Numerical simulation method for nitrogen and phosphorus release of lake and reservoir sediment

Abstract

The application discloses a numerical simulation method for nitrogen and phosphorus release of lake and reservoir sediment, and comprises the following steps: step 1, nitrogen and phosphorus migration process division; step 2, calculation of sedimentation flux; step 3, calculation of lithogenic flux; step 4, calculation of release flux; and step 5, optimization of release flux calculation. The method fully considers the dynamic resuspension process of the sediment and the response of the oxidation-reduction environment in the nitrogen and phosphorus release process of the sediment in the deep-water lake and reservoir, realizes continuous dynamic simulation of the nitrogen and phosphorus sedimentation-lithogenesis-re-release process at the water-sediment interface, more scientifically and accurately simulates the nitrogen and phosphorus endogenous release law, and effectively improves the calculation precision of the nitrogen and phosphorus release process, which plays an important role in improving the water safety guarantee of the deep-water lake and reservoir in China.

Description

Technical Field

[0001] This invention belongs to the field of aquatic ecosystem protection technology, and in particular relates to a numerical simulation method for nitrogen and phosphorus release from lake and reservoir sediments. Background Technology

[0002] The exchange of matter between sediments and overlying water is a crucial component of aquatic ecosystems. Surface sediments contain more numerous and complex physical, chemical, and biological processes than the overlying water. Deep lakes exhibit thermal stratification in summer, vertically forming a mixed layer, thermocline, and hysteresis layer from top to bottom. Under the combined effects of thermal stratification and oxygen depletion by polluted sediments, the bottom of the reservoir rapidly depletes oxygen, creating an anoxic environment with dissolved oxygen (DO) concentrations less than 2 mg / L. Large amounts of pollutants are released from the sediments, leading to problems such as fish deaths, increased eutrophication, and severe odors in the discharged water.

[0003] Currently, research on the endogenous release of nitrogen and phosphorus from lake and reservoir sediments, both domestically and internationally, generally employs model estimation methods. These methods, based on mathematical models, simulate and extrapolate the migration and transformation processes of nitrogen and phosphorus at the overlying water-sediment interface. However, this calculation method has significant limitations in practical applications. The crucial dynamic process of sediment resuspension is often neglected during the calculation. As a vital link connecting sediments and overlying water for material exchange, sediment resuspension is influenced by multiple factors, including particle size distribution and flocculation sedimentation, resulting in significant spatiotemporal heterogeneity in its release flux. Existing models often simplify this to a fixed release rate, failing to accurately characterize the complex impact of resuspended particles on nitrogen and phosphorus migration and transformation under different hydrodynamic conditions. Furthermore, the nitrogen and phosphorus release processes under different redox conditions also face computational bottlenecks. From the aerobic layer at the sediment-overlying water interface to the deeper anaerobic environment, nitrogen and phosphorus undergo a series of complex biochemical processes, including nitrification, denitrification, phosphate adsorption and desorption, and microbial mineralization. The various processes are interconnected and mutually restrictive, but existing models mostly use empirical parameters, resulting in inaccurate calculations. Therefore, how to achieve a more scientific and accurate calculation of nitrogen and phosphorus release processes from sediments is a technical problem that urgently needs to be solved in this field. Summary of the Invention

[0004] The purpose of this invention is to provide a numerical simulation method for nitrogen and phosphorus release from lake and reservoir sediments to solve the above-mentioned technical problems.

[0005] To achieve the above objectives, the present invention provides the following technical solution:

[0006] This invention discloses a numerical simulation method for nitrogen and phosphorus release from lake and reservoir sediments, the method comprising the following steps:

[0007] Step 1: Classification of Nitrogen and Phosphorus Migration Processes: Based on the sedimentary diagenesis theory, the migration process of particulate nitrogen and phosphorus organic matter in overlying water and sediment is generalized into sedimentation, release, and diagenesis processes. Each process is quantitatively described using fluxes, namely sedimentation flux, release flux, and diagenesis flux. The sediment has a two-layer structure: the upper layer is connected to the overlying water body and is an aerobic layer, while the lower layer is an anaerobic layer. The sedimentation flux is the amount of particulate organic nitrogen and particulate organic phosphorus received by the upper and lower sediment layers from the overlying water. The release flux is the amount of dissolved nitrogen and phosphorus from the upper sediment layer returning to the overlying water. The diagenesis flux is the amount of particulate matter converted to dissolved form, occurring in the lower sediment layer.

[0008] Based on the ease of degradation of particulate organic matter, particulate organic matter in sediments is divided into 3G categories, denoted as G1, G2, and G3, respectively. G1 represents rapid degradation, G2 represents moderate degradation, and G3 represents difficult degradation. Based on the ease of reaction between particulate organic matter and other substances, particulate organic matter in sediments is divided into active particulate organic matter and insoluble particulate organic matter.

[0009] Step 2, Settlement flux calculation: Gth i The sedimentation fluxes of particulate organic nitrogen and particulate organic phosphorus are expressed as follows:

[0010] J PON,i =FNLP i ·WS LN ·LPON+FNRP i ·WS RN ·RPON+∑ x＝c,d,g FNB x,i ANC x ·WS x ·B x (1)

[0011]

[0012] in:

[0013]

