Characterization of Silica Sand-Based Pervious Bricks and Their Performance under Stormwater Treatment

Abstract

The acceleration of urbanization has disrupted natural water cycles, resulting in increased impervious urban surfaces and non-point source pollution from stormwater runoff. Addressing urban stormwater recharge has become crucial. This study introduces a novel silica sand-based permeable filtration material, investigating its surface characteristics, pore structure, permeability, and pollutant interception capabilities. The results demonstrate that hydrophilic binder coating modification of the permeable surface sand aggregate, combined with hydrophilic inorganic additives, having a porous structure with an average pore size of less than 50 μm and a porosity between 15% and 35%, significantly enhances surface hydrophilicity, achieving a permeation rate of up to 6.8 mL/(min·cm²). Moreover, it shows exceptional filtration and anti-clogging properties, achieving over 98% suspended solids interception and strong resistance to fouling. Dynamic biofilm formation experiments using simulated rain and domestic wastewater explore biofilm morphology and function on silica sand filtration well surfaces. Mature biofilms sustain COD removal efficiency exceeding 70%, with levels consistently below 50 mg/L, NH₄⁺ decreasing to 2 mg N/L, and total nitrogen maintained below 10 mg N/L. The system features anoxic, anoxic, and aerobic zones, fostering synergistic organic matter and nitrogen removal by diverse microorganisms, enhancing pollutant mitigation. Silica sand-based permeable filtration material effectively mitigates urban stormwater runoff pollutants—suspended solids, organic matter, and nitrogen—offering an innovative solution for sponge city development and rainwater resource management.

1. Introduction

As global urbanization accelerates, a large amount of farmland, forests, and other natural permeable layers are being replaced by impermeable surfaces such as urban buildings, roads, and public spaces. This leads to a rapid increase in impermeable surfaces in cities, altering the mechanisms of the natural water cycle. Consequently, urban water environmental issues, such as non-point source pollution caused by stormwater runoff, are becoming more prominent [1]. Research indicates that urban non-point source pollution carried by stormwater runoff has become a significant cause of the deterioration of urban water environments [2,3]. Moreover, the annual pollution load from urban non-point source pollution is equivalent to five times the annual pollution discharged by wastewater treatment plants, with the concentration of various pollutants being essentially the same as those in untreated urban sewage. Controlling urban non-point source pollution has become a critical global environmental concern [4].

A permeable filtration system with permeable materials as the main component is an important solution to address the pollution of stormwater runoff and achieve the replenishment of urban rainwater [5]. However, high permeability, high pollutant interception capability, and anti-clogging performance are key issues that need to be resolved in the application of permeable filter bricks [6]. Domestic and international urban permeable pavements primarily use millimeter-level large-pore gravity permeable materials, such as permeable concrete, permeable asphalt, and polyurethane stone paving [7,8,9]. Although this approach to permeability is straightforward, the complex pore structure and uneven pore size can easily become clogged by debris and dust, leading to a short lifespan in terms of permeability [10]. Additionally, millimeter-level large pores generally do not provide effective filtration, allowing most pollutants in urban stormwater runoff to pass through the permeable materials and potentially contaminate groundwater [11,12,13]. In contrast, micrometer-level porous permeable materials offer better filtration effects by intercepting granular pollutants and even providing adsorption effects. However, these materials are more prone to clogging [14,15].

