Direct-connection double-membrane system for water supply plant

Abstract

The utility model provides a direct-connection double-membrane system for a water supply plant and belongs to the technical field of sewage treatment devices. An ultrafiltration water inlet tank is communicated with a self-cleaning filter through an ultrafiltration water inlet pump; the output end of the self-cleaning filter is communicated with an ultrafiltration membrane module; the output end of the ultrafiltration membrane unit is communicated with a reverse osmosis / nanofiltration membrane unit through a high-pressure pump; the output end of the ultrafiltration membrane module is also communicated with an ultrafiltration backwashing water tank; the output ends of the reverse osmosis / nanofiltration membrane modules are jointly communicated with a reverse osmosis nanofiltration water producing tank; and the reverse osmosis nanofiltration water producing tank and the ultrafiltration backwashing water tank are jointly communicated with the mixing well. The process flow is shortened, and the problem that microorganisms breed in the pool is thoroughly avoided; the cleaning frequency of the nanofiltration / reverse osmosis membrane is reduced, and the service life and the performance of the nanofiltration / reverse osmosis membrane element are ensured; intermediate link equipment is reduced, so that the control is simple, and the investment and operation cost of a double-membrane system are reduced; the occupied area of the membrane system is reduced; the reverse osmosis / nanofiltration system makes full use of the ultra-filtration produced water residual pressure, and the energy consumption of the operation of the double-membrane system is reduced.

Description

Technical Field

[0001] The utility model relates to the technical field of water treatment devices, in particular to a direct-connected double-membrane system for a water supply plant. Background Art

[0002] A dual-membrane system consists of two systems: pressure ultrafiltration (PUS) and reverse osmosis (RO) / nanofiltration (NF). In conventional dual-membrane processes, the UF product water enters the RO / NF inlet tank, where the effective volume requires a residence time greater than the 30-minute design flow rate of the inlet pump for the subsequent membrane treatment process. This long retention time and the inevitable presence of dead water within the tank lead to microbial growth within and on the tank walls. To ensure the proper functioning of the subsequent RO membranes, enhanced sterilization and cleaning, as well as the installation of safety filters, are necessary to maintain the operation of the subsequent RO / NF system. The high cost of non-oxidizing biocides, the frequent replacement of safety filters, and the frequent cleaning of NF / RO membranes not only increase system operating costs but also reduce the lifespan and performance of the NF / RO membrane elements. Although some projects have added stainless steel sheeting to the UF product tank walls to improve surface finish and slow microbial growth, this increased investment does not completely resolve the microbial growth issue; it only mitigates it.

[0003] The existing conventional dual-membrane process places a 5-micron security filter behind an ultrafiltration membrane with an average pore size of 0.02 microns. This filtration measure primarily addresses the issue of secondary contamination in the ultrafiltration product water, protecting the subsequent reverse osmosis / nanofiltration membrane system. Furthermore, the security filter requires a low-pressure inlet pump to provide operating pressure. In summary, the conventional dual-membrane treatment process in a waterworks follows the following sequence: "ultrafiltration inlet tank - ultrafiltration inlet pump - self-cleaning filter - pressure ultrafiltration - ultrafiltration product tank - reverse osmosis / nanofiltration inlet pump - security filter - high-pressure pump - reverse osmosis / nanofiltration membrane - product tank." The disadvantages of this process include a lengthy process, susceptibility to secondary biological contamination in the ultrafiltration product tank, numerous equipment components, complex control, high investment and operating costs, and a large footprint. Utility Model Content

[0004] The purpose of the present utility model is to provide a direct-connected double-membrane system for a water supply plant to solve at least one technical problem existing in the above-mentioned background technology.

