Small Scale Wastewater Treatment Method for Rainwater Garden Based on Photovoltaic Photocatalysis Technology

Conventional small-scale sewage treatment mostly adopts the principle of biological contact oxidation process, which has limited scope of application, low degradation rate of pollutants in sewage, and poor sewage treatment effect. Based on this, a new small-scale sewage treatment method was proposed by introducing photovoltaic photocatalysis technology and taking S sponge community in Ningnan County, Liangshan Prefecture as an example. First, hydrothermal method is used to prepare photocatalyst to provide basic guarantee for sewage treatment. Secondly, the rainwater garden model is simulated and designed to obtain the composition and content of small-scale sewage. On this basis, a rainwater garden photovoltaic photocatalytic water purification system is designed, which establishes an A/O integrated process by connecting the anoxic zone (A section) and aerobic zone (O section) in series to fully achieve the goal of sewage treatment. Experimental analysis shows that the new method can significantly improve the degradation rate of organic pollutants in rainwater garden wastewater, with a degradation rate of over 99% and a maximum degradation time of 10.97 seconds, indicating that the application effect of this method is good.


Introduction
The development of industrialization has led to various problems associated with industrial sewage and domestic sewage [1].There are many sources of wastewater generation, including industrial production, domestic water, and even rainfall, which may introduce pollutants into the water body.Rainwater contains a large number of pollutants, and rainwater runoff will be involved in a large number of pollutants and discharged into the nearest water body.There are a lot of nitrogen and phosphorus elements in the rainwater pollutants, which will cause serious eutrophication in the water body.
After research, some scholars listed urban rainwater runoff as the third largest pollution source, accounting for 9% of river pollution.At this stage, most of the traditional urban planning concepts focus on the rapid drainage of rainwater.However, as a large amount of urban sewage is discharged to the downstream water system, it has a great impact on the infrastructure construction of the downstream water system [2].On the one hand, it has destroyed the infrastructure construction of the downstream water system and polluted the urban environment, while also causing water shortages in the city.
In response to the above situation, the current conventional sewage treatment technologies typically involve activated carbon adsorption method where the activated carbon has a developed pore structure and a huge specific surface area inside that has good treatment effects on different types of sewage.Biofilm treatment technology mainly utilizes biofilms attached to the surface of fillers to continuously absorb and oxidize algae and organic matter in water.However, for high concentration organic wastewater, biofilm poisoning and failure often occur due to the limitation of current technological application.Membrane separation method uses a permeable membrane as the filtering layer to filter pollutants in water under the action of energy, concentration, or chemical potential difference, thereby achieving the goal of sewage treatment.However, there are still many problems in application, such as the selectivity of the membrane, which makes it not widely applicable, and the recycling and reuse process of the membrane is relatively complex, with the most important being the high energy consumption in the membrane separation process.General oxidation method refers to the oxidation reduction reaction between oxygen or air as an oxidant and pollutants in water under high temperature and pressure conditions, in order to achieve the goal of sewage treatment.The final products after treatment are CO2 and H2O.Its oxidation efficiency is high, but the requirements for equipment are too strict, the initial investment in technology is too large, and the cost is high.
To some extent, rainwater gardens can effectively remove harmful substances from sewage and reduce their content The rainwater garden includes gravel, planting soil, overlay, sand, and aquifers.Rainwater garden can remove pollutants in water through substrate and plants, and the removal capacity is high [3].At this stage, the research on small-scale sewage treatment in Rainwater Garden is gradually mature, which can effectively purify sewage.However, the traditional small-scale sewage treatment method in Rainwater Garden is still insufficient in practical application, and the degradation rate of pollutants in sewage cannot be significantly improved [4].Photovoltaic photocatalysis technology can produce hydrogen and CO2 through photolysis of water, and reduce organic carbon compounds, which can provide important technical support for sewage treatment.Based on this, this paper introduces the photovoltaic photocatalysis technology to carry out the research on small-scale sewage treatment methods of rainwater garden.

