I. Reverse Osmosis Membrane Fouling
1. Membrane fouling caused by damage to the reverse osmosis membrane performance
(1) Polyester reinforced non-woven fabric, approximately 120μm thick; (2) Polysulfone porous intermediate support layer, approximately 40μm thick; (3) Polyamide ultra-thin separation layer, approximately 0.2μm thick.
Based on its performance structure, the possible reasons for damage to the reverse osmosis membrane performance are as follows:
(1) Improper maintenance of new reverse osmosis membranes; (2) Storage time exceeding 1 year under compliant maintenance; (3) Improper maintenance of reverse osmosis membranes in the shutdown state; (4) Ambient temperature below 5℃; (5) System operating under high pressure; (6) Improper shutdown operation.
2. Membrane fouling caused by frequent changes in water quality
Changes in raw water quality compared to the design water quality increase the pretreatment load. Due to the increase in inorganic substances, organic substances, microorganisms, particulate matter, and colloids in the influent, the probability of membrane fouling increases.
3. Membrane fouling caused by untimely cleaning and incorrect cleaning methods
During use, in addition to the normal attenuation of membrane performance, untimely cleaning and incorrect cleaning methods are also important factors leading to serious membrane fouling.
4. Failure to add chemicals correctly
During the use of composite polyamide membranes, due to the poor chlorine resistance of polyamide membranes, the failure to correctly add chlorine and other disinfectants during use, coupled with insufficient user attention to microbial prevention, easily leads to microbial contamination.
Membrane elements are blocked by foreign objects or the membrane surface is worn (such as sand grains). In this case, the detection method should be used to detect the elements in the system, find the damaged elements, modify the pretreatment, and replace the membrane elements.
II. Phenomena of Reverse Osmosis Membrane Fouling
During reverse osmosis operation, due to the selective permeability of the membrane, certain solutes accumulate near the membrane surface, resulting in membrane fouling.
Common fouling signs include: one is biological fouling (symptoms appear gradually). Organic deposits are mainly living or dead microorganisms, hydrocarbon derivatives, natural organic polymers, and all carbon-containing substances. Initially, it manifests as an increase in desalination rate, an increase in pressure drop, and a decrease in water production. Another is colloidal fouling (symptoms appear gradually). During membrane separation, the concentration of metal ions and changes in solution pH may cause metal hydroxide (mainly represented by Fe(OH)3) deposition, causing fouling. Initially, it manifests as a slight decrease in desalination rate, which gradually increases, and finally the pressure drop increases and water production decreases. There is also particulate fouling. During the operation of the reverse osmosis system, if the security filter has a problem, it will cause particulate matter to enter the system, causing particulate fouling of the membrane.
Initially, it manifests as an increase in concentrate flow rate, little change in desalination rate in the initial stage, a gradual decrease in water production, and a rapid increase in system pressure drop. Finally, there is also chemical scaling (symptoms appear quickly). When the feed water contains high concentrations of Ca2+, Mg2+, HCO3-, CO32-, SO42-, etc., CaCO3, CaSO4, MgCO3, etc., will be deposited on the membrane surface. It is characterized by a decrease in desalination rate, especially in the last section, and a decrease in water production.
Membrane fouling is the main reason for the decrease in membrane permeation flux. This includes the increase in membrane filtration resistance caused by the blockage of membrane pores and macromolecular solutes; the adsorption of solutes on the inner walls of the pores; and the formation of a gel layer on the membrane surface, increasing the mass transfer resistance. The deposition of components in the membrane pores will cause the membrane pores to shrink or even become blocked, effectively reducing the effective area of the membrane. The additional resistance generated by the fouling layer formed by the deposition of components on the membrane surface may be much greater than the resistance of the membrane itself, making the permeation flux independent of the membrane's own permeability. This effect is irreversible, and the degree of pollution is related to the membrane material, the solvent in the retention liquid, and the concentration, properties, solution pH, ionic strength, charge composition, temperature, and operating pressure of the macromolecular solute. When the pollution is serious, the membrane flux can decrease by more than 80%.
In system operation, membrane fouling is a very thorny problem. Its appearance causes a significant decrease in the removal rate, water permeability, and membrane flux of the reverse osmosis device, while increasing the operating pressure of each section, increasing operating and maintenance costs, and seriously affecting the service life of the membrane and the development and utilization of reverse osmosis technology.
For every membrane device, people hope that it can play its role to the maximum extent, hoping for the highest desalination rate, the largest water permeability, and the longest possible lifespan. To achieve the above three points, the water quality is crucial. Therefore, the raw water entering the membrane device must have good pretreatment. Reasonable pretreatment is very important for the long-term safe operation of the reverse osmosis device. With pretreatment that meets the reverse osmosis feed water quality requirements, the water production flow rate can be maintained stable; the desalination rate can be maintained at a certain value for a long time; the product water recovery rate can remain unchanged; the operating cost is minimized; and the membrane service life is longer, etc. Specifically, reverse osmosis pretreatment is to achieve: (1) Prevent pollution on the membrane surface, that is, prevent suspended impurities, microorganisms, colloidal substances, etc., from adhering to the membrane surface or blocking the water flow channels of the membrane element. (2) Prevent scaling on the membrane surface. During the operation of the reverse osmosis device, due to the concentration of water, some sparingly soluble salts are deposited on the membrane surface, so it is necessary to prevent the formation of these sparingly soluble salts. (3) Ensure that the membrane is free from mechanical and chemical damage to ensure that the membrane has good performance and a sufficiently long service life.
Even though the slurry undergoes various pretreatment measures, after long-term use, deposits and scaling may still occur on the membrane surface, causing the membrane pores to become blocked and the water production to decrease. Therefore, regular cleaning of the polluted membrane is necessary. However, the reverse osmosis membrane system cannot wait until the pollution is very serious before cleaning, as this will increase the cleaning difficulty, increase the number of cleaning steps, and prolong the cleaning time. It is necessary to correctly grasp the cleaning time and remove the dirt in time.
