Control method for slowing down differential pressure rise of reverse osmosis security filter

Abstract

The invention discloses a control method for slowing down differential pressure rise of a reverse osmosis security filter, and belongs to the technical field of water treatment. The invention aims to solve the problem that a filter is polluted and blocked due to microorganism breeding caused by an anaerobic environment caused by continuous addition of a reducing agent. According to the method, monitoring points are arranged at an ultrafiltration water outlet and a reverse osmosis water inlet, and a reducing agent dosing pump is intermittently started and stopped according to the logical relationship between the real-time residual chlorine concentration and a preset threshold value: the reducing agent dosing pump is started when the residual chlorine at any monitoring point exceeds the standard, and the reducing agent dosing pump is stopped when the two points both reach the standard. The core innovation lies in that the system actively maintains dissolved oxygen and trace residual chlorine in water during the shutdown period of the dosing pump, an aerobic and micro-oxidation synergistic antibacterial environment is formed, and breeding of anaerobic microorganisms is inhibited from the source. The invention further discloses a system for executing the method. The invention can effectively slow down the differential pressure rise of the security filter, prolong the equipment maintenance period and obviously reduce the operation cost.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of water treatment, in particular to a control method for slowing down the pressure difference rise of a reverse osmosis security filter. BACKGROUND

[0002] In the industrial fields of thermal power generation, chemical industry, etc., high-purity chemical make-up water is the key to ensure the safe and stable operation of boilers and related thermal equipment. Reverse osmosis (RO) technology is the mainstream process for preparing high-purity water at present, and the core component of reverse osmosis membrane is a precision component highly sensitive to chemical oxidants. In order to prevent microorganisms in water from breeding on the membrane surface to form biological fouling and affect water production efficiency and service life, it is necessary to add oxidizing bactericides such as sodium hypochlorite to the raw water for pretreatment before entering the reverse osmosis device.

[0003] However, if these bactericides residues (usually in the form of residual chlorine) directly contact the reverse osmosis membrane, it will cause irreversible chemical damage to the polyamide material of the membrane, and seriously shorten its service life. Therefore, in the prior art, a standard protection measure is to add reducing agents such as sodium bisulfite to the water before the water flow enters the reverse osmosis device to neutralize and remove the residual oxidants in the water quality.

[0004] In the general practice of the prior art, in order to ensure the absolute safety of the reverse osmosis membrane, the operator usually adopts the strategy of continuous, even excessive addition of reducing agents. Although this method can reliably remove residual chlorine, it also brings an unexpected and difficult to solve serious side effect: it not only neutralizes the residual chlorine, but also consumes all the valuable dissolved oxygen in the water quality, thus creating a stable, anaerobic environment inside the pipeline and equipment from the security filter to the reverse osmosis device.

[0005] This anaerobic environment provides an ideal breeding condition for specific anaerobic microorganisms such as sulfate-reducing bacteria (SRB). These microorganisms use sulfate in water and excess reducing agents as "food", multiply in large quantities in the relatively slow-flowing areas inside the security filter, and secrete highly viscous extracellular polymers to form biofilms that are difficult to remove. These biofilms will quickly adhere to and clog the micropores of the filter core of the security filter, causing the pressure difference between the inlet and outlet of the security filter to rise sharply in a short time.

[0006] This phenomenon eventually forces the operator to frequently interrupt the system operation to replace the expensive security filter, and accordingly increases the frequency of chemical cleaning of the downstream reverse osmosis system. This not only significantly increases the operation and maintenance cost of the entire water treatment system, but also reduces the automation level and operation stability of the system due to frequent intervention operation. Therefore, how to effectively protect the reverse osmosis membrane from oxidative damage while avoiding the secondary microbial fouling problem caused by excessive reduction is a technical problem that needs to be solved in the field. SUMMARY

[0007] The technical problem to be solved by the present application is that in the existing reverse osmosis water treatment process, in order to protect the reverse osmosis membrane from damage by oxidizing agents (such as residual chlorine), a continuous addition of reducing agents is usually used, but this will consume the dissolved oxygen in the water, forming an anaerobic environment. This anaerobic environment, combined with excess reducing agents, will promote the proliferation of anaerobic microorganisms such as sulfate-reducing bacteria, produce metabolites attached to the surface of the equipment, and further cause the pressure difference of the security filter to rise rapidly, increasing the system cleaning frequency and operation cost.

