Method for removing nitrate in interflow in mountainous and hilly areas

Abstract

The invention discloses a mountainous and hilly area interflow nitrate removal method which comprises the following steps: S1, preparing wood chips, pretreating the wood chips, and performing domestication culture of denitrifying bacteria by adopting the pretreated wood chips to obtain wood chips with microorganisms attached to the surfaces; preparing a biochar raw material or biochar; s2, a denitrification reactor is prepared, a water distribution layer, a filler layer and an anti-suspension layer are laid in the denitrification reactor in sequence, and the filler layer is composed of the wood chips with microorganisms attached to the surfaces and the biochar raw material or biochar; s3, introducing interflow to be treated into the denitrification reactor, and carrying out denitrification reaction; and S4, degassing and precipitating the reacted fluid to obtain nitrate-removed interflow. The method can effectively remove the nitrate in the interflow with high NO3 <->-N and low organic carbon, and provides technical support for the treatment of the interflow.

Description

Technical Field

[0001] The present invention relates to the technical field of water treatment, and particularly relates to a method for removing nitrate from subsurface flow in mountainous and hilly areas. Background Art

[0002] Nitrogen is an essential nutrient for plant growth, and at the same time, nitrogen is also an important source of water eutrophication. Dissolved nitrogen compounds in water exist in three main forms: nitrate (NO3 - -N), ammonium salt (NH4 + -N), and organic nitrogen. Among them, NO3 - -N is the most difficult to remove from water, and NH4 + -N will also be converted into NO3 - -N when it comes into contact with dissolved oxygen (DO) during the water transportation process. High NO3 - -N pollution accelerates the growth of algae in the process of eutrophication, and has adverse effects on human health directly or indirectly by consuming DO in the water body. NO3 - -N participates in the nitrogen cycle of nature, exists abundantly in nature, is easily absorbed by organisms and is also easily excreted. Although the physiological toxicity of nitrate nitrogen is relatively small, due to its conversion to NO2 with higher toxicity in anoxic environments - -N, its harm to the environment and food chain cannot be underestimated; at the same time, the anoxic environment brought about by water eutrophication is likely to lead to the interruption of the food chain, the degradation of habitats in rivers, lakes and coastal waters, and an increase in N2O emissions. Therefore, formulating strategies for removing NO3 - -N from surface water, subsurface flow and groundwater is crucial.

[0003] NO3 - -N in soil is an important source of NO3 - -N in surface water, groundwater and even the global biogeochemical system. The main runoff pathways of nitrogen fertilizers in many agricultural activities are ammonia volatilization and conversion to NO3 - -N entering the ecosystem, making the soil, especially the purple soil in farmland in the southwestern subtropical region, an important source of NO3 - -N pollution. Different from other regions, purple soil has a relatively developed subsurface flow system, that is, in ways such as agricultural irrigation, rainfall and groundwater migration, NO3 - -N is extremely easy to migrate to the watershed water body through underground pathways, further causing NO3 - -N pollution such as water eutrophication.

[0004] Existing NO3 - -N removal methods remain at the purification of nitrogen pollution by wastewater treatment plants (WWTPs), constructed wetlands and ecological ditches, lacking efficient and economical methods for subsurface flow NO3 --N pollution purification method, and the above common methods are more targeted at surface water pollution. Compared with surface non-point source pollution, there are certain technical problems in the control difficulty and purification effect of subsurface flow pollution. Taking WWTP as an example, although WWTP sewage treatment can effectively reduce the high concentrations of COD and NH4 + -N in domestic sewage, in principle, it is to maintain an appropriate carbon-nitrogen ratio of activated sludge and promote the carbon-nitrogen mineralization of organisms. However, for subsurface flow sewage with low COD and high NO3 - -N, it is difficult to collect the fluid and remove it biologically. Ordinary sewage purification methods are not applicable, and there is an urgent need for a sewage treatment method that is suitable for high concentrations of NO3 in subsurface flow of subtropical mountain purple soil - -N, and at the same time can effectively remove COD with a low concentration. Summary of the Invention

[0005] In view of the above problems, the present invention aims to provide a method for removing nitrate from subsurface flow in mountainous and hilly areas.

