Life cycle assessment of construction and renovation of sewer systems using a detailed inventory tool

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. Comprehensive life cycle inventories (LCIs) for sewers construction and renovation were obtained by first identifying all processes involved after interviewing construction experts and reviewing sewer construction budgets from a Catalan company; and second transforming the processes into masses of materials and energy usage using construction databases. In order to run the life cycle impact assessment (LCIA) the materials and energy typologies from the inventories were matched to their corresponding equivalents into available LCI databases. Afterwards the potential impacts were calculated through the use of LCIA characterization factors from ReCiPe. Life cycle assessment (LCA) was run several times to assess the construction of a 1-km-long sewer with varying pipe materials, life spans for each material, diameters, transport distances, site-specific characteristics, and pipe deposition options. The environmental impacts generated by construction and renovation of a 1 km Polyvinylchloride (PVC) pipe with a diameter of 40 cm are mainly associated with pipe laying and backfilling of the trench. The evaluation of several pipe materials and diameters shows that the exclusion of renovation would underestimate the impacts by 38 to 82 % depending on the pipe materials and diameters. Including end-of-life phase for plastic pipe materials increases climate change (up to an extra 71 %) and human toxicity (up to an extra 147 %) impacts (among all diameters). The preferred pipe materials from an environmental point of view are precast concrete and High-Density Polyethylene (HDPE). Site-specific characteristics (specially the presence of rocky soil and asphalt placement) and material life span have a high influence on the overall environmental profile, whereas changes in transport distances have only a minor impact (<4 %). Environmental impacts during the construction and renovation of sewers are subject to differences in material type, site-specific characteristics and material life span. Renovation of sewers has a large influence on all potential environmental impacts and costs and, hence, should not be omitted in LCA studies. The treatment and disposal processes of plastic pipes at the end of their life has to be accounted in LCA studies.


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Human activities in households and industries consume large amounts of water which have to be treated before

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Three type of trenches can be considered for the construction of sewers depending on the site characteristics.

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Rectangular trenches are applied for rocky soils and for most compact soils (Figure 2a and b). When the 86 excavated soil is soft a trapezoidal trench is applied (Figure 2c). If asphalt is placed in the upper part of the 87 trench deeper trenches are needed (Figure 2a). The trench type shown in Figure 2b is the selected for the analysis 88 conducted in this paper, even though the Excel ® spreadsheet tool created considers the three options.

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which also includes its transport.

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Each phase comprises different processes such as materials production and transport, consumed diesel from 96 machinery work and disposal (incineration or landfilling). Work area cleaning phase includes the energy 97 consumption process. Excavation phase includes diesel consumption by the machines and transportation of the 98 excess material to a landfill. Pipe laying phase includes the production of the pipe, its transport to the workplace, 99 the diesel consumed during the pipe laying and water consumed to proof its reliability. Backfilling phase 100 considers the granite sand extraction/production, its transport to the workplace and diesel and water consumed 101 during its placement. Asphalt placement phase includes the asphalt production, its transport to the workplace and 102 its placement. The soil distribution around the work accounts for the diesel consumed.   The goal of this life cycle assessment was to compare the environmental impacts from the construction and 113 renovation of several sewer system typologies and to determine which are the phases, processes and related 114 parameters contributing the most to the environmental impacts. As a starting point a hypothetical sewer system 115 with a length of 1 km and a PVC pipe with a diameter of 40 cm was evaluated (this is the initial system from 116 now on). It was considered that the sewer is located in a non-urban area without traffic (no asphalt placement in

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After evaluation of the environmental impacts of the initial sewer system defined above, a sensitivity analysis 131 was conducted to evaluate which parameters involved in the sewer construction and renovation (parameters in 132 grey in Table 2) have the highest influence on the different environmental impact categories. The parameters 133 included in the sensitivity analysis are the type of pipe material (PVC, HDPE, precast concrete and reinforced 134 concrete), diameter of the pipes (ranging from 20 to 160 cm), transport distances for materials (ranging from 0 to 135 100 km) and the characteristics of the working area (location, type of soil and asphalt placement). Special 136 emphasis was put in analyzing the influence of pipe renovation, which is directly related to the life span of the 137 materials. In the analysis, variability related to the life span of pipes was included. For PVC, an average lifetime 138 of 25 ± 5 years was assumed. For HDPE, a lifetime of 40 ± 10 years was considered, and finally, for precast 139 concrete and reinforced concrete pipes, a lifetime of 70 ± 20 years was assumed. Even though pipe suppliers 140 normally specify longer life span ranges, construction companies and water agencies experience shorter life 7 spans in practice (Blosser et al., 2003). Hence, the ranges assumed in this paper were defined after personal 142 communication with the construction company Voltes S.L.U. (Catalonia) with more than 60 years of experience 143 in the field. Precast concrete and reinforced concrete pipes have the same weight. Certain materials that are used, 144 such as ductile iron, or pipe configurations, such as oval pipes, were not considered here. Finally, transportation 145 was considered for the environmental analysis but not for the economic analysis.

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With regards to renovation, the excavation process was considered as the excavation of compact soil. In addition, 147 the energy consumed during pipe extraction from the trench was considered to be the same as during its laying.