Preparation of photocatalyst
Before small-scale sewage treatment in Rainwater Garden, it is necessary to prepare a highly adaptable photocatalyst according to the actual demand of photovoltaic photocatalysis technology and the characteristics of rainwater garden sewage, so as to provide basic support for subsequent sewage treatment.
In this paper, photocatalyst was prepared by hydrothermal method.Hydrothermal method is a particularly common method in the preparation of photocatalysis materials, which uses liquid reagents such as water and ethylene glycol as solvents, and solid materials such as inorganic salts as solutes to dissolve and recrystallize in a pressure sealed container [5].Compared with other preparation methods, it has the characteristics of low cost and easy to obtain suitable stoichiometric substances and crystal forms.For example, most materials that are difficult to prepare by conventional methods for some intermediate, metastable and special phases can be generated by hydrothermal method.The crystal shape, grain size, grain shape and other characteristics of the material can be controlled by changing the experimental temperature and pressure of the hydrothermal method.The materials prepared by the hydrothermal method have the characteristics of complete grain particles and light agglomeration [6].Preparation of GO urea precursor: add 20mg graphene oxide powder (GO) to 38.5mL deionized water, ultrasonic for 60min, and make it uniformly dispersed.Add 2.0g urea (urea) to the ultrasonic GO dispersion, and stir at room temperature for 30min to make it completely dissolved in the above GO dispersion.At this time, the concentrations of GO and urea in the precursor are 0.5mg/mL and 50 mg/mL [7]respectively.
Preparation of composite foam material: cut the commercial melamine resin foam (MF) into 1.5x1.5x2.0cmsize blocks, soak all of them in the graphene urea solution, ultrasonic and static treatment for 30min, the main purpose is to make the urea evenly dispersed on the MF surface [8].The foam with evenly adsorbed urea was placed in a 10 mL crucible and fully wrapped up in tin foil paper.Under Ar gas atmosphere, the heating temperature is not over 500℃, and the heating rate is not more than 2.5℃ / min for 5h.After heating, the catalyst is naturally cooled down to room temperature, and absolute ethanol is repeatedly clarified to remove C3N4 on MF to prevent the failure of the catalyst during the use process, and the foam photocatalyst [9] can be obtained in 55℃ vacuum drying for 10h.
To prepare the Ti3C2x solution, Ti3C2x Clay was added to the 100mL deionized water, shake it for 30min, ultrasonic treat it in argon for 2h, and centrifuge it at 3500r/min for 1h to obtain Ti3C2x colloidal solution [10].Take a certain amount of colloidal solution, add it to the centrifuge tube for drying, and calculate the concentration through the mass change.The concentration of Ti3C2x is generally about 10mg/mL [11].
Put the prepared Ti3C2x solution into a beaker containing 20mg/L rhodamine B (RhB) solution, and stir at a speed of 150r/min, without light, adsorption for 30 min [12].Then, under the irradiation of 300W xenon lamp, sample every 30 minutes, measure the content of Agt and RhB through ICP and liquid ultraviolet visible spectrophotometer respectively, and the distance between the lamp source and TCM is 15.0 cm.
In the process of photocatalyst preparation reaction, the solution is constantly concentrated and the volume fluctuates greatly, the concentration should be converted back to the original system when calculating the pollutant concentration [13].The degradation efficiency of reduced silver ions and organic matter is defined as the ratio of the pollutant content in the solution to the initial material content at a certain time, and its calculation expression is: Among them, c represents the concentration of the solution measured after a period of reaction; c0 indicates the concentration of pollutants in the original solution; v0 is the volume of solution; v represents the volume of solution measured after a period of reaction.Through calculation, the efficiency of photocatalytic reduction of silver ions and organic matter degradation [14]was obtained.

2.2.Simulated design rainwater garden model
After completing the preparation of the aforementioned photocatalyst materials, the next step is to simulate and design a rainwater garden model to obtain the structure of the rainwater garden, providing a reference basis for subsequent photovoltaic photocatalytic wastewater treatment.Rainwater Garden is a shallow concave green space next to a greenhouse flower house, which collects roof rainwater and ground rainwater.It has the function of ecological sustainable control and utilization of rainwater.Roof rainwater is relatively clean and has few impurities, and can be directly discharged into the water storage system through waste flow and filtration; The surface rainwater has many impurities and complex pollution sources.After being discarded and filtered, it is precipitated and discharged into the water storage system, making it an ecologically sustainable facility.Rainwater gardens regulate, purify, and utilize rainwater runoff through the comprehensive effects of plants, covers, and sand, ensuring sufficient purification of rainwater and ultimately achieving rainwater circulation and supplementing groundwater resources.At the same time, it also has the function of rainwater storage, improving environmental landscape, and regulating local microclimate.In this process, rainwater brings economic and ecological benefits.The sewage treatment system of Rainwater Garden is a sewage purification system that can treat industrial and domestic wastewater in daily life again, reduce water waste, and enhance the secondary utilization of water resources.

