Previously we had discussed the removal of color from drinking water. We will now discuss the removal of color from textile wastewaters.
The textile industry is familiar with ozone and comfortable with its use. Ozone has been used for successfully for removal of color from textile waste water streams. Ozone has also been used in the dyeing process itself in a step called stripping. In the stripping process some of the color is removed from the textile and the fabric is dyed again. This process is used to even out the color on the textile. Depending on the effect desired this can be done more than once.
In wastewater treatment, ozone is often used in conjunction with biological treatment systems such as activated sludge. Organic dyes are mostly refractory due to their large molecular size and they can be poorly removed by adsorption on activated sludge. In some cases ozone has been used before the biological process, but mainly after biological treatment. If the wastewater is hardly biodegradable or toxic to activated sludge pretreatment is an option.
Ozone can be used prior to a biological process since it has a tendency to convert organic molecules into smaller more biodegradable species. This can enhance the efficiency of the biological process. In addition, ozone treatment of wastewater increases the oxygen content of the water (unconverted oxygen and ozone that decomposes back to oxygen that was mixed with the water) which results in improvement in aerobic processes. While this benefit is well known in the literature it is difficult to practically apply since the amount of improvement is difficult to predict and pilot studies involving ozone and biological processes are difficult to carry out. In textile wastewater processes, a 20-30% improvement in the action of the biological system has been observed.
The effect of ozone on improving biodegradability and reducing toxicity is worth noting in terms of the effect of the treated water on the receiving stream. Where the treated water is tested for toxicity, the impact of the treatment process on this parameter must be considered. Destroying one organic molecule, but creating more toxic ones in a treatment process has been observed, for example the ozonation of MTBE without any additional agents or treatment processes can result in a more toxic wastewater. Another consideration is the presence of surfactants and the need to remove these compounds from the water. In some locales surfactant concentrations are tightly controlled and must be kept under 1 ppm. This creates an additional demand for oxidant. Some textile wastewaters contain both color and surfactants.
Ozone is effective in removing the color from all dyes used in textile processing. The amount of ozone can vary depending on a number of factors: how much color was removed in the biological process, the type of dye used, where ozone is applied in the process, etc. Knowing the proper amount of ozone required to meet the color removal objective for the receiving water body is critical to the economics of the ozone system. In general it is not easy to predict the amount of ozone required, so in virtually all cases where specific previous experience is not available, pilot testing is employed.
Tosik has shown that about 1 mg ozone/mg dye is required to achieve 95% color removal, although this ratio varies by dye type. The ratio increases to about 1.5 for 100% removal. Reaction times were on the order of 10 minutes. In the textile industry a typical dosage might be 15 mg/l post biological treatment, but the levels could easily reach 25 mg/l. It is important to note that the ozone dose only needs to make the dye compound uncolored and not necessarily completely mineralize the material.
Example of Ozone Color Removal
We recently looked at a situation where a textile plant in the US wanted to reuse their wastewater. Color levels reached 150 as measured by the ADMI method. We did not reach the pilot plant stage for this project, but we did a cost analysis on the basis of dosing the water with 25 mg/l of ozone. The flow ranged from 1 to 1.5 MGD. The required amount of ozone would be ont eh order of 300 lbs/day.
For the example discussed above the, following system costs have been estimated at $467,000 installed.
 Removal of Dissolved Organic and color from dying Wastewater by Pre-Ozonation and Subsequent Biological Treatment, Takahashi, Nobuyuki; Kumagai, Tomoyo; Ozone: Science and Engineering, 28: 199-205
 Dyes Color Removal by Ozone and Hydrogen Peroxide: Some Aspects and Problems, R. Tosik, Ozone: Science and Engineering 27: 265-272