Life cycle assessment of construction and renovation of sewer systems using a detailed inventory tool
- Serni Morera
- Christian Remy
- Joaquim Comas
- Lluís Corominas
Table of Contents
Description Of Sewer Pipes Construction And Renovation Phases And Processes
- Figure 1
- Fig. 2 Rules for the calculation of the trench depth, a with traffic in the upper part in a rectangular trench, bwithout traffic in the upper trench in a rectangular trench, c without traffic in the upper part of the trench in a trapezoidal trench. Option b is the option used for the hypothetical sewer system. DN is the diameter of the pipe. β is the angle between the trench wall and the soil horizontal (β = 90° rectangular trench, β < 90° trapezoidal trench)
- Life Cycle Assessment
- Goal And Scope Definition
- Inventory Analysis
- Environmental Impact Assessment
- Inventory Tool For Sewer Systems
Environmental Impact Profile And Costs For The Initial Hypothetical Sewer System
- Fig. 5 LCIA results for the initial hypothetical sewer system with a length of 1 km and a PVC pipe with a diameter of 40 cm (analysis includes construction and renovation). The results include a single construction and two renovations over 70 years of operation. For each impact category, the results are split into the construction and renovation phases. Left part of each bar (construction or renovation) relates to the phases and the right part of each bar relates to the processes. Total impact for each category is the sum of construction and renovation bars
- Fig. 6 Costs for the different phases of the initial hypothetical sewer system construction and renovation (two renovations needed over 70 years)
- Influence Of Pipes (Materials And Diameters)
- Influence Of Transport Distances
- Fig. 8 Influence of site-specific characteristics (soft and rocky soil vs compact, asphalt placement when a road is constructed vs no asphalt, and urban vs non-urban setting) on the environmental impacts. The impact of each site-specific characteristic is evaluated for three different diameters (40, 80, and 140 cm). Each diameter has its own baselinewith the characteristics of the hypothetical sewer (compact soil, non-urban setting, and no-road construction)
- Fig. 9 Effect of including deposition on the LCIA of a 1-km sewer pipe (positive percentages mean increased impacts whereas negatives percentages correspond to decreased impacts). Different materials and diameters are evaluated. Baselines (no deposition included) are different for each material and pipe diameter
- Influence Of Life Span
- Influence Of Including Renovation Or Not
Purpose ::: The objective was to provide comprehensive life cycle inventories for the construction and renovation of sewers. A detailed inventory was provided with multiple options of pipe materials, diameters and site-specific characteristics, and was embedded into the Excel®-based tool SewerLCA. The tool allows for life cycle evaluation of different sewers. It was applied to determine the most important phases, processes, and related parameters involved in the construction and renovation of sewers from an environmental and economical perspective.
Human activities in households and industries consume large amounts of water which have to be treated before being returned to the freshwater ecosystems. Above 80 % of population in 14 of the EU Member states are connected to urban wastewater treatment plants (WWTPs) (European Environment Agency 2013), which have the function of removing contaminants from the used water (i.e., wastewater). Sewer systems are the elements that collect and transport wastewater from households and industries to the WWTPs. Despite the important role they have, their construction generates environmental impacts associated to the direct emissions generated on-site and to the production of energy and resources required. Life cycle assessment (LCA) is a widespread tool to assess the environmental impacts from urban water systems (UWS) (Loubet et al. 2014) , including drinking water treatment, distribution systems, sewer systems, and WWTPs. Many of the studies published so far include the operation but neglect (or partially consider) the environmental load of the construction and end-of-life phases. The review from Loubet et al. (2014) concluded that only 3 out of 18 studies included the pipe materials (either in sewers or in drinking-water distribution networks) in the inventory phase and none considered the civil works involved in the construction of sewer systems. Risch et al. (2015) is the first study that compared in detail the environmental impacts from the construction and operation of a case-study which included a sewer system and a WWTP. Risch et al. (2015) provided a detailed inventory including pipe materials and civil works for the construction of a specific sewer system.