[0014] In the formula: J PON,i and J POP,i They are respectively the Gth i Sedimentation flux of nitrogen and phosphorus from particulate organic matter, g / (m 2 ·d); FNLP i For the Gth i The percentage of organic nitrogen in active particles of class 3G; FPLP i For the Gth i The percentage of active particulate organophosphorus phosphorus in Class 3G active particulate organophosphorus phosphorus; WS LNand WS LP , , represent the settling velocities of organic nitrogen and phosphorus in the active particles, respectively, in m / d; LPON and LPOP represent the concentrations of organic nitrogen and phosphorus in the active particles, respectively, in g / m³. 3 FNRP i For the Gth i The percentage of sparingly soluble particulate organic nitrogen in class 3G and FPRP i For the Gth i The percentage of insoluble particulate organophosphates in Class 3G; WS RN and WS RP , , represent the settling velocities of sparingly soluble particulate organic nitrogen and phosphorus, respectively, in m / d; RPON and RPOP represent the concentrations of sparingly soluble particulate organic nitrogen and phosphorus, respectively, in g / m³. 3 FNB x,i For the Gth i The percentage of particulate organic nitrogen in algae of type x is the same as that in algae of type 3G; FPB x,i For the Gth i The percentage of particulate organic phosphorus in algae of type x is 3G; c, d, and g represent cyanobacteria, diatoms, and green algae, respectively; ANC x and APC x These represent the ratios of nitrogen to carbon in algae and the ratio of phosphorus to carbon in algae, respectively; WS x B represents the net settling velocity of algae, in m / d. x Algal biomass, g / m 3 WS TSS ρ4 represents the settling rate of suspended sediment, in m / d; PO4P represents the soluble phosphate concentration, in g / m2. 3 ;γ i γ is a binary indicator variable; when i = 1, γ i The value is 1, and when i = 2, 3, γ i =0;

[0015] Step 3, Calculation of diagenetic flux: First, calculate the kinetic equation for organic nitrogen in 3G-type particles:

[0016]

[0017] In the above equation, G3 type particulate organic nitrogen does not generate diagenetic flux, while the diagenetic flux generated by the reacting G1 and G2 type particulate organic nitrogen is:

[0018]

[0019] In the formula: J N Nitrogen diagenetic flux, g / (m 2 ·d); G PON,i It is the Gth layer in the lower part of the sediment. iNitrogen concentration of particulate organic matter, g / m 3 ;K PON,i It is the Gth layer in the lower sediment layer at 20℃ i decay rate of particulate organic nitrogen, d -1 ; For K PON,i The temperature regulation constant; T is the sediment temperature, °C; ω is the burial rate, m / d; H2 is the depth of the lower sediment layer, m;

[0020] The kinetic equation for organophosphorus compounds of type 3G is:

[0021]

[0022] Similarly, in the above equation, G3 type particulate organophosphorus phosphorus does not generate diagenetic flux, while the diagenetic flux generated by the reacting G1 and G2 type particulate organophosphorus phosphorus is:

[0023]

[0024] In the formula: J P Phosphorus diagenetic flux, g / (m 2 ·d); G POP,i It is the Gth layer in the lower part of the sediment. i Concentration of particulate organic phosphorus, g / m 3 ;K POP,i It is the Gth layer in the lower sediment layer at 20℃ i decay rate of particulate organophosphorus compounds, d -1 ; For K POP,i Temperature regulation constant;

[0025] Step 4, Calculation of Release Flux: The formula for calculating the release flux of nitrogen and phosphorus is as follows:

[0026] J aq =s(C1-C0) (10)

[0027] In the formula: J aq To release flux, g / (m 2 •d); s is the surface mass transfer coefficient, m / d; C1 is the concentration of dissolved nitrogen or phosphorus in the upper layer of sediment, g / m 3 C0 represents the concentration of dissolved nitrogen or phosphorus in the overlying water body, in g / m³. 3 ;

[0028] The mass conservation equations for the upper and lower layers of the sediment are as follows:

[0029]

[0030] fd1+fP1=1 (13)

[0031] fd2+fP2=1 (14)

[0032] In the formula: H1 is the thickness of the upper aerobic layer of the sediment, in meters; H2 is the thickness of the lower anaerobic layer of the sediment, in meters; C2 is the concentration of dissolved nitrogen or phosphorus in the lower sediment layer, in g / m³. 3 KL 12 ω represents the diffusion rate of the dissolved portion between the upper and lower layers of sediment, in m / d; 12 ρ represents the particle mixing velocity between upper and lower sediment layers, in m / d; fd0 is the percentage of dissolved matter in the overlying water, fd1 is the percentage of dissolved matter in the upper sediment layer, fp1 is the percentage of particulate matter in the upper sediment layer, fd2 is the percentage of dissolved matter in the lower sediment layer, and fp2 is the percentage of particulate matter in the lower sediment layer. J M Nitrogen (M=N) or phosphorus (M=P) diagenetic flux, g / (m 2 ·d);

[0033] Solving equations (10), (11), and (12) together yields the expression for the released flux:

[0034]

[0035] in:

[0036]

[0037] In the formula: r 21 The ratio of the concentration of material in the lower sediment layer to that in the upper sediment layer; r * 21 This represents the ratio of the concentration of sediment in the upper sediment layer to the concentration of substances in the overlying water.

[0038] Step 5: Optimize the release flux calculation: Based on the intensity of disturbance at the bottom of the sediment, further optimize the release flux calculation formula.

[0039] Furthermore, step 5 involves optimizing the flux calculation formula based on the intensity of sediment bottom disturbance, specifically including the following scenarios:

[0040] 1. When there is no disturbance at the bottom of the sediment, the mixing rate of particles between the upper and lower layers is negligible, and the burial effect is much smaller than the diffusion effect. This process is "dissolution-driven," i.e., ω < <KL 12 fd1+s fd1,ω< <KL 12 fd2, the flux release formula becomes:

[0041]

[0042] Right now:

[0043]

[0044] 2. When there is strong disturbance at the bottom of the sediment, the changes in nitrogen and phosphorus concentrations in the overlying water over time are obtained by combining indoor experiments, the nitrogen and phosphorus release fluxes in the sediments at different flow velocities are calculated, and the release flux calculation formula is optimized.

