Low-temperature domestic sewage treatment system

Abstract

The application provides a low-temperature domestic sewage treatment system, which comprises a pretreatment unit, a heat recycling unit, a water treatment unit, a heating unit, a design evaluation unit, a first tail water treatment pipeline and a second tail water treatment pipeline; the heat recycling unit is a double-pipe countercurrent heat exchanger; the heating unit is arranged in the water treatment unit; the design evaluation unit comprises the design of the heating unit, the design of the heat recycling unit, the calculation and evaluation of the annual average unit water heat energy consumption of the sewage treatment system, the annual average heat recycling amount and the annual average heat recycling rate; the length of the double-pipe countercurrent heat exchanger is accurately calculated by using an iterative calculation model, the performance meets the standard in the extreme case, and the waste is reduced to the maximum extent; based on a sinusoidal temperature fluctuation model, an annual average heat energy consumption and heat recycling rate evaluation system is constructed, the annual energy saving benefit of the system is accurately quantified, the operation cost is effectively reduced, and a scientific basis is provided for the efficient treatment and energy management of low-temperature domestic sewage.

Description

Technical Field

[0001] This invention relates to the field of heat recovery and utilization technology in low-temperature domestic sewage treatment, and specifically to a low-temperature domestic sewage treatment system. Background Technology

[0002] Domestic wastewater treatment mainly relies on the biodegradation of microorganisms, and the metabolic activity of these microorganisms is highly sensitive to temperature. In winter or in cold regions, a decrease in influent temperature can lead to a significant drop in the rate of biochemical reactions, severely affecting the quality of the effluent.

[0003] In theory, heating the influent is the most direct way to maintain efficient system operation. However, due to the high specific heat capacity of wastewater and the typically huge flow rate, direct heating requires extremely high energy consumption, making it economically infeasible. Therefore, although heating can solve the problem of low-temperature failure, in actual wastewater treatment engineering applications, the high energy cost means that there are very few cases of using active heating methods. This forces many treatment plants to face the dilemma of reduced efficiency or even difficulty in meeting standards during low-temperature seasons.

[0004] To overcome this energy consumption bottleneck, existing technologies attempt to recover waste heat from tailwater using heat exchangers. While this reduces energy consumption to some extent, significant shortcomings remain in system design and energy efficiency assessment, hindering its widespread application. First, the design of shell-and-tube countercurrent heat exchangers lacks precision. Existing designs are often based on empirical values ​​or single operating points, lacking precise iterative calculation methods for specific target heat exchange. This results in heat exchange area designs that are either insufficient to meet the requirements or too large, leading to material waste.

[0005] Second, energy consumption assessments are detached from dynamic realities. Traditional energy consumption assessments typically use static parameters, ignoring the dynamic changes in ambient temperature and influent water temperature with the seasons (such as sinusoidal fluctuations). This calculation method cannot accurately reflect the system's annual operating energy consumption, leading to significant discrepancies between the estimated annual energy consumption and the actual energy-saving effect.

[0006] Therefore, there is an urgent need for a system that can accurately calculate heating power, optimize the design of shell-and-tube countercurrent heat exchangers, and accurately assess annual average heat energy consumption and heat recovery rate based on a dynamic temperature model, so as to make low-temperature wastewater heating economically feasible in engineering. Summary of the Invention

[0007] To address the aforementioned technical problems, this invention aims to provide a low-temperature domestic wastewater treatment system. The system includes: a pretreatment unit, a heat recovery and reuse unit, a water treatment unit, a heating unit, a design evaluation unit, a first effluent treatment pipeline, and a second effluent treatment pipeline. The heat recovery and reuse unit is a coaxial counter-current heat exchanger. The heating unit is located within the water treatment unit. The design evaluation unit includes the design of the heating unit, the design of the heat recovery and reuse unit, and the calculation and evaluation of the average annual heat energy consumption per unit volume of wastewater, the average annual heat recovery amount, and the average annual heat recovery rate of the wastewater treatment system. When heat exchange is required, the effluent undergoes heat exchange in the coaxial counter-current heat exchanger within the heat recovery and reuse unit, and is finally discharged through the first effluent treatment pipeline. When heat exchange is not required, the effluent is directly discharged through the second effluent treatment pipeline.

