How Can Environmental Impacts of Water Demand Management Policies on a Wastewater System be Assessed?

As applying water demand management policies (WDMPs) can affect urban water infrastructures like wastewater systems from various aspects, this study proposes a novel approach to address the environmental impacts of applying WDMPs for the rst time. Baharestan city, in Iran’s Isfahan province, is considered as a case study where various WDMPs, such as public awareness campaigns and water pressure management, are applied to confront water scarcity and to reduce water demand. To this end, the life cycle assessment (LCA) method is used to assess the environmental burdens of utilizing WDMPs in the operation stage of a real wastewater collection network (WWCN) and wastewater treatment plant (WWTP) under different WDMPs. A detailed inventory is considered, including detailed repairing of sewer pipelines and their maintenance, replacement of manhole covers, civil works, road rehabilitation, energy consumption, chemical usage, transportation, and air emissions of both the WWCN and WWTP, in various phases. The environmental assessment is conducted in SimaPro software using the ReCipe method. Based on results, a signicant part of the environmental impacts accounts for energy consumption which has strong effects on most midpoint categories and has a great change in different scenarios. Besides, the results demonstrate that the environmental effects driven by the WWTP are eightfold the corresponding WWCN’s impacts. Overall, the outcomes from the present study revealed that implementing WDMPs can lead to diminishing environmental impacts. For instance, reducing wastewater production up to 68% can decrease 18% of the WDMPs’ environmental impacts of the wastewater system in the operation stage.


Introduction
Although freshwater is crucial to human survival, economic growth, and ecosystem service provision (Shahangian et al. 2021b), ensuring water security has currently emerged as a crucial policy challenge that water utilities will meet in the future (Shahangian et al. 2021a, b). Over the past years, rapid urban population growth and climate change, as the main drivers of water scarcity, led policymakers to adopt strategies to control or reduce water consumption (Ramsey et  Several studies have investigated different aspects of applying WDMPs on one or more than one part of the urban water services, and they solely represented a percentage of water consumption reduction ( )) only considered reuse, recycling, and rainwater harvesting as policies for confronting water scarcity, it should be noted that effects of other WDMPs, like water pressure management, not only on water systems but also on wastewater systems are as important to assess as the mentioned policies' environmental impacts.
Among previous research that has addressed the environmental impacts of wastewater systems using Summing up from the literature review, only a handful of research considered the impacts of applying WDMPs on wastewater systems. These studies have been primarily focused on analyzing the changes in qualitative, quantitative, and hydraulic parameters of the sewer networks, and there is no evidence to address the impacts of applying WDMPs on WWTPs from different aspects. Also, there is still a large research gap in assessing the environmental impacts of WDMPs, other than reuse and recycling of greywater and rainwater harvesting, on wastewater systems through a detailed inventory dataset, which was neglected in previous studies. Moreover, limited research addressed the environmental impacts of both WWCNs and WWTPs simultaneously. Besides, only a few studies focused on the environmental effects of wastewater systems in the operation stage. Hence, this study attempts to bridge all the abovementioned research gaps by assessing the environmental impacts of implementing WDMPs on a real wastewater system in the operation stage through a novel approach using the LCA method and considering a detailed data inventory.

Methodology
LCA is a practical and helpful tool with a global point of view for addressing the environmental aspects throughout a product or service's life cycle. LCA allowed the quanti cation of the system's environmental effects, such as global warming and fossil resource scarcity. This method includes four main steps: (1) De nition of goal and scope, which speci es the system boundaries and aim of the research; (2) Providing Life Cycle Inventory (LCI), which encompasses the collection of different types of data (inputs and outputs) considered in the system boundaries; (3) Life Cycle Impact Assessment (LCIA), in which the collected data are assessed based on the assessment methods and LCA software (ReCipe method in SimaPro software is used in the current study); and (4)  In the following sub-sections, the steps of the LCA are presented, which align with ISO standards used to assess the environmental impacts of WDMPs on the wastewater system.