[0045] The formula for calculating the nitrogen and phosphorus release fluxes from sediments at different flow velocities in the indoor experiment is:

[0046]

[0047] In the formula: J aq1 The release flux obtained from the experiment is expressed in g / (m³). 2 ·d); V is the volume of the overlying water in the device, in L; C t,m C represents the nitrogen and phosphorus concentrations at the m-th sampling point, in mg / L. t,0 The nitrogen and phosphorus concentrations at the time of the 0th sampling are in mg / L; C t,i-1 V represents the nitrogen and phosphorus concentrations at the (i-1)th sampling, in mg / L; i-1 Let L be the volume of the (i-1)th sample; C be the volume of the sample. a The concentration of nitrogen and phosphorus in the added water sample is mg / L; A is the contact area between the overlying water and the sediment in the experimental setup, m². 2 t represents the incubation time, d;

[0048] Based on the calculation results, a scatter plot of nitrogen and phosphorus emission fluxes as a function of flow velocity was obtained. After fitting, the relationship between nitrogen and phosphorus emission fluxes and flow velocity v was obtained as follows:

[0049] J aq1 =f(v) (21)

[0050] Substituting the obtained relationship between nitrogen and phosphorus release flux and flow rate into formula (10), the mass transfer coefficient s is calculated as follows:

[0051]

[0052] The optimized formula for calculating the release flux is then obtained as follows:

[0053]

[0054] 3. The nitrogen and phosphorus release flux at the overlying water-sediment interface is affected by the dissolved oxygen concentration of the overlying water. That is, as the thermal stratification of the lake and reservoir forms, develops, stabilizes, and recedes, the dissolved oxygen concentration at the bottom of the water body gradually changes from anoxic or anaerobic to aerobic, and the nitrogen and phosphorus release flux changes accordingly. Therefore, combined with indoor experiments, the curves of the change of nitrogen and phosphorus concentration in the overlying water over time under different dissolved oxygen concentrations are obtained. The nitrogen and phosphorus release flux of sediments under different dissolved oxygen concentrations in the indoor experiments are calculated, and then the formula for calculating the release flux is optimized.

[0055] According to formula (20), the nitrogen and phosphorus release fluxes of sediments under different dissolved oxygen concentrations in the indoor experiment were calculated. Based on the calculation results, a scatter plot of the nitrogen and phosphorus release fluxes as a function of dissolved oxygen concentration was obtained. After fitting, the relationship between nitrogen and phosphorus release fluxes and dissolved oxygen was obtained as follows:

[0056] J aq1 =F(DO, A, B) (26)

[0057] In the formula: DO represents dissolved oxygen concentration, mg / L; A and B represent constants in the formula fitting.

[0058] Substituting the experimentally obtained relationship between nitrogen and phosphorus release flux and dissolved oxygen concentration into formula (10), the mass transfer coefficient s is calculated as follows:

[0059]

[0060] The optimized formula for calculating the release flux is then obtained as follows:

[0061]

[0062] Furthermore, the specific steps for obtaining the curves of nitrogen and phosphorus concentration in the overlying water over time at different flow rates, combined with indoor experiments, include:

[0063] S1: Spread the sediment evenly in the indoor water tank and let it stand for 1 hour before the experiment to prevent the sediment from becoming suspended.

[0064] S2: Set the flow rate of the indoor water tank to 0.1 m / s and conduct a nitrogen and phosphorus release experiment from the sediment;

[0065] S3: Sample collection and measurement: Collect 100 mL water samples at 0, 30, 60, 90, 120, 150, 180, 240, 300 and 360 min after the start of the experiment, avoiding disturbing the sediment. Then transfer the water samples to clean centrifuge tubes and measure the nitrogen and phosphorus concentrations.

[0066] S4: Continue to switch the flow rate of the indoor water tank to 0.2m / s;

[0067] S5: Collect samples at a flow rate of 0.2 m / s according to step S3 and determine the nitrogen and phosphorus concentrations;

[0068] S6: Similarly, follow the steps above to complete the experiments at different flow rates;

[0069] S7: Charting: Organize the nitrogen and phosphorus concentration data measured at different time points under different flow rates, and plot the nitrogen and phosphorus concentrations as the vertical axis and time as the horizontal axis to create curves showing the change of nitrogen and phosphorus concentrations over time at different flow rates.

[0070] Furthermore, the specific steps for obtaining the curves of nitrogen and phosphorus concentrations in the overlying water over time under different dissolved oxygen concentrations, combined with indoor experiments, include:

[0071] S1: Spread the sediment evenly in the indoor water tank and let it stand for 1 hour before the experiment to prevent the sediment from becoming suspended.

[0072] S2: Set the dissolved oxygen concentration in the indoor water tank to 2 mg / L and conduct a nitrogen and phosphorus release experiment on the sediment;

[0073] S3: Sample collection and measurement: Collect 100 mL water samples at 0, 30, 60, 90, 120, 150, 180, 240, 300 and 360 min after the start of the experiment, avoiding disturbing the sediment. Then transfer the water samples to clean centrifuge tubes and measure the nitrogen and phosphorus concentrations.

[0074] S4: Continue to switch the dissolved oxygen concentration in the indoor water tank to 3 mg / L;

[0075] S5: Collect samples with a dissolved oxygen concentration of 3 mg / L according to step S3 and determine the nitrogen and phosphorus concentrations;

[0076] S6: Similarly, follow the steps above to complete the experiments under different dissolved oxygen concentrations;

[0077] S7: Charting: Compile the nitrogen and phosphorus concentration data measured at different time points under different dissolved oxygen concentrations, and plot the nitrogen and phosphorus concentrations as the vertical axis and time as the horizontal axis to create curves showing the change of nitrogen and phosphorus concentrations over time under different dissolved oxygen concentrations.

[0078] The beneficial effects of this invention are: the method described in this invention fully considers the dynamic resuspension process of sediments and the response of the redox environment during the release of nitrogen and phosphorus from sediments in deep-water lakes and reservoirs, and realizes continuous dynamic simulation of the nitrogen and phosphorus sedimentation-diagenesis-re-release process at the overlying water-sediment interface, thereby simulating the endogenous release law of nitrogen and phosphorus more scientifically and accurately, effectively improving the calculation accuracy of the nitrogen and phosphorus release process, and playing an important role in improving the water security of deep-water lakes and reservoirs in my country.

