3.6. Lifecycle Environmental Impacts
We considered the optimal cluster size linked to a single water storage tank in order to compare the lifecycle environmental impacts between the proposed system (permeable pavement with stormwater harvesting) and the current scenario (traditional drainage system and conventional water supply). Thus, we considered a community size of 612 people, a paved area of 0.1% (which is equal to 28,505 m²), a stormwater tank with 500 m³ capacity, and a community water consumption of 91,702 L/day (as seen in Table 2
The functional unit—water supply (in m³) for the community over the 20-year time horizon—was calculated based on the results shown in Section 3.1
and was equal to 669,864 m³. All input and output data of the proposed system and the current scenario were quantified and are presented in Tables S7 and S8 (in the Supplementary data)
. Chlorination for the water treatment and piping to handle overflow were considered in the quantification. The impacts related to cleaning the permeable pavement were estimated considering the use of a sweeper consuming approximately five litres of diesel per hour. We considered that each maintenance took six hours to be completed.
compares the environmental impacts (characterisation) through the midpoint categories selected for this study, as stated in Section 2.3
. Most impacts caused by the current scenario (traditional drainage system and conventional water supply) were reduced due to the implementation of the permeable pavement system and stormwater utilisation.
Regarding the global warming category, the permeable pavement and stormwater utilisation scenario presented a reduction of 20.6% in CO2-equivalent emissions compared to the current scenario. The polystyrene production was the largest contributor of the permeable pavement scenario to this category, i.e., 9.8% of the total emissions. The high polystyrene production is due to the high number of pipes necessary to the stormwater system. The current scenario, in turn, showed the heat production process to be the most impacting for the global warming category, i.e., 27.0% of emissions. The high figure found for this process is mainly due to the production of hot mix asphalt (HMA), which is used as a conventional pavement surface course.
In the fine particulate matter formation category, the impact for the permeable pavement scenario was 33.3% lower compared to the current scenario. In the permeable pavement scenario, diesel processing was the process found to most contribute to the impacts in this category, i.e., 14.3% of equivalent PM2.5
emissions. In the current scenario, the heat production process again exhibited the most significant impact, i.e., 24.5% of equivalent PM2.5
emissions. The 33.3% higher result observed for the conventional scenario was mainly due to the use of hot mixture asphalt (HMA) in the conventional pavement surface, which demands high heat production and is responsible for increasing the levels of PM2.5
]. The fine particulate matter formation category is related to air pollution and a negative impact on human health, ranging from respiratory symptoms to hospitalisations and deaths, represented by the PM2.5
-equivalent unit, i.e., particulate matter with a diameter smaller than 2.5 μm [40
In the water consumption category, the impact for the scenario with permeable pavement and stormwater utilisation was 98.6% lower than the conventional scenario. This expressive result could be explained by the potable water production with conventional treatment in the current scenario, with this process having the greatest impact (50.9%). Also, the process of distribution of this potable water by the water utility contributed to the greater damages. In this category, the water consumed for potable purposes (showering, washing machines, drinking and cooking, washing up, etc.) was omitted in the assessment, as explained in Section 2.4
Regarding the human carcinogenic toxicity category, permeable pavements greatly reduced the impacts (64.1%). The treatment of slag from unalloyed electric arc furnace steel was the process with the greatest impact in both scenarios. This process is related to the final disposal of slag in the landfill. In the human noncarcinogenic toxicity category, the treatment of sulfidic tailing was the most impacting in the permeable pavement scenario (57.5%), while the treatment of spoil from hard coal mining was the most impacting in the conventional scenario (24.2%).
In the fossil resource scarcity category, the permeable pavement showed a growth of 20.9% in the kg oil equivalent emissions due to the additional stormwater system proposed, with necessary pipes, connections, tanks, and valves to use the stormwater harvested. Such components use larger quantities of petroleum. Petroleum production was the most impacting process in both scenarios (22.1% in the permeable pavement scenario and 19.7% in the conventional scenario).