The development of permeable materials currently faces two contradictory principles: one is that permeability requires a high porosity rate, while compression resistance requires high density; the other is that permeability requires large pores, while filtration requires small pore diameters. Sand-based permeable bricks offer high permeability and cost-effectiveness and have been widely used as pavement bricks abroad [16,17]. A common method to improve the hydrophilicity of the silica sand surface is through coating. Currently, practical silica sand modification coatings generally use heat treatment and resin coating processes to achieve better compressive strength while ensuring permeability [18,19,20]. Sand-based permeable bricks are made from desert aeolian sand, either as the main aggregate or surface aggregate, with an average diameter of 0.05–1 mm. These sands are bonded together after being coated and modified with hydrophilic binders, such as hydrophilic epoxy resins, hydrophilic polyurethane resins, and hydrophilic acrylic resins. During the coating modification of the original sand, coupling agents containing silicon, titanium, or phosphorus elements are used to enhance the binding force between the hydrophilic binder and the original sand. Additionally, hydrophilic inorganic materials, such as diatomite, bentonite, and perlite, as well as weather-resistant additives like light stabilizers and antioxidants, are added to the permeable surface. This ensures the hydrophilicity, mechanical, and chemical stability of the permeable surface and extends the service life of the permeable bricks. Although significant research has been conducted on improving the permeability, stability, and strength of these bricks, there is still a lack of focus on their pollutant interception capacity, biofilm formation characteristics, and the performance of microorganisms in removing organic matter, nitrogen, and other typical pollutants [21,22].

Thus, this study aims to develop a novel type of silica sand-based permeable and filterable material, which was hypothesized to overcome the aforementioned limitations, resulting in enhanced surface hydrophilicity, improved pollutant removal efficiency, and greater resistance to clogging. It focuses on examining the significant characteristics of the new material, including its surface properties and pore structure, and further investigates its permeability and pollutant interception capabilities. In addition, through dynamic biofilm experiments utilizing simulated rainwater, the morphology, microbial species composition, and functional attributes of the biofilm on the surface of silica sand filter bricks were systematically investigated. Furthermore, this study encompasses an in-depth analysis of the internal flow dynamics, pollutant micro-circulation, and oxygen transmission within the permeable and filterable wells constructed from this material, along with a simulation of oxygen self-enrichment processes. This study would provide a robust theoretical foundation for effective pollution control and practical applications concerning suspended solids, organic matter, and nitrogen elements carried by urban rainwater runoff.

2. Materials and Methods

2.1. Silica Sand-Based Permeable Filter Bricks

The material is made from coated silica sands, utilizing special resins to form bricks without sintering processes. The modification of pre-coated membranes using a new hydrophobic polymer. The coated modified sand particles, as shown in Figure 1A–C, are formed without sintering and possess the dual functions of water permeability and filtration, suitable for use as road bricks (Figure 1D). The silica sand-based permeable bricks used in this study have dimensions of 900 × 450 × 100 mm (Figure 1D). These bricks feature a composite structure consisting of a fine upper permeable layer (5 mm) and a coarser lower permeable layer, with a celadon gray surface layer.

To enhance the strength of micro-particles, a new hydrophobic polymer is used for pre-coating modification. The aeolian sand particles are pre-coated with this polymer, and a reactive coupling agent and a binder are then applied to the surface of the sand, forming a stable chemical bond and a thin, hard shell pre-coated membrane. This improved their sphericity by one grade. Simultaneously, the surface activity and binding force of the aeolian sand are enhanced. A hydrophilic composite binder, rich in carboxyl and hydroxyl functional groups, is used for secondary coating. This allows the hydrophilic groups to adhere to the surface of individual particles, forming hydrophilic-coated sand. This reduces the contact angle between water and the coated sand interface, achieving hydrophilic modification.

Through the use of this coating technology, a multi-level enhanced roughness microstructure is formed on the surface of the coated sand particles, giving the material super-hydrophilic characteristics and enabling micrometer-level pores to exhibit high-efficiency water permeability. The outcome is a micron-level silica sand-based permeable product with a smooth and dense surface, where 90% of the pore diameters are ≤75 μm.

2.2. Silica Sand-Based Filter Well

The high-quality permeable masonry modules mentioned above were used to construct a honeycomb-shaped silica sand water purification and filtration wall structure. This structure consists of six interconnected, distributed unit partitions (Figure 1E). The water filtration well built with this filtration structure (Figure 1F) features a bottom layer of breathable and waterproof sand, which achieves passive natural aeration and increases the dissolved oxygen in the water body. This facilitates oxidation contact and biofilm formation, thereby enabling the biological purification of the water body.

