Stormwater runoff treatment substrate and stormwater bioretention system constructed by using substrate

Abstract

A stormwater runoff treatment substrate includes a lower pyrite substrate layer and an upper biochar substrate layer. The lower pyrite substrate layer includes pyrite, oyster shell powder, and sandy materials in a volume ratio of 10:5:85. The upper biochar substrate layer includes biochar or activated carbon, organic nutrient soil, and sandy materials in a volume ratio of 20:3:77.

Claims

1. A stormwater bioretention system comprising a cell body, wherein a gravel drainage layer, a transition layer, a pyrite substrate layer, a biochar substrate layer, a woodchip protective layer, a ponding zone, a perforated water collection pipe, a raised water outlet pipe, and an overflow pipe, wherein: the gravel drainage layer is disposed at a bottom of the cell body; the transition layer is disposed above the gravel drainage; the pyrite substrate layer is disposed above the transition layer, and wherein the pyrite substrate layer comprises a mixture of pyrite, oyster shell powder, and a first quartz sand in a volume ratio of 10:5:85; the biochar substrate layer is disposed above the pyrite substrate layer, and wherein the biochar substrate layer comprises a mixture of biochar, organic nutrient soil, and a second quartz sand in a volume ratio of 20:3:77; the woodchip protective layer is disposed above the biochar substrate layer; the ponding zone is disposed above the woodchip protective layer; the perforated water collection pipe is disposed in the gravel drainage layer, and the perforated water collection pipe is connected to the raised water outlet pipe; a height of the raised water outlet pipe is equal to a height of a top of the pyrite substrate layer, and an the overflow pipe is disposed at a top opening of the cell body.

2. The stormwater bioretention system of claim 1, wherein an edge of an entrance of the cell body is provided with a runoff guidance slope, and a woodchip of the woodchip protective layer is 1-2 cm long.

3. The stormwater bioretention system of claim 1, wherein the transition layer is a sand layer with a particle size larger than the pyrite substrate layer and smaller than the gravel drainage layer.

4. The stormwater bioretention system of claim 1, wherein a permeability coefficient of the biochar substrate layer is between 200 mm/h and 600 mm/h; and a permeability coefficient of the pyrite substrate layer is between 300 mm/h and 600 mm/h.

5. The stormwater bioretention system of claim 1, wherein the biochar substrate layer comprises the mixture of 20 vol. % of the biochar, 3 vol. % of the organic nutrient soil, 3 vol. % of 5-10 mesh of the second quartz sand, 7 vol. % of 10-20 mesh of the second quartz sand, 40 vol. % of 20-35 mesh of the second quartz sand, 17 vol. % of 30-60 mesh of the second quartz sand, and 10 vol. % of 60-120 mesh of the second quartz sand.

6. The stormwater bioretention system of claim 1, wherein the pyrite substrate layer comprises the mixture of 10 vol. % of the pyrite with a particle size of 1-3 mm, 5 vol. % of the oyster shell powder with a sheet length of 1-3 mm, 3 vol. % of 5-10 mesh quartz, 2 vol. % of 10-20 mesh of the first quartz sand, 35 vol. % of 20-30 mesh of the first quartz sand, 25 vol. % of 30-60 mesh of the first quartz sand, and 20 vol. % of 60-120 mesh of the first quartz sand.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The disclosure is described hereinbelow with reference to accompanying drawings, in which the sole FIGURE is a structural diagram of a stormwater bioretention system of the disclosure.

(2) In the drawings, the following reference numbers are used: 1. Runoff guidance slope; 2. Woodchip protective layer; 3. Upper biochar substrate layer; 4. Lower pyrite substrate layer; 5. Transition layer; 6. Gravel drainage layer; 7. Overflow pipe; 8. Perforated water collection pipe; 9. Raised water outlet pipe; 10. Cell body; and 11. Ponding zone.

DETAILED DESCRIPTION

(3) To further illustrate, embodiments detailing a stormwater runoff treatment substrate are described below. It should be noted that the following embodiments are intended to describe and not to limit the disclosure.

(4) The concept of the disclosure is that compared with wastewater, stormwater has different hydraulic and water quality conditions and physical and chemical properties. First, the stormwater pollutant concentration is lower, which can adapt to the trait of the less electron supply rate of pyrite system; Second, stormwater treatment facilities are usually operated intermittently, which can provide sufficient hydraulic retention time in the dry period, to meet the characteristics of long retention time for pyrite denitrification; Thirdly, pyrite denitrification is mainly autotrophic denitrification, and its sludge yield is lower, which helps to maintain the good hydraulic performance of bioretention and meets the needs of low maintenance of bioretention facilities; Fourth, pyrite is cost-effective and durable, does not need complex material synthesis or processing, and can turning waste into treasure, which meets the requirements of simple structure and economic saving of stormwater bioretention system.

