Investigation of the Performance of an Anaerobic Membrane Bioreactor in the Treatment of Mixed Municipal Sludge Under Ambient, Mesophilic and Thermophilic Operating Conditions

摘要

Anaerobic membrane bioreactors (AnMBRs) may provide a sustainable treatment technologyfor the digestion of mixed municipal sludge based on their ability to achieve elevatedvolatile solids (VS) conversion and a net positive energy balance. However, AnMBRs mayhave throughput limitations particularly when operated at high solids inventories due tomembrane fouling. This study characterized the anaerobic digestion bioprocess and themembrane performance under various operating conditions and identified the foulant modeusing the classical ‘Hermia-Field’ blocking models while operating the system under lowcrossflow and low transmembrane pressures. Using the in-vitro yeast estrogen screen (YES)bioassay the sludge and permeate quality was assessed for its estrogenic potential. Thepermeate was analysed for a group of environmentally relevant trace organic compounds.The study used a side-stream pilot scale AnMBR with two parallel negative tubular membranesand a flow through anaerobic digester (AD) as a control. Each reactor was fed inparallel from a common mixed sludge source. The tubular ultrafiltration membranes werepolyvinylidene difluoride based with a nominal pore size of 20 nm, operated at a crossflowof 1–1.2 m/s and a transmembrane pressure of 34–54 kPa. Four operating conditionsthat included different SRT:HRT ratios under ambient (25 C), mesophilic (35 C) andthermophilic (55 C) temperatures were investigated.The main AnMBR advantage over the AD is the ability to decouple and independentlycontrol the system SRT and HRT resulting in increased throughput, at lower HRT, whilemaintaining a low food to microorganisms ratio by increasing the SRT. This operationalstrategy was used and under mesophilic conditions, the AnMBR showed a 54 and 64 %volatile solids conversion at SRT:HRT of 30:15 and 21:7 days, with loading rates of 2.1 ±0.4 and 3.7 ± 0.9 kg COD/m3 · d, respectively. Under ambient and thermophilic operatingconditions, the SRT/HRT ratio was adjusted considering the system kinetics and theAnMBR showed a 49 and 55 % volatile solids conversion at SRT/HRT of 40:8 and 22:7days, with loading rates of 3.5 ± 0.4 and 3.4 ± 0.8 kg COD/m3 · d, respectively. Under allthe operating conditions the AnMBR was operated at more than double the loading rateand showed an improvement of 13–30 % increase in volatile solids conversion, comparedto the AD. A comparison of the energy balance between the AnMBR and AD showeda net positive energy balance for the AnMBR when operated at mesophilic and ambienttemperatures but not during thermophilic operation. The AD proved sustainable underambient operation only.The membrane performance showed a median sustainable flux of 6–7 ± 2 LMH which wasmaintained through inter-permeation cycle rest and clean-in-place (CIP) strategy. Thefrequency of the CIP increased with an increase in SRT, TS and during ambient operationdue to an increase in viscosity of the mixed liquor. The dominant fouling mode was foundto be cake fouling under all operating conditions and primarily of a reversible type. Thepermeability decline was assessed and found to be affected primarily by TS, SRT/HRTratio and extracellular polymeric substances (EPS) protein to polysaccharide ratio.The estrogenicity of the feed sludge was found to persist in anaerobically treated mixedmunicipal sludges and the AnMBR permeate. This was corroborated by the permeatechemical analysis which found a significant contribution from a small number of estrogens.