Introduction

In recent years, permeable pavements have become one of the most frequently used low impact development (LID) techniques (Hernandez-Crespo et al. 2019; Liu et al. 2019; Rahman et al. 2015; Kia et al. 2019). This infiltration-based technology consists of structural layers with relatively high porosity to allow rainwater to pass through its surface and underlying layers, including a permeable pavement surface, aggregate subbases, and sometimes includes geotextiles and underdrains, and eventually infiltrate into the natural soil or discharge into a drainage system (Imran et al. 2013; Kuruppu et al. 2019; Chandrappa and Biligiri 2016).

In addition to good hydrological effect in reducing runoff, delaying peak flow, increasing infiltration rate, and recharging the ground water (Rowe et al. 2010; Newman et al. 2014; Fassman and Blackbourn 2015; Collins et al. 2010; Brown and Borst 2014), permeable brick paving system (PPS) also exhibits a good decontamination potential in improving water quality (Boogaard and Lucke 2019; Chu and Fwa 2019; Drake et al. 2013; Jiang et al. 2015). Many scholars had researched the management performances of PPS. Fassman E A et al. found that the runoff reduction rate and flood peak reduction rate of PPS were 72% and 89% in the power clay circumstance, respectively (Fassman and Blackbourn 2015), while Collins K A et al. found that the runoff reduction rate and flood peak reduction rate of PPS were 38–66% and 60–77% in the clay circumstance (Collins et al. 2010). There were more researches on runoff rainwater quality for permeable pavement and showed that PPS were highly effective in the mitigation of pollutants in rainwater runoff; it not only removed suspended solids but also decreased total phosphorus and heavy metals (Boogaard and Lucke 2019; Chu and Fwa 2019; Drake et al. 2013; Jiang et al. 2015). For example, research has shown that suspended solids, total phosphorus, and lead can be decreased by up to 64%, 74.2%, and 79% (Legret et al. 1996).

At present, in China, except the bricks in which the infiltration of storm water is through open joints filled with clear aggregate, there are more and more applications of bricks that can infiltrate themselves (Lin et al. 2020; Lin et al. 2019; Yang et al. 2019). Obviously, the role of surface bricks is becoming more and more important. However, fewer researches focused on the surface layer of PPS, namely, the surface permeable brick. In fact, the effect of permeable brick on water purification was very significant. As the surface layer of PPS, the permeable brick was the first layer that contacts directly with the runoff. The characteristics of the brick would remarkably affect the runoff quality (Zhang et al. 2018; Li et al. 2017), especially for permeable bricks that can permeated. Li et al found the shale brick had a good removal effect on some pollutants in runoff (Li et al. 2017). Furthermore, the composition of the material in the brick itself would also directly affect the quality of the rainwater. Some permeable brick containing cement is alkaline in nature, and it released hydroxide ions and carbonate ion when in contact with rainwater which increase the pH of the effluent (Chandrappa and Biligiri 2016; Zhang et al. 2018). Researches showed that the pH values rapidly increased at first, followed by a gradual decrease and eventually stabilized at about 10.77 compared with 7.40 pH values of raw water for the cement brick (Zhang et al. 2018). The pollution status and cleaning characteristics of bricks had the greatest relationship to the overall removal rate of pollutants. Moreover, permeable bricks did not have the ability to self-purify, pollutants accumulated in the bricks as time increased. During a complete rainfall event, the pollutants trapped in the initial rainwater might be washed away in the middle and late stages of rainfall, causing secondary pollution of the environment. Hence, it is necessary to strengthen the research on surface permeable brick, especially in decontamination.

Nowadays, there are three common used permeable bricks in Jiangsu province, China, namely, steel slag permeable brick, ceramic permeable brick, and cement permeable brick. The processes and raw materials of the three permeable bricks were different. For instance, ceramic permeable brick was manufactured by sintering process, cement permeable brick and steel slag permeable brick were manufactured by pressure forming, and the raw materials for the latter two were cement and steel slag, which were very different in characteristic. Obviously, the performance of the brick was affected by the nature of the brick. Considering the different manufactured processes and raw materials, it is inevitable to investigate decontamination ability of three kinds of permeable bricks.