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Additionally, during pipe extraction and laying during renovation, granite sand losses of 10% were considered.

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Finally, it was assumed that concrete pipes and excess soil are disposed of in landfills, located at 30 km.,

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whereas plastic pipes are incinerated (a process that involves energy recovery).

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The inventory was carried out following the steps identified in

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The calculations related with the trench characteristics and the volumes to be excavated and backfilled were 167 estimated using the guidelines proposed in "Installing pipes for distribution, irrigation and sanitation according 168 to current legislation" (Adequa-Grupo Uralita, 2007). Table 1 shows the rules used to calculate the width of the 169 trenches, and Figure 2 provides guidance on to calculate the depth of the trenches.

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The tool was implemented in an Excel® spreadsheet and incorporates all parameters and options required for

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[place Table 2 in here]

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Results of the inventory analysis stage are summarized in Table S1 from supplementary information.

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With regards to the construction phase (left side of double bars for construction and renovation in Figure 5), pipe 212 laying (which also includes PVC pipes production) is the major contributor to the CC, FD and HT categories, 213 with a 55%, 63% and 54% share respectively. Backfilling represents 44% and 42% of the PM and the HT 214 impacts, respectively. With regards to the renovation phases, besides pipe laying, with a contribution to the 215 impact of 52% in CC, 56% in PM and 80% in FD categories, the deposition of trench materials significantly 216 contributes to the impacts (particularly for CC and HT, with a share of 34% and 61% respectively).

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When analyzing the contribution of the processes (right side of double bars for construction and renovation in 226 Figure 6 shows that the sewer pipe renovation of the initial sewer system (90,480 €) (two renovations are 227 included) is more expensive than its construction (73,970 €). The increase of costs is related to pipe laying 228 because pipes are changed twice during the life span of the sewer. Analyzing the different phases, it is possible 229 to see that for construction, backfilling, which includes the price of the granite sand, machines, water used and 230 labor force, is the most expensive phase followed by pipe laying, which includes the pipe, machines, water and 231 labor force. In addition, during the renovation phases, pipe laying is the most expensive process followed by     the diameter increases. For small diameters (< 50 cm) the difference between these groups is less than 100% and 255 for large diameters (> 90 cm) increases up to 150%.

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For PM, reinforced concrete has the highest impact followed by PVC, precast concrete and HDPE. Differences 257 between PVC and reinforced concrete are constant and have a higher impact by approximately 40% for 258 reinforced concrete. However, when comparing PVC against the other materials, differences appear with larger 259 diameters (> 90 cm., between 134-155% for HDPE and 58-100% for precast concrete) because the impact per kg 260 of PVC is higher than HDPE in addition to the lower life span for PVC.

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For FD, there are two types of materials that follow different trends. The first type includes concrete-based pipes, 262 and the second type is plastic pipes. Plastic sewers have a 1.5 to 3 times higher impact compared with that of 263 concrete sewers because plastic requires energy during its production and transport phases and also includes the    As shown in Figure S1, varying the transport distances (from 0 to 100 km) of excess materials from the 282 construction site to landfill and from suppliers to the construction site result in less than 4% change for all impact 12 categories compared to the initial hypothetical sewer system. The influence of PVC pipe transportation was even 284 lower (results not shown). By looking into Table S1, it can be seen that the influence of transport distances is 285 even lower as pipe diameter increases.   disposal (e.g., transport, incineration) and also the recovery of feedstock energy from plastic material. The effect 300 of taking into account the disposal process (incineration for PVC and HDPE with electricity production and a 301 specific landfill for construction materials for precast concrete and reinforced concrete) is shown compared with 302 the exact same sewer but without considering disposal (0%) (Figure 9).

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[place Figure 9 in here]

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As shown in Figure 9, including the disposal process adds between 28 to 71% of the impact to CC for plastic 305 pipes, which is mostly due to CO 2 emissions from incineration. The partial recovery of electricity from the   The results presented in section 3.2 already indicate that including renovation for PVC pipes has a significant 327 contribution. Table S2, including a summary of all the sensitivity analysis, shows how the non-inclusion of 328 renovation would underestimate the impacts from 40 to 80% for different pipe materials and diameters.

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The following conclusions can be drawn from the results presented: 332  Renovation of pipes after their technical life span has expired greatly influences all environmental and 333 cost impacts during the lifetime of a sewer system; in the initial hypothetical sewer system, the 334 renovation has an impact between 55 to 77% to the total environmental impact depending on the 335 studied impact.

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14  The environmental impacts generated during the construction are mainly associated with pipe laying 337 and backfilling of the trench. During the renovation apart from backfilling and pipe laying also the 338 trench deposition phase has a high influence in the results.

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 In the initial hypothetical sewer system, the pipe material production process has an impact between 30 340 and 60% depending on the impact category for construction and between 33 and 74% for renovation.

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 A proper life span selection for the pipes is crucial because the results greatly change, ranging from a 342 reduction of the impact of 51% to an increase of 61%.           β is angle of the no-underpinned trench wall measured from the horizontal.