Nutrients and trace organic contaminants removal by an anoxic-aerobic membrane bioreactor (mbr): lab-, pilot- and full-scale investigations

Increasingly stringent environmental regulations and freshwater shortage are key drivers for a worldwide trend of introducing advanced technologies for wastewater treatment, particularly in removing nutrients (i.e., nitrogen and phosphorous) and trace organic contaminants (TrOCs). Membrane bioreactor (MBR) is a compact process that employs membranes for effective solid-liquid separation, which in turn brings about additional advantages such as decoupling of hydraulic retention time (HRT) and sludge retention time (SRT), maintenance of higher mixed liquor solids concentration (MLSS) than the conventional activated sludge (CAS) process and potentially better removal of resistant contaminants in a single step. The anoxic-aerobic MBR process combines bioreactors harbouring different redox conditions and thus facilitates efficient removal of nutrients, and potentially that of TrOCs. This thesis aims to evaluate the performance of an anoxic-aerobic MBR in terms of nutrient and TrOC removal at lab-, pilotand full-scale installations. The dynamics of bacterial communities in the MBR system and the corresponding removal performance under different operating conditions have been assessed. The robustness of the anoxic-aerobic MBR during simulated ‘hazardous events’ i.e., deviations in operating conditions is also evaluated. Simultaneous nitrogen and TrOC removal (a set of 30 TrOCs) by a laboratory scale anoxic-aerobic MBR was demonstrated. In this study, biodegradation was demonstrated as the main TrOC removal mechanism, with aerobic degradation playing a major role. Low oxidation reduction potential (ORP) regimes (i.e., anoxic) were conducive to biodegradation of some TrOCs, but it may only aid in biosorption in absence of internal recirculation between the anoxic-aerobic zones. Metagenomic approach using pyrosequencing of 16S rRNA genes revealed the bacterial communities developed in the anoxic-aerobic MBR system. Internal recirculation between the aerobic and anoxic bioreactors was observed to be a key driving force shaping the bacterial communities in the anoxic-aerobic MBR. Insights into the shifts in bacterial communities along with the changes in removal efficiencies under different operating conditions have been provided. A more diverse bacterial community was noted during operation without sludge withdrawal (‘infinite’ SRT) than during an SRT of 25d. However, with a few exceptions, the bulk organic, nitrogen and TrOC removal performance were similar under the SRTs investigated, suggesting that the shorter SRT investigated in this study (25 d) was adequate for the development of functional bacterial groups in the MBR. Potential bacterial groups participating in TrOC degradation were identified. During comparison of bulk organics, nutrients and TrOC removal performance by a fulland a pilot-scale MBR from real wastewater originating from a resort town, the pilot-scale MBR demonstrated a very similar COD reduction as the full-scale MBR. However, given the significantly higher MLVSS concentration and presence of additional anoxic and aerobic bioreactors in the full-scale plant, the removal of nutrients, particularly that of phosphorous, and a few resistant TrOCs by the full-scale MBR was significantly higher. Both TN and TrOC removals were facilitated by a delicate combination of multiple redox zones in the bioreactors. Simulated hazardous events, namely, aeration and power failure, and chemical shock (ammonia and bleach) were found to alter pH and/or ORP of the mixed liquor and inhibit biomass growth, thus affecting the removal of bulk organics, nutrients and TrOCs. Chemical shocks generally exerted greater impact on MBR performance than aeration/power failure events, with ammonia shock exerting the greatest impact. The removal of hydrophilic TrOCs that are resistant and/or occur at high concentrations in wastewater were notably affected by the hazardous events. MBR treatment effectively reduced estrogenic activity in wastewater; however, chemical shocks were observed to temporarily increase the endocrine activity of the effluent. Depending on the chemical shock-dose and the applied membrane flux, hazardous events can exacerbate membrane fouling. Except for ammonia shock, recovery of the MBR performance was achieved within 72 h of hazardous events.