[0079] The present invention will now be described in further detail with reference to the accompanying drawings and specific embodiments. Attached Figure Description

[0080] Figure 1 This is a schematic diagram illustrating the relationship between subsidence flux, release flux, and diagenetic flux.

[0081] Figure 2 The graph shows the change of nitrogen and phosphorus concentrations over time at different flow rates.

[0082] Figure 3 This is a scatter plot showing the nitrogen and phosphorus release fluxes as a function of flow velocity. Detailed Implementation

[0083] Example 1

[0084] This embodiment discloses a numerical simulation method for nitrogen and phosphorus release from lake and reservoir sediments, the method comprising the following steps:

[0085] Step 1: Division of nitrogen and phosphorus migration processes:

[0086] Based on the sedimentary diagenesis theory, the migration process of particulate nitrogen and phosphorus organic matter in the overlying water-sediment system is generalized into three processes: sedimentation, release, and diagenesis. Each process is quantitatively described using fluxes: sedimentation flux, release flux, and diagenesis flux. Sediment has a two-layer structure: the upper layer is a thin, generally aerobic layer connected to the overlying water, about 0.1 cm thick; the lower layer is always anaerobic, about 10 cm thick. Sedimentation flux represents the amount of particulate organic nitrogen and particulate organic phosphorus received from the overlying water by both sediment layers. Release flux represents the amount of dissolved nitrogen and phosphorus from the upper sediment layer returning to the overlying water. Diagenesis flux represents the amount of particulate matter converted to dissolved form, which usually occurs in the lower sediment layer. The relationship between the three fluxes is as follows: Figure 1 As shown.

[0087] Based on the ease of degradation, particulate organic matter in sediments is classified into 3G categories, denoted as G1, G2, and G3, respectively. G1 represents rapid degradation (half-life of approximately 20 days), G2 represents moderate degradation (half-life of approximately 360 days), and G3 represents recalcitrant degradation (no significant decay). Based on the ease with which particulate organic matter reacts with other substances, it is further classified into reactive particulate organic matter and insoluble particulate organic matter.

[0088] Step 2, Settlement flux calculation:

[0089] Sedimentation is a crucial process connecting the water quality and sediments of overlying water bodies. The main sources of sediments in these bodies are external loads and debris from dead aquatic organisms. (G) i The sedimentation fluxes of particulate organic nitrogen and particulate organic phosphorus are expressed as follows:

[0090] J PON,i =FNLP i ·WS LN ·LPON+FNRP i ·WS RN ·RPON+∑ x＝c,d,g FNB x,i ANC x ·WS x ·B x (1)

[0091]

[0092] in:

[0093]

[0094] In the formula: J PON,i and J POP,i They are respectively the Gth i Sedimentation flux of nitrogen and phosphorus from particulate organic matter, g / (m 2 ·d); FNLP i For the Gth i The percentage of organic nitrogen in active particles of class 3G; FPLP i For the Gth i The percentage of active particulate organophosphorus phosphorus in Class 3G active particulate organophosphorus phosphorus; WS LN and WS LP , , represent the settling velocities of organic nitrogen and phosphorus in the active particles, respectively, in m / d; LPON and LPOP represent the concentrations of organic nitrogen and phosphorus in the active particles, respectively, in g / m³. 3 FNRP i For the Gth i The percentage of sparingly soluble particulate organic nitrogen in class 3G and FPRP i For the Gth i The percentage of insoluble particulate organophosphates in Class 3G; WS RN and WS RP , , represent the settling velocities of sparingly soluble particulate organic nitrogen and phosphorus, respectively, in m / d; RPON and RPOP represent the concentrations of sparingly soluble particulate organic nitrogen and phosphorus, respectively, in g / m³. 3 FNB x,i For the Gth i The percentage of particulate organic nitrogen in algae of type x is the same as that in algae of type 3G; FPB x,i For the Gth i The percentage of particulate organic phosphorus in algae of type x is 3G; c, d, and g represent cyanobacteria, diatoms, and green algae, respectively; ANC x and APC x These represent the ratios of nitrogen to carbon in algae and the ratio of phosphorus to carbon in algae, respectively; WS x B represents the net settling velocity of algae, in m / d. x Algal biomass, g / m 3 WS TSS ρ4 represents the settling rate of suspended sediment, in m / d; PO4P represents the soluble phosphate concentration, in g / m2. 3 ;γ i γ is a binary indicator variable; when i = 1, γ i The value is 1, and when i = 2, 3, γ i It is 0.

[0095] Step 3, Calculation of diagenetic flux:

[0096] Diagenesis involves the conversion of settled particles into a dissolved state in the lower layers of sediments, generating diagenetic flux. This diagenetic flux is also a source of sedimentary flux. The kinetic equation for organic nitrogen in 3G-type particles is:

[0097]

[0098] In the above equation, G3 type particulate organic nitrogen does not generate diagenetic flux, while the diagenetic flux generated by the reacting G1 and G2 type particulate organic nitrogen is:

[0099]

[0100] In the formula: J N Nitrogen diagenetic flux, g / (m 2 ·d); G PON,i It is the Gth layer in the lower part of the sediment. i Nitrogen concentration of particulate organic matter, g / m 3 ;K PON,i It is the Gth layer in the lower sediment layer at 20℃ i decay rate of particulate organic nitrogen, d -1 ; For K PON,i The temperature regulation constant; T is the sediment temperature, °C; ω is the burial rate, m / d; H2 is the depth of the lower sediment layer, m.