[0008] Furthermore, the design of the heating unit refers to calculating the heating power of the heating unit. Heating power of the heating unit The design calculations fully consider the heat demand and heat loss during system operation. Specifically, It consists of two parts: one is the power required to heat the wastewater to the set temperature of the water treatment unit. Secondly, the heat loss power of the water treatment unit. .

[0009] Heating power of the heating unit The calculation is shown in equation (1): Equation (1); The The calculation is shown in equation (1-1): Equation (1-1); In extreme cases, The The calculation is shown in equation (1-2): Equation (1-2); In the formula: Heating power of the heating unit, kJ / d; The power required to heat wastewater to the set temperature of the water treatment unit, in kJ / d; : Heat loss power of water treatment unit, kJ / d; Specific heat capacity of wastewater, kJ / (kg·℃); : System processing traffic, m 3 / d; Wastewater density, kg / m³ 3 ; Water treatment unit set temperature, °C; Wastewater effluent temperature from the heat recovery and reuse unit, in °C; The overall heat transfer coefficient of the water treatment unit is taken as... ; : Surface area of ​​the outer container of the water treatment unit, in m² 2 ; : Annual lowest ambient temperature, °C; Ambient temperature, °C.

[0010] Through this calculation model, the system can accurately determine the total heating power required under extreme low temperature conditions (such as ambient temperature of 0°C and inlet water temperature of 2°C), ensuring stable system operation.

[0011] Furthermore, the heat recovery and reuse unit design involves iteratively calculating the tube length of the coaxial counter-current heat exchanger. The iterative approach is as follows: Under the lowest temperature environment, by setting initial values ​​and an error threshold, the target heat transfer rate is iteratively calculated and compared with the theoretical heat transfer rate, and the error is compared. During the iteration process, the length of the previous iteration is corrected using a step size coefficient to obtain the calculation formula for the length of the next iteration, ensuring convergence accuracy. Finally, the tube length of the coaxial counter-current heat exchanger that meets the set error is obtained. The calculation process is as follows: Step S1: Set the initial setting value for the tube length of the shell-and-tube counterflow heat exchanger. Allowable error threshold Iteration step size coefficient Assume that the flow velocities of the tailwater and wastewater in the shell-and-tube countercurrent heat exchanger are the same, both being the system's processing flow rate.

[0012] Step S2: Temperature difference parameter calculation: The inlet and outlet temperatures of the tailwater flowing back from the heating unit to the heat recovery and reuse unit, as well as the inlet and outlet temperatures of the wastewater flowing from the pretreatment unit to the heat recovery and reuse unit, are related to the flow rates of the tailwater and wastewater in the shell-and-tube counter-current heat exchanger. Assuming the flow rates of the tailwater and wastewater are the same, the logarithmic mean temperature difference between the inlet and outlet on the same side of the shell-and-tube counter-current heat exchanger is first calculated. As shown in equation (2): Equation (2); In the formula, The calculation is shown in equation (2-1): Equation (2-1); when At that time, take ; Step S3: Forward Iterative Calculation: Based on the current length of the shell-and-tube counter-current heat exchanger, i.e., the... The tube length of the next iteration of the coaxial counterflow heat exchanger (in (current iteration number) and known system processing throughput The heating time of the sewage through the shell-and-tube counter-current heat exchanger was calculated sequentially. Target heat exchange (Based on temperature rise requirements) and theoretical heat exchange capacity (Based on the heat transfer coefficient of the shell-and-tube counterflow heat exchanger) ).