Treatment of waste natural gas burned in production flare was the process with the most significant impact on the terrestrial acidification category in the permeable pavement scenario. For the conventional scenario, heat production was the most impacting (31.1%). The permeable pavement scenario achieved a 40.2% reduction in equivalent SO2 emissions for this category compared to the conventional scenario.
The other categories (mineral resources scarcity, land use, marine ecotoxicity, freshwater ecotoxicity, terrestrial ecotoxicity, ozone formation, and stratospheric ozone depletion) did not present relevant damages, as seen in the endpoint approach. Figure 4
shows the normalised environmental impacts for the two scenarios, showing midpoint impact categories with contributions to the endpoint approach.
shows the ReCiPe grouped endpoint categories (human health, ecosystems, and resources). Damage to human health was observed as the most impacting. The comparison of the two scenarios in a single score is shown in Figure 6
, showing which scenario has the highest potential for environmental impact by totalling the weighted data for each impact category. An overall reduction of approximately 47% in environmental impact was obtained using the proposed system. Water consumption, fine particulate matter formation, and global warming (damage to human health) were the most impacting categories.
Although most studies found in the literature did not take into account the use of stormwater infiltrated in permeable pavements, several authors found a better environmental performance for the lifecycle of permeable pavements compared to traditional pavements. Liu et al. [21
] showed better lifecycle performance for permeable asphalt pavement compared to dense-graded asphalt pavement in terms of energy consumption, global warming potential, acidification potential, smog formation potential, and human toxicity potential. Lu et al. [24
], in turn, concluded that porous asphalt pavement did not show a significant advantage in reducing energy consumption and greenhouse gas emissions compared to the normal dense asphalt due to the heating required to produce asphalt. In the study of Lu et al. [24
], the permeable feature of pavement was not considered, explaining the nondifference between the environmental impacts of the pavements.
3.7. Lifecycle Cost Analysis
shows the initial costs for permeable pavement with a stormwater harvesting system. The total initial cost, including materials and labour, was equal to £3,248,476.40, representing £113.96/m². The structure of the permeable pavement was responsible for 72.9% of the total initial costs, while the drains and the components necessary for the stormwater system (pipes, connections, tanks, and valves) were responsible for 6.6% and 20.5%, respectively.
The electricity consumption to pump stormwater was calculated by multiplying the power of the water pumps (kW) by the daily time of usage (four hours), resulting in 8.83 kWh/day. We considered the use of a 3 HP water pump. The pumping costs were calculated by multiplying the total amount of energy consumed during the lifecycle of the system by the average rate that Glasgow residents pay to their energy utilities. The tariff used was £0.17681 per kWh [46
]. The annual costs of cleaning the pavement twice-yearly were £600.00. The energy tariff and the costs of cleaning the pavement were corrected through the lifecycle analysis, according to Section 2.4
The economic benefit of the decrease in potable water consumption in the buildings was then assessed using Equation (2). The water tariff considered was equal to £2.422 [47
]. The average economic benefit calculated for the first year was £28,023.75. The water tariff was also corrected using the lifecycle analysis, according to Section 2.4
shows the initial costs for a conventional (impermeable) pavement with a traditional drainage system. The total initial cost, including materials and labour, was equal to £2,766,076.82, representing £97.04/m². Compared to the permeable pavement with a stormwater harvesting system, the initial cost of the conventional system was 14.9% cheaper. The structure of the pavement was responsible for 84.6% of the total initial cost, while the drainage components were responsible for 15.4% of the cost.
Considering the economic benefit of the decrease in potable water consumption (discounting the costs to pump stormwater and maintain the pavement), the discounted payback period of the proposed system would be 16.9 years. Therefore, the system can be considered economically feasible since its lifespan is equal to 20 years.
Studies that consider the hydraulic system necessary for stormwater harvesting are unusual. In the study developed by Vaz et al. [25
], the initial cost for the construction of a traditional pavement was 35.7% lower compared to the implementation of a permeable pavement (including the hydraulic system). However, unlike our study, the drainage system required for the traditional pavement was not considered by the authors. The discounted payback ranged from 7 to 15 years.