Resumen de Hybrid systems for wastewater treatment in the framework of circular economy. Coupling biological and membrane technologies for a sustainable water cycle
The increasing water demand coupled to the depletion of natural water sources has raised the need to investigate and develop in wastewater treatment and reuse. Even more, the application of circular economy principies to water cycle has highlighted the need to see wastewater as a source of water and resources. Therefore, hybridization of already developed technologies can help achieve circular economy goals. Moreover, these hybrid systems that take the best of each technology are capable to gain to the limitations of current conventional treatments. Thus, in this thesis, different hybrid systems have been developed and tested (at bench and pilot scales) for wastewater treatment, both urban and industrial.
On one hand, three upflow anaerobic sludge blanket (UASB) reactors with different configurations: flocculent biomass, flocculent biomass and membrane solids separation and granular biomass and membrane solids separation (UASB-AnMBR), were operated to compare start-up, solids hydrolysis and effluent quality. The challenges of this work were both the low temperature and the low COD content. A really quick start-up was observed for the three reactors and was attributed to the previous acclimation of the seed sludge. The UASB configurations with membrane retained the solids in the reactor increasing solids hydrolysis efficiency. Moreover, flocculent biomass promoted slightly higher hydrolysis than granular one. Therefore, a configuration based on flocculent UASB-AnMBR was appropriate for the treatment of urban wastewater with low COD content at 10°C. ' On the other hand, a single-stage AnMBR for the treatment of cheese whey and its co-digestion with cattle slurry was investigated with the aim of potentially recovering water and energy. High COD removal (91% ± 7%) was achieved with a biogas production of 0.2-0.9 m3 biogas/kg COD removed. Therefore, high energy recovery could be obtained when using this process with a mean value of 2.4 kWh/kg COD removed. Although energy recovery was directly validated, severallimitations were detected regarding water reuse. Those limitations comprised high salt concentration in the permeate, which should be removed prior to its reuse.
Moreover, petrochemical wastewater pre-treatment was optimised with the final objective of water recycling. lt consisted in a coagulation-flocculation (CF) step followed by a moving bed biofilm reactor (MBBR) aimed to decrease suspended solids (SS) and organic content. In this case, only the first part of the hybrid system was optimised, membrane units were not included in this work. CF tests showed a decrease in wastewater turbidity but no significant DOC removal. Wastewater was then treated by MBBR. In MBBR, high sCOD removal efficiency (80-90%) was maintained. The MBBR proved to be also effective when treating raw wastewater as well as when feed wastewater effluent proportions were changed. The obtained results showed that MBBR was a suitable technology for petrochemical wastewater pre-treatment.
Finally, a novel treatment strategy for landfillleachate aimed to decrease its environmental impact was studied. The system consisted in a membrane bioreactor (MBR) pre-treatment aimed to remove COD, N and SS. lt was followed by a combined reverse osmosis-electrodialysis reversa! (RO-EDR) treatment aimed to remove salts and decrease brine volume. MBR decreased inorganic carbon by 92 ± 8% and achieved N removal of 85%. RO achieved a recovery of 84% and rejections of above 95%. EDR unit treating RO brine achieved a recovery of 67%. Thus, average recovery of the whole system was above 90%. lt is important to highlight that end-of-life RO regenerated membranes were used in this study. This fact, together with the low volume of brine (<10%) helped decrease the environmental impact of leachate treatment.
Hence, this thesis was conducted from an applied research approach, aimed to reduce the gap between basic technology development and industrial implementation.
