Anaerobic co-digestion of microalgae and primary sludge in an anaerobic membrane bioreactor for resource recovery (biogas and bionutrients) from urban wastewater

Continuous primary energy consumption has motivated the scientific community to search for less resource-demanding technologies and alternative renewable and eco- friendly energy sources that could substitute fossil fuels. Microalgae cultivation in combination with anaerobic technologies arose as a promising and sustainable technology in the field of wastewater treatment. Microalgae biomass valorisation through anaerobic digestion (AD) allows energy production from waste streams. Since microalgae AD presents some drawbacks that hinder process efficiency, anaerobic co-digestion (ACoD) with carbon-rich substrates such as waste paper or sludge has been studied as an alternative to increase anaerobic process efficiency. The present study consists in the long-term evaluation of raw microalgae and primary sludge ACoD. For this purpose, the ACoD technology was included in an environmentally sustainable framework in which AD was used first for organics removal from settled raw wastewater; microalgae cultivated in a membrane photobioreactor pilot plant were used for nutrients removal from the AD effluent; and microalgae biomass and primary sludge were co-digested in an anaerobic membrane bioreactor (AnMBR). Microalgae and primary sludge were co-digested at laboratory and pilot-scale and different operating conditions were evaluated considering biological processes, membrane filtration and microbial community involved. Microalgae and primary sludge ACoD was studied first at lab-scale, operating at different solids retention time (SRT), organic loading rate (OLR) and temperature. Results indicated that, the higher the SRT and the OLR, the higher the methane production, working in an AnMBR. Laboratory results also showed that thermophilic AnMBR achieved higher methane yield than the mesophilic one. Nevertheless, economic and energetic balances carried out to know which are the best operating conditions to scale up the process from laboratory to pilot-scale showed that 70 d SRT and mesophilic conditions were the recommendable ones to operate the ACoD pilot plant. Then, based on these results, the ACoD pilot plant was operated during a year at 70 d SRT and 35 oC. The pilot plant AnMBR achieved 62% organic matter biodegradability and showed high stability in terms of pH and volatile fatty acids. Filtration process was assessed, indicating that applying gas sparging and a backwash cycle every two filtration cycles avoided irreversible fouling formation. Microalgae ACoD has demonstrated to increase methane production compared to microalgae mono-digestion. Adding an easily biodegradable substrate as primary sludge to microalgae biomass had a synergetic effect on AD, creating an adapted microbial community capable of degrading both substrates through syntrophic pathways in which syntrophic microorganisms as Smithella or W5 degrade intermediates (especially propionate) and enhancing methane production, mainly carried out by aceticlastic methanogens as Methanosaeta. Hydrolytic and fermenters microorganisms involved in protein degradation (e.g. Coprothermobacter, Fervidobacterium, members of Synergistaceae family) and cellulose degradation (e.g. Defluviitoga, Thermogutta) also had an important role in both substrates degradation. This work also assessed potential nutrient recovery from ACoD effluents (permeate and digestate). Nitrogen was recovered from permeate with 99% efficiency using a hydrophobic polypropylene hollow-fibre membrane contactor. Phosphorus was not recovered from permeate since 74% was precipitated during AD process. Laboratory experiments with ACoD digestate carried out in Dewar reactors demonstrated that a composting process after ACoD can be applied, generating a stable and sanitised composted material that could be used as a soil improver. This thesis provides novel information on raw microalgae and primary sludge ACoD, since this process was studied not only at laboratory-scale, but also at pilot-scale, which is a necessary step for future applications at industrial-scale. High stability and high substrates degradation, corresponding to high methane yield, were achieved co-digesting both substrates due to the synergy effects found between microorganisms and also due to the use of AnMBR technology, avoiding the application of costly pretreatments. This process is enclosed in a circular economy scenario in which resources (biogas, nutrients and water) are being recovered from urban wastewater.