How to Design the Pretreatment Process of Reverse Osmosis System
Table of Contents
Due to the wide variety of raw water, its composition is also very complex. According to the requirements of the primary process design parameters, such as the quality of raw water and the recovery rate of the RO system, the appropriate pretreatment process system is selected to reduce the fouling and scaling of the RO membrane, prevent the reduction of RO membrane desalination rate and water production rate, especially for the current shortage of water sources and deterioration of water quality. Choosing the right pretreatment system will directly affect the function of the whole water treatment system. As we all know, the failure of RO system operation is mostly due to the imperfect part of the pretreatment system. The raw water must be strictly treated to ensure the regular operation of the reverse osmosis process.
What are the water sources for the reverse osmosis system?
Reverse osmosis water sources generally include surface water and groundwater. Surface water contains a wide range, such as rivers, lakes, reservoirs, oceans, etc. At the same time, groundwater is formed by the percolation of rainwater and surface water through strata existing in soil and rock strata. Surface water and groundwater are collectively referred to as natural water. In addition, with the advancement of water treatment technology, industrial water sources such as municipal secondary sewage and power plant cooling wastewater have also become new ways.
When natural water, municipal water, and industrial wastewater can be used as reverse osmosis water sources, there is the problem of water source selection: checking the available water sources and their corresponding characteristics to select a more economical and feasible water source. These factors should be considered when selecting a water source: ① safety of the water intake point; ② adequacy of water quantity; ③ quality of the water source; ④ water intake requirements (such as the structure of the intake, depth of the intake well); ⑤ treatment requirements (including the cost and flexibility of waste liquid disposal); ⑥ water delivery and water distribution requirements.
The water source selection will directly affect the determination of the water treatment process and the cost of water treatment. The selected water source should ensure a continuous water quantity supply and stable water quality. Strengthening water source management and water quality monitoring is conducive to discovering the impact of water quality changes on the water treatment process to find and solve problems in time.
What is the feed water quality of the reverse osmosis system?
The reverse osmosis water treatment system has high requirements for feed water due to the limitations of the structure, material, and desalination mechanism of the reverse osmosis membrane elements. The following table reflects reverse osmosis influent’s most critical water quality parameters.
| RO Feed Water Quality Requirements | |||
| Item | Unit | Limit Value | Consequences of non-compliance |
|---|---|---|---|
| Turbidity | NTU | <1 | Sludge, colloidal fouling, and clogging |
| SDI | mg/L | <5 | Sludge, colloidal fouling, and clogging |
| Hardness | mg/L | Consider with alkalinity | Insoluble salt scaling |
| Alkalinity | mg/L | Consider with hardness | Carbonate scaling |
| COD | mg/L | <60 | Organic pollution |
| BOD | mg/L | <10 | Organic pollution |
| Iron | mg/L | <0.05 | Iron pollution |
| Magnesium | mg/L | <0.1 | Manganese pollution |
| Insoluble salt | mg/L | Concentrated water side unsaturated | Insoluble salt scaling |
| Oil, surfactant | mg/L | Non-detectable | Oil pollution, flux decay |
| pH | / | 3~10 | Membrane element hydrolysis |
| Chloride | mg/L | <0.1 | Irreversible oxidation |
(Some of the indicators in the table must be considered according to different water sources)
For the characteristics of pollutants and the requirements of reverse osmosis feed water quality, the standard processes can be selected as follows:
- Common processes to remove suspended solids and colloids include coagulation and precipitation, fiber rotary filtration, media filtration, etc.
- The commonly used processes to remove hardness include dosing softening + precipitation, resin softening, and adding scale inhibitors.
- Common COD reduction processes include ozone oxidation, aerated biofilter, activated carbon filtration, etc.
By understanding the characteristics of each process and knowing how to select and combine these processes, you can design a reasonable pretreatment system. The long-term stable operation of the reverse osmosis system is not only the correct operation of the operation and maintenance personnel but also the strict and reasonable design of the designers to create all this.
What is the purpose of pretreatment of the reverse osmosis system?
Reasonable pretreatment is significant for the long-term safe operation of the reverse osmosis unit, the purpose of which is usually to
Remove suspended solids and reduce turbidity.
- Control the growth of microorganisms.
- Inhibit and control the deposition of micro-soluble salts.
- Adjustment of feed water temperature and pH.
- Remove organic matter.
- Control precipitation of metal oxides and silica.
What is the target of the pretreatment of reverse osmosis system?