In this study, ceramic brick, cement brick, and steel slag brick were chosen to investigate the influencing factors on their decontamination ability such as rainfall intensity and contaminant concentration. The pollution status referred to the cumulative pollution inside the permeable brick, and cleaning characteristics referred to the pollutant discharge of the brick scoured by rainwater were further discussed. The objectives and novelties of this research were to explore the properties of brick and external rainfall factors on the decontamination ability of permeable bricks, which could offer a new perspective for the decontamination research of pervious bricks.

Materials and methods

Experimental setup

The test device for the permeable brick paving system was shown in Fig. 1. The dimensions of the device were 0.8 m × 0.8 m × 0.5 m. A total of 16 permeable bricks measuring 20 cm × 20 cm × 5.0 cm were placed in close proximity. A 10-mm-diameter outlet pipe was set at the bottom of the permeable brick system. An overflow pipe with a diameter of 10 mm was set at the top of the device.

Fig. 1
figure 1

Schematic of the device

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The prepared rainwater was lifted by a water pump and evenly distributed by a rotating water distributor. The rainwater was sampled after passing through the ceramic permeable brick and the paving system. Samples were taken every 10 min for a total of 2 h, while the outlet pipe of the pavement had a stable outflow. The water sample was collected using a 500-mL polyethylene sampling bottle, and all samples were measured within 48 h.

Pervious brick

The three previous bricks measured 20 cm × 20 cm × 5.5 cm were purchased from Nanjing local building materials market. All the three bricks can be divided into two layers in a common configuration with small constituent materials located on the top and large materials located on the bottom, and the upper layer has a thickness of about 1.0 cm with relatively smaller diameter of the particle, and the lower layer is about 4.5 cm thick (Fig. 2a).

Fig. 2
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Three permeable bricks. a Ceramic, b cement, and c steel slag

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The related properties of three permeable bricks including permeability coefficient and porosity all meet the requirements of the Chinese national standard, permeable paving bricks, and permeable paving flags (GB/T 25993-2010). The main properties of three bricks are shown in Table 1.

Table 1 The main properties of three permeable bricks
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Simulated rainfall

The simulated rainfall was carried out according to the situation in Suzhou, the test was based on the 2-h rainfall duration, and the rainfall intensity was altered by adjusting repetition period of rainfall of 5a, 20a, and 50a.

According to the existing monitoring data (Gong et al. 2019; Wang et al. 2019), the concentrations of various pollutants in the artificially prepared rainwater were shown in Table 2.

Table 2 Concentrations of runoff rainwater pollutants
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Characterization of the cleaning

In the cleaning process, The sample was taken every 5 min, and then the average concentration was calculated. The effluent removal rate was defined and calculated to characterize the flushing difficulty of pollutants, and the pollution status and cleaning characteristics of permeable brick.

$$ \eta =\frac{C_i-{C}_{\mathrm{e}}}{C_{\mathrm{i}}}\times 100\% $$

where η is the effluent removal rate, percent; Ci is the concentration of influent of rainwater, milligram per liter; and Ce is the mean concentration of effluent of washing water, milligram per liter.

Test methods

According to the Chinese National Standard Methods (SEPA of China 2002), SS was determined by the gravimetric method (GB 11901-89); COD was determined by the fast digestion-spectrophotometric method (HJ/T 399-2007); concentrations of NH3-N, TN, and TP were determined by the spectrophotometric method; and concentrations of Zn, Cu, Pb, and Cd were determined by atomic absorption spectrophotometry. According to linear transverse winding test of Standard Test Method for Microscopical Determination of Parameters of the Air-Void System in Hardened Concrete (ASTM C457), the chord size distribution was measured by using air void analyzer(Rapid air 457, Germany).

The main experimental equipment was as follows: air void analyzer (Rapid air 457, Germany), UV spectrophotometer (L5S, China), high-pressure steam sterilization pot (YX280A, China), peristaltic pump (MP-15RN, China), magnetic stirrer (Chijiu84-1, China), and atomic absorption spectrophotometer (TAS-990, China). All the water used in the experiment was ultra-pure water made by an ultra-pure water machine (Buisafer-20TAB, China)

In order to ensure the accuracy of the results, all the data in this test have been repeated more than 3 times, and the average value was taken.