Focusing on pipe networks (either for sewers or for drinking-water distribution), three studies have addressed in detail their construction following a life cycle approach. Piratla et al. (2012) compared four pipe materials (molecular oriented polyvinylchloride (PVC-O), polyvinylchloride (PVC), high-density polyethylene (HDPE), and ductile iron) in terms of CO 2 emissions from their manufacturing and assumed a life span of 50 years for all materials evaluated. Du et al. (2013) compared six pipe materials (PVC, ductile iron, concrete, HDPE, reinforced concrete, and cast iron) in terms of global warming potential (GWP) and considered pipe production, transport, installation, and use phases and assumed a service life of 30 years for all pipe materials. Petit-Boix et al. (2014) followed a multi-criteria LCA approach (involving several potential impact categories) to evaluate pipes made from PVC, concrete and HDPE, and assumed that plastic pipes had to be replaced once every 100 years. The three studies differ in the phases and processes considered in the construction of the pipe networks, did not model in detail renovation of the infrastructure and made different assumptions regarding their life span. Construction and renovation (particularly for sewers) are not systematically included in LCA studies of UWS due to the limited availability of comprehensive life cycle inventories which account for different diameters, a variety of characteristics of the site, multiple materials with varying life span and several options for pipe disposal. Hence, the objective of this study is to provide comprehensive life cycle inventories for the construction and renovation of sewer systems. This inventory was embedded into the SewerLCA Excel ® -based tool, which allows for effective life cycle evaluation of different sewer typologies. By using the tool it was possible to determine which are the most important phases, processes and related parameters involved in the construction and renovation of sewers from an environmental and economical point of view.
Description Of Sewer Pipes Construction And Renovation Phases And Processes
The sewer system construction process can be divided into six different phases (Fig. 1a): (1) the working area is cleaned; (2) the trench is excavated and underpinned; (3) the pipe is layed at the bottom of the trench above a layer of draining material; (4) the trench is backfilled with granite sand until 30 cm above the pipe and normal sand, which is taken from the same workplace, or sand taken from elsewhere; (5) if there is a road on top of the trench, a layer of asphalt is included, and (6) the unused excavated soil is distributed around the working area or transported and deposited in a landfill.
Three types of trenches can be considered for the construction of sewers depending on the site characteristics. Rectangular trenches are applied for rocky soils and for most compact soils (Fig. 2a, b) . When the excavated soil is soft a trapezoidal trench is applied (Fig. 2c) . If asphalt is placed in the upper part of the trench deeper trenches are needed (Fig. 2a) . The trench type shown in Fig. 2b is the selected for the analysis conducted in this paper, even though the Excel ® spreadsheet tool created considers the three options.
To renovate a sewer, six different phases are considered ( Fig. 1b): (1) breaking the asphalt layer and deposition of the material; (2) excavation of the trench; (3) extraction of the old pipe; (4) substitution with the new pipe; (5) backfilling the trench and (6) including a layer of asphalt when is necessary. Finally, during the renovation, it is also necessary to include the end-of-life processes, i.e., to extract and deposit/ incinerate the pipe, which also includes its transport.
Each phase comprises different processes such as materials production and transport, consumed diesel from machinery work and disposal (incineration or landfilling). Work area cleaning phase includes the energy consumption process. Excavation phase includes diesel consumption by the machines and transportation of the excess material to a landfill. Pipe laying phase includes the production of the pipe, its transport to the workplace, the diesel consumed during the pipe laying and water consumed to proof its reliability. Backfilling phase considers the granite sand extraction/production, its transport to the workplace and diesel and water consumed during its placement. Asphalt placement phase includes the asphalt production, its transport to the workplace and its placement. The soil distribution around the work accounts for the diesel consumed.