[0008] To solve the above technical problems, the present application provides a new control method and system, which aims to accurately control the addition of reducing agents, effectively protecting the reverse osmosis membrane and significantly inhibiting the microbial fouling problem caused by the anaerobic environment.

[0009] The first aspect of the present application provides a control method for slowing down the pressure difference rise of a reverse osmosis security filter, which is applied to a chemical makeup water treatment system comprising an ultrafiltration device, a security filter and a primary reverse osmosis device. The core of the method is to change the continuous addition of reducing agents to an intermittent, on-demand response closed-loop control. It determines the start and stop of the reducing agent dosing pump according to the logical relationship between the real-time water quality data and the preset threshold, so as to realize intelligent regulation and control of the system operating environment.

[0010] Specifically, the method comprises the following steps:

[0011] Firstly, the real-time residual chlorine concentration of the first preset monitoring point in the reverse osmosis system is obtained. In a preferred embodiment, the first preset monitoring point is located at the outlet of the ultrafiltration device that supplies water to the reverse osmosis system, and its data reflects the initial residual chlorine state of the water after pretreatment and entering the subsequent conveying pipeline.

[0012] Secondly, the real-time residual chlorine concentration of the second preset monitoring point in the reverse osmosis system is obtained. The second preset monitoring point is located at the inlet pipe of the primary reverse osmosis device, which is the last line of defense to protect the reverse osmosis membrane, and its data is directly related to the safety of the membrane element.

[0013] Then, the control system executes a judgment logic according to the obtained two real-time residual chlorine concentrations. When the real-time residual chlorine concentration at the first preset monitoring point is greater than the first preset threshold value, or the real-time residual chlorine concentration at the second preset monitoring point is greater than the second preset threshold value, the reducing agent dosing pump is started. This "or" logic ensures that as long as the residual chlorine of any key position is over-standard, the protection program is triggered.

[0014] In another condition, when the real-time residual chlorine concentration at the first preset monitoring point is less than or equal to the first preset threshold value, and the real-time residual chlorine concentration at the second preset monitoring point is less than or equal to the second preset threshold value, the reducing agent dosing pump is stopped. This "and" logic ensures that only when the residual chlorine of the whole system is within the safe range, the dosing is stopped, avoiding the waste of reagent.

[0015] The innovation of the present application is that, during the stoppage of the reducing agent dosing pump, instead of pursuing zero residual chlorine concentration, the real-time residual chlorine concentration at the first preset monitoring point is actively maintained within a preset bacteriostatic operation interval, for example, 0.01 mg / L to 0.1 mg / L, by regulating the front-end water treatment process. This bacteriostatic operation interval can effectively inhibit the reproduction of sulfate-reducing bacteria in the reverse osmosis system (especially in the pipelines and security filters) by utilizing the slight residual chlorine itself, thereby breaking the anaerobic environment required for their breeding and slowing down the formation of biological fouling from the root. At the same time, due to the discontinuous dosing of reducing agent, a higher dissolved oxygen is retained in the water, further inhibiting the growth of anaerobic bacteria.

[0016] In a specific embodiment, to achieve precise control, the first preset threshold value can be set to 0.1 mg / L, and the second preset threshold value can be set to 0.05 mg / L. At this time, the start-stop logic of the dosing pump can be defined by the following mathematical model:

[0017] Let the real-time residual chlorine concentration at the outlet of the ultrafiltration device be C UF , the real-time residual chlorine concentration at the inlet pipe of the primary reverse osmosis device be C RO , the first preset threshold value be C , and the second preset threshold value be C

[0018] The start condition is that when C , the reducing agent dosing pump is started.

[0019] The stop condition is that when C , the reducing agent dosing pump is stopped.

[0020] The reducing agent used can be sodium bisulfite or sodium sulfite solution.

[0021] The second aspect of the present application provides a control system for slowing down the pressure difference rise of a reverse osmosis security filter, which is used to execute the aforementioned method, and comprises:

[0022] A first monitoring unit is configured to obtain the real-time residual chlorine concentration at the first preset monitoring point.

[0023] A second monitoring unit is configured to obtain the real-time residual chlorine concentration at the second preset monitoring point.