[0101] The kinetic equation for organophosphorus compounds of type 3G is:

[0102]

[0103] Similarly, in the above equation, G3 type particulate organophosphorus phosphorus does not generate diagenetic flux, while the diagenetic flux generated by the reacting G1 and G2 type particulate organophosphorus phosphorus is:

[0104]

[0105] In the formula: J P Phosphorus diagenetic flux, g / (m 2 ·d); G POP,i It is the Gth layer in the lower part of the sediment. i Concentration of particulate organic phosphorus, g / m 3 ;K POP,i It is the Gth layer in the lower sediment layer at 20℃ i decay rate of particulate organophosphorus compounds, d -1 ; For K POP,i Temperature regulation constant.

[0106] Step 4, Calculate the release flux:

[0107] Compared to sedimentation flux and diagenetic flux, the calculation of release flux is more complex. Dissolved matter produced during diagenesis returns to the overlying water as release flux. Factors influencing release flux include basal diagenesis, the distribution of granular and dissolved matter between different sedimentary layers (including the upper aerobic layer and the lower anaerobic layer), particle mixing between the two layers, and dissolved matter diffusion. Each influencing factor is complex. The formulas for calculating nitrogen and phosphorus release flux are:

[0108] J aq =s(C1-C0) (10)

[0109] In the formula: J aq To release flux, g / (m 2 •d); s is the surface mass transfer coefficient, m / d; C1 is the concentration of dissolved nitrogen or phosphorus in the upper layer of sediment, g / m 3 C0 represents the concentration of dissolved nitrogen or phosphorus in the overlying water body, in g / m³. 3 .

[0110] The mass conservation equations for the upper and lower layers of the sediment are as follows:

[0111]

[0112] fd1+fP1=1 (13)

[0113] fd2+fP2=1 (14)

[0114] In the formula: H1 is the thickness of the upper aerobic layer of the sediment, in meters; H2 is the thickness of the lower anaerobic layer of the sediment, in meters; C2 is the concentration of dissolved nitrogen or phosphorus in the lower sediment layer, in g / m³. 3 KL 12 ω represents the diffusion rate of the dissolved portion between the upper and lower layers of sediment, in m / d; 12 ρ represents the particle mixing velocity between upper and lower sediment layers, in m / d; fd0 is the percentage of dissolved matter in the overlying water, fd1 is the percentage of dissolved matter in the upper sediment layer, fp1 is the percentage of particulate matter in the upper sediment layer, fd2 is the percentage of dissolved matter in the lower sediment layer, and fp2 is the percentage of particulate matter in the lower sediment layer. J M Nitrogen (M=N) or phosphorus (M=P) diagenetic flux, g / (m 2 ·d).

[0115] Solving equations (10), (11), and (12) together yields the expression for the released flux:

[0116]

[0117] in:

[0118]

[0119] In the formula: r 21 The ratio of the concentration of material in the lower sediment layer to that in the upper sediment layer; r * 21 It is the ratio of the concentration of the upper sediment layer to the concentration of the overlying water.

[0120] Step 5, Optimize Flux Calculation:

[0121] The formula for calculating the release flux is further optimized based on the intensity of disturbance at the bottom of the sediment. This includes the following specific scenarios:

[0122] 1. In the absence of disturbance at the bottom of the sediment, the mixing rate of particles between the upper and lower sediment layers is negligible, and the burial effect is much smaller than the diffusion effect. This process is "dissolution-driven," and the sediment release flux is directly related to the diagenetic flux and the nitrogen and phosphorus concentrations in the overlying water, i.e., ω < 0.05. <KL 12 fd1+s fd1,ω< <KL 12 fd2, the flux release formula becomes:

[0123]

[0124] Right now:

[0125]

[0126] 2. When there is strong disturbance at the bottom of the sediment, it is necessary to optimize the formula for calculating the release flux by combining the research results of laboratory experiments. The specific process is as follows:

[0127] 1) Through indoor experiments, the curves of nitrogen and phosphorus concentrations in the overlying water changing over time at different flow rates were obtained. The specific steps are as follows:

[0128] S1: Spread the sediment evenly in the indoor water tank and let it stand for 1 hour before the experiment to prevent the sediment from becoming suspended.

[0129] S2: Set the flow rate of the indoor water tank to 0.1 m / s and conduct a nitrogen and phosphorus release experiment from the sediment;

[0130] S3: Sample collection and measurement: Collect 100 mL water samples at 0, 30, 60, 90, 120, 150, 180, 240, 300 and 360 min after the start of the experiment, avoiding disturbing the sediment. Then transfer the water samples to clean centrifuge tubes and measure the nitrogen and phosphorus concentrations.

[0131] S4: Continue to switch the flow rate of the indoor water tank to 0.2m / s;

[0132] S5: Collect samples at a flow rate of 0.2 m / s according to step S3 and determine the nitrogen and phosphorus concentrations;

[0133] S6: Similarly, follow the steps above to complete the experiments at different flow rates;

[0134] S7: Chart Plotting: Organize the nitrogen and phosphorus concentration data measured at various time points under different flow rates, and plot the nitrogen and phosphorus concentrations as the vertical axis and time as the horizontal axis to create curves showing the change of nitrogen and phosphorus concentrations over time at different flow rates. The curve plotted in this embodiment is as follows: Figure 2 As shown.

[0135] 2) Calculate the nitrogen and phosphorus release fluxes from sediments at different flow velocities in the indoor experiment using the following formula:

[0136]

[0137] In the formula: J aq1 The release flux obtained from the experiment is expressed in g / (m³). 2 ·d); V is the volume of the overlying water in the device, in L; C t,m C represents the nitrogen and phosphorus concentrations at the m-th sampling point, in mg / L. t,0 The nitrogen and phosphorus concentrations at the time of the 0th sampling are in mg / L; C t,i-1 V represents the nitrogen and phosphorus concentrations at the (i-1)th sampling, in mg / L; i-1 Let L be the volume of the (i-1)th sample; C be the volume of the sample. a The concentration of nitrogen and phosphorus in the added water sample is mg / L; A is the contact area between the overlying water and the sediment in the experimental setup, m². 2 t represents the incubation time, d.