Key points analysis of rainwater garden
Rainwater garden is a rainwater management system composed of shallow green space, substrate, plants, underground pipe network, rainwater storage tank and irrigation system.

System construction
According to the surrounding conditions and functional needs, construct one or more shallow concave green spaces in the green space that are 50 centimeters to 100 centimeters below the surrounding ground level.Calculate the acceptable rainwater through the volume of the constructed rainwater garden, and then connect the rainwater from the surrounding municipal roads to the shallow concave green spaces through surface, surface open ditches, and underground pipe networks.Multiple permeable blind pipes are buried below the surface of shallow green spaces to accelerate rainwater infiltration rate.At the same time, permeable blind pipes are connected to underground water storage tanks, which store rainwater for green space irrigation.Various plants that can adsorb and reduce pollutants are planted in shallow green spaces to provide rainwater purification function.Install overflow wells in shallow green spaces to connect with municipal pipelines, and a complete rainwater garden system has been established.

2.1.2.Matrix improvement
The substrate is generally composed of soil, gravel, coarse sand or coarse sand, and is the main place for filtering rainwater.It also provides nutrients and growth carriers for various water resistant plants.When rainwater containing pollutants flows in, the substrate will physically remove some of the pollutants from it.The commonly used matrix ratio in Shanghai is to mix and mix 50% clay, 20% gravel with a particle size of 2cm to 5cm, and 30% coarse sand, and replace the surface soil of shallow concave green spaces with 30cm to 50cm.Each region can adjust according to local soil conditions to ensure the infiltration rate of the soil.

Plant selection
Plants are an important component of rainwater gardens, and the removal of pollutants mainly relies on the absorption of plants.Plants with high biomass, well-developed roots, and multiple growth cycles have become the target of selection.At the same time, due to the fact that plants in the rainwater garden will be soaked in the initial rainwater and come into contact with pollutants with high concentrations, plants should have both water and drought resistance, strong pollution resistance, disease and pest resistance, and consider reducing mosquito breeding.It is also necessary to choose some plant varieties with mosquito repellent effects.These plants can directly absorb and utilize various nutrients that can be utilized in wastewater, adsorbing heavy metals and toxic substances.At present, the commonly used plants in rainwater gardens are mostly sedge and grass plants, and perennial herbaceous wetland plants are widely used.In addition to considering ecological habits, the selection of plants for rainwater gardens should also consider the rational allocation of plant varieties, avoiding direct competition between species, and forming a planting structure with reasonable structure, sound functions, and stable population.Strong stress resistance, developed root systems, lush stems and leaves, long growth cycles, high economic and ornamental value should be considered as the criteria for plant selection.At the same time, in winter, plants are prone to withering, death, or growth and dormancy.Therefore, in plant configuration design, it is important to pay attention to the combination of evergreen and fallen leaves.

2.2.1.4.Operation mode
After the establishment of the rainwater garden system, during rainfall, the rainwater from surface runoff is diverted to shallow green spaces for temporary storage.When the rainfall is less than the volume of shallow green spaces, the urban drainage system does not work.The temporarily stored rainwater begins to infiltrate through the improved substrate and enters the underground storage tank through the underground rainwater blind pipe system.Plants planted in shallow green spaces can adsorb and reduce pollutants in rainwater that have not yet infiltrated.Thus, it plays a role in interception and purification.When there is a short-term heavy rainfall and the shallow green space is filled with rainwater, it will enter the municipal pipeline network through the set overflow well.The temporarily stored rainwater in the shallow green space will seep down within 72 hours.