2.3. Surface Morphology and Pore Structure Analysis

The morphology of the samples was investigated using scanning electron microscopy (SEM, Hitachi SU8020, Tokyo, Japan) after coating with gold (Au). Elemental analysis was conducted using inductively coupled plasma mass spectrometry (ICP-MS) with extraction by 0.1% dilute nitric acid. Energy-dispersive X-ray spectroscopy (EDS) was employed to determine the elemental compositions of hydration products. X-ray photoelectron spectroscopy (XPS, Thermo Escalab 250Xi, Waltham, MA, USA) was utilized to analyze the elements on the surface layer of the permeable brick and their respective valence distributions.

For contact angle measurements, two-microliter droplets of diiodomethane (CH2I2, 99%, Sigma–Aldrich GmbH, Darmstadt, Germany) were placed onto the prepared test filter positioned on an X–Y stage. The contact angle of the liquid droplet on the PTFE foam coating filter surface was measured using a liquid–solid contact angle analyzer (DSA100; KRÜSS^®, Hamburg, Germany) equipped with a high-speed camera.

The water permeability of the permeable and filter bricks was tested according to Chinese standard GB/T 25993–2010 [23] based on Darcy’s Law. The apparent porosity of the prepared permeable bricks was determined using the Archimedes drainage method. Detailed methods for water permeability and porosity of the permeable bricks followed those outlined in a previous study [3]. All of the experiments were conducted in triplicate, and the results presented in the text are the mean values.

2.4. Biofilm Formation

The dynamic biofilm formation experiment utilized a continuous natural method to cultivate biofilms using synthetic wastewater. The study monitored changes in chemical oxygen demand (COD), ammonium (NH₄⁺), and total nitrogen (TN) over a 27-day period. Detailed parameters and results are summarized in Table S1.

The influent for the experiment consisted of a mixture of domestic wastewater and tap water in a ratio of 1:9. Throughout the cultivation period, water samples were collected every other day for analysis of water quality parameters, including nitrate (NO₃⁻), nitrite (NO₂⁻), NH₄⁺, TN, and COD. Upon reaching biofilm maturity, the biofilm structure and protozoa were observed using a microscope and scanning electron microscope (SEM).

2.5. Other Analysis Methods

Suspended solids (SSs) were determined using a 0.45 μm paper filtration followed by a dry weighing method. Turbidity was measured using a spectrophotometric method. Total nitrogen (TN) was analyzed using the alkaline potassium persulfate digestion UV (ultraviolet) spectrophotometric method. Chemical oxygen demand (COD) was assessed using a COD quick-analysis apparatus (Lian-hua Tech. Co., Ltd., 5B-1, Shenzhen, China). NO₃⁻, NO₂⁻, and NH₄⁺ were determined according to the standard method for the examination of water and wastewater.

3. Results and Discussion

3.1. Characteristics of Silica Sand-Based Permeable Bricks

3.1.1. Surface Morphology and Structural Analysis

As depicted in Figure 2, the silica sand-based permeable brick consists of a permeable base layer (incorporating water flow channels) and a permeable surface layer that overlays the base layer. The aggregate components of the permeable base layer include pebbles, mineral wool, ceramsite, perlite, and expanded vermiculite, with an average particle diameter ranging from 3 to 7 mm (Figure 2B). This was larger than the aeolian sand used in a previous study, which was characterized by a very uniform particle size distribution, ranging between 0.08 and 0.63 mm [24]. These aggregates are bonded together using cement, silicate, or phosphate, ensuring the permeable base layer possesses strong mechanical strength and compressive capacity. To enhance permeability and reduce water resistance, the permeable base layer is designed with two to five interconnected water flow channels, each with a diameter of 1 to 5 cm (Figure 2B). This structural design effectively reduces water resistance and significantly improves the overall permeability of the brick.