[0138] Based on the calculation results, a scatter plot of nitrogen and phosphorus emission fluxes as a function of flow velocity can be obtained. After fitting, the relationship between nitrogen and phosphorus emission fluxes and flow velocity v is obtained as follows:

[0139] J aq1 =f(v) (21)

[0140] The scatter plot obtained in this embodiment is as follows: Figure 3 As shown in the figure, the relationship between nitrogen release flux and flow rate is obtained through fitting as follows:

[0141] J aq1 =90.39Exp(2.73v) (22)

[0142] The goodness of fit is expressed as the goodness of fit; the closer the goodness of fit is to 1, the better the fit. The goodness of fit in the above formula is Rfit. 2 =0.9564, where R is the correlation coefficient.

[0143] The relationship between phosphorus release flux and flow rate is as follows:

[0144] Jaq1 = 4.92 + 8.03v + 2.08v 2 (twenty three)

[0145] Its goodness of fit is R 2 =0.8981.

[0146] 3) Substitute the experimentally obtained relationship between nitrogen and phosphorus release flux and flow rate, i.e., formula (20), into formula (10) to solve for the mass transfer coefficient s:

[0147]

[0148] The optimized formula for calculating the release flux is then obtained as follows:

[0149]

[0150] 3. The nitrogen and phosphorus release fluxes at the overlying water-sediment interface are affected by the dissolved oxygen concentration in the overlying water. Specifically, as the thermal stratification of the lake / reservoir forms, develops, stabilizes, and recedes, the dissolved oxygen concentration at the bottom of the water body gradually changes from anoxic or anaerobic to aerobic, and the nitrogen and phosphorus release fluxes change accordingly. It is necessary to optimize the flux calculation formula by incorporating the results of laboratory experiments. The specific process is as follows:

[0151] 1) Through indoor experiments, the curves of nitrogen and phosphorus concentrations in the overlying water changing over time under different dissolved oxygen concentrations were obtained. The specific steps are as follows:

[0152] S1: Spread the sediment evenly in the indoor water tank and let it stand for 1 hour before the experiment to prevent the sediment from becoming suspended.

[0153] S2: Set the dissolved oxygen concentration in the indoor water tank to 2 mg / L and conduct a nitrogen and phosphorus release experiment on the sediment;

[0154] S3: Sample collection and measurement: Collect 100 mL water samples at 0, 30, 60, 90, 120, 150, 180, 240, 300 and 360 min after the start of the experiment, avoiding disturbing the sediment. Then transfer the water samples to clean centrifuge tubes and measure the nitrogen and phosphorus concentrations.

[0155] S4: Continue to switch the dissolved oxygen concentration in the indoor water tank to 3 mg / L;

[0156] S5: Collect samples with a dissolved oxygen concentration of 3 mg / L according to step S3 and determine the nitrogen and phosphorus concentrations;

[0157] S6: Similarly, follow the steps above to complete the experiments under different dissolved oxygen concentrations;

[0158] S7: Charting: Compile the nitrogen and phosphorus concentration data measured at different time points under different dissolved oxygen concentrations, and plot the nitrogen and phosphorus concentrations as the vertical axis and time as the horizontal axis to create curves showing the change of nitrogen and phosphorus concentrations over time under different dissolved oxygen concentrations.

[0159] 2) Based on formula (20), the nitrogen and phosphorus release fluxes of sediments under different dissolved oxygen concentrations in the indoor experiment were calculated. Based on the calculation results, a scatter plot of the nitrogen and phosphorus release fluxes as a function of dissolved oxygen concentration was obtained. After fitting, the relationship between nitrogen and phosphorus release fluxes and dissolved oxygen was obtained:

[0160] J aq1 =F(DO, A, B) (26)

[0161] In the formula: DO represents dissolved oxygen concentration, mg / L; A and B represent constants in the formula fitting.

[0162] 3) Substitute the relationship between nitrogen and phosphorus release flux and dissolved oxygen concentration obtained from the experiment into formula (10) to solve for the mass transfer coefficient s:

[0163]

[0164] The optimized formula for calculating the release flux is then obtained as follows:

[0165]

[0166] Finally, it should be noted that the above description is only used to illustrate the technical solution of the present invention and not to limit it. Although the present invention has been described in detail with reference to the preferred arrangement, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solution of the present invention without departing from the spirit and scope of the technical solution of the present invention.

