Assessing energy benefits of operating anaerobic digesters at low temperature with solids pre‐ozonation

Conference Dates

June 5-10, 2016


Further to the Kyoto Protocol, many European countries have introduced investment subsidies for the development of anaerobic digestion and biogas technologies. These subsidies (up to 40% of project costs) supported the installation of hundreds of biogas plants in Europe over the last 3-4 years (Grim et al., 2015). Anaerobic digestion is an effective means of reducing waste biosolids from activated sludge wastewater treatment, the handling and disposal of which can reach up to 60% of the operational cost of a treatment plant (Liu, 2003). The objective of the current research is develop new anaerobic digestion processes that can further reduce biosolids and enhance energy production. Conventionally, anaerobic digesters are operated at 35 ºC or higher temperatures in order to overcome the rate-limiting step of hydrolysis. However, the energy expenditure for heating anaerobic digesters may be high in northern countries like Canada where the treatment process temperature remains below 12 ºC for 4-6 months per year. In a recent laboratory study, we have shown that it was possible to operate batch anaerobic digesters at 20 ºC by synergistically combining them with ozonation of waste activated sludge (WAS) solids. The integration of solids ozonation and anaerobic digestion at 20 ºC resulted in a volatile suspended solids (VSS) destruction of 67%, while conventional anaerobic digestion at 35 ºC only achieved a 47% destruction. The solids loadings and hydraulic retention times (HRT) were the same for both digesters, but the solids retention time (SRT) was 20 days (same as the HRT) in the conventional digester and 27 days for the ozonated digester to maintain similar steady-state VSS concentrations. The increase in VSS destruction enhanced methane production from 53.1 mL methane/g VSSin for the 35 ºC-conventional digester to 69.2 mL methane/g VSSin for the 20 ºC-ozonated digester, a 30% increase. The energy expenditures of anaerobic digesters operated at 35 ºC, and at 20 ºC with solids ozonation were characterized from literature values to determine the energy sustainability of these operational scenarios (Ruggeri et al., 2015). The benchmark digester considered received 1,000 m3/d of WAS at 3% total solids (TS) and 10 ºC (Figure 1). The modeling considered heat losses from the digester, the heat transfer efficiency between influent and effluent (70%), the biogas conversion into electricity to mix the digesters and produce ozone (35% energy conversion), and solids thickening. Calculations found the 20 ºC-ozonated digester to produce 20% more energy than the 35 ºC-conventional digester (net energy balance: +147 GJ/day and +129 GJ/d, respectively). Assuming operation of the ozonated digester at 10 ºC with similar performance, the increase in net energy production over the conventional reactor reached 32% (+170 GJ/d). The results demonstrate that higher energy gains can be achieved with anaerobic digestion at low temperature by combining it with WAS ozonation. As both scenarios had an Energy Sustainability Index (ESI = energy produced/ direct energy; the energy necessary to run the process, namely electricity and heat) (Ruggeri et al., 2015) above 1, they can be judged potentially energy sustainable. However, the higher energy returns and VSS destruction of the 20 ºC-ozonated digester suggest energy efficacy and economic gains for this approach.

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