[0013] The heating time is calculated as shown in equation (3). : Equation (3); The result obtained from equation (3) Substituting into equation (4), the target heat exchange is calculated as shown in equation (4): Equation (4); The result obtained from equation (2) The result obtained by formula (3) Substitute into equation (5) to calculate the theoretical heat exchange, as shown in equation (5): Equation (5); Step S4 Error Judgment: Calculate the theoretical heat exchange as shown in equation (6). Heat exchange with target absolute error between : Equation (6); Step S5 Iterative Update: Determine Is it less than the allowable error threshold? : like If the iteration stops, output the current length. As the final design length; like Then, adjust the length to obtain the length of the next iteration. Then return to step S3; In the above formula: : Initial setting value for the tube length of the shell-and-tube counterflow heat exchanger, in meters; Allowable error threshold, range of values ; Iteration step size coefficient, range of values ; : Tailwater inlet temperature of the heat recovery and reuse unit, °C; : Tailwater outlet temperature of the heat recovery and reuse unit, °C; Wastewater inlet temperature of the heat recovery and reuse unit, °C; Wastewater effluent temperature from the heat recovery and reuse unit, in °C; : No. The tube length of the shell-and-tube countercurrent heat exchanger in the next iteration, in meters; Heating time, d; Pipe radius of the shell-and-tube counter-current heat exchanger, in meters; Target heat exchange, kJ; Theoretical heat exchange, kJ; Logarithmic mean temperature difference between the inlet and outlet on the same side of a shell-and-tube countercurrent heat exchanger, in °C; : Temperature difference ratio at both ends of a shell-and-tube countercurrent heat exchanger, dimensionless; The overall heat transfer coefficient of the shell-and-tube counter-flow heat exchanger is taken as... .

[0014] Furthermore, the iteration length in step S5 The calculation is shown in equation (7): Equation (7); In the formula: : No. The tube length of the shell-and-tube countercurrent heat exchanger in the next iteration is in meters (m).

[0015] Furthermore, the design evaluation unit performs an annual average unit water volume heat energy consumption test. The calculation approach is as follows: an annual temperature model based on a sine function is constructed to characterize the periodicity of ambient temperature changes over time. An instantaneous heating power function is then constructed using the temperature model, and the daily heating power is integrally calculated over the heating period to derive the system's average annual heat energy consumption and average annual heat energy consumption per unit volume of water.

[0016] Average annual heat energy consumption per unit of water volume The calculation steps are as follows: Step 1: To construct a sinusoidal temperature function, first calculate the temperature parameters in the function: Obtain the annual maximum temperature of environmental soil and wastewater influent. With the lowest temperature of the year Calculate their respective average values. With amplitude : Equation (8); Equation (9); Then, by using equations (8) and (9), the relationship between heating time and other parameters is constructed. The changing sinusoidal temperature function: Equation (10); Equation (11); In the formula: Average annual ambient temperature, °C; : Annual amplitude of ambient temperature, °C; The highest annual ambient temperature, in °C; : Annual lowest ambient temperature, °C; : Annual average temperature of wastewater influent to the heat recovery and reuse unit, °C; : Annual amplitude of wastewater influent temperature in the heat recovery and reuse unit, °C; The highest annual temperature of the wastewater influent to the heat recovery and reuse unit, in °C; : The lowest annual temperature of wastewater influent to the heat recovery and reuse unit, in °C; Wastewater inlet temperature of the heat recovery and reuse unit, °C; Ambient temperature, °C; : Fixed value, taking the value 365 days; Heating time, d; The result obtained from equation (11) Substituting into equation (1-2) yields .

[0017] Step 2: Calculate the effluent temperature of the heat recovery and reuse unit. : Based on the calculated design length of the shell-and-tube counterflow heat exchanger tubes Combined with the overall heat transfer coefficient of the shell-and-tube counterflow heat exchanger Pipe radius of shell-and-tube counter-flow heat exchanger Specific heat capacity of wastewater And the set temperature of the water treatment unit as the effluent inlet temperature. Calculate the influent temperature of wastewater in the heat recovery and reuse unit. Varying wastewater effluent temperature of heat recovery and reuse unit : Equation (12); In equation (12), the heat exchange efficiency factor is... The calculation is shown in equation (12-1): Equation (12-1); In the above formula: Wastewater effluent temperature from the heat recovery and reuse unit, in °C; Water treatment unit set temperature, °C; Heat exchange efficiency factor, dimensionless; : Design length of tubes in a shell-and-tube counter-current heat exchanger, in meters; The result obtained from equation (12) Substituting into equation (1-1) yields .

