Application and improvement of reverse osmosis desalination technology

The reverse osmosis desalination water treatment process adopts a physical method without phase change. It has the characteristics that the traditional water treatment method does not have in many aspects: its system is simple, the operation is convenient, the operating cost is low, no acid or alkali is consumed, and the corresponding corrosion and There are also fewer environmental pollution problems; the product water quality is stable, without fluctuations, which is extremely beneficial to the normal operation of the secondary mixed bed.

(1) Improved sand filter media to remove iron ions

From the analysis data of the past two years, due to the fluctuation of iron ions in the raw water, the iron content can reach up to 0.60 ppm. The membrane supplier’s requirements for the influent water quality of the roll composite membrane: the maximum iron content is 0.10ppm. Therefore, high-quality natural manganese sand with a height of 400mm and a particle size of 0.5-0.6mm is added to the original sand filter.
The iron ions in natural water have two forms: divalent iron and trivalent iron. Because Fe2+ has strong reducibility, it is easily oxidized to Fe3+, and Fe3+ undergoes hydrolysis reaction in water to form insoluble compound Fe(OH)3 colloid , Block 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 divalent iron to trivalent iron. As long as the pH value of the water is greater than 5.5, Fe2+ can be oxidized to Fe3+ by contact with manganese sand. The reaction is as follows:
4MnO2 + 3O2 = 2Mn 2O7
Mn2 O7+ 6 Fe2+ + 3H2O = 2MnO2+ 6 Fe3+ +OH-
The generated Fe3+ is immediately hydrolyzed to form flocculent iron hydroxide precipitates, and the Fe(OH)3 precipitates are removed after being filtered by manganese sand. Therefore, the manganese sand filter layer plays a dual role of catalysis and filtration. From the operational analysis data after the transformation, the iron content at the outlet of the sand filter can be controlled below 0.04ppm, which meets the water inlet requirements of the RO membrane element.

Salt Water Desalination System

(2) Improvement of ultrafilter operation mode

The pretreatment of reverse osmosis feed water includes two aspects: one is to prevent the pollution and blockage of the membrane and the inside of the pipeline by suspended solids, colloids and microorganisms; the other is to prevent the precipitation and scaling of insoluble salts.
Ultrafiltration is a pressure-driven membrane separation technology. Under the pressure difference as the driving force, the solvent (water) and small soluble particles in the solution permeate the membrane from the high-pressure feed liquid side to the low-pressure side, which is generally called the filtrate, while the large particle size components are blocked by the membrane , Thereby increasing the concentration in the filtrate. In the process of ultrafiltration membrane separation, as the flow velocity reaches the surface of the membrane, solutes are trapped by the membrane and accumulate, making the concentration of solutes on the membrane surface gradually higher than the concentration of the feed liquid. The main obstacles in the ultrafiltration process are concentration polarization and membrane fouling. Normally, the permeation volume of ultrafiltration increases with the increase of temperature and feed rate, but decreases with the increase of feed concentration. As we all know, concentration polarization is an inevitable result of the ultrafiltration process. In order to control concentration polarization, reduce pollution, and increase the ultrafiltration flux.
During the renovation process, two low-pressure circulating pumps 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 water flow rate of the ultrafilter and increase the membrane surface water flow rate. It is used to eliminate a part of the concentration polarization layer, so that the solute in the cross section can be taken away by the water flow in time, further reducing the thickness of the concentration polarization layer, and increasing the permeation flux of the ultrafilter. In addition, the operation mode of the UF module has also been improved. On the basis of the original backwashing, an automatic rapid flushing process of the UF membrane surface has been added to further reduce the frequency of chemical cleaning of the UF membrane element.