Claims

1. A numerical simulation method of nitrogen and phosphorus release from lake and reservoir sediment, characterized in that, The method comprises the following steps: Step 1, nitrogen and phosphorus migration process division: according to the sediment diagenetic theory, the migration process of particulate nitrogen and phosphorus organic matter in overlying water-sediment is generalized as a settling process, a release process and a diagenetic process, each process is quantitatively described by using a flux, i.e. a settling flux, a release flux and a diagenetic flux; wherein the sediment is a double-layer structure, the upper layer is connected with the overlying water body and is an aerobic layer, and the lower layer is an anaerobic layer; the settling flux is the amount of particulate organic nitrogen and particulate organic phosphorus received by the upper and lower layers of the sediment from the overlying water; the release flux is the amount of dissolved nitrogen and phosphorus in the upper layer of the sediment returned to the overlying water; and the diagenetic flux is the amount of particulate state converted into dissolved state, which occurs in the lower layer of the sediment; According to the difficulty of degradation of particulate organic matter, the particulate organic matter in the sediment is divided into 3G, which are represented by G1, G2 and G3 respectively, wherein G1 is fast degradation, G2 is moderate degradation, and G3 is difficult degradation; according to the difficulty of reaction of particulate organic matter with other substances, the particulate organic matter in the sediment is divided into active particulate organic matter and difficult-to-dissolve particulate organic matter; Step 2, Sedimentation flux calculation: Gth i The sedimentation flux of particulate organic nitrogen and particulate organic phosphorus is expressed as follows: J PON,i = FNLP i • WS LN • LPON + FNRP i • WS RN • RPON +∑ x＝c,d,g FNB x,i • ANC x • WS x • B x (1) Wherein: In the formula: J PON,i and J POP,i They are respectively the Gth i Sedimentation flux of nitrogen and phosphorus from particulate organic matter, g / (m 2 ·d); FNLP i For the Gth i The percentage of organic nitrogen in active particles of class 3G; FPLP i For the Gth i The percentage of active particulate organophosphorus phosphorus in Class 3G active particulate organophosphorus phosphorus; WS LN and WS LP , , represent the settling velocities of organic nitrogen and phosphorus in the active particles, respectively, in m / d; LPON and LPOP represent the concentrations of organic nitrogen and phosphorus in the active particles, respectively, in g / m³. 3 FNRP i For the Gth i The percentage of sparingly soluble particulate organic nitrogen in class 3G and FPRP i For the Gth i The percentage of insoluble particulate organophosphates in Class 3G; WS RN and WS RP , , represent the settling velocities of sparingly soluble particulate organic nitrogen and phosphorus, respectively, in m / d; RPON and RPOP represent the concentrations of sparingly soluble particulate organic nitrogen and phosphorus, respectively, in g / m³. 3 FNB x,i For the Gth i The percentage of particulate organic nitrogen in algae of type x is the same as that in algae of type 3G; FPB x,i For the Gth i The percentage of particulate organic phosphorus in algae of type x is 3G; c, d, and g represent cyanobacteria, diatoms, and green algae, respectively; ANC x and APC x These represent the ratios of nitrogen to carbon in algae and the ratio of phosphorus to carbon in algae, respectively; WS x B represents the net settling velocity of algae, in m / d. x Algal biomass, g / m 3 WS TSS ρ4 represents the settling rate of suspended sediment, in m / d; PO4P represents the soluble phosphate concentration, in g / m2. 3 ;γ i γ is a binary indicator variable; when i = 1, γ i The value is 1, and when i = 2, 3, γ i =0; Step 3, diagenetic flux calculation: first, the kinetic equation of 3G particulate organic nitrogen is calculated as: In the above equation, G3 particulate organic nitrogen does not produce diagenetic flux, and the diagenetic flux produced by G1 and G2 particulate organic nitrogen is: where: J N is the nitrogen flux into the rock, g / (m 2 ·d); G PON,i is the concentration of the G i th class of particulate organic matter nitrogen in the sediment underlying layer, g / m 3 ; K PON,i is the decay rate of the G i th class of particulate organic nitrogen in the sediment underlying layer at 20°C, d -1 ; is the temperature adjustment constant of K PON,i ; T is the sediment temperature, °C; ω is the burial rate, m / d; H2is the depth of the sediment underlying layer, m; The kinetic equation of 3G particulate organic phosphorus is: Similarly, in the above equation, G3 particulate organic phosphorus does not produce diagenetic flux, and the diagenetic flux produced by G1 and G2 particulate organic phosphorus is: wherein: J P is the phosphorus lithogenic flux, g / (m 2 ·d); G POP,i is the concentration of the G i th class of particulate organic matter phosphorus in the sediment underlying layer, g / m 3 ; K POP,i is the decay rate of the G i th class of particulate organic phosphorus in the sediment underlying layer at 20°C, d -1 ; is the temperature adjustment constant for K POP,i ; Step 4, release flux calculation: the calculation formula of the release flux of nitrogen and phosphorus is: J aq = s(C1-C0)(10) wherein: J aq is the release flux, g / (m 2 ·d); s is the surface mass transfer coefficient, m / d; Ci is the concentration of dissolved nitrogen or phosphorus species in the upper layer of the sediment, g / m 3 ; Co is the concentration of dissolved nitrogen or phosphorus species in the overlying water body, g / m 3 ; The mass conservation equations of the upper and lower layers of the sediment are as follows: fd1+fP1=1 (13) fd2+fP2=1 (14) where H1 is the thickness of the overlying aerobic layer of sediment, m; H2 is the thickness of the underlying anaerobic layer of sediment, m; C2 is the concentration of dissolved nitrogen or phosphorus species in the underlying layer of sediment, g / m 3 ; KL 12 is the diffusive velocity of the dissolved fraction between the overlying and underlying layers of sediment, m / d; ω 12 is the mixing velocity of the particulate fraction between the overlying and underlying layers of sediment, m / d; fd0 is the percentage of total material that is dissolved in the overlying water, fd1 is the percentage of total material that is dissolved in the overlying layer of sediment, fp1 is the percentage of total material that is particulate in the overlying layer of sediment, fd2 is the percentage of total material that is dissolved in the underlying layer of sediment, and fp2 is the percentage of total material that is particulate in the underlying layer of sediment; J M is the flux of nitrogen (M = N) or phosphorus (M = P) to the rock, g / (m 2 ·d); The expression of the release flux is obtained by jointly solving formulas (10), (11) and (12): Wherein: where: r 21 is the ratio of the concentration of the substance in the subsurface sediment layer to the concentration of the substance in the overlying water; r * 21 is the ratio of the concentration of the substance in the subsurface sediment layer to the concentration of the substance in the overlying water; Step 5, release flux calculation optimization: according to the disturbance intensity at the bottom of the sediment, the release flux calculation formula is further optimized. 2.The lake reservoir sediment nitrogen and phosphorus release numerical simulation method according to claim 1, characterized in that, The release flux calculation formula is further optimized according to the disturbance intensity at the bottom of the sediment in step 5, which specifically includes the following several cases:

1. In the absence of disturbance at the sediment bottom, the mixing rate of the upper and lower layers of the sediment particles is ignored, and the burial effect is much smaller than the diffusion effect. This process is "dissolved state driven", that is, ω << KL 12 fd1 + fd1, ω << KL 12 fd2, the release flux formula becomes: That is:

2. In the case of strong disturbance at the bottom of the sediment, the nitrogen and phosphorus concentration curves of the overlying water with time under different flow rates are obtained by combining laboratory experiments, the nitrogen and phosphorus release fluxes of the sediment under different flow rates are calculated, and the release flux calculation formula is further optimized; The formula for calculating the nitrogen and phosphorus release fluxes of the sediment under different flow rates in the laboratory experiment is: where: J aq1 is the release flux obtained from the experiment, g / (m 2 ·d); V is the volume of overlying water in the device, L; C t,m is the concentration of nitrogen and phosphorus at the mth sampling, mg / L; C t,0 is the concentration of nitrogen and phosphorus at the 0th sampling, mg / L; C t,i-1 is the concentration of nitrogen and phosphorus at the i-1th sampling, mg / L; V i-1 is the volume of the i-1th sampling, L; C a is the concentration of nitrogen and phosphorus in the added water sample, mg / L; A is the contact area of overlying water and sediment in the experimental device, m 2 ; t is the incubation time, d; According to the calculation results, the scatter plot of the nitrogen and phosphorus release fluxes with the flow rate is obtained, and the relationship between the nitrogen and phosphorus release fluxes and the flow rate v is obtained by fitting: J aq1 = f(v) (21) The relationship between the nitrogen and phosphorus release fluxes and the flow rate v is substituted into formula (10) to solve the mass transfer coefficient s: Then the optimized release flux calculation formula is obtained as follows: 3、The nitrogen and phosphorus release flux of the overlying water-sediment interface is affected by the concentration of dissolved oxygen in the overlying water, that is, as the thermal stratification of the lake or reservoir is formed, developed, stabilized and disappeared, the concentration of dissolved oxygen at the bottom of the water body gradually changes from anoxic or anaerobic to aerobic, and the nitrogen and phosphorus release flux changes accordingly. In combination with the laboratory experiment, the nitrogen and phosphorus concentration curves of the overlying water with time under different concentrations of dissolved oxygen are obtained, the nitrogen and phosphorus release fluxes of the sediment under different concentrations of dissolved oxygen in the laboratory experiment are calculated, and the release flux calculation formula is optimized; According to formula (20), the nitrogen and phosphorus release fluxes of the sediment under different concentrations of dissolved oxygen in the laboratory experiment are calculated, according to the calculation results, the scatter plot of the nitrogen and phosphorus release fluxes with the change of the concentration of dissolved oxygen is obtained, and through fitting, the relationship between the nitrogen and phosphorus release fluxes and the concentration of dissolved oxygen is obtained as follows: J aq1 = F(D0, A, B) (26) In the formula, DO represents the concentration of dissolved oxygen, mg / L; A and B represent constants in the formula fitting; The relationship between the nitrogen and phosphorus release fluxes and the concentration of dissolved oxygen obtained by the experiment is substituted into formula (10) to solve the mass transfer coefficient s as follows: The optimized release flux calculation formula is obtained as follows:

3. The numerical simulation method for nitrogen and phosphorus release of lake and reservoir sediment according to claim 2, characterized in that, The specific steps of obtaining the nitrogen and phosphorus concentration curves of the overlying water with time under different flow rates in combination with the laboratory experiment include: S1: The sediment is laid flat in the indoor water tank, and is allowed to stand for 1 hour before the experiment starts, so that the sediment is no longer suspended; S2: The flow rate of the indoor water tank is set to 0.1 m / s, and the nitrogen and phosphorus release experiment of the sediment is carried out; S3: Sample collection and determination: 100 mL of water sample is collected at 0, 30, 60, 90, 120, 150, 180, 240, 300 and 360 min after the start of the experiment, the sediment is avoided to be disturbed, then the water sample is transferred to a clean centrifuge tube, and the nitrogen and phosphorus concentrations are determined; S4: The flow rate of the indoor water tank is switched to 0.2 m / s; S5: The samples under the flow rate of 0.2 m / s are collected and the nitrogen and phosphorus concentrations are determined according to step S3; S6: The experiment under different flow rates is completed according to the above steps; S7: Chart drawing: The nitrogen and phosphorus concentration data measured at different time points under different flow rates are arranged, the nitrogen and phosphorus concentrations are taken as the vertical coordinates, and the time is taken as the horizontal coordinate, and the nitrogen and phosphorus concentration curves with time under different flow rates are drawn.

4. The numerical simulation method for nitrogen and phosphorus release of lake and reservoir sediment according to claim 2, characterized in that, The specific steps of obtaining the nitrogen and phosphorus concentration curves of the overlying water with time under different concentrations of dissolved oxygen in combination with the laboratory experiment include: S1: The sediment is laid flat in the indoor water tank, and is allowed to stand for 1 hour before the experiment starts, so that the sediment is no longer suspended; S2: The concentration of dissolved oxygen in the indoor water tank is set to 2 mg / L, and the nitrogen and phosphorus release experiment of the sediment is carried out; S3: Sample collection and determination: 100 mL of water sample is collected at 0, 30, 60, 90, 120, 150, 180, 240, 300 and 360 min after the start of the experiment, the sediment is avoided to be disturbed, then the water sample is transferred to a clean centrifuge tube, and the nitrogen and phosphorus concentrations are determined; S4: The concentration of dissolved oxygen in the indoor water tank is switched to 3 mg / L; S5: The samples under the concentration of dissolved oxygen of 3 mg / L are collected and the nitrogen and phosphorus concentrations are determined according to step S3; S6: The experiment under different concentrations of dissolved oxygen is completed according to the above steps; S7: Chart drawing: collate the nitrogen and phosphorus concentration data measured at different time points under different dissolved oxygen concentrations, draw the nitrogen and phosphorus concentration-time curve under different dissolved oxygen concentrations with nitrogen and phosphorus concentration as the vertical coordinate and time as the horizontal coordinate.

Images