Understand the characteristics of local water quality, perform chemical analysis of pollutants, and select the best cleaning agent and cleaning method based on the results of the analysis to provide a basis for finding the best method under specific feed water conditions;
a. Product water volume is 5%-10% lower than normal.
b. To ensure sufficient product water volume, the water supply pressure is increased by 10%-15% after temperature correction.
c. The water quality conductivity (increased salt content) is increased by 5%-10%.
d. Multi-stage RO system, with a significant increase in pressure drop across different stages.
First, perform system backwashing; then perform negative pressure cleaning; mechanical cleaning if necessary; then chemical cleaning; ultrasonic cleaning if possible; online electric field cleaning is a good method, but it is expensive; because chemical cleaning is more effective, other methods are somewhat difficult to implement, and although the names and methods of use of the agents provided by various suppliers are different, their principles are roughly the same. For example, our company currently uses membrane cleaning agents MC2 and MA10.
The cleaning steps are as follows:
Cleaning a single-stage system: (1) Prepare the cleaning solution; (2) Input the cleaning solution at a low flow rate; (3) Circulation; (4) Soaking; (5) High-flow water pump circulation; (6) Rinsing; (7) Restart the system.
Cleaning for specific pollutants includes: cleaning sulfate scale, cleaning carbonate scale, cleaning iron and manganese pollution, cleaning organic pollution, etc.
III. Proper membrane maintenance
Maintenance of new reverse osmosis membranes New reverse osmosis membrane elements are usually immersed in a 1% NaHSO3 and 18% glycerol aqueous solution and stored in a sealed plastic bag. If the plastic bag is not broken, storage for about one year will not affect its lifespan and performance. Once the plastic bag is opened, it should be used as soon as possible to avoid adverse effects on the element due to the oxidation of NaHSO3 in the air. Therefore, the membrane should be opened as close to use as possible. During non-production periods, the maintenance of the reverse osmosis system is a relatively important issue. The following methods can be used.
(1) System shutdown for a short period (1-3 days): Before shutdown, first rinse the system at low pressure (0.2-0.4 MPa) and high flow rate (approximately equal to the system's water production), for 14-16 minutes; maintain the normal natural water flow, allowing water to flow into the concentrate stream.
(2) System shutdown for more than one week (ambient temperature above 5℃): Before shutdown, first rinse the system at low pressure (0.2-0.4 MPa) and high flow rate (approximately equal to the system's water production), for 14-16 minutes; perform chemical cleaning according to the methods for chemical cleaning of the system in the reverse osmosis system operating instructions; after the chemical cleaning is complete, rinse the reverse osmosis membrane clean; prepare a 0.5% formalin solution, input it into the system at low pressure, and circulate for 10 minutes; close all system valves and seal; if the system is shut down for more than 10 days, the formalin solution must be replaced every 10 days.
(3) Ambient temperature below 5℃: Before shutdown, first rinse the system at low pressure (0.2-0.4 MPa) and high flow rate (approximately equal to the system's water production), for 14-16 minutes; where possible, raise the ambient temperature to above 5℃, and then perform system maintenance according to method 1; if it is not possible to raise the ambient temperature, then: use low pressure (0.1 MPa) and a flow rate of 1/3 of the system's water production for continuous flow to prevent the reverse osmosis membrane from freezing, and ensure that the system runs for 2 hours per day; after cleaning the reverse osmosis membrane according to methods (2) and (3) in 1, remove the reverse osmosis membrane and move it to a place where the ambient temperature is above 5℃, immerse it in a prepared 0.5% formalin solution, and turn it over every two days; the water in the system pipes should be drained to prevent damage to the system due to freezing.
IV. Avoid high-pressure operation of the membrane
Residual gas remains in the system during startup and shutdown, causing the system to operate under high pressure. The pressure gauges before and after the filter in the system are used to monitor the pressure drop of the filter element, while the primary and final pressure gauges are used to monitor the pressure drop of the RO membrane module. Adjust the inlet valve and concentrate valve to ensure operating pressure and recovery rate. If the product water flow rate or total flow rate decreases during operation, or if the pressure difference between the primary and intermediate stages increases significantly compared to the initial pressure difference at the start of operation (using the initial operating data of the new reverse osmosis membrane module as the standard), the system needs to be rinsed or cleaned to ensure the performance and safety of the membrane module.
(1) After the equipment is drained, when restarting, if the gas is not completely discharged, rapid pressurization operation should not be performed. The remaining air should be discharged under system pressure before gradually increasing the pressure.
(2) If the joint between the pretreatment equipment and the high-pressure pump is not well sealed or leaks (especially if the micron filter and subsequent pipelines leak) and the pretreatment water supply is insufficient, such as if the micron filter is blocked, some air will be sucked in due to the vacuum at the poorly sealed location. The micron filter should be cleaned or replaced, and the pipelines should be ensured to be leak-free.
(3) Check whether each operating pump is running normally and whether the flow rate is the same as the specified value, and compare it with the pump operating curve to determine the operating pressure.
V. Pay attention to shutdown operations
(1) Rapid depressurization during shutdown without thorough rinsing. Because the concentration of inorganic salts on the concentrate side of the membrane is higher than that of the raw water, it is easy to form scale and pollute the membrane. When preparing to shut down, gradually reduce the pressure to about 3 bar and rinse with pretreated water for 14-16 minutes.
(2) Adding chemical reagents when preparing to shut down will cause the reagents to remain in the membrane and membrane shell, causing membrane pollution and affecting the service life of the membrane. Adding reagents should be stopped.
