Development of a process model and life cycle assessment for a large water resource recovery facility and comparison of biosolids process upgrade options

Life cycle assessment (LCA) (ISO 2020) is a valuable tool to assess environmental performance and can form the basis for the environmental criteria when comparing alternatives based on triple-bottom-line principles. Conducting an effective LCA requires the development of a life cycle inventory (LCI) for the alternatives under consideration. The LCI for existing systems can be developed based on existing data, but this becomes difficult when comparing alternatives where sufficient data for all the alternatives under consideration might not be available. Modeling of the alternatives is one viable method to develop the LCI to the level of detail needed for sufficient life cycle impact assessment (LCIA) of each alternative (that has substantiative differences in environmental performance).

Process modeling is routinely applied to size, characterize, and quantify the important commodity inputs for wastewater treatment systems such as energy (electricity and natural gas) and chemicals (Fernandez-Arevalo et al. 2017; Solon et al. 2017; Iqbal et al. 2020). These results can form the basis for the development of appropriate LCI's when wastewater management systems are being evaluated, and thus supporting the LCA's used for characterizing their relative environmental performance (Ko et al. 2024). Models for the alternatives under consideration can provide data needed for cost analyses and performance evaluations while also providing the LCI input–output data needed to conduct an LCA (Wang et al. 2012; Corominas et al. 2013; de Faria et al. 2015; Meneses et al. 2015; Iqbal et al. 2020; Lam et al. 2022). Ko et al. (2024) provide an extensive review of LCA and process modeling related to wastewater management decision-making.

The Great Lakes Water Authority (GLWA) is currently considering options to upgrade the biosolids management system at their water resource recovery facility (WRRF). In support of this effort, the University of Michigan is conducting an assessment of the decision-making processes that can be used for GLWA's organizational decision-making. Preliminary results indicate that coupling process modeling with LCA can provide a solid basis for the environmental component when assessing upgrade options in a sustainability framework (Ko et al. 2024). In this paper, we present the results of using a plant-wide model for the GLWA WRRF, one of the largest treatment plants in the world, to develop the LCI used for subsequent LCIA analysis comparing biosolids treatment alternatives.

The biosolids management system consists of thickening, dewatering, and either drying or incineration. Primary clarifier sludge (PCS) and waste activated sludge (WAS) are separately thickened in gravity thickener complexes A and B, respectively. The thickened sludges are blended. Approximately three-quarters of the biosolids are dewatered and processed in a rotary drum drying facility (biosolids drying facility, BDF) producing pellets for land application. The remaining biosolids are dewatered and incinerated in multiple hearth units, with the ash transported to a landfill. A small portion (approximately 8%) of the dewatered sludge also undergoes lime treatment and is directly transferred to the landfill.

The characteristics of the GLWA WRRF influent and effluent for the period from April 2017 through December 2019 are summarized in Table 1. The effluent limits for the GLWA WRRF are 25 mg/L for 5-day biochemical oxygen demand (BOD₅), 30 mg/L for total suspended solids (TSS), and total phosphorus (TP) of 0.6 mg P/L for the summer (April–September) and 0.7 mg P/L during the winter (October–March), all on a monthly average basis.

GLWA had previously evaluated options to improve future biosolids management to achieve three primary objectives: (1) address ageing incinerators, (2) minimize the amount of biosolids landfilled, and (3) reduce greenhouse gas (GHG) emissions. Maximizing energy and resource recovery in the sludge treatment process was also a core objective. Various biosolids management technologies, such as anaerobic digestion and thermal hydrolysis process (THP), were proposed and reviewed (CDMSmith 2020). Five biosolids management options are evaluated here, relative to the baseline of the existing system, including four anaerobic digestion options, along with composting, to provide an additional conventional alternative (Table 2). Further details on these options are presented in the Supplemental Material.

Introducing anaerobic digestion can significantly improve biosolids management by reducing the mass of biosolids to be managed and creating an energy source in the form of biogas production. THP supplements anaerobic digestion, as a pre-digestion step, to increase biosolids destruction and enhance biogas production. Phosphorus recovery via struvite production in the side stream, which can recover phosphorus and reduce the amount recycled to the liquid process stream, is also enhanced by anaerobic digestion through the release of phosphorus from the digested biosolids. Alternative 2 uses anaerobic digestion and recovers phosphorus from the digester centrate/filtrate through struvite formation. Alternative 3 introduces THP to increase biogas production and further reduce the final volume of biosolids. Alternative 4 involves anaerobic digestion of only the more digestible PCS (compared to the less digestible WAS) to minimize construction costs but with less biogas production and sludge volume reduction. Anaerobic digestion of only PCS also reduces phosphorus release in the digester and makes phosphorus recovery in the side stream unnecessary, as the plant-wide model results verified. Alternative 5 pretreats only WAS with THP to increase its biodegradability and subsequent biogas production in anaerobic digestion. The advantages and disadvantages of anaerobic digestion and THP are presented in Table 3 (Metcalf et al. 2014; Barber 2016; Urgun-Demirtas et al. 2022). Alternative 1, which replaces incineration with composting to maximize use of the drying facility and manages the remaining biosolids through composting, is included to provide an alternative, conventional option.