Life Cycle Assessment
The environmental assessment was conducted following the ISO 14040 and 14044 standards (ISO 14040 2006; ISO 14044 2006) which define four stages: (1) goal and scope definition, (2) inventory analysis, (3) environmental impact assessment, and (4) interpretation. Even though process-based LCA is a very time and data-intensive process (Murray et al. 2008 ), this approach was followed by including specific process of construction and renovation of sewers. Being aware the hybrid LCA would reduce uncertainty from the selection of the boundaries by covering the entire supply chain (Noori et al. system. DN is the diameter of the pipe. β is the angle between the trench wall and the soil horizontal (β = 90°rectangular trench, β < 90°t rapezoidal trench) 2015), for this particular study, the analytical benefits of hybridization did not outweight its costs and hence was not applied (Udo de Haes et al. 2004 ).
Goal And Scope Definition
The goal of this life cycle assessment was to compare the environmental impacts from the construction and renovation of several sewer system typologies and to determine which are the phases, processes, and related parameters contributing the most to the environmental impacts. As a starting point a hypothetical sewer system with a length of 1 km and a PVC pipe with a diameter of 40 cm was evaluated (this is the initial system from now on). It was considered that the sewer is located in a non-urban area without traffic (no asphalt placement in the upper part of the trench). The pipe was installed in a compact soil zone within a rectangular trench with no underpinning (Fig. 2b) . Distances of 50, 30, and 25 km were selected for the transport of granite sand and sand, asphalt, and pipe distributors (personal communication with Voltes S.L.U. company), respectively, which also coincides with the assumptions taken in Petit-Boix et al. (2014) . It was assumed that the surface to be cleaned was the double of the trench surface in the upper part. The functional unit of the entire study was the construction and regular renovation of a sewer system of 1 km length during 70 years of operation. The construction of the sewer plus two renovations were considered since the life span of PVC pipes was 25 years and the functional unit was 70 years. The system boundaries are shown in Fig. 3 and include direct emissions generated on-site (e.g., air emissions to the ecosphere from diesel combustion) and indirect emissions related to diesel and material production, pipe material deposition, and transport. Even though the Excel ® -based tool provides estimations of water consumption during the construction process, it has no effects in this study since the water is considered that is directly extracted from the nature.
After evaluation of the environmental impacts of the initial sewer system defined above, a sensitivity analysis was conducted to evaluate which parameters involved in the sewer construction and renovation (parameters in grey in Table 2 ) have the highest influence on the different environmental impact categories. The parameters included in the sensitivity analysis are the type of pipe material (PVC, HDPE, precast concrete, and reinforced concrete), diameter of the pipes (ranging from 20 to 160 cm), transport distances for materials (ranging from 0 to 100 km), and the characteristics of the working area (location, type of soil, and asphalt placement). Special emphasis was put in analyzing the influence of pipe renovation, which is directly related to the life span of the materials. In the analysis, variability related to the life span of pipes was included. For PVC, an average lifetime of 25 ± 5 years was assumed. For HDPE, a lifetime of 40 ± 10 years was considered, and finally, for precast concrete and reinforced concrete pipes, a lifetime of 70 ± 20 years was assumed. Even though pipe suppliers normally specify longer life span ranges, construction companies, and water agencies experience shorter life spans in practice (Blosser et al. 2003) . Hence, the ranges assumed in this paper were defined after personal communication with the construction company Voltes S.L.U. (Catalonia) with more than 60 years of experience in the field. Precast concrete and reinforced concrete pipes have the same weight. Certain materials that are used, such as ductile iron, or pipe configurations, such as oval pipes, were not considered here. Finally, transportation was considered for the environmental analysis but not for the economic analysis.
With regards to renovation, the excavation process was considered as the excavation of compact soil. In addition, the energy consumed during pipe extraction from the trench was considered to be the same as during its laying. Additionally, during pipe extraction and laying during renovation, granite sand losses of 10 % were considered. Finally, it was assumed that concrete pipes and excess soil are disposed of in landfills, located at 30 km., whereas plastic pipes are incinerated (a process that involves energy recovery).