The SEM images of the permeable surface layer (Figure 2) reveal a porous structure with an average pore diameter of less than 50 μm and a porosity ratio ranging from 15% to 35%. This porosity ratio is higher than previously reported values for pavement aggregates with recycled asphalt and polymer-modified pervious concrete, which ranged from 20% to 30% [25,26]. Generally, pervious concrete with a void content greater than 15% is considered acceptable [27]. Moreover, there is an inverse relationship between porosity and density, with typical porosity values varying from 15% to 35% [27]. Thus, the new pavement exhibited enhanced permeability.

Although permeable concrete pavement construction is an emerging technology due to its infiltration and environmental benefits, it is still characterized as a low-strength material because of its interconnected porous structure and high porosity [28,29]. The surface layer of the permeable brick is bonded with a hydrophilic binder that has film-forming and modifying capabilities. This binder contains hydrophilic groups such as hydroxyl, carboxyl, sulfonic acid, or amine groups, depending on the specific binder used. X-ray photoelectron spectroscopy (XPS) analysis of the Si 2p and C 1s on the surface layer of the permeable brick confirmed these functional groups. As shown in Figure 3A,B, the presence of Si-C bonds indicates that stable covalent bonds have formed between sand particles, coupling agents, and binders, ensuring that the hydrophilic binder remains intact during the use of the permeable brick. Additionally, the presence of C-C and C-O bonds suggests a higher concentration of oxygen-containing functional groups, such as hydroxyl and carboxyl groups, on the surface layer. These functional groups increase the surface energy of the permeable surface layer and enhance its hydrophilicity.

3.1.2. Permeability Performance

According to the Young–Laplace equation, when water comes into contact with a hydrophilic surface (where the water contact angle θ is less than 90°), it generates a downward additional pressure ΔP, which encourages water to penetrate into the pores (Figure 4A).

$$\Delta P={\displaystyle \frac{2{\gamma }_{1,2}}{R}}={\displaystyle \frac{4{\gamma }_{1,2}\cos \theta }{d}}$$

(1)

where γ_1,2 represents the interfacial tension between phases 1 and 2; R is the radius of curvature, and d is the pore diameter.

During natural rainfall or artificial watering, water within the interconnected and semi-connected pores of the brick moves downward due to the combined effects of capillary action and gravity. The super-hydrophilic nature of the permeable brick’s surface significantly enhances the additional pressure when water contacts it, enabling rapid water infiltration into the brick.

Permeability rate tests have shown that the brick’s permeation rate can reach up to 6.8 mL/(min·cm²). This remarkable permeability is primarily attributed to the brick’s highly porous structure, where pores with diameters ≤ 75 μm constitute approximately 90% of the total porosity. Additionally, the pore structure exhibits variability across the permeable surface layer and the permeable base layer, contributing to the brick’s overall permeability.

Previous studies have demonstrated that bricks with a higher content of large particles tend to have fewer contact points between particles, leading to increased porosity and, consequently, a higher permeability coefficient. A positive correlation between permeability and porosity was consistently observed in these studies [30]. In this study, the silica sand-based bricks displayed excellent permeability and a well-developed porous structure, which can be attributed to the optimal particle size distribution of the coated aeolian sand particles.

As depicted in Figure 4B, the pore diameter and porosity of the permeable brick gradually increase from the permeable surface layer towards the permeable base layer, following the direction of water flow. According to Darcy’s law, this gradient results in a progressive increase in water flux (J) and a decrease in membrane resistance (R) along the flow path. Consequently, as water penetrates and moves through the permeable brick, the flow rate increases while resistance decreases, ensuring the brick’s exceptional permeability performance.

$$J={\displaystyle \frac{\epsilon \pi r_p^2\Delta p}{8\mu L}}={\displaystyle \frac{\Delta p}{\mu R}}$$

(2)

where ϵ represents the porosity, r_p is the pore diameter, Δp is the external pressure, μ is the viscosity of the permeating liquid, and L is the total length the liquid permeates through.