[0005] In order to achieve the above purpose, the present invention adopts the following technical solutions:

[0006] The utility model provides a direct-connected double-membrane system for a water supply plant, comprising:

[0007] The ultrafiltration water inlet pool is connected to multiple self-cleaning filters through multiple ultrafiltration water inlet pumps;

[0008] The output ends of the plurality of self-cleaning filters are connected to a main pipe and then connected to a plurality of ultrafiltration membrane modules;

[0009] The output end of each ultrafiltration membrane module is connected to each reverse osmosis / nanofiltration high-pressure pump through a main pipe and is connected to the corresponding reverse osmosis / nanofiltration membrane module;

[0010] The output end of each ultrafiltration membrane module is connected to the ultrafiltration backwash water tank through a main pipe and then through a branch pipe;

[0011] The output end of each reverse osmosis / nanofiltration membrane assembly is commonly connected to a reverse osmosis nanofiltration water production pool;

[0012] The reverse osmosis nanofiltration water production pool and the ultrafiltration backwash water pool are connected to a mixing well.

[0013] Furthermore, a water inlet regulating valve is provided on the water inlet pipe of the ultrafiltration membrane module.

[0014] Furthermore, an ultrafiltration water inlet total electromagnetic flowmeter is provided on the communication pipe between the ultrafiltration water inlet pump and the self-cleaning filter.

[0015] Furthermore, a water inlet flow meter is provided on the input pipe of the ultrafiltration membrane module, or a water production flow meter is provided on the output pipe of the ultrafiltration membrane module.

[0016] Furthermore, an ultrafiltration water production main pipe pressure transmitter is provided on the communication pipe between the reverse osmosis / nanofiltration high-pressure pump and the ultrafiltration membrane module.

[0017] Furthermore, an ultrafiltration excess water electromagnetic flowmeter is provided on the connecting pipe between the ultrafiltration membrane module water production main pipe and the ultrafiltration backwash water tank.

[0018] Furthermore, an ultrafiltration excess water pipeline regulating valve is provided on the connecting pipe between the ultrafiltration membrane module water production main pipe and the ultrafiltration backwash water tank.

[0019] Furthermore, the ultrafiltration backwash water pool is connected to the ultrafiltration membrane module via an ultrafiltration backwash water pump.

[0020] Furthermore, an output pipe of each reverse osmosis / nanofiltration membrane module is provided with a reverse osmosis / nanofiltration produced water electromagnetic flowmeter.

[0021] Furthermore, a reverse osmosis concentrated water electromagnetic flowmeter is provided on the output pipe of each reverse osmosis / nanofiltration membrane module.

[0022] The beneficial effects of the utility model are as follows: shortening the process flow and completely avoiding the problem of microbial growth in the water pool; reducing the frequency of nanofiltration / reverse osmosis membrane cleaning and ensuring the service life and performance of nanofiltration / reverse osmosis membrane elements; reducing intermediate link equipment, making control simpler, and reducing the investment and operating costs of the dual membrane system; reducing the area occupied by the membrane system; the reverse osmosis / nanofiltration system fully utilizes the residual pressure of ultrafiltration water production and reduces the energy consumption of the dual membrane system operation.

[0023] Additional aspects and advantages of the present invention will be partially given in the following description, which will become apparent from the following description or be understood through practice of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the following briefly introduces the drawings required for use in the description of the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0025] Figure 1 This is a structural block diagram of the direct-connected double-membrane system of a water supply plant described in an embodiment of the present utility model.

[0026] Figure 2 This is a PID control structure diagram of a direct-connected double-membrane system in a water supply plant according to an embodiment of the present invention.

[0027] Figure 3 This is a control principle diagram of a direct-connected double-membrane system in a water supply plant according to an embodiment of the present utility model.

[0028] Among them: 1-ultrafiltration water inlet pool; 2-ultrafiltration water inlet pump; 3-self-cleaning filter; 4-ultrafiltration membrane assembly; 5-reverse osmosis / nanofiltration high-pressure pump; 6-reverse osmosis / nanofiltration membrane assembly; 7-ultrafiltration backwash water pool; 8-reverse osmosis nanofiltration water production pool; 9-mixing well. DETAILED DESCRIPTION

[0029] The following describes embodiments of the present invention in detail, examples of which are shown in the accompanying drawings, wherein the same or similar reference numerals throughout represent the same or similar elements or elements having the same or similar functions. The embodiments described below with reference to the accompanying drawings are exemplary and are intended only to explain the present invention and are not to be construed as limiting the present invention.

