B5 Research Project
Improving biogas production of sewage sludge Report at a glance
RACE for Business Research theme B5: Anaerobic digestion for electricity, transport and gas ISBN: 978-1-922746-47-4 Industry Report Improving Biogas Production of Sewage Sludge October 2023
Project team Griffith University • • •
Prof P. Kaparaju N. Ordeu M. Rybachuk
RMIT University • •
Prof . R. Parthasarathy Prof. N. Eshtiaghi
• •
I. Al Waili T. Das
•
M. Haris
Citations Kaparaju, P., Parthasarathy, R., Eshtiaghi, N., Al Waili, I., Ordeu, N., Rybachuk, M., Das, T., Haris, M. (2023). Improving Biogas Production of Sewage Sludge. Prepared for RACE for 2030 CRC.
Project partners
Acknowledgements We would like to thank the Industry Reference Group participants from the following organisations: City of Gold Coast, Hunter Water, SA Water, Water Corporation WA, Water Service Association Australia (WSAA)
Although the IRG members and partners have provided valuable inputs and feedback throughout the project, the findings and recommendation included in this report do not necessarily reflect the views of each individual member.
Acknowledgement of Country The authors of this report would like to respectfully acknowledge the Traditional Owners of the ancestral lands throughout Australia and their connection to land, sea and community. We recognise their continuing connection to the land, waters and culture and pay our respects to them, their cultures and to their Elders past, present, and emerging.
What is RACE for 2030? RACE for 2030 CRC is a 10-year cooperative research centre with AUD350 million of resources to fund research towards a reliable, affordable, and clean energy future. https://www.racefor2030.com.au
Disclaimer The authors have used all due care and skill to ensure the material is accurate as at the date of this report. The authors do not accept any responsibility for any loss that may arise by anyone relying upon its contents.
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Report at a glance What is the report? This report summarises the results of the project which undertook a techno-economic analysis on improving biogas production from anaerobic digestion of pretreated sewage sludge. In this project, two pretreatment processes – Thermal Hydrolysis (TH) of pretreated sludge and Wet Air Oxidation (WAO) of treated anaerobic digested sludge – to determine the net energy generation, improvements in sludge dewaterability and overall economics of retrofitting these technologies.
Why is it important? Annually, Sydney Water generates 58 GWhel of electricity and 61 GWhth of heat in cogeneration plants from the biogas (~60% methane) produced from its anaerobic digestion (AD) facilities. The produced biogas can supply 4060% of a treatment plant’s electricity requirement but the residual organic rich solids (biosolids) and the anaerobic digestate still contain residual energy (25-30%). Thus, Sydney Water needs to establish if the remaining energy can be economically recovered, as this could also reduce the mass of sludge requiring disposal, thus reducing costs and greenhouse gas (GHG) emissions. Sydney Water is constructing a thermal hydrolysis plant at the St Mary’s wastewater treatment facility and wants to understand the relative performance of the two available treatment technologies, TH and WAO.
What did we do? • • •
•
A critical literature review on the effect of TH and WAO treatments on methane production from waste activated sludge (WAS) and anaerobic digestate, respectively, was completed. Chemical composition and methane potential tests of WAS Concentrate (with and without TH pretreatment) and with digestate concentrate (with and without WAO at two different oxygen loading levels) were performed. Two Aspen Plus models of AD were used to evaluate the techno-economic feasibility of WAO and TH treatments on biogas production and energy requirements. Four scenarios were evaluated: Scenario 1 - business as usual (AD treatment of a concentrated WAS, separation of digestate into solids (concentrate) and liquid (centrate) fractions, with 300 km transport of digestate solids (biosolids) for final disposal, liquid fraction disposal to a wastewater treatment plant (WWTP) and biogas used for cogeneration in a combined heat and power (CHP) plant with renewable electricity and heat were consumed behind-the-meter); Scenario 2 – Scenario 1 with TH of WAS before AD; Scenario 3 – Scenario 1 with AD biosolids treated with WAO with 100% oxygen loading in a gravity pressure vessel; and Scenario 4 – Scenario 1 with AD biosolids treated with WAO with 20% oxygen loading in a gravity pressure vessel. Using the process simulation models developed in Aspen Plus, mass and energy balance simulations were constructed in Microsoft® Excel spreadsheet to evaluate the techno-economic analyses. The techno-economic spreadsheet tool allowed the optimisation of the process parameters such as loading rate, temperature and, residence time and evaluate its impact on economic viability. Metrics for all four scenarios included total capital investment (TCI), operating cost net present value (NPV) and internal rate of return (IRR)
What difference will it make? The techno-economic analyses showed that an AD plant with TH would be a better option than an AD plant with WAO treatment. However, both technologies are 1.9 to 2.7 times more expensive than the existing base case scenario. Both experimental and simulation studies suggest that further optimisation of the treatment conditions for WAO in terms of solids loading, treatment temperature and oxygen concentration needs to be determined. A pilot-scale study could examine the effects of the above process parameters before scaling up scenarios could be evaluated. As only a few studies on gravity pressure vessel (GPV) were performed and the data is limited or seldom reported in literature, we recommend that Sydney Water further investigates optimising the process parameters for GPV technology.
What next? Given the potential of enhanced sludge treatment to assist with increasing biogas production from WWTPs, thus increasing their energy self-sufficiency and reducing sludge disposal costs and GHG emissions, further work may assist with identifying the level of capital support required to make TH or WAO projects cost neutral for wastewater treatment companies.
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