[0024] A reducing agent dosing unit comprises a controllable reducing agent dosing pump.

[0025] and a controller.

[0026] The controller is electrically connected with the first monitoring unit, the second monitoring unit and the reducing agent dosing unit, and is internally programmed with the aforementioned control logic, which can compare the real-time residual chlorine concentration data received from the monitoring unit with the first preset threshold value and the second preset threshold value, and automatically send instructions to start or stop the reducing agent dosing pump according to the judgment result.

[0027] The present application provides a control method for slowing down the pressure difference rise of a reverse osmosis security filter, which has the following beneficial effects:

[0028] 1. By changing the continuous dosing to intermittent control, the present application stops adding reducing agent during most of the operation time, thereby preserving the dissolved oxygen in the water quality and avoiding the formation of anaerobic environment. At the same time, the trace residual chlorine maintained by the system can continuously inhibit microbial activity. This synergistic effect of aerobic and micro-aerobic oxidation prevents the massive reproduction of anaerobic bacteria (such as sulfate-reducing bacteria) that cause biological fouling, thereby significantly slowing down the pressure difference rise rate of the security filter.

[0029] 2. Since the formation and development of biological fouling in the system are effectively inhibited, the present application directly prolongs the maintenance period of key consumables and equipment units in the system. Due to the slowed down pressure difference rise of the security filter, the frequency of replacing the filter element is greatly reduced. At the same time, the biological load entering the primary reverse osmosis device is reduced, which also prolongs the chemical cleaning period of the reverse osmosis membrane system, thereby reducing the system downtime maintenance time.

[0030] 3. The control method of the present application brings significant economic benefits. On the one hand, the reducing agent is only added during a short period of time when the residual chlorine concentration is abnormal, and the total consumption is greatly reduced compared to the traditional continuous dosing method. On the other hand, the reduction of filter element replacement and chemical cleaning frequency also directly reduces the cost of spare parts and related maintenance labor costs, thereby reducing the overall operation cost of the system.

[0031] 4、The application adopts double monitoring point closed loop automatic control, and converts residual chlorine management from open loop operation depending on artificial experience into automatic process based on real-time data decision. The system can independently cope with upstream water quality fluctuation, and can complete abnormal response and automatic recovery without artificial intervention, greatly improving operation stability, reliability and automation level of the whole chemical make-up water treatment system.

[0032] 5、The double preset monitoring point layout of the application constructs a hierarchical and feedforward safety protection system. The ultrafiltration effluent monitoring point located in the upstream plays a "early warning" role, and can start protection measures in advance before the polluted water mass reaches the reverse osmosis membrane. The inlet monitoring point located in the downstream is the final "defence line", and ensures that the water quality entering the membrane element is absolutely safe. The double insurance design provides more careful and reliable protection for the expensive reverse osmosis membrane compared with single point control. BRIEF DESCRIPTION OF DRAWINGS

[0033] Fig. 1 It is a structure schematic diagram of the chemical make-up water treatment system of the embodiment of the application.

[0034] Fig. 2 It is a control method flow chart of the embodiment of the application.

[0035] Fig. 3 It is a controller internal function module schematic diagram of the embodiment of the application. DETAILED DESCRIPTION

[0036] In order for those skilled in the art to better understand and implement the application, a preferred embodiment is described in detail below with reference to the accompanying drawings. This embodiment is intended to illustrate the technical solutions of the application in detail, but the protection scope of the application should not be limited by this embodiment.

[0037] REFERENCE Figs. 1 to 3 The application provides a control method for slowing down pressure difference rise of a reverse osmosis security filter. The system aims to achieve technical balance between ensuring reverse osmosis membrane safety and inhibiting microorganism breeding through an intelligent intermittent dosing strategy.

[0038] The chemical make-up water treatment system is specifically constructed as follows: the water flow to be treated first enters an ultrafiltration device, removes impurities such as suspended solids and colloids in the water through membrane separation. The water flowing out of the ultrafiltration device then enters a security filter for final precision filtration to protect downstream equipment. The water filtered by the security filter is finally delivered to a primary reverse osmosis device for core desalination treatment.