Results and discussion

Characterization of the brick

There are many irregular voids and pores inside the permeable bricks to improve the permeable effect. Naturally, porosity and chord size distribution are two important indicators characterizing the filtration of bricks. Smaller porosity and reasonable chord size distribution was beneficial to improve the brick filtration effect (Yong et al. 2013).

The permeability coefficient of upper layer of ceramic brick, cement brick, and slag brick was 3.1 × 10−2, 3.6 × 10−2, and 2.6 × 10−2 cm/s, which was highly consistent with the porosity of 24.82%, 26.72%, and 24.29% with goodness of fit being 90.42%. The reason for the correlation might be that the rainwater mainly flowed through the pores inside the brick, and the porosity was positively correlated to the seepage velocity.

The chord size distribution of the ceramic brick was shown in Fig. 3. The upper layer of ceramic permeable brick on the cross-section had a smaller proportion of pores in the 0.5–4.0-mm specification than lower layer, while the two layers had a porosity of 24.82% and 58.66%, respectively (Fig. 3). Obviously, the presence of larger pore size increased the porosity of the lower layer, and the distribution of the pores, small pores lying on the top and big pores being on the bottom, objectively made the upper layer function as a sieve, and upper layer may play a more critical role in purification process (Winston et al. 2016).

Fig. 3
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Chord size distribution of ceramic permeable brick

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The chord size distribution of the upper layer of three permeable bricks was shown in Fig. 4, in which chord size was used to represent the size of the pore, and the chord length frequency was used to represent the number of pores. As shown in Fig. 4, the upper layer of ceramic permeable brick did not contain large particles of 1.5–3.0 mm, and the pore ratio of 0.5–1.5 mm is smaller than cement and slag bricks, which meant the ceramic brick had more small pores. The more uniform and dense pore distribution may be caused by sintering and melting of ceramic raw materials.

Fig. 4
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Chord size distribution of upper layer of three bricks

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Although the permeability coefficient was closely related to the porosity of upper layer, however, the purification effect was more likely to relate with chord size distribution; therefore, the rationality of the chord size distribution needs to be further analyzed combining with the comparison of filtering effect

The influence of concentrations

Pollutant removal rates of three permeable bricks under different contaminant concentrations are shown in Fig. 5. As can be seen form Fig. 5, with the increase of contaminant concentrations, the removal rate was basically stable except for the significant decrease in NH4+-N, TN. The removal rates of NH4+-N, TN decreased significantly with the increase of contaminant concentrations. When the contaminant concentrations increased from low concentration to high concentration, the average removal rate of the pollutants decreased by 16.57% and 13.45%, respectively. NH4-N and TN were mainly removed by the adsorption of the brick materials, and the permeable brick had a poor adsorption effect on these pollutants (Zhang et al. 2018; Li et al. 2017; Niu et al. 2016; Bean et al. 2007). For example, the removal rate of NH4+-N by three kinds of permeable bricks was only 5–15%, which meant the bricks had weak resistance to impact load of NH4+-N. The removal rate of NH4+-N rapidly decreased because the adsorption capacity of the brick was limited in the case of an increase of concentrations. The contaminant concentrations had little effect on the removal rates of SS, TP, Cu, Zn, Pb, and Cd. When the contaminant concentrations increased from low concentration to high concentration, the average removal rate merely decreased by 2.80%, 4.54%, 2.86%, 2.17%, 2.44%, and 2.51%, respectively. This might result from the large removal capacity of these pollutants by permeable bricks. High concentration would only shorten the filtration lifespan without affecting the removal efficiency. Combining the removal rates of different pollutants, we could conclude that contaminant concentrations had a great influence on pollutants with lower removal rate and had little effect on pollutants with higher removal rate. The physical interception inside the permeable brick played a major role in the removal of SS, TP, and heavy metals in the permeable brick (Liu et al. 2019; Jiang et al. 2015; Niu et al. 2016); these pollutants were mainly filtered by the voids and pores inside permeable bricks. At the low concentration of the pollutants, the internal voids in the permeable bricks were gradually occupied during the rainfall process, which was helpful in retaining more pollutants, which meant a process of improving efficiency and a certain buffer capacity for the removal of these kinds of pollutants. Thereby increasing the concentrations would not have a significant effect relying on the buffer capacity (Liu et al. 2019; Sansalone et al. 2012).

