Removal of trace organic contaminants by anaerobic membrane bioreactors

Anaerobic membrane bioreactor (AnMBR) can potentially be the core technology to facilitate the transformation of existing wastewater treatment plants to biorefineries, where clean water, renewable energy, and nutrients can be simultaneously extracted from wastewater. AnMBR is an integration of the membrane filtration process with anaerobic digestion. Organic substances are biodegraded to produce methane rich biogas, which can be subsequently converted to electricity to offset energy consumption of wastewater treatment facilities. Anaerobic treatment converts nutrients (e.g. nitrogen and phosphorus) to more chemically available forms to improve subsequent recovery. As such, wastewater is treated and can serve for different reuse applications with the integration of additional processes for further effluent purification. Nevertheless, the tech-economic feasibility of AnMBR for the treatment of and resource recovery from municipal wastewater is challenging due to its dilute nature. Thus, several advanced techniques, such as forward osmosis, have been strategically used to pre-concentrate municipal wastewater to achieve the carbon level suitable for efficient AnMBR treatment. In addition to organic matter, wastewater pre-concentration can also enrich inhibitory substances, such as inorganic salts and sulphur, and thus, negatively affecting the AnMBR performance. This thesis aimed to investigate the performance of AnMBR and its hybrid system for wastewater treatment and resource recovery. By simulating scenarios in wastewater pre-concentration, the effects of salinity and sulphur build-up in the influent on the performance of AnMBR were comprehensively investigated. In addition, AnMBR was integrated with membrane distillation (MD), which is a thermally driven membrane process, to advance wastewater treatment and resource recovery. Results from this thesis show that salinity build-up in the influent from negligible to 15 g/L (as NaCl) significantly affected the performance of AnMBR regarding biogas production and contaminant removal. Salinity build-up to the level above 10 g/L NaCl could reduce the activity and growth of anaerobic digester and thus reduce the removal of bulk organic matter as indicated by the measurement of chemical oxygen demand (COD). Nevertheless, biogas production was slightly reduced within the range of 0.4 – 0.5 L/g CODloaded with methane composition stabilising at 58 – 65% when the influent salinity was increased up to 15 g/L NaCl. Of the 33 trace organic contaminants (TrOCs) investigated, the elevated salinity reduced the removal of most hydrophilic compounds, but did not significantly affect the removal of hydrophobic chemicals. The accumulation of a few persistent TrOCs in the sludge phase was observed, but such accumulation did not vary significantly as salinity increased. In this thesis, the impact of sulphur content on the performance of AnMBR was also investigated with an emphasis on the biological stability, contaminant removal, and membrane fouling. Removal of 38 TrOCs that are ubiquitously present in municipal wastewater by AnMBR was specifically elucidated. Results show that basic biological performance of AnMBR regarding biomass growth and COD removal was not affected when the influent COD/SO42- ratio was maintained higher than 10. Nevertheless, the content of hydrogen sulphide in the produced biogas increased significantly and membrane fouling was exacerbated with sulphur addition. Moreover, sulphur increase considerably affected the removal of some hydrophilic TrOCs and their residuals in sludge during AnMBR operation. By contrast, no significant impact on the removal of hydrophobic TrOCs was noted with sulphur addition to AnMBR. A direct contact MD process was integrated with AnMBR to simultaneously recover energy and produce high quality water for reuse from wastewater. Results showed that AnMBR could produce 0.3 – 0.5 L/g CODadded biogas with a stable methane content of approximately 65%. By integrating MD with AnMBR, COD and phosphate were almost completely removed. Removal of the 26 selected TrOCs by AnMBR was compound specific, but the MD process could complement AnMBR removal, leading to an overall efficiency from 76% to complete removal for these compounds. The results also show that, due to complete retention, organic matter (such as humic-like and protein-like substances) and inorganic salts accumulated in the MD feed solution and therefore resulted in significant fouling of the MD unit. As a result, the water flux of the MD process decreased continuously. Nevertheless, membrane pore wetting was not observed. Results from this thesis provide unique insights to the further development of AnMBR, particularly for the treatment of pre-concentrated municipal wastewater. Additional processes should be applied to control the wastewater salinity less than 10 g/L NaCl and manage the wastewater COD/SO42- ratio above 10 to avoid inhibitory effects on AnMBR. Moreover, contaminant accumulation in the feed solution should be addressed when a membrane separation process is used to purify AnMBR effluent