Modeling nutrient removal and energy consumption in an advanced activated sludge system under uncertainty.

Activated sludge models are widely used to simulate, optimize and control performance of wastewater treatment plants (WWTP). For simulation of nutrient removal and energy consumption, kinetic parameters would need to be estimated, which requires an extensive measurement campaign. In this study, a novel methodology is proposed for modeling the performance and energy consumption of a biological nutrient removal activated sludge system under sensitivity and uncertainty. The actual data from the wastewater treatment plant in Slupsk (northern Poland) were used for the analysis. Global sensitivity analysis methods accounting for interactions between kinetic parameters were compared with the local sensitivity approach. An extensive procedure for estimation of kinetic parameters allowed to reduce the computational effort in the uncertainty analysis and improve the reliability of the computational results. Due to high costs of measurement campaigns for model calibration, a modification of the Generalized Likelihood Uncertainty method was applied considering the location of measurement points. The inclusion of nutrient measurements in the aerobic compartment in the uncertainty analysis resulted in percentages of ammonium, nitrate, ortho-phosphate measurements of 81%, 90%, 78%, respectively, in the 95% confidence interval. The additional inclusion of measurements in the anaerobic compartment resulted in an increase in the percentage of ortho-phosphate measurements in the aerobic compartment by 5% in the confidence interval. The developed procedure reduces computational and measurement efforts, while maintaining a high compatibility of the observed data and model predictions. This enables to implement activated sludge models also for the facilities with a limited availability of data.

Comprehensive evaluation of the carbon footprint components of wastewater treatment plants located in the Baltic Sea region.

Finland and Poland share similar environmental interests with regard to their wastewater effluents eventually being discharged to the Baltic Sea. However, differences in the influent wastewater characteristics, treatment processes, operational conditions, and carbon intensities of energy mixes in both countries make these two countries interesting for carbon footprint (CF) comparison. This study aimed at proposing a functional unit (FU) which enables a comprehensive comparison of wastewater treatment plants (WWTPs) in terms of their CF. Direct emissions had the highest contribution (70%) to the total CF. Energy consumption dominated the total indirect emissions in both countries by over 30%. Polish WWTPs benefitted more from energy self-sufficiency than Finnish plants as a result of higher electricity emission factors in Poland. The main difference between indirect emissions of both countries were attributed to higher chemical consumption of the Finnish WWTPs. Total pollution equivalent removed (TPErem) FU proposed enabled a better comparison of WWTPs located in different countries in terms of their total CF. High correlations of TPErem with other FUs were found since TPErem could balance out the differences in the removal efficiencies of various pollutants. Offsetting CF was found a proper strategy for the studied WWTPs to move towards low-carbon operation. The studied WWTPs could reduce their CF from up to 27% by different practices, such as selling biofuel, electricity and fertilizers. These findings are applicable widely since the selected WWTPs represent the typical treatment solutions in Poland, Finland and in the Baltic Sea region.

Energy neutrality versus carbon footprint minimization in municipal wastewater treatment plants.

This work aimed to compare the carbon footprint (CF) of six full-scale wastewater treatment plants (WWTPs). The CF was estimated in the range of 23-100 kg CO2e per population equivalent. In the total CF, the direct emissions held the highest share (62-74%) for the plants with energy recovery from biogas. In the plants depending entirely on the power grid, the indirect emissions due to energy consumption dominated the total CF (69-72%). The estimated CF was found highly sensitive towards the choice of N2O emission factors. A dual effect of external substrates co-digestion on the CF has been presented. After co-digestion, the overall CF decreased by 7% while increasing the biogas production by 17%. While applying the empirical model, the level of energy neutrality was strongly related to the ratio of the indirect to direct emissions.

Strategies for mitigating nitrous oxide production and decreasing the carbon footprint of a full-scale combined nitrogen and phosphorus removal activated sludge system.

Nitrous oxide (N2O) emitted from biological nutrient removal activated sludge systems contributes significantly to the total carbon footprint of modern wastewater treatment plants. In the present study, N2O production and emissions were experimentally determined in a large-scale plant (220,000 PE) employing combined nitrogen (N) and phosphorus (P) removal. As a modelling tool, the Activated Sludge Model 2d (ASM2d) was extended with modules describing multiple N2O production pathways and N2O liquid-gas transfers. The new model was calibrated and validated using the results of laboratory experiments and full-scale measurements. Different operational strategies were evaluated following the proposed model-based procedure. Heterotrophic denitrification was found to be the predominant pathway of N2O production under both anoxic and aerobic conditions. This behaviour could primarily be attributed to the predominant abundance of heterotrophic denitrifiers over nitrifiers. Simulations revealed that the optimal solution for minimizing liquid N2O production is to set the dissolved oxygen concentration in the aerobic zone from 1 to 2 mg O2/L and to enhance the mixed liquor recirculation rate (MLR) (>500% of the influent flowrate) while not compromising effluent standards. Regarding the actual conditions, the potential reduction in the carbon footprint was estimated to be 10% by applying the proposed operational strategy. The results suggest that considerable improvements can be achieved without substantial upgrades and increased costs.

Strategies for achieving energy neutrality in biological nutrient removal systems - a case study of the Slupsk WWTP (northern Poland).

The paper presents a model-based evaluation of technological upgrades on the energy and cost balance in a large biological nutrient removal (BNR) wastewater treatment plant (WWTP) in the city of Slupsk (northern Poland). The proposed upgrades include chemically enhanced primary sludge removal and reduction of the nitrogen load in the deammonification process employed for reject water treatment. Simulations enabled to estimate the increased biogas generation and decreased energy consumption for aeration. The proposed upgrades may lead the studied WWTP from the energy deficit to energy neutrality and positive cost balance, while still maintaining the required effluent standards for nitrogen. The operating cost balance depends on the type of applied coagulants/flocculants and specific costs of electric energy. The choice of the coagulant/flocculent was found as the main factor determining a positive cost balance.