[0030] Those skilled in the art will understand that, unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by those skilled in the art in the art to which this invention belongs.

[0031] It should also be understood that terms, such as those defined in commonly used dictionaries, should be understood to have a meaning consistent with their meaning in the context of the prior art and will not be interpreted in an idealized or overly formal sense unless as defined herein.

[0032] Those skilled in the art will understand that, unless otherwise stated, the singular forms "a," "an," "said," and "the" used herein may also include plural forms. It should be further understood that the term "comprising" used in the specification of the present utility model refers to the presence of the stated features, integers, steps, operations, elements, and / or components, but does not preclude the presence or addition of one or more other features, integers, steps, operations, elements, and / or groups thereof.

[0033] In the description of this specification, reference to the terms "one embodiment," "some embodiments," "example," "specific example," or "some examples" means that the specific features, structures, materials, or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present invention. Moreover, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in any one or more embodiments or examples. In addition, those skilled in the art may combine and integrate different embodiments or examples described in this specification, as well as features of different embodiments or examples, unless they are mutually inconsistent.

[0034] In this specification, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Therefore, a feature specified as "first" or "second" may explicitly or implicitly include at least one such feature. In the description of this utility model, "plurality" means two or more, unless otherwise specifically defined.

[0035] In the description of this specification, the terms "center", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", "outside", etc., indicating the orientation or position relationship, are based on the orientation or position relationship shown in the accompanying drawings. They are only for the convenience of describing the present technology and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, be constructed and operate in a specific orientation. Therefore, they should not be understood as limiting the present technology.

[0036] Unless otherwise specified or limited, the terms "installed," "connected," "connected," and "disposed" should be understood broadly. For example, they may refer to fixed connection or disposition, detachable connection or disposition, or integral connection or disposition. Those skilled in the art will understand the specific meanings of these terms in this technology based on specific circumstances.

[0037] To facilitate understanding of the present invention, the present invention will be further explained below with reference to specific embodiments in conjunction with the accompanying drawings, and the specific embodiments do not constitute a limitation on the embodiments of the present invention.

[0038] Those skilled in the art should understand that the drawings are merely schematic diagrams of embodiments, and the components in the drawings are not necessarily necessary for implementing the present invention.

[0039] like Figures 1 to 3 As shown, in this embodiment, a direct-connected double-membrane system for a water supply plant is provided, including: an ultrafiltration water inlet pool 1 is connected to a plurality of self-cleaning filters 3 through a plurality of ultrafiltration water inlet pumps 2; a plurality of ultrafiltration membrane modules 4 are connected to the output main pipe of each of the self-cleaning filters 3; the output end of the ultrafiltration membrane module 4 is connected to a water production main pipe and then connected to each reverse osmosis / nanofiltration membrane module 6 corresponding to the high-pressure pump through a reverse osmosis / nanofiltration high-pressure pump 5; the ultrafiltration main pipe also has a branch pipe connected to an ultrafiltration backwash water pool 7; the output end of each reverse osmosis / nanofiltration membrane module 4 is commonly connected to a reverse osmosis nanofiltration water production pool 8; the reverse osmosis nanofiltration water production pool 8 and the ultrafiltration backwash water pool 7 are commonly connected to a mixing well 9.

