Development of an evaluation method for hydropower energy recovery in wastewater treatment plants: Case studies in Ireland and the UK

Abstract

Wastewater management is an energy intensive sector that is reliant on fossil fuels in many countries. This paper examines the potential for improvements in the sustainability of the sector through energy recovery using hydropower turbines at the outlets of wastewater treatment plants (WWTPs). Flow and head data in outlet pipes were collected from over 100 plants in Ireland and the UK. An evaluation method was developed to estimate the potential power outputs and payback periods of hydropower energy recovery schemes at the plants. A sensitivity analysis was carried out to illustrate the effect of variations in flow, turbine selection, electricity pricing and financial incentives on power output and payback period. Seasonal changes in precipitation and extreme rainfall events were found to impact the selection of turbine design flow. A method of optimising design flow to cater for flow variation without impeding on turbine efficiency was developed. In addition, the impact of estimated future flow rate changes (due to demographic and climate changes) on turbine design flow was investigated. For plants with planned or expected future growth, it was found that an increased design flow rate to allow for higher future flows is more economically viable than using present flow rates at a plant. With regard to turbine selection, the Kaplan turbine was found to have the greatest potential power output but the pump-as-turbine had the lowest cost per kilowatt. Finally, electricity pricing was found to have a major impact on the economic viability of hydropower energy recovery. For the majority of the EU-28 countries investigated, using the power output directly on-site at the plants was found to have a shorter economic payback period than selling the electricity to the grid to earn revenue from feed-in tariffs.