To ensure the water recovery rate of the reverse osmosis system, the quality of the product water, the stability of the product water flow, the minimization of the operating costs, as well as the optimization of the membrane life, etc., it is necessary to carry out a perfect pretreatment, the objectives are
- To prevent pollution on the RO membrane surface, i.e., try to remove suspended solids, microorganisms, colloidal substances, and organic matter as much as possible to prevent these substances from depositing on the membrane surface or fouling in the water flow channel of the membrane element.
To prevent scaling on the surface of the RO membrane, it is necessary to inhibit the deposition of insoluble salts such as CaCO3, CaSO4, BaSO4, SrSO4, CaF2, and iron, manganese, aluminum, silicon compounds on the membrane surface as much as possible.
- Prevent RO membrane from physical and chemical damage; that is, it is necessary to avoid the effects of high temperature, extremely acidic or alkaline water, oxidizing agents, etc., on the membrane.
What are the regular pretreatment methods for reverse osmosis systems?
Depending on the specific raw water conditions and application requirements, pretreatment methods include the following, generally used in combination.
Chemical oxidation
Chemical oxidation treatment is usually a chemical method that uses strong oxidants to decompose organic pollutants in water. At the same time, some oxidants can remove color, taste, odor, iron, enzymes, and algae from the water. Some oxidizers are also good bactericides.
The chemical oxidant treatment process can be combined with coagulation, filtration, adsorption, and other unit operations to achieve good treatment results. The oxidants commonly used in industrial water treatment are chlorine, sodium hypochlorite, chlorine dioxide, potassium permanganate, ozone, hydrogen peroxide, etc.
Since oxidants very easily oxidize the aromatic polyamide reverse osmosis membrane, it is necessary to pay attention to the use of appropriate reduction methods in the pretreatment process to prevent the residual oxidant from entering the reverse osmosis system channel and causing oxidation damage to the membrane.
Chemical softening
Chemical softening is a process by which the hardness contained in the water is removed by forming insoluble compounds under the action of appropriate chemicals based on the principle of chemical precipitation and the direction of solubility product. The most common method in water treatment is the precipitation of calcium and magnesium ions, followed by the precipitation of oxychlorides of metal ions. Chemical softening is usually combined with flocculation, precipitation, or clarification.
Common softening agents for water treatment include lime, soda, toxic sodium, trisodium phosphate, disodium hydrogen phosphate, etc. According to different types of raw water quality, various agents can be combined for treatment. In general, the lime softening method is used for water with high hardness and high alkalinity, the lime-soda ash method is used for water with high hardness and low alkalinity, and the lime-gypsum method is used for water with high alkalinity and negative hardness (i.e., total alkalinity is greater than total hardness).
The use of chemical agents for softening must be correct. After softening treatment, the pH value of the water will generally increase, and the pH value should be reduced to a suitable value according to the situation.
Coagulation - flocculation
Coagulation refers to adding a certain amount of chemical agents to the water. These chemical agents hydrolyze in the water, collide with the colloidal particles in the water, and undergo electrical neutralization, resulting in adsorption, bridging, and net capture, thereby forming large floc particles and settling out of the water, which plays a role in reducing particle suspension and colloid.
Sometimes the coagulant alone does not achieve the desired effect; auxiliary chemicals can be added to improve the coagulation effect. This additive is called a coagulant aid. The coagulant aid cannot produce the same effect as the coagulant but can only enhance the floc structure and make the floc particles larger, stronger, and heavier. Therefore, using coagulant aid alone can not play an excellent effect.
Since the surface of most aromatic polyamide reverse osmosis membranes is negatively charged as a whole, if the amount of cationic coagulant or coagulant aid added in the pretreatment process is excessive, the excess cationic substances may be adsorbed on the membrane surface, resulting in ionic fouling of the membrane, which will be challenging to restore effectively. Therefore, adequate measures must be taken to monitor the accurate addition of coagulants and coagulant aids and to control the pH range.
Media filtration
Media filtration refers to separating water from water by using silica sand, garnet, or anthracite as media so that water passes through the bed composed of these media under gravity or pressure, and the particulate pollutants in the water are blocked by the media.
The design of a media filter mainly considers the selection of filter material type, particle size selection, filtration rate, retention time, cleaning method, backwash flow rate and time, backwash expansion space, and other parameters.
