Reducing energy demand by the combined application of advanced control strategies in a full scale WWTP

Operational optimization at the Nenäinniemi wastewater treatment plant's tertiary disc filter phosphorus removal installation to reduce chemical consumption

Instrumentation, automation and control in municipal wastewater treatment can result in large resource, cost and energy savings. Feedforward and feedback control algorithms were implemented together with turbidity and phosphorus analysers to control the chemical dose at the tertiary stage of the Nenäinniemi wastewater treatment plant, consisting of coagulation, flocculation and microsieve filtration. This optimization lowered the coagulant dose by 70% and the polymer dose by 36% compared to manual adjustments of the chemical dosing. Effluent total phosphorus (TP) and total suspended solids (TSS) concentrations were lowered by 20-30%. With the control system in operation, the annual savings in coagulant and polymer were in the range of 100 and 1.4 tons, respectively. Conducting automated CIP on the media at an economical break-even interval of approximately 20 days was also important to further lower energy usage and operational costs.

Evaluating the potential impact of energy-efficient ammonia control on the carbon footprint of a full-scale wastewater treatment plant

An assessment was performed for elucidating the possible impact of different aeration strategies on the carbon footprint of a full-scale wastewater treatment plant. Using a calibrated model, the impact of different aeration strategies was simulated. The ammonia controller tested showed its ability in ensuring effluent ammonia concentrations compliant with regulation along with significant savings on aeration energy, compared to fixed oxygen set point (DO_sp) control strategies. At the same time, nitrous oxide emissions increased due to accumulation of nitrification intermediates. Nevertheless, when coupled with the carbon dioxide emissions due to electrical energy consumption for aeration, the overall carbon footprint was only marginally affected. Using the local average CO₂ emission factor, ammonia control slightly reduced the carbon footprint with respect to the scenario where DO_sp was fixed at 2 mg·L^-1. Conversely, no significant change could be detected when compared against the scenarios where the DO_sp was fixed. Overall, the actual impact of ammonia control on the carbon footprint compared to other aeration strategies was found to be strictly connected to the sources of energy employed, where the larger amount of low CO₂-emitting energy is, the higher the relative increase in the carbon footprint will be.

Improving the quality of wastewater treatment plant monitoring by adopting proper sampling strategies and data processing criteria

Monitoring is a crucial operation for plant management. However, proper sampling procedures and data processing criteria are not always adopted. Wastewater treatment plants work under dynamic conditions, which poses a challenge for a correct performance assessment. The aim of this work is to analyse some important aspects of wastewater sampling and data processing, to identify case by case methods which should to be adopted in order to obtain reliable and consistent information on plant performance. The study was conducted through simulations and real data analyses. It turned out that: a) the preferable 24-hour composite sampling procedure is the flow-proportional mode; in addition, the required sampling frequency (i.e. the number of sub-samples to be taken to make the 24-h composite sample) increases as the percentage of population discontinuously discharging the monitored substance decreases; b) a Variability Index was defined to help find the minimum sampling frequency (i.e. the number of 24-h composite samples per year) for the calculation of annual mass flows with an acceptable uncertainty; and c) criteria were proposed for the identification of pseudo-steady state periods needed to calculate reliable mass balances and plant performance indicators.