Cooling and purification effect of permeable pavements

Rapid urbanization and the expansion of impervious areas have led to increasing urban flooding risks and runoff pollution problems (Qi et al. 2019; Ferrari et al. 2020). Urban stormwater runoff has not only contained pollutants such as suspended solids (SS), heavy metals, organic matter, total nitrogen (TN) and total phosphorus (TP) (Wang et al. 2015; Wu et al. 2015), but there has also been thermal pollution of water bodies caused by high-temperature stormwater runoff discharged into receiving water bodies (Li et al. 2020), resulting in lower dissolved oxygen concentrations, higher nitrogen and phosphorus concentrations, and massive aquatic-animal die-off from hypoxia, which has endangered the health of rivers and lakes and damaged water ecosystems (Xu et al. 2020a, 2020b, 2020c). Flooding could significantly raise the moisture content of materials in pavements to saturation, which is known to degrade the mechanical qualities of most paving materials (Awwad 2021; Nawir & Mansur 2022). Therefore, the study of pollutant removal and heat reduction in the initial runoff of pavement is very important for the environmental health assessment of surface water and ecological protection of river water in China. To manage stormwater problems in cities, various technical measures have been widely developed, including the Low Impact Development concept (LID) in the United States (Mohajerani et al. 2017), the Sustainable Drainage Concept (SUDS) in the United Kingdom (Mbanaso et al. 2019), Water Sensitive Urban Design (WSUD) in Australia (Morison & Brown 2011), the ABC Water Program in Singapore (Hopkins et al. 2018) and the Sponge City in China (Barbosa et al. 2012). Permeable pavement is a common LID technology with an open void structure in different structural layers that enables continuous infiltration of rainwater, cutting the effluent temperature by 4.4–5.2 °C (Xu et al. 2020a, 2020b, 2020c), and is easy to maintain, repair and renovate as a source control measure without occupying urban landscape space.

The permeable pavement is a paving technology based on full infiltration, including permeable surface layer, gravel base layer, geotextile and bedding layer (Guan et al. 2021). When generating heavy rainfall, rainwater first infiltrates into the pavement storage area and then discharges into rivers or infiltrates into the surrounding soil through catchment pipes and eventually into groundwater, which plays an important role in water quality and quantity regulation and maintains the stability of water ecosystems (Martins Vaz et al. 2020; Winston et al. 2020). It has been shown (Liu & Borst 2018) that permeable pavements are more effective in removing pollutants attached to suspended solid particles (Pb, Al, Fe, N and P) than dissolved pollutants (Cu, Zn and Mn), and the content of Na, K and V in the effluent of permeable concrete pavement (PC) is significantly higher than that of permeable asphalt pavement (PAP) and permeable brick pavement (PICP) and the reduction rates of stormwater runoff for different types of permeable pavement runoff reduction rates correlate well with rainfall intensity (Zhang et al. 2018), and the rainfall reduction rates of permeable pavements range from 10.21% to 32.26% under the conditions of rainfall return period of 1–10 a (years) (Xu et al. 2020a, 2020b, 2020c). It has also been found (Al-Hasan et al. 2020) that asphalt materials with polymers and recycled aggregates are highly effective in improving the quality and performance of asphalt pavements, providing an experimental basis for studying the performance and pollutant reduction capacity of different materials for permeable pavements. For the study on the effect of urban stormwater runoff heat pollution control, some scholars believed that its heat pollution mainly comes from the heat exchange process during rainfall on urban impervious substrates, and the heat pollution load was influenced by multidimensional factors such as substrate type, impervious area ratio, rainfall characteristics and climatic conditions, and the land use type and development intensity should be reasonably controlled during urbanization (Ferrari et al. 2020); some scholars also believed that stormwater runoff reduced through low impact development techniques (green roofs, rain gardens, depressed green spaces and permeable pavements) infiltrated into the lower structural layers, and since the lower structural layers not directly affected by solar radiation are usually cooler than the surface layers, the average temperature of the water discharge decreased by 4.4–13.2 °C (Mohajerani et al. 2017; Li et al. 2020). However, since there were many types of permeable pavements and different pavement forms had various effects on the treatment of different pollutants and heat, it is important to screen out permeable roads suitable for different regions and functional areas for the application of low impact development techniques to treat urban stormwater runoff and alleviate water pollution.

