Simulation-Based Assessment of the Impacts of Implementing Resource Recovery in a Full-Scale Wastewater Treatment Plant

Simulation-Based Assessment of the Impacts of Implementing Resource Recovery in a Full-Scale Wastewater Treatment Plant H.L. Ivan*, A. Shenk* * Future Energy Center, Mälardalen University, Universitetsplan 1, Västerås, 72223 SE (E-mail: [email protected]; [email protected]) Abstract As the human population continues to increase, core infrastructure such as waste management and food production must adapt to ensure sustainability. Nitrogen, phosphorus, and water can create a link between wastewater treatment plants and the agricultural industry, with one providing the raw materials for the other. The ongoing rebranding of wastewater treatment plants as “water resource recovery facilities” illustrates the shift in focus from contaminant removal to material and energy recovery. Much research today is focused on the technologies and methods to support this recovery, but less information is available regarding how recovery may affect the core operation of the wastewater treatment plant. Simulations of two recovery scenarios were conducted to investigate the effects of removing waste activated sludge and reject water on the operation of the plant considering energy consumption, effluent quality, and emissions in the biological treatment. Keywords (maximum 6 in alphabetical order) Circular economy; nitrogen recovery; water resource recovery; wastewater treatment. INTRODUCTION The wastewater industry has been undergoing a transition from focusing on treatment to focusing on resource recovery since around 2009 (Soares 2020), with recent regulatory support further driving the transition (EU 2024/1765). As urbanization and industrialization continue, freshwater sources are dwindling, and the amount of wastewater increases. The shift to resource recovery allows for not only increased efficiency and sustainability of the wastewater treatment plants (WWTPs), but also for the recovery of raw materials essential for other industries. Energy may be recovered from organic material within the plant to make the treatment process more efficient and sustainable. With proper treatment, the water can be reused in agriculture or other industries to decrease pressure on freshwater resources. Finally, nutrients like nitrogen and phosphorus can be recycled from wastewater into forms useful in agriculture, which can decrease the environmental impacts associated with mining phosphorus and the Haber Bosch process for nitrogen fertilizer production (Lorick, Harder, and Svanström 2021). Current research tends to focus on technologies and methods to recover materials and energy from wastewater treatment plant streams, but there is information lacking on the impact removing these materials has on the process. Incorporation of new technologies will change the energy and mass balance of the wastewater treatment plants, and there will likely be a trade-off between the benefits (better effluent quality, more biogas production) and the drawbacks (energy use, emissions). This work utilizes simulations and extrapolation of assumptions where necessary to provide a preliminary investigation into the effects of two different wastewater recovery facility schemes: (1) reject water extract and pyrolysis of digestate and (2) pyrolysis of waste activated sludge. The biochar material from pyrolysis and the reject water will be used as fertilizing material at an agricultural site on the wastewater treatment plant property, thus eliminating the cost for transportation of the material. MATERIALS AND METHODS The BSM2G plant-wide model (Flores-Alsina et al. 2014) was used to simulate the WWTP and evaluate the impact of recovering materials from the desired process streams on the performance of the process. This model has the same layout as the BSM2, described by Gernaey (2014), but utilizes the ASMN model (Hiatt and Grady 2008) for modelling the biological processes and includes estimations for greenhouse gas emissions in several parts of the process. More details can be found in Flores-Alsina et al. (2011). Two sets of simulations were performed, each simulation starting from the same conditions and simulating 609 days of operation. The first set involved the recovery of different percentages of the reject water stream, effectively reducing the flow rate of reject water returning to the influent, while the second involved recovering different percentages of the waste activated sludge (WAS) stream resulting in a reduced flow rate of WAS entering the thickener and being digested. Evaluation parameters integrated into the model were used, such as the effluent quality index (EQI), in combination with additional data to evaluate the impacts of the proposed modifications. The total direct secondary treatment emissions were calculated as the sum of the equivalent CO2 emissions generated in biomass respiration, BOD oxidation, and denitrification and the equivalent CO2 consumed in nitrification as in Flores-Alsina et al. (2011). All parameters were reported as averages over the evaluation period. The energy consumption