Goal and scope de nition
The main goal of the present research is an integrated assessment of the environmental impacts resulting from implementing different real and hypothetical scenarios of applying WDMPs on a real wastewater system, including both the WWCN and the WWTP, in the operation stage. To this end, as a novel starting point in studying WDMPs impacts on urban infrastructures, a thorough LCA is conducted via a detailed inventory of data on the wastewater system. In this regard, the following objectives are considered: Evaluating the environmental impacts of different scenarios by applying WDMPs (e.g., water-e cient appliances) Identifying hotspots by comparing the environmental impacts of different phases considered for both the WWCN and WWTP.
Highlighting the environmental contribution of the WWCN and the WWTP to the wastewater system.
Providing a thorough inventory of material, transportation, machinery, air emissions, energy, and chemical for the operation stage.
To compare the environmental consequences of implementing WDMPs which are a combination of different scenarios on the wastewater system, a whole life cycle of this system is modeled. The timeframe of the assessment is several years into the future, until the end of 2036. System boundaries contain the process and activities included in LCA (ISO 2006). In de ning the system boundary, the main objective is how to choose the process and parts of the life cycle that are relevant to the aim of the study (Corominas et al. 2020). Considering the purpose of this study, only the environmental impacts of applying WDMPs on the existing wastewater system in its operation stage are therefore considered. In this paper, the operation stage of the wastewater system includes gas emissions to air, network failures, electricity usage, chemical consumption, and transportation. The system boundary of the WWCN and WWTP's phases are shown in Fig. 1(a) and Fig. 1(b), respectively.
The functional unit (FU) that is used to determine the inputs and outputs in the LCA method must be commensurate with the aim of the study and the system boundaries (ISO 2006). Therefore, various types of FU can be chosen, such as per capita and pollution load (Hajibabaei et al. 2018;Corominas et al. 2020). Regarding the de ned goal and scope in the present research, FU has been considered 1 m 3 of wastewater discharged into the WWCN, treated in the WWTP, and nally goes out as e uent and sludge. The de ned FU is constant in different scenarios. The nal results will also be changed by the whole wastewater production volume in the system's lifetime. The considered lifespan for both the WWTP and the WWCN is assumed to be 19 years (from 2018 to 2036) based on their real lifespan in the case study.

Case study and compared scenarios
Using a real case study (Baharestan), the applicability of the presented novel approach was examined to evaluate the environmental effects of applying WDMPs on the wastewater system. Baharestan is a city with 86011 inhabitants in 2018, located in Isfahan province, Iran (the location of the case study is provided in the supplementary information (SI)). The Baharestan's gravity sewer system was made of HDPE pipes with a minimum 200 mm diameter. Stormwater is collected in a separate pipeline system, which is not in the de ned system boundary. The central wastewater treatment plant comprises primary, secondary, and complementary treatment (Further information about the type of wastewater treatment used in this work can be found in the SI). The solid residue for disposal from the wastewater system is transferred to land lls. The e uent is also discharged into the arti cial lake near the town, and the sludge is transferred to the depot site. Due to severe drought and water scarcity in Isfahan province (Gohari et al. 2013;Pishyar et al. 2018) and the high population growth rate in Baharestan city, water pressure management has been applied since 2018 to reduce water consumption by decreasing the water distribution network's pressure to a lower than the desired range, as one of the WDMPs in this city.
The difference between per capita sewage production caused by reducing water demand under related WDMPs is considered in separate scenarios listed below.
Scenario 0 (base scenario): In this scenario, the sewage production is considered 165.4 liters per person per capita per day (lpcd), which is the average of the last seven years in the case study. This scenario does not include any reduction in wastewater production, where only the population growth rate is considered. Indeed, it is assumed that no WDMPs are applied.
Scenario 1: This scenario that is representative of the current situation of the study area involves water pressure management and awareness-raising and educational campaigns. Based on Baharestan water and wastewater utility (2019), the implementation of this scenario has been reduced 8.84% of water demand, and consequently, sewage production during the rst year. Besides, given the report of Isfahan Water and Wastewater Company (2019), it is expected that wastewater will be reduced by 20% during ten years. Therefore, the maximum long-term reduction percentage in this scenario is assumed to be 20%. reviewing the previously published literature in this eld, the nal reduction percentage is considered 30% at the end of the system's lifetime. It should be noted that all households would not accept this policy simultaneously. Thus, it has been assumed that households adopt water-e cient appliances over time by raising public awareness. As a result, the percentage of wastewater reduction starts at 10% in the rst year and will reach 30% at the end of the system's lifetime. . This scenario is case-speci c and depends on social and economic conditions. In this paper, 8% is considered as the average percentage of water consumption reduction for the rst year, reaching 18% in 19 years. Scenario 4: This scenario is a hypothetical combination of the above scenarios applied to highlight the environmental effects of applying WDMPs on the wastewater system, making it easier to compare the results. Therefore, considering a more signi cant reduction percentage to demonstrate the impacts of applying WDMPs on water consumption reduction above the growing population rate could be helpful. Table 1 shows the trend of wastewater production per capita in every scenario and different periods until the end of the system's lifetime. In this table, the highest reduction percentage in wastewater production is 68% (from 163.39 lpcd in 2016 to 52.28 lpcd, average of 94 lpcd) by 2036, which occurred in the hypothetical scenario 4.

