Reverse osmosis water treatment equipment: improvement of reverse osmosis desalination system

1. Back pressure method to balance the water flux distribution of the system

Reverse osmosis is a membrane separation process powered by pressure gradient. It is the reverse process of natural osmosis. The increase of feed water pressure increases the water flux of the membrane, but the increase of pressure does not affect the amount of salt permeation. Under the condition of constant salt permeation, the increase of water flux will decrease the salt content in the product water.

Since the device uses UF components as the pretreatment of the reverse osmosis desalination device, the SDI value of the UF permeated water is low, and it can operate 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, and the pure driving pressure at the concentrated water end is very low. This is mainly because the permeation of concentrated water is higher than the concentrated water pressure due to the friction loss of the membrane element. 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 will produce a small amount of fresh water with high salt content. Under such conditions, the concentration polarization of the front-end membrane element is serious, It will adversely affect the salt content of the product water, and may also accelerate the fouling rate of the membrane.

When the RO membrane elements in the original design were arranged in 5×3, the membrane elements in the first section accounted for 62.5% of the total, but the water production accounted for 85% of the total, that is, 34.0t/h. During this renovation, the system was redesigned, the parameters of the whole system were adjusted, and the original 5×3 array was split into two sets of 3×1 arrays to operate in parallel. A pressure gauge is installed between the outlet and the product water outlet of the second stage, and a manual regulating valve is installed to balance the water volume of the first and second stage products. This extreme water flux distribution in a multi-stage system is corrected by adjusting the operating parameters of each stage by increasing the product water back pressure. In the actual operation process, the need to increase the inter-stage pressure is relatively small, only need to add a back pressure of about 0.02Mpa to the first-stage permeate water to improve the fresh water flux distribution and make it reach the specified 75% ratio 25% of the produced water is distributed, and 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 permeability of dissolved gases such as CO2 in water is almost 100% due to the characteristics of the membrane element, 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 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 contains almost only HCO3-. In view of the above situation, before the RO membrane security filter, in addition to adding NaHSO3 reducing agent and AF200ul scale inhibitor, adding NaOH at the same time to adjust the pH value of reverse osmosis feed water to 8.2-8.3, so that reverse osmosis can remove the maximum free CO2 Improve the desalination rate of reverse osmosis to a certain extent, and finally increase the fixed yield of the mixed bed. At the same time, the RO product water tank in the original design is cancelled, the RO product water is directly sent to the first-level desalinated water tank, and a duplex liquid alkali respirator is installed on the top of the middle to prevent the secondary pollution of the RO product water by CO2 and other gases in the atmosphere. , reduce the burden of mixed bed ion exchange.

3. Increase the RO shutdown pure water flushing process

Using forward osmosis is also a flushing method. When the RO system is shut down, the pure water from the outlet of the mixed bed is introduced to replace and flush the membrane surface. Since the salt content of the pure water at the outlet of the mixed bed is much lower than that of the RO product water, the product water on the RO product water side can be made to stop 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 have penetrated into the pores of the membrane and adsorbed on the membrane surface become easily removed, and the concentration polarization of the membrane surface can be reduced in 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 disturbance of feedwater pretreatment conditions and changes in feedwater composition, especially when surface water is used as raw water, bacteria and microorganisms in water will still lead to RO The membrane surface is fouled and blocked, resulting in a decrease in the water production of the system, an increase in the pressure difference, an increase in energy consumption, and a shortening of the service life of the membrane element. Combined with this transformation, we selected FILMTEC BW30-365FR anti-fouling membrane from American DOW company. This element adopts FILMTEC’s unique structure of “increasing the number of membranes and shortening the length of the membrane” and the surface finish of the membrane is 40% higher than that of ordinary membrane elements.

According to relevant information, the membrane element has:

  1.  the optimal design of the water supply channel: the water in the water supply channel is in a state of highly turbulent flow, which reduces concentration polarization and reduces the deposition of pollutants on the membrane surface;
  2. a wide water supply channel, Improve the clarity of the membrane;
  3. The membrane has strong stability and is a membrane element with strong anti-pollution performance.

From the data after the transformation is completed and put into operation:

  1. The fouling blocking speed of the anti-pollution membrane is small, the water supply resistance is small, and the energy consumption is low;
  2. It reduces the number of chemical cleanings and prolongs the service life of the membrane elements (the RO membrane element has not been chemically cleaned for 11 months since it was put into use, and the cleaning period before the membrane replacement is 50 days)
  3. The system’s salt rejection rate is relatively stable (98%)

Xi’an CHIWATEC Water Treatment Technology is a high-tech enterprise specialized in various water processing devices. Aside from these individual products, which cover a number of types and series, we can also help with related comprehensive engineering projects. Thanks to our hard work and dedication upon our founding, we are now one of the fastest-developing water treatment equipment manufacturers in Western China.

Further reading:

Filmtec RO membrane bw30365

Do you have a water treatment project we can help with

Designing,machining,installing,commissioning, customize and one-stop service

    We will answer your email shortly!