3.1.3. Drainability Performance

The water filtration capability of the permeable brick was tested, revealing that it can remove 98% of suspended solids from water and possesses strong anti-clogging properties. Compared to traditional permeable materials, the surface pore diameter of this permeable brick is smaller, effectively intercepting large particulate matter with diameters greater than 50 μm. Under the hydraulic flushing action of surface runoff, the intercepted large particles migrate with the water flow, keeping the surface of the permeable brick clean and ensuring the removal of 98% of suspended solids. Smaller particles and dissolved organic pollutants enter the pores of the permeable brick; however, due to the brick’s fast permeation rate, these particulate pollutants do not adhere to the pore walls under the rapid flushing action of the water flow. Furthermore, the strong hydrophilicity of the modified sand particles forms a hydration membrane on the pore walls, inhibiting the adhesion and accumulation of organic pollutants and microorganisms. These combined effects allow the permeable brick to avoid clogging even with long-term use.

In addition, while most previous studies on pervious pavers focused on the removal of solids, COD, and heavy metals [31], this study found that the permeable brick also effectively intercepts oil pollutants. As shown in Figure 5A, the contact angles of the permeable brick surface with 1,2-dichloroethane, 1,2-diiodomethane, petrol, methylbenzene, and hexadecane all exceed 150°, demonstrating super-oleophobic properties. This phenomenon occurs because the permeable surface layer of the brick, when wetted by water, forms a hydration membrane on the particle surfaces, reducing the contact area between oil pollutants and the surface. This finding provides a new direction for the application of permeable bricks in environmental remediation and stormwater management.

3.2. SS and COD Removal Performance

Rainwater collected on 17 November 2020 from road runoff on the west side of the roundabout on Xingsheng South Road in the Economic and Technological Development Zone of Miyun, Beijing (GPS: 40°21′31.72″ N, 116°48′28.67″ E), was utilized in experimental testing to evaluate the efficacy of silica sand-based permeable bricks in removing SS and COD. On the collection day, conditions included light rain with temperatures ranging from 13 °C to 8 °C, accompanied by a north wind at levels 1–2, following a dry period exceeding 15 days. During the experiment, a flow rate of 1.6–1.8 mm/min was maintained, corresponding to 96–108 mm/hr, which approximates the peak intensity of a once-in-a-year 10 min rainfall event in Jinan.

Table 1. The performance of SS and COD removal for the silica sand-based permeable material (n = 3).

	Influent Concentration (mg/L)	Effluent Concentration (mg/L)	Removal Efficiency (%)
SS	606 ± 85.6	54 ± 21	91.1 ± 6.8
COD	297 ± 42.4	18.1 ± 8.9	93.9 ± 4.6

demonstrates that the silica sand-based permeable material achieved notable removal efficiencies of 91.1% for SS and 93.9% for COD. These removal efficiencies significantly exceed those reported in comparable studies. For example, Teymouri et al. reported reductions of 69.75%, 68%, and 69% in COD, TSS, and turbidity, respectively, in iron slag pervious concrete due to its porous structure and reduced pore size, facilitating enhanced pollutant capture from urban runoff [31]. In their study, urban runoff with a COD concentration of 980 mg/L decreased to 752 mg/L after passing through the control sample, representing a reduction of approximately 23%.

3.3. Biofilm Formation and Pollutant Removal Performance of the Silica Sand-Based Permeable Bricks

3.3.1. Biofilm Formation

The process of biofilm attachment and maturation on silica sand-based permeable bricks unfolds through distinct stages: (1) Microorganism Transport: Initially, microorganisms are transported to the brick surface within the first three to four days. This transport occurs via hydraulic dynamics, diffusion forces, Brownian motion, gravity, and sedimentation, forming a thin water film on the surface; (2) Reversible Attachment: Over the subsequent two to three days, a light tan water film develops on the brick, which can be partially washed away with water. Planktonic microbial cells adhere to the surface, where some are dislodged by hydraulic forces while others firmly attach; (3) Irreversible Attachment: As time progresses, the biofilm deepens in color and remains adherent even after rinsing with water. At this stage, microbial cells produce polysaccharide matrices that bind them tightly together; (4) Maturation: With continuous water supply and aeration, biofilm development continues. Microscopic examination reveals the presence of bacterial flocs, protozoa (such as Vorticella), and indicator organisms like rotifers (Figure S1), and the SEM observation revealed a significant presence of microorganisms, specifically rod-shaped bacteria, on the surface of the material (Figure 6), clearly indicating the completion of biofilm formation.

