Reasons for the increase in pH value of demineralized water
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Our company has recently installed a new concentrated water recovery system. At the beginning of its operation, a problem arose: the desalinated water (with a pH value of around 4.2), after being transported over a long distance of about 1200 meters, had its pH value rise to around 8.1. Initially, it is suspected that the reasons could be 1) there is a large amount of rust on the inner surface of the pipes, 2) a hydrogen evolution reaction occurred, or 3) some other reactions. Those with experience in this area are requested to analyze and determine the exact cause.
There is little data available, and the specific processing methods are not clear either, so it’s difficult to say. The change from acidic to alkaline water can be explained from these aspects: 1. Initially, the pH value was 4.2, meaning the water was slightly acidic; during transportation, this weak acidity dissolved. For example, carbonic acid may decompose during transportation due to changes in process conditions, resulting in an increase in pH value. 2. Alkaline substances may mix in along the pipeline. 3. There are strong base weak salts in the water; these decompose when process conditions change during transportation. 4. Conduct ion analyses before and after transportation to observe any changes in the relative amounts of positive and negative ions, which can help identify the cause. Rust in the pipes is iron oxide; it’s not clear whether it will dissolve into iron ions at a pH value of 4.2, thereby causing the pH value to rise. The situation is complex, and my explanation might be incorrect; please bear with me.
The desalination process is a reverse osmosis membrane-based desalination method
Is your reverse osmosis membrane desalination process caused by the addition of substances such as reducing agents, scale inhibitors, or oxidizing agents when the concentrated water enters the reverse osmosis membranes?
Scale inhibitors were added; nothing else was added