The Advanced Oxidation Processes (AOP), in broad sense, are a set of chemical treatment procedures designed to remove organic materials in wastewater by oxidation through reactions with hydroxyl radicals (OH*). To produce these radicals chemicals like ozone, hydrogen peroxide, UV light are used in combination. The ozone-peroxide combination has been known to produce high-energy hydroxyl radicals and are more effective in decolorizing textile effluents than other types of AOP, i.e. UV-Peroxide. These radicals are produced in situ by the following reaction:
2O3 + H2O2 → 2OH* + 3O2
It, however, has has some limitations resulting from ozone stability, which is affected by the presence of salts, pH and temperature. Ozone-Peroxide AOP is also expensive, but they produce no sludge. It is also possible to use AOP as a pre-treatment to biological treatment where the substances are broken down into simpler forms which can be biodegraded in the biological process.
Membrane treatments like ultra-filtration (UF) or micro-filtration (MF), or adsorption techniques like activated carbon treatments are used sometimes as an alternative to advanced oxidation processes. These processes are always used in combination with some other process as these are used as final polishing steps. Some dyes are not biodegradable and therefore are not affected by the biological processes. These dyes can be removed by membrane or adsorption treatments.
As the demand for water reuse is rising due to scarcity of water, it is recommended that, for achieving consistently high quality treated effluent, it is advisable to use AOP – either as a pre-treatment step to biological treatment to alter the biodegradability of primarily stable dye molecules, or as a post-treatment to biological treatment as a final polishing step. There are also studies which suggest that the combination of conventional activated sludge treatment (biological treatments) followed by sand filtration or activated carbon filter (physical treatments) have high organics removal and color removal rates with low operational difficulty and nominal chemical cost.
Below is the basic process flow diagram of a treatment process. First, the primary treatment takes place through coagulation or flocculation following which settling of wastewater takes place in a gravity settler. The wastewater then undergoes biological treatment. After the biological treatment, the water is transferred into a solid/liquid separator, from where it is transferred into one of three different available options as illustrated in the process diagram.