Silica sand-based permeable bricks facilitate biofilm formation due to several key attributes: their inherent hydrophilicity supports initial attachment and growth of biofilm. The bricks’ permeability enhances water circulation and nutrient exchange, crucial for sustaining biofilm development. Moreover, their filtration capability intercepts suspended solids, providing surfaces for microorganism attachment and facilitating the adsorption and decomposition of organic matter.

Furthermore, the robust structural integrity of silica sand-based permeable bricks ensures long-term stability, maintaining an environment conducive to sustained biofilm growth. While the bricks themselves do not possess direct biological degradation capabilities, they serve as a favorable substrate for microbial colonization, thereby enabling effective pollutant removal through microbial activity. In conclusion, silica sand-based permeable bricks demonstrate significant potential for application in ecological engineering and environmental management, particularly in enhancing water quality through biofilm-mediated pollutant degradation processes.

3.3.2. COD and Nitrogen Removal

The biofilm on the surface of the silicon sand filter brick can be sequentially divided into anoxic/anaerobic layers, aerobic layers, and attached water layers from the inside out. Each layer acts as a micro-reactive zone, and their combined action significantly enhances the biofilm’s efficiency in removing pollutants, particularly organic pollutants and ammonia nitrogen, facilitating complete reactions. Figure 7 illustrates the simple mechanism by which the biofilm in the silicon sand filter well removes pollutants.

The biofilm effectively removes organic pollutants. Initially, organic pollutants are adsorbed in the attached water layer, after which they are decomposed by aerobic bacteria in the aerobic layer, and then enter the anaerobic layer for anaerobic decomposition reactions. The flowing water layer washes away the aged biofilm, allowing the growth of new biofilm, thereby purifying the wastewater through a cyclical process. For the simulated stormwater, diluted domestic wastewater was used in this experiment. After the biofilm matures, the removal rate of COD is maintained at over 70%, with overall levels below 50 mg/L. This result is supported by previous studies, which indicate that adding different fine-grained adsorbents such as zeolite, perlite, quartz, vermiculite, and pumice to pervious concrete improves its ability to reduce stormwater and wastewater pollution [32,33,34].

The structure of anoxic/anaerobic layers, aerobic layers, and the attached water layer plays a crucial role in the removal of NH₄⁺, NO₂⁻, and NO₃⁻. As illustrated in Figure 6B, the inner side of the biofilm forms an anaerobic reaction zone where denitrifying bacteria (anaerobic bacteria) thrive, while the outer side forms an aerobic reaction zone where nitrifying bacteria (aerobic bacteria) proliferate. In the aerobic reaction zone, organic matter is decomposed into inorganic compounds through aerobic nitrification reactions. Concurrently, in the anaerobic reaction zone, denitrifying bacteria perform denitrification, converting nitrogen into nitrogen gas, thus completely removing it from the water body.

In the simulated rainwater, the diluted wastewater was used in this experiment, once the biofilm was successfully established, the concentration of ammonia nitrogen in the water gradually decreased to 2 mg/L, and the TN concentration was maintained below 10 mg N/L (

Table 2. Changes in the NH₄⁺, TN and COD in treating the simulated stormwater.