Activated carbon adsorption
Activated carbon adsorption is a method of removing one or more harmful substances in water by adsorption on a solid surface using activated carbon’s porous properties. Activated carbon adsorption has a good effect on removing organic matter, colloids, microorganisms, residual chlorine, and odor in water. At the same time, activated carbon has a particular reduction effect and a good removal effect on oxidants in water.
The adsorption function of activated carbon has a saturation value. When the saturation value and adsorption capacity are reached, the adsorption function of the activated carbon filter will be significantly reduced. Therefore, it is necessary to analyze the adsorption capacity of activated carbon, replace activated carbon in time or disinfect and restore by high-pressure steam. At the same time, the organic matter adsorbed on the surface of activated carbon may become a nutrient source or hotbed for bacterial reproduction. Therefore, the problem of microbial reproduction in activated carbon filters is also worthy of attention. Regular disinfection is necessary to control bacterial reproduction.
Disc filter or mesh filter
The disc filter, also known as a laminated filter, is a parallel combination of filter units. The filter unit consists of ring-reinforced plastic filter discs with grooves or edges. When the wastewater is filtered, the wastewater enters from the outside, and the rim formed by the grooves or edges on the adjacent filter disc intercepts the solids in the water. During backwashing, the water flows from the inside to the outside of the annular filter disk, and the pollutants retained on the filter disc are flushed and discharged through the wastewater outlet.
The disc filter has the characteristics of a large processing function, stable operation, and easy control. It is divided into semi-automatic and full-automatic flushing, and the latter is easier to operate with modular control. It is mainly used for pretreatment of ultrafiltration, which plays a safety role in ultrafiltration.
The mesh filter is mainly a self-cleaning filter, which is a kind of precision equipment that uses the filter mesh to intercept the impurities directly, suspended matter, particles, etc., in the liquid and, at the same time, reduce the turbidity of the water, reduce the fouling, and ensure the regular operation and service life of other equipment in the system. It has the feature of automatic wastewater discharge. Raw water flows into the water inlet end of the self-cleaning filter, then passes through the fine mesh from the inside out and flows out of the water outlet of the filter. During this process, impurities accumulate on the inner surface of the screen and form a filter cake. The gradual accumulation of filter cake causes the pressure difference between the inside and outside of the screen to increase slowly. A self-cleaning process is initiated when the pressure difference reaches the preset value (0.5 bar) set by the differential pressure switch.
The self-cleaning filter has a high degree of automation and is precise and reliable. It adopts automatic filtration technology, and unique stainless steel brushes to clean the filter mesh, which can remove dirt, and consumes less water (not more than 3% of the total flow rate) when backwashing. It does not require bypass and can still supply water without interruption, which is more advantageous than the disk-type filter.
Microfiltration and ultrafiltration
Microfiltration and ultrafiltration are pressure-driven separation processes that can remove almost all suspended solids from water. Ultrafiltration can also remove some of the dissolved organic matter, and the specific removal efficiency depends on the organic molecular weight and the molecular weight cutoff of the ultrafiltration membrane.
Chlorine or sodium hypochlorite disinfectant is used in the backwashing process of microfiltration membranes and ultrafiltration membranes. Because of the oxidative property of chlorine, a reducing agent (commonly used sodium sulfite or sodium bisulfite) should be added to the back end of the ultrafiltration or the front end of the RO influent to protect the RO membrane from oxidation by the oxidant.
Security filtration
To prevent the suspended particles not wholly removed or newly generated in the pretreatment from entering the reverse osmosis system and to protect the high-pressure pump and reverse osmosis membrane, a cartridge-type security filter is usually installed before the reverse osmosis water inlet. The filter material mainly includes sintered filter tube, melt-blown fiber filter element, and honeycomb filter element.
The pressure gauge should be set for the inlet and outlet water of the security filter. When the pressure difference between the inlet and outlet water reaches the limit value during operation, the filter element should be replaced in time. Due to the low cleaning recovery efficiency of the filter element, it is recommended to use a disposable filter element.
The above is a detailed discussion of how to design the reverse osmosis membrane pretreatment system. The pretreatment of the reverse osmosis system must be reasonably adopted, and raw water quality should be comprehensively analyzed before configuration pretreatment. The practice has proved that high-quality pretreatment and more conservative process design usually make the reverse osmosis system run better and enhance the adaptability to water quality fluctuations. Although a conservative design leads to higher initial investment costs, its total operating expenses over the years will be reduced, and a reasonable conservative design is value for money. Therefore, in applying a reverse osmosis system, the investment and operation costs should be considered comprehensively to strive for maximum cost performance.
