Wastewater Treatment Plant Operation: Simple Control Schemes with a Holistic Perspective

Abstract

In this paper, a control approach for improving the overall efficiency of a wastewater treatment plant (WWTP) is presented. It consists of a cascaded control system that uses a global performance indicator as the controlled variable to drive the plant to operating conditions that satisfies trade-offs involved in the WWTP operation, improving the global performance of the plant. The selected global performance indicator is the N/E index that measures the ratio between the amount of nitrogenated compounds eliminated (kgN) and the energy (kWh) required to achieve that goal. This index links the variables of the activated sludge process with the energy consumed in the whole plant, thus the control strategy takes actions based on plantwide considerations. An external Proportional Integral (PI) controller changes the DO set point according to the N/E index and the basic dissolved oxygen (DO) control scheme in the activated sludge process follows this reference changes varying the aeration intensity. An outer loop with an event-based controller is used to compute the index values when the DO concentration is driven to excessively low limits, preventing long operation periods in this undesirable condition. Simple proportional integral controllers (PI) are used to adapt the strategy to the automation systems available in WWTPs. The implementation in the Benchmark Simulation Model 2 (BSM2) demonstrates the potential of the proposed approach. The results show the possibilities of the N/E index to be used as an indicator of global performance of WWTPs. It provides a link between water line objectives and energy consumption in the whole plant that can be exploited to introduce plantwide considerations in alternative control strategies formulated to drive the plant to operating conditions that optimize the overall process efficiency.

Life Cycle Assessment of Municipal Wastewater Treatment Processes Regarding Energy Production from the Sludge Line

“…Therefore, the overall efficiency of the WWT facility should be examined as a multifactor problem in a complex way in order to ensure that the holistic perspective is achieved [2]. Life Cycle Assessment (LCA) is a suitable tool that can be applied in this field for the evaluation of environmental aspects [3].…”

Eco-Efficiency Assessment of Control Actions in Wastewater Treatment Plants

“…WWTPs comprise several units working at different time scales: primary/secondary clarifiers for physical separation, activated sludge reactors for biological treatment, anaerobic digesters for sludge stabilization, thickener/flotation units, dewatering systems, and storage tanks. The configuration of the plant with sequential unit processes constituting the water line, the sludge line, and the gas line and numerous returns of mass flows produce an integrated system with several units interconnected in an intricate manner [2,9]. Moreover, the influent load exhibits fluctuations associated with human activity in the catchment, or rainfall in the case of combined sewage systems, and seasonal effects due to temperature and influent flowrate variations.…”

“…Recent works of the research group [2,9] address the improvement of the overall efficiency of wastewater treatment plants, considering the compromise between energy consumption, as one part of operation costs, and environmental costs. The Benchmark Simulation Model 2 (BSM2), that is a widely accepted simulation model [10], is used as a reference plant in both cases, even though the analysis and methodology could be extended to real applications.…”

“…The Benchmark Simulation Model 2 (BSM2), that is a widely accepted simulation model [10], is used as a reference plant in both cases, even though the analysis and methodology could be extended to real applications. A holistic perspective of WWTP control system performance is introduced in [9], where an optimizing PI-control strategy is used to regulate an overall efficiency index (N/E index), that represents the ratio between the amount of nitrogenated compounds eliminated (kgN) and the energy (kWh) consumed in the whole plant. Moreover, a dynamic evaluation of an index that measures the ratio between environmental benefits of WWTP, and the energy required to achieve that goal, is used to optimize the operation of the plant.…”

“…Moreover, the evaluation of dynamic performance is focused, in this case, on capturing seasonal effects of temperature on effluent quality and energy recovery from biogas. The idea of the application of a plant-wide control strategy to introduce a holistic perspective into WWTP operation strategy to improve eco-efficiency is adopted from [9]. In [9] overall performance was considered, but control actions were limited to activated sludge process (ASP).…”