in the WWTP was calculated as the sum of the aeration energy, mixing energy, pumping energy, and heating energy, while the energy generated in the WWTP was taken as the energy produced from the biogas in the digester. These energy terms were used as defined in Gernaey (2014). When recovering reject water, it was considered that the water and its nitrogen content would replace traditional irrigation water and fertilizer, and thus the energy required to produce these. An average energy cost of 0.57 kWh per m3 drinking water was assumed based on performance of the drinking water plant in Sweden (Eriksson 2023), which aligns with average energy consumption in other countries (Plappally and Lienhard V 2012). The energy requirements for producing Haber Bosch nitrogen fertilizers were taken as 30 GJ per ton NH3 (Rouwenhorst et al. 2020). Additional energy required to process the sludge to produce biochar is mainly the energy to dry the sludge which depends on the water content, and the energy to pyrolyze. These values depend on the composition of the sludge and are not yet available to be included in this analysis. RESULTS AND DISCUSSION The extraction, or recovery, of reject water (RW) from the WWTP resulted in a decrease in the energy consumed by the WWTP, without significant changes to the energy generated. This can be seen in Figure 1 for different extraction volumes. These changes are predominantly due to a decrease in the aeration energy required in the secondary biological treatment, while the sludge production is largely unaffected. Also shown in Figure 1 is the potential energy that can be saved by utilizing the RW for irrigation and fertilization; the drinking water (DW) energy savings are negligible while the energy required to produce equivalent amounts of nitrogen using the Haber Bosch (HB) process can approach 10GJ·day-1. In Sweden an average of 84 kg N fertilizer per hectare was used in 2021 (“Nitrogen Fertilizer Use per Hectare of Cropland,” n.d.), the nitrogen recovered by utilizing 90% of the RW stream could cover roughly 1000 hectares. Looking beyond the energy flows, in the case of 90% RW recovery the total direct secondary treatment emissions are reduced by 19% from the baseline case with no recovery, while the EQI is reduced by roughly 17%. This shows that extracting the RW has no obvious negative effects on the ability of the WWTP to perform its primary objective. A similar analysis was conducted for the scenario considering the removal of waste activated sludge (WAS), where the changes in energy consumption and generation in the WWTP can be seen in Figure 2. In this case the energy consumed does not vary significantly, some small changes to the aeration and heating energy requirements contribute to the minor decrease, while the energy generated decreases by larger amount as the amount of WAS recovered from the WWTP increases. The removal of WAS reduces the amount of sludge that is digested, and subsequently the amount of energy that can be obtained from the production of biogas. The impact of recovering WAS on the biological treatment is minimal, with 7% reduction in EQI and 10% reduction in total direct secondary treatment emissions when 90% of WAS is recovered. Figure 1: Energy terms in the WWTP and potential conservation for increasing RW recovery. Figure 2: Energy terms in the WWTP for increasing WAS recovery percentages. In both scenarios, biochar is produced for food production as a solid fertilizer. In the first scenario, where RW is recovered, digested sludge is the feedstock for pyrolysis. In the second scenario both digested sludge and WAS would be processed into biochar, biogas, and bio-oil. The energy required by, and produced from, drying and pyrolysis is not yet included in these results as composition of the sludge streams are still being analyzed. The largest energy requirement in the pyrolysis of sewage sludge is the drying, thus the energy efficiency is closely tied to the solids content. Self-sustainable operation of pyrolysis is possible up to 78% water content when utilizing the bio-oil and biogas for the drying and heating requirements (Barry et al. 2019). It has been suggested that a combined system of anaerobic digestion with pyrolysis could produce a surplus of energy at an organic content of 70% in the sludge (Li and Feng 2018). Upon further analysis of the sewage sludge streams, these energies will be calculated for the simulated scenarios to more concretely address the question of the energy balance of the system. CONCLUSION From the preliminary results it appears that materials in the form of WAS and RW may be recovered from the WWTP without negatively impacting the quality of the water treatment. Recovering RW decreased the energy requirements of the plant and did not meaningfully affect the energy produced. The recovery of WAS, however, reduced the energy generated from the anaerobic digestion more than it reduced the energy consumed in the WWTP. Utilizing digested sludge does not affect the WWTP as the material is recovered at the point where it would be transported. 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