(3) Improvement of reverse osmosis desalination system

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

Reverse osmosis is a membrane separation process driven by pressure gradient, which 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 permeation. In the case of constant salt permeation, an increase in water flux will reduce the salt content in the product water.
Since this device uses UF components as the pretreatment of the reverse osmosis desalination device, the SDI value of UF permeate is low, and it 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 water inlet end is very high, and the pure driving pressure at the concentrated water end drops to a very low level. This is mainly due to the friction loss of the membrane element causing the infiltration of concentrated water to be higher than the pressure of concentrated water. Therefore, the front-end membrane element will operate under high water flux and high recovery rate, while the end-end membrane element will produce a small amount of fresh water with higher salt content. Under such conditions, the concentration difference of the front-end membrane element will be serious. 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 membrane elements of the first stage account for 62.5% of the total, while the water production accounted for 85% of the total, that is, 34.0t/h. During this transformation, the system design was re-designed, the parameters of the entire system were adjusted, and the original 5×3 arrangement was split into two sets of 3×1 arrangements to operate in parallel. At the same time, extreme water flux analysis may occur for the multi-stage system. Install a pressure gauge and a manual regulating valve between the outlet and the second-stage product water outlet to balance the first and second-stage product water volume. Adjust the operating parameters of each stage by increasing the product water back pressure to correct this extreme water flux distribution in the multi-stage system. In the actual operation process, the required increase of inter-stage pressure is relatively small, only need to add about 0.02Mpa back pressure on the first stage of the produced water to improve the fresh water flux distribution to reach the specified 75% ratio With 25% of the produced water distribution, the quality of the product water has also been 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 characteristics of the membrane element determine that the permeability of dissolved gases in water such as CO2 is almost 100%, and the permeability of HCO3- decreases with the increase of PH value.
It can be seen from the relationship between the degree of ionization of carbonic acid and the PH value of 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 almost only contains HCO3-. In view of the above situation, in this transformation, before the RO membrane security filter, in addition to adding NaHSO3 reducing agent and AF200ul scale inhibitor, at the same time, adding NaOH to adjust the reverse osmosis feed water PH value to between 8.2-8.3, so that the reverse osmosis can remove the maximum free CO2 Increase the desalination rate of reverse osmosis to a certain extent, and finally increase the fixed yield of the mixed bed. At the same time, cancel the RO product water tank in the original design, and the RO product water directly enters the first-level demineralized water tank, and install a double liquid caustic respirator at the top of the middle to prevent the secondary pollution of the RO product water by CO2 and other gases in the atmosphere , To reduce the burden of mixed bed ion exchange.

3. Increase the RO shutdown pure water flushing 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 salt content of the pure water at the outlet of the mixed bed is much lower than the RO product water, the product water on the RO product water side can be stopped after the shutdown Begin to permeate the membrane and move to the low-concentration pure water side. Due to the movement of water, the pollutants that invade the pores of the membrane and adsorb on the membrane surface become easy to remove. In the flowing state, the concentration polarization of the membrane surface can be reduced. Phenomenon, reduce the pollution of the membrane surface.

4. Use anti-pollution film

Although the purpose of pretreatment of the water supply is to reduce the pollution of the RO membrane surface, due to the disorder of the water pretreatment conditions and changes in the composition of the water supply, especially the use of surface water as raw water, bacteria and microorganisms in the water will still cause RO Fouling occurs on the membrane surface, which causes a decrease in the water production of the system, and an increase in pressure difference causes an increase in energy consumption and shortens the service life of the membrane element.
Combined with this transformation, the American DOW company FILMTEC BW30-365FR anti-fouling membrane was selected. This element adopts FILMTEC’s unique structure of “increasing the number of membranes and shortening the length of the membrane” and the characteristics of membrane surface smoothness increased by 40% compared with ordinary membrane elements. According to relevant information, the membrane element has: ①Optimal water supply channel design: the water in the water supply channel is in a highly turbulent state, which reduces concentration polarization and reduces the deposition of pollutants on the membrane surface; ②Wide water supply channel , Improve the clarity of the membrane; ③The membrane has strong stability and is a membrane element with strong anti-pollution performance.

