During the use of the equipment, in addition to the normal attenuation of performance, the attenuation of equipment performance is more serious due to pollution. The common pollution of EDI high-purity water equipment mainly includes chemical scale, organic matter and colloidal pollution, and microbial contamination. The symptoms of different pollution are different. The symptoms of membrane fouling proposed by different membrane companies are also somewhat different.
In the project, we found that the duration of pollution is different, and the symptoms are different. For example, when the film is contaminated with calcium carbonate scale, the pollution time is one week, the main performance is the rapid decrease of the salt rejection rate, the pressure difference increases slowly, and the water production does not change significantly. The cleaning with citric acid can completely restore the performance. The pollution time is one year (a pure water machine), the salt flux increases from the initial 2mg/L to 37mg/L (the raw water is 140mg/L to 160mg/L), and the water production decreases from 230L/h to 50L/h. After washing with citric acid, the salt flux was reduced to 7 mg/L, and the water production increased to 210 L/h.
In addition, pollution is often not single, and the symptoms of its performance are also different, making the identification of pollution more difficult.
Identification of pollution types should be based on the combination of raw water quality, design parameters, pollution index, operational records, equipment performance changes and microbiological indicators:
(1) Colloidal pollution: When colloidal contamination occurs, it is usually accompanied by the following two characteristics: A. The microfilter is blocked quickly in the pretreatment, especially the pressure difference increases rapidly, and the B and SDI values are usually above 2.5.
(2) Microbial contamination: When microbial contamination occurs, the total number of bacteria in the permeated water and concentrated water of the RO equipment is relatively high, and it must not be maintained and disinfected as required.
(3) Calcium scale: It can be judged according to the raw water quality and design parameters. For carbonate type water, if the recovery rate is 75%, the scale inhibitor is added at the time of design, the LSI of the concentrate should be less than 1; the LSI of the concentrate should be less than zero when no scale inhibitor is added. No calcium scale is produced.
(4) The performance change of different parts of the test component can be judged by inserting a 1/4 inch PVC plastic tube into the assembly.
(5) Determine the type of pollution based on changes in equipment performance.
(6) Acid washing (such as citric acid, dilute HNO3) can be used to judge the calcium scale according to the cleaning effect and the cleaning liquid, which is further confirmed by the analysis of the composition of the cleaning liquid.
(7) Chemical analysis of the cleaning solution: take raw water, clean the original solution, and wash the liquid, and analyze the three samples.
After determining the type of contamination, it can be cleaned as in Table 1 and then disinfected. When the type of contamination cannot be determined, it is usually cleaned by washing (3) disinfecting 0.1% HCl (pH 3).