[0040] The inlet pipe of the ultrafiltration membrane module 4 is equipped with an inlet regulating valve. The connecting pipe between the ultrafiltration inlet pump 2 and the self-cleaning filter 3 is equipped with an ultrafiltration inlet total electromagnetic flowmeter. The input pipe of the ultrafiltration membrane module 4 is equipped with an inlet flowmeter, or the output pipe of the ultrafiltration membrane module is equipped with a production water flowmeter. The connecting pipe between the ultrafiltration module 4 and the reverse osmosis / nanofiltration high-pressure pump 5 is equipped with an ultrafiltration production water main pressure transmitter. The connecting pipe between the ultrafiltration membrane module 4 and the ultrafiltration backwash tank 7 is equipped with an ultrafiltration excess production water electromagnetic flowmeter. The connecting pipe between the ultrafiltration membrane module 4 and the ultrafiltration backwash tank 7 is equipped with an ultrafiltration excess production water pipeline regulating valve. The ultrafiltration backwash tank 7 is connected to the ultrafiltration membrane module 4 via the ultrafiltration backwash pump. Each output pipe of the reverse osmosis / nanofiltration membrane module 6 is equipped with a reverse osmosis / nanofiltration production water electromagnetic flowmeter. An output pipe of each reverse osmosis / nanofiltration membrane assembly 6 is provided with a reverse osmosis concentrated water electromagnetic flowmeter.

[0041] At the water plant's front end, pretreated effluent enters the ultrafiltration (UF) inlet tank for storage and conditioning. The UF inlet pump pumps the effluent to a self-cleaning filter, which further traps suspended solids larger than 100-200 microns. The effluent then enters the pressure-type UF membrane modules (preferably at least four UF modules). The output from each module is then combined into a single main pipe. Two streams are then directly split from this main pipe. One stream, sufficient to meet the RO / NF production requirements, is piped directly to the inlet of the RO / NF high-pressure pump. A smaller stream of excess output, used for UF backwash, is also piped directly above the maximum level of the UF backwash tank. This ensures that the backwash pump's draw from the tank during UF backwash operation does not affect the inlet flow and pressure of the RO / NF system, maintaining stable system pressure. Finally, the RO / NF output water and excess UF output water are mixed through a weir and then flow by gravity to the plant's final clear water tank.

[0042] In this embodiment, the configuration and control method of the direct connection system are described as follows:

[0043] The direct-connect system features the same instrument types and quantity as the conventional connection configuration for the ultrafiltration and reverse osmosis / nanofiltration modules. The difference lies in the instrumentation changes at the junction between the two systems: the UF inlet pressure transmitter, as used in the conventional connection, is installed on the UF product water main. Additionally, an electromagnetic flowmeter and regulating valve are installed on the branch line carrying the UF excess product water to the backwash tank. The key equipment, valves, instrument configuration, and control methods for the direct-connect system are shown in Table 1.

[0044] Table 1

[0045]

[0046]

[0047] In this embodiment, the key steps of the direct connection system are described as follows:

[0048] (1)Ultrafiltration, reverse osmosis / nanofiltration units produce normal water:

[0049] Part of the water produced by the ultrafiltration system goes to the reverse osmosis / nanofiltration system for treatment, and the other part goes to the ultrafiltration backwash pool. Then the two parts of water go to the clear water pool after passing through the mixing well.

[0050] During the initial system operation, the inlet regulating valve of each ultrafiltration module is set to a certain opening, while the total water output of the ultrafiltration system is monitored to meet the overall system requirements. As operation increases, the transmembrane pressure differential of the ultrafiltration increases, and the ultrafiltration inlet pump increases its operating frequency according to the set value of the total pressure transmitter of the ultrafiltration water production line (adjustable by approximately 50kPa). The subsequent reverse osmosis / nanofiltration system high-pressure pump inlet water is drawn from the ultrafiltration constant-pressure water production main. During the commissioning period, after the recovery rate of each module is adjusted according to the concentrate end stop valve, it can be put into automatic operation. As the system operates, the transmembrane pressure differential will gradually increase, and the high-pressure pump will operate at a variable frequency according to the water production set by the module.

[0051] (2) Ultrafiltration unit backwash:

[0052] When the UF system's water production cycle (60-90 minutes, adjustable) is complete, the backwash process begins, following the same procedures as the conventional non-indirect connection method. During backwash, the UF modules in that group are unable to filter water. However, by increasing the opening of the inlet regulating valves in other modules, the peak flux of the UF membranes can be utilized to meet the system's total water production needs. Furthermore, the UF system's excess water production is sufficient to meet the backwash demand in real time, and backwash water is drawn from the backwash reservoir. This process does not significantly affect the pressure and flow rate in the UF water mainline.

