Why does reverse osmosis require pre-treatment?

Author: Hongjie Water

The reverse osmosis system includes three parts: pre-treatment of raw water, reverse osmosis device, and post-treatment. The reverse osmosis system has specific requirements for the pre-treatment of raw water. Due to the wide variety of raw water and its complex composition, a suitable pre-treatment process system is selected to meet the requirements of the main process design parameters such as raw water quality and reverse osmosis system recovery rate. This reduces fouling and scaling of the reverse osmosis membrane, and prevents a decrease in the desalination rate and water production rate of the reverse osmosis membrane. Especially in response to the increasingly scarce water source and deteriorating water quality, a correct pre-treatment system is chosen, It will directly affect the functionality of the entire water treatment system. As is well known, the failure of the reverse osmosis system is mostly caused by the incomplete function of the pre-treatment system. To ensure the normal operation of the reverse osmosis process, strict pre-treatment of the raw water is necessary.

The main factors that reduce the performance of reverse osmosis membranes are: 

1. The membrane undergoes chemical degradation, such as the destruction of aromatic polyamides by oxidizing agents such as chlorine and strong acids and bases;

2. Scaling of insoluble salts on the membrane surface;

3. The membrane is blocked by suspended solids and colloidal pollutants from influent water;

4. Membrane fouling and membrane degradation occur after adhesion and erosion by microorganisms, fungi, algae, etc;

5. Large molecule organic matter blocks membrane fouling and small molecule organic matter is adsorbed by the membrane.

The efficiency and lifespan of reverse osmosis are closely related to the pre-treatment effect of raw water. The purpose of pre-treatment is to reduce the pollution, scaling, and damage of the membrane caused by influent water, thereby optimizing the system's water production, desalination rate, recovery rate, and operating cost. Therefore, good pre-treatment is crucial for the long-term safe operation of reverse osmosis devices. Its purpose is subdivided into:

1. Remove suspended solids and reduce turbidity;

2. Control the growth of microorganisms;

3. Control the deposition of slightly soluble salts;

4. Adjustment of inlet water temperature and pH;

5. Removal of organic matter;

6. Precipitation control of metal oxides and silicon.

The goal of reverse osmosis membrane pretreatment

In order to ensure the water recovery rate, permeate water quality, stable permeate water flow rate, low operating costs, and optimized membrane service life of the reverse osmosis system, it is necessary to carry out comprehensive pre-treatment. The specific goals are:

1. To prevent pollution on the membrane surface, it is necessary to remove suspended solids, microorganisms, colloidal substances, and organic matter as much as possible, in order to prevent these substances from depositing on the membrane surface or clogging the water flow channel of the membrane element;

2. To prevent scaling on the membrane surface, it is necessary to minimize the deposition of insoluble salts such as CaCO3, CaSO3, BaSO3, SrSO3, CaF2, as well as iron, manganese, aluminum, silicon compounds, etc. on the membrane surface;

3. To prevent physical and chemical damage to the membrane, it is necessary to try to avoid the impact of high temperature, acidic or alkaline water, oxidants, etc. on the membrane.