A process model of the existing GLWA WRRF was developed using SUMO21 software and calibrated to the existing facility to characterize the baseline scenario and to simulate the alternative scenarios (Dynamita 2019). SUMO2S was used as it includes simulation of the sulfur and iron cycles, along with phosphorus removal via ferric chloride addition, and the reactions of sulfur and metals in anaerobic digesters. The model was calibrated using routine daily operating data over a 2-year and 9-month period from April 2017 to December 2019. A prior model of the HPO-activated sludge system developed in SUMO21 to characterize biological phosphorus removal (Jun et al. 2023) and on detailed wastewater characterization conducted in 2017 and 2018 (Yang et al. 2019) was imported into SUMO2S and expanded to incorporate the entire plant. Further details on the calibrated model are provided in the Supplemental Material.

The system boundary used for the LCA was ‘gate to grave,’ as shown in Figure 1 (companion process flow diagrams for the five alternatives evaluated are presented in the Supplemental Material). The liquid boundary included wastewater treatment from the entry of influent wastewater to the discharge of treated effluent. The sludge stream boundary encompassed the whole sludge treatment and disposal system, from sludge generation to the disposal of biosolids in the landfill and the land application of the dried sludge pellets. Although not shown in Figure 1, electricity consumption by the influent pump station was included, but vehicles and fuel required for applying the pellets to farmland in land application were excluded due to a lack of data. This study focused only on the operation phase. Considering that the lifespan of a WRRF is 50–100 years, there is a proportionally minor environmental impact resulting from the construction and end-of-life phases. As reported by Corominas et al. (2020), the environmental impacts of wastewater treatment plants (WWTPs) are concentrated in the operation phase for most treatment technologies, except for options such as constructed wetlands. Rebello et al. (2021) reviewed 111 papers and reported that 57% considered only the operation phase. A total of 1 m³ of wastewater treated was selected as the functional unit, consistent with others, as Rebello et al. (2021) reported that 66% of WRRF LCA studies used volume of wastewater as the functional unit.

LCI

Foreground information collected for the LCI was obtained from GLWA WRRF operation data and process modeling results, while background data were gathered from the USLCI and Ecoinvent 3 databases. The LCI was constructed in three stages: (1) process model calibration of the current system (baseline) using GLWA WRRF operation data, (2) acquisition of foreground information for alternatives using the plant-wide simulation results, and (3) utilization of literature data and background information from USLCI and Ecoinvent 3. Specific data sources and background databases are presented in Table 4 and the Supplemental Material. The heat requirement for anaerobic digestion was calculated using the method demonstrated by Metcalf et al. (2014). The amount of bulking agent used in composting was 380 kg/ton, based on Rostami et al. (2020). The inventory from Lam et al. (2022) was used to estimate the LCI for the land application of biosolids pellets produced in drying facilities, as presented in the Supplemental Materials. GHG emissions were calculated using IPCC protocols (IPCC 2019). Nitrous oxide (N₂O) emissions from the liquid process stream were assumed to be negligible as the HPO-activated sludge system is operated at a relatively low solids residence time (SRT) of 2.5 days and does not generally nitrify. Further information on the development of the LCI is presented in the Supplemental Material.

LCIA

TRACI 2.1 V1.06/US 2008 was used to calculate impacts. This method can be used to examine several impact categories; in this analysis, the primary focus was evaluating global warming, eutrophication, carcinogenics, ecotoxicity, respiratory effects, and fossil fuel depletion. The TRACI method can also facilitate the analysis of ozone depletion, smog, acidification, and non-carcinogenics. SimaPro 9.3.0.3 was the LCA software used to assess the environmental impacts of both the current GLWA WRRF baseline and the alternatives.