[0053] (3) Forward flushing after backwashing of the ultrafiltration unit:

[0054] After each water production cycle, backwashing often involves a forward flush. This flush uses raw water from the raw water pump to rinse the membrane surface. Because flush water does not permeate the membrane fibers, the resistance loss through the module is lower than during normal water production. Without any system adjustments, at a given total water production pressure, the flush water volume flowing through this module will be greater than during normal production. Therefore, during this operation, the raw water pump not only provides the forward flush water volume but also meets the water needs of the subsequent systems, resulting in a total water supply greater than that provided during normal filtration and backwashing. First, the opening of the water production module's inlet valve can be further increased to reduce the resistance of the water production module. Second, the opening of the forward flush module's inlet regulating valve can be decreased to increase the flush module's resistance and reduce the amount of flush water discharged. Third, the opening of the ultrafiltration excess water valve can be automatically adjusted to ensure the water volume entering the subsequent reverse osmosis / nanofiltration system. These three factors ensure relatively stable water volume and pressure in the subsequent systems.

[0055] (4) Chemical cleaning process of ultrafiltration unit:

[0056] The ultrafiltration system will be chemically cleaned after running for about one month. The chemical cleaning process lasts about two hours, during which backwashing or flushing will occur, resulting in the two sets of ultrafiltration modules being unable to filter and produce water, affecting the water volume of the subsequent reverse osmosis / nanofiltration system. At this time, one of the reverse osmosis / nanofiltration modules needs to be stopped (the number of shutdowns can be calculated based on the reduced water volume).

[0057] (5) Reverse osmosis / nanofiltration module positive flushing process:

[0058] The reverse osmosis / nanofiltration modules undergo a daily maintenance flush and each startup and shutdown. This flush uses the reverse osmosis / nanofiltration system's produced water, drawn directly from the reservoir. Even if the modules involved in the flush are not producing water properly, because the ultrafiltration water production mainline to each reverse osmosis / nanofiltration module is connected in parallel, each module operates at a constant flow rate. Even if one module fails to produce water, it will not significantly affect the pressure and water production of the ultrafiltration water production mainline. Excess ultrafiltration flow caused by the failure of the forward flush modules to produce water can be adjusted and discharged by increasing the opening of the regulating valve in the ultrafiltration excess water production pipeline.

[0059] (6) Chemical cleaning process of reverse osmosis / nanofiltration module:

[0060] The reverse osmosis / nanofiltration cleaning cycle is approximately 1-3 months, lasting 3-6 hours (the specific cleaning cycle and duration are determined by the contamination situation). While one module is undergoing chemical cleaning, the other modules are minimized from participating in the positive flushing process. Therefore, the system operates the same as the positive flushing process described above. However, if one module is currently being flushed, meaning two modules are unable to produce water normally, the ultrafiltration system will have excessive water supply, and one ultrafiltration module will need to be shut down (the number of ultrafiltration modules to be shut down can be calculated based on the actual water production capacity of the reverse osmosis / nanofiltration module).

[0061] (7) Ultrafiltration and nanofiltration straight pipe connection control algorithm:

[0062] The difficulty in connecting ultrafiltration and nanofiltration (UF) straight pipes lies in maintaining the relative stability of the pressure and flow in the connecting pipes. When either the UF or RO / NF modules undergo flushing or chemical cleaning, the pressure in the UF water main fluctuates, causing significant pressure variations in the high-pressure pumps of each RO / NF system while operating at a constant flow rate. Flow matching between the UF and RO / NF systems is also crucial to ensure efficient water production. Therefore, maintaining a tight pressure and flow range during these six critical steps ensures safe and stable system operation.

[0063] This control section primarily involves two aspects: pressure and flow. Pressure stabilizes the system, while flow ensures production. Flow is calculated using a reverse calculation, i.e., the target production. Pressure is compensated using a forward compensation method, ultimately achieving relative stability in pressure and flow.

