In the textile industry, wastewater treatment is crucial to ensure environmental compliance. Many times, after undergoing biological treatment and the addition of polyaluminum chloride or ferrous sulfate with alkali, the treated wastewater exhibits a pale yellow color. What causes this phenomenon, and how can it be effectively treated? In this blog, TAIRAN CHEMICAL will delve into the reasons behind the yellowing of wastewater after dyeing and printing wastewater treatment, especially focusing on the use of ferrous sulfate in the treatment process.
Dyeing and printing wastewater typically contains high levels of organic compounds, deep color, and high alkalinity. If the wastewater appears yellow even after treatment with ferrous sulfate and alkali, there are two possible reasons. First, there may be some undeposited inorganic metal ions present in the dyeing and printing wastewater. Second, the yellow color might be attributed to the presence of organic compounds. To address undeposited inorganic metal ions, the wastewater can be adjusted by adding alkali or sodium sulfide to facilitate the precipitation of these ions.
If the yellow color is due to organic compounds, advanced oxidation or adsorption methods need to be considered for effective removal.
In some cases, the initial color of the wastewater is yellow, and after treatment with ferrous sulfate, the color fades, only to reappear as yellow. This could be due to a recoloring phenomenon associated with the use of certain reactive dyes. Therefore, many dyeing and printing wastewater treatment processes incorporate a biological treatment step.
Recently, we encountered a challenge in treating dyeing wastewater. Even after using ferrous sulfate, sodium hydroxide, and polyaluminum chloride, some red color remained in the effluent, with a pH of 8. Why is it that some of the red color remains unremoved, and should we consider switching to other chemicals?
The addition sequence of ferrous sulfate and sodium hydroxide can impact the treatment effectiveness. To address this issue, we recommend the following:
Add sodium hydroxide at the beginning of the treatment process, followed by ferrous sulfate. Ensure that the pH remains around 9 after adding ferrous sulfate. Starting with sodium hydroxide will elevate the pH to above 12, creating a strong alkaline environment that can break down the molecular structure of some colored groups. This transforms difficult-to-treat colored groups into more manageable ones. Initially, during the reaction with ferrous sulfate, the pH will be around 11, which is more favorable for the color removal process, especially for stubborn colored groups.
Replace sodium hydroxide with lime. This not only reduces the cost of chemicals but also improves the treatment efficiency.
By following these guidelines, it's possible to achieve compliance with color discharge standards and effectively treat dyeing and printing wastewater. For more insights on wastewater treatment and chemistry-related topics, stay tuned to TAIRAN CHEMICAL's blog.