The inventory was carried out following the steps identified in Fig. 4 . Comprehensive life cycle inventories for sewers construction and renovation were obtained after interviewing construction experts and reviewing sewer construction budgets from the Catalan company Voltes S.L.U. (Catalonia). The construction budgets include in detail the amount of resources (materials, energy, machinery, etc.) required to execute the work. The public database BEDEC from the Construction Technology Institute of Catalonia (Banc BEDEC, accessed in August 2013) was also used to obtain detailed information of each material or element (a pipe is for instance an element included in the database). The Banc BEDEC database supplies technical and economic information regarding all kind of elements used in the construction market. It includes detailed information of 2,026 construction items including prices (before taxes). For example, the database provides information from a Bconcrete pipe^of a specific diameter. The information includes the weight and the price per unit (in this case, linear meter). Since concrete pipes are heavy, the database also adds the right handling machinery, in this case a crane, assuming the cost of the rental, the estimated time of use of the machinery and the diesel consumption per hour.
The calculations related with the trench characteristics and the volumes to be excavated and backfilled were estimated using the guidelines proposed in BInstalling pipes for distribution, irrigation, and sanitation according to current legislation( Adequa-Grupo Uralita 2007). Table 1 shows the rules used to calculate the width of the trenches, and Fig. 2 provides guidance on to calculate the depth of the trenches.
Environmental Impact Assessment
The types of materials and energy sources from the inventories were matched to their corresponding equivalents in the Ecoinvent database (Weidema et al. 2013 ). The potential environmental impacts were calculated through the use of LCIA characterization factors related to four impact categories from ReCiPe (H) 1.09 (Goedkoop et al. 2013) . The climate change (CC) category (measured as emissions of CO 2 equivalent) evaluates the emission of greenhouse gases that capture part of the irradiation reflected on the earth from the sun, which increases the temperature of the surface. The human toxicity (HT) category (measured in kilogram 1.4-dichlorobenzene (DB) equivalents) takes into account the environmental persistence and accumulation in the human food chain and toxicity of toxic substances related to human activities. The particulate matter formation (PM) category (measured as PM10 equivalents) evaluates the emission of small particles that can enter into the human body and negatively affect human health. Finally, the fossil depletion (FD) category (measured in kilogram of oil equivalent) considers the depletion of fossil fuels from hydrocarbons. All inventories used for the materials and energy production processes in this study were taken from Ecoinvent 3 (Weidema et al. 2013 ) except the inventories related to the materials deposition, which were taken from Ecoinvent 2.1 (Frischknecht et al. 2005) . 3 Results and discussion
Inventory Tool For Sewer Systems
To facilitate the creation of material and energy inventories and the assessment of LCA impacts for the construction and renovation of sewers, a tool that works automatically was created. This tool automates steps two to four described in Fig. 4 .
The tool was implemented in an Excel ® spreadsheet and incorporates all parameters and options required for sewer system construction, renovation, and end-of-life of sewer systems (Table 2) . Once the required input data are introduced, the tool automatically estimates the material and energy inventory, the LCA impacts (CC, HT, PM, and FD) and costs. The environmental impacts can be estimated either per considered materials or per considered processes. A prototype version of the SewerLCA tool, which is not intended to be used for commercial purposes, is provided as supplementary information.
Results of the inventory analysis stage are summarized in Table S1 (Electronic supplementary material).
Environmental Impact Profile And Costs For The Initial Hypothetical Sewer System
The construction and renovation of a 1-km PVC pipe with a diameter of 40 cm generates environmental impacts (Fig. 5) . For CC the overall impact represents 3.11·10 5 kg CO 2 eq., for HT 5.63·10 4 kg 1.4-DB eq., for PM 5.03·10 2 kg PM-10 eq., and for FD 1.14·10 5 kg of oil eq. depletion (in all categories this number is the sum of the contribution of both construction and renovation phases). Renovation of the sewer has a larger impact compared with that of construction, which is 2.2 times higher for CC, 3.2 higher for HT, 1.4 higher for PM, and 1.6 higher for FD (this accounts for two renovations). Except for CC, impacts from one renovation do not equal those from one construction, being larger for HT (renovation includes incineration of the PVC pipe which generates emissions of arsenic, barium, manganese, selenium, and vanadium into water, among others, associated with the combustion or the production of chemicals used during the incineration process) and smaller for PM and FD (e.g., 90 % of the granite sand is reused during renovation).
