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Chemical and Biological Treatment of the Wastewater of Dyestuff Intermediates

The aim of dye intermediates wastewater treatment is to reduce CODCr and chroma in water and recover organic matter, waste acid and inorganic salt in wastewater. According to different water quality and discharge requirements, different treatment methods are adopted.

Ⅰ. Chemical treatment of dye intermediates' wastewater

Chemical treatment method mainly uses chemical reaction to change the structure of harmful substances in wastewater, so as to achieve the purpose of recovery or decomposition removal. Chemical treatments are often used in combination with physical or biological processes.

1. Oxidation method

The purpose of the oxidation method is to decolorize and remove CODCr by destroying the chromophore group or dye molecular structure through the oxidation of strong oxidant. Commonly used oxidation methods include pharmaceutical oxidation, electrolysis, O3 oxidation, photooxidation, wet air oxidation and so on. Reagent oxidation method can be used chlorine gas, hydrogen peroxide as oxidant.

2. Reduction method

Azo dyes can be reduced. But sometimes the aromatic amines produced are carcinogens. For anthraquinone dyes, the reduction is reversible for the above reasons. The reduction method of dye intermediate wastewater is not widely used in the treatment of dye wastewater.

3. Incineration method

The incineration of dye intermediate wastewater is the most effective method to treat extremely high concentration organic wastewater by air deep oxidation at high temperature, and it is the most easy method to realize industrialization.

High concentration wastewater with CODCr greater than 1.0×105 mg/L, calorific value greater than 1.0×104 kJ/kg, operating temperature in the furnace is 900~1000℃, residence time is 3~4 s, air residual coefficient is 1.2~1.4, the wastewater can be directly burned. The main purpose of dye intermediates wastewater treatment in China is to recover inorganic salts.

At present, the main problems of incineration treatment in China are: low heat recovery rate, and many incineration devices cannot operate due to high operating costs. Foreign advanced incineration systems are equipped with waste heat recovery and waste gas pollution control devices, which are conducive to reducing energy consumption and eliminating secondary pollution.

Ⅱ. The biological treatment of dye intermediates' wastewater

Biological treatment is a conventional method of wastewater treatment and also one of the commonly used methods of dye wastewater treatment. The commonly used biological methods include activated sludge method, biofilm method, oxidation pond method and anaerobic biological method.

The wastewater contains amines, phenols and so on, which can be treated by biological method. For acidic and alkaline wastewater, it can be neutralized first and then treated by biological method; Azo dye and sulfide dye wastewater can be treated by reduction and oxidation to reduce its toxicity, and then treated by biological method. There are also reports of degradation of azo dyes by bacteria-algal symbiosis. The wastewater first enters the anaerobic pond to break the azo double bond, and then enters the aerobic pond to degrade aromatic amines.

In recent years, due to the rapid development of the dye industry, in order to improve the performance of dyes, dye products have gradually developed to the direction of oxidation resistance, photolysis resistance and biodegradation resistance. More and more materials are difficult to biodegrade in dye intermediates' wastewater, which brings certain difficulties to biological treatment.

The most important thing for the treatment of dye wastewater by biological method is to solve the problem of biodegradability of the wastewater. At present, the combination of enhanced biological method and physicochemical method is generally adopted.