Although the removal effects of permeable bricks and permeable concrete with N, P, SS, heavy metals, organic matter and other physical pollutants have been reported by scholars, less research has been conducted on the removal, evaluation and control effects of various permeable pavements with runoff pollutants and heat for multi-occasion applications. In this study, permeable cement blocks (CP-B-Cement), steel slag permeable blocks (SP-B), ceramic permeable blocks (CP-B-Ceramic), permeable asphalt pavement (PAP) and impermeable pavement (IAP) were used as research objects to investigate purification and based on the results of the above study, the contribution ratio and key factors of cooling and purification are discussed, in order to provide a scientific basis and engineering parameters for the ecological pollution control technology of stormwater runoff in China and the design of urban roads according to local conditions.

The impervious asphalt pavement structure was composed of surface layer, base layer and soil layer from top to bottom: the surface layer was made of impervious asphalt with a thickness of 50 mm; the base layer was cement-stabilized gravel with a thickness of 300 mm; and the soil layer was 100 mm thick. Temperature sensors were set at different locations in the experimental system: the impervious asphalt pavement had temperature sensor S1 at the outlet, and the permeable pavement had temperature sensors at the overflow outlet and the bottom of each structural layer to monitor the surface layer outlet temperature T1, base layer outlet temperature T2, bedding layer temperature T3 and water sample temperature T4, respectively. The water sample sampling port was set at the bottom of each device, and the samples were taken every 15 min in the first 1 h and every 30 min in the next 60 min according to the change of rainfall intensity.

According to the pollution characteristics of urban road stormwater runoff, dissolved nitrogen and phosphorus content dominate in road rainfall runoff (Wang et al. 2015; Wu et al. 2015). It was also found (Yuan et al. 2019) that total nitrogen and total phosphorus concentrations were significantly reduced in urban stormwater runoff after grassy swale treatment, and the removal of heavy metal ions reached 30.95%–97.39%. For this reason, the four pollutants TP, TN, Zn and Pb were selected as the object of this experiment. The rainwater used for the test was taken to be prepared manually, and the specific concentrations are shown in Table 2.

The removal effect of permeable pavement types on heavy metal Pb varied widely, with cement permeable pavement outflow concentration of 0.15 mg · L⁻¹, which was higher than for the other permeable pavements, and ceramic permeable tiles had the best removal effect on Pb, up to 67.6%, but the average outflow concentration was still higher than 0.1 mg · L⁻¹, which could not reach the Surface Class V water standard. Compared with impervious pavement, the removal rates of Zn by cement permeable pavement, steel slag permeable pavement, ceramic permeable pavement, and permeable asphalt pavement increased by 52.8%, 55.7%, 62.3% and 70.2%, respectively, and the average concentration of Zn in the effluent of all four types of permeable pavement was lower than 0.3 mg · L⁻¹, which was much lower than the Surface Class V water standard (2.0 mg · L⁻¹). This phenomenon may be caused by the high concentration of TP and Zn in the inlet water of the permeable asphalt pavement. During the simulated rainfall, the rainwater containing high concentrations of TP and Zn gradually filled the pores in the pavement structure and reached saturation, reducing the pollutant removal efficiency and resulting in the concentrations of TP and Zn in the effluent water quality exceeding the Class V threshold of surface water.

