Techno-economic analysis on improving biogas from anaerobic digestion on pre-treated sewage sludge
Purpose of project
A techno-economic analysis on improving biogas production from anaerobic digestion of pre-treated sewage sludge.
Sydney Water treats sewage sludge through anaerobic digestion and is constructing a thermal hydrolysis plant at St Marys Wastewater treatment Plant. In this project, a techno-economic analysis including biodegradability and net energy generation will be compared for thermally-hydrolysed pre-treated sludge and wet air oxidation treated digested sludge. Sydney Water treats 500 GL of sewage and produces 180,000 tonnes of biosolids from its anaerobic digestion (AD) facilities annually. The generated biosolids are trucked inland and reused as compost or soil conditioner. However, due to long trucking distance of around 300 km, transport costs of biosolids are high ($100/tonne) and also lead to significant fossil fuel carbon emissions. Reduction in the volume of biosolids produced will have a direct impact on trucking costs and carbon emissions.
Annually, Sydney Water generates 58 GWhel and 61 GWhth in cogeneration plants from the biogas (~60% methane) produced from its AD facilities. The produced biogas can supply 40–60% of a tertiary treatment plant’s electricity requirement.
Wastewater treatment plants (WWTPs) in general consume a significant amount of energy, representing about 1% of grid energy across Australia. In 2019/20, Sydney Water’s WWTP consumed 188 GWh of grid energy costing $23 million. Surplus electricity produced through AD facilities will reduce the energy cost for WWTP, avoid carbon emissions and generate additional heat that can be used for other purposes. There is potential for WWTPs to become net energy generators rather than significant energy consumers.
Sydney Water is constructing a thermal hydrolysis pre-treatment (THP) plant to reduce biosolids quantity and improve biosolids quality. However, current thermal pre-treatment technologies such as the THP process are energy intensive and result in only a small positive net energy generation. Therefore, this project aims to assess the energy balance of wet air oxidation process (WAOP) and compare it to the THP process in terms of net energy generation. The project outputs will be used by Sydney Water to evaluate the feasibility of these technologies at their WWTPs. If the project outcomes show that the application of the WAOP pre-treatment process results in a positive net energy generation, then further research will be considered by Sydney Water to optimise the WAOP operational conditions for energy recovery, provide an improved financial and process model and determine biosolids quantity and quality.
Impact of project
If the WOAP technology is implemented, this project has the potential to result in additional renewable energy generation, emissions reductions in the form of reduced scope 1 and 2 emissions, and an improvement in the reliability of product (biosolids) quality. If WOAP is financially and technically feasible, the technology is likely to be implemented within the next five years so will have a short-term impact. Sydney Water generates about 58 GWhel per year from its 10 biogas facilities. This is enough to power 7000 homes. It is expected that WAOP technology at one WWTP will double biogas production and thereby increase power by 5 GWhel.
Sydney Water and most other utilities are state regulated and therefore customer bills are not directly controlled by the utility. Customer bill reductions shown below are therefore an extrapolation. Net value from the 5 GWhel generated each year is about $0.3 million per year and from reduced biosolids transport is about $14 million per year. Considering market of 4 million Sydney Water customers, this equates to a saving of about $3.70/customer.
This project will result in an emissions reduction of about 4,000 tonnes CO2-e by offsetting grid electricity and 6,500 tonnes CO2-e per year by reducing trucking requirements.
WAOP may also improve the dewaterability (and therefore the reliability and quality) of solid residue as is the case for the thermal hydrolysis process. This may provide access to new markets resulting in less distance to transport the biosolids as a pathogen free product
Project partners – industry and research
Griffith University (Lead), RMIT, Sydney Water
Industry Reference Group members
City of Gold Coast, Hunter Water, SA Water, Water Corporation WA, Water Service Association Australia (WSAA)