Evaluating environmental effects of different anaerobic wastewater treatment systems by using LCA approach

Abstract

Enhancing effluent quality has been the primary goal of wastewater treatment (WWT) since the invention of the activated sludge technique in 1914. As stricter discharge limitations are enforced to save the environment and public health, this focus has changed. One potential way to mitigate the environmental implications of WWT systems, including energy consumption, solid waste creation, and greenhouse gas emissions, is to implement a better method that includes the recovery of resources obtained from wastewater. However, the environmental implications and advantages of enacting such a significant shift remain unclear and require extensive examination. By eliminating contaminants, the discipline of WWT is thus evolving into a phenomenon that encompasses the pursuit of resource recovery and a circular economy. By assessing the environmental sustainability of novel technologies and procedures, life cycle assessment, or LCA, is vital to this developing phenomenon. As the first step of this thesis, a comprehensive literature review was conducted. Although numerous LCA studies on wastewater treatment systems have been published in recent years, none of the existing reviews have focused specifically on anaerobic treatment applications. In this section, LCA studies addressing anaerobic wastewater treatment were examined, with the structure organized as follows: first, publication trends in LCA and wastewater treatment were summarized, highlighting the increase in studies since 2000. Subsequently, the reviewed studies were analyzed according to the methodological stages of LCA, namely goal and scope definition, life cycle inventory (LCI), life cycle impact assessment (LCIA), and life cycle interpretation. Thereafter, existing LCA studies and economic assessments of anaerobic treatment systems were discussed, followed by perspectives for future research in this field. The reviewed studies indicate that anaerobic wastewater treatment systems exhibit lower environmental impacts in most categories compared to conventional systems. Owing to their energy recovery mechanisms, anaerobic systems generally require less energy than aerobic systems. However, if dissolved methane in anaerobic effluents is not captured, it significantly contributes to global warming potential (GWP) and increases toxicity. Similarly, if nutrient recovery is not implemented following anaerobic treatment—for instance through aeration systems or alternative methods—eutrophication potential (EP) may rise. Another contributor to increased EP is the direct discharge of anaerobic effluents into receiving waters without reuse. While most existing LCA studies compare a particular anaerobic process with conventional activated sludge (CAS) systems, studies comparing anaerobic technologies with each other remain scarce. Furthermore, integrating LCA with additional tools such as techno-economic analysis (TEA) and social assessments is considered essential for more comprehensive evaluations. The detailed literature review also revealed a lack of LCA studies on anaerobic forward osmosis (AnFO) systems, which have gained prominence in recent years. For this reason, the second part of the thesis focuses on these systems. In the second part of the thesis, a comprehensive life cycle assessment (LCA) was performed for an Anaerobic Forward Osmosis Membrane Bioreactor (AnFOMBR) treating municipal wastewater. The main objective of this study was to evaluate the environmental impacts of different draw solution (DS) types and concentrations and to determine the optimal operating conditions that balance treatment efficiency with environmental sustainability. Conducted in accordance with ISO 14040 and 14044 standards, this LCA aimed to quantify the environmental burdens associated with different DS scenarios and to provide insights into the design and operation of forward osmosis (FO)-based wastewater treatment systems. The impact categories assessed included global warming potential (GWP), ozone depletion potential (ODP), terrestrial acidification (AP), freshwater and marine eutrophication (FEP and MEP), human toxicity (HT), and ecotoxicity. The first comparative analysis in this section examined the environmental performance of NaCl and MgCl₂ at 0.5 M and 1 M concentrations. The results showed only minor differences between these draw solutions. In terms of GWP, MgCl₂ consistently exhibited slightly higher values than NaCl, primarily due to its more energy-intensive production and transportation; however, the differences were negligible. This finding suggests that draw solution choice exerts little influence on GWP, while factors such as sludge management, membrane production and disposal, and energy demand are more decisive. Regarding ODP, both solutions had minimal and comparable impacts, indicating that the chemicals used in the AnFOMBR system do not significantly contribute to stratospheric ozone depletion. By contrast, terrestrial acidification effects were more pronounced, with MgCl₂ showing slightly higher values due to its greater energy requirements during production. Differences became more evident in eutrophication categories: high-concentration NaCl (1 M) resulted in the highest freshwater and marine eutrophication impacts, whereas NaCl at 0.5 M had the lowest. While biogas recovery partially mitigated these effects by offsetting synthetic fertilizer use, phosphorus and nitrogen release remained critical concerns. In the human toxicity categories, MgCl₂—particularly at 1 M—caused the highest impacts in both carcinogenic and non-carcinogenic toxicity, followed by MgCl₂ at 0.5 M, whereas NaCl at 0.5 M showed the lowest impact. These differences were linked to the resource- and emission-intensive production processes of MgCl₂. Overall, the choice of draw solution (NaCl vs. MgCl₂, 0.5 M vs. 1 M) was found to influence certain categories such as eutrophication and human toxicity, while exerting negligible effects on GWP and ODP. In the second analysis of this section, it the comparison was expanded to four draw solutions: NaCl, MgCl₂, sodium sulfate (Na₂SO₄), and monoammonium phosphate (MAP, NH₄H₂PO₄). Results showed that MgCl₂ use led to approximately 6.5% higher GWP than NaCl (increasing from 0.492 to 0.527 kg CO₂-eq), whereas NaCl showed a 2.7% decrease across concentrations. ODP values remained consistently low across all scenarios, with no significant differences. Acidification potential (AP) was 28–40% higher for MgCl₂ and Na₂SO₄ compared to MAP, primarily due to sulfur-related emissions and fossil fuel-based energy use. Nutrient-related impacts were most pronounced in MAP and MgCl₂ scenarios, driven by phosphorus and nitrogen concentrations in the effluent; among NaCl and MgCl₂, NaCl at 1 M yielded the highest eutrophication potential. Furthermore, MgCl₂ presented 15–20% higher human toxicity impacts than NaCl, linked to its resource-intensive production processes and the chemical use (e.g., EDTA, acetonitrile) in membrane cleaning. Energy consumption emerged as a critical factor: for most draw solutions, system energy demand was 2.43 kWh/m³ due to regeneration, while MAP required only 0.4 kWh/m³, eliminating the need for regeneration and thereby lowering GWP. However, this advantage came at the expense of 25–30% higher human toxicity and freshwater ecotoxicity, attributable to nutrient-related effects. These results highlight the trade-off between energy efficiency and environmental health risks in draw solution selection. The third part of the thesis presents a comparative LCA of three anaerobic wastewater treatment technologies—AnMBR, AnFOMBR, and UASB—using the ReCiPe midpoint method. Operational inputs, emissions, waste generation, and credits from energy and nutrient recovery were assessed. The results demonstrated that AnFOMBR achieved the best overall performance, with the lowest impact scores in 9 out of 11 categories. In terms of GWP, AnFOMBR (8.49 kg CO₂-eq) performed 33.7% better than AnMBR (12.8 kg CO₂-eq) and 10.5% better than UASB (9.49 kg CO₂-eq), owing to lower electricity consumption (0.34 kWh vs. 0.865 kWh in AnMBR) and high nutrient removal efficiencies. In freshwater eutrophication, AnFOMBR (1.63 kg P-eq) reduced impacts by 67.7% compared to AnMBR and by 76.8% compared to UASB, correlating with its 85% phosphorus removal efficiency versus 30% in AnMBR and 10% in UASB. For marine eutrophication, AnFOMBR (1.24 kg N-eq) performed 90% better than AnMBR and 83% better than UASB, emphasizing the crucial role of nutrient removal in mitigating aquatic ecosystem impacts. AnFOMBR also exhibited the lowest ODP values and ozone formation impacts affecting human health, attributed to reduced fossil fuel use and chemical demand. For fine particulate matter formation and terrestrial acidification, AnFOMBR achieved 45% and 36.5% lower impacts than AnMBR, respectively. Although differences in ecotoxicity and human toxicity were smaller, AnFOMBR consistently demonstrated the lowest impacts. In human health, expressed in Disability-Adjusted Life Years (DALYs), AnFOMBR reduced impacts by 42% compared to AnMBR, while UASB showed intermediate performance. In ecosystem quality, AnFOMBR yielded 67% lower species loss than both UASB and AnMBR. Finally, in resource depletion, AnFOMBR incurred the lowest additional cost (0.76 USD), outperforming both UASB (0.77 USD) and AnMBR (0.89 USD). These results demonstrate that AnFOMBR not only reduces environmental burdens across multiple categories but also minimizes long-term resource use, positioning it as the most sustainable option among the three anaerobic technologies. In conclusion, this LCA study shows that while the type and concentration of draw solution exert a measurable influence on certain impact categories, the overall environmental footprint of AnFOMBR systems is more strongly shaped by factors such as energy consumption, chemical cleaning, membrane materials, and sludge management. The findings provide valuable insights for the sustainable design and optimization of next-generation anaerobic wastewater treatment systems. Moreover, the results highlight that system-level improvements, combined with the use of environmentally advantageous draw solutions such as MAP and comprehensive LCA-based evaluations, could significantly reduce the ecological footprint of AnFOMBR, positioning it as a promising alternative for sustainable wastewater treatment in the future.