Application and Improvement of Reverse Osmosis Desalination Technology

The reverse osmosis desalination treatment process uses a physical method without phase change. It has many features that traditional water treatment methods do not have: its system is simple, the operation is convenient, the operating cost is low, and it does not consume acid and alkali. There are also few environmental pollution problems; the product water quality is stable and there is no fluctuation of high and low, which is extremely beneficial to the normal operation of the secondary mixed bed.

1. Sand filter material improved to remove iron ions

According to the analysis data in the past two years, due to the influence of the iron ion content in the raw water by the flood season of the Yangtze River and the steady rate of dosing in the water plant, the iron content can be as high as 0.60ppm. The membrane supplier’s water quality requirements for the roll composite membrane: the maximum iron content is 0.10ppm. Therefore, in this transformation, high-quality natural manganese sand with a height of 400mm and a particle size of 0.5-0.6mm was added to the original sand filter.
Iron ions in natural water have two forms of iron and ferric iron. Because Fe2 + has strong reducibility, it is easily oxidized to Fe3 +. Fe3 + undergoes a hydrolysis reaction in water to form the insoluble compound Fe (OH) 3 colloid , Blocking the water channel of the membrane element. The main component of natural manganese sand is manganese dioxide, which is a good catalyst for the oxidation of ferric iron to ferric iron. As long as the PH value in the water is greater than 5.5, Fe2 + can be oxidized into Fe3 + by contact with manganese sand. The reaction is as follows:
4MnO2 + 3O2 = 2Mn2O7
Mn2O7 + 6 Fe2 + + 3H2O = 2MnO2 + 6 Fe3 + + OH-
The generated Fe3 + is immediately hydrolyzed to produce flocculent ferric hydroxide precipitate, and the Fe (OH) 3 precipitate is removed after being filtered by manganese sand. Therefore, the manganese sand filter layer plays a dual role of catalysis and filtration. From the operation analysis data after the transformation is completed, the iron content at the outlet of the sand filter can be controlled below <0.04ppm, which meets the water inlet requirement of the RO membrane element.

2. Improvement of ultrafilter operating mode

The pretreatment of reverse osmosis influent water includes two aspects: one is to prevent the suspended matter, colloids and microorganisms from fouling and blocking the inside of the membrane and pipeline, and the other is to prevent the precipitation and scaling of insoluble salts. In this transformation, the UF-IB9 type internal pressure hollow fiber ultrafiltration membrane produced by Shandong Zhaoyuan Membrane Engineering Equipment Factory was still selected according to the original design.
Ultrafiltration is a pressure-driven membrane separation technology. Under the action of the pressure difference as the driving force, the solvent (water) and small solute particles in the solution pass through the membrane from the high-pressure feed liquid side to the low-pressure side, which is generally called the filtrate, and the large-diameter components are blocked by the membrane , So that the concentration in the filtrate is increased. In the process of ultrafiltration membrane separation, the solute that reaches the membrane surface with the flow rate accumulates due to the interception of the membrane, so that the solute concentration on the membrane surface is gradually higher than the main concentration of the feed liquid. The main obstacles in the ultrafiltration process are concentration polarization and membrane fouling. Generally, the amount of ultrafiltration penetration increases with increasing temperature and feed rate, but decreases with increasing feed concentration. As we all know, concentration polarization is an inevitable result of the ultrafiltration process. In order to control the concentration polarization to reduce pollution and increase the ultrafiltration flux. In this transformation process, the two low-pressure circulation pumps in the original design were retained, and the raw water pipe network was directly connected to the raw water pump suction port to increase the output head of the raw water pump to increase the inlet flow rate of the ultrafilter and increase Large membrane surface water flow rate. It is used to eliminate part of the concentration polarization layer, so that the cross-sectional solute can be taken away by the water flow in time, further reduce the thickness of the concentration polarization layer, and increase the permeation flux of the ultrafilter.
In addition, this modification also improves the operation mode of the UF module. On the basis of the original backwashing, an automatic rapid washing process of the UF membrane surface is added, which further reduces the chemical cleaning frequency of the UF membrane element.

