversion efficiency into bio-lipids was 8.2%. An acid condition with a process pH 3.5-4.5 was favorable to the fermentation of T. dermatis. The TH sludge liquor could also be combined with glucose to provide the organics and nutrients for co-fermentation, which enabled the preferable acid condition (pH 3.5-4.5) for oleaginous yeast and improved the conversion of waste organics to bio-lipids to 12.8%. The TH-treated sludge can also undergo acidogenic fermentation under an anaerobic condition to produce SCFAs. TH pretreatment resulted in rapid abiotic hydrolysis, and the SCFAs yield was 122.0 mg COD/g VS after fermentation. TH of the sludge followed by one-time alkali treatment further increased the SCFAs production from fermentation (TH&Alk‒FM) to 258.1 mg COD/g VS, even 22.8% higher than that of enhanced fermentation of non-treated sludge with the daily alkaline pH adjustment (NT‒FMpH10). According to the bioinformatics analysis, the TH&Alk‒FM and NT‒FMpH10 fermented sludge had simple but different microbial communities. The dominant genera in TH&Alk‒FM were Unclassified Ruminococcaceae (18.9%) and Unclassified Porphyromonadaceae (22.3%), which belonged to classes Clostridia and Bacteroidia, respectively. NT‒FMpH10 was dominated by Tissierella (23.7%) and Proteiniborus (13.5%), belonging to sole Clostridia. A novel contactless MD system was developed by introducing an air gap between the hydrophobic membrane and feed solution to largely improve the ammonia recovery efficiency and eliminate the membrane fouling problems. Results show that the overall NH3 transfer coefficient by contactless MD was (1.49 ± 0.05) × 10-5 m/s, more than 7 times greater than that of conventional MD. In continuous operation with a short retention time of 20 min, a high NH3 recovery (> 80 %) from the fermented sludge liquor and a clean membrane without foulant attachment were maintained, while the conventional MD system failed in 12 h due to severe membrane fouling. After NH3 recovery, the sludge liquor was turned into a preferable organic carbon source for utilization. This innovative contactless MD can recover ammonia from various waste liquids.