Environmental and economic performance evaluation of municipal wastewater treatment plants in India: a life cycle approach

Life cycle assessment (LCA) was used to evaluate the environmental impacts associated with wastewater treatment plants (WWTPs). Moreover, an economic evaluation was also addressed using life cycle cost (LCC) approach. Emissions associated with electricity production for operating the WWTPs, emissions from the treated effluent and hazardous heavy metals emissions have been identified as the main contributors to the overall environmental impact. Among the WWTPs considered, soil biotechnology (SBT) obtained the lowest environmental impact in all the evaluated impact categories, except for eutrophication potential. While the aerated lagoons (AL) system presented the worst results due to the high electricity and chemicals consumption. Moreover, the results obtained from the evaluation of benefit from treated effluent reuse clearly indicate that there is a drop in the toxicity potential when the rate of effluent reuse is increased. On the other hand, the present worth of SBT was estimated to be Rs. 40 million/millions of litres per day (MLD) which is the highest as compared to other technologies. Membrane bioreactor (MBR) is the second highest (Rs. 24.7 million/MLD), which is mainly contributed by civil, electro-mechanical and membrane cost. The results of LCA and LCC provide specific insights about the factors which play a major role during the life cycle of wastewater treatment technology and its associated impacts. This is an Open Access article distributed under the terms of the Creative Commons Attribution Licence (CC BY 4.0), which permits copying, adaptation and redistribution, provided the original work is properly cited (http://creativecommons.org/licenses/by/4.0/). doi: 10.2166/wst.2019.110 om https://iwaponline.com/wst/article-pdf/79/6/1102/617125/wst079061102.pdf 2020 Sheetal Kamble (corresponding author) Anju Singh Environmental Engineering and Management, National Institute of Industrial Engineering (NITIE), Mumbai, India E-mail: sheetaljkamble@gmail.com Absar Kazmi Department of Civil Engineering, Indian Institute of Technology, Roorkee, Uttarakhand, India Markus Starkl Competence Centre for Decision Aid in Environmental Management, University of Natural Resources and Life Sciences (BOKU), Vienna, Austria


INTRODUCTION
Economic losses from inadequate sanitation may slow down economic growth, as costs from pollution and health impacts were estimated as 6.4% of gross domestic product (WHO & UNICEF ). The generation of wastewater is increasing because of population growth and improved living standards in many countries. Wastewater treatment plants (WWTPs) have been designed and operated to reduce the pollution of wastewater and to minimize the adverse impacts on environmental quality and human health (Wang et al. ). Advanced wastewater treatment (WWT) and the associated reclamation of water is a necessary and critical function to protect both human health, the natural/aquatic environment providing for reduced overall water demand through reuse. municipal wastewater is only 22,963 MLD (CPCB ). Because of the hiatus in WWT capacity, about 38,791 MLD of untreated wastewater (62% of the total wastewater) is discharged directly into nearby water bodies, leaving a big gap in the treatment of municipal wastewater. In the near future, there is a need for extensive sewerage network for collecting and transporting the sewage generated to the WWTP for the treatment. The stricter implementation of policies, examination of latest technology and regular observations and measurements (O&M) of existing WWTPs, promoting decentralized treatment plants for treatment of households, rural, and urban sewage and reuse of treated sewage for non-potable purposes like flushing, gardening, etc. would surely help the municipal authorities to achieve better wastewater management (Singh et al. ).
Life cycle assessment (LCA) and life cycle costs (LCC) applied to wastewater treatment and management LCA is a standardized and sophisticated tool to 'compile and evaluate the inputs, outputs and the potential environmental impacts of a product/process/service system throughout its life cycle' (ISO a). LCA has been used in the field of wastewater treatment for two decades. Several studies applied LCA to conventional activated sludge (CAS) based technologies (Kalbar et al. a, b, ; Holloway et al. ), natural based systems (Garfi et Lin et al. () investigated the economic and environmental profiles of three alternative nitrogen removal and recovery routes integrated into WWTPs, including conventional nitrification-denitrification, anammox, and ion exchange. There are few other studies that used LCA and economic analysis methods for investigating WWT systems and processes (Meneses et al. ).
The goal of this study was to evaluate environmental impacts of six WWT technologies, namely: sequencing batch reactor (SBR), membrane bioreactor (MBR), moving bed biofilm reactor (MBBR), activated sludge process (ASP), soil biotechnology (SBT) and aerated lagoons (AL) using LCA. Another objective was to carry out LCC of six technologies under consideration.