3. Improvement of reverse osmosis desalination system

1. The water flux distribution of the back pressure method balance system

Reverse osmosis is a membrane separation process driven by a pressure gradient. It is the reverse process of natural osmosis. The increase in water supply pressure increases the water flux of the membrane, but the increase in pressure does not affect the salt penetration. In the case of constant salt penetration, the increase in water flux will reduce the salt content in the product water.
Since the device uses UF components as the pretreatment of the reverse osmosis desalination device, the UF permeated water has a lower SDI value and can be operated at a higher water flux.
Under the same water flux, the pure driving pressure of the system will produce a large gradient, that is, the pure driving pressure at the inlet end is very high, while the pure driving pressure at the concentrated end drops to very low. This is mainly due to the friction loss of the membrane element causing the penetration of the concentrated water to be higher than the concentrated water pressure. Therefore, the front-end membrane element will operate in a state of high water flux and high recovery rate, while the end-end membrane element produces a small amount of fresh water with a high salt content. Under such conditions, the concentration of the front-end membrane element is highly polarized. It has an adverse effect on the salt content of the product water, and may also accelerate the fouling speed of the membrane.
When the original design RO membrane elements are arranged in 5 × 3, the first-stage membrane elements account for 62.5% of the total, while the water production accounts for 85% of the total, which is 34.0t / h. During this transformation, the system design is re-designed, the parameters of the entire system are adjusted, and the original 5 × 3 arrangement is split into two sets of 3 × 1 arrangement in parallel operation. At the same time, extreme water flux analysis may be generated for multi-stage systems. Install a pressure gauge between the outlet and the second-stage product water outlet, and manually adjust the valve to balance the water volume of the first and second-stage products. Adjust the operating parameters of each section by increasing the product water back pressure, and correct this extreme water flux distribution in a multi-stage system. In the actual operation process, the increased pressure between the sections is relatively small. It is only necessary to add a back pressure of about 0.02Mpa to the first section of production water to improve the distribution of fresh water flux to reach the specified 75% ratio. 25% of the produced water is distributed, and the water quality of the product water is also improved. So far the RO system desalination rate is 98.6%.

2. Adjust the PH value of RO feed water to remove free CO2

The permeability of dissolved gas such as CO2 in water is almost 100%, and the permeability of HCO3- decreases with the increase of PH value.
It can be seen from the relationship between the ionization degree of carbonic acid and the PH value in water that bicarbonate in water is unstable, and it can exist in three forms: HCO3-, CO32-, and CO2 + H2CO3. When the pH is about 8.3, the solution contains almost only HCO3-. In view of the above situation, in addition to adding NaHSO3 reducing agent and AF200ul scale inhibitor before the RO membrane security filter, NaOH was added to adjust the pH value of the reverse osmosis feedwater to 8.2-8.3 to maximize the removal of free CO2 by reverse osmosis To a certain extent, increase the desalination rate of reverse osmosis, and ultimately increase the fixed yield of mixed bed. At the same time, the RO product water tank in the original design is canceled. The RO product water directly enters the first-level desalination tank, and a double liquid alkali respirator is installed on the top of the middle to prevent secondary pollution of the RO product water by gases such as CO2 in the atmosphere. , Reduce the burden of mixed bed ion exchange.

3. Increase RO parking pure water washing process

The use of forward osmosis is also a flushing method. When the RO system is stopped, the pure water at the outlet of the mixed bed is introduced to replace and rinse the membrane surface. Since the pure water at the outlet of the mixed bed has a much lower salt content than the RO product water, the product water on the RO product water side can be stopped after the shutdown Start to move through the membrane to the low-concentration pure water side. Due to the movement of water, the contaminants that enter the pores of the membrane and adsorb on the membrane surface become easy to remove, and the concentration polarization of the membrane surface can be reduced under the flowing state. Phenomenon, reduce the pollution of the membrane surface.

4. Choose anti-pollution film

Although the purpose of pretreatment of feedwater is to reduce the pollution of RO membrane surface, due to the disorder of feedwater pretreatment conditions and changes in feedwater composition, especially the use of surface water as raw water, bacteria and microorganisms in the water will still cause RO Fouling phenomenon occurs on the membrane surface, which results in a decrease in the system water production, an increase in pressure difference causes an increase in energy consumption, and shortens the service life of the membrane element.
Combined with this transformation, we selected the FILMTEC BW30-365FR anti-fouling membrane of the US DOW company. This element adopts the unique structure of FILMTEC “increasing the number of membranes and shortening the length of the membrane” and the membrane surface finish is 40% higher than that of ordinary membrane elements. . According to relevant information, the membrane element has: ① optimal water supply channel design: water in the water supply channel is highly turbulent, reducing concentration polarization and reducing the deposition of pollutants on the membrane surface; ② wide water supply channel , Improves the clarity of the membrane; ③ The membrane has a strong stability and is a membrane element with strong anti-pollution performance.

