At present, the mainstream purification water preparation process on the market includes the following three kinds:

(1) Pretreatment + first stage reverse osmosis (RO) + second stage reverse osmosis (RO)

(2) Pretreatment + first stage reverse osmosis (RO) + electric deionization (EDI)

(3) Pretreatment + first stage reverse osmosis (RO) + second stage reverse osmosis (RO) + electric deionization (EDI)

The pretreatment unit is mainly composed of multi-media filter, softener and activated carbon filter. The function of multi-media filter is to remove suspended solids and particles larger than 10μm in raw water, requiring SDI value of water production ≤5; The role of softener is to use raw water through sodium cation exchange resin, reduce the hardness of water to avoid reverse osmosis membrane fouling, so as to achieve the purpose of reverse osmosis membrane to maintain good performance; The role of activated carbon filter is to adsorb organic matter in water, residual chlorine, oxidant, pigment and other substances, to prevent reverse osmosis membrane oxidation. The residual chlorine value of water production is less than 0.05mg/L. Activated carbon filters require regular pasteurization to reduce the risk of microbial growth. In general, if the raw water conductivity, hardness is low, pretreatment can not use softener. If the conductivity and hardness of raw water are high, it is recommended to use softener for pretreatment to improve the quality of pre-treated water and extend the chemical cleaning cycle and service life of RO.

Electric deionization (EDI) is generally followed by reverse osmosis system, which is a modern new separation technology, combining electrodialysis and ion exchange resin desalting technology, secondary desalting RO water, water conductivity <0.07μS/cm. The concentrated water of EDI unit was also reused before reflux RO, and the recovery rate was 90~95%. EDI device has many advantages, such as no chemical regeneration, no consumption of acid and alkali, saving resources; It can operate continuously and produce water stably and efficiently. In the system design, compact structure, can save space; Convenient operation and management, convenient maintenance, low operating cost; It can be pasteurized at high temperature to reduce the risk of microbial growth.