Environmental and geometric optimisation of cylindrical drinking water storage tanks

Urban water cycle construction processes are an important element to consider when assessing the sustainability of urban areas. The present study focuses on a structural and environmental analysis of cylindrical water tanks. The goal is to optimise cylindrical water tanks from both an environmental (environmental impacts due of life cycle assessment (LCA)) and a geometric perspective (building material quantities for construction purposes depending on the tank characteristics). A sample of 147 cases was defined based on different positions (buried, superficial and partially buried), dimensions (combinations of heights and radii) and storage capacities (between 100 and 10,000 m3). A structural analysis was conducted for a defined set of cases to determine the quantities of steel and concrete required for its construction. The environmental impacts of the entire life cycle were assessed through a life cycle assessment (LCA). Additionally, environmental standards (the less impactful option for each dimension assessed: geometry, storage capacity and position) defined in the study were applied to realistic cases to evaluate potential environmental savings. The LCA shows that materials are the main contributor to environmental impacts (more than transport, installation and end of life cycle stages). For this reason, the results of the structural and environmental assessments coincide. Taller water tanks have shown to be less impactful (60 to 70 % less impact for a 10.000-m3 tank). Regarding the position, superficial water tanks have shown to have between 15 and 35 % less impact than buried ones. The environmentally preferred water storage capacity is between 1000 and 2500 m3, being between 20 and 40 % less impact. For instance, an 8000-m3 tank would emit 1040 t of CO2 eq. Applying the environmental standards 170.5 t of CO2 eq could be saved (16 % of the total amount). The results of this study show that among the cases analysed, superficially positioned cylindrical water tanks of 8.5 m in height and of between 1000 and 2500 m3 in storage capacity present fewer impacts. The use of these standards in municipal water tanks construction projects may significantly reduce environmental impacts (10 to 40 %) in all impact categories.


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Typically, mid-sized cities (10,000 to 50,000 inhabitants) include several water tanks that perform one or 92 more of the following functions: flow regulation, pressure regulation and supply security. Municipal 93 water tanks that service mid-sized and large cities are typically constructed of concrete, given that this is 94 the most common material used to construct large tanks. According to their geometries, water tanks can 95 take various shapes. The most common are rectangular and cylindrical in geometry. Among all possible 96 configurations, the cylindrical form serves as the best structural configuration and allows for a greater 97 optimisation of materials, as it offers the smallest perimeter for a given height and volume (CEDEX, 98 2010). For this reason, the analysis performed in this paper focuses on cylindrical configurations as a first 99 step. However, cylindrical tanks cannot always be installed due to urban form limitations. Thus, an 100 analysis of rectangular tanks will also be of interest to the reader.

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The goal of the present article is to optimise cylindrical water tanks from a geometric and environmental 112 perspective. In defining environmental standards (the less impactful option for each dimension assessed: 113 geometry, position and storage capacity), we aim to reduce environmental effects of drinking water tank 114 construction. The specific goals of the study are:

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 To select a number of representative cases of cylindrical water tanks that present realistic ranges 116 of volumes, dimensions and positions (buried, superficial and partially buried).

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 To assess the geometric and environmental optimisation of water tank cases analysed using the 118 LCA methodology.

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 To identify the best water tank (based on dimension, position and volume) among the cases 120 studied for each volume and to define a curve for the calculation of environmental impacts of the 121 optimal cases.

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 To apply the assessment methodology to three case studies.

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The results of the present study will serve as new information on the environmental impacts of  The following variables were considered to assess the geometric and environmental optimisation of 130 cylindrical water tanks: (1) position in relation to ground level, (2) storage capacity (tank volume) and (3) 131 dimensions (in terms of height and radius). Following CEDEX (2010), for constructive reasons, a 30 cm-132 thick wall is generally used when designing water tanks (given the minimum distance between walls 133 needed to set the reinforcement and to cast the formworks). Therefore, a fixed logical 30 cm-wall 134 thickness value was used.
Three different positions in relation to the ground level were considered: superficial (S; 0% of the tank 136 underground), partially buried (P; 50% underground) and buried (B; 100% underground). For each, seven 137 different volumes were analysed (in m 3 ) (100; 500; 1,000; 2,500; 5,000; 7,500 and 10,000), covering the 138 range of water tanks commonly used in small and mid-sized municipalities (Agbar, 2013). Finally, for 139 each tank position and volume, seven different heights (and radii) were studied. The following heights 140 were considered (in m) (2.0, 3.0, 4.0, 5.0, 6.0, 7.0 and 8.0), and each was allocated an additional covering 141 of 0.5 m for construction reasons. All values considered in each case studied are summarised in Table 1  In total, 147 different water tanks were analysed.

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To distinguish between the different case studies, the following nomenclature beginning with C

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In turn, a 6.5 m-tall, 1,000-m 3 superficial cylindrical tank would be expressed as "CS10006". The functional unit is the reference value that all cases compared must be referred to. This is a basic 150 element of an LCA that must be properly defined. For this study, the functional unit considered is one 151 cubic meter of water storage capacity, including the production, transport, installation and end of life of 152 the water storage tank for 50 years.

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Because each structure analysed was composed of reinforced concrete, no differences were considered 154 regarding the lifespan of each structure in our comparisons. However, the authors estimate a municipal 155 water tank lifespan of approximately 50 years. This lifespan was used, for instance, in Vargas-Parras et al.

