March 21, 2024     Posted by :

Water Quality Requirements

Reverse osmosis as a new type of pure physical desalination process, due to the structure of the reverse osmosis membrane element, material, desalination mechanism and other conditions of the limitations of the reverse osmosis equipment on the feed water has high conditions requirements:

Temperature conditions between 1 and 45 ℃;.

The pH value must be within the range of 2 to 11.

Organic matter content (COD, mg/L) should be less than 1.5.

Turbidity (NTU) should be controlled below 1.0; and

Silt density index (SDI value) <4.0; and

Residual chlorine content <0.1mg/L (actual control at 0); and

Iron content (mg/L): Fe<0.05 when dissolved oxygen>5mg/L;

Silicon dioxide content (mg/L): SiO2 <100 in concentrated water; LSI: pHb-pHs <0; and

Sr, Ba and other ions that are easy to form insoluble salts: Ipb<0.8Ksp.


The latter three by adding scale inhibitors can be appropriate to improve its value.


If one or more of the above indicators do not meet the standard, the reverse osmosis membrane will cause the following effects:

1, RO reverse osmosis membrane contaminated by metal oxides.

2, colloid contamination.

3, RO reverse osmosis membrane fouling.

4, suspended solids fouling RO reverse osmosis membrane.

5, organic matter and microorganisms and other pollution, resulting in increased COD in the water.


In turn, the entire reverse osmosis device to cause adverse effects:

Reduce the reverse osmosis pure water system of water production; reduce the reverse osmosis pure water system of water quality; increase the reverse osmosis equipment operation of energy consumption, including raw water, electricity consumption; increase the running cost of water treatment, including reverse osmosis scale inhibitor, resin regeneration salt, other water treatment chemicals.


Reverse osmosis pretreatment plays an irreplaceable role in it. When the pretreatment is not good, reverse osmosis water quality is seriously substandard, and the time is too long, it will lead to reverse osmosis membrane element irreversible physical and chemical damage, greatly shorten the service life of reverse osmosis membrane element.


The purpose of reverse osmosis pretreatment is to solve the following problems to ensure the stable operation and service life of reverse osmosis equipment.

First, to prevent scaling on the membrane surface (including CaCO3, CaSO4, SrSO4, CaF2, SiO2, iron and aluminum oxides, etc.).

Second, to prevent the fouling of organic substances.

Third, to prevent the fouling of microorganisms.

Fourth, to prevent colloidal substances and suspended solids fouling.

Fifth, to maintain the stability of the reverse osmosis device water production.

The process flow for industrial RO system

Raw water tank
Storing raw water, used to precipitate large sand particles and other precipitable substances in the water. At the same time, buffer the impact of unstable water pressure in the raw water pipe on the water treatment system. (Such as water pressure is too low or too high caused by the reaction of the pressure sensing).

Raw water pump
Constant system water supply pressure, stabilize the water supply.

Multi-media filter
Adopting the filter with multiple filtration layers, the main purpose is to remove the sediment, rust, colloidal substances, suspended solids and other particles above 20um contained in the raw water, and manual valve control or full-automatic controller can be selected to carry out a series of operations, such as backwashing, positive flushing, and so on. Ensure the quality of water produced by the equipment and extend the service life of the equipment.

Activated carbon filter
The system adopts shell activated carbon filter, activated carbon can not only adsorb electrolyte ions, but also carry out ion exchange adsorption. The activated carbon adsorption can also make the potassium permanganate oxygen consumption (COD) from 15mg/L (O2) down to 2-7mg/L (O2), in addition, due to the adsorption effect so that the surface is adsorbed by the increase in the concentration of replicates, and thus also play a catalytic role, remove the pigment in the water, odor, a large number of biochemical organics, reduce the residual chlorine value of water and pesticide pollutants and remove the water of the trihalogenated compounds (THM), as well as other pollutants. pollutants in the water. At the same time, the equipment has a self-maintenance system and low operating costs.