[0018] Step 3: Calculate the heating power of the heating unit. : Heating power is supplied by the wastewater from Upgraded to The power and water treatment unit power loss are composed of the power calculated in step one. and the result calculated in step two Substituting into equation (1) and rearranging, we get: Equation (13).

[0019] Step 4: Integrate and calculate the average annual heat energy consumption and the average annual heat energy consumption per unit volume of water: right Integrating over the heating period yields the average annual heat energy consumption. and average annual heat energy consumption per unit of water : Equation (14); Equation (15); in: Equation (14-1); Equation (14-2); : The baseline term for heating power, kJ / d; : Fluctuation term of heating power, kJ / d; Average annual thermal energy consumption, kJ; Average annual heat energy consumption per unit of water volume, kJ / m³ 3 .

[0020] The design evaluation unit innovatively introduces a time-based approach. A sinusoidal function model is used to simulate environmental and water temperature changes, and the annual average heat energy consumption per unit of water volume is calculated. The calculations include: Constructing a temperature function: Based on the annual maximum and minimum temperatures, calculate the average value (avg) and amplitude (amp), and construct... and The sinusoidal wave function.

[0021] Daily power recovery modeling: Based on the formula for the outlet water temperature of a shell-and-tube counter-current heat exchanger, calculate the power required to heat wastewater to the set temperature of the water treatment unit. The formula is decomposed into a baseline term. and fluctuation term ,in Energy consumption reflects the average temperature difference. Energy consumption reflects the fluctuation range.

[0022] Integration: The power function is integrated over the effective heating period. This invention derives an exact analytical expression containing inverse trigonometric functions, allowing direct calculation of the average annual heat energy consumption. This leads to the derivation of the average annual heat energy consumption per unit of water volume. .

[0023] The design evaluation unit further calculates the system's average annual heat recovery. To quantify energy-saving effects, the calculation approach is as follows: A daily recovered power function is constructed using the iteratively calculated length of the coaxial counter-current heat exchanger and a sinusoidally varying temperature function. This function is then integrated over the heating period to accurately calculate the total annual recovered heat energy of the system. The calculation steps include constructing the daily recovered power function... The average annual heat recovery is obtained by integrating the heat recovery during the heating period. This evaluation method comprehensively considers the impact of the heat transfer coefficient, tube length, and annual temperature fluctuations on the recovery rate of the shell-and-tube countercurrent heat exchanger.

[0024] Furthermore, the design evaluation unit calculates the system's average annual heat recovery. The specific steps are as follows: ① Calculate daily recovery power : The daily recovery power is constructed as shown in equation (16): Equation (16); In the formula: Daily power recovery, kJ / d; ②During the heating period Perform integral calculation of average annual heat recovery : Equation (17); Average annual heat recovery, kJ.

[0025] Furthermore, the design evaluation unit calculates the system's average annual heat recovery rate. The specific steps are as follows: system annual average heat recovery rate Definition formula: Equation (18); The design evaluation unit calculates the system's average annual heat recovery rate. The method quantifies energy-saving effects by comprehensively considering the heat transfer coefficient, tube length, and annual temperature fluctuations of the coaxial countercurrent heat exchanger on the recovery efficiency.

[0026] The calculated , And all intermediate parameters are substituted into the defined formula to calculate the system's average annual heat recovery rate. .

[0027] Compared with existing technologies, the present invention has the following significant advantages: 1. Precision and Scientific Nature of the Design Method: This invention abandons the traditional empirical estimation method and, through an iterative calculation model, can accurately solve for the optimal length of the shell-and-tube counter-flow heat exchanger based on the specific target heat exchange and fluid physical properties. This method not only ensures that the performance of the shell-and-tube counter-flow heat exchanger meets the standards under extreme winter conditions, but also avoids the waste of materials and space caused by design errors.

[0028] 2. Authenticity and Dynamism of Energy Consumption Assessment: The assessment model established in this invention overcomes the limitations of static calculations, using a sine function to simulate the annual periodic changes in environmental and influent water temperatures. This makes the calculated results of annual average heat energy consumption and annual average heat energy consumption per unit of water volume more consistent with actual operating patterns, significantly reducing errors.