[0006] The technical solution of the present invention is as follows:

[0007] A method for removing nitrate from subsurface flow in mountainous and hilly areas, comprising the following steps:

[0008] S1: Prepare wood chips, and pre-treat the wood chips. Use the pre-treated wood chips for the acclimation and cultivation of denitrifying bacteria to obtain wood chips with microorganisms attached to the surface; prepare biochar raw materials or biochar;

[0009] S2: Prepare a denitrification reactor, and sequentially lay a water distribution layer, a packing layer, and an anti-suspension layer in the denitrification reactor. The packing layer is composed of the wood chips with microorganisms attached to the surface and the biochar raw materials or biochar;

[0010] S3: Introduce the subsurface flow to be treated into the denitrification reactor for denitrification reaction;

[0011] S4: Degas and precipitate the reacted fluid to obtain subsurface flow with nitrate removed.

[0012] Preferably, in step S1, the pre-treatment of the wood chips specifically includes the following sub-steps: rinse the wood chips with clean water to remove the dust on the surface of the wood chips, soak them in a sodium hydroxide solution with a mass concentration of 1.5% for 18 h, then rinse them with deionized water until neutral, and finally dry them at 80 °C for 48 h.

[0013] Preferably, in step S1, the biochar raw material is corn straw, and the biochar is obtained by pyrolyzing the biochar raw material at 300-500 °C.

[0014] Preferably, in step S2, the denitrification reactor is made of acrylic, and a light-shielding cloth is provided on the surface of the denitrification reactor to avoid light.

[0015] Preferably, in step S2, the volume ratio of the sawdust with attached microorganisms to the biochar is 7:3.

[0016] Preferably, in step S2, both the water distribution layer and the suspended matter release layer are laid with quartz sand.

[0017] Preferably, in step S2, a nylon screen is further provided between the packing layer and the anti-suspension layer.

[0018] Preferably, in step S3, when introducing the subsurface flow to be treated into the denitrification reactor, the water is introduced in an upward flow manner.

[0019] Preferably, in step S3, the denitrification reaction is carried out under a variety of different environments.

[0020] Preferably, it further includes calculating the removal efficiency of NO3 - -N, the removal rate of nitrate, and the NO3 - -N load per unit volume of water to evaluate the denitrification performance of the denitrification reactor:

[0021]

[0022]

[0023]

[0024]

[0025] In the formula: NRE is the removal efficiency of NO3 - -N in the water body, %; C i1 and C o1 are the NO3 - -N concentrations at the inlet and outlet of the denitrification reactor, respectively, mg N / L; NRR is the removal rate of nitrate in the water body, mg N / (L·d); HTR is the hydraulic retention time, d; h is the height of the denitrification reactor, cm; A is the cross-sectional area of the denitrification reactor, cm 2 ; n e is the medium porosity; Q is the influent flow rate, L / d; NO3 - -N load is the NO3 - -N load per unit volume of water, mg N / (L·d); V e is the effective volume of the denitrification reactor, L.

[0026] The beneficial effects of the present invention are as follows:

[0027] Through the synergistic effect of biochar and wood chips and in combination with domesticated denitrifying agents, the present invention enhances the microbial activity of the system, improves the NO3 - -N removal efficiency, reduces the generation of by-products, and the whole process features high efficiency, sustainability, environmental friendliness, and excellent economy, providing technical support for solving the problem of high nitrate nitrogen and low organic carbon pollution in subsurface flow. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for the description of the embodiments or the prior art. Obviously, the drawings in the following description are only some embodiments of the present invention. For those of ordinary skill in the art, other drawings can be obtained based on these drawings without creative efforts.

[0029] Figure 1 It is a schematic structural diagram of a denitrification reactor in a specific implementation;

[0030] Figure 2 It is a schematic diagram of the overall structure of the device for denitrification treatment in a specific implementation;

[0031] Figure 3 It is a schematic diagram of the nitrogen concentration results of denitrification reactors with different fillers in a specific implementation;

[0032] Figure 4 It is a schematic diagram of the effluent nitrogen removal rate results of denitrification reactors with different fillers in a specific implementation;

[0033] Figure 5 It is a schematic diagram of the CH4 and CO2 release flux results of denitrification reactors with different fillers in a specific implementation;

[0034] Figure 6 It is a schematic diagram of the NO2 release flux results of denitrification reactors with different fillers in a specific implementation;

[0035] Figure 7 It is a schematic diagram of the effluent COD results of denitrification reactors with different fillers in a specific implementation. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0036] The present invention will be further described below in conjunction with the accompanying drawings and embodiments. It should be noted that, without conflict, the embodiments in the present application and the technical features in the embodiments may be combined with each other. It should be pointed out that unless otherwise specified, all technical and scientific terms used in the present application have the same meaning as commonly understood by those of ordinary skill in the technical field to which the present application belongs. As used in the disclosure of the present invention, words such as "including" or "comprising" and the like are intended to mean that the elements or items appearing before the word cover the elements or items listed after the word and their equivalents, without excluding other elements or items.

