The film of membrane element will be contaminated by inorganic salt, microbial, colloidal solid and not soluble organics after a period of running. These contaminants are deposited on the surface of membrane film, which cause the standard flux rate and salt rejection rate decline or even deteriorate.

Membrane system needs to chemical clean when the following conditions occurrence:

• Standardization water production permeate flux is lowered than 10-15%.
• The difference of system pressure between inflow water and concentrated water reach the 1.5 times of initial value.
• Standard salt permeate rate increase more than 10-15%.

The types of cleaning agents will be determined by quality of feed water. The membrane system needs acid cleaning following by alkali cleaning as the system mainly contaminated by the inorganic scaling. On the other hand, the membrane system needs alkili-acid-alkali sequence cleaning process, as the system is mainly fouled by organic matters

There are six steps in the cleaning process:

1. Flush the membrane element. Rinsing with the permeate water under low pressure and high flow velocity.
2. Preparation of cleaning solution Pump mixed, preheated the cleaning solution to the vessel at conditions of low flow rate.
3. Recycle. Membrane system should be cleaned with low pressure and high flux water. At the initial five minutes, circulation cleaning should be controlled with 1/3 of designed flow rate. Then the flux should be adjusted 2/3 of designed flux to full value in each ten minutes. The time of the recycle usually is 1 hour. Measure the pH of solution and difference of the pressure.
4. Soak and recycle. Slowly turn the circulation pump off and allow to soak the element. The soaking time is 1-12 hours depending on the situation of the contamination.
5. Rinse. Feed the clean solution into the membrane element and cleaning device. The rinsing time is about 20-60 minutes. Once the pH and conductivity of concentrated flow is similar with the feed flow, the rinsing process can be stopped.
6. ⑥Trail. After turning on the system, the permeate should be drained until the water quality meets the requirements.

When the pH value changes 0.5, operator should add cleaning agents. It is strongly recommended to clean the system separately by using one type of cleaning solution. Before alerting the cleaning agent, the membrane system should be thoroughly washed by permeate water. Usually, the period of acid cleaning is no more than 8h and of alkali cleaning is no more than 24-36h.