SBR
In this study, a small-scale, 2.5 MLD capacity SBR plant designed for greater organic, as well as nutrient, removal was selected for the analysis.

MBR
A 0.8 MLD capacity plant was selected. Low-pressure membrane filtration, either microfiltration or ultrafiltration is used to separate effluent from activated sludge. The membrane is immersed in the reactor.

MBBR
A 2 MLD MBBR plant was selected. MBBR technology provides cost-effective treatment with minimal maintenance since MBBR processes self-maintain an optimum level of productive biofilm. Additionally, the biofilm attached to the mobile bio-carriers within the system automatically responds to load fluctuations.

ASP
The ASP is one of the most commonly used technologies for the secondary treatment of sewage in India. In this study, 1 MLD capacity plant was selected.

SBT
A 1.5 MLD plant designed to recycle sewage was studied. The plant consists of a treatment bed made from specially prepared soil with required mineral additives. The bed is of about 2.5 m in height spread over an area of about 1,700 m 2 . The floor and the walls of the containment are waterproofed with HDPE sheets with side anchoring.

AL
A 1.4 MLD plant was selected. Artificial aeration is provided to oxidize the organic matter. In aerobic lagoons, all the suspended solids present in the wastewater are in suspension, and complete mixing takes place.
The unit processes and operations of the analyzed WWTPs are depicted in Table 1.

METHODOLOGY OF THE STUDY
The LCA modelling was carried out with the help of GaBi software (v.6.2). The LCA methodology used in this study is in accordance with the international standards ISO 14040-44 series (ISO a, b). The ISO 14040-44 determines four stages for LCA as follows: goal and scope definition, life cycle inventory, life cycle impact assessment, interpretation for the study. The following sections provide a brief description of the four LCA procedures utilized in this study.

Functional unit
In the current study, 1 m 3 of treated wastewater was chosen as a functional unit which is the most commonly used functional unit (Corominas et al. a).

System boundaries and assumptions made in this study
Previous studies have shown that construction and demolition phases of mechanized WWTPs have negligible impacts (less than 5% of impacts compared with overall life cycle impacts of the WWTP) compared with the operation phase (Pasqualino et al. ). Thus, this study has taken into account only the operation phase. The system boundary of the present study comprised unit processes related to wastewater treatment, sludge treatment, sludge disposal and transportation to a landfill site. The inputs were influent, electricity, chemicals, and diesel. The data for electricity consumption were collected from plant operators, and background data were used from the Ecoinvent database to assess the environmental impacts. The energy process used for modelling was Indian electricity grid mix, medium voltage. Direct process emissions which are biogenic in nature were excluded from the analyses because they belong to the short CO 2 cycle and do not contribute to climatic change (Coats et al. ). For transportation of sewage sludge to the landfill site, the distance of 50 km was assumed.

Life cycle inventory
Following the goal and scope definition, life cycle inventory analysis was conducted. Life cycle inventories were generated based on several on-site visits to WWTPs. Table 1 presents a summary of the operation phase life cycle inventory for WWT technologies.

Life cycle impact assessment (LCIA)
LCIA represents the third phase. In this study, the CML 2001 (November 2013) method was used for LCIA as it gives a separate score for each type of environmental impact. The results of the LCIA of WWTPs for different selected impact categories are presented below in Table 2. The results of the comparative assessment of six plants are shown in Figure 1.

Interpretation of results
Finally, the interpretation of the results allows identifying the hot spots in the process as well as recommending options to reduce the environmental burdens. Impacts were mainly caused by consumption of fossilbased electricity and chemicals, as well as wastewater effluents from WWTPs are major contributors due to the remaining nutrients emissions and the emission of heavy metals from disposed sludge. Concerning impact indicator results, it is found that electrical consumption by the WWTPs makes the most significant contribution to global impact categories such as abiotic depletion potential (ADP), global warming potential (GWP), acidification potential (AP) and photochemical  ozone creation potential (POCP). The WWTP itself does not impose a direct impact on the local environment. Its environmental impact is ascribed to the production of the electricity (Ioannou-Ttofa et al. ). This was owing to the extraction and burning of fossil fuels, which releases pollutants and carbon dioxide to the environment.

