Development of bioelectrochemical system based constructed wetland technology for efficient wastewater treatment and resource recovery

Bioelectrochemical systems (BES), also known as microbial electrochemical technology (MET), has emerged as a potential sustainable technology for wastewater treatment and complimentary electricity generation. BES consists of a set of technologies focused on the interaction between microbes and conductive materials/electrodes, which leads to a high catalytic rate at the microbe-electrode interface. The primary benefit of the favourable interaction between microbes and conductive materials is that it can boost microbial metabolism in an electron acceptor deficient anaerobic environment. This pioneering technology has shown great potential for wastewater treatment with simultaneous electricity generation along with other environmental applications. In the last decades, BES has been incorporated into constructed wetland (CW) technology to form an integrated hybrid technology, i.e., CW-BES. The CW-BES is the most innovative scalable development so far in the field of BES for wastewater treatment and other environmental applications. The CW-BES merger assists in overcoming the challenges associated with both technologies at an individual level. An anaerobic environment mainly dominates CW, leading to slow treatment performance and requiring a larger land footprint; nevertheless, BES integration assists in improving its treatment efficiency and additional functionalities. The CW-BES merger is still in its infancy stage. It requires a better understanding of many aspects of microbe-electrode interaction to remove various types of pollutants, such as organic/inorganic, recalcitrant, etc. It also needs further comprehensive investigation on electricity generation enhancement and practical application of generated electricity along with its applications. This PhD thesis aims to establish a deeper understanding of anaerobic microbe-electrode interaction in CW-BES to remove various pollutants from wastewater such as carbon, nitrogen, sulphate, toxic heavy metals and dyes, as well as operational problems like clogging, hydraulic retention time, and organic loading rates. The study also focussed on determining a way of harvesting energy, its storage and practical application of bioenergy generated from CW-BES. Thus, this study is divided into five stages to explore a different set of CW-BES technology for achieving sustainability in wastewater treatment. In the first stage, this study has focused on treating carbonaceous and nitrogenous (ammonium nitrogen) wastes, both high energy and cost demanding pollutants, from wastewater using CW BES. CW removal of carbon and ammonium nitrogen is a challenge due to the anaerobic environment and slow treatment rate. Oxidation of both carbon and ammonium nitrogen requires sufficient electron acceptors in an anaerobic environment. Hence, in the first stage, CW was incorporated into one of the BES technologies, a microbial fuel cell (MFC). The presence of electrodes in the anaerobic environment of CW-MFC acted as an artificial electron acceptor and assisted efficient electron transfer resulting in high treatment performance. Further, in the same study for anaerobic ammonium nitrogen removal, the study developed a new process where the electrode at the anode (anaerobic environment) of CW-MFC acts as an artificial electron acceptor. Based on electrode dependent anaerobic ammonium oxidation, the process was named the electroanammox process. This process provided a new pathway for ammonium nitrogen removal from wastewater in an anaerobic environment. The study was conducted in open and closed circuit conditions in a CW-MFC to better understand the electron flow role. This was further compared with the traditional CW. Based on the higher performance of open-circuit CW-MFC over the traditional CW, a CW filled only with conductive materials without electrical connections (electroactive CW) was then assessed to treat chromium, a typical heavy metal pollutant. The performance of electroactive CW was constant throughout, with chromium removal of 99.9%. The study exhibited a strong link between microbes and conductive material for this removal. In contrast, the presence of the conductive material decreased toxicity build up in the system and allowed microbial activity at a higher loading of chromium. The redox gradient developed in electroactive wetland assisted the system's electron flow, leading to better microbial metabolism. As sulphur is a very well-known redox metabolite in BES, a further study was conducted in CW-MFC to emphasise the interrelation between sulphate and a conductive material in electron transfer and treatment performance. CW-MFC performance was examined in the presence and absence of sulphate in wastewater. This indicated that sulphate acts as a redox coupling that significantly influences other CW-MFC treatment processes compared to the CW-MFC with no sulphate in the wastewater. The redox metabolites of sulphur improved the electron transfer mechanism and influenced treatment performance. Based on the outcome of previous stages, more complex wastewater consisting of various common pollutants and other, more recalcitrant pollutants was employed for the next stage. This was to examine the CW-BES technology's versatility to resist clogging at different loads, toxicity tolerance, and treatment performance. A variety of BES technologies integrated with CW were tested in this stage, such as CW-microbial electrolysis cell (MEC), CW-MFC, and electroactive wetland. The result indicated that the redox coupling of CW-BES has a significant influence on its treatment performance. Based on the favourable redox potential, the treatment performance increased, decreasing the toxicity build-up in the system with low sludge generation. The other significant outcome of the study was even at higher loading of recalcitrant pollutants, microbiology in terms of diversity and communities was not affected. Hence, CW-BES was found to be a versatile technology even at higher loading with recalcitrant pollutants. In the final stage, storage and the practical application of generated electricity from CW-MFC was explored. Since CW-MFC generates low power, scaling up the generated energy for real applications is a challenge. Therefore, the generated electricity from CW-MFC was stored using a power management system (PMS), mainly designed to store low power from CW-MFC. Further, the stored energy was used to operate an air-pump to provide aeration at the cathode of CW-MFC. The result showed significant improvement in treatment performance after aerating the cathode while decreasing the system's internal resistance. Thus, the study overall demonstrated practical application of bioenergy generated from the CW-MFC system. Economically, this study's advanced technology can provide a sustainable method for wastewater treatment while addressing the water-energy nexus, a prime focus in the current era. Overall, the study attempted to provide a low-cost sustainable, innovative approach for wastewater treatment to keep a greener and cleaner environment.