The article presents a comprehensive study that applies the Life Cycle Assessment (LCA) methodology to evaluate the environmental impacts of various wastewater and sludge treatment methods. Wastewater treatment plants (WWTPs) play a crucial role in reducing environmental pollution and recovering valuable resources like water, nutrients, and energy. However, these processes can themselves have considerable environmental footprints due to energy consumption and emissions. This study focuses on assessing and comparing the environmental performance of four prominent wastewater treatment technologies—Activated Sludge Process (ASP), Sequential Batch Reactor (SBR), Constructed Wetlands (CWs), and Up-Flow Anaerobic Sludge Blanket (UASB)—as well as sludge treatment techniques like anaerobic digestion (AD), lime stabilization (LS), and their variations.

India serves as the regional scenario for the analysis due to its pressing wastewater management challenges. With urban wastewater production reaching over 61,000 MLD and treatment capacity lagging significantly behind, nearly 62% of untreated wastewater is discharged into water bodies. This situation underscores the urgent need to evaluate and improve treatment technologies from an environmental sustainability perspective. The study aims to identify environmental “hotspots” across the life cycle of wastewater and sludge treatment processes and provide insights into the most sustainable options for developing countries like India.

The LCA methodology used in this research adheres to ISO 14040/14044 standards and employs Simapro 9.1 software with the Ecoinvent v3.6 database. The functional units considered are 1 cubic meter of treated wastewater for WWT methods and 1 kg (dry weight) of sludge for sludge treatment. The boundaries of the LCA are set using a “gate-to-grave” approach, covering all processes from the point wastewater enters a treatment plant to the final disposal of treated sludge. The study includes direct emissions, electricity use, material inputs, and avoided products (such as recovered fertilizers or energy).

Each wastewater treatment technology has unique characteristics. The Activated Sludge Process (ASP) is widely used in Indian urban areas and is known for its high efficiency in removing BOD, COD, and total suspended solids (TSS). It involves aeration tanks, settling tanks, and sludge return mechanisms. The sludge produced is typically thickened and treated through anaerobic digestion. The Constructed Wetlands (CWs) are nature-based solutions that use wetland vegetation and microbes for treatment. They are low-energy systems but have limitations related to land use and greenhouse gas emissions like methane and nitrous oxide. In this study, Phragmites karka is considered as the vegetation used.

The Sequential Batch Reactor (SBR) is a variation of the ASP, where all treatment steps—equalization, aeration, and clarification—occur in the same reactor but in a batch mode. While it provides high nutrient removal, it is energy-intensive due to frequent aeration and automation needs. The UASB system is an anaerobic process that treats wastewater in a suspended-growth reactor, producing biogas in the process. Though it has lower energy demands and sludge production, its nutrient removal efficiency is limited.

Regarding sludge treatment, the study compares four methods: basic anaerobic digestion (AD), AD with pretreatment (e.g., thermal or mechanical), lime stabilization (LS), and LS with energy recovery. The performance of these processes is evaluated in terms of their energy needs, emissions to air, water, and soil, and the benefits of avoided products (such as electricity or fertilizer recovered from sludge).

The Life Cycle Impact Assessment (LCIA) stage uses the IMPACT 2002+ methodology, which combines midpoint and endpoint categories for a holistic evaluation. Midpoint impact categories include global warming potential, toxicity (human and ecological), eutrophication, acidification, respiratory effects, and fossil resource depletion. Endpoint categories are grouped into four damage areas: human health (in DALYs), ecosystem quality, climate change, and resource use (in MJ).

The findings indicate that SBR has the highest overall environmental impact due to its high electricity consumption. It scores highest in categories like global warming, human toxicity, acidification, and fossil fuel depletion. Electricity used in SBR contributes to emissions such as carbon dioxide, nitrogen oxides, sulfur dioxide, and heavy metals, significantly affecting environmental indicators. ASP follows SBR in terms of environmental burden, primarily due to similar energy requirements for aeration. UASB and CWs show comparatively lower impacts, with CWs even demonstrating a negative global warming potential due to carbon sequestration by wetland vegetation.

In terms of toxicity, SBR again performs poorly, showing the highest human and ecological toxicity scores, largely attributed to emissions from fossil-fuel-based electricity generation. Conversely, constructed wetlands and UASB demonstrate lower toxicity levels. For eutrophication potential, UASB has the worst performance because it lacks efficient nutrient removal, releasing more nitrogen and phosphorus into the environment. SBR has the lowest eutrophication potential due to its enhanced nutrient removal capabilities.

Respiratory impacts, caused by particulate emissions, are also most significant in SBR, followed by ASP. Fossil depletion results mirror the electricity use patterns, with SBR consuming the most non-renewable energy resources. On the other hand, CWs consume minimal electricity and have negligible impacts on fossil fuel depletion, making them the most sustainable option in this regard. When endpoint impact categories are analyzed, SBR scores the highest in damage to human health, ecosystems, climate, and resources. Constructed wetlands again come out as the least damaging technology in all endpoint indicators. UASB performs better than ASP and SBR in most categories, particularly due to its biogas recovery and lower energy demands. The study also analyzes sludge treatment scenarios and finds that anaerobic digestion with pretreatment is the most environmentally favorable. This is due to the energy recovered and reduced transportation and landfilling needs. Lime stabilization, especially without energy recovery, ranks as the least favorable due to chemical usage and lack of energy offset.

The conclusions emphasize that there is no universally superior wastewater treatment technology; each has strengths and weaknesses depending on the environmental category considered. However, in the context of developing countries like India, constructed wetlands and UASB offer more sustainable options when land availability and operational simplicity are taken into account. The findings advocate for integrated treatment systems that combine low-energy processes and effective sludge management with energy recovery. Moreover, the study underlines the importance of local data for LCA modeling, which is often a limitation due to reliance on global databases not tailored to regional conditions.

Overall, this research contributes valuable insights into sustainable wastewater management. It supports informed decision-making by highlighting the trade-offs between treatment performance and environmental impacts. The authors recommend further research incorporating long-term monitoring, economic analysis, and the development of localized emission databases to enhance the applicability of LCA in national wastewater planning.