Strict feedwater pretreatment is the prerequisite for the successful operation of reverse osmosis, and microbial pollution is always an important risk for reverse osmosis systems. Microbial pollution is not just for reverse osmosis membranes. It may occur in every link of the pretreatment system and affect system operation. Preventing microbial pollution is one of the important tasks in the pretreatment stage of reverse osmosis feedwater.
1. System Overview
A power plant adopts reverse osmosis membrane technology to perform pre-desalination, and then prepares qualified demineralized water through secondary ion exchange desalination. The source water comes from the surface water of the reservoir. The pretreatment of the reverse osmosis system adopts clarification and multi-stage filtration treatment. Shown in Figure 1.
The flocculant used in the clarification treatment is polyaluminum chloride. In order to control the microorganisms in the system, a biocide was added to the clarification pond entrance for sterilization and algae killing. The biocide was initially used as chlorine dioxide. Due to the unstable operation of the chlorine dioxide generator, It was replaced with sodium hypochlorite in 2013. The reverse osmosis device consists of two rows, using a two-stage combination mode. The single row design has a force of 150t / h, and each row is equipped with 4 security filters. The security filter element uses an imported brand high-flow folding filter element with a filtration accuracy of 5μm.
2.The abnormal pressure difference of the security filter
The reverse osmosis system was put into operation in 2010. During operation, the pressure difference of the security filter was monitored. When the pressure difference between the inlet and outlet of the security filter exceeded 0.3 MPa, the filter element was replaced. After the system is put into operation, the normal use period of the security filter element is about 3 months, and the longest can be up to half a year. Since April 2013, the pressure difference of the security filter has often risen abnormally, and the service life of the filter element has become shorter: when the reverse osmosis system is out of service for a period of time, it is put into operation, or the fine sand filter is put into operation again after backwashing, and the security filter The pressure difference of the device will rise quickly beyond the allowable value within a short time. The situation was more serious in the spring and summer of 2014, and the shortest use time of the filter element was only two weeks. During the monitoring of the activated carbon filter and the fine sand filter's water pollution index (SDI), they were in the qualified range, and no overshoot was found. The subsequent differential pressure of the subsequent reverse osmosis device also increased, but within the allowable range, the second stage operation There is no significant change in the pressure rise rate.
3. Judgment of microbial pollution in pretreatment system
3.1 Investigation of pretreatment system
An increase in the differential pressure of the security filter indicates a fouling of the filter element. Under normal circumstances, the causes of filter element fouling are nothing more than sludge particle clogging and microbial contamination. Investigate the operation of various equipment in the pretreatment system. The clarification tank and the air scrubbing filter are operating normally. The turbidity of the effluent of the filter is less than 0.5NTU. The historical SDI data of the effluent of the activated carbon filter and the fine sand filter are in the range of 4 ~ 5. Reverse osmosis water requirements, no sludge or filter material powder retention phenomenon was found on the SDI test membrane, but green slime attachment was observed on the filter material surface at the observation hole of the fine sand filter. Take out the polluted filter element and inspect it. There is a heavier odor. The inside (water inlet side) of the filter element cylinder is clean, and the outer side (water outlet side) has light yellow spots. No signs of debris clogging are found. The system inspection results initially point to microbial contamination.
3.2 Filter element fouling detection
In order to further determine the cause of the fouling, a scanning electron microscope and an energy spectrometer were used to conduct a microscopic inspection of the fouling filter element. The freshly removed dirt plug filter element is kept in a wet state and sealed, and then sent to the detection mechanism. During the test, the folded filter membrane is cut to a size of about 3cm ¡Á 4cm, and the three layers of the filter membrane are separated according to the process structure of the filter membrane. The water inlet side of the layer is enlarged and observed, and the elemental analysis of the fouling material is performed by a energy spectrometer. Figures 2 and 3 are the scanned images of the first layer (surface layer) and the second layer enlarged by 250 times, respectively.