Ion Softening System/Dosing System
In order to concentrate and discharge dissolved solids and utilize fresh water in R/O device, in order to prevent the concentration of CaCO3, MgCO3, MgSO4, CaSO4, BaSO4, SrSO4, SiSO4 from precipitating out of the concentrated water side of the concentrated water side of the last membrane module of the RO device, which is greater than the equilibrium solubility constant, and to damage the original membrane properties, an ion softening system should be used prior to the entry into the reverse osmosis (RO) membrane module. Before entering the reverse osmosis membrane module, use ion softening device or put appropriate amount of scale inhibitor to prevent the crystal precipitation of carbonate, SiO2, sulfate.

Precision Filter
The use of precision filters to remove the residual suspended solids, non-curved particles and colloids in the feed water makes the RO system and other subsequent equipment run more safely and reliably. The filter element is 5um meltblown filter element, the purpose is to remove the impurities larger than 5um that are missed by the upper filtration unit. It prevents them from entering the reverse osmosis unit and damaging the surface of the membrane, thus damaging the desalination performance of the membrane.

Reverse osmosis system
A reverse osmosis unit uses sufficient pressure to separate the solvent (generally water) in a solution by passing it through a reverse osmosis membrane (or semi-permeable membrane), because the process is in the opposite direction of natural osmosis, hence the name reverse osmosis. Reverse osmosis method can be adapted to all kinds of salt content of raw water, especially in the high salt content of water treatment works, can obtain very good technical and economic benefits. The desalination rate of reverse osmosis method is increased, the recovery rate is high, the operation is stable, the floor space is small, the operation is simple, the reverse osmosis equipment removes most of the bacteria, colloid and large molecular weight organic matter while removing the salt.

Ozone sterilizer/ultraviolet sterilizer (optional)
Kill the bacteria produced by secondary pollution to ensure that the finished water health indicators.

Raw water → raw water tank → raw water pump → multi-media filter (quartz sand filter) → activated carbon filter → soft water processor (add scale inhibitor device) → precision filter → high-pressure pump → a reverse osmosis (RO) device → ultraviolet sterilization device (ozone sterilization device) → water point

Industrial Reverse osmosis system for each unit function

1.1 Pre-treatment equipment
Pretreatment equipment consists of raw water pressurization pump, quartz sand filter, active carbon filter, and automatic water softener and fine filter five units.
1.1.1 Raw water pressurizing pump
When the raw water is tap water, the tap water pressure should be greater than the quartz sand filter, activated carbon filter, automatic water softener and fine filter of the total resistance of water flow (water pressure difference), and can meet the requirements of entering the reverse osmosis mainframe water pressure, so it is necessary to set up a pressurized pump to meet the water pressure requirements.
1.1.2 quartz sand filter
Because the turbidity of tap water is generally 3 to 5 degrees, the water also contains a small amount of suspended solids and sediment. Therefore, it is necessary to set up a quartz sand filter to reduce the turbidity of the water to no more than 1 degree, to prevent clogging of the reverse osmosis membrane.
1.1.3 Activated carbon filter
Mainly removes free chlorine and organic matter in water to prevent free chlorine from oxidizing the reverse osmosis membrane. At the same time, it can also remove the odor in the water and improve the taste of pure water for drinking.
1.1.4 Automatic water softener
When the process water passes through the automatic sodium ion exchanger, Ca2+, Mg2+ and other cations in the water are exchanged with Na+ in the exchanger to reduce the hardness of the water, so that the water quality is softened.
1.1.5 Precision filter
The filter is the last filtration device before the tap water enters the membrane, it can effectively remove the sand filter, carbon filter has not removed the clean material larger than 5μm, can intercept the sand filter, carbon filter loss of broken sand and activated carbon powder, etc., so as to effectively protect the RO membrane is not subjected to or less polluted.

