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The cause of yellow of printing and dyeing wastewater after treatment


Printing and dyeing wastewater generally has the characteristics of high organic content, deep color and high alkalinity.
If it is treated with ferrous sulfate and liquid alkali, there are two possibilities.

  1. Some inorganic metal ions in printing and dyeing wastewater that have not been precipitated.
  2. The color of organic matter; for the removal of inorganic metal ions that have not been precipitated, alkali can be adjusted and sodium sulfide can be added to precipitate the inorganic metal ions.

If the organic matter in printing and dyeing wastewater makes the color pale yellow, it is necessary to consider advanced oxidation or adsorption.

The original color of waste water is yellow. After ferrous treatment, the color will fade and then yellow again. The reason may be a reverse color phenomenon, which is related to the use of some reactive dyes
It seems that the color of the front is removed, and the color of the back is similar to that of the original wastewater, so most of the dyeing wastewater treatment processes will adopt biochemical treatment.

The treatment effect of ferrous sulfate on red and green wastewater is worse, but the chroma can be reduced to the discharge standard as long as the reaction conditions are suitable.
In this case, the following suggestions are made:

  1. Add sodium hydroxide to the front end and ferrous to the back end, and keep the pH at about 9 after ferrous addition.
    After sodium hydroxide is added, the pH value can reach more than 12. Under the condition of strong alkali, the molecular structure of some colored groups can be destroyed, and the difficult colored groups can be changed into the easy colored groups.
    Moreover, the pH value will be about 11 at the beginning of reaction with ferrous, and the decolorization effect of ferrous above pH 11 will be better than that of pH 8, especially for some refractory colored groups.
  2. Replacing sodium hydroxide with lime can not only reduce the cost of agent, but also improve the treatment effect.

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