Question

Scientists detected COVID-19 in Boston’s sewer system. The city decides to disinfect the effluent of its wastewater treatment plant. Please describe the procedures and the required chemical agents to measure chlorine concentration in water.

Answer 1

Disinfection is usually the tertiary treatment applied to waste water for microbial treatment after the initial processes of physical, chemical and biological treatments. Disinfection has to meet two objectives:

1. Primary DIsinfection: to kill any pathogens in water
2. Secondary or Residual Disinfection: to prevent pathogen regrowth in the water during the period before it is used.

Different chemical agents can be used for both the stages, but the most commonly used method of disinfection is chlorination or free chlorine disinfection, because it is cheap, reliable and easy to use.

Free chlorine in water is developed by dosing with either chlorine gas (Cl2(g)), sodium hypochlorite (NaOCl) or calcium hypochlorite (Ca(OCl)2). The dosed chemical reacts in the water to produce dissolved chlorine gas (Cl2(aq)), hypochlorous acid (HOCl) and hypochlorite (OCl-), which all contribute to the free chlorine concentration.

Out of these forms of free available chlorine, hypochlorous acid (HOCl) is the most effective (being about 80 times more effective than hypochlorite (OCl-)). For this reason, pH value of water during chlorination is generally maintained slightly less than 7 (kept acidic), so as to keep dissociation of HOCl to minimum. Moreover, chlorine will immediately react with ammonia present in water to form various chloramines, which are less effective than free chlorine. The free chlorine remaining in water after about 15-30 minutes of contact between water and the chlorine dose is called residual chlorine. Presence of residual chlorine ensures an effective check over the reduction of pathogens in water to the safe limit.

Procedures to determine residual chlorine:

1. Starch-iodide method: It depends on the oxidizing power of the residual chlorine to convert potassium iodide (KI) to free iodine (I2), which gives a blue colour in the presense of starch solution. If this iodine released is titrated with a standard solution of a reducing agent i.e. sodium thiosulphate (Na2S2O3), the blue colour disappears and the total residual chlorine os quantitatively measured. Chemical reactions are represented as follows:

1. Orthotolodine Method: It involves the use of orthotolodine, a colorimetric indicator for chlorine residuals. Orthotolodine solution when added to water sample is oxidized by the presense of residual chlorine to produce a yellow colored compound. The colour is matched with standard colour discs in a residual chlorine comparator and the concentration of chlorine is obtained therefrom.
2. Orthotolidine-Arsinite Method (OTA): This method is based on the fact that free residual chlorine reacts instantaneously with orthotolidine requiring less than 15 seconds for the development of yellow colour whereas combined chlorine reacts slowly requiring 5 minutes for full colour development. The test is split up into two parts:
   1. In the first part, total residual chlorine is measured after the lapse of full 5 minutes
   2. in the second part, sodium arsenite solution which is a reducing agent is added within 5 seconds after orthotolidine is added. This gives time for only free residual chlorine to oxidize the orthotolidine. Sodium arsenite solution quickly reduces chloromines stopping further reaction, so that the yellow colour developed is principally due to free chlorine.

The combined residual chlorine is obtained by difference betweenthe two results:

Combined Residual Chlorine = Total Residual Chlorine - Free Residual Chlorine