Rainwater Garden Model Design
The sewage treatment system of Rainwater Garden is a sewage purification system that can treat industrial and domestic wastewater in daily life again, reduce water waste, and enhance the secondary utilization of water resources.The rainwater garden model device simulated and designed in this paper consists of two parts, namely, rainwater garden permeable material and container barrel, as shown in Figure 1 and Figure 2.   1 and Figure 2, the rainwater garden permeable material model is modified from a mesh barrel, which belongs to a permeable tank and contains multiple hierarchies.The main function of the rainwater garden container bucket is to store water.Through the role of the filter screen and the detention area, the pollutants in the sewage are preliminarily purified and the suspended solids [15]are adsorbed.The matrix proportion in the rainwater garden is shown in Table 1.As shown in Table 1, for the matrix ratio of rainwater garden designed in this paper, the rainwater garden runoff water quality characteristic information data is input into the simulation model built by manual configuration to obtain the sewage composition and content.

Design of photovoltaic photocatalytic water purification system for rainwater garden
After completing the simulation design of the rainwater garden, the next step is to design a photovoltaic photocatalytic water purification system.Through the water purification system, the goal of small-scale sewage treatment in the rainwater garden will be fully realized.The photovoltaic photocatalytic technology water purification system is a multifunctional system based on photovoltaic power generation and photocatalytic oxidation technology, as well as the principle of spectral band division utilization.It fully utilizes solar energy to treat wastewater.The working principle diagram is shown in Figure 3.The ultraviolet light in the solar spectrum is used for photocatalytic treatment of wastewater, while visible and near-infrared light are used for photovoltaic power generation to drive water pumps, causing simulated wastewater to circulate within the system.Far infrared light is basically carried away by the simulated wastewater in the form of heat, thereby maintaining a lower operating temperature of the battery module.
In the formula, Pin is the rated power and Iin is the rated short-circuit current.
FF is a parameter that measures the quality of the battery P-V junction and the series resistance.Its definition is: The spectral characteristics of solar cells refer to the relationship between the collected photogenerated current and the response of specific incident wavelengths of sunlight under sunlight irradiation.Similar to photocatalytic reactions, light irradiation excites electron transitions on semiconductor silicon, collecting the transition electrons through electrodes for power generation.The photoelectric conversion rate during the process is mainly limited by light absorption, carrier transfer, and carrier reception.In solar cells, only photons with energy greater than the forbidden bandwidth Eg of the semiconductor material can be absorbed by the material to generate electron hole pairs and generate current.Photons with energy less than Eg are absorbed and only converted into heat to heat up the battery.The spectral response of silicon batteries ranges from 400 to 1100nm, reaching its peak at a wavelength of 850nm and capable of absorbing 76% of light energy.
The increase in operating temperature of solar cells will have different effects on the electrical performance parameters of solar cells, thereby affecting the efficiency of power generation of the panel.Moreover, when the temperature exceeds a certain limit, it will cause long-term damage to the battery.Therefore, battery manufacturers generally provide the specific operating temperature and maximum operating temperature of the battery, and the design of the heat dissipation subsystem should meet the required battery operating temperature.