With regards to the construction phase (left side of double bars for construction and renovation in Fig. 5) , pipe laying (which also includes PVC pipes production) is the major contributor to the CC, FD, and HT categories, with a 55, 63, and 54 % share respectively. Backfilling represents 44 and 42 % of the PM and the HT impacts, respectively. With regards to the renovation phases, besides pipe laying, with a contribution to the impact of 52 % in CC, 56 % in PM and 80 % in FD categories, the deposition of trench materials significantly contributes to the impacts (particularly for CC and HT, with a share of 34 and 61 %, respectively).
When analyzing the contribution of the processes (right side of double bars for construction and renovation in Fig. 5 ), the production of PVC (both for construction and renovation) is the primary source of impact for CC (around 50 % for both construction and renovation), FD (60 % for construction and 74 % for renovation) and for HT (53 % share in the construction and 33 % in the renovation). Trench material deposition impacts are primarily driven by PVC incineration. Diesel burned in machines is the major contributor to the PM impact category (39 % contribution on the construction and 46 % on the renovation) together with the production of PVC (30 % for construction and 43 % for renovation). Either looking at the phases or processes, the non-inclusion of the renovation phase results in underestimation of the environmental impacts between 58 and 77 % depending on the category. Figure 6 shows that the sewer pipe renovation of the initial sewer system (90,480 €) (two renovations are included) is more expensive than its construction (73,970 €). The increase of costs is related to pipe laying because pipes are changed twice during the life span of the sewer. Analyzing the different phases, it is possible to see that for construction, backfilling, which includes the price of the granite sand, machines, water used, and labor force, is the most expensive phase followed by pipe laying, which includes the pipe, machines, water, and labor force. In addition, during the renovation phases, pipe laying is the most expensive process followed by backfilling and excavation because only a small part of new granite sand is replaced, whereas new pipes must be acquired each time. From a life cycle cost point of view, costs for renovation should also be included in the infrastructure asset management because they are higher than that for construction. Hence, the phases contributing the most to the environmental impacts are also the most expensive ones. Trench width depends on the pipe size and the working area needed. The trench width depends on the pipe diameter (because more extra space will be needed as bigger is the diameter), the necessity to underpin the trench or not and the angle between the trench wall and the soil horizontal. Information from Adequa-Grupo Uralita (2007) OD the outside diameter of the pipe in meters, β angle of the nounderpinned trench wall measured from the horizontal
Influence Of Pipes (Materials And Diameters)
Different pipe materials were evaluated (PVC, HDPE, precast concrete, and reinforced concrete) for diameters ranging from 20 to 160 cm (Fig. 7) , while maintaining the remaining characteristics of the initial sewer system. Regarding CC, PVC sewers always have a larger impact than concrete (reinforced concrete and precast concrete) and HDPE sewers. PVC results in 40 to 55 % larger impacts compared with that of HDPE depending on the impact category. The large differences between PVC and HDPE are explained by their different life spans (25 years implying two renovations against 40 years implying 1 renovation) and because for all of the studied categories except for FD, the impact generated per kilogram of tube produced is larger for PVC (it is worth noting that weight for PVC and HDPE were assumed to be the same).
Comparing PVC and concrete pipes, the relative differences are the largest and increase with diameter (up to 299 % for the FD impact category for a 150-cm diameter PVC pipe). For HT, two different groups of pipe materials can be distinguished. Significantly larger impacts are estimated for PVC and reinforced concrete, whereas smaller impacts are obtained for HDPE and precast concrete. Larger impacts are associated with materials production because the emissions to the air of mercury during PVC production and reinforcing steel contribute mostly to the HT impact. This difference becomes more evident as the diameter increases. For small diameters All the parameters considered in the sensitivity analysis conducted in this study are colored in grey (<50 cm) the difference between these groups is less than 100 % and for large diameters (>90 cm) increases up to 150 %. For PM, reinforced concrete has the highest impact followed by PVC, precast concrete, and HDPE. Differences between PVC and reinforced concrete are constant and have a higher impact by approximately 40 % for reinforced concrete. However, when comparing PVC against the other materials, differences appear with larger diameters (>90 cm, between 134 and 155 % for HDPE and 58-100 % for precast concrete) because the impact per kilogram of PVC is higher than HDPE in addition to the lower life span for PVC.
