Chlorine is a common household and industrial effluent that ultimately ends up in treatment plants. Here, chlorine reacts with various other chemicals and their compounds adding to the problems at the treatment plant. In this article I discuss some reactions of chlorine that occur both in wastewater and in laboratory tests that is important to those associated with wastewater treatment.
Residual Chlorine In Wastewater
As mentioned earlier home cleaning and disinfecting products as well as chemical and pharmaceutical industry effluents discharge a huge amount of chlorine in wastewater. And since this wastewater contains other chemicals and compounds, the reactions of chlorine can disrupt the functions of microscopic aquatic organisms necessary for treatment. For example in laboratories, ortho toluidine, which is found in the effluent of the textile and dye industry, produces a yellow colored compound. Since there are many such complex compounds in wastewater there can be multiple compounds formed due to the reactions of chlorine having the potential to affect treatment.
Chemical Reactions With Chlorine From Bleaching Powder
Bleaching powder is a common household disinfecting and cleaning agent. Unfortunately all the doses of chlorine end up in the drain that leads to the wastewater treatment plant. The contaminated water at the treatment plant contains several chemicals, some of which can be potassium, sodium, iodine, calcium along with their compounds. All of these can have their own reactions with chlorine and produce compounds that complicate the treatment process. Consider this scenario for example. If there is acetic acid in wastewater, of which there is a high chance, then it will react with the effluent bleaching powder and liberate chlorine from it. The liberated chlorine can then react with potassium iodide to liberate iodine from it. The following is the chemical formula representation of this reaction.
CaOCl2 + 5CH3COOH -> (CH3COO)2Ca + Cl2 + H2O
Here CaOCl2 (bleaching powder) reacts with Acetic Acid ( 5CH3COOH ) to produce Calcium Acetate ((CH3COO)2Ca) and chlorine (Cl2). The calcium acetate thus liberated actually acts as a growth inhibitor that affects bacteria. Certain types of bacteria and other microscopic organisms are useful for the decomposition process during treatment. The acetate can destroy these otherwise necessary bacteria thus affecting treatment.
Cl2 + 2KI -> 2KCl + I2
The liberated chlorine is then free to react with other substances, for example Dipotassium diiodide. Dipotassium dichloride is produced along with iodine. If this escapes the treatment process and contaminates water, then it becomes difficult to purify water.
2Na2S2O3 + I2 -> Na2S4O6 + 2NaI
The iodine produced by the previous reaction, reacts with Sodium Thiosulfate (2Na2S2O3) to produce Na2S4O6 (Sodium Tetrathionate) and Disodium Diiodide (2NaI).
The above explained reactions are an example of the different chain reactions possible in wastewater. There are many such reactions running in parallel in wastewater. Hence it becomes necessary to identify the possible reactions so that the treatment process can be suitably adapted.