2.3.2.Composition of Photovoltaic Photocatalytic Water Purification System
The components of the rainwater garden photovoltaic photocatalytic water purification IOP Publishing doi:10.1088/1755-1315/1303/1/0120319 system designed in this paper are shown in Table 2.As shown in Table 2, it is the composition structure of the photovoltaic photocatalytic water purification system designed in this paper.The photocatalytic water purification system is simulated through the self-assembled reaction tank.The reaction tank is composed of acrylic plate and SSR-256 neutral silicone structural adhesive.Set the size of reaction tank as 100mm×100mm×50mm [16].When water washes the surface of the photocatalyst, the photocatalyst will generate a certain piezoelectric potential.The generation of piezoelectric potential can greatly enhance the photocatalytic performance [17]of the above prepared catalyst to some extent.Small water pumps are arranged in the reaction tank for small-scale water energy circulation.Sewage constantly washes the surface of photocatalyst, resulting in different piezoelectric potentials [18].The photovoltaic photocatalytic water purification system, based on the preparation of catalyst with high photocatalytic activity, uses the piezoelectric potential generated by water flow to further enhance the photovoltaic photocatalytic activity and accelerate the degradation efficiency of organic wastewater [19].In general, photocatalysts undergo a very complex reaction process when degrading structurally stable organic compounds.There are various types of intermediates produced during the reaction process, and different reaction conditions can also have a significant impact on the reaction products.However, after sufficient reaction time, the majority of organic compounds will be completely mineralized into CO2 and H2O, while the halogen atoms, sulfur atoms, phosphorus atoms, and nitrogen atoms in organic compounds will be converted into X-, SO4 2-, respectively, Inorganic salts such as PO4 3 -and NO3-will ultimately eliminate the harmfulness of organic matter as the degradation reaction is completed.In order to avoid the problem of incomplete treatment, based on this, the photovoltaic photocatalytic water purification system is started, and the water purification system is set to adopt an integrated anoxic/aerobic (A/O) treatment process, and comprehensively purify the small-scale sewage of the rainwater garden.The A/O integrated treatment process of the water purification system consists of a series of anoxic zone (section A) and aerobic zone (section O).The sewage first enters the anoxic section, where the following biological reaction process [20] with poor solubility, difficult to be directly degraded and large molecular weight in sewage into soluble, easy to degrade and small molecular weight organic matter, improve the biodegradability of sewage, and also help to reduce the oxygen demand [21]of aerobic biochemical process.B.Receive the sewage circulating back through the system from the aerobic section, and the denitrifying bacteria will reduce the nitrate nitrogen in the small-scale sewage to N2 under the anoxic environment and discharge it out of the water purification system, so as to achieve the removal of TN in the sewage [22].The sewage then enters the aerobic section with aeration measures and is degraded dissimilarly.At the same time, the nitrifying bacteria also oxidize the NH3-N in the sewage to nitrate nitrogen so as to provide sufficient reaction raw materials [23]for the smooth denitrification process in the anoxic section.Next, the sewage flows slowly from the aerobic section to the sedimentation tank for sludge water separation.Some newly proliferated activated sludge returns to the anoxic section to ensure that the number of microorganisms in the anoxic section is in dynamic balance.The residual sludge is discharged from the system for subsequent treatment, and the effluent can be reused or discharged up to standard [24].
The A/O integrated treatment process flow of the water purification system is shown in Figure 4.As shown in Figure 4, the integrated sewage treatment process designed in this paper is simple, with few structures, strong impact resistance, and can achieve simultaneous removal of organics and TN in sewage.After proper pretreatment, the raw sewage directly enters the anoxic section, providing sufficient carbon source for the denitrification process.During the denitrification process of the photovoltaic photocatalytic water purification system, it can also compensate for a part of the alkalinity consumed, so the demand for carbon source and alkali of the system is greatly reduced, and it can even not be added according to the actual sewage treatment situation, effectively reducing the operating cost [25].The anoxic section of the photovoltaic photocatalytic water purification system has the function of biological selector, which can inhibit the proliferation of filamentous bacteria to some extent and avoid the expansion of impurities in the sewage.
On this basis, after being treated by the rainwater garden photovoltaic photocatalytic water purification system, the reduction rate of small-scale sewage is calculated, and then the reduction efficiency of each stage of sewage treatment is described to obtain the reduction and elimination effect of photovoltaic photocatalytic water purification system.The calculation expression of sewage reduction rate is: Among them, Ma refers to the pollutant content when small-scale sewage enters the rainwater garden; Mc refers the pollutant content when the small-scale sewage of rainwater garden is discharged.Through the calculation expression, the different reduction effects of different sewage combinations are obtained.Based on the combined reduction effect of sewage, continuously adjust the operation mode of the photovoltaic photocatalytic water purification system, and comprehensively achieve the goal of small-scale sewage treatment in the rainwater garden.