[0039] To implement the control method of this invention, a reducing agent dosing system is installed in the pipeline between the downstream of the security filter and the upstream of the first-stage reverse osmosis unit 30. This system can precisely add reducing agent to the water flowing into the first-stage reverse osmosis unit according to instructions. The automated operation and logical decision-making of the entire system are coordinated and managed by a central controller, which can be a programmable logic controller (PLC) or part of a distributed control system (DCS).

[0040] To achieve precise closed-loop control, the system sets up online water quality monitoring points at two key locations. The first preset monitoring point is located on the outlet pipe of the ultrafiltration unit to obtain the real-time residual chlorine concentration of the water leaving the ultrafiltration treatment unit. The second preset monitoring point is located on the inlet pipe of the first-stage reverse osmosis unit, physically positioned after the dosing point of the reducing agent dosing system, ensuring direct and accurate monitoring of the final residual chlorine concentration in the water that will come into contact with the reverse osmosis membrane elements.

[0041] The control method described in this invention is essentially a set of logic programs executed by a controller. The core of this method lies in dynamically deciding the operating state of the dosing pump in the reducing agent dosing system based on real-time data obtained from the two preset monitoring points. This decision-making process can be precisely described by the following state function:

[0042] In this function, the symbols are defined as follows:

[0043] P status : Represents the operating status of the reducing agent dosing pump. It is a binary variable, where "1" defines the operating status and "0" defines the shutdown status.

[0044] C UF : Represents the real-time readings measured at the first preset monitoring point located at the outlet of the ultrafiltration device.

[0045] Residual chlorine concentration.

[0046] C RO : Represents the real-time residual chlorine concentration measured at the second preset monitoring point located at the inlet pipe of the first-stage reverse osmosis unit.

[0047] This represents the upper limit threshold for residual chlorine concentration control set for the first preset monitoring point, i.e., the first preset threshold.

[0048] This represents the upper limit threshold for residual chlorine concentration control set for the second preset monitoring point, i.e., the second preset threshold. This value is also a key action threshold for protecting the reverse osmosis membrane.

[0049] The combination of the system composition and the core control model lays a foundation for subsequent detailed execution process and principle elaboration.

[0050] In the embodiment, in order to realize accurate execution of the foregoing control method, key hardware deployment and control parameters in the system are carefully set. Reasonable configuration of the hardware and careful selection of the parameters are the cornerstone for reliable operation of the technical solution of the application.

[0051] In order to realize continuous and accurate monitoring of the residual chlorine concentration in the water quality, the first and second preset monitoring points relied on by the application are both configured with high-precision online residual chlorine analyzers. Specifically, the analyzer at the first preset monitoring point is physically installed on the water outlet main pipe of the ultrafiltration device, and its function is to capture the residual chlorine baseline value of the water quality that has just completed pretreatment and is about to enter the subsequent pipeline system. The analyzer at the second preset monitoring point is installed on the water inlet pipeline of the primary reverse osmosis device, and its installation position is after the injection point of the reducing agent dosing system and before the reverse osmosis membrane assembly, so as to ensure that its measurement value can truly and finally reflect the water quality state about to contact the reverse osmosis membrane.

[0052] The specific value of the first preset threshold value S and the second preset threshold value S is determined based on the dual consideration of system safety and operation efficiency. In the embodiment, the second preset threshold value S is strictly set to 0.05 mg / L. The threshold value is set according to the high sensitivity of the reverse osmosis membrane element to oxidizing agents, and setting this value at a very low level is the final safety line to protect the membrane element from any cumulative oxidative damage. Any residual chlorine higher than this concentration is considered a direct threat to the primary reverse osmosis device.

[0053] Correspondingly, the first preset threshold value S UF_high is set to 0.1 mg / L in the embodiment. This value is slightly higher than the second preset threshold value S RO_high , and the technical intention is to establish a “warning” mechanism. When the residual chlorine concentration of the ultrafiltration device outlet water exceeds 0.1 mg / L, even if the residual chlorine of the primary reverse osmosis device inlet water has not exceeded the standard at this time, this condition indicates that the upstream chlorination process has fluctuated or abnormally. The addition of the reducing agent is triggered in advance, which can neutralize the water quality with excessive residual chlorine before it reaches the reverse osmosis membrane, providing valuable time buffer for system response, and embodying a layered and progressive protection strategy.