5. The mixed bed operates at a constant flow rate to increase the fixed yield

In the original design, RO product water is directly sent to downstream users through a Ф1500 mixed bed. As the water consumption of downstream users is constantly changing, the operating flow rate of the mixed bed fluctuates high and low.
When the height of the resin layer is constant, the total ion content of the inlet water is constant, and the effluent index of the mixed bed is constant, the main factor affecting the operation of the ion exchange device is the change of the operating flow rate. When the flow rate is too low, the liquid film on the resin surface is thicker, and the water flows into a stagnant state, and the ions in the water are not easily exchanged, which makes the effluent water quality poor; appropriately increase the flow rate to speed up the ion diffusion rate and increase the possibility of ion exchange. This has been improved accordingly. 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 reduction of the resin working exchange capacity.
In this transformation, the original pipe rack pipe position is used, a return pipeline is added, and a circulation automatic regulating valve is equipped. The return water enters the desalinated water tank, so that the operating flow rate of the mixed bed is maintained in the range of 30-40m/h. Compared with before the transformation, the fixed yield of the mixed bed can be increased from 3600~4000 tons before the transformation to about 8000 tons.

(4)Comparison before and after the renovation project

before the renovation project

How to carry out a renovation project

The iron ion content of the influent water is much higher than the requirement (<0.1ppm), up to 0.60ppm, which causes blockage of the membrane element channel.

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 in the feed water is controlled below 0.04ppm, which meets the water feed requirements of membrane elements

Concentration polarization and membrane fouling of ultrafilters, Concentration polarization and membrane fouling exist

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 water flow rate of the ultrafilter and increase the water flow rate on the membrane surface. The phenomenon of concentration polarization and membrane fouling is eliminated, and the permeation flux of the ultrafilter is improved. Reduce the frequency of chemical cleaning of UF membrane elements.

Uneven proportion of fresh water effluent

When the original design RO membrane elements are arranged in 5×3, the membrane elements of the first stage account for 62.5% of the total, while the water production accounts for 85% of the total. The proportion of fresh water is uneven, resulting in concentration polarization. Split the original 5×3 arrangement into two sets of 3×1 arrangement in parallel operation, install a pressure gauge and a manual regulating valve between the first-stage product water outlet and the second-stage product water outlet. The fresh water flux distribution reaches the specified 75% to 25% production water distribution, and the product water quality is also improved. The current RO system desalination rate is 98.6%.

The cleaning cycle of the reverse osmosis membrane is too short, the membrane is polluted seriously, and the system cleaning cycle is 50 days

Replace the ordinary reverse osmosis membrane BW30-365 with anti-pollution BW30-365FR. The system has been running continuously for 12 months, and there is no sign of cleaning.

Pure water flushing process without RO shutdown

Using forward osmosis, when the RO system is shut down, the pure water at the outlet of the mixed bed is introduced to flush the membrane surface, so that the pollutants that invade the pores in the membrane and adsorb on the membrane surface can be easily removed, and the membrane surface can be reduced in the flowing state The phenomenon of concentration polarization. Reduce the pollution of the membrane surface.

The flow rate of the mixed bed is unstable. Because the user’s water consumption is constantly changing, the running flow rate of the mixed bed fluctuates high and low.

The fixed yield of the mixed bed is 3600~4000 tons. Equipped with a return line, equipped with a circulating automatic regulating valve, and the return water enters the desalinated water tank. Keep the running flow rate of the mixed bed in the range of 30-40m/h. The fixed yield of the mixed bed is increased to 7000 tons.

5. Data after the transformation is completed and put into operation

1. The anti-pollution membrane has low fouling speed, low water supply resistance and low energy consumption;
2. Reduced the number of chemical cleanings and prolonged the service life of membrane elements (the RO membrane elements have not been chemically cleaned for 11 months since they were put into use, and the cleaning cycle before membrane replacement is 50 days)
3. The desalination rate of the system is relatively stable (98%)

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