For FD, there are two types of materials that follow different trends. The first type includes concrete-based pipes, and the second type is plastic pipes. Plastic sewers have a 1.5 to 3 times higher impact compared with that of concrete sewers because plastic requires energy during its production and transport phases and also includes the embedded (fossil) energy in the form of crude oil.
Overall, the obtained results show that environmental impacts are lower for precast concrete and HDPE pipes. This fact is due to the longer life of concrete and HDPE compared with that of PVC and also because the production of PVC pipes (per kilogram of material) has a greater impact than other materials. In terms of CC, this statement is in agreement with Du et al. (2013) , where it was also shown that concrete and HDPE pipes have the lowest contribution to CC. However, Du et al. (2013) obtained higher CO 2 eq emissions than the ones obtained in this study. For instance, for a ≈30-cm PVC pipe, Du et al. (2013) estimated 3,600 kg CO 2 eq·km −1 ·year −1 and this study obtained 2,500 kg CO 2 eq·km −1 ·year −1 , and still Du (2013) did not consider incineration which would result in even 38 % higher emissions. In fact, the emission factors used in Du et al. (2013) for the production of PVC result in 19 kg CO 2 ·(kg PVC) −1 compared to 2.72 kg CO 2 · (kg PVC) −1 applied in this study using the Ecoinvent database. In contrast to our statement, Petit-Boix et al. (2014) concluded that PVC pipes have the lowest environmental impacts. Their results differ from ours mainly because they did not include the end-of-use processes of the renovation phase. In this study incineration of PVC and HDPE pipes were modeled, and more energy is recovered during the incineration process for the HDPE pipe since it has a much higher heating value than PVC (41.84 and 20.92 MJ·kg −1 , respectively). Piratla et al. (2012) concluded as well that HDPE pipes production and installation result in lower CO 2 emissions than PVC pipes.
Influence Of Transport Distances
As shown in Fig. S1 (Electronic supplementary material), varying the transport distances (from 0 to 100 km) of excess materials from the construction site to landfill and from suppliers to the construction site result in less than 4 % change for all impact categories compared to the initial hypothetical sewer system. By looking into Table S1 (Electronic supplementary material) it can be seen that the influence of transport distances is even lower as pipe diameter increases.
The influence of changing site-specific characteristics (soil type in construction area, asphalt placement need, and urban or non-urban setting) on the initial hypothetical sewer is shown in Fig. 8 . For the initial hypothetical sewer (with PVC pipes) changing from compact to soft soil does not make a difference on any of the impact categories, whereas changing to rocky soil increases the impacts between 9 and 34 % depending on the impact category. With increased diameter size (80 and 140 cm), the percentage of change decreases because the contribution of the tube laying and backfilling increases. When covering the trench with asphalt the impacts increase by around 20 % for CC and HT, and up to 35 and 55 % for PM and FD, respectively. Again, this influence decreases as the diameter increases. The environmental burden from constructing the sewer on an urban or a non-urban setting does not show a significant difference.
Considering the disposal of pipes at the end of their life enables the inclusion of both the additional impacts of disposal (e.g., transport, incineration) and also the recovery of feedstock energy from plastic material. The effect of taking into account the disposal process (incineration for PVC and HDPE with electricity production and a specific landfill for construction materials for precast concrete and reinforced concrete) is shown compared with the exact same sewer but without considering disposal (0 %) (Fig. 9 ). As shown in Fig. 9 , including the disposal process adds between 28 and 71 % of the impact to CC for plastic pipes, which is mostly due to CO 2 emissions from incineration. The partial recovery of electricity from the heating value of plastic materials in incineration does not offset the negative impacts from incineration emissions. For HT, the additional impact of disposal is even more pronounced, with an increase of 74-147 % for PVC compared with the baseline. For PM and FD, including the disposal phase is less important and adds only 1-8 % for PVC to the impacts, and in the case of HDPE, the impact decreases between 5 and 15 % because fewer resources are used and more energy is obtained in the HDPE incineration. For concrete-based materials, the impact of including disposal is marginal (<5 %) for all four impact categories, which is essentially because disposal only includes additional transport to the landfill and no subsequent emissions or energy recovery.
