Prevention and treatment of calcium carbonate scaling in reverse osmosis operation

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In the daily operation of reverse osmosis, a common failure factor is calcium carbonate scaling. The judgment, prevention, judgment and treatment of calcium carbonate scaling can be handled in the following ways: Index LSI and S&DSI Index for judging calcium carbonate scaling tendency For brackish water with TDS≤10,000mg/L, the Langrier index (LSI) is used as an indicator of the scaling possibility of CaCO3. In the LSI= PHC-PHS formula, pHC is the pH value of concentrated water and pHS is the pH value when CaCO3 is saturated. When LSI≥0, CaCO3 scaling will occur. When most natural water is untreated, the LSI will be positive. For high-salinity brackish or seawater sources with TDS > 10,000mg/L, the Steve and David Saturation Index (S&DSI) is used as an indicator of the scaling potential of CaCO3. In the formula S&DSI= PHC-PHS, pHC is the pH value of concentrated water, and pHS is the pH value when CaCO3 is saturated. When S&DSI is ≥0, CaCO3 scaling will occur. When most natural water sources with high salt content are not treated, S&DSI is generally positive. In order to prevent CaCO3 scaling, the S&DSI must be made negative by adding acid. If the CaCO3 is prevented from settling by adding scale inhibitor, the S&DSI can be positive.

Prevention of calcium carbonate scaling

(1) Raw water quality survey: Input the raw water quality test results into the reverse osmosis design software, automatically calculate the LSI or S&DSI index, and formulate the scale prevention plan according to the value;

(2) pretreatment softening process: the softening process mainly includes lime softening, lime soda softening, resin softening, and the appropriate softening process is selected according to the amount of water treated and the economy;

(3) Adding acid and scale inhibitor to RO water: reserve acid and scale inhibitor dosing equipment and pipelines during design;

(4) Regularly test the electrical conductivity of the single-branch membrane shell of the first and second stages of the reverse osmosis system. If the electrical conductivity of the second stage is found to be abnormally increased, it is necessary to investigate the cause in time. Determination of calcium carbonate scaling

(1) Water quality investigation: if the alkalinity and hardness of RO inlet water are high, and the LSI or S&DSI index calculated by measuring the pH value of concentrated water is greater than zero, it indicates that there is a risk of scaling;

(2) Operation data survey: the general system will show that the pressure difference of the second stage is larger, the water production is lower, and the conductance of the second stage water production is higher;

(3) The pickling effect is obvious: when hydrochloric acid or citric acid is used for cleaning, the pH of the cleaning solution rises faster, and the water production and desalting rate recover after pickling;

(4) Remove the end cover: there is white or yellowish inorganic scale on the surface of the last membrane element or end plate in the end segment, and the inorganic scale can be dissolved by acid. After calcium carbonate scaling, timely chemical cleaning is necessary, and pickling is generally recommended; Hydrochloric acid is recommended for pickling. If the scale is very serious, pay attention to it when cleaning online:

a) Pickling solution pH≥3;

b) Low flow circulation dissolution, large flow circulation is strictly prohibited to prevent inorganic scale scratching the desalting layer;

c) Add acid in time to maintain the pH of the cleaning solution.

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