Real time control for reduced aeration and chemical consumption: a full scale study

Quantification of energy and cost reduction from decreasing dissolved oxygen levels in full-scale water resource recovery facilities

Aeration systems often lack the efficiency to maintain a desired residual dissolved oxygen (DO) concentration in the tank in part because little consideration is given to the dynamic daily and seasonal loading conditions. Although advanced aeration controllers exist, the majority of plants have DO set points typically based on common practice and literature values rather than site-specific conditions, which can result in DO set points higher than those necessary to meet treatment objectives. DO set point reduction strategies have primarily been proposed through either static or dynamic simulations. In this study, the substantial improvements associated with DO set point reduction are demonstrated at full scale. A yearlong characterization of full-scale aeration dynamics captured the effect of diurnal and seasonal fluctuations on oxygen transfer and energy demand and so facilitated the estimation of the potential savings of DO reduction strategies. Full-scale validation provided direct evidence of DO reduction strategies inducing an overall enhancement of oxygen transfer efficiency along the different bioreactors, while confirming that energy savings as high as 20% were feasible. This study quantifies the influence of oxygen transfer efficiency on operating choices and site-specific conditions (control strategy, loading conditions, and influent flow variability). PRACTITIONER POINTS: We quantified the energy reduction and cost savings associated with a DO reduction in an aeration tank. For each 0.2 mg/L of DO decreased, the average power demand reduction per unit water treated exceeded 17%. Field measurements of dynamic alpha values eliminate the uncertainty in estimating aeration energy and cost savings from DO variations.

Dynamic air supply models add realism to the evaluation of control strategies in water resource recovery facilities

This paper introduces the application of a fully dynamic air distribution model integrated with a biokinetic process model and a detailed process control model. By using a fully dynamic air distribution model, it is possible to understand the relationships between aeration equipment, control algorithms, process performance, and energy consumption, thus leading to a significantly more realistic prediction of water resource recovery facility (WRRF) performance. Consequently, this leads to an improved design of aeration control strategies and equipment. A model-based audit has been performed for the Girona WRRF with the goal of providing a more objective evaluation of energy reduction strategies. Currently, the Girona plant uses dissolved oxygen control and has been manually optimised for energy consumption. Results from a detailed integrated model show that the implementation of an ammonia-based aeration controller, a redistribution of the diffusers, and the installation of a smaller blower lead to energy savings between 12 and 21%, depending on wastewater temperature. The model supported the development of control strategies that counter the effects of current equipment limitations, such as tapered diffuser distribution, or over-sized blowers. The latter causes an intermittent aeration pattern with blowers switching on and off, increasing wear of the equipment.

Water quality permitting: From end-of-pipe to operational strategies

End-of-pipe permitting is a widely practised approach to control effluent discharges from wastewater treatment plants. However, the effectiveness of the traditional regulation paradigm is being challenged by increasingly complex environmental issues, ever growing public expectations on water quality and pressures to reduce operational costs and greenhouse gas emissions. To minimise overall environmental impacts from urban wastewater treatment, an operational strategy-based permitting approach is proposed and a four-step decision framework is established: 1) define performance indicators to represent stakeholders' interests, 2) optimise operational strategies of urban wastewater systems in accordance to the indicators, 3) screen high performance solutions, and 4) derive permits of operational strategies of the wastewater treatment plant. Results from a case study show that operational cost, variability of wastewater treatment efficiency and environmental risk can be simultaneously reduced by at least 7%, 70% and 78% respectively using an optimal integrated operational strategy compared to the baseline scenario. However, trade-offs exist between the objectives thus highlighting the need of expansion of the prevailing wastewater management paradigm beyond the narrow focus on effluent water quality of wastewater treatment plants. Rather, systems thinking should be embraced by integrated control of all forms of urban wastewater discharges and coordinated regulation of environmental risk and treatment cost effectiveness. It is also demonstrated through the case study that permitting operational strategies could yield more environmentally protective solutions without entailing more cost than the conventional end-of-pipe permitting approach. The proposed four-step permitting framework builds on the latest computational techniques (e.g. integrated modelling, multi-objective optimisation, visual analytics) to efficiently optimise and interactively identify high performance solutions. It could facilitate transparent decision making on water quality management as stakeholders are involved in the entire process and their interests are explicitly evaluated using quantitative metrics and trade-offs considered in the decision making process. We conclude that the operational strategy-based permitting shows promising for regulators and water service providers alike.