[0064] like Figure 3 As shown, the output of the ultrafiltration system is set to: Fud, the design safety pressure of the nanofiltration inlet water is: Pd, the nanofiltration set output is: Fd, and the straight pipe connection loss output is: Fl.

[0065] 1) Fud=Fd+Fl

[0066] 2) Fl = Fw + ΔF, where Fw is the backwash water flow rate. During backwash water replenishment, it is a constant. When water replenishment is not required, Fw = 0. ΔF is the system loss flow rate, which is the inevitable flow loss caused by instruments, pipelines, pressure, etc. This value is uncertain but very small.

[0067] 3) Pd = α*(Fd + ΔFp), where α is the flow-pressure conversion coefficient, ΔFp is the flow-pressure conversion pressure loss difference, and Pi is the initial pressure value (i.e., the pressure when the straight pipe flow is 0). It is a constant in engineering and depends on the installation height of the pressure gauge. In the same pipeline, the change in flow can be expressed by this formula.

[0068] 4)Pd=α*(Fud-Fw+ΔF+ΔFp)-Pi

[0069] 5)Pd=α*(Fud-Fw+ΔFt)-Pi where ΔFt=ΔF+ΔFp

[0070] 6) Pd + Pi = α * (Fud - Fw + ΔFt)

[0071] Let P = Pd + Pi. Since Pi is a constant, it can be concluded that to stabilize Pd within a certain range, it is only necessary to stabilize P. In practice, P is the real-time value of the pressure transmitter of the ultrafiltration water main.

[0072] 7) P = α*(Fud-Fw+ΔFt)

[0073] Equation 6 shows that P is primarily determined by the backwash feed flow rate and ΔFt. ΔFt can be considered a noise factor. Although small, flow rates are cumulative. Over time, accumulated flow can cause pressure to deviate significantly from the design value. In a straight pipe connection, it's essential to ensure that P remains stable within a certain range, which also offers the advantage of convenient operation. To maintain P within a certain range, Fud - Fw + ΔFt must be within a certain range.

[0074] 8) Fud-Fw+ΔFt∈[Fud-Fw+ΔFt1-ΔFt2,Fud-Fw+ΔFt1+ΔFt2], where Fud-Fw, ΔFt1, ΔFt1 are all greater than 0, ΔFt=ΔFt1±ΔFt2; ΔFt1, ΔFt2 are only for the convenience of mathematical conversion and have no engineering significance.

[0075] 9) Fud-Fw+ΔFt∈[Fud+ΔFt1′-ΔFt2, Fud+ΔFt1′+ΔFt2], where ΔFt1′=ΔFt1-Fw.

[0076] From Equation 9), it can be seen that if the pressure is stable within a certain range, it can be kept within the certain range by simply configuring the two parameters ΔFt1' and ΔFt2.

[0077] In actual production operation, users only need to configure the values ​​of ΔFt1 and ΔFt2 on the host computer. This value is mainly related to the diameter of the straight pipe connection, liquid properties, pressure sensor installation height, etc.

[0078] In summary, the direct-connected dual-membrane system for the water supply plant described in the embodiment of the present invention eliminates the ultrafiltration water production pool. The subsequent system directly draws water from the ultrafiltration water production mother pipe. The water flow in the pipe has a flushing flow rate, there is no dead zone, and there is no problem of biological secondary contamination. Therefore, the biological and organic contamination of the reverse osmosis / nanofiltration membrane elements is reduced, and the frequency of membrane cleaning is reduced, thereby ensuring the performance and service life of the membrane elements (the service life of the reverse osmosis / nanofiltration membrane elements is directly related to the cumulative cleaning time); the elimination of low-pressure water inlet pumps and safety filters and other equipment reduces equipment investment, while shortening the process flow, simplifying control, and avoiding the replacement of filter elements and the operating costs caused by the operation of low-pressure pumps; in addition, the control method of the direct-connected system ensures the stability of the entire dual-membrane system. When the number of ultrafiltration groups is less than 4, when one ultrafiltration group is backwashed, the total ultrafiltration water production cannot meet the reverse osmosis / nanofiltration water inlet. It is necessary to reduce the number of reverse osmosis / nanofiltration operating groups to maintain the pressure of the ultrafiltration water production pipe and achieve front-to-back flow matching. The direct-connect system ultrafiltration backwash pump also takes water directly from the ultrafiltration water production mother pipe.

