The color of organic molecules or substances in water starts to fade upon contact with ozone, due to their breakdown. Hence, ozone applications serve as a great solution for color removal in wastewater from several industries.
Wastewater from the textile industry contains a significant amount of color and COD pollution, making it difficult for water treatment plants to meet discharge standards. Although aerobic biological treatment is the primary method used for treatment in the textile industry, it often fails in the color removal from wastewater that contains dyestuffs. Consequently, alternative solutions are required to effectively tackle color and COD pollution in textile wastewater.
The color of textile products plays a crucial role in their commercial success, but the color can fade due to various external factors such as washing, light exposure, dry cleaning, and sweat. To improve the resistance of dyestuffs to these factors, modifications are made to their structures, resulting in increased fastness. However, these modifications have made it challenging for traditional wastewater treatment systems to effectively remove these dyestuffs, as noted by O’Neill et al. (1999) and Cooper (1993).
To make the COD removal and color removal from textile wastewater effective, a significant level of oxidation is necessary. Therefore, in the textile industry, a wastewater project must ensure that the ozone generator meets specific standards.
Textile companies operate their wastewater treatment facilities around the clock, seven days a week, and the daily capacity of wastewater can reach up to 15,000 m3 in large-scale textile companies. However, when treating such high flow rates with ozone, the contact time between the ozone gas and wastewater becomes very limited.
To ensure effective treatment of textile wastewater with ozone, a minimum gas concentration of 11% is necessary. Achieving this concentration requires feeding the ozone generator with oxygen rather than relying on dry air, as no known technique or production method can produce ozone gas at 11% concentration from dry air.
There are 3 primary reasons behind employing ozone in the treatment of textile wastewater, as follows:
1) Removal of COD and BOD
2) Color removal
3) Odor removal
A successful ozone gas project can only be achieved with a well-designed treatment plan. It is important to note that the ozone generator is not a standalone solution for purification, but rather a complementary tool.
The potent oxidizing property of ozone have paved the way for its widespread use in wastewater treatment, owing to its success in controlling color and odor in drinking water, resulting in a significant market demand.
Ozone systems offer the advantage of low operating costs and require no personnel costs. They have been found to provide a success rate of 85-98% for color removal and 40-55% for COD removal.
The use of ozone is a popular method in various industrial plants including painting, textile, pharmaceuticals, and others in many countries. Ozone is not only used for color and odor control but also as a “bleach” in industries such as pulp and paper production, kaolin production, and textile production.
The effectiveness of ozone treatment in controlling color in effluent depends on:
- The feed’s color values
- Ozone dosage
- Type of wastewater. (Usually, even with a particularly high ozone dosage, color values do not decrease below 200 Pt-Co units.)
- Temperature of the wastewater. (Optimum results can be obtained when treating wastewater with a lower temperature, especially when using effluent from existing treatment processes rather than from the equalization tank.)
- Other wastewater characteristics which are also impacted by ozone. (There will be better results if BOD, COD and SS have already been decreased in a previous treatment level.)
Optimal color removal results can be obtained through prior treatment of wastewater to reduce other characteristics, enabling the ozone’s oxidizing effect to be solely or predominantly focused on color removal. The temperature of the wastewater should also be maintained below 30°C to ensure optimal solubility and physical conditions for the treatment process.