1.2 Reverse osmosis host
1.2.1 Water inlet conditions
The reverse osmosis host should be operated under the following water inlet conditions. It is very important to check whether your raw water is within this limit. Reverse osmosis feed water conditions that do not meet this standard will result in permanent and irrecoverable contamination and damage to the membrane packaged elements.
Minimum influent pressure: 2.8KG/CM2
Minimum influent flow rate: 1.5t/h
Water temperature: 4oC~45oC
PH range: 4~9
Hardness: 17mg/L (as CaCO3)
Turbidity: ≤1 degree
Total dissolved solids: TDS<1000mg/L
Iron: <0.1mg/L
Free chlorine: <0.5mg/L
Manganese: <0.05mg/L
Organic matter: COD<1mg/L

1.2.2 Relationship between inlet water temperature and water production capacity

The rated water yield of the equipment is set assuming an influent water temperature of 25oC. The water yield of the reverse osmosis system decreases as the feed water temperature decreases. In general, for every 1oC decrease in water temperature, the water production will decrease by 3%.


1.2.3 Names and functions of components High-pressure pump – high-pressure pump using southern vertical multi-stage centrifugal high-pressure water pump, which is an important component of the RO host, its role is to give the RO membrane conveying a certain amount of a certain pressure of the water source. Use should ensure that no idling, not long-term overload operation, often exclude air as required, should ensure that the electrical part of the dry. High-pressure pump water throttle valve – installed in the high-pressure pump after the stainless steel shut-off valve, its main role is to regulate and control the RO membrane water flow, it and the thick water regulator valve used in conjunction with the adjustment of the membrane pipe pressure and water supply. It should be partially closed when starting, and then slowly open after the pump starts, the opening degree should be moderate, and then adjust the indicators. RO membrane vessel – now using a stainless steel membrane vessel, the installation of the two ends of the end, should be coated with a layer of propylene glycol (glycerol) on the rubber O-ring, so that easy to disassemble, but also increase the sealing. Beware of damaging the seals during maintenance. RO membrane – RO membrane is the key component of the reverse osmosis host, plays a decisive role in the water yield and quality of the equipment, this equipment is a first-class selection of the U.S. Hydronics ESPA4-4040 low-pressure composite membrane, the second level of selection of Dow membrane. Conductivity meter – its main role is to display the conductivity of pure water when the equipment is running, the unit of measurement is μs/cm. Concentrated water adjustment valve – the valve is an important element of the reverse osmosis host, its main role is to regulate the pressure in the membrane tube to achieve the purpose of regulating the proportion of pure water and concentrated water. It is used in conjunction with the throttle valve of the high-pressure pump to better adjust the pressure in the membrane pipe, pure water production, etc. The valve should be opened before the equipment is turned on. The valve should be opened to a certain degree before the equipment is turned on to prevent the membrane pressure from suddenly rising above the limit when the equipment is started. Membrane flushing solenoid valve – Its main function is to open regularly, let concentrated water flow through, reduce the pressure in the membrane tube, increase the flow rate in the membrane tube, to achieve the purpose of flushing membrane, it is an important part of the reverse osmosis host. Pure water and concentrated water flow meter – Its function is to measure pure water and concentrated water flow rate, the unit of measurement is gallons per minute (GPM) or liters per minute (LPM). Through these two flow meters, you can clearly grasp the equipment at that time the amount of water production and concentrated water flow, you can visualize the ratio of pure water and concentrated water, in order to help us adjust the system condition according to the required ratio and production. Pressure gauge before high-pressure pump (0 ~ 10.5kg/cm2) – This gauge mainly displays the pressure of raw water before it enters the high-pressure pump. The table and the microporous membrane fine filter inlet pressure gauge with observation, you can determine whether the fine filter filter element failure or whether it should be cleaned. When the pressure difference is large, it indicates that the filter element should be cleaned, and if it still fails to meet the requirements after cleaning, it should be replaced. Pre-membrane and Thick Water Pressure Gauge (0~28kg/cm2) – The pre-membrane pressure gauge shows the water pressure when water enters the membrane, and the thick water pressure gauge shows the pressure from the last membrane outlet to the front of the thick water regulating valve. Two pieces of table with observation, you can know the pressure difference, which is very important in the actual operation. When adjusting the operating parameters, the membrane pressure gauge should be used as the basis for adjusting the system pressure, especially when the system pressure is at the upper limit and the membrane differential pressure is large and large, more attention should be paid to this point. Low-pressure pressure switch – The function of the low-pressure pressure switch is to set the value of water pressure according to the system process requirements to control the host operation or stop the insurance device. When the inlet water supply pressure is lower than the system set value, the pressure switch will automatically shut down the mainframe to prevent the high-pressure pump from idling when there is a lack of water or no water, which may cause damage to the pump. The unit uses a pressure switch that has been parameterized internally. It is set at a minimum value of roughly 0.35kg/cm2. Electrical control box – The inlet solenoid valve of the reverse osmosis mainframe, the low-pressure pressure switch, the flushing solenoid valve, and the start and stop of the high-pressure pump are all controlled by the DDG403b microcomputer controller, which has an input power supply of 220 volts. The electric control box is equipped with switches, fuses, microcomputer controllers, contactors, thermal protectors and other components.