GWP
The energy consumption for the operation of WWTPs is found to be the largest contributing factor for CO 2 emissions and GWP. Being the technology with the highest electricity demand, AL reported worst on this impact category followed by MBBR. Reasons for the higher energy requirements of AL are because this system processing 1 m 3 of wastewater consumes more electricity and chemicals than others. Next, due to the high consumption of electricity and chemical dosing, the CO 2 and GHG emissions to air and hence the emissions related GWP has been found to be more for the MBBR plant under study.
Further, sometimes, due to the use of aerators or mechanical stirrers in MBBR to ensure that the beds are moving for uniform treatment make the energy consumption.

AP and ADP
AP is mainly because of SO 2 and NOx emissions from coal combustion, which generates electricity for operating the plants. Coal consumption also has a major contribution to ADP (fossil). Similarly, for ADP (fossil), AL is found to have the highest AP and ADP as compared with other technologies.

EP
The EP of a WWTP is mostly associated with the emissions to water, mainly due to the phosphorus (P), nitrogen (N) and to a lower extent, degradable organics in wastewater effluent. This is not surprising since WWTPs' discharge acting as source point is one of the main contributors to aquatic eutrophication worldwide, and the eutrophication impact would have been worse in the absence of WWT (Renou et al. ).
The MBR (0.0000291 kg PO 4 3-Eq) has the lowest EP value as compared to other technologies, which matches with values (for nutrient removing systems) reported by Gallego et al. (). ASP has the highest EP (0.000816 kg PO 4 3-Eq) because there is negligible removal of nutrients in the ASP system. Thus, the EP impact can be decreased immediately by implementing more sophisticated technology to enhance the nutrient removal efficiency (however, generally with an increase of other environmental impacts).

ODP
This category mainly refers to the emission of gases that reduce the ozone layer (principally CFC-11, CFC-12 and Halon 1301) and these emissions are found to be minimal in this study.

POCP
The values obtained for POCP in the current study are far smaller from consideration.

Toxicity potentials (FAETP, HTP, MAETP and TETP)
Ecotoxicity potential is mostly dependent on the heavy metals released in the air, water and soil environment, for which in the considered WWTPs there is no special provision for heavy metal removal. However, some removal takes place through the physicochemical and biological processes. Further, the disposal of sludge containing heavy metals contributed substantially to the ecotoxicity impact categories.

FAETP
Concerning to FAETP, the emissions of metals that take place during electricity production dominate the impact for all the secondary reactors. Far from the item energy use, the discharge of treated water is the second element, mainly to the release of Zn, Ni and Cu to the aquatic environment.

HTP
It is mainly because of the release of heavy metals in water, air and the soil environment. In this study, SBT has the lowest HTP (0.020238 Kg DCB-Equiv).

MAETP
The current study revealed that MAETP contributes most to the overall impacts, the result is in agreement with the results of earlier studies (Kalbar et al. b, a, ; Kamble et al. ). The characterization factors in this category (for chemical consumption, sludge production, energy consumption, etc.) are generally higher than those of other impact categories. AL presented the highest environmental impact in MAETP since this treatment scheme had the highest electricity consumption.

TETP
It is dominated by the presence of heavy metals in the sludge being Zn, Pb, and Cu as the main contributors. This contribution is directly dependent on the quantity of sludge produced. AL system is found to have the highest TETP (0.0141 kg 1,4-DCB-Eq). The TETP for MBR (0.0001) is almost negligible mainly because, very much less sludge is generated during the WWT process. The reason for this is the MBR's ability to operate at much longer sludge retention times (Sadr et al. ).
In sum, it can be said that the impact of WWTP is more dependent on design and how a plant is operated. Even for similar technologies, there can be a huge difference in the performance depending upon the operation of the plant; this fact has already been reported by Kamble et al. ().