[0054] The reducing agent dosing system as the execution unit of the method, its core is a metering pump directly controlled by the controller. The metering pump is connected with a medicine box storing the reducing agent (sodium bisulfite solution in this embodiment). After receiving the real-time data from the two online residual chlorine analyzers, the controller will collect the C UF and C RO values at a high frequency (for example, once per second) and compare them with the preset and values. Once the result of the logical judgment meets the start condition, the controller immediately sends a digital signal instruction to the reducing agent dosing system to drive the metering pump to work; otherwise, when the stop condition is met, the controller sends an instruction to stop the reducing agent dosing system, thereby forming a complete and automatic closed-loop control circuit.

[0055] In this embodiment, the detailed execution process of the control method is embodied as a closed-loop process automatically switched between different operating modes led by the controller. The process seamlessly combines the daily operation of the system with the abnormal response, realizing intelligent management of water quality.

[0056] After the system is started, it defaults to the "normal monitoring mode". In this mode, the controller sets the operating state P status of the reducing agent dosing pump to the initial value "0", that is, the reducing agent dosing system is in the stop state and does not add any reducing agent to the water quality. This mode is not passive waiting, but a core technical link of the invention. At this time, through accurate regulation of the chlorination unit (not shown in the figure) upstream of the ultrafiltration device, the real-time residual chlorine concentration C UF measured at the first preset monitoring point at the outlet of the ultrafiltration device is actively maintained within a preset bacteriostatic operating interval, for example, 0.01 mg / L to 0.1 mg / L. This ensures that the water entering the subsequent pipeline retains weak but effective oxidation ability, and also maintains the dissolved oxygen content in the water, creating an unfavorable environment for the growth of microorganisms.

[0057] The controller collects the real-time residual chlorine concentrations C UF and C RO from the first and second preset monitoring points at a very high frequency, for example, once per second or higher. The collected data is immediately sent to the preset logical judgment module and compared with the set threshold and values in real time to detect any residual chlorine fluctuations that may threaten the safety of the system.

[0058] When the residual chlorine concentration at any monitoring point exceeds the safety limit due to changes in upstream water quality during system operation, the system will automatically switch from the "normal monitoring mode" to the "emergency dosing mode". The specific triggering logic is as follows: once the controller detects or Either of the two conditions is true, the running state P status Switched from "0" to "1". The command is sent to the reducing agent dosing system to drive its metering pump to start immediately, starting to add reducing agent to the water flow into the primary reverse osmosis device.

[0059] After the reducing agent dosing pump starts running, the controller does not suspend its monitoring task, but continues to collect the values of C UF and C RO at the same frequency, closely tracking the neutralization effect of reducing agent on residual chlorine. With the addition of reducing agent, the residual chlorine concentration at the two monitoring points will quickly decrease.

[0060] When the residual chlorine in the water quality is fully neutralized, the system confirms that it has fully recovered to a safe state, and automatically switches back from "emergency dosing mode" to "normal monitoring mode". The judgment condition for this switch is more stringent, and both residual chlorine concentrations at the two monitoring points must meet the safety requirements. Specifically, only when the controller confirms and Two conditions are met at the same time, the running state P status Switched from "1" to "0", and the command to shut down the reducing agent dosing pump is issued. At this point, a complete response and recovery of the closed-loop control process is completed, and the system returns to the normal monitoring mode of efficient, energy-saving and microbial growth inhibition.

[0061] The technical solution of the present application can achieve the expected effect of slowing down the pressure difference rise of the security filter. The fundamental reason is that it overturns the traditional control concept of water quality chemical environment in reverse osmosis pretreatment, and builds a system environment that is not conducive to the survival of specific microorganisms, thereby inhibiting the formation of biological fouling from the source.

[0062] In the traditional continuous dosing of reducing agent process, excessive reducing agent (such as sodium bisulfite) not only neutralizes the residual chlorine, but also consumes valuable dissolved oxygen in the water quality, thereby creating a stable anaerobic environment in the entire pipeline system and equipment, especially in areas where the water flow is slow. This anaerobic environment is an ideal breeding ground for anaerobic microorganisms such as sulfate-reducing bacteria (SRB). These microorganisms use naturally occurring sulfate and excess reducing agent in water as raw materials for metabolism, reproduce in large numbers and secrete sticky extracellular polymers, forming biofilms that firmly adhere to the inner wall of the pipeline, the surface of the security filter and even the surface of the reverse osmosis membrane, which is one of the core reasons for system fouling and rapid pressure difference rise.

