Energy and Nutrient Recovery from Organic Wastes Through Anaerobic Digestion and Digestate Treatment

Download or read book Energy and Nutrient Recovery from Organic Wastes Through Anaerobic Digestion and Digestate Treatment written by Mianfeng Zhang and published by . This book was released on 2017 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: Renewable energy with its benefits of reducing greenhouse gas emissions and substituting depleted fossil energy plays more and more important role nowadays. Anaerobic digestion, which converts biodegradable materials through a series of biochemical conversion processes, can contribute a significant portion of renewable energy. Meanwhile, as an alternative pathway for organic waste treatment process, anaerobic digestion can successfully achieve waste treatment and renewable energy production in well-controlled anaerobic digestion system. However, the effluent from anaerobic digesters usually contains ammonia and residual organic nitrogen, as well as other nutrients, salts and organic compounds that require proper treatment in order to reduce the environmental impact and recover values. Thus, digester effluent treatment is important for the development and deployment of anaerobic digestion technologies. In order to develop an efficient organic waste to renewable energy conversion process, a thorough study including feedstock characterization, pilot scale and large scale anaerobic digester facility feasibility test, effluent treatment process efficiency evaluation and life cycle assessment of large scale anaerobic digester facility was conducted. In feedstock characterization, eight types of organic wastes including tomato waste, tomato pomace, rice straw, commercial food waste, supermarket vegetable waste and egg liquid waste were selected as representations of various organic waste sources in California. All the samples were analyzed for their physical characteristics and chemical compositions including total solids (TS), volatile solids (VS) and nutrient contents and the biogas potentials of these selected organic wastes were investigated in batch anaerobic digestion test. As a result, all the selected waste streams were determined to have relatively good biogas potentials. Commercial food waste and supermarket vegetable waste had high biogas yield of 937 ml/g VS and 809 ml/g VS. On the other hand, rice straw, chicken manure and cardboard had relatively lower biogas yields of 565 ml/g VS, 447 ml/g VS and 396 ml/g VS, respectively. Although, biogas potentials varied based on the characteristics of the waste streams, all the selected waste were considered to be good feedstocks for anaerobic digestion due to their high organic content. Co-digestion of multiple organic waste can effectively adjust C/N ratio of the feedstocks and improve biogas production. Co-digestion of manure and food waste was studied in a pilot-scale high solids, two phase, thermophilic anaerobic digestion system. A pilot scale anaerobic digester was operated for five-month with mixed food wastes and dairy manure as feedstock. During the five-month operation, dairy manure had an average total solid content of 32% and average volatile solid content of 16%. Food waste had lower average solid content of 25% but higher average VS/TS ratio of 94%. The overall average biogas yield was 613 ml/g VS and average methane content was 62%. As a result of ammonia accumulation during the operation, ammonia concentration increased over the five months and reached up 4,172 mg/L after 10-week operation. High ammonia concentration may cause inhibition to microbial activity and reduce biogas production, thus proper treatment process would be required for long term operation. An integrated system consisting of biological treatment and membrane separation was investigated and developed to recover the nutrients from the effluent of anaerobic digesters. The system design includes a continuous aerobic process to reduce organic content and remove odor and then following two steps of membrane separation (microfiltration and reverse osmosis) for nitrogen recovery. Microfiltration could effectively remove chemical oxygen demand (COD), total suspended solids (TSS) and total solids (TS) with removal efficiency of 85%, 89% and 72%, respectively. Reverse osmosis (RO) could effectively recover nitrogen from the effluent with removal efficiency of 99%. The integrated system was able to successfully recover 73% of the ammonia nitrogen from the digester effluent into concentrate stream of RO, which only accounted for 16% of the initial volume. The UC Davis Renewable Energy Anaerobic Digester (READ) facility was evaluated for the feasibility and stability of the performance of a large scale anaerobic digester system. During one year start-up operation, READ facility was able to successfully process various organic waste streams and maintain stable functional conditions. A life cycle assessment (LCA) study was also conducted to evaluate the energy and environmental impact of READ facility. The annual Green House Gas (GHG) emissions of READ was estimated to be 98.2 metric tons CO2e/year for processing approximately 13,000 tons of organic waste per year. A carbon intensity (CI) value of 5.39 CO2e/MJ of the electricity fuel produced by READ was also determined as outcome of this LCA study. This CI value indicates that anaerobic digester systems can not only be an effective waste treatment process but also become a feasible approach to meet low carbon fuel standard (LCFS).