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Thus, the resulting total impact of the tank was divided by its total capacity for each impact category 158 (impact/m 3 of water stored). The final volume required for storing water will depend on different service 159 factors such as the number of inhabitants.

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Life cycle stages of the system assessed along with system boundaries and different elements considered 161 for each stage are shown in Figure 1. Note that the operation phase has been excluded, as it varies 162 considerably across cases, and especially due to water pumping differences (section 1). It is evident that

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Following common sector practices, 30 MPa concrete and B500S steel materials were used in this study.

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Moreover, the tank was placed in a general exposure class IIB (exteriors in the absence of chlorides, 194 subject to rainwater action, in areas with an average annual rainfall level of less than 600 mm), which,

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The following six midpoint impact categories were considered: abiotic depletion potential (ADP),

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From the plant to the installation site, a distance of 130 km was used for the reinforcement of steel bars.

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To apply the results, three water storage tanks were considered as case studies. Table 2 lists 228 characteristics of the three case studies. These case studies were based on data on real water tanks. An optimal geometric configuration is given for the solution that uses fewer materials for a given water

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The results show that the tallest tank (CP1008) has an impact equivalent to roughly half that of the 301 shortest (CP1002) tank for the 100 m 3 water tanks and equivalent to roughly one third that of the 10,000

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However, increasing tank height while reducing tank radius is only useful to a certain point.

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As shown in Figure 6, superficial water tanks present the lowest environmental impacts across all impact 320 categories (between 15 and 35% less for 100 m 3 water tanks and between 20 and 35% less for 10,000 m 3 321 water tanks). Superficially placed water tanks do not require soil excavation and landfill transport. For 322 this reason, superficial water tanks constitute the environmentally preferred option. These results are also 323 consistent with those presented in section 3.1, as superficial water tanks require less reinforcing steel.

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Though not considered in this study, it may be possible to use excess soil for tank wall reinforcement 325 purposes to at least partially mitigate the impacts from the transport and landfilling of this material.

Capacity optimisation 342
Finally, the environmental impacts per cubic meter of storage capacity were determined for each volume 343 for the most optimal cases shown in sections 3.2.1 and 3.2.2 (the tallest superficially placed tanks).

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For the rest of the impact categories (CED, ADP, EP and POCP), the lowest environmental impacts 361 correspond with the 1,000 m 3 water tank, being significantly higher for volumes lower than 500 m 3

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(between 5 to 30% higher) and larger than 5,000 m 3 (between 5 and 20% higher). In this case, water tanks

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Nonetheless, material contributions vary depending on the volume and impact category. Steel 383 contributions to environmental impacts of ADP are higher than those for GWP (approximately 50% to 384 75% for ADP as opposed to 30 to 60% for GWP). Additionally, because water tanks with larger storage 385 capacities require more reinforcing steel, steel contributions are higher for higher volumes (nearly 60% 386 for 10,000 m 3 as opposed to 30% for 100 m 3 for GWP). For POCP, steel impact percentages are higher 387 than those of the other impact categories (between 70 and 85% for each volume), explaining 388 differentiation shown in its curve in Figure 6.

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Another significant contributor to environmental impact levels for each case pertains to material 390 transport, representing roughly 10% of the total value. It must be highlighted that this element is highly 391 related to the quantity of materials required for construction, reinforcing the importance of concrete and

Environmental assessment of case studies 402
The results of three case study assessment are presented in Figure 9. The environmental impacts of the 403 optimal water tank are significantly lower (between 10 and 40%) for all of the cases. Additionally,

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GWP=Global warming potential, ODP=Ozone layer depletion potential, POCP=Photochemical oxidation 414 potential, CED=Cumulative energy demand 415 For water tanks 1 and 2, environmental impact reductions are attributable to lower quantities of 416 reinforcing steel required for their construction (roughly 30% less impact). By contrast, an environmental 417 impact reduction of only approximately 5% is found in the case of concrete material use. For water tank 418 3, these environmental savings are less significant, all falling below a 5% reduction. For the three cases, 419 processes of excavation were disregarded (along with their environmental impacts), as the optimal water 420 tank found in the sample assessed is superficially positioned.

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In absolute terms, the optimisation of these water tanks would reduce emissions by between 19.2 (for the 422 400 m 3 water tank) and 170.5 t of CO2 equivalents (for the 8,000 m 3 water tank).

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While these environmental savings cannot be applied to existing water tanks, applying environmental

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After analysing a sample of 147 cases, it is concluded that the superficially placed, 8.5 meter tall water 430 tank (and its corresponding radius according to volume) with a storage capacity of between 1,000 and 431 2,500 m 3 performs the best environmentally.

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It is environmentally preferable to position water tanks superficially rather than underground, as less

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Water tank structural optimisation is essential provided that reinforcing steel and concrete required for 442 construction are the elements that contribute the most to tank environmental impacts (between 30 and 443 75% of the global impact for steel and between 7 and 50% for the global impact of concrete).

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Regarding water storage capacities, water tanks with volumes ranging between 1,000 and 2,500 m 3 are 445 environmentally preferable, as the relative quantity of steel and concrete required for tank construction        water tanks with 100, 500, 1,000, 2,500, 5,000, 7,500 and 10,000 m3 of storage 646 capacity depending on their height. 647 and GWP for 8.5 m tall 100 and 10,000 m3 cylindrical and partially buried water tanks. 662 663 Figure 9. Estimation of environmental effects of three real drinking water tanks based 664 on the present situation and a hypothetical optimal water tank of the same volume.