Renewable energy from wastewater grown microalgae - a concept for nutrient recycling and sustainable energy recovery

Abstract: Wastewater is an excellent source of nutrients and energy. By treating the wastewater in a clever way, it is possible to both recycle nutrients and recover energy, and thereby create opportunities for a sustainable system. In conventional wastewater treatment where the activated sludge process is used, aeration is needed and a lot of biomass is produced. In regions with warmer climate, or if low-valuable heat is available for heating this technique could be replaced with an anaerobic one. With anaerobic techniques, less sludge is produced, the need for aeration is decreased and potentially more energy can be recovered. An increased interest in renewable energy sources has put demand on finding suitable substrates for production of biobased fuels. For this purpose, algal biomass presents interesting characteristics. Algae use sunlight, carbon dioxide and nutrients for growth. One of the hurdles with production of algal biomass is nutrient supply but this can be solved by growing algae in wastewater. In this way two problems are solved: the algae are supplied with nutrients at the same time as nutrient reduction in the wastewater is accomplished. In this study, the feasibility of integrating an algal step in a wastewater treatment system was evaluated based on a series of laboratory experiments. Further, a concept for wastewater treatment including an Anaerobic Moving Bed Biofilm Reactor (AnMBBR) and algal cultivation was compared to an existing wastewater treatment plant (WWTP). Nutrient reduction over the algal cultivation showed more than 97% reduction of phosphate and more than 84% reduction of ammonium. Algal harvesting experiments showed that it was possible to efficiently separate algal biomass and treated water by sedimentation for 30 minutes after flocculation by addition of ferric chloride and cationic polymer. These experiments also showed that it was possible to meet the discharge limits for P-tot (0.3 mg/L), N-tot (10 mg/L) and COD (70 mg/L). Harvesting efficiency of up to 96% was achieved. Methane potential from primary sludge was found to be 295 NmL/gCOD and for untreated microalgae, dominated by Scenedesmus sp., 95-108 NmL/gCOD. In batch tests, no synergistic effects could be seen for co-digestion of algae and primary sludge. The methane yield for algal biomass was increased by 46% when pretreated at 120°C for 30 minutes and by 74% when pretreated at 170°C for 30 minutes. Evaluation of the proposed concept showed that the ratio between primary sludge and algae would be 32:68 on volatile suspended solids basis, if algae are grown 12 months per year. Compared to a conventional WWTP which uses the activated sludge process, the yield of methane was 35% higher without pretreatment, and up to 75% higher if pretreatment is applied. Finally, it was found that microalgae have a great potential for biogas production compared to some energy crops. The energy potential of algae was found to be 60-160 MWh/(ha year) depending on pretreatment and cultivation period (8-12 months/year).