From the data after the completion of the transformation and operation:
1. The anti-pollution membrane has a small clogging speed, low water supply resistance, and low energy consumption;
2. Reduced the number of chemical cleanings and prolonged the service life of the membrane elements (RO membrane elements have not been chemically cleaned for 11 months since commissioning, and the cleaning period before membrane replacement is 50 days)
3. The system desalination rate is relatively stable (98%)

4. mixed bed constant flow rate operation, increase the fixed yield

In the original design, RO product water was sent directly to downstream users through a Ф1500 mixed bed. Because the water consumption of downstream users is constantly changing, the operating flow rate of the mixed bed is suddenly high and low.
When the height of the resin layer is fixed, the total ion content of the influent is fixed, and the mixed bed effluent index is fixed, the main factor affecting the operation of the ion exchange device is the change in the operating flow rate. When the flow rate is too low, the resin film on the surface of the resin is thick, and the water flows into a stagnation state. The ions in the water are not easy to exchange, which makes the effluent water quality poor; appropriately increasing the flow rate speeds up the ion diffusion speed and increases the possibility of ion exchange. Correspondingly improved. However, the flow rate is too high, the contact time is short, and the exchange reaction is incomplete; at the same time, the thickness of the working layer increases, which will cause the deterioration of the effluent water quality and the decrease of the resin working exchange capacity.
In this transformation, the original pipe rack tube position is used, a return line is installed, a circulation automatic regulating valve is equipped, and the return water is fed into the desalination tank to keep the mixed bed operating flow rate in the range of 30-40m / h. Compared with before modification, the fixed yield of mixed bed can be increased from 3600-4000 tons before modification to about 8000 tons.

5. conclusion

Through technical improvements, the system’s operating effect has been significantly enhanced. The details before and after the transformation are summarized in the following table:

Iron ion The iron ion content in the inlet water is much higher than the requirement (<0.1ppm), up to 0.60ppm, which causes the membrane element channel to block. The original sand filter is supplemented with high-quality natural manganese sand with a height of 400mm and a particle size of 0.5-0.6mm. The iron content of the influent is controlled below <0.04ppm, which meets the requirement of membrane element influent.
The concentration polarization and membrane pollution of the ultrafilter exist the phenomenon of concentration polarization and membrane pollution. The raw water pipe network is directly connected to the suction port of the raw water pump to increase the output head of the raw water pump to increase the inlet flow rate of the ultrafilter and increase Large membrane surface water flow rate. The phenomenon of concentration polarization and membrane pollution is eliminated, and the permeation flux of the ultrafilter is improved. Reduced the frequency of chemical cleaning of UF membrane elements.
Uneven ratio of freshwater effluent When the original design RO membrane elements are arranged in 5 × 3, the first-stage membrane element accounts for 62.5% of the total, while the water production accounts for 85% of the total. The uneven ratio of freshwater effluent results in polarization polarization . Split the original 5 × 3 arrangement into two sets of 3 × 1 arrangement and operate in parallel. Install a pressure gauge between the first and second product water outlets and manually adjust the valve. The distribution of fresh water flux reaches the specified 75% to 25% product water distribution, and the water quality of the product water is also improved. The current RO system desalination rate is 98.6%.
The reverse osmosis membrane cleaning cycle is too short, and the membrane is seriously polluted. The system cleaning cycle is 50 days. The ordinary reverse osmosis membrane BW30-365 is replaced by the anti-fouling BW30-365FR. The system has been running continuously for 12 months. There is no sign of cleaning.
The RO parking pure water washing process does not make use of positive osmosis. When the RO system is stopped, the pure water at the outlet of the mixed bed is introduced to rinse the membrane surface, so that the contaminants that enter the pores of the membrane and adsorb on the membrane surface become easily removed. In the flow state, the concentration polarization phenomenon on the membrane surface can be reduced. Reduce the pollution of the membrane surface.
The flow rate of the mixed bed is unstable. Because the water consumption of downstream users is constantly changing, the running flow rate of the mixed bed is suddenly high and low. The fixed yield of mixed bed is 3600-4000 tons. Install a return line, equipped with an automatic circulation regulating valve, and return water into the desalination tank. Keep the mixed bed operating flow rate in the range of 30 ~ 40m / h. The fixed yield of mixed bed was increased to 7000 tons.

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