Evaluation of benefit from reuse of the treated effluent
Properly treated wastewater can be reused for various purposes to provide ecological benefits, reduce the demand for potable water and augment water supplies (Mo & Zhang ). The cost of treating 1 KLD (kilolitre per day) of sewage costs about INR (Indian Rupees) 18 to 20, while the cost of treated water lies between INR 40-60, thereby posing a profitable option (Singh et al. ).
In the current study, treated effluent was used to replace tap (fresh) water, and the benefits are gained from water saving. In conclusion, is it understood that the total lifecycle benefit from reuse of the tertiary treated effluent is much higher than the life cycle energy consumption for the tertiary treatment. This indicates that the implementation of the tertiary WWT facility is beneficial. The results of SBT and MBR with and without reuse of effluent are shown in Figures 2 and 3, respectively.
The main environmental concerns obtained through the LCA are linked to increased toxicity impacts from the chosen end use of wastewater and related recovery products. The results obtained clearly indicate that there is a drop in the toxicity potential when the rate of effluent reuse is increased.

Life cycle costing -LCC
In this study, capital cost and O&M costs have been considered. Capital cost includes civil, mechanical, electrical and any other related items during the construction of the plant. The capital cost also includes land cost estimated as per the Rs. 0.02 lakh per m 2 . O&M costs incurred usually account for labour requirement, energy, chemicals, repair and replacement of electro-mechanical materials. LCC was calculated using the IS  () and IS  () Parts 1 and 2 methodologies of Bureau of Indian Standards. The current study employed the present worth (PW) method for LCC.

PW
The uniform present worth factor (UPWF) is used to estimate the present worth of O&M cost spent every year. The following equation is used to estimate the total present worth of the WWTPs: where i is the interest rate, and T is the economic life of the asset. For this study, economic life is defined as 50 years and the interest rate is taken as 12%. The costs data for the six WWTPs under study and the results of LCC are presented in Table 3.
The PW of SBT was estimated to be Rs. 40 million/MLD which is the highest compared to other technologies. This high cost is due to the very large land requirement and high-density polyethylene liner cost. MBR has the second highest (Rs. 24.7 million/MLD) which is mainly contributed by civil, electro-mechanical costs and membrane cost.
Although high capital and O&M costs have been pointed out as a major drawback for MBRs' widespread implementation, in this case, such costs are counteracted again by the environmental damage avoided, due to the total absence of solids in MBR permeates and their high treatment quality (Castillo et al. ). MBR technology is found to be comparable or sometimes better than upgraded MBBR and SBR plants to produce effluent of excellent quality, as the water quality produced by other WWTPs is insufficient (Singh & Kazmi ). MBBR is economical,  but the removal of TSS and BOD5 is unsatisfactory and produces a medium quality effluent.
In fact, if the legislation of discharge standards are tightened further, such as conventional and other technologies are not able to meet the demand, then MBR is the only viable option. According to Starkl et al. (), trade-offs between pollutant removal and costs were not possible, as lacking fulfilment was extreme; either legal requirements were violated or costs were excessive.
The main findings from the LCA and LCC study are as follows: • Energy is very often a great source of an impact as the Indian electricity mix is mainly carbon-based. In this study, energy consumption is a central contributor to the environmental profile of the studied plants, in that it contributes to different impact categories in varying degrees.
• The potentials of the impact categories dominated by energy consumption are heavily influenced by electrical grid-mix. Plants operating in countries with high levels of fossil fuels in the electrical grid-mix may exhibit a higher GWP than a plant with similar energy consumption rates operating in a country with a greener electrical grid-mix.
• The organic loading rate had the largest influence on energy consumption rates because it is directly related to the oxygen demand and, subsequently, the required aeration power.
• In comparison to conventional technologies, natural technologies such as SBT has been proposed as a better alternative with lower environmental impacts and reduced pollutant loads as these technologies are highly efficient for heavy metal removal and have low energy demand; nevertheless, implementation of these technologies requires large areas of land.
• LCA and LCC are determined as being the most appropriate environmental assessment and economic tools, respectively, for system evaluation.
• The results of this study can be used to design and simulate new technologies compared with the existing ones. This work intends to provide the decision support to the decision makers through identification of the important components that influence the life cycle impacts as well as through providing a reasonable estimation of the environmental impact of the WWTPs.

CONCLUSIONS
A comprehensive LCA and LCC have documented in this study can help decision makers to take sustainable decisions taking into account both environmental and economic aspects of WWTPs. The results obtained from the LCA study provide insightful information about the potential environmental impacts triggered by every aspect of the operation of the treatment processes. Whilst there are some uncertainties in the data quality, different operating performance parameters, system boundaries, background inventories and different LCIA methodologies may significantly vary the results of an LCA. This study reflects the need for the development of exhaustive and relevant Indian life cycle inventories to add to the Indian database.