Filtración directa por membranas para potenciar la recuperación de recursos de las aguas residuales

Resumen

Direct membrane filtration (DMF) is a potential alternative to boost resource recovery within municipal wastewater (MWW) treatment field. This technology aims to enhance particulate matter recovery from MWW by implementing a membrane filtration unit as a previous step to the biological secondary treatment, thereby reducing the energy requirements associated to the aerobic oxidation of organic matter in activated sludge systems. Additionally, the particulate organics retained and concentrated in the DMF tank can be used to increase methane production via anaerobic digestion, enhancing the energy balance of the overall wastewater treatment. This PhD thesis aimed to determine the most suitable membrane technology (microfiltration, ultrafiltration, or dynamic membranes) to perform the direct filtration of municipal wastewater. Additionally, different influent types (i.e. raw wastewater and primary settler supernatant) and operating conditions (i.e. solids concentration, transmembrane flux, and supporting material pore size for dynamic membranes) were studied in order to determine effective design and operational strategies to minimize fouling while maximizing resource recovery in the long-term operation. To this aim, a membrane-based pilot plant equipped with commercial membrane modules was operated for more than 3 years, which was fed with MWW coming from a full-scale municipal wastewater treatment facility. The relationship between membrane pore size and influent particles size distribution was found to be one of the major fouling controllers, identifying UF technology much more suitable than MF for DMF of MWW. Operating at relatively high suspended solid concentrations in the membrane tank (between 6 ¿ 11 g L-1) allowed minimizing membrane fouling. Applying low/middle transmembrane fluxes (around 10 LMH) also kept fouling propensity in low levels, i.e. severe fouling intensities were observed when the transmembrane flux was increased. Both tested influent sources (i.e. raw wastewater and primary settler supernatant) showed similar fouling propensities when operating at adequate suspended solids concentrations in the membrane tank. Sludge retention times below 3 days resulted in negligible organic matter losses due to biodegradation in the membrane tank. Organic matter was found to be the major fouling promotor during filtration (permeability losses from 75 to 99% depending on the porous membrane employed). UF treatment scheme generated a high quality permeate meeting European discharge requirements for non-sensible discharge environments. Moreover, promising results were obtained from preliminary energy, economic, and carbon footprint analysis. Sludge filterability tests identified the total solid concentration as the dominant parameter affecting sludge resistance to filtration when treating non-biological sludge. The time to filter filterability methodology was identified as the most convenient one compared to the capillary suction time and specific resistance to filtration due to its higher sensibility when treating non-biological sludge. A simple and generic model was proposed to predict/capture transmembrane pressure dynamics during UF operation treating raw MWW and primary settler supernatant. This mathematical model was able to properly predict membrane fouling within the range of operating conditions evaluated. Dynamic membranes were identified as a potential alternative for DMF of MWW, mainly due to reduced investment, replacement, and maintenance cost compared to porous membranes. Raw MWW treatment allowed to self-form a dynamic membrane in the short-term. Since the permeate quality was not significantly improved by using more restrictive supporting materials, the use of a unique supporting material layer with 5 µm of pore size was established in this study as the most convenient alternative. Unfortunately, the permeate quality produced was far from meeting the European discharge standards, while the energy and carbon footprint improvements that this membrane technology could achieve compared to primary settling where not high enough to justify the replacement of the later in existing municipal facilities.