The influence pattern of road runoff concentration on the removal rate of Zn and Pb was similar to that of P. Moreover, the purification effect of permeable asphalt pavement was significantly higher than that of the other types of permeable pavement under different pollution loads, and the removal effect of Pb was improved to 65.7%, 78.2%, 81.7% and 84.1% for the four types of permeable pavement at high concentrations, which was 15%–25% higher compared with the purification effect for pollutants at low concentrations. The permeable cement pavement had the lowest removal rate (74.1%) for Zn, and the permeable asphalt pavement was 1.21, 1.13 and 1.09 times more effective than the cement, steel slag and ceramic permeable pavements, respectively, in purifying the infiltrated runoff for different concentrations of pollutants under high concentration pollution loads, and the permeable pavement types had different purification effects for different concentrations of pollutants.

Rainfall intensity also had a greater influence on the removal rate of heavy metals Zn and Pb. When the rainfall recurrence period increased from 0.5 to 3 a, the removal rate of Pb decreased in the different types of permeable pavement by about 12.7%–21.1%; the decrease was the largest for permeable cement pavement, about 21.3%, the removal rate of Zn was the highest for permeable asphalt pavement and the least influenced by rainfall intensity, and the removal rate of permeable cement pavement was the most influenced by rainfall intensity, decreasing by about 24.3% when the rainfall recurrence period increased from 0.5 to 3 a. The degree of influence on different pollutants by rainfall intensity was in the following order: Zn > Pb > TN > TP, and the degree of influence of different permeable pavements by rainfall intensity was CP-B-Cement > SP-B > PAP > CP-B-Ceramic.

Permeable pavements can effectively reduce harmful pollutant loads in road stormwater runoff while controlling runoff, but their purification effects are influenced by many random factors, such as road runoff water quality, rainfall characteristics and seasonal climate. In this study, the four types of permeable pavements showed advantages in all aspects of road runoff control over impervious pavements (Figures 3 and 6), which was similar to the results of Hernández et al. (2019). Ceramic permeable pavements showed the highest average pollution load reduction (65.3%), which was higher than that of steel slag permeable pavements (61.7%), cement permeable pavements (59.4%), permeable asphalt pavements (47.9%) and impermeable pavements (7.4%) in that order.

In general, the generated surface load of N, P and organic matter became higher with the increase of rainfall intensity, and the removal rate of TN by permeable asphalt pavement was even lower than 20% under the condition that the rainfall return period reaches 3 a. The study of Wang et al. (2019) found that the increase in rainfall intensity increased the infiltration rate of rainwater in permeable pavement, and reduced the hydraulic retention time between runoff and permeable materials, and reversible adsorption sites were occupied rapidly with the rise of infiltration, which led to a gradual increase in the concentration of various pollutants in the effluent. The results of this study showed that compared with other pollutants, the removal effect of TP and Zn showed a relatively stable trend under different rainfall intensities, and surface adsorption was the main removal pathway for phosphate and Zn, which was also revealed by similar studies; the impact load resistance of ceramic permeable pavement was relatively stronger than that of the other pavements, mainly due to the fact that ceramic permeable tiles were fired at high temperatures, and the surface of the pavement had a large specific surface area as well as a complex micro- and mesoporous structure, resulting in more effective adsorption sites. Although the average removal rate of pollutants in stormwater runoff from permeable pavement in this study ranged from 36.3% to 84.5%, the content of some pollutants (TN and Pb) still exceeded the limits of the surface water environmental quality standard (GB3838-2002). Obviously, it is difficult to achieve effective removal of different pollutants by infiltration of rainwater through permeable pavement alone, and other technical measures must be combined.