The first layer (surface layer) is the water inlet layer, and the filtration accuracy is relatively low. From the enlarged image, the pores between the filter fibers in the surface layer are clear, and there are few attachments, and there is basically no clogging. Elemental analysis of the attachments showed that the main elements were C, O, Si, and Al, and their mass fractions were 27.56%, 38.52%, 25.13%, and 1.89%, respectively.
The second layer of filter pores is smaller than the first layer, and there are more flocculent attachments between the fiber pores. The element detection of the flock-like attachments showed that the mass fractions of C, O, Si, and Al were 57.81%, 16.53%, 10.21%, and 2.87%, respectively, and carbon was the majority. It is inferred that the main content of the flock-like attachments was For organic matter.
Figures 4 and 5 are scanned images of the third layer at 250x and 5000x magnification, respectively.
Compared with the first two layers, the third layer has a color change in appearance, which is yellow-green. The image shows that the filter pores in the third layer are smaller, and there are many fibrous attachments between the fiber pores, which basically occupy more than 50% of the pore space. The morphological characteristics of the attachments are similar to those in the second layer. The test results of the elements of filiform attachments showed that the mass fractions of C, O, Si, and Al reached 58.94%, 26.92%, 5.28%, and 4.03%, respectively, and carbon accounted for the majority.
Scanning electron microscope observation results show that foreign matter does indeed adhere to the inside of the filter membrane of the filter element, which mainly occurs in the second and third layers of the filter membrane, especially the third layer is the most serious. The attachments occupy almost half of the pore space, which should cause filtration. The direct cause of the high pressure difference of the device. The results of elemental analysis of the attachment showed that the mass fraction of carbon and oxygen accounted for more than 70%, and there were also a small amount of silicon and aluminum elements. Judging from the element ratio, the attached matter inside the filter element is mainly organic matter. Combined with the filter element being tested, it has a strong stench and yellow-green patches on the surface of the water outlet. It can be confirmed that microbial contamination is the main cause of the filter element blockage.
The microbial contamination of the security filter element reflects that the sterilization and algae treatment in the pretreatment stage was not thorough enough. Theoretically, there is a risk of microbial contamination in the equipment of each link of the pretreatment system. The presence of green slime on the surface of the filter material of the fine sand filter indicates that this risk has indeed occurred. Although the activated carbon filter has not been sampled and tested, the severity of the pollution from the fine sand filter It seems that the possibility of microbial contamination is high.
4. Analysis of causes of microbial pollution
Microorganisms are present in almost all surface waters, but sporadic microorganisms do not form colonies and do not pose a threat to microbial contamination. There are many factors that cause microbial pollution, such as water source, temperature, season, treatment method, and operating conditions. As far as the power plant is concerned, the occurrence of microbial contamination is mainly the result of a combination of killing treatment, operation mode, seasonal factors, and system defects.
(1) Inadequate killing treatment in the pre-processing stage. From the time point of view, microbial contamination occurred after the biocide was changed from chlorine dioxide to sodium hypochlorite, reflecting that the insufficient dosage of sodium hypochlorite caused incomplete killing. In terms of disinfection performance, sodium chlorate will be weaker than chlorine dioxide next time under the same dosage conditions. After using sodium hypochlorite in power plants, the residual chlorine concentration in the clarification tank effluent is generally around 0.1mg / L. Refer to relevant industry standards, such as sanitary standards for drinking water. The residual chlorine in the factory water is required to be ≥0.3mg / L, and the design specifications for circulating cooling water treatment require that the residual chlorine content in the water be controlled to be 0.1 ~ 0.5mg / L when continuous chlorine is added. To maintain the continuous lethality of the clarifier effluent during the long stay in the subsequent filters, clear ponds and filters, the 0.1mg / L residual chlorine in the clarifier effluent is obviously low, as the residual chlorine content is consumed and decayed , The inactivation of the water body disappears, and the active organisms reproduce and accumulate again in a suitable environment.