Fig. 5
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Pollutant removal rates of three permeable bricks under different concentrations

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Ceramic permeable brick had a higher removal rate for SS and TP, indicating that the uniform and dense pore distribution was instrumental in the retention of particulates (Yong et al. 2013). The chord size distribution of ceramic permeable brick was good at filtering the particulate matter, so the pore size distribution is reasonable for the filtration of SS.

The pH changes before and after filtration of three permeable bricks are shown in Fig. 6. As shown in Fig. 6, the pH of the cement and steel slag permeable bricks had a certain increase because that both of them belonged to the non-sintered bricks, which contained a certain amount of cement. During the brick forming process, the hydration reaction of the cement produced alkaline substances such as calcium hydroxide, and the contact between rainwater and alkaline substances or washing out of alkaline materials lead to an increase of the pH after filtration (Zhang et al. 2018; Li et al. 2017). Contrary to the non-sintered permeable bricks, the pH increase of ceramic permeable brick was not obvious because the high temperature in sintering allowed raw material to be fully melted without the generation of alkaline materials.

Fig. 6
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The pH changes of three permeable bricks

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The influence of rainfall intensity

Change of effluent concentrations of three permeable bricks under different rainfall intensities is shown in Fig. 7. As shown in Fig. 7, with the increase of rainfall intensity, the effluent concentration increased, and the removal efficiency of various permeable bricks on various pollutants showed a downward trend. This was mainly because the increase of rainfall intensity improved the seepage velocity of rainwater in the brick, which would reduce hydraulic retention time and contact opportunity between rainwater and brick materials (Liu et al. 2019; Niu et al. 2016).

Fig. 7
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Change of effluent concentrations of three permeable bricks under different rainfall intensities

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In terms of specific pollutants, the removal rates of NH4-N, TN and COD decreased significantly with the increase of rainfall intensity; when the repetition period of rainfall increased from 5 to 50 years, the average removal rate of the three pollutants in three bricks decreased by 9.71%, 11.34%, and 9.64%, respectively. These three pollutants are mainly removed by the adsorption of the brick materials (Liu et al. 2019; Drake et al. 2013; Jiang et al. 2015; Niu et al. 2016; Razzaghmanesh and Borst 2019), and the seepage velocity of the rainwater in the permeable brick reduced the adsorption effect. The rainfall intensity had little effect on the removal rates of SS, TP, Cu, Zn, Pb, and Cd; when the rainfall return period increased from 5 to 50 years, the removal rate merely decreased by 4.55%, 5.72%, 5.50%, 4.17%, 4.70%, and 4.48%, respectively. The SS was mainly removed by the physical interception (Jiang et al. 2015; Niu et al. 2016), and other pollutants were easily adsorbed on the SS or removed along with the filtering of the SS (Jiang et al. 2015; Drake et al. 2014; Sansalone et al. 2012; Liu et al. 2016), The removal rate of these pollutants was less affected by rainfall intensity. It might result from the large removal capacity of these pollutants by permeable bricks. The seepage velocity of rainwater inside the brick was no longer the main factor restricting its removal efficiency.

In terms of specific bricks, ceramic permeable brick had the advantage of better removal in SS, TP, Cu, Zn, Pb, and Cd, especial for the SS and TP, in the background of higher removal rate; the change of removal rate caused by rainfall intensity had less influence on these targets. That is, ceramic permeable brick had better adaptability to these targets in the change of rainfall intensity. However, the removal efficiency of NH4+-N, TN and COD by ceramic permeable brick was not high, and TN and COD efficiency were the lowest in three bricks, which indicated the poor adaptability to these pollutants. Therefore, brick selection under different rainfall intensities should be considered by the classification of pollutants

Combining the removal rates of different pollutants, we could conclude that rainfall intensity had a great influence on pollutants with lower removal rate and had little effect on pollutants with higher removal rate.