Water and wastewater systems use 3–4% of the total energy in the US and emit 45 million tons of GHGs annually (EPA 2013). In Michigan, wastewater treatment uses 55% of the energy in water and wastewater systems (MWEA 2017). As a result of this substantial electricity consumption, the energy mix of the regional electrical supply has a significant contribution to the overall environmental impact of a WRRF and is crucial to consider in an LCA. Detroit Edison (DTE) Energy, which supplies electricity to the GLWA WRRF, plans to reduce CO₂ emissions by 85% by 2035, aiming for net zero by 2050 (DTEEnergy 2022). Therefore, a sensitivity analysis of the alternatives was conducted based on DTE's projected energy mix, as shown in Table 5. The 2022 energy mix statistics were obtained from the DTE webpage (DTEEnergy 2023). The energy mix predictions from 2027 to 2050 were established using data from the 2022 DTE Electric Integrated Resource Plan (DTEEnergy 2022). The sensitivity analysis in this study examined how electricity energy mix changes would influence the relative life cycle environmental impacts of the biosolids management alternatives evaluated.

Table 6 summarizes the principal outputs for the baseline and the five alternatives. Additional results for direct GHG emissions, energy, and material consumption are provided in the Supplemental Materials. Plant effluent values for TSS and BOD₅ meet NPDES permit requirements (data are not shown) and, as indicated in Table 6, effluent TP values also comply with the NPDES permit values of 0.6 mg P/L (April–September) and 0.7 mg P/L (October–March). While phosphorus was released during anaerobic digestion for Alternatives 2, 3, and 5, the released phosphorus is recovered in the form of struvite, which reduces the concentration of P in the return flow to the liquid stream. In Alternative 4, only PCS flows to the anaerobic digesters, and WAS is sent directly to the dewatering process after thickening. An aerobic basin is provided in this alternative, upstream of the WAS thickener, to limit P release, thereby reducing the concentration of P in the overflow and in the return flow. Consequently, the effluent TP concentration for all alternatives is estimated to be similar to or lower than the baseline, complying with NPDES limits.

Elimination of incineration for the composting alternative (Alternative 1) resulted in a 20% reduction in global warming impacts (Figure 3). Implementation of anaerobic digestion (Alternatives 2–5) resulted in an additional reduction in global warming impact of about 20% due to the production of biogas, biosolid volume reduction from anaerobic digestion thereby reduced energy consumption in the BDF, and reduced N₂O emissions from decreased land application (Figure 4(a)). The total reduction in global warming impacts for the digestion alternatives was generally about 40% compared to the baseline, with modest differences between the four digestion alternatives.

Eutrophication impacts were reduced modestly for all alternatives compared to the baseline, with the largest reduction for the anaerobic digestion alternatives (Alternatives 2–5). These predicted reductions were largely a result of predicted reductions in effluent TP discharges (Table 6), with modest contributions from the elimination of the incinerators and reduced land application for the anaerobic digestion alternatives (Figure 4(b)).

Elimination of incineration resulted in a reduction in respiratory effects, while biogas recovery from anaerobic digestion generated an impact reduction for Alternatives 2–5 (Figure 4(e)). Elimination of incineration also resulted in a reduction of fossil fuel depletion impacts (Figure 4(f)).

A hotspot analysis graphically presents the contribution of each unit process within the facility to each impact category. Hotspot analysis for the baseline presented in Figure 5 highlights the important role of the elimination of incineration in reducing impacts for all categories except eutrophication. Incineration is a significant contributor to all impact categories except eutrophication. Drying and land application are also significant contributors to global warming, eutrophication, respiratory effects, and fossil fuel depletion. Reductions in biosolids volumes by anaerobic digestion also resulted in reduced drying and land application, which contributed to impact reductions in several categories.

As discussed above, the principal objectives GLWA was seeking to achieve by modifying the biosolids management system at their WRRF are to (1) address ageing incinerators, (2) minimize the amount of biosolids landfilled, and (3) reduce GHG emissions. All five alternatives evaluated achieve the first objective. All five alternatives also reduce GHG emissions (Figure 3, expressed here as global warming potential), but the greatest reduction is achieved with the anaerobic digestion alternatives (Alternatives 2–5). The anaerobic digestion alternatives also reduce the total mass of solids needing to be disposed of (Figure 2(a)), reduce net fuel consumption, and generally achieve reduced electrical energy consumption similar to the composting alternative (Alternative 1) relative to the baseline (Figure 2(c)). Some of the anaerobic digestion alternatives also offer the possibility to recover phosphorus, for example, as struvite (Figure 2(d)) (Lam et al. 2022). The LCIA results (Figure 3) indicate that anaerobic digestion provides superior performance to the baseline and composting (Alternative 1) for several other environmental impact categories (Hospido et al. 2005; Xue et al. 2019; Morelli et al. 2020). This indicates that the GHG reduction benefit offered by a transition to anaerobic digestion does not require a tradeoff between GHG emission reductions and the potential for increases in other environmental impacts. In short, anaerobic digestion provides several advantages over the range of factors considered in this analysis and no disadvantages.

