How to adjust pH in coagulation sedimentation, and how to select chemicals
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Physical-chemical coagulation and sedimentation require pH adjustment (lime, liquid alkali, sulfuric acid are sometimes used); appropriate chemicals and dosages must be selected to ensure effective coagulation (options include PAC+PAM, lime+ferrous ions+PAM). 1. What should the pH value generally be controlled at? What chemicals are used to adjust pH? 2. Which types of coagulants are generally used? How to combine them for better results? 3. What is the best way to adjust pH and add chemicals?
We deal with acidic wastewater; previously lime was used to adjust the pH value, but since lime generates a large amount of sludge, we have now switched to liquid alkali (sodium hydroxide). The pH value for coagulation sedimentation is generally maintained between 7 and 8.
Check what the main metal ions are, and determine the pH based on the minimum solubility product; it’s usually around 8-10
I have previously dealt with leather wastewater; the pH of the raw wastewater ranged from 5 to 8. Lime was used to adjust the pH, while the coagulation agents employed were lime + ferrous sulfate + PAM. Lime is hydrated lime, mixed with water to about 5% concentration ； Ferrous sulfate is a liquid with a concentration of around 12% ； PAM uses non-ionic types. The sequence of adding chemicals is to first add lime to adjust the pH to around 10, then add ferrous sulfate to bring the pH to 7–8, and finally add PAM ； It offers excellent coagulation and decolorization effects.
1. The pH value required for discharge must be kept between 6 and 9. If it is below 6, alkaline substances (lime, caustic soda, liquid caustic) must be used for adjustment; if it is above 9, acidic substances (sulfuric acid, hydrochloric acid) must be used for adjustment. 2. Commonly used coagulants include PAC, aluminum sulfate, ferrous sulfate, etc. For ordinary wastewater, PFS or PAC can be used directly for coagulation and sedimentation. 3. The best method for adding chemicals is to use liquids (solid substances need to be dissolved first); mechanical stirring or aeration can be employed as methods of mixing
This post was last edited by zhaolijun on 2015-6-10 09:22. 1. The pH value required for emissions must be kept between 6 and 9. Therefore, values below 6 require the use of alkaline substances (lime, caustic soda flakes, liquid caustic soda) for adjustment ； Values above 9 require adjustment using acidic substances (sulfuric acid, hydrochloric acid). 2. Commonly used coagulants at present include PAC, PFS, aluminum sulfate, ferrous sulfate, etc. For ordinary wastewater, PFS or PAC can be used directly for coagulation and sedimentation. 3. The best method for adding chemicals is to use liquid form (solid substances need to be diluted first); for mixing, mechanical stirring or aeration can be employed.