Some researchers have found that stormwater runoff with higher pollution concentrations leads to higher concentrations in permeable pavement outflow (You et al. 2019), and similar studies have shown (Jiang et al. 2015) that pollutant concentrations have a greater effect on pollutants with low removal rates and a smaller effect on pollutants with high removal rates. In this study, the removal efficiency of TP, Pb and Zn was increased for all four permeable pavements under extreme rainfall conditions at high concentrations, and only the effluent concentration of TN was gradually increasing, mainly because adsorption was the main way of nitrogen removal from permeable pavements, and the structural layers composed of asphalt, cement, steel slag, ceramic, gravel and coarse sand were designed only considering strength, drainage or other road performance, and the selection of the adsorption capacity of the materials was poor, and the limited adsorption sites made each construction material approach or reach the adsorption saturation amount in a short period of time. A large number of studies have shown (D. S. Li et al. 2019; J. Li et al. 2019) that root nitrate nitrogen was generally removed by ion exchange, chemical reduction or biological denitrification, and the amount reduced by adsorption is low (<10%), resulting in a low and unstable removal rate of TN from stormwater runoff. Under extreme rainfall events with high concentrations, although road material voids may retain certain pollutants by trapping or adsorption, they were still retained inside the permeable pavement and may accumulate and be released leading to secondary pollution of the environment if not properly maintained at a later stage.

Previous studies found that LID facilities had the potential to mitigate thermal pollution from stormwater runoff (Sounthararajah et al. 2017; Rommel et al. 2021). D. S. Li et al. (2019) and J. Li et al. (2019) also showed that the synergy of multiple sponge-city infrastructures was more efficient in reducing thermal pollution than a single technology, with the average removal rates of thermal pollution from stormwater runoff for permeable asphalt, cement, steel slag and ceramic pavements being 19.9%, 27.5%, 30.1% and 27.4%, mainly for volume reduction and heat exchange to reduce the temperature of stormwater runoff. However, other studies suggested that their own material properties, construction system and water storage capacity were usually the main reasons for heat reduction (Wang et al. 2019). In this study, the reduction rate of runoff heat pollution from steel slag permeable pavement could reach 30% in the early stage, which was 8.6 °C higher than the outlet temperature of impervious pavement under the same temperature runoff and was lower than that of the rain garden studied by Xu et al. (2020a, 2020b, 2020c), but higher than permeable brick and permeable asphalt pavement, etc. Steel slag permeable pavement could increase convective heat exchange and promote the removal of thermal pollution due to its rough surface and strong thermal conductivity, and the higher the initial infiltration runoff temperature, the higher the thermal pollution reduction rate. The reduction rate of thermal pollution on impervious pavement and other permeable pavements was 3.1%–27.1%, which was at a low level. At the beginning of rainfall, the internal structural layer of permeable pavement was in a non-saturated state, and the percolation process of the permeable surface layer was ‘water absorption – saturation – percolation’. When the inflow rate was greater than the percolation rate, rainwater was mostly stored in the water storage area, and the ‘storage – slow release’ process of rainwater played a major role in the volume reduction of permeable pavement. In the ‘slow release’ process, the temperature of the structural layer material was lower than the runoff temperature, and the deeper the layer, the lower the temperature of the structural material, the ‘stored’ runoff inside the permeable pavement aquifer and steel slag, gravel, ceramic, cement and asphalt mix. Li et al. (2013) showed that when the thermal conductivity of the material increased from 0.6 to 2.6 W/m/K, the average temperature of the surface layer decreased by about 7.0 K. The strong thermal conductivity could make the heat accumulated in the road surface layer be rapidly introduced into the lower layer of the structure, and in this study, K_SP-B > K_CP-B-Ceramic > K_CP-B-Cement > K_PAP; the surface layer material properties enabled materials such as steel slag and ceramics to increase convective heat exchange and faster heat exchange with the contacted rainwater runoff. In addition, the infiltration rate of rainwater had an important effect on the reduction of runoff pollutants and heat; the higher the rainfall intensity, the smaller the heat removal from the permeable pavement, which was consistent with the studies of Xu et al. (2020c).