(5) The preparation of stormwater runoff treatment substrate of the disclosure:

(6) The preparation of stormwater runoff treatment substrate has high requirements. If the ratio is inappropriate, it may lead to several problems, such as uneven ratio can lead to facility collapse and particle leakage; Too small particle size will lead to poor hydraulic conditions and blockage; Too coarse particle size will lead to low pollutant removal capacity, poor water retention and difficult for plants survival.

(7) The following example provides a substrate ratio considering various needs, which can be suitable for common stormwater quality and quantity conditions:

(8) In the upper biochar substrate layer, the volume ratio is: 5-10 mesh quartz sand 3%, 10-20 mesh quartz sand 7%, 20-35 mesh quartz sand 40%, 30-60 mesh quartz sand 17%, 60-120 mesh quartz sand 10%, powdered biochar 20% and organic peat soil 3%; The D50 after mixing is about 0.48 mm.

(9) In the lower pyrite substrate layer, the volume ratio is: 5-10 mesh quartz sand 3%, 10-20 mesh quartz sand 2%, 20-30 mesh quartz sand 35%, 30-60 mesh quartz sand 25%, 60-120 mesh quartz sand 20%, 1-3 mm pyrite 10% and oyster shell powder 5%; The D50 after mixing is about 0.51 mm.

(10) The quartz sand mentioned above can also be replaced by river sand.

(11) The key to the preparation of stormwater runoff treatment substrate is to select sandy materials with different particle sizes and gradations in a certain proportion. This certain range of particle size distribution will make the permeability coefficient of mixed materials is into a suitable range. The permeability coefficient is tested by the equal head method. The permeability coefficient of the lower pyrite substrate layer is not less than 300 mm/h, and the permeability coefficient of the upper biochar substrate layer is not less than 200 mm/h, both are not more than 600 mm/h. This range of permeability coefficients can make the bioretention system not only meet the needs of water reduction and plant growth but also ensure good pollutant reduction capacity.

(12) As shown in the sole FIGURE, the stormwater bioretention cell of the disclosure comprises a cell body 10, in which there are gravel drainage layer 6, transition layer 5, lower pyrite substrate layer 4, upper biochar substrate layer 3, woodchip protective layer 2, and ponding zone 11 from bottom to top. A perforated water collection pipe 8 is installed in the gravel drainage layer 6, and the perforated water collection pipe 8 is connected to the raised water outlet pipe 9, the elevation of the raised water outlet pipe 9 is equal to the top height of the lower pyrite substrate layer 4, and an overflow pipe 7 is installed at the top of the cell body 10.

(13) A runoff guidance slope 1 is set along the edge of cell body 10. The woodchips of the woodchip protective layer 2 are 1-2 cm long, and the woodchip can choose bark. The transition layer 5 is a sand layer with a particle size larger than the lower pyrite substrate layer and smaller than the gravel drainage layer, which is used to prevent particles from the lower pyrite substrate layer from leaking into the gravel drainage layer and blocking the perforated water collection pipe.

(14) The purpose of activation at the initial operation of the stormwater bioretention cells is to accelerate the maturity of the microorganism in the cells (This step can also be abandoned and make the bioretention accept natural rainfall to mature). Specifically, before the first operation, a culture medium with tap water or stormwater as solvent immersed in the lower pyrite substrate layer is inversely introduced from the raised water outlet pipe 9 to promote the proliferation of sulfur autotrophic denitrification microorganisms in the lower pyrite substrate layer. The components of the culture medium are 0.2 g/L KNO.sub.3, 0.05 g/L NH.sub.4Cl, 0.5 g/L Na.sub.2S.sub.2O.sub.3.Math.5H.sub.2O and 0.02 g/L KH.sub.2PO.sub.4. After the long-term operation, when the treatment effect of stormwater bioretention cell decreases, the maintenance can be applied by renovating the upper biochar substrate layer, adding organic materials such as peat soil, or replacing the top woodchip protective layer using new woodchips.