Experiment preparation
The S Sponge Community in Ningnan County, Liangshan Prefecture is selected as the engineering example of this experimental study.The community was built in the 1990s and was included in the key reconstruction project of Liangshan Sponge Community.There are five buildings in Sponge Community S, covering an area of 15.25hm2, with building density of 9.12%, greening rate of 51.78%, and current comprehensive rainfall runoff coefficient of 0.50.The location of the community is superior and the transportation is convenient.Rainwater Garden is located in the north of Sponge Community S. The catchment area in the research scope of this paper includes its surrounding area and relevant municipal pipe network, with a total area of 8516m2, a runoff control area of 764m2, a width of 12m, and an effective storage depth of 0.2m.The Rainwater Garden is located in Ningnan County, Liangshan Prefecture, which is located in the Sichuan Basin.Its geographical coordinates are between 102 °27 ′ 44 ″ -102 °55 ′ 09 ″ E and 26 °50 ′ 12 ″ -27 °18 ′ 34 ″ N. The climate of the area where Rainwater Garden located is prominent, belonging to a semi humid climate.
Winter and summer alternate clearly.The climate changes greatly in different seasons, and the spring is warm and humid; It is dry and rainy in summer; It is dry and windy in autumn; Winter is cold and foggy.The annual average temperature is 17 ℃.Rivers and lakes have developed water systems with concentrated rainfall.The maximum rainfall occurs in June, July, August and September, with an annual average rainfall of 526.7mm.For the land condition, rainwater garden and related areas are flat, without special landform.The regional soil types are mainly loam, sandy soil and clay.The specific soil data are shown in Table 3.As shown in Table 3, it is the soil seepage performance data of rainwater garden.The overall terrain of the community is high in the middle and low in the south and north.Rainwater, sewage, etc. collected in the rainwater garden and the catchment area in the northern half of the community are input into the municipal drainage pipe network of Qibin Avenue through a collection pipe channel, and the collection mode is rainwater sewage combined system; The rainwater in the southern half of the community is transported to the municipal pipe network of Jiuzhou Road through a collection pipe channel, and the collection mode is the rainwater sewage separation system.Before the transformation of the sponge city project in the community, the internal green space rate of the community was low, and the main plants were sophora japonica, pine and cypress, willow, ginkgo, rose, lawn and other types.

Experimental content
The initial concentration of simulated wastewater prepared in the experiment is 5-20 mg/L, and the concentration of the catalyst used is 0.2 g/L.This experiment completes the self operation experiment of the system by connecting a solar controller and a battery.Quantitative simulated pollutants and catalysts were placed in 8L deionized water and thoroughly stirred under dark conditions for 40 minutes to achieve adsorption equilibrium, which was used in the experiment; Pour 8L of simulated sewage into the water tank of the system, turn on the water pump, and operate the system under dark conditions for 10 minutes.
After the system runs stably, remove the shading and start the photovoltaic photocatalytic degradation experiment.Test the electrical performance of the system and its catalytic degradation effect on pollutants during the experimental process.Before conducting comparative testing, first test the electrical performance of the constructed system to demonstrate its effective performance and purification effect.According to the above formula (2), it can be seen that short-circuit current affects conversion efficiency.Therefore, using short-circuit current and conversion efficiency as indicators, the maximum short-circuit current and conversion rate that the system can achieve are compared with the standard results to demonstrate the operational feasibility of the system.Then the degradation rate and degradation rate of organic pollutants are selected as the evaluation indicators of this comparative experiment.The higher the degradation rate, the better the small-scale sewage treatment effect of Rainwater Garden is, and the more in line with the requirements of green chemistry; The higher the degradation rate, the higher the treatment efficiency and the better the performance of this method.Set the wastewater treatment method based on photovoltaic photocatalysis technology proposed in this article as the experimental group, and set conventional method 1 and conventional method 2 as control group A and control group B for comparative analysis.

Result analysis 3.3.1. System electrical performance testing
The maximum short-circuit current measured by the system and the conversion efficiency were calculated according to formula (2).The obtained results were compared with the standard results, as shown in Table 4.According to the results obtained from Table 4, the maximum short-circuit current of the photovoltaic photocatalytic water purification system can reach 1.127A, which can far exceed the standard value (0.968A);And the conversion efficiency of electrical energy can reach 98.9%, which is higher than the standard value (95.7%).Based on the analysis of the above results, it can be concluded that the system can operate effectively, achieve efficient electricity conversion, provide a solid electricity foundation for its purification work, and achieve small-scale sewage treatment in rainwater gardens, exerting the purification effect of the system.