In this study, the purification and cooling performance of four commonly used permeable pavements was evaluated by simulating rainfall with different characteristics (intensity, pollution load and runoff temperature). However, detailed studies have not been conducted on the pathways of pollutant removal in permeable pavements through void configuration and material properties, pollutant accumulation in the pavement, and the risk of groundwater contamination; the adsorption between stormwater runoff and permeable pavements and their removal efficiency for heavy metals, nutrients, organic matter and thermal pollution are important for long-term (longer rainfall cycles and more rainfall) operation. Therefore, our future research will focus on long-period experimental studies and the contribution ratios of different mechanisms (biotransformation, adsorption and energy transfer) to the removal of each pollutant from stormwater runoff by permeable pavement systems of different configurations.

Choosing a reasonable type of underlayment can reduce rainwater runoff pollution. Wang et al. (2019) monitored permeable pavement in an outdoor test site of Guangzhou construction and found that permeable pavement surface materials could effectively decrease environmental heat pollution load, of which ceramic permeable tiles effectively reduced about 10 °C, while permeable concrete could only reduce about 5 °C. Compared with impervious roads, permeable pavement rainwater runoff was through infiltration into the grass-roots level, and its temperature was lower than the surface ambient temperature as the grass-roots level was not directly affected by solar radiation; a reasonable choice of rainwater control and utilization technology could also reduce rainwater runoff heat pollution, and green roofs collected heat and could reduce 15%–51% of the total urban heat through continuous use, 21%–44% of the total runoff and 33%–82% of the pollution load, with significant deterrent effect. In the process of sponge city development and construction, priority should be given to the use of source decentralized rainwater control and utilization technologies to control rainwater runoff pollution, such as permeable roads, rain gardens, green roofs, grass-planting ditches and recessed green areas. On the one hand, such source processing technologies could effectively control initial runoff with higher temperature, as the runoff infiltrated to the structural layers with lower temperature and fully contacted with the materials of each structural layer for heat energy exchange to reduce road runoff thermal pollution; on the other hand, these source treatment measures continued to increase the removal effect on the initial runoff of different pollutants, directly reducing the pollution load of stormwater runoff. In the receiving water body areas that were sensitive to temperature changes, we can try to establish relevant specification standards and control systems for road runoff thermal pollution in the process of land development and planning design to maintain the ecological balance of water bodies.

• (1)

  The purification efficiency of four permeable pavements was in the order of CP-B-Ceramic > SP-B > CP-B-Cement > PAP, the removal rate of TN for all permeable pavements was lower than 60%, and the effluent water quality concentration was higher than the surface V water threshold, indicating that the initial stormwater runoff was infiltrated through permeable pavements or discharged directly into rivers and lakes, which had a certain risk of pollution to surface and groundwater.

• (2)

  When the return period increased from 0.5 to 3 a, the temperature of permeable asphalt, cement, steel slag and ceramic pavement outflow increased by 2.4, 1.5, 0.7 and 1.8 °C, respectively. The cooling characteristics and pollutant reduction capacity of permeable pavement both diminished with increasing rainfall intensity; the higher the rainfall intensity, the lower the pollutant reduction capacity.

• (3)

  The runoff heat pollution reduction of four permeable pavements was in the order of SP-B > CP-B-Cement > CP-B-Ceramic > PAP. Heat pollution reduction rate increased with the rise of runoff temperature, and the highest heat reduction efficiency of cement permeable pavement was up to 36.25% when the inlet water temperature was 40 °C. The surface material characteristics of steel slag and ceramic enabled the convective exchange of heat and accelerated the contact with rainwater runoff for heat exchange. In addition, the effect of stormwater infiltration rate on the reduction of runoff pollutants and heat needed to be further investigated.

This research work was supported by Xinjiang Biomass Solid Waste Resources Technology and Engineering Center of China (KSUGCZX2022); Lianyungang Key Research and Development Plan (Social Development) Project of China (SF2130); Postgraduate Research & Practice Innovation Program of Jiangsu Province (SJCX23_1814); a Project Funded by the National First-class Disciplines (PNFD) and a Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD); Natural Science Foundation of the Jiangsu Higher Education Institutions of China (22KJB560001).

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.