[0037] The present invention provides a method for removing nitrate from subsurface flow in mountainous and hilly areas, comprising the following steps:

[0038] S1: Prepare wood chips, and pre-treat the wood chips. Use the pre-treated wood chips for the acclimation and cultivation of denitrifying bacteria to obtain wood chips with microorganisms attached to the surface; prepare biochar raw materials or biochar.

[0039] In a specific embodiment, the pre-treatment of the wood chips specifically includes the following sub-steps: Rinse the wood chips with clean water to remove the dust on the surface of the wood chips, soak them in a sodium hydroxide solution with a mass concentration of 1.5% for 18 h, then rinse them with deionized water until neutral, and finally dry them at 80 °C for 48 h. It should be noted that the drying step in this embodiment is to remove microorganisms. In addition to the preferred temperature and time in this embodiment, other temperature and time parameters that can achieve this purpose can also be used.

[0040] In a specific embodiment, when carrying out the acclimation and cultivation of denitrifying bacteria, add a microbial agent to a liquid medium prepared from the pre-treated wood chips and deionized water, and anaerobically cultivate it at room temperature for 15 d to make the microorganisms attach to the surface of the wood chips.

[0041] In a specific embodiment, the biochar raw material is corn straw, and the biochar is obtained by pyrolyzing the biochar raw material at 300 - 500 °C. Optionally, when preparing the biochar, air-dry the corn straw, cut it into sections, put it into a carbonization furnace, heat it up to the target temperature (300 °C, 400 °C, 500 °C) and keep it until no gas overflows, and then cool it and take it out to obtain the biochar.

[0042] S2: Prepare a denitrification reactor, and sequentially lay a water distribution layer, a packing layer, and an anti-suspension layer in the denitrification reactor. The packing layer is composed of the wood chips with microorganisms attached to the surface and the biochar raw materials or biochar.

[0043] In a specific embodiment, the denitrification reactor is made of acrylic, and a light-shielding cloth is provided on the surface of the denitrification reactor to avoid light. In this embodiment, setting the light-shielding cloth on the surface of the denitrification reactor can prevent the growth of algae.

[0044] In a specific embodiment, the volume ratio of the sawdust with microorganisms attached to the surface to the biochar is 7:3.

[0045] In a specific embodiment, both the water distribution layer and the suspended layer are laid with quartz sand. Optionally, a nylon sieve is further provided between the filler layer and the anti-suspension layer. In this embodiment, setting the nylon sieve can prevent the biochar particles from being washed away.

[0046] In a specific embodiment, a sampling port is installed every 10 cm in the filler area corresponding to the denitrification reactor.

[0047] S3: Introduce the subsurface flow to be treated into the denitrification reactor to carry out the denitrification reaction.

[0048] In a specific embodiment, when introducing the subsurface flow to be treated into the denitrification reactor, the water inlet adopts an upward flow mode.

[0049] In a specific embodiment, when carrying out the denitrification reaction, it is carried out under a variety of different environments. Optionally, it includes the dry-wet alternation environment and the extreme environment. In a specific embodiment, the influent flow rate gradually increases from 18 mL / h during startup to a maximum of 288 mL / h, and the elevated NO3 - -N concentration during the drought period, the dry-wet alternation environment, and the high-flow subsurface flow during the rainstorm period are simulated at specific stages.

[0050] S4: Degas and precipitate the reacted fluid to obtain the subsurface flow after nitrate removal.

[0051] In a specific embodiment, the method for removing nitrate from the subsurface flow in the mountain and hilly areas of the present invention further includes calculating the removal efficiency of NO3 - -N, the removal rate of nitrate, and the NO3 - -N load of unit volume of water to evaluate the denitrification performance of the denitrification reactor:

[0052]

[0053]

[0054]

[0055]

[0056] Where: NRE is the removal efficiency of NO3 - -N in water body, %; C i1 and C o1 are the NO3 - -N concentrations at the inlet and outlet of the denitrification reactor respectively, mg N / L; NRR is the removal rate of nitrate in water body, mg N / (L·d); HTR is the hydraulic retention time, d; h is the height of the denitrification reactor, cm; A is the cross-sectional area of the denitrification reactor, cm 2 ; n e is the porosity of the medium; Q is the influent flow rate, L / d; NO3 - -N loading is the NO3 - -N loading per unit volume of water, mg N / (L·d); V e is the effective volume of the denitrification reactor, L.

