Treatment of sewage sludge by anaerobic membrane bioreactor technology

Abstract

Wastewater treatment requires a substantial amount of energy to meet the discharge criteria. Energy can be recovered by anaerobic digestion of the produced sludge, which has a favorable impact on the energy balance. Conventional anaerobic digesters are constructed as completely mixed reactors operated at sludge retention times (SRTs) (< 30 days) to maximize solids conversion into biogas and sustain the methanogenic activity inside the reactor. In order to achieve a sufficient reduction of volatile solids (VS), anaerobic digesters are often constructed with huge volumes. Effective substitutes for conventional anaerobic digesters for the digestion of sludge are anaerobic membrane bioreactors (AnMBRs). Simply, AnMBR system is made up with the combination of a membrane and an anaerobic reactor. AnMBRs produce high-quality effluent, have a lower environmental impact, are resistant to toxic substrates, and have a high ability to transform carbonated organic molecules into biogas. AnMBRs can be operated at long SRT independent from hydraulic retention time (HRT), which allows the biomass to retain in the reactor for a longer time, thus results in higher digestion performance and biogas production. As an alternative to primary clarifier, high-rate activated sludge (HRAS) system, referred as adsorption stage (A-stage), was used since more organic matter can be recovered by A-stage. The biogas produced during the digestion of each sludge type and methane content were measured. The permeate quality was assessed. The filtration performance of an ultrafiltration (UF) membrane was also observed. The membrane area was 0.012 m2 and the flux was 5 L/m2.h. Morphological analyses were conducted to make a broader evaluation of membrane fouling. This study makes a comparative evaluation of the biogas production, treatment performance, and filtration performance of primary sludge (PS) and adsorption stage sludge (A-sludge) treated by AnMBR under mesophilic conditions. Finally, a plant-wide chemical oxygen demand (COD) mass balance was conducted to evaluate the COD conversion of each sludge type. Biogas production for PS was observed to be higher than for A-sludge, with average volumes of 5908 ± 352 and 5486 ± 238 mL/day, respectively. However, A-sludge contained a greater methane percentage (73%) in biogas than PS (62%). Similar COD removal efficiencies were obtained for each sludge type, approximately 96% for PS and 97% for A-sludge. Stable digester conditions in the digester were obtained, considering the optimum volatile fatty acids (VFA) to alkalinity ratio of nearly 0.08 found for each sludge type. Total nitrogen (TN) removal efficiency was 52.5% for PS, while nearly 19% was achieved for A-sludge. High total phosphorus (TP) removal efficiencies of 97% and 82% were acquired for PS and A-sludge, respectively. Total suspended solids (TSS) removal efficiency for each sludge was more than 99% thanks to the membrane, and almost no solids and fecal coliforms were found in the permeate. Extracellular polymeric substances (EPS) were found higher in AnMBR treating A-sludge. In correlation with this higher EPS, a higher capillary suction time (CST) value (293 ± 11 sec) was observed for the anaerobic sludge fed with A-sludge. Average transmembrane pressure (TMP) was higher for A-sludge (223 ± 51 mbar) in comparison to PS (171 ± 53). Morphological analyses of membranes were conducted following the operation period. Environmental scanning electron microscopy (ESEM) analysis revealed that a denser cake layer was observed on the membrane of the system fed with A-sludge, which may be correlated with the higher EPS content in the sludge of the system fed with A-sludge. A plant-wide COD mass balance was conducted in the study and revealed that A-stage integration can convert 34.5% of COD in the wastewater into methane, while primary clarifier integrated with AnMBR can recover only 19.9% of COD into methane. Consequently, in terms of energy efficiency, integration of AnMBR with A-stage instead of primary clarifier can be applied and contributes to the energy efficiency of wastewater treatment plants (WWTPs).