Nitrogen Removal to Minimize Energy Consumption of the Two WWTPs Choutrana II and Menzel Bourguiba in Tunisia

The control of the aeration time and the dissolved oxygen in a reactor plays an important role in the management of the plant and the reduction of energy consumption. For this, two full-scale municipal wastewater treatment plants were chosen. The first plant (Choutrana II) designed for 600,000 population-equivalents (p.e), consists of a primary treatment step (screening, grit and grease removal), followed by four parallel oxidation ditches with a total volume of 60,000 m3. Aeration is ensured by air diffuser. The second plant (Menzel Bourguiba) designed for 140,000 p.e consists of a primary treatment stage (screening, grit and grease removal), followed by anoxic tanks and two aerobic reactors with a total volume of 14,000 m3. Aeration is ensured by six surface aerators which are operated episodically to create aerobic and anoxic conditions. Currently, the aeration management is controlled by a timer with a maintained oxygen concentration at a given level in the aerobic reactor. The nitrogen removal is done during air-on and air-off periods. The process can be seen as a succession of aeration periods followed by an anoxic period. This category of plant operation is far from the optimal and raises energy consumption and operative costs. The purpose of this study is to implement a new strategy to control aeration taking into account the variation of the daily organic load received by plants (feedforward control) and to create favorable conditions for nitrification and denitrification steps (feedback control). The Activated Sludge Model 1, which is calibrated and validated by respirometry tests, is used. Simulation results show the potentialities of the chosen strategy, which could save 28% of aeration energy and could adjust the aerobic and anoxic times. As first results, daily aeration time could be considered enough to produce treated water conform to standard terms. For Choutrana II plant, the results obtained have allowed to achieve a nitrogen removal rate of 85% and an output concentration between 6 and 12 mg/L. Rates of nitrification and denitrification were estimated at 4 mg NO3-N/L/h and 6 mg NH4-N/L/h, respectively. At the same time, aeration periods (on/off) are connected with daily organic load received by the plant and created favorable conditions for nitrification–denitrification. A good agreement between input biological loads, aeration profile and nitrogen removal is approved.

Optimal aeration control in a nitrifying activated sludge process

An important tool to minimise energy consumption in activated sludge processes is to control the aeration system. Aeration is a costly process and the dissolved oxygen level will determine the efficiency of the operation as well as the treatment results. What aeration control should achieve is closely linked to how the effluent criteria are defined. This paper explores how the aeration process should be controlled to meet the effluent discharge limits in an energy efficient manner in countries where the effluent nitrogen criterion is defined as average values over long time frames, such as months or years. Simulations have been performed using a simplified Benchmark Simulation Model No. 1 to investigate the effect of different levels of suppressing the variations of the effluent ammonium concentration. Optimisation is performed where the manipulated variable for aeration (the oxygen transfer coefficient, K(L)a) is minimised with the constraint that the average daily flow-proportional ammonium concentration in the effluent should reach a desired level. The optimisation results are compared with constant dissolved oxygen concentrations and supervisory ammonium control with different controller settings. The results demonstrate and explain how and why energy consumption can be optimised by tolerating the ammonium concentration to vary around a given average value. In these simulations, the optimal oxygen peak-to-peak amplitude range between 0.7 and 1.8 mg/l depending on the influent variation and ammonium level in the effluent. These variations can be achieved with a slow ammonium feedback controller. The air flow requirements can be reduced by 1-4% compared to constant dissolved oxygen set-points. Optimal control of aeration requires up to 14% less energy than needed for fast feedback control of effluent ammonium.