[0057] When denitrification reaction is carried out under multiple different environments, the weighted score is calculated by the following formula to evaluate the denitrification performance under different environments:

[0058]

[0059] N i = NRE i ×W i (4)

[0060] Where: WPI is the weighted score; n is the number of different environments; N i is the actual contribution of nitrate removal in the i-th environment; Di is the number of days in the i-th environment; D is the total number of days in different environments; NRE i is the nitrate removal rate in the i-th environment; W i is the weighted factor in the i-th environment.

[0061] In a specific embodiment, the weighted factor in the stable or recovery stage is 1, the weighted factor in the drought stage is 2, the weighted factor in the wet-dry alternating stage is 1.5, and the weighted factor in the rainfall stage is 1.25.

[0062] In a specific embodiment, taking the subsurface flow in the mountainous and hilly rural areas in the middle and upper reaches of the Yangtze River as an example, the nitrate removal method for subsurface flow in mountainous and hilly areas described in the present invention is used to remove nitrate therein.

[0063] In this embodiment, the denitrification reactor is as Figure 1 shown, and the overall device combining degassing and precipitation is as Figure 2As shown. The denitrification reactor is made of acrylic, with a diameter of 10 cm and a height of 60 cm, and is wrapped with light-proof cloth to prevent the growth of algae; a 5-cm-thick quartz sand water distribution layer is provided at the bottom of the denitrification reactor, and 70% by volume of wood chips with attached microorganisms and 30% by volume of biochar raw materials or biochar mixtures are filled in the middle (4 groups of reactors are formed according to different selected raw materials. Among them, the group that selects biochar raw materials is recorded as WBR, the group that selects biochar prepared at a pyrolysis temperature of 300 °C is recorded as B300, the group that selects biochar prepared at a pyrolysis temperature of 400 °C is recorded as B400, and the group that selects biochar prepared at a pyrolysis temperature of 500 °C is recorded as B500), and it is compacted at an incremental height of 5 cm until the total depth of the filter medium is 50 cm. A 400-mesh nylon sieve is placed on top of the packing to prevent the biochar particles from being washed away, and 5 cm of quartz sand is added on top to prevent the biochar particles from floating. A degassing chamber of the same size is added after the denitrification reactor in the overall device. The purified sewage enters the degassing chamber through the pipeline under the ecological ditch reaction chamber. The liquid rises and the dissolved gas is discharged. When it reaches the upper pipeline, a part of the liquid is circulated to the denitrification reactor for cyclic purification, and the remaining part enters the sedimentation tank, and the particulate matter in the denitrification reactor is precipitated, and then the purified water is discharged from the outlet below.

[0064] The wood chips with attached microorganisms are obtained through the following steps: Cut the wood waste into pieces with a length of 10 - 20 mm and a width of 2 - 10 mm. Rinse with clean water to remove surface dust, soak in 1.5% NaOH for 18 h, then rinse with deionized water until neutral, and dry in an 80 °C oven for 48 h. Add the bacterial agent to a sterilized beaker, keep the concentration of the microbial bacterial agent at 1 g / L (according to the bacterial agent instruction manual), add the pretreated wood chips and deionized water to prepare a liquid medium (the specific nutrient components of the medium are the same as those in the subsurface flow to be treated), and carry out the domestication culture of denitrifying bacteria and make the microorganisms attach to the surface of the wood chips. Collect the upper liquid every 72 h and then replace the upper liquid medium, and carry out anaerobic culture at room temperature of about 25 °C for 15 d. The biochar is purchased from the biochar made of corn straw by Nanjing Zhironglian Technology Co., Ltd.