1.3 Low-voltage power distribution device

The device input power supply 380 volts, the output has four 15A three-phase air switch and a 10A single-phase air switch, constituting the output power supply 380 volts in four circuits, respectively, on the raw water pressurization pump, 1 # high-pressure pumps, 2 # high-pressure pumps and cleaning pumps power supply. Another circuit 220 volts, sand filter, carbon filter and automatic water softener power supply. In order to make the raw water pressurization pump and reverse osmosis host to start at the same time, the low voltage distribution device is equipped with a contactor and thermal relay, with a control line connected to the reverse osmosis host to start components.

Industrial reverse osmosis system operation

2.1 Pre-treatment equipment and reverse osmosis host power on and off

2.1.1 Before starting the equipment, the operator should first familiarize himself with the name and function of each instrument on the electric control panel of the reverse osmosis host, the installation position of the instrument on the panel and the circuit diagram.

2.1.2 If the equipment is started for the first time, it should open the drain valve on the outlet pipe of the activated carbon filter, observe the drain with the naked eye, and close the drain valve after determining that there is no carbon in the water at the end of the water quality is clean. Then open the microporous membrane fine filter lower drain valve, to be clean water, close the drain valve.

2.1.3 Check the pretreatment equipment and reverse osmosis host components are normal, adjust the valve switch state, to ensure that the operation of the water circuit is smooth. Always open the pressurized water pump inlet valve, slightly open the pump bypass pipe return valve, the opening should be moderate. Close the control valve on the sand filter bypass pipe and open the sand filter inlet and outlet valves. At the same time, close the control valve on the carbon filter bypass pipe and open the carbon filter inlet and outlet valves. Open the thick water regulating valve of reverse osmosis host, and adjust the throttling valve after the high-pressure pump to a moderate state. Open the exhaust water valve at the top of the precision filter, and then close the exhaust water valve after the water pressure of the equipment is stabilized.

2.1.4 Close all four three-phase air switches and one single-phase air switch inside the low-voltage distribution device to send power.

2.1.5 Insert the power plugs of sand filter, carbon filter and full-automatic water softener into 220-volt sockets to turn on the power supply, and then adjust the controller to reach the operating state.

2.1.6 Reverse osmosis host electric control panel power switch, from OFF off position rotating to ON open position, the power supply is turned on so that the inlet solenoid valve opens, pressurized water pumps and high-pressure water pumps start at the same time.

2.1.7 reverse osmosis host power switch is open, microcomputer controller internal settings will flush solenoid valve immediately open, reverse osmosis host automatic flushing. Every time the power supply is turned off and started, automatic flushing is carried out once. When the power supply is turned off, the timing is canceled and restored to the zero position, and the timing is re-timed for the next startup.

2.1.8 When the water pressure is lower than the setting value of low pressure switch during the operation of the equipment, the low pressure switch operates, and the internal setting of the microcomputer controller shuts down the high pressure pump of the host. When the water pressure is restored, the host high-pressure pump starts automatically.