Day	NH4+ (mg N/L)	TN (mg N/L)	COD (mg/L)
1	13.3	18.2	106.4
3	12.9	17.1	106.3
4	12.4	16.7	105.8
5	11.9	16.5	102.5
7	9.3	15.8	83.2
9	8.8	14.5	80.3
11	8.6	14.2	77.4
13	8.4	14.0	75.3
15	6.1	11.9	64.4
17	4.6	10.2	50.3
19	2.9	8.5	38.2
21	1.5	7.2	27.1
23	1.3	7.3	28.0

), meeting the water quality requirements for landscape environmental use.

3.4. Implications

Permeable pavers are eco-friendly building materials primarily composed of cement, sand, slag, fly ash, and other sustainable materials. These pavers are formed under high pressure without the need for high-temperature firing. Silica sand-based bricks, made from sand as the raw material, are formed by extrusion molding without sintering. They possess water permeation and filtration capabilities, making them energy-saving and environmentally friendly.

The sand particles on the permeable surface of these bricks are modified using a hydrophilic binder coating, introducing numerous hydrophilic functional groups, such as carboxyl, hydroxyl, sulfonic acid, and ammonium. The results suggest that these groups increase surface energy and improve hydrophilicity. By altering the surface tension of water, the surface becomes denser, resolving the conflict between strength and permeability. Consequently, silica sand-based permeable materials exhibit high strength and rapid permeability, resist blockage by dust, and maintain long-term permeability.

The results of this study also indicate that permeable bricks have a significant interception effect on oil pollutants, presenting a broad opportunity for their application in treating oil-contaminated wastewater. Notably, once the biofilm matures, the COD removal efficiency for the simulated stormwater in this study is maintained at over 70%, with overall levels below 50 mg/L. Additionally, NH₄⁺ in the water gradually decreases to 2 mg N/L, and TN remains below 10 mg N/L, meeting water quality requirements for landscape environmental use.

In addition, using silicon sand-based filter bricks, multi-stage filter walls and self-purifying storage pools can be constructed, with breathable and impermeable sand laid at the bottom to create vertical air flow and energy channels between the water body and the strata. By utilizing this structure, gas (such as CO₂, O₂) and heat exchange is achieved through the temperature difference and diffusion effect between the strata, water body, and atmosphere, realizing natural ventilation without the need for power. This structure increases the dissolved oxygen in the water body and achieves biological purification of the water through intergranular contact oxidation and biofilm formation.

In practical applications, silica sand-based permeable bricks can be used to construct permeable bioreactors, artificial wetlands, and other ecological projects. Through the action of the biofilm, these bricks effectively remove organic matter and ammonia nitrogen from water bodies, thereby improving water quality. Furthermore, their permeability and filtration characteristics help to reduce the pressure on urban drainage systems, enabling effective management and utilization of rainwater. In summary, pervious bricks represent a new type of ecological product with functions including water permeation, water storage, and humidity adjustment. They can alleviate the pressure of excessive rainwater on urban roads, reduce organic matter and nitrogen pollutants, and enhance the quality of urban life, offering significant prospects for development and application.

4. Conclusions

The silica sand-based permeable brick features a porous structure characterized by super-hydrophilic surface properties, rapid permeation rate, and fine surface pores, achieving a notable permeability of up to 6.8 mL/(min·cm²). These attributes underscore its robust water filtration capabilities, with an ability to intercept over 98% of suspended solids and exhibit strong resistance to clogging. Furthermore, its underwater super-oleophobic properties enhance its effectiveness in capturing oil pollutants upon contact with water. The brick’s unique structure promotes microbial attachment and growth on honeycomb walls and within pores, thereby maximizing microbial surface area without compromising permeability, playing a crucial role in enhancing organic matter removal under anaerobic conditions, denitrification of nitrates and nitrites in anoxic zones, and oxidation of ammonia nitrogen and organic matter in aerobic zones. Overall, this study demonstrated that the silica sand-based permeable bricks demonstrated superior water permeability and pollutant interception capabilities, making them promising materials for stormwater management. Further studies in exploring the long-term performance and durability of silica sand-based permeable bricks with various environmental conditions in real-world applications are suggested, which could provide more practical valuable insights for sustainable urban planning.