(15) It is assumed that the ratio of the surface area of the stormwater bioretention cell to the catchment area of the cell is 1:20, and the runoff coefficient is 0.75. When the cell deals with low-intensity rainfall (It is defined as the rainfall within 12 hours is not more than 14.9 mm), the stormwater collected in the system enters the cell body through the runoff guidance slope, large particles and suspended solids are intercepted by the woodchip protective layer, stormwater infiltrates into the upper biochar substrate layer, and the upper biochar substrate layer adsorbs ammonia nitrogen and organic matter. At the same time, the lower part of the upper biochar substrate layer uses organic matter for heterotrophic denitrification to remove part of nitrate nitrogen. Then the stormwater enters the lower pyrite substrate layer which the pyrite is used for autotrophic denitrification to remove nitrogen, and the generated iron ions are complexed with dissolved phosphate for phosphorus removal. At this time, because the total flow received by the bioretention cell is less, the hydraulic load is low and the dissolution of organic matter is less. When the rainfall stopped, the remaining nitrate was removed by autotrophic denitrification in the lower pyrite substrate layer. In the next rainfall, the treated stormwater in the lower pyrite substrate layer will be replaced by new stormwater, to continue the above pollution reduction steps.

(16) When dealing with high-intensity rainfall, the removal process is similar to that of low-intensity rainfall. However, due to the larger runoff volume generated during high-intensity rainfall and the permeability coefficient of the upper biochar substrate layer is in the range of 200-600 mm/h, the stormwater will not permeate immediately, but will gradually gather in the ponding zone, making the stormwater bioretention cells operate at full hydraulic load. This will increase the water head difference, moisture content and infiltration rate of the system, strengthen the scouring and organic matter dissolution, greatly improve heterotrophic denitrification, and offset the low rate of nitrogen removal using pyrite only, making the stormwater bioretention cells still has excellent pollutant removal efficiency under heavy stormwater even.

(17) Comparative Test

(18) 1. Test of Pollutant Leakage at the Beginning of Operation

(19) Compared with the traditional sand bioretention system and the woodchip modified bioretention system, the stormwater bioretention system of the disclosure has the advantage of low pollutant leaching. Taking tap water as the influent, we compared and tested the indicators such as Kjeldahl nitrogen (TKN), nitrate nitrogen (NO.sub.3.sup.N), nitrite nitrogen (NO.sub.2.sup.N), total nitrogen (TN), total phosphorus (TP), chemical oxygen demand (COD) and ultraviolet absorbance at 254 nm (UV.sub.254) of the above three bioretention cells. The test results of pollutant leakage after one month of initial operation of the cells are shown in Table 1.

(20) TABLE-US-00001 TABLE 1 Mean value of pollutant leaching concentration (mg/L) Kjeldahl Nitrate Bioretention system nitrogen nitrogen TN TP COD UV.sub.254 Bioretention system of 0.48 0.43 0.95 0.42 19.4 0.199 disclosure Traditional sand 0.73 0.90 1.71 0.52 26.7 0.325 bioretention system Woodchip modified 0.92 0.48 1.43 1.56 65.3 0.595 bioretention system

(21) It can be seen from Table 1 that the pollutant leaching concentration of the disclosure is significantly lower than that of the two existing bioretention systems.

(22) Although total iron and sulfate will be generated during pyrite-based autotrophic denitrification or oxidation, the concentration of these two by-products is very low in the bioretention system of disclosure, the net leaching of sulfate is generally not more than 10 mg/L.

(23) Except for the first two operations during the start up phase, the total iron generation is stably below 0.3 mg/1, which meets the requirements for total iron in class III water body of Quality standard for groundwater of China (GBT-14848-2017).

(24) 2. Pollutant Removal Efficiency Test

(25) To test the pollutant removal efficiency, we test the stormwater bioretention system of the disclosure, the traditional sand bioretention system and the woodchip modified bioretention system using synthetic stormwater.

(26) Assuming that the service area ratio of facilities is 1:20, the runoff coefficient is 0.75, the rainfall duration is 2 h, and the rainfall is 25 mm. 10 large-scale rainfall events are simulated and the pollutant removal efficiency is calculated. The results of pollutant removal are shown in Table 2.

(27) TABLE-US-00002 TABLE 2 Average removal efficiency of simulated runoff pollutant (%) Kjeldahl Nitrate Bioretention system nitrogen nitrogen TN TP COD Bioretention system of 85.2 41.7 67.4 80.3 76.3 disclosure Traditional sand bioretention 63.7 1.8 39.1 45.2 68.5 system Woodchip modified 78.0 54.6 68.2 16.0 35.6 bioretention system

(28) It can be found that this disclosure can achieve excellent pollutant removal performance even under heavier rainfall events. Compared with the traditional sand bioretention system, the pollutant removal performance of this disclosure was higher. Compared with the woodchip-modified system, although nitrate removal in the current disclosure was slightly lower, the TN removal performance was almost equal, and this new disclosure achieved significantly higher COD and TP removal performance.

(29) It will be obvious to those skilled in the art that changes and modifications may be made, and therefore, the aim in the appended claims is to cover all such changes and modifications.