[0065] In this embodiment, except for the start-up stage, the 4 groups of denitrification reactor systems operate in 8 stages during operation. The 1st to 4th stages (P1 - P4) are respectively: the influent flow rates are increased from 18 mL / h during the start-up period to 36 mL / h, 72 mL / h, 144 mL / h, 288 mL / h; the 5th to 8th stages (P5 - P8) are: when the influent flow rate is 72 mL / h, simulate the NO3 in the subsurface flow during the dry period --N concentration increases, wet-dry alternation environment, recovery after wet-dry alternation, and an influent flow rate of 1152 mL / h is used to simulate high-flow subsurface flow during rainstorm periods. In 8 separately detected systems, the NO3 - -N removal effect was investigated. By collecting water samples and gas samples twice a week during the reactor operation, the stability of the ecological ditch reactor under different environmental conditions was judged. During fixed time periods at different times, the water quality indicators and by-products in the reactor were detected. The test results are as Figures 3 - 4 shown.

[0066] From Figure 3 and Figure 4 it can be seen that when the influent flow rate increased from 72 mL / h to 144 mL / h, the NO3 - -N removal effect of the 4 groups of reactors showed a trend of first decreasing and then increasing. Eventually, when the system was stable, the NO3 - -N removal rates of the 4 groups of reactors could all reach 86% - 89%; when the influent flow rate increased to 288 mL / h, the NO3 - -N removal effect of the 4 groups of reactors also showed a trend of first decreasing and then increasing. The NO3 - -N removal rates of the B300, B500, and B400 reactors finally stabilized at 65%, 61%, and 59% respectively, and the NO3 - -N removal rate of the WBR was the lowest, at 50% during the stable period; when the influent flow rate increased to 1152 mL / h, the NO3 - -N removal rates of the 4 reactors decreased sharply to below 50%. When the system was stable, the removal rates of the B500, WBR, and B400 reactors decreased to 41%, 33%, and 31% respectively, and the NO3 - -N removal rate of the B300 was the lowest, dropping to 24%. When the influent flow rate increased, the HRT became shorter, and the NO3 - -N removal effect became worse.

[0067] From Figure 3 it can be seen that during operation at different flow rates, when the system reached stability, the effluent NH4 + -N concentration was mostly below 1 mg / L and all below 1.5 mg / L, indicating that there would be no situation of excessive NH4 + -N concentration when the effluent of the 4 denitrification systems was stable. However, during operation at influent flow rates of 144 mL / h and 288 mL / h, there would be a situation of increasing effluent NH4 + -N concentration, which might be because when the NO3 - -N load increased, the nitrate dissimilatory reduction to ammonium (DNRA) reaction occurred.

[0068] Table 1 Effluent conditions of reactors with different fillers at different times

[0069]

[0070]

[0071]

[0072] As can be seen from Table 1, as the HRT decreases, even when the NO3 - -N load increases, the NO3 - -N removal rate gradually decreases, which is related to the insufficient reaction time between denitrifying microorganisms and NO3 - -N, the loss of biofilm due to water flow scouring, and the loss of organic carbon. At stages P2 and P5, when the influent flow rate is 36 mL / h and the NO3 - -N concentrations are 30 mg / L and 50 mg / L respectively, the NRR at stage P5 is significantly higher than that at stage P2. Except for the B400 reactor, when the other three groups of denitrifying reactors reached stability at stages P2 and P5, the NO3 - -N removal rate reached over 95%, and the HRT of all four groups of reactors was higher than 30 h. This is because when the HRT is long enough, as the influent NO3 - -N concentration increases, denitrifying microorganisms can carry out denitrification reactions with more NO3 - -N

[0073] In addition, under the condition of high-flow operation, B500 has the best NO3 - -N removal rate. As mentioned above, this is not only related to microbial abundance and electron transport systems, but may also be because the biochar particles pyrolyzed at 500 °C are finer and have a larger specific surface area for microorganisms to carry out denitrification reactions with NO3 - -N

[0074] From Figure 5 it can be seen that under different operating conditions, there are certain differences in the CO2 and CH4 release fluxes generated by the four systems:

[0075] For CO2, when the influent flow rate is 72 mL / h, the CO2 release fluxes of all systems are relatively low, less than 200 mg m -2 h -1 . When the influent flow rate remains unchanged and the influent NO3 - -N increases to 50 mg / L, the CO2 release flux will first increase and then decrease. The highest CO2 release flux is 812 mg m -2 h -1 for B300. When the systems of the four groups of reactors gradually stabilize, the CO2 release fluxes all drop to 200 mg m -2 h-1 The following is the situation when the four groups of reactors undergo wet-dry alternation. Except for the B300 reactor, the CO2 emission fluxes of the other three groups of reactors during wet-dry alternation operation are all below 200 mg m -2 h -1 below. Under anoxic or anaerobic conditions, the main reason for the production of CO2 in the bioreactor is the degradation of organic matter. When the influent nitrate concentration increases, microorganisms need to degrade more organic matter for denitrification reactions. Especially when the nitrate concentration suddenly changes, the microbial system will have a short-term maladjustment. At this time, the microorganisms degrade more organic matter or show irregular degradation, so the CO2 emission flux increases. When wet-dry alternation occurs, the CO2 emission flux does not seem to be very different from that during stable operation, and there will be sudden increases and fluctuations, especially in the WBR system with biochar added. This is because when the system is dry, there is no water flow in the system, and there is less water flow attached to the packing, making it difficult for microorganisms to degrade organic matter; while the porous structure of biochar can retain some pore water dissolved with organic matter, and the organic matter can continue to degrade to produce CO2.

[0076] For CH4, when the influent flow rate is 72 mL / h, the CH4 emission fluxes of each system are all relatively low, below 10 mg m -2 h -1 . When the influent flow rate remains unchanged and the influent NO3 - -N increases to 50 mg / L, the CH4 emission flux shows irregular emission. The highest CH4 emission flux is in the B300, which is 41 mg m -2 h -1 . When the four groups of reactors undergo wet-dry alternation, except for the B500 and B300 reactors, the CH4 emission fluxes of the other two groups of reactors during wet-dry alternation operation are all below 10 mg m -2 h -1 below. Analyzing the reasons, generally there are two main ways for CH4 production: one is that complex organic matter produces organic acids with simple molecular structures under the action of bacteria, and these organic acids are directly utilized by CH4-producing bacteria to produce CH4, or the organic acids are further degraded to generate CO2 and H2, and CO2 and H2 are generated under the action of CH4-producing bacteria; the other is that complex organic matter directly produces CO2 and H2 under the action of bacteria without going through the acid production process, and then CH4 is generated under the action of CH4-producing bacteria.

[0077] From Figure 6 it can be seen that when the influent flow rate is 72 mL / h and the influent nitrate nitrogen concentration is 30 mg / L, the N2O emission fluxes of the four groups of reactors are all very low, all below 1000 ug m -2 h -1As can be seen from the previous text, at this time, the nitrate nitrogen removal rates of the four groups of reactors all reached over 94%, and there was no accumulation of nitrite nitrogen in the effluent, indicating that denitrification in the four groups of reactors could proceed completely during this operation stage without producing excessive by-products. When the influent flow rate remained constant and the influent nitrate nitrogen concentration was 50 mg / L, except for individual operation dates of WBR and B500, the N2O release fluxes of the four groups of reactors were still at a low level, all lower than 1000 μg m -2 h -1 ; During this operation stage, the highest N2O release flux was released by WBR, which was 2670 μg m -2 h -1 . When the four groups of reactors experienced the wet-dry alternation, the N2O release fluxes of all reactors increased, which was mainly related to the incomplete denitrification reaction in the reactors. The N2O release flux released by the B400 reactor was less than that of the other three denitrification systems, which might be related to the fact that biochar could change the redox characteristics of the system and promoted the denitrification reaction under adverse conditions.

[0078] From Figure 7 it can be seen that there were significant differences in the short-term and long-term COD concentrations of the effluents of the four groups of reactors, but the differences in the COD effluent concentrations among the four groups of reactors were not significant. The COD effluent characteristics of the four groups of reactors in the early stage of operation were that the initial effluent concentration was very high and then decreased rapidly. In the first 7 days of operation of the four groups of reactors, the COD effluent concentration was not the highest value, which might be because the insoluble substances in the wood chips were not completely dissolved by NaOH. From the 9th to the 15th day of reactor operation, the effluent COD increased, and even exceeded 1000 mg / L at the highest. From the 15th to the 26th day of system operation, the COD concentration of the reactor effluent decreased sharply to below 200 mg / L, showing a fluctuating downward trend; after 83 days of reactor operation, the effluent COD concentration dropped below 50 mg / L; after 103 days of reactor operation, the effluent COD concentration dropped below 20 mg / L.

