25 common solutions of reverse osmosis membrane technology problem (Part 4)

19. What are the effects of pH on removal rate, water production, and membrane life?

The pH range corresponding to reverse osmosis membrane products is generally 2-11, and pH has little effect on the membrane performance itself, which is one of the significant characteristics different from other membrane products. However, the characteristics of many ions in water are greatly affected by pH. For example, when weak acids such as citric acid are in a non-ionic state under low pH conditions, they dissociate and become ionic at high pH values. Due to the high charge level of the same ion, the removal rate of the membrane is high. If the charge level is low or not, the removal rate of the membrane is low. Therefore, pH has a significant impact on the removal rate of certain impurities.

20. What is the relationship between TDS of incoming water and conductivity?

When obtaining the inlet conductivity value, it must be converted into TDS value so that it can be input during software design. For most water sources, the ratio of conductivity to TDS is between 1.2 and 1.7. For ROSA design, a ratio of 1.4 is used for seawater and a ratio of 1.3 is used for brackish water conversion, which usually yields a good approximate conversion rate.

21.How to know if the membrane has been contaminated?

The following are common symptoms of pollution:

1. Under standard pressure, water production decreases
2. In order to achieve standard water production, it is necessary to increase the operating pressure v
3. The pressure drop between inlet and concentrated water increases by v
4. The weight of membrane components increases by v
5. Significant changes in membrane removal rate (increase or decrease)
6. When the component is removed from the pressure vessel, pour water onto the inlet side of the vertical membrane component. Water cannot flow through the membrane component and only overflows from the end face (indicating complete blockage of the inlet channel).

22. How to prevent the growth of microorganisms inside the original packaging of membrane components?

When the protective solution becomes cloudy, it is likely due to the growth of microorganisms. Membrane components protected with sodium bisulfite should be inspected every three months. When the protective solution becomes cloudy, the components should be removed from the sealed storage bag and re soaked in fresh protective solution with a concentration of 1% (by weight) food grade sodium bisulfite (not activated by cobalt), soaked for about 1 hour, and re sealed. The components should be drained before repackaging.

23. What are the water inlet requirements for RO membrane components?

In theory, entering the RO and IX systems should not contain the following impurities:

1. Suspended solids, colloids, calcium sulfate, algae, bacteria, oxidants, such as residual chlorine, etc
2. Oil or lipid substances (must be below the lower detection limit of the instrument)
3. Chromium complexes of organic matter and iron organic matter
4. Metal oxides such as corrosion products of iron, copper, and aluminum

23. What impurities can RO membranes remove?

RO membrane can effectively remove ions and organic matter. Reverse osmosis membrane has a higher removal rate than nanofiltration membrane. Reverse osmosis can usually remove 99% of the salt in the feed water, and the removal rate of organic matter in the feed water is ≥ 99%.

24. How do you know what cleaning method to use for your membrane system?

In order to achieve the best cleaning effect, it is very important to choose targeted cleaning agents and cleaning steps. Incorrect cleaning can actually deteriorate system performance. Generally speaking, for inorganic scaling pollutants, it is recommended to use acidic cleaning solutions, microbial or organic pollutants, and alkaline cleaning solutions are recommended.

Why is the pH value of RO produced water lower than the pH value of influent water?

When the balance between CO2, HCO3-, and CO3=is understood, the best answer to this question can be found. In a closed system, the relative content of CO2, HCO3-, and CO3=varies with pH value. Under low pH conditions, CO2 accounts for the majority, with HCO3- predominating in the medium pH range and CO3=predominating in the high pH range. Due to the ability of RO membrane to remove soluble ions but not soluble gases, the CO2 content in RO production water is basically the same as that in RO influent. However, HCO3- and CO3=can often be reduced by 1-2 orders of magnitude, which can break the balance between CO2, HCO3-, and CO3=in influent. In a series of reactions, CO2 will combine with H2O to undergo the following reaction equilibrium transfer until a new equilibrium is established.

If the influent contains CO2, the pH value of the RO produced water will always decrease. For most RO systems, the pH value of the reverse osmosis produced water will decrease by 1-2 times. When the influent alkalinity and HCO3- are high, the pH value of the produced water will decrease even more.

A very small amount of incoming water, containing less CO2, HCO3- or CO3=, results in less variation in the pH value of the produced water. In some countries and regions, there are regulations for the pH value of drinking water, generally ranging from 6.5 to 9.0. According to our understanding, this is to prevent corrosion of the water supply pipeline. Drinking low pH water itself will not cause any health problems. As is well known, many commercially available carbonated beverages have a pH value between 2 and 4.