Influence Of Life Span
Given the defined life span ranges for each material, the selection of the highest or the lowest values (see Section 2.2.1) (compared to the average value assumed) greatly affects the obtained results (Fig. 10) . PVC increases between 20 and 40 % of the impact depending on the category when using the lowest life span. The increase is even larger for HDPE, between 40 and 60 % (but still lower absolute values compared to PVC pipes). For concrete materials, the selection of the highest life span value represents a decrease in the environmental impacts between 40 and 50 % depending on the category. The combination of the effect of the diameter together with the life span results in differences lower than 12 % (differences for each studied characteristic between the black, light grey, and dark grey). This means the influence of the selection of life span is large no matter the pipe diameter.
Influence Of Including Renovation Or Not
The results presented in Section 3.2 already indicate that including renovation for PVC pipes has a significant Fig. 8 Influence of site-specific characteristics (soft and rocky soil vs compact, asphalt placement when a road is constructed vs no asphalt, and urban vs non-urban setting) on the environmental impacts. The impact of each site-specific characteristic is evaluated for three different diameters (40, 80, and 140 cm) . Each diameter has its own baseline with the characteristics of the hypothetical sewer (compact soil, non-urban setting, and no-road construction)
Environmental impacts during the construction and renovation of sewers are subject to differences in material type, site-specific conditions and material life span. More specifically, the following conclusions can be drawn:
& Renovation of pipes, after their technical life span has expired, greatly influences all environmental impacts and costs; for the initial hypothetical sewer system evaluated (1 km PVC pipe of 40 cm diameter; non-urban area; compact soil; rectangular trench without underpinning; no asphalt placement), renovation is responsible for 55 to 77 % of all environmental impacts (among all categories). & The environmental impacts generated during the construction are mainly associated with pipe laying (which includes the production of the pipes) and backfilling of the trench. During the renovation, in addition to backfilling and pipe laying, the trench deposition phase has a high influence on the results. & For the initial hypothetical sewer system, the pipe material production process represents 30 to 60 % of the construction environmental impacts (among all categories) and 33 to 74 % of the renovation impacts. & An accurate accounting for the life span of the pipes is crucial. The uncertainty ranges evaluated in this study result in reductions of the impacts down to 51 % or increments up to 61 %. & The construction and renovation environmental impacts are lower for precast concrete and HDPE pipes due to the longer life of concrete and HDPE compared with that of PVC and also because the production of PVC pipes (per kilogram of material) has a greater impact than other materials. & Site-specific characteristics can influence the construction and renovation of sewers. Asphalt placement, on top of the trench, increases the environmental impacts from 20 to 55 % (among all categories). The presence of rocky soil (as opposed to compact or soft soil) can increase the impacts from 9 to 34 %. & Including the disposal process for plastic pipes adds up to an extra 71 % to climate change impacts and up to an extra 147 % to human toxicity impacts. & All calculations shown in this paper were obtained thanks to the SewerLCA tool, which was developed to facilitate the development of material and energy inventories for the construction and renovation of sewers and the calculation of environmental impacts and costs. This tool can be easily expanded and adapted to include other processes, which might be relevant in other countries.
SewerLCA tool can easily be upgraded to account for more trench backfilling options (e.g. combination of concrete and granite sand), the incorporation of more pipe materials, diameters and shapes (besides the circular ones). The tool includes the construction of manholes but not the pumping stations. Another interesting upgrade of the tool would be the inclusion of different types of uncertainties in the processes considered. Still the tool has a potential to become widely applicable to obtain detailed and complete inventories and to estimate the environmental impacts of sewer systems.