Unlocking energy flexibility of municipal wastewater aeration using predictive control to exploit price differences in power markets

To accommodate the increasing amount of renewable energy sources in electricity grids it is crucial to utilize the flexibility of all electricity consumers. Municipal wastewater treatment plants consume approximately 1 % of the electricity consumption of a country's total electricity consumption, to reduce nutrient concentrations from the incoming wastewater before discharging the water back into the environment. In this paper, a novel economic optimal control strategy is proposed for unlocking the available energy flexibility in wastewater treatment. The strategy suggests that the power consumption in wastewater treatment can be flexible since the water is treated in large tanks with long retention times where specialized aeration equipment is repeatedly switched on and off. By controlling these switching times with respect to nutrient concentrations, electricity consumption can be predicted and shifted in time and hence provide short-term demand side flexibility. The proposed principle is used to reduce the operating costs of a wastewater treatment plant by enabling the flexibility to distribute the aeration load to periods with less expensive power prices. The performance of the proposed method is demonstrated for the operation of a single wastewater treatment plant and the strategy is backtested on Nord Pool market data from 2019. This shows modest savings of 1.15% if only the dayahead market is considered. However, if the regulating and special regulating power prices are included in the optimization the realized savings are in the magnitude of 7.23% and 27.32%, respectively. Thus this study is considered as a step towards exploiting flexibility for the benefit of reducing the combined taxation and aeration cost of wastewarer treatment.