The objective of this paper is to demonstrate the importance of incorporating more realistic energy cost models (based on current energy tariff structures) into existing water resource recovery facilities (WRRFs) process models when evaluating technologies and cost-saving control strategies. In this paper, we first introduce a systematic framework to model energy usage at WRRFs and a generalized structure to describe energy tariffs including the most common billing terms. Secondly, this paper introduces a detailed energy cost model based on a Spanish energy tariff structure coupled with a WRRF process model to evaluate several control strategies and provide insights into the selection of the contracted power structure. The results for a 1-year evaluation on a 115,000 population-equivalent WRRF showed monthly cost differences ranging from 7 to 30% when comparing the detailed energy cost model to an average energy price. The evaluation of different aeration control strategies also showed that using average energy prices and neglecting energy tariff structures may lead to biased conclusions when selecting operating strategies or comparing technologies or equipment. The proposed framework demonstrated that for cost minimization, control strategies should be paired with a specific optimal contracted power. Hence, the design of operational and control strategies must take into account the local energy tariff.
The goal of this paper is to show the effect of primary sedimentation on the chemical oxygen demand (COD) and solids fractionation and consequently on the carbonaceous and energy footprints of wastewater treatment processes. Using a simple rational procedure for COD and solids fraction quantification, we quantify the effects of varying fractions on CO2 and CO2-equivalent mass flows, process energy demand and energy recovery. Then we analysed two treatment plants with similar biological nutrient removal processes in two different climatic regions and quantified the net benefit of gravity separation before biological treatment. In the cases analysed, primary settling increases the solid fraction of COD that is processed in anaerobic digestion, with an associated increase in biogas production and energy recovery, and a reduction in overall emissions of CO2 and CO2-equivalent from power importation.
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