Reliable energy recovery in an existing municipal wastewater treatment plant with a flow-variable micro-hydropower system

Abstract

A micro-hydropower (MHP) system with a flow-variable turbine was tested for over one year to investigate its applicability for small-scale municipal wastewater treatment plants (WWTP) with severe flow fluctuations. The applied MHP was designed as a semi-Kaplan, equipped with only adjustable turbine blades without guide vanes, hence it is simple in its mechanical structure and is inexpensive while providing high-level performance. To exploit as much hydro-energy as possible, a maximum water level tracking control scheme in the forebay tank was employed and the turbine blade angle was accurately adjusted corresponding to the oncoming flow rate, which allows water to hit the blade in the best direction for maximum efficiency. Despite its micro-scale (12.3kW at design conditions of 4.30m net head and 0.35m3/s flow rate), the applied MHP can work stably over a wide range of flows from 57% to 123% of the rated design flow, with the highest turbine efficiency of 91.3% and its corresponding overall electric efficiency of 80.3%. Even as the flow rate decreases to 23% of the design flow, the turbine still runs but at relatively lower efficiencies. Because of wide flow adjustability, the tested MHP can generate power even at extreme flow rates so that an almost complete amount (95.8%) of WWTP’s total effluent was used to produce 68.1MWh annually. In addition, as compared with similar WWTP-based hydropower systems in South Korea, the tested MHP achieved 1.78–2.80 times higher normalized electricity in both flow rate and net head, indicating a more efficient use of the flow and head. These results should draw new interest in the WWTP-based MHP, which was considered unfeasible in the past in Korea due to its low efficiency.