Research on Microbial Pollution and Prevention of Reverse Osmosis Water Treatment System

From the proposal of using reverse osmosis technology to desalinate seawater in 1953 to its commercial operation in the 1960s, and after more than 50 years of development today, reverse osmosis water treatment technology has been successfully applied in many fields. At the beginning, reverse osmosis technology was only used for seawater desalination, but later gradually expanded to areas such as brackish water desalination, food processing, pharmaceutical and hygiene, beverage purification, and ultra pure water preparation, resulting in high economic benefits.

During the operation of the reverse osmosis water treatment system, if the system design is unreasonable or the operation control is improper, membrane fouling will inevitably occur. Among the several types of membrane pollution (sedimentation pollution, microbial pollution, colloid pollution, etc.), microbial pollution has its particularity, and the operational difficulties it causes in reverse osmosis water treatment are relatively serious. At present, the problem of microbial contamination of membranes is becoming increasingly prominent in the operation of reverse osmosis water treatment systems in China.

1. The generation and harm of microbial pollution

1.1 Causes

Biological pollution refers to the phenomenon of microorganisms accumulating at the membrane water interface, thereby affecting system performance. Microbial contamination is the result of complex interactions between membrane materials, flow parameters (such as dissolved matter, flow rate, pressure, etc.), and microorganisms. Microbial pollution is basically a problem of biofilm growth.

The main source of microbial pollution is RO feed water. Due to the presence of microorganisms in both surface water and shallow groundwater, if the pre-treatment system is not functioning properly and effectively, microorganisms will enter the RO module, and the moist and dark interior of the RO module can provide an ideal environment for microbial growth. If not killed before entering the reverse osmosis system, these microorganisms will use the reverse osmosis membrane as a carrier to reproduce and grow with the help of the nutrient salts in the reverse osmosis concentrated water section. Under warm and hot conditions, The growth of microorganisms is even faster, and within a few days, a biofilm layer can be formed on the surface of the reverse osmosis membrane, leading to a rapid increase in the pressure difference between the inlet and outlet water of the reverse osmosis system, a rapid decrease in water production and desalination rate, and pollution of the product water. On the other hand, pre-treatment may also be a source of microbial contamination, such as excessive flocculants that assist in removing suspended solids, providing a suitable growth environment for microorganisms.

In the RO system, the main presence is aerobic bacteria, and generally no fungi or molds are found. The distribution of aerobic bacteria varies at different stages of the system, as shown in Table 1. It can be seen that the raw water tank is the main site for bacterial growth, and secondly, there is bacterial growth inside the RO processor (due to the organic materials of the membrane providing certain conditions for bacterial growth)

1.2 Hazards

At present, the commercialized reverse osmosis membrane materials mainly include two categories: acetate fiber and polyamide. The cellulose acetate membrane device is currently a commonly used and economical reverse osmosis device in ultra pure water manufacturing systems. But one of its major drawbacks is its poor resistance to microbial erosion. Although polyamide membranes can resist microbial erosion, pollution issues still exist.

The proliferation of a large number of microorganisms within the membrane and components will result in three adverse consequences: the microorganisms will swallow the reverse osmosis membrane, the desalination layer will be eroded, resulting in a decrease in desalination rate, a shortened membrane life, damage to the integrity of the membrane structure, and even major system failures (only for CA membranes); The second is the massive reproduction and metabolism of microorganisms, which produce a large amount of colloidal substances, causing the membrane to be blocked and increasing the pressure drop of water supply, resulting in a decrease in water flow rate; Thirdly, it will cause an increase in the total number of bacteria in the production water, leading to a decrease in the quality of the product; The fourth is that biofilms (sticky mud) are insoluble in acids and difficult to dissolve in alkalis, and are almost unaffected by the shear force of water flow. Even if washed frequently, they cannot be washed away. Disinfection and sterilization are also difficult to completely remove sticky mud.

An important feature of microorganisms is their ability to quickly biochemical and genetic regulate changes in nutrient hydrodynamics or other conditions. Therefore, biological pollution is more harmful than non active colloidal pollution or mineral scaling.

The widely used TFC reverse osmosis membrane currently uses polyacrylamide as its key material, which does not have strong resistance to oxidizing substances. Therefore, users generally control the redox potential of the reverse osmosis inlet, allowing the membrane to work in an environment without oxidants. Bacteria and other microorganisms attach to the surface and channel network layer of the membrane, and multiply in large quantities based on the nutritional components in the water. Many literature suggests that this type of pollution appears to occur in the middle of the membrane, but in reality, it is often the entire system flooding together.

2. Prediction and Simple Identification Methods for Microbial Pollution

(1) Measure the total bacterial count (TBC) of reverse osmosis feed water, concentrated water, and reverse osmosis product water from the raw water inlet, pre-treatment stages, and calculate the bacterial change value. If a significant increase in TBC is found in concentrated water, it indicates that there may be sludge formation on the reverse osmosis membrane.

(2) The organic matter in water supply can not only form membrane pollution, but also serve as a nutrient for bacterial growth. So organic matter (represented by total organic carbon, abbreviated as TOC) can be monitored, and the membrane manufacturer prompts to control TOC<2mg/L (represented by C). 2mg/LTOC is roughly equivalent to 5mg/L of total organic biomass. This value will not cause organic fouling between membranes during normal operation.