Several anaerobic digestion alternatives were evaluated in this analysis, including varying the sludges digested (PCS, WAS, or both) and whether THP is included. While modest differences were noted between these options, the results did not identify a systematic advantage for any of the anaerobic digestion alternatives that were evaluated. All achieve the objectives of allowing the ageing incinerators to be retired from service, coupled with reduced GHG emissions and reduced volume of ultimate solids requiring disposal. Some of the anaerobic digestion alternatives offer other advantages. The anaerobic digester volume required to digest PCS alone (Alternative 4) is about half the volume required to digest both PCS and WAS (Alternative 2), thereby reducing capital and operating costs substantially. Likewise, the anaerobic digester volume required to digest PCS alone (Alternative 4) is similar to that needed with THP to digest both PCS and WAS (Alternative 2), thereby reducing capital and operating costs because pre-dewatering and THP are avoided. Even less digester volume is required if WAS alone is treated with THP and anaerobic digestion (Alternative 5), but pre-dewatering and THP facilities are still needed. An economic analysis would be required to determine the relative cost-effectiveness of digesting only PCS (Alternative 4) or processing WAS only through THP and digestion (Alternative 5). In addition, an assessment of the relative quality of the dried product between the alternatives would also be needed. In any event, the results of the evaluation indicate that factors other than environmental impact, such as cost and product quality, are likely to influence the selection of the specific anaerobic digestion option implemented (Kamble et al. 2019; Diaz-Elsayed et al. 2020).

The fact that all four anaerobic digestion alternatives meet the core objectives outlined by GLWA, and with negligible differences in environmental impacts, suggests a phased implementation strategy. A facility allowing for the digestion of both PCS and WAS could be laid out, along with space allocated to THP. Then, the laid-out components of the overall facilities could be implemented depending on GLWA's needs and objectives. For example, initially, GLWA might only construct the volume needed to digest PCS. Digested PCS would then be blended with thickened WAS prior to dewatering and drying. Such a facility could be upgraded in the future to allow digestion of both PCS and WAS by adding either additional digesters (possible since space was allocated for the possibility) or by adding THP. Alternatively, GLWA could implement facilities to treat WAS through THP and digestion, and the THP and digestion facilities could be expanded in the future if THP treatment and digestion of both sludge streams became desirable. The initial design of the facility could be developed before a final decision was made concerning the facilities to be constructed in the initial phase. The potential for future upgrades is preserved with this approach because sufficient space is allocated for all options, and the design accommodates later additions.

LCA is an effective tool to assess the relative environmental performance of alternatives. This case study assessed the combination of process modeling to develop the LCI needed to complete an LCIA for five biosolids management options in comparison to the existing (baseline) process at the GLWA WRRF. The overall objectives of the biosolids management upgrade are to (1) address the impacts of the ageing incinerators at the facility, (2) minimize the amount of biosolids landfilled, and (3) reduce GHG emissions. All five alternatives considered (composting and four anaerobic digestion alternatives) were able to meet these biosolids management system upgrade objectives.

• 1. Analysis of the mass and energy balances indicates advantages for all five alternatives, including solids mass reduction for the four anaerobic digestion alternatives (modest increase for the composting alternative 0, net fuel consumption reduction (greater for anaerobic digestion alternatives), and net reduction in electricity for sludge treatment compared to the baseline. Phosphorus recovery through struvite production was possible for the anaerobic digestion options where WAS would be digested.

• 2. Assessment of environmental impacts based on six impact categories (global warming, eutrophication, carcinogenics, ecotoxicity, respiratory effects, and fossil fuel depletion) indicated reduced impacts for all alternatives for all categories compared to the baseline. Impact reductions were less for the composting alternative compared to the anaerobic digestion alternatives but were similar for the four anaerobic digestion alternatives. Both the mass and energy balance and environmental impact assessment indicated consistent advantages for the anaerobic digestion alternatives compared to both the baseline and the composting alternatives.

• 3. Similarity of the mass and energy balances and environmental impacts for the four anaerobic digestion alternatives evaluated suggests that the selection of the specific anaerobic digestion option to be implemented can be based on other factors, such as cost and operational factors.

• 4. Coupling mass and energy balances based on process modeling and environmental impact assessment based on LCA allowed the development of a phased approach to implement anaerobic digestion at the GLWA WRRF.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.