Detailed Explanation of Four Processes for Pharmaceutical Ultrapure Water Preparation

There are four main processes for pure water preparation:

1. Raw water – Pretreatment – Anti-ion exchange – Anion exchange – Anion exchange – Mixed bed – Pure water;

2. Raw water – Pretreatment – Electrodialysis – Cation exchange – Anion exchange – Anion exchange – Mixed bed – Pure water;

3. Raw water – Pretreatment – Weak acid bed – Reverse osmosis – Raw water – Anion exchange – Anion exchange – Mixed bed – Pure water;

4. Raw water – Pretreatment – Weak acid bed – Reverse osmosis – Degassing – Mixed bed – Pure water 

Process 1, the full ion exchange method, and process 2, the electrodialysis-ion exchange method, are commonly used pure water processes. The process selection depends on the salinity of the raw water. When the feed salinity is 300 mg/L, process 1 is economically viable. When the feed salinity is 500-600 mg/L, the output is only half of the input water, with the remainder consumed in resin bed washing and regeneration. When the salinity reaches 800-1000 mg/L, the pure water output is only 14% of the input water; at this point, the ion exchange water production cycle is short, regeneration is extremely frequent, and normal water supply is difficult.

Process 2 combines the advantages of electrodialysis and ion exchange to obtain high-quality pure water. When the feed salinity is 500-600 mg/L, process 2 is more technically and economically reasonable than process 1, and its advantages become more significant with higher salinity.

Processes 3 and 4 are characterized by the presence of a reverse osmosis unit, which can remove 85%-90% of salts, achieving a higher desalination rate than electrodialysis. Furthermore, it offers excellent sterilization, pyrogen removal, and COD reduction. In pure water preparation systems, reverse osmosis serves as primary desalination before the resin bed. If the water hardness is high, a softening bed or weak acid bed is often installed after the mechanical filter to remove calcium and magnesium ions. Some pure water systems directly use it as pretreatment for mixed beds. In this case, to minimize the amount of alkali used during mixed bed regeneration, a degassing tower is often installed to remove carbon dioxide from the water. The pure water quality is significantly higher than that of processes 1 and 2, with a resistivity reaching 8 MΩ·cm. However, the pretreatment requirements for water are more stringent, and the equipment investment and operating costs are higher than those of processes 1 and 2. Moreover, due to the complexity of the reverse osmosis system, the operation and maintenance requirements are very strict, making its application difficult for small and medium-sized pharmaceutical plants.