[0029] 3. Visualization and Quantification of Energy-Saving Benefits: This system, through a clearly defined annual average heat recovery rate (EPR) calculation process, can intuitively quantify the energy-saving benefits brought by the heat recovery and reuse unit. As shown in the example data, under typical annual temperature fluctuations, the system can achieve an annual average heat recovery rate of approximately 72%. This indicator provides strong data support for the system's economic analysis and green, low-carbon operation, demonstrating the system's extremely high application value in the field of low-temperature wastewater treatment. Attached Figure Description

[0030] Figure 1 This is a schematic diagram of the low-temperature domestic sewage treatment system based on heat energy recovery in the embodiment; In the diagram: 1. Pretreatment unit, 2. Heat recovery and reuse unit, 3. First tailwater treatment pipeline, 4. Water treatment unit, 5. Heating unit, 6. Design evaluation unit, 7. Second tailwater treatment pipeline. Detailed Implementation

[0031] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0032] The technical solution of the present invention will be further explained below with reference to implementation examples.

[0033] Example like Figure 1 As shown, the low-temperature domestic sewage treatment system provided in this embodiment includes: a pretreatment unit 1, a heat recovery and reuse unit 2, a first effluent treatment pipeline 3, a water treatment unit 4, a heating unit 5, a design evaluation unit 6, and a second effluent treatment pipeline 7. The heat recovery and reuse unit 2 is a coaxial counter-current heat exchanger. The design evaluation unit 6 includes the design of the heating unit 5, the design of the heat recovery and reuse unit 2, and the calculation and evaluation of the annual average heat energy consumption per unit volume of sewage treatment system, the annual average heat recovery amount, and the annual average heat recovery rate. The dashed arrows in the figure represent the sewage flow direction, and the solid arrows represent the effluent flow direction.

[0034] Wastewater sequentially passes through pretreatment unit 1 to remove impurities, heat recovery and reuse unit 2 for preliminary heating, and water treatment unit 4 to meet standards. When the temperature of the effluent inlet of the shell-and-tube counter-current heat exchanger is lower than the temperature of the wastewater inlet, the treated effluent is discharged through the second effluent treatment pipe 7. When the temperature of the wastewater inlet of the shell-and-tube counter-current heat exchanger is lower than the temperature of the effluent inlet, the treated effluent undergoes heat exchange in the heat recovery and reuse unit 2 (shell-and-tube counter-current heat exchanger) and is then discharged through the first effluent treatment pipe 3.

[0035] Temperature sensors are installed at both ends of the inlet and outlet of the shell-and-tube countercurrent heat exchanger, as well as in the water treatment unit and the surrounding soil environment. By calculating the tube length of the shell-and-tube countercurrent heat exchanger, the heating power of the heating unit, and the annual average heat recovery and annual average heat energy consumption per unit volume of water in the treatment system, the design and operating parameters of the shell-and-tube countercurrent heat exchanger can be optimized to improve the heat recovery rate and reduce energy waste. Through the system's heat energy consumption assessment method, the annual effective heat energy and the annual total input heat energy can be accurately calculated, thereby obtaining the system's heat energy utilization efficiency, which provides a scientific basis for system optimization.

[0036] In this embodiment, the first step is to perform a "design calculation of the heating power of the heating unit," calculating the required power of the heating unit based on factors such as actual heating demand. The next step is to calculate the tube length of the coaxial counter-current heat exchanger in the heat recovery and reuse unit. Using relevant principles and data, the heat recovery and reuse unit is designed, determining its structure and parameters. The heat loss during system operation is analyzed and evaluated, taking into account factors such as heat dissipation, to optimize system energy efficiency.

[0037] Wastewater was simulated in the laboratory.

[0038] 1. Parameter settings Water treatment unit set temperature: 20℃; Specific heat capacity of wastewater ; Wastewater density ; : System processing traffic ; : Surface area of ​​the outer container of the water treatment unit, 30m² 2 ; Pipe radius of the shell-and-tube counter-current heat exchanger: 0.025m; Overall heat transfer coefficient of a shell-and-tube counter-current heat exchanger ; Overall heat transfer coefficient of the water treatment unit .