(2) Defects in process flow. Because the fine sand filter and reverse osmosis device are newly added on the basis of the activated carbon filter + ion exchange desalination system, the activated carbon filter precedes the fine sand filter in the process of the reverse osmosis pretreatment system. This unconventional process arrangement allows the clear water effluent to be removed in the activated carbon filtration link even if it contains a sufficient amount of residual chlorine. The biocide cannot reach the fine sand filter and the security filter. The microorganisms behind the activated carbon bed cannot be killed. Long-term operation The result is that a large number of microorganisms accumulate in the fine sand filter and become the source of microbial contamination of the security filter and reverse osmosis membrane.
(3) Long-term system outage promotes microbial contamination. Restricted by less salt water consumption, reverse osmosis and pretreatment systems are often shut down for more than 24 hours. In the case of incomplete sterilization in the pretreatment stage, the static state of the water flow provides a better environment for the accumulation and reproduction of microorganisms in the water body. Activated carbon filter The fine sand filter and security filter are susceptible to trapping pollutants. During the outage, microorganisms grow faster. Microbial metabolism forms biological mucosa. Biological mucosa has strong viscosity and is hardly affected by the shear force of water flow. Once formed, it is difficult to remove it by conventional means such as backwashing and forward washing, and accumulate to form biological slime for a long time. The security filter has high filtration accuracy and is sensitive to the flux change caused by microbial growth. The macro performance is that the filter element pressure differential rises rapidly in the short term. With the continuous rise of water temperature in spring and summer, the speed of microbial reproduction accelerates, and the phenomenon of filter blockage becomes more apparent. obvious.
5. Handling countermeasures and effects
Based on the cause analysis, the power plant formulated measures for microbial pollution treatment and prevention and control in the reverse osmosis pretreatment system based on the actual situation of the system.
(1) First, the equipment and pipelines from the air scrubbing filter to the security filter process section (except the activated carbon filter) are subjected to a comprehensive impact sterilization and algae treatment.
(2) Chemically clean the contaminated fine sand filter. The chemical cleaning scheme includes sterilization, alkaline washing, and backwashing processes to ensure that the attached biological slime is completely removed.
(3) Improve the control standard of residual chlorine at the outlet of the clarifier to 0.5 ~ 1.0mg / L, ensure the vitality of the subsequent system water, and take into account the increase of high residual chlorine on the burden of activated carbon, and control the residual chlorine content in the activated carbon filter to not exceed 0.3. mg / L.
(4) Add a fungicide dosing point at the outlet of the activated carbon filter for secondary sterilization treatment, and add sodium hypochlorite to control microbial contamination in subsequent systems. During the operation, control the residual chlorine at the inlet of the security filter at 0.1 ~ 0.3mg / L, and control the amount of reducing agent added to ensure that the residual chlorine at the inlet of the reverse osmosis membrane is less than 0.1mg / L.
(5) Reduce the outage time of the reverse osmosis system, each outage time does not exceed 24h, and try to avoid microbial reproduction and accumulation during the outage.
The above-mentioned treatment measures were carried out separately according to the degree of implementation difficulty. In May 2015, the secondary sterilization and dosing equipment of the activated carbon filter was reformed. Operation, the second sterilization treatment continues to be put into operation. From the operation situation after the transformation, the control and prevention and control measures have achieved good results. As of July 15, 2015, the security filter element has been used continuously for more than two months, and the pressure difference of the filter has remained in a good condition.
Strict feedwater pretreatment is the prerequisite for the successful operation of reverse osmosis. With the application of microfiltration, ultrafiltration and other precision filtration equipment more and more mature, more and more mature, colloids and suspended particles, as well as macromolecular organic matter on the reverse osmosis membrane. The threat has been greatly reduced, and microbial contamination has always been an important risk for reverse osmosis systems. Microbial pollution is not just for reverse osmosis membranes. It may occur in every link of the pretreatment system and affect system operation. Preventing microbial pollution is one of the important tasks in the pretreatment stage of reverse osmosis feedwater. Because microbial pollution is affected by many factors such as water source, season, treatment method, and equipment conditions, there is no uniform standard for reference for its prevention and control. Operators need to choose a suitable treatment method according to the actual situation, strengthen monitoring, and continuously summarize experience to form A set of safe and effective prevention and control methods suitable for you.