2.1.9 to be reverse osmosis host flushing end, appropriate adjustment of high pressure pump pump throttle valve, control membrane water pressure in front of not more than 15kg/cm2. at the same time adjust the thick water regulating valve, control the flow of pure water. After meeting the above conditions, then the thick water adjustment valve and high pressure pump throttle valve with the use of adjustment, so that the equipment recovery rate in the appropriate range. At any time, do not close the thick water adjustment valve completely, otherwise it will make the reverse osmosis host pressure suddenly rise, causing damage to the equipment or endangering the safety of the operator.

Industrial reverse osmosis system maintenance

3.1 In the night shutdown, can be used as raw water tap water on the quartz sand filter and activated carbon filter backwash. Because there is a certain pressure tap water, can still enter the sand filter and carbon filter through the pressurized water pump pump body and reflux valve.

3.2 According to the quality of raw water and the operation of the equipment, according to the user’s needs, can be set to run the automatic water softener cycle and time.

3.3 The quartz sand or active carbon in the sand filter or carbon filter is recommended to be replaced about once a year.

3.4 Precision filter, drain once a week, PP filter element in the precision filter, it is recommended to replace about once a month.

3.5 If it is not due to the temperature and pressure factors caused by the water production gradually reduced by 15%, or water quality gradually decline in excess of the standard, then the reverse osmosis membrane needs to be chemically cleaned.

3.6 In operation, due to a variety of reasons will occasionally appear some failure problems, problems should be viewed in detail after the operation records, analyze the causes of failure.

The number of daily operation should be recorded


Operation precautions:

(1), the hydrolysis of the reverse osmosis membrane is easy to cause the deterioration of the performance of the reverse osmosis device, for this reason, it is necessary to strictly control the PH value of the water, and the PH value of the feed water is within the range of 3-11.

(2), when the amount of injected sodium hypochlorite is not enough to make the free chlorine in the feed water can not be measured, in the reverse osmosis device on the membrane module there will be slime occurs, reverse osmosis device differential pressure will increase. However, for polyamide membrane, it is necessary to strictly control the free chlorine entering the membrane module. Strictly control the amount of free chlorine into the membrane module, more than the specified value will lead to the oxidative decomposition of the membrane.

(3), if the FI value exceeds the standard water supply reverse osmosis device as feed water, the surface of the membrane module will be attached to the dirt, which must be cleaned to remove the dirt.

(4), excessive feed water flow will make the membrane module early deterioration, so the feed water flow should not exceed the design standard value. In addition the flow rate of concentrated water should try to avoid less than the design standard value, in the concentrated water flow rate is too small under the conditions of operation, will make the reverse osmosis device? Installation of the pressure vessel uneven flow and due to excessive concentration and precipitation of dirt on the membrane module.

(5), reverse osmosis device of high-pressure pumps even if there is a very short period of time to interrupt the operation of the device may fail.

(6), reverse osmosis inlet pressure to maintain a proper margin, otherwise due to the lack of proper compaction, desalination rate will be reduced.

(7), reverse osmosis device should be stopped when the low-pressure feed water to replace the water in the reverse osmosis device. This is to prevent the precipitation of silica in the shutdown (in the winter when the water temperature drops).

(8), need to pay attention to the security filter pressure difference. The reason for the sharp rise in differential pressure is mainly due to the security filter turbidity leakage. On the contrary, there is a sharp drop in differential pressure due to the breakage of precision filter elements, as well as the security filter element fastening screws loose.

(9), when the reverse osmosis device inlet and outlet pressure difference exceeds the standard, indicating that the membrane surface has been contaminated or the water supply flow in the design value above. If the flow rate adjustment can not solve the problem of differential pressure, the membrane surface should be cleaned.

(10), in the summer water temperature is high, the water flow is too much, sometimes have to reduce the operating pressure, which will lead to a decline in the quality of produced water. In order to prevent this, you can reduce the number of membrane modules, while the operating pressure is still maintained at a high level.


