Different materials of purified water equipment membranes have different pH ranges, such as acetate fiber membranes with a pH range of 3-8, aromatic polyamide membranes with a pH range of 4-10, and DuPont nylon hollow fiber membranes with a pH range of 1.5-12. When the pH value of the feed solution exceeds the usage limit of the membrane, it will cause harmful effects such as hydrolysis and aging of the membrane, resulting in a decrease in water production and a sustained decrease in membrane performance, until the membrane is damaged. Usually, the pH value during the operation of acetate fiber membrane should be controlled between 4-7, while the pH value during the operation of directional polyamide membrane should be controlled between 3-11. The allowable pH range for composite membrane is about 2-11.