[0079] Although the above describes the specific implementation methods of the present invention in conjunction with the accompanying drawings, it does not limit the scope of protection of the present invention. Technical personnel in the relevant field should understand that on the basis of the technical solution disclosed in the present invention, various modifications or variations that can be made by technical personnel in this field without creative work should be included in the scope of protection of the present invention.

Claims

1. A water plant direct-connected double membrane system, characterized in that: include: The ultrafiltration water inlet pool (1) is connected to a plurality of self-cleaning filters (3) via a plurality of ultrafiltration water inlet pumps (2); The output ends of the plurality of self-cleaning filters (3) are connected to a main pipe and then connected to a plurality of ultrafiltration membrane modules (4); The output end of each ultrafiltration membrane assembly (4) is connected to each reverse osmosis / nanofiltration high-pressure pump (5) through a main pipe and is connected to the corresponding reverse osmosis / nanofiltration membrane assembly (6); The output end of each ultrafiltration membrane assembly (4) is connected through a main pipe and then connected to an ultrafiltration backwash water tank (7) through a branch pipe; The output end of each reverse osmosis / nanofiltration membrane assembly (6) is commonly connected to a reverse osmosis nanofiltration water production pool (8); The reverse osmosis nanofiltration water production pool (8) and the ultrafiltration backwash water pool (7) are connected to a mixing well (9).

2. The water supply plant direct-connected double-membrane system according to claim 1, characterized in that: A water inlet regulating valve is provided on the water inlet pipe of the ultrafiltration membrane assembly (4).

3. The water supply plant direct-connected double-membrane system according to claim 1, characterized in that: An ultrafiltration water inlet total electromagnetic flowmeter is provided on the communication pipe between the ultrafiltration water inlet pump (2) and the self-cleaning filter (3).

4. The water supply plant direct-connected double-membrane system according to claim 1, characterized in that: A water inlet flow meter is provided on the input pipe of the ultrafiltration membrane assembly (4), or a water production flow meter is provided on the output pipe of the ultrafiltration membrane assembly.

5. The water supply plant direct-connected double-membrane system according to claim 1, characterized in that: An ultrafiltration water production main pipe pressure transmitter is provided on the communication pipeline between the reverse osmosis / nanofiltration high-pressure pump (5) and the ultrafiltration membrane assembly (4).

6. The water supply plant direct-connected double-membrane system according to claim 1, characterized in that: An ultrafiltration excess water electromagnetic flowmeter is provided on the communicating pipe between the water production main pipe of the ultrafiltration membrane assembly (4) and the ultrafiltration backwash water pool (7).

7. The water supply plant direct-connected double-membrane system according to claim 1, characterized in that: An ultrafiltration excess water production pipeline regulating valve is provided on the communicating pipeline between the ultrafiltration membrane assembly (4) water production main pipe and the ultrafiltration backwash water pool (7).

8. The water supply plant direct-connected double-membrane system according to claim 1, characterized in that: The ultrafiltration backwash water pool (7) is connected to the ultrafiltration membrane assembly (4) via an ultrafiltration backwash water pump.

9. The water supply plant direct-connected double-membrane system according to claim 1, characterized in that: An output pipe of each reverse osmosis / nanofiltration membrane assembly (6) is provided with a reverse osmosis / nanofiltration water production electromagnetic flowmeter.

10. The water supply plant direct-connected double-membrane system according to claim 1, characterized in that: An output pipe of each reverse osmosis / nanofiltration membrane assembly (6) is provided with a reverse osmosis concentrated water electromagnetic flowmeter.

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