[0079] Combined with the research on the NO3 - -N removal effect, it can be seen that although the COD:NO3 - -N in the sewage was as low as 5:1 (usually should be 20:1), the reactor system using biochar raw materials or pyrolytic biochar compounded with wood chips as the base filler could not only achieve a NO3 - -N removal efficiency of over 70%, but also could reduce the COD concentration as much as possible (the influent COD was about 500 mg / L, and the effluent COD was below 20 mg / L, and the COD removal efficiency exceeded 90%). This indicated that the present invention could effectively remove agricultural subsurface flow wastewater with low COD and high NO3 - -N.

[0080] The above are only the preferred embodiments of the present invention, and do not impose any form of limitation on the present invention. Although the present invention has been disclosed above with the preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art can make some changes or modifications to equivalent embodiments of equivalent changes within the scope of the technical solution of the present invention by using the above-disclosed technical content. However, as long as it does not depart from the content of the technical solution of the present invention, any simple modification, equivalent change and modification made to the above embodiments according to the technical essence of the present invention still fall within the scope of the technical solution of the present invention.

Claims

1. A method for removing nitrates from subsurface flow in mountainous and hilly areas, characterized in that, It includes the following steps: S1: Prepare wood chips, and pre-treat the wood chips. Use the pre-treated wood chips for the acclimation and cultivation of denitrifying bacteria to obtain wood chips with microorganisms attached to the surface; prepare biochar raw materials or biochar; S2: Prepare a denitrification reactor, and sequentially lay a water distribution layer, a packing layer, and an anti-suspension layer in the denitrification reactor. The packing layer is composed of the wood chips with microorganisms attached to the surface and the biochar raw materials or biochar; S3: Introduce the subsurface flow to be treated into the denitrification reactor for denitrification reaction; S4: Degas and precipitate the reacted fluid to obtain the subsurface flow after nitrate removal.

2. The nitrate removal method for subsurface flow in mountain and hilly areas according to claim 1, characterized in that In step S1, the pre-treatment of the wood chips specifically includes the following sub-steps: rinse the wood chips with clean water to remove the dust on the surface of the wood chips, soak them in a sodium hydroxide solution with a mass concentration of 1.5% for 18 h, then rinse them with deionized water until neutral, and finally dry them at 80 °C for 48 h.

3. The nitrate removal method for subsurface flow in mountain and hilly areas according to claim 1, wherein In step S1, the biochar raw material is corn straw, and the biochar is obtained by pyrolyzing the biochar raw material at 300 - 500 °C.

4. The nitrate removal method for subsurface flow in mountainous and hilly areas according to claim 1, wherein, In step S2, the denitrification reactor is made of acrylic, and a light-shielding cloth is provided on the surface of the denitrification reactor to avoid light.

5. The nitrate removal method for subsurface flow in mountain and hilly areas according to claim 1, characterized in that In step S2, the volume ratio of the wood chips with microorganisms attached to the surface to the biochar is 7:

3.

6. The nitrate removal method for subsurface flow in mountain and hilly areas according to claim 1, wherein In step S2, both the water distribution layer and the anti-suspension layer are laid with quartz sand.

7. The nitrate removal method for subsurface flow in mountainous and hilly areas according to claim 1, characterized in that, In step S2, a nylon screen is also provided between the packing layer and the anti-suspension layer.

8. The method for removing nitrate from subsurface flow in mountainous and hilly areas according to claim 1, wherein In step S3, when introducing the subsurface flow to be treated into the denitrification reactor, the water inlet mode is upward flow.

9. The method for removing nitrate from subsurface flow in mountain and hilly areas according to claim 1, characterized in that, In step S3, when carrying out the denitrification reaction, it is carried out under a variety of different environments.

10. The nitrate removal method for subsurface flow in mountain and hilly areas according to any one of claims 1-9, characterized in that, It also includes steps to evaluate the denitrification performance of the denitrification reactor by calculating the removal efficiency of NO3 - -N, the removal rate of nitrate, and the NO3 - -N loading of unit volume of water: Where: NRE is the removal efficiency of NO3 - -N in water body, %; C i1 and C o1 are the NO3 - -N concentrations at the inlet and outlet of the denitrification reactor, mg N / L; NRR is the removal rate of nitrate in water body, mg N / (L·d); HTR is the hydraulic retention time, d; h is the height of the denitrification reactor, cm; A is the cross-sectional area of the denitrification reactor, cm 2 ; n e is the porosity of the medium; Q is the influent flow rate, L / d; NO3 - -N loading is the NO3 - -N loading per unit volume of water, mg N / (L·d); V e is the effective volume of the denitrification reactor, in L.

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