Inventory analysis
Based on the collected data, all the inputs and outputs needed for the LCA of a system (e.g., materials and processes) are quanti ed. To reach this goal, a list of the inventory related to the operation stage of the wastewater system was prepared. The required data is mainly obtained from Baharestan city's water and wastewater utility. Secondary data, as well as missing primary data, was also achieved using scienti c literature, interviews, and site visits. In addition, Ecoinvent 3.5 database was used for modeling sub-process data.

Life Cycle Inventory (LCI) of WWCN
In the operation stage of the WWCN, replacing defective manhole covers (including materials, construction of covers, transportation, and installation), unclogging sewer pipes and manholes, repairing sewer pipes failures (including excavation, back lling, compacting, etc.), reconstruction of the asphalt layer, and gas emissions to air including CH 4  , respectively (the equations can be found in SI). It is worthwhile to note that the hydraulic parameters driven by hydraulic modeling for every sewer pipe were used in mentioned equations to calculate gas production. The data collected from the sewer network in the operation stage is categorized into four separate phases.
First, sewer blockage phase: This phase contains blockage in the main sewer pipelines, siphon of the sewer branch pipes, and manholes. The blockage rate in sewer pipelines (main and branches) differs based on the amount of wastewater in ow to the network. In other words, more reduction in wastewater ow leads to more blockages, which mostly occurs because of lower velocities in pipelines and more sedimentation. Due to the lack of data collected from Baharestan water and wastewater utility for manhole blockage, the rate of this one was considered xed in different scenarios.

Comparing the environmental midpoint impacts of different phases in the WWCN
The environmental impacts of applying WDMPs on the WWCN in four different phases, explained in section 2.2.1, are compared via eighteen separate midpoint categories. Fig. 2 illustrates the environmental impacts of various phases of the WWCN in the base scenario, scenario 1 (current situation), scenario 2, scenario 3, and scenario 4 (with the maximum reduction in wastewater ow), respectively. Further and extended comparisons are conducted between the base scenario and scenario 4, which have the maximum difference in wastewater production. The numerical results of the other scenarios can be found in SI. Based on the results, the gas emissions phase has a signi cantly higher impact on global warming. Although the concentration of pollutants in scenario 4 is more than the base scenario, there are fewer environmental effects of gas emissions, due mainly to a signi cant decrease in wastewater in ow.
Comparing Fig. 2(a) to Fig. 2(e) shows that decreasing wastewater production volume leads to an increase in environmental impacts of applying WDMPs in the blockage phase. For instance, considering that a Waterjet machine is used to unclog the blockages in the main pipelines, the growth of the blockage rate makes an increase in water consumption from approximately 34% (around 3800 m 3  reported that direct gas emissions from a WWCN do not contribute signi cantly to overall LCA results, while the results of the present study are opposite. The probable reason for this difference is that their study included the construction stage, while the gas emissions phase mostly contributes to the operation stage.

Comparing the environmental midpoint impacts of different phases in the WWTP
The environmental impacts of implementing WDMPs on the WWTP in the operation stage in different phases from scenarios 0 to 4 are depicted in Fig. 3 using separate midpoint categories. Energy consumption is one of the most signi cant parts of water and wastewater systems, which has great environmental impacts (Beygi et al. 2021). As shown in Fig. 3, the energy consumption phase has a more considerable contribution to most midpoint impact categories, in line with Akhoundi and Nazif (2020). Furthermore, the amount of energy consumption decreases in scenario 4 in comparison with the base scenario. The impact of the energy phase on the global warming is reduced from around 18.4 million kg CO 2 eq. to 8.5 million kg CO 2 eq. from scenario 0 to scenario 4 with an average wastewater production of 165.4 to 94 lpcd during 19 years, respectively.
The gas emissions phase has considerable effects on stratospheric ozone depletion and global warming categories because gases such as CO 2 , CH 4 , and N 2 O have a high global warming potential (IPCC 2014), which is considered in the assessment of the environmental impacts of the current study. Moreover, the transportation phase signi cantly affects a speci c midpoint category (i.e., land use). Considering the reduction in treated wastewater e uent amount, the environmental impacts of the transportation phase reduce approximately 52% from the base scenario to scenario 4.

