Modelling to Lower Energy Consumption in a Large WWTP in China While Optimising Nitrogen Removal

Abstract

In the last decade, China has sharply tightened the monitoring values for wastewater treatment plants (WWTPs). In some regions with sensitive discharge water bodies, the values (24 h composite sample) must be 1.5 mg/L for NH4-N and 10 mg/L for total nitrogen since 2021. Even with the previously less strict monitoring values, around 50% of the wastewater treatment plants in China were permanently unable to comply with the nitrogen monitoring values. Due to the rapid changes on-site to meet the threshold values and the strong relation to energy-intensive aeration strategies to sufficiently remove nitrogen, WWTPs do not always work energy-efficiently. A Chinese WWTP (450,000 Population equivalents or PE) with upstream denitrification, a tertiary treatment stage for phosphorus removal and disinfection, and aerobic sludge stabilisation was modelled in order to test various concepts for operation optimisation to lower energy consumption while meeting and undercutting effluent requirements. Following a comprehensive analysis of operating data, the WWTP was modelled and calibrated. Based on the calibrated model, various approaches for optimising nitrogen elimination were tested, including operational and automation strategies for aeration control. After several tests, a combination of strategies (i.e., partial by-pass of primary clarifiers, NH4-N based control, increase in the denitrification capacity, intermittent denitrification) reduced the air demand by up to 24% and at the same time significantly improved compliance with the monitoring values (up to 80% less norm non-compliances). By incorporating the impact of the strategies on related processes, like the bypass of primary settling tanks, energy consumption could be reduced by almost 25%. Many of the elaborated strategies can be transferred to WWTPs with similar boundary conditions and strict effluent values worldwide.