[0039] 2. Heating unit power calculation Simulates the worst-case scenario of starting up in extreme low temperatures during winter.

[0040] Wastewater effluent temperature from the heat recovery and reuse unit: 2℃; The lowest ambient temperature of the year is 0℃. The heating unit heating power The calculation is as follows:

[0041] First, calculate the heat loss power of the water treatment unit: ; ; Secondly, calculate the power required to heat the wastewater to the set temperature of the water treatment unit: ; ; Calculate the heating power of the heating unit: .

[0042] 3. Length design of the shell-and-tube countercurrent heat exchanger in the heat recovery and reuse unit The extreme condition parameters are as follows: Wastewater inlet temperature of the heat recovery and reuse unit: 2℃; Wastewater effluent temperature from the heat recovery and reuse unit: 15℃; The tailwater inlet temperature of the heat recovery and reuse unit is 20℃. The effluent temperature of the heat recovery and reuse unit is 7℃.

[0043] Calculation process: Step 1 (Initialization): Setting the initial value of the tube length for the shell-and-tube counterflow heat exchanger Allowable error threshold Iteration step size coefficient The flow rates of wastewater and effluent are the same, both being the system's treatment flow rate.

[0044] Step Two (Parameter Calculation) ): Calculate the temperature difference ratio at both ends of a shell-and-tube counterflow heat exchanger : ; because The logarithmic mean temperature difference is taken as the extreme value, that is, the logarithmic mean temperature difference between the inlet and outlet on the same side of the shell-and-tube counterflow heat exchanger. : .

[0045] Steps three to five (demonstration of iterative calculation): Based on system-level traffic processing Perform iterations: The first iteration ( ): Heating time ; Take 3.14; Target heat exchange ; Theoretical heat exchange ; error ; Update length ; ...after multiple iterations... Eventual convergence: When the error condition is met, the design length of the tubes in the shell-and-tube counterflow heat exchanger can be calculated. It is approximately 82.80m.

[0046] 4. Annual average heat energy consumption and heat recovery rate assessment The lowest ambient temperature of the year is 0℃. The highest annual ambient temperature is 25℃. Average annual ambient temperature: 12.5℃; Annual amplitude of ambient temperature, 12.5℃; The annual minimum temperature of the wastewater inlet of the heat recovery and reuse unit is 2℃. The highest annual temperature of the wastewater inlet of the heat recovery and reuse unit is 23℃. The annual average temperature of the wastewater inlet to the heat recovery and reuse unit is 12.5℃. : Annual amplitude of wastewater influent temperature of the heat recovery and reuse unit, 10.5℃.

[0047] Average annual heat energy consumption per unit of water volume )calculate: First, calculate the design pipe length. At that time, the heat exchange efficiency factor of the shell-and-tube counterflow heat exchanger : ; Calculate the baseline term With fluctuation term : ; ; ; ; ; ; ; .

[0048] Calculate the average annual heat energy consumption : ; .

[0049] Calculate the average annual heat energy consumption per unit of water volume : .

[0050] Calculate the average annual heat recovery : ; .

[0051] Calculate the average annual heat recovery rate : .

[0052] This application overcomes the limitations of traditional empirical design, achieving precise design of the coaxial counter-current heat exchanger. Traditional wastewater treatment systems often rely on empirical estimations for coaxial counter-current heat exchangers, which can easily result in an undersized area (insufficient heat exchange) or an oversized area (wasted costs). This embodiment introduces an iterative calculation model with a step size coefficient of 0.5 and an error threshold of 1 kJ. After multiple iterations, the system precisely determined the optimal design length of the coaxial counter-current heat exchanger to be 82.80 m. This precise mathematical model calculation ensures performance compliance under extreme winter conditions and avoids material and space waste from the outset, thus enhancing the patent's commercialization value.