No.  Cause Countermeasure
1 Reduction in the number of membrane modules Operate according to the designed number of membrane modules
2 Low pressure operation Operate according to designed baseline pressure
3 Pressure sealing of membrane module occurs When operating under conditions that greatly exceed the reference pressure, the membrane module will become compressed and must be replaced.
4 Decrease in operating temperature Operate at design temperature 25oC
5 Under high recovery rate conditions When operating at a recovery rate of more than 75%, the amount of concentrated water will decrease, so the concentration rate of the water in the membrane module will increase, resulting in a serious decline in the quality of the feed water. As the osmotic pressure of the feed water increases, the amount of permeated water decreases. In severe cases, salt scale will precipitate on the membrane surface. Water must be produced according to the design recovery rate.
6 Metal oxides and dirt adhering to the membrane surface Perform daily low-pressure flushing
7 Differential pressure rise in the reverse osmosis unit during operation. Improve the operation and management of pretreatment devices and improve the quality of reverse osmosis water

Cleaning of membrane modules with pharmaceuticals

After a period of normal operation of the reverse osmosis system, the reverse osmosis membrane elements will be contaminated by suspended or insoluble salts that may be present in the feed water, and physical or chemical cleaning is required when the following symptoms occur:

  • A 10 to 15% decrease in water production from normal values at normal feedwater pressure;
  • A decrease of 10 to 15% from the normal value by temperature calibration in order to maintain normal water production;
  • A decrease of 10 to 15% in the quality of produced water and an increase of 10 to 15% in the salt permeability;
  • An increase of 10-15% in feedwater pressure;
  • A significant increase in the differential pressure between the various sections of the system (there may be no instrumentation to detect this parameter).


Cleaning unit system generally take the following six steps:

  1. Preparation of cleaning solution

Add the agent to clean water, and the preparation water must be RO product water or deionized water that removes hardness and is free of excessive metals and residual chlorine. Ensure that the solution is thoroughly mixed and homogenized and that the temperature and PH are adjusted to the desired values.

  1. Low flow rate input of cleaning solution

First use the cleaning pump to mix the cleaning solution once, preheat the cleaning solution should be at a low flow rate. Then to the lowest possible cleaning fluid pressure replacement component of the raw water, the pressure only to reach enough to replenish the feed water to the concentrated water pressure loss can be, that is, the pressure must be low enough not to produce significant osmotic water production. The low-pressure displacement operation minimizes the re-precipitation of fouling onto the membrane surface and, as appropriate, discharges a portion of the concentrated water to prevent dilution of the cleaning solution.

  1. Recycling

When the raw water is replaced, the concentrated water line should appear in the cleaning fluid, so that the cleaning fluid cycle back to the cleaning tank and ensure that the cleaning fluid temperature is constant.

  1. Soak

Stop the cleaning pump and allow the membrane element to be completely immersed in the cleaning solution. Sometimes 1 hour of element immersion is enough, but for stubborn contaminants, need to extend the immersion time, such as immersion 10 ~ 15 hours or soak overnight. In order to maintain the temperature of the immersion process, a very low circulation flow rate can be used (about 10% of the flow rate shown in Table 1).

5, high-flow pump circulation

Circulate at the flow rate listed in Table 1 for 30 to 60 minutes. High flow rate can rinse off the pollutants washed down by the cleaning solution. If the contamination is serious, please use more than 50% of the flow rate specified in Table 1 will help cleaning, in high flow conditions, there will be too high pressure drop problem, the maximum allowable pressure drop of the unit parts for 1bar (15psi), the maximum allowable pressure drop of multi-element pressure vessel for the maximum allowable pressure drop of 3.5bar (50psi), in order to exceed the first as a limit.


Low-pressure flushing with clean water (RO product water or deionized water that removes hardness and is free of metal ions such as iron and chlorine), and rinsing solution in the flushing system can be used with pre-treated, qualified produced water, unless there is a corrosion problem. In order to prevent precipitation, the minimum rinsing temperature is 20oC.

industrial reverse osmosis system

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