[0063] The control method of the present application first changes continuous dosing to intermittent dosing, stopping the addition of reducing agent during most of the operating time. The direct chemical consequence of this change is that the original dissolved oxygen in the water is completely preserved, so that the entire water system from the ultrafiltration device to the primary reverse osmosis device is maintained in an aerobic environment. The aerobic environment has a natural and strong inhibitory and killing effect on strict anaerobic microorganisms such as sulfate-reducing bacteria, fundamentally destroying their living basis.

[0064] More importantly, the present application does not simply stop dosing, but actively maintains the real-time residual chlorine concentration at the outlet of the ultrafiltration device within a pre-set antibacterial operating interval that is accurately calculated, for example 0.01 mg / L to 0.1 mg / L, under normal monitoring mode. This trace amount of continuously existing residual chlorine is safe and controllable for reverse osmosis membranes (because of downstream monitoring and dosing protection), but it is a continuously effective low-concentration disinfectant for microorganisms in the pipeline and equipment. It can penetrate the cell wall of microorganisms, destroy the activity of the internal enzyme system, and thus directly inhibit their growth and reproduction, preventing the initial formation of biofilm.

[0065] Although embodiments of the present application have been shown and described, it is to be understood that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the present application, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A control method for mitigating the rise in differential pressure of a reverse osmosis security filter, characterized in that, Includes the following steps: Obtain the real-time residual chlorine concentration at the first preset monitoring point in the reverse osmosis system; Obtain the real-time residual chlorine concentration at the second preset monitoring point in the reverse osmosis system; When the real-time residual chlorine concentration at the first preset monitoring point is greater than the first preset threshold, or the real-time residual chlorine concentration at the second preset monitoring point is greater than the second preset threshold, the reducing agent dosing pump is activated. When the real-time residual chlorine concentration at the first preset monitoring point is less than or equal to the first preset threshold, and the real-time residual chlorine concentration at the second preset monitoring point is less than or equal to the second preset threshold, the reducing agent dosing pump is shut down.

2. The method according to claim 1, characterized in that, The first preset monitoring point is located at the outlet of the ultrafiltration device that supplies water to the reverse osmosis system, and the second preset monitoring point is located at the inlet pipe of the first-stage reverse osmosis device.

3. The method according to claim 1 or 2, characterized in that, During the period when the reducing agent dosing pump is shut down, the water quality entering the reverse osmosis system is controlled to maintain the real-time residual chlorine concentration at the first preset monitoring point within the preset antibacterial operating range.

4. The method according to claim 3, characterized in that, The residual chlorine within the preset antibacterial operating range is used to inhibit the growth of sulfate-reducing bacteria in the reverse osmosis system.

5. The method according to claim 3, characterized in that, The preset antibacterial operating range is 0.01 mg / L to 0.1 mg / L.

6. The method according to claim 1, characterized in that, The first preset threshold is 0.1 mg / L, and the second preset threshold is 0.05 mg / L.

7. The method according to claim 2, characterized in that, The step of starting the reducing agent dosing pump is determined by the following starting conditions: the real-time residual chlorine concentration at the outlet of the ultrafiltration device is set to C. UF The real-time residual chlorine concentration at the inlet pipe of the first-stage reverse osmosis unit is C. RO ; Set the first preset threshold to The second preset threshold is When the condition is met At that time, start the reducing agent dosing pump.

8. The method according to claim 2, characterized in that, The step of stopping the reducing agent dosing pump is determined by the following stopping condition: the real-time residual chlorine concentration at the outlet of the ultrafiltration device is set to C. UF The real-time residual chlorine concentration at the inlet pipe of the first-stage reverse osmosis unit is C. RO ; Set the first preset threshold to The second preset threshold is When the condition is met When this happens, the reducing agent dosing pump should be shut down.

9. The method according to claim 1, characterized in that, The reducing agent is sodium bisulfite or sodium sulfite.

10. The method according to claim 1, characterized in that, The method is applied to a chemical makeup water treatment system, which includes at least an ultrafiltration unit, a security filter, and a first-stage reverse osmosis unit connected in sequence.

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