[0053] This application ensures stable system operation under extremely unfavorable winter conditions. Low temperature is a critical factor affecting microbial activity. This embodiment simulates an extreme low-temperature start-up scenario with ambient soil temperature of 0°C and influent water temperature of only 2°C. The system accurately calculates the heat loss of the water treatment unit and the thermal power required to heat the wastewater. Finally, the required heating power under extreme conditions is determined. This demonstrates that a large energy supply is required to ensure the water treatment temperature is reached under extreme conditions, and also verifies the engineering application prospects of this system.

[0054] This application quantifies the outstanding energy-saving and emission-reduction benefits ( (Up to 72%). The example incorporates a sinusoidal temperature function that varies with time to calculate the annual average heat energy consumption. The system's annual average heat energy consumption per unit volume of water. Only about Average annual heat recovery Approximately The system's average annual heat recovery rate (calculated using the formula) is... The heat recovery rate reached 72%. The over 70% heat recovery rate provides highly convincing data support for "energy-saving benefit visualization", proving that the system has extremely high economic feasibility and green and low-carbon value in engineering.

[0055] The data from the implementation cases in this application fully demonstrate that this invention abandons static parameter evaluation, adopts a dynamic model, and successfully constructs a highly efficient system that is "predictable in energy consumption, does not waste equipment, and does not fail in winter".

[0056] The embodiments described above are merely examples of several implementations of the present invention, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the invention. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these modifications and improvements all fall within the scope of protection of the present invention.

Claims

1. A low-temperature domestic sewage treatment system, characterized in that, The system includes: a pretreatment unit, a heat recovery and reuse unit, a water treatment unit, a heating unit, a design evaluation unit, a first tailwater treatment pipeline, and a second tailwater treatment pipeline; the heat recovery and reuse unit is a shell-and-tube countercurrent heat exchanger; the heating unit is located in the water treatment unit; the design evaluation unit includes the design of the heating unit, the design of the heat recovery and reuse unit, and the calculation and evaluation of the annual average heat energy consumption per unit volume of water, the annual average heat recovery amount, and the annual average heat recovery rate of the wastewater treatment system.

2. The low-temperature domestic sewage treatment system according to claim 1, characterized in that, The design of the heating unit refers to calculating the heating power of the heating unit. As shown in equation (1): Equation (1); The The calculation is shown in equation (1-1): Equation (1-1); In extreme cases, The The calculation is shown in equation (1-2): Equation (1-2); In the formula: Heating power of the heating unit, kJ / d; The power required to heat wastewater to the set temperature of the water treatment unit, in kJ / d; : Heat loss power of water treatment unit, kJ / d; Specific heat capacity of wastewater, kJ / (kg·℃); : System processing traffic, m 3 / d; Wastewater density, kg / m³ 3 ; Water treatment unit set temperature, °C; Wastewater effluent temperature from the heat recovery and reuse unit, in °C; The overall heat transfer coefficient of the water treatment unit is taken as... ; : Surface area of ​​the outer container of the water treatment unit, in m² 2 ; : Annual lowest ambient temperature, °C; Ambient temperature, °C.

3. The low-temperature domestic sewage treatment system according to claim 2, characterized in that, The design of the heat recovery and reuse unit involves iterative calculation of the tube length of the coaxial counter-current heat exchanger. The calculation process is as follows: Step S1: Set the initial setting value for the tube length of the shell-and-tube counterflow heat exchanger. Allowable error threshold Iteration step size coefficient ; Step S2: First, calculate the logarithmic mean temperature difference between the inlet and outlet on the same side of the shell-and-tube counter-flow heat exchanger. As shown in equation (2): Equation (2); In the formula, The calculation is shown in equation (2-1): Equation (2-1); when At that time, take ; Step S3, calculate the heating time as shown in equation (3). : Equation (3); The target heat exchange is calculated as shown in equation (4): Equation (4); The theoretical heat exchange is calculated as shown in equation (5): Equation (5); Step S4, calculate the theoretical heat exchange as shown in equation (6). Heat exchange with target absolute error between : Equation (6); Step S5, Iterative Update: like If the iteration stops, output the current length. As the final design length; like Then, adjust the length to obtain the length of the next iteration. Then return to step S3; In the above formula: : Initial setting value for the tube length of the shell-and-tube counterflow heat exchanger, in meters; Allowable error threshold, range of values ; Iteration step size coefficient, range of values ; : Tailwater inlet temperature of the heat recovery and reuse unit, °C; : Tailwater outlet temperature of the heat recovery and reuse unit, °C; Wastewater inlet temperature of the heat recovery and reuse unit, °C; Wastewater effluent temperature from the heat recovery and reuse unit, in °C; : No. The tube length of the shell-and-tube countercurrent heat exchanger in the next iteration, in meters; Heating time, d; Pipe radius of the shell-and-tube counter-current heat exchanger, in meters; Target heat exchange, kJ; Theoretical heat exchange, kJ; Logarithmic mean temperature difference between the inlet and outlet on the same side of a shell-and-tube countercurrent heat exchanger, in °C; : Temperature difference ratio at both ends of a shell-and-tube countercurrent heat exchanger, dimensionless; The overall heat transfer coefficient of the shell-and-tube counter-flow heat exchanger is taken as... .

