The reverse osmosis membrane element is used as a deep filtration method, and the surface of the reverse osmosis membrane element will inevitably have colloids, microorganisms, impurity particles and insoluble salts precipitated on its surface. , eventually need to be cleaned, but the length of the cleaning cycle is different. However, online cleaning, as a means of cleaning and maintenance, impact sterilization and regular protection of reverse osmosis system, is powerless in the face of heavy pollution of reverse osmosis membrane elements. At this time, it is necessary to perform offline cleaning of reverse osmosis membrane elements.

Although there will be a certain degree of surplus in the design of the reverse osmosis system to ensure that in an emergency, the safety production will not be caused by insufficient water supply due to the decrease of the water production or desalination rate of the reverse osmosis system and the increase of the pressure difference of the reverse osmosis system. Threats, but in fact it is precisely because of the existence of these surpluses that sometimes hidden faults cannot be manifested in time, which may eventually turn into heavy pollution of reverse osmosis membrane elements.

The concept of severe pollution of reverse osmosis membrane elements refers to the surface adhesion, deposition pollution of RO membranes caused by suspended solids, colloids, organic matter, microorganisms and other particles contained in the influent water of the reverse osmosis system, or the chemical ion components in the water are on the membrane surface due to Concentration polarization and other factors cause the formation of chemical scales after the ion product is greater than the solubility product. Severe pollution means that the single-stage pressure difference after pollution is more than 2 times the single-stage pressure difference value at the initial stage of system operation, the water production of the reverse osmosis system has dropped by more than 30%, or the weight of a single reverse osmosis membrane element exceeds the normal value by more than 3 kg. Condition.　　

Heavy pollution is often the superposition of heavy physical pollution and heavy chemical pollution, and in some cases, the two are associated at the same time.
The reasons and characteristics of heavy pollution of reverse osmosis membrane elements are different because of the different water quality and treatment methods used in different places, so the reasons for RO membrane elements are also different. The common causes and characteristics of pollution are as follows: 1 Design and manufacturing defects of water treatment process system

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

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

③ COD in source water is >3 without removal measures;

(4) The second stage of the reverse osmosis unit whose designed flux exceeds the standard is fouled and the pressure difference rises rapidly, the water production declines and the desalination rate drops rapidly.

The above ①②③ design defects may lead to an increase in the pressure difference in the first stage of the reverse osmosis system; ③ may also lead to an increase in the second stage pressure difference, all of the above design defects may lead to a decrease in the water production of the system.

2 Improper selection of reverse osmosis additives and other consumables

①The type or dosage of reverse osmosis scale inhibitor is improperly selected, such as: the type of reverse osmosis scale inhibitor is not compatible with the source water, for example, the source water contains aluminum ions, iron ions, or iron, aluminum, etc. are used in the pretreatment and dosing. As flocculant, try to choose organic anti-scaling agent for reverse osmosis, and try to avoid using polycarboxylic acid type; the dosage of reverse osmosis anti-scaling agent is generally between 2-4 ppm (calculated by the amount of water inflow), and can not exceed 6 ppm at most etc., the improper selection of scale inhibitor type and dosage will lead to a rapid decrease in system water production and a significant increase in system pressure difference;

② The quality of the reverse osmosis security filter element must be ensured during the selection process. The filter element that claims to be 5 microns but the actual accuracy cannot reach it is very unfavorable for the safe operation of the reverse osmosis membrane element;

③ Improper selection of flocculants and coagulants in the reverse osmosis pretreatment system will cause serious pollution of membrane elements, and improper selection of bactericides will seriously damage the desalination rate of the reverse osmosis system; (contains Cl-).

3 System abnormalities caused by sudden changes in water treatment operating conditions

Generally, the operation of the reverse osmosis device deteriorates when the content of suspended solids in the source water increases due to climate change, or the type of source water changes (for example, the change from groundwater to tap water leads to an increase in the residual chlorine content in the reverse osmosis influent). form heavy pollution.

4 System operation and operation management problems

①Do not operate according to the operating procedures;

② The adjustment of equipment status parameters is not timely;

③ Improper adjustment of operating parameters;

④The dosage of the added agent has not been optimized;

⑤ The system is not cleaned in time when the system reaches the cleaning conditions;

⑥ The system is not adjusted on time;

⑦ Lack of operation management summary.

5 The water supply of the water treatment system is tight

In addition to the above situations, in many enterprises, due to the shortage of water supply in the water treatment system, even if the operating parameters of the reverse osmosis device deteriorated and the system was polluted, due to the shortage of water supply, timely cleaning could not be carried out, resulting in system pollution. Gradually serious, resulting in heavy pollution.