Quality of cleaning water

Changes in pH of three brick washing effluents was shown in Fig. 8. As could be seen from Fig. 8, the effluent pH increased at first and then tended to be stable, especially for the cement permeable brick and steel slag brick, which was also due to the contact of water with alkaline substances (Chandrappa and Biligiri 2016; Zhang et al. 2018).

Fig. 8
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Changes in pH of three brick washing effluents

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The effluent removal rate of pollutants in cleaning effluent of three permeable bricks is shown in Fig. 9. As can be seen from Fig. 9, different trends in the removal rate of pollutants were presented in cleaning effluent.

Fig. 9
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The effluent removal rate of pollutants in cleaning effluent of three permeable bricks. a Steel slag brick, b ceramic brick, and c cement brick

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Except for the COD, TN and NH4+-N, the concentration of the conventional contaminants in the cleaning effluent was stable in the later stage with the removal rate being in the range of 95–99%, while the relatively high concentration made it easy to be washed out in the early stage. It can be seen that these pollutants were not easily washed out. In particular, it was pointed out that the effluent removal rate of SS was maintained at a high level in the whole cleaning process, indicating that SS was difficult to be cleaned out. Thus, it is easy to block the internal voids and pores inside the permeable brick, which may be one of the main reasons for the clogging of brick after long term use (Liu et al. 2019; Niu et al. 2016; Sansalone et al. 2008; Beecham et al. 2009). The concentration of COD, TN and NH4+-N in the cleaning effluent decreased with the cleaning time continued; the effluent removal rate of pollutants increases gradually, indicating that the three pollutants trapped in the permeable brick were easy to be washed out in the cleaning process. In comparison, COD was less likely to be washed out among the three pollutants.

Combining with the effect of decontamination, this falls into a paradox that the pollutants easily removed in the filtration process are not easy to be washed out, and the pollutants difficultly removed are easily flushed out. The soluble pollutants such as COD, TN and NH4+-N was difficult to be removed but easy to be washed out. Therefore, the treatment of these kinds of pollutants would be the difficulty and hot spot in the research of permeable brick.

Conclusions

In this study, ceramic permeable brick, cement permeable brick, and steel slag permeable brick were selected to study their decontamination performance, the influencing factors such as rainfall intensity and contaminant concentration were investigated, and the cleaning characteristics of the permeable brick was discussed further. Based on the results from this study, the following conclusions could be drawn.

  1. (1)

    From the perspective of cross-section structure, three bricks all can be divided into the upper layer and the lower layer. Small pores lying on the top and big pores being on the bottom objectively makes the upper layer a sieve, which might play a more critical role in purification process. The uniform and dense pore distribution of ceramic permeable brick was instrumental in the retention of particulate matters.

  2. (2)

    Contaminant concentration and rainfall intensity had little effect on pollutants with higher removal rate but had a great influence on pollutants with lower removal rate. SS, TP, Cu, Zn, Pb, and Cd were less influenced by the change of the contaminant concentration and rainfall intensity.

  3. (3)

    The dissolved pollutants such as NH4+-N and TN were mainly removed by the chemical adsorption, while the permeable brick had a poor adsorption effect and weak resistance to impact load, the removal rate of these pollutants rapidly decreased because the adsorption capacity was limited in the case of an increase of concentrations or repetition period of rainfall.

  4. (4)

    Non-sintered bricks containing a certain amount of cement increased the pH after filtration, because alkaline substances such as calcium hydroxide was produced owing to the hydration reaction of the cement during the brick forming process. Ceramic permeable brick had not obvious effect on pH since the high temperature in sintering allowed raw material to be fully melted without the generation of alkaline materials.

  5. (5)

    The trapped SS was not easily washed out; this might be responsible for the clogging of bricks after long term use. The soluble pollutants such as COD, TN, and NH4-N were difficult to be removed but easy to be washed out. Therefore, the treatment of this kind of pollutants was the difficulty for permeable brick.