Analysis of degradation rate results
The MATLAB simulation analysis software was used to measure the degradation rate of various organic pollutants in the small-scale sewage of the rainwater garden after the application of the three methods.The SPSS statistical analysis software is used to make integrated statistics on the measurement results and draw a column comparison chart of the degradation rate as shown in Figure 4.By analyzing Figure 5, it can be seen that there are significant differences in the degradation rates of organic matter in wastewater among the three treatment methods.Among them, after applying the method proposed in this article, the degradation rates of six organic pollutants were significantly higher than the other two methods, and the degradation rates of organic compounds in wastewater all reached over 99%.The degradation rate of organic matter in wastewater by the two comparison methods has always been lower than 98%.The degradation rate of the other two methods is lower than 98%.Comparing the results of the three methods, it is not difficult to see that the small-scale sewage treatment method proposed in this paper for rainwater garden is highly feasible, which can effectively improve the sewage problem, comprehensively degrade pollutants in the water, with good water purification effect and high pollutant reduction capacity.

Analysis of degradation time results
In order to further analyze the performance of the method proposed in this article, validation was conducted on the degradation rate.The degradation rate is determined by the degradation time, therefore, statistical analysis is conducted on the degradation time of different methods.As the experimental testing was conducted in MATLAB, the tic function was used to record the degradation time.The degradation time of different methods is shown in Table 5. Analyzing the results obtained from Table 5, it can be seen that there is a significant difference between the results obtained by the proposed method and the control method.The degradation time of the proposed method for six organic pollutants is significantly lower than that of the other two conventional methods.The degradation time of the proposed method is maintained within 10 second, and the longest degradation time is 10.97 seconds; The degradation time of the other two methods is both above 21.8 seconds, with conventional method 1 having a minimum degradation time of 21.89 seconds and conventional method 2 having a minimum degradation time of 31.84 seconds, both of which are higher than the degradation time of the method proposed in this article.
From the above analysis, it can be seen that the small-scale sewage treatment method for rainwater gardens proposed in this article based on photovoltaic photocatalysis technology not only has a high degradation rate, but also can complete the degradation of organic pollutants in sewage in a short time, quickly complete sewage treatment work, and maintain a high sewage degradation rate, comprehensively degrade water pollutants, achieve good water purification effect, and effectively improve sewage problems.

Conclusion
In the current trend of gradual scarcity of water resources, efficient sewage treatment is crucial.Under this background, this paper carried out a comprehensive study on small-scale sewage treatment methods of rainwater garden based on photovoltaic photocatalysis technology.Photovoltaic photocatalysis technology is based on spectral segmentation and utilization technology.It effectively combines photocatalytic oxidation technology with photovoltaic power generation technology, and achieves the goal of using solar energy to treat sewage.In the process of treatment, ultraviolet spectrum is used for photocatalytic degradation of wastewater.Visible spectrum and near-infrared spectrum are used to generate electricity on the battery surface to provide power for wastewater recycling.Far infrared spectrum is absorbed by wastewater to keep the battery working at a lower operating temperature.Through the above discussion, the quality and efficiency of sewage treatment have been improved, the degradation effect of organic pollutants in wastewater is better, the process flow is simple, and the input cost is less, which meets the requirements of green chemistry at this stage.

Figure 1 .
Figure 1.Permeable Material Model Structure of Rainwater Garden

Figure 2 .
Figure 2. Model Structure of Rainwater Garden Container BarrelAs shown in Figure1and Figure2, the rainwater garden permeable material model is modified from a mesh barrel, which belongs to a permeable tank and contains multiple hierarchies.The main function of the rainwater garden container bucket is to store water.Through the role of the filter screen and the detention area, the pollutants in the sewage are preliminarily purified and the suspended solids[15]are adsorbed.The matrix proportion in the rainwater garden is shown in Table1.

Figure 4 .
Figure 4. A/O Integrated Treatment Process Flow of Photovoltaic Photocatalytic Water Purification System

14 Figure 5 .
Figure 5.Comparison Results of Organic Pollutants Degradation Rate of Small scale Sewage in Rainwater Garden

Table 1 :
Matrix Proportioning Design of Rainwater Garden

Table 2 :
Components of rainwater garden photovoltaic photocatalytic water purification system

Table 3 :
Seepage performance data of rainwater garden soil

Table 4
Maximum Short Circuit Current and Conversion Rate

Table 5 :
Comparison Results of Degradation Time for Three Methods