(3) A simple method to test whether it is microbial contamination is to scrape a small portion of the pollutant from the surface and burn it on a flame, with the same odor as hair burning.

3. Microbial pollution prevention and control

For the RO water treatment system, it is necessary to set up comprehensive measures to kill microorganisms in the pre-treatment of the RO process system in order to fundamentally control microbial pollution.

Regarding the prevention and control of microbial pollution. The traditional view holds that the microbial fouling of RO membrane components mainly comes from surface water, while the fouling from groundwater is relatively light. At the same time, it is believed that when the microbial content in the inlet water of the RO device is<10000 cfu/ml, it is relatively safe. Therefore, when choosing to design the RO process system, the attention given by the application is often insufficient. Domestic examples indicate that certain RO membrane components in groundwater systems have been subjected to varying degrees of microbial fouling. This is because the system is equipped with a raw water tank with a capacity of 1-2 hours for water storage regulation (such as in the power industry), which is prone to microbial growth. Among membrane components that need to be cleaned due to microbial fouling within six months, a single 8-inch roll membrane component with severe fouling has a wet weight of 35kg, and a normal wet weight of 16.4kg, The pollutants are light yellow and transparent, with a burning odor of wool.

The method to prevent microbial pollution is usually to take effective sterilization treatment measures, including conventional methods such as chlorine gas and NaClO, ClO2, KMnO4, H22O2, O3, ultraviolet radiation, etc. The key to control is to select suitable fungicides and maintain sufficient contact time. For chlorine fungicides, the dosage is generally based on the residual chlorine content in the influent>1mg/L, and the appropriate residual chlorine amount is controlled according to different reverse osmosis membranes. In addition, both oxidizing and non oxidizing fungicides (such as Na2S2O5, NaHSO50mg/L, and isothiazolinone 15-25mg/L) can be used for regular, alternating impact, and high-dose sterilization, which can kill most microorganisms in the system and even penetrate the biological slime film adhered to the system, playing a killing and stripping role. Furthermore, it is necessary to strictly control the organic matter content in the water supply (expressed as total organic carbon (TOC) not exceeding 2mg/L) to prevent bacterial growth and reproduction. It is better to pay attention to monitoring the total bacterial count (TBC) in the water at all stages of the reverse osmosis system for effective prevention. When symptoms of biological pollution are found (pressure difference increases by 10%, water production decreases by 10%), timely cleaning measures should be taken (including cleaning the pre-treatment system and RO system) to avoid worsening pollution.

Regularly sterilize, usually rinse with 1% -3% formaldehyde solution for 15 minutes to kill bacteria. During the shutdown period of the RO system, it is required to use formaldehyde and wash it every 2 days. In addition to using formaldehyde, 0.2% H22O2 can also be used for sterilization.

It is generally believed that water treated with activated carbon will contain a large amount of microorganisms. But for the activated carbon treatment process, as long as the backwashing frequency and replacement frequency are adjusted properly, it can also prevent microbial pollution

4. Conclusion

(1) Microbial contamination of reverse osmosis membranes in reverse osmosis water treatment systems is more severe among various membrane fouling. It has the characteristics of rapid development, difficult to completely remove the formed biofilm, easy to block the membrane, causing a rapid increase in pressure difference between the inlet and outlet water of the reverse osmosis system, a rapid decrease in water production and desalination rate, and may contaminate the product water, and even damage the membrane.

(2) Microbial pollution can be predicted by monitoring changes in the total bacterial count (TBC) value of concentrated water and the total organic carbon (TOC) value of feedwater, and it can be easily determined whether it is microbial pollution by burning pollutants.

(3) We need to strengthen the importance of pre-treatment, disinfection, and sterilization of groundwater as raw water to prevent microbial contamination in RO devices.

(4) In addition to conventional methods, the prevention and control of microbial pollution can also be achieved through regular, alternating, and high-dose sterilization with oxidizing and non oxidizing fungicides. Strictly control the organic matter content in water supply (expressed as total organic carbon (TOC) not exceeding 2mg/L) to prevent bacterial growth and reproduction. For the activated carbon treatment process, as long as the backwashing frequency and replacement frequency are adjusted properly, microbial pollution can also be prevented.

The above is the research on microbial pollution and prevention in reverse osmosis water treatment systems that Hongjie Water brings to you. We hope it can help you!

Shenzhen Hongjie Water Technology Co., Ltd. is an integrated high-tech enterprise specializing in product research and development, production, sales, engineering design, installation and debugging, technical consulting, and sales of supporting materials in the fields of industrial water treatment and drinking water treatment. Our main products include the design, manufacturing, installation, debugging, and maintenance of water treatment equipment, including pure water equipment, ultra pure water equipment, reverse osmosis equipment, GMP purified water equipment, EDI deionized water equipment, domestic sewage equipment, industrial wastewater equipment, softened water equipment, and reclaimed water reuse equipment, all of which are integrated into an environmentally friendly high-tech manufacturer.