4. The low-temperature domestic sewage treatment system according to claim 3, characterized in that, Iteration length in step S5 The calculation is shown in equation (7): Equation (7); In the formula: : No. The tube length of the shell-and-tube countercurrent heat exchanger in the next iteration is in meters (m).

5. The low-temperature domestic sewage treatment system according to claim 3, characterized in that, Average annual heat energy consumption per unit of water volume The calculation steps are as follows: Step 1: Calculate temperature parameters: Equation (8); Equation (9); Construction with heating time The changing sinusoidal temperature function: Equation (10); Equation (11); In the formula: Average annual ambient temperature, °C; : Annual amplitude of ambient temperature, °C; The highest annual ambient temperature, in °C; : Annual lowest ambient temperature, °C; : Annual average temperature of wastewater influent to the heat recovery and reuse unit, °C; : Annual amplitude of wastewater influent temperature in the heat recovery and reuse unit, °C; The highest annual temperature of the wastewater influent to the heat recovery and reuse unit, in °C; : The lowest annual temperature of wastewater influent to the heat recovery and reuse unit, in °C; Wastewater inlet temperature of the heat recovery and reuse unit, °C; Ambient temperature, °C; : Fixed value, taking the value 365 days; Heating time, d; The result obtained from equation (11) Substituting into equation (1-2) yields ; Step 2: Calculate the effluent temperature of the heat recovery and reuse unit. : Equation (12); In the formula, the heat exchange efficiency factor The calculation is shown in equation (12-1): Equation (12-1); In the formula: Wastewater effluent temperature from the heat recovery and reuse unit, in °C; Water treatment unit set temperature, °C; Heat exchange efficiency factor, dimensionless; : Design length of tubes in a shell-and-tube counter-current heat exchanger, in meters; The result obtained from equation (12) Substituting into equation (1-1) yields ; Step 3: Calculate the heating power of the heating unit. : Equation (13); Step 4: Integrate and calculate the average annual heat energy consumption and the average annual heat energy consumption per unit volume of water: right Integrating over the heating period yields the average annual heat energy consumption. and average annual heat energy consumption per unit of water : Equation (14); Equation (15); in: Equation (14-1); Equation (14-2); : The baseline term for heating power, kJ / d; : Fluctuation term of heating power, kJ / d; Average annual thermal energy consumption, kJ; Average annual heat energy consumption per unit of water volume, kJ / m³ 3 .

6. The low-temperature domestic sewage treatment system according to claim 5, characterized in that, The design evaluation unit calculates the system's average annual heat recovery. The specific steps are as follows: ① Calculate daily recovery power : The daily recovery power is constructed as shown in equation (16): Equation (16); In the formula: Daily power recovery, kJ / d; ②During the heating period Perform integral calculation of average annual heat recovery : Equation (17); Average annual heat recovery, kJ.

7. The low-temperature domestic sewage treatment system according to claim 6, characterized in that, The design evaluation unit calculates the average annual heat recovery rate. The specific steps are as follows, with an average annual heat recovery rate. Definition formula: Equation (18); The calculated and Substitute into the definition formula to calculate the system's average annual heat recovery rate. .

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