Technical discussion: What is the pH of the desalinated water?
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This post was last edited by hesonchang214 on 2012-7-26 at 17:20. I would like to ask fellow sea water professionals: what is the typical pH value of the desalinated water produced by the mixed-bed systems in your companies?
Deionized water with a conductivity of less than 0.2 μS/cm, and a pH value of 6.5–7.5.
The pH of the desalinated water produced by the mixed-bed system is generally between 5.5 and 7.0, with it rarely exceeding 7.0.
It’s the same in our company as well; the deionized water produced by the mixed-bed system has a pH level between 5.8 and 7. However, I learned that another company’s pH level is between 7 and 9, which puzzles me. Of course, logically speaking, a higher pH level should result in less corrosion of the pipes.
The output water from the mixed-bed system cannot be too high due to the rapid dissolution of carbon dioxide during testing; it is generally between 5.5 and 7.0.
The pH value of the desalinated water at the outlet of the mixed-bed is generally between 6.5 and 7.5; if it falls below 7.0, ammonia should be added to adjust the pH value, otherwise it may cause corrosion of the equipment or pipes.
This post was last edited by cdpulin on 2012-2-21 at 10:45. The application of electrochemical theory is used to study the relationship between pH and conductivity in pure water-grade desalinated water. For deionized water with a conductivity of less than 0–2 μS/cm, its pH value must be within the range of 6.3 to 7.9. For pure water-grade deionized water, for every 1-unit decrease in pH value, the conductivity increases to approximately 10 times its original value. The pH value of desalinated water is analyzed by manual sampling, and the error mainly stems from the influence of air.
Project Category Conductivity uS/cm Na+ ug/l pH SiO2 ug/l Hardness umol/l Turbidity mg/l Fresh water 0～6 Effluent from cation exchange resin 1～500 0 Effluent from anion exchange resin 0.2～10 1～500 7.0～9.5 0 First-stage deionized water 0.1～5 0.5～300 7.0～9.2 0 Second-stage deionized water 0～1.0 0～20 5.6～7.0 2～20 0 Third-stage deionized water 0～1.0 0～20 1～20 0
Ammonia is generally added to the effluent from the mixed-bed system to adjust the pH to 8.5–9, which reduces corrosion in the pipes.
If sampling for measurement is used, there will be errors; it is recommended to use an online pH meter, with a value generally below 7. Regarding pipeline corrosion, the addition of ammonia is commonly used; for shorter pipelines, materials such as stainless steel or UPVC can be employed. This post was last edited by li*ch1968 on 2009-4-16 at 16:31.]
Generally, the pH of the effluent is below 7, and ammonia needs to be added for adjustment
The output water from a mixed-bed system is generally between 6 and 7
This post was last edited by cdpulin on 2012-2-21 at 10:46. “The deionized water produced by the mixed-bed system has a pH between 5.8 and 7, but from what I’ve learned, a certain company’s deionized water has a pH between 7 and 9.” Is that the pH value of that company’s deionized water after ammonia treatment?
Under normal operation, the pH value at the outlet of the mixed-bed should be 6.5–7.0; it may be lower when the system is not functioning properly. However, issues with the resin ratio, or problems with the regeneration equipment or chemicals, can also cause the pH to drop. However, generally, desalinated water must first be stored in a desalination tank for a certain period of time before being pumped into the make-up water system using a desalination pump. If the desalination tank is not isolated from air, carbon dioxide in the air will dissolve into the desalinated water, causing its pH value to drop as low as 5.5
The pH value at the outlet of the mixed-bed should be maintained between 6.5 and 7.0
RO membranes cannot remove gas, so carbon dioxide in the water can pass to the product water side where it dissolves and hydrolyzes, resulting in an acidic pH. Using conventional surface water or groundwater (with a pH generally between 6.5 and 7.5), if the pH is not adjusted before the first stage of RO treatment, the resulting water will be slightly acidic due to the carbon dioxide that has passed through it; its pH is usually between 5.5 and 6.5, although this value is affected by temperature and atmospheric pressure. In secondary RO processes, steps are generally taken to remove this carbon dioxide beforehand (using a decarburizer or by adding alkali to adjust the pH to around 8.5). When a decarburizer is used, the produced water is usually slightly acidic; however, this is not necessarily the case when alkali is used, as it depends on the pH of the feed water after adjustment, and the fluctuations in pH are somewhat greater in the latter case.
An outlet water concentration of 6–9 is considered normal for mixed-bed systems. Here, the pH of the produced water is generally kept between 7 and 9 to meet production requirements ； An excessively high or low pH in the mixed-bed is closely related to the ratio of resin; adjusting this ratio appropriately can change the pH of the produced water.
Here, the conductivity of RO water is around 7, with a pH of 5.8 to 6
The pH value of the effluent is generally around 5.5 to 7; ammonia is added to adjust it to a theoretical pH of 8.8 to 9.2, while an actual pH range of 8.5 to 9.5 is sufficient
I have a question: Can I add caustic soda?
The pH value at the outlet of the mixed-bed is controlled between 6.5 and 7.0.