As a deep filtration means, the surface of reverse osmosis membrane element will inevitably remain colloids, microorganisms, impurity particles and insoluble salts on its surface. Therefore, once put into use, the reverse osmosis devices used in many fields will eventually need to be cleaned, but the length of cleaning cycle is different. However, as a means of reverse osmosis system cleaning and maintenance, impact sterilization and regular protection, on-line cleaning is powerless in the face of severe pollution of reverse osmosis membrane elements. At this time, it is necessary to clean the reverse osmosis membrane elements off-line.

Although there will be a certain degree of margin in the design of reverse osmosis system to ensure that there will be no threat to safe production due to insufficient water supply due to the decrease of water production or desalination rate of reverse osmosis system and the increase of differential pressure of reverse osmosis system in case of emergency, But in fact, it is because of the existence of these margins that sometimes hidden faults can not be displayed in time, which may eventually evolve into severe pollution of reverse osmosis membrane components.

one

Concept of severe pollution of reverse osmosis membrane elements

It refers to the surface adhesion and Deposition Pollution of RO membrane caused by suspended solids, colloids, organic matter, microorganisms and other particles contained in the influent of reverse osmosis system, or the formation of chemical scale after the ion product is greater than the solubility product due to concentration polarization and other factors on the membrane surface. Severe pollution refers to the situation that the single section pressure difference after pollution is more than 2 times of the single section pressure difference at the initial stage of system operation, the water production of reverse osmosis system decreases by more than 30%, or the weight of single reverse osmosis membrane element exceeds the normal value by more than 3kg.

Severe pollution is often the superposition of severe physical pollution and severe chemical pollution. In some cases, they are accompanied at the same time.

two

Causes and characteristics of severe pollution of reverse osmosis membrane components

Due to the different water quality of water sources and different treatment methods, the causes of RO membrane elements are also different. The common pollution causes and characteristics are as follows:

one

Design and manufacturing defects of water treatment process system

① For the reverse osmosis pretreatment filter, when the source water is not well water, the equipment filtration speed is greater than 8.5m/h, while the equipment filtration speed in the surface water design is greater than 8m / h, resulting in exceeding the standard of SDI of reverse osmosis inlet water;

② The source water contains colloids, suspended solids and other impurities, but there are no removal measures in the design;

③ COD in source water is more than 3 without removal measures;

④ If the design flux of reverse osmosis membrane element exceeds the standard, the scaling and differential pressure of the second section of the reverse osmosis device increase rapidly, and the water production attenuation and desalination rate decrease rapidly.

The above ① ② ③ design defects may increase the pressure difference in the first section of the reverse osmosis system; ③ It may also lead to the increase of the second section differential pressure, and all the above design defects may lead to the reduction of the system water production.

two

Improper selection of reverse osmosis additives and other consumables

① Improper selection of reverse osmosis scale inhibitor type or dosage, such as: the type of reverse osmosis scale inhibitor is incompatible with the source water, such as aluminum ion and iron ion in the source water or iron, aluminum and other flocculants are used in pretreatment dosing. Try to select organic reverse osmosis scale inhibitor and avoid polycarboxylic acid type as much as possible; The dosing amount of reverse osmosis scale inhibitor is generally between 2-4ppm (calculated according to the water inflow), and can not exceed 6ppm at most. Improper selection of scale inhibitor type and dosing amount will lead to rapid decrease of system water production and obvious increase of system differential pressure;

② The quality of the filter element of the reverse osmosis security filter should be reliable in the selection process. The filter element known as 5 microns and the actual accuracy can not reach is very unfavorable to the safe operation of the reverse osmosis membrane element;

③ Improper selection of flocculant and coagulant aid in reverse osmosis pretreatment system will cause serious pollution of membrane elements, while improper selection of bactericide will seriously damage the desalination rate of reverse osmosis system; (containing Cl --).

three

System abnormality caused by sudden change of water treatment operating conditions

Generally, it is due to the increase of suspended matter content in source water due to climate change, or the change of source water type (for example, the change from groundwater to tap water leads to the increase of residual chlorine content in reverse osmosis inlet water), which worsens the operation condition of reverse osmosis device and forms severe pollution.

four

System operation management

① Do not operate according to the operating procedures;

② The adjustment of equipment status parameters is not timely;

③ Improper adjustment of operating parameters;

④ The metering of added chemicals is not optimized;

⑤ Do not clean the system in time when the system reaches the cleaning conditions;

⑥ The system is not adjusted on time;

⑦ Lack of operation management summary.

five

Water supply shortage of water treatment system

In addition to the above, in a considerable number of enterprises, due to the shortage of water supply in the water treatment system, even if the operation parameters of the reverse osmosis device deteriorate and the system is polluted, the system cannot be cleaned in time due to the shortage of water supply, resulting in the gradual severity of system pollution and severe pollution.