Comparing the environmental impacts between the WWCN and WWTP
The endpoint impact categories of the WWTP in comparison with the WWCN are illustrated in Fig. 4. The LCA results demonstrate that the WWTP has more signi cant environmental effects than the WWCN in the operation stage, in line with Slagstad and Brattebø (2014), to the extent that the environmental impacts of the WWTP are approximately eightfold the WWCN. One of the most in uential phases of the WWTP is energy consumption, which affects the overall results as well and makes the WWTP's environmental impacts higher than the WWCN. As depicted in Fig. 4, human health mostly contributes to the endpoint impact categories, which accounts for approximately 90% of total environmental impacts for both the WWCN and WWTP in every scenario. Indeed, it can be seen in the results that most inventory data are dangerous for human health. Also, the weight of midpoint categories which contribute to human health is higher than the weight of them for other endpoint categories. As observed in Fig. 4, the environmental effects of applying WDMPs decreased by reducing wastewater production, which can lead to positive in uences on both the WWCN and the WWTP from the environmental perspective.

Comparing the environmental impacts of different scenarios
Although the reduction in wastewater production and population growth have reverse effects, the overall environmental impacts of applying WDMPs on the wastewater system are reduced (Shown in Fig. 5

Discussion And Conclusions
This study is conducted with the aim of assessing the environmental effects of implementing WDMPs on a real WWCN and WWTP in the operation stage of the system. To reach this purpose, a comprehensive inventory of gas emissions, chemical and energy consumption, repairing pipes, manhole covers, and transportation is considered. The LCA method is applied to the assessment in order to compare various scenarios with different amounts of wastewater production. In this regard, SimaPro software, ReCipe midpoint and endpoint method is used. The overall results show that applying WDMPs could reduce the environmental burden of the wastewater system, which is an important issue in today's world. To explain more, the environmental impacts of applying WDMPs on the wastewater system decrease 18% from the base scenario to scenario 4 with a 68% reduction in wastewater production. As the numerical results of the assessment show, in the WWCN, the manhole cover phase has signi cant environmental impacts that are neglected in previous studies. To explain, the raw material for manholes, as well as the construction, transportation, and installation process, played a signi cant role in the environmental burdens.
It is essential to consider the repair of the network's breakages and blockages. A major challenge in this case study was the pipeline blockages. The blockages in pipelines increase by reducing wastewater in ow, the reduced velocity of discharges in pipes, and augmentation of sedimentation. Therefore, the environmental impacts of this phase increase in scenarios with lower wastewater in ow. The results emphasize that bitumen paving which is ignored in most previous studies, should be considered in the LCA of sewer systems, even in small amounts of the asphalt layer. Also, the results show gas emission was a crucial phase in the operation stage of the WWCN. The concentration of BOD 5 , wastewater quantity, velocity in pipes, and wastewater retention time in pipelines change the amount of gas production in alternative scenarios. The gas emission phase mainly in uences the global warming midpoint category. The total environmental burden of this phase reduced 53% in around 68% of in ow reduction (from scenarios 0 to 4). The energy consumption in the operation stage of the wastewater system was the most signi cant phase. The environmental effects of this phase were reduced around 54% from the base scenario to scenario 4 (with the maximum in ow reduction).
Moreover, results demonstrate that the environmental impacts of the WWTP are approximately eight times greater than the WWCN in the operation stage. In the end, the total environmental impacts of WDMPs in scenarios are as below: Scenario 0> scenario 3> scenario 1> scenario 2> scenario 4. In other words, a scenario with lower wastewater quantity had lower environmental impacts. Thus, from an environmental standpoint, scenario 4 has the best performance, but is this scenario perfect from other perspectives, too? Therefore, it is recommended to consider other aspects of applying WDMPs on wastewater systems, such as social and economic aspects, to reach a more particular viewpoint. Also, the supposed case study's wastewater system is a new one, and there is no data of the system's renovation stage. Therefore, it is recommended for future studies to take into account the renovation stage and make the system boundary wider, which could affect the overall results. In addition, considering both water and wastewater systems in an environmental assessment will help decisionmakers select a better WDMP for their case study. Thus, it could be worthwhile to include other parts of a water supply system, such as a pump station, water treatment plant, water distribution networks, in a system boundary of thorough research. Environmental impacts of WWCN's phases of a) the base scenario, b) scenario 1, c) scenario 2, d) scenario 3, e) scenario 4; all by the ReCipe midpoint method