Chlorination is cheap, reliable, and present in the residual form to protect against further contamination. Hence, chlorine has become a good choice for disinfecting drinking water.
Chlorine can be applied in other forms like chlorine gas, chloramines, and chlorine dioxide. However, bleaching powder [ Ca(OCl)2 ] is the cheapest commonly used form of disinfecting water. This is available in powder or tablet form.
Bleaching powder contains (30-35)% of chlorine when freshly made. Chlorine escapes and the percentage of chlorine decreases over time. Hence, it is a good idea to use fresh bleaching powder and to check the chlorine content in it.
Commercial Bleaching powder does not Just contain [ Ca(OCl)2 ] (Calcium hypochlorite)
The bleaching powder is made by passing chlorine gas (Cl2) over the slaked lime [ Ca(OH)2 ]. So, there is calcium chloride [ CaCl2] as a byproduct. So, the commercial bleaching powder is a mix of:
- Bleaching powder/ Calcium hypochlorite [ Ca(OCl)2 ]
- calcium chloride [ CaCl2]
- slaked lime [ Ca(OH)2 ]
Hence, the mixture varies with manufacture and the available chlorine lies between 30 to 35% when freshly made. The available chlorine content in a commercial bleaching powder decreases over time.
The available chlorine in the commercial bleaching powder is determined in a lab with Iodometric titration.
Chlorine Demand
The amount of chlorine consumed in the oxidation of the organic and inorganic impurities, before disinfection is the chlorine demand of water. Hence, we apply chlorine dose to water as some free residual chlorine must be present for disinfection.
In formula, Chlorine Demand = Chlorine Dose – Free Residual Chlorine (F.R.C)
Estimation of Chlorine Demand with Bleaching Powder
In simple, it can be done in the following steps:
- Step 1: Collect the water and make multiple samples of it.
- Step 2: Put different doses of bleaching powder in it (Ex. 0.5,1.0,1.5,2.0 milligrams/liter as required)
- Step 3: Give sufficient contact time (Ex: 30 to 60 minutes)
- Step 4: Measure the chlorine content in water (F.R.C) using test kits or in a lab
- Step 5: The dose of bleaching powder that gives the chlorine (F.R.C) in water of 0.2 mg/l is the dose of the chlorine.
- Step 6: If not getting an F.R.C., then increase the bleaching powder dose and try again. If more F.R.C is seen, decrease the dose and try again until target F.R.C is achieved.
Factors affecting disinfection with chlorine
Many factors will affect the disinfection with chlorine:
- Turbidity
- Ammonia presence
- Water pH
- Water Temperature
- Time of contact
- Type of pathogens
- concentration of chlorine/pathogen
Why the pH of water is important?
The bleaching powder [ Ca(OCl)2 ] when dissolved in water becomes HOCl and OCl. The HOCl and OCl are the compounds that actually do the oxidation of impurities and disinfection,
However, HOCl is 40 to 80 times more efficient at killing pathogens than OCl.
The presence of HOCl and OCl is dependent on pH of water as illustrated in the figure.
- At pH near 11, there will be no HOCl.
- At pH near 4, there will be no OCl.
- pH between 4 and 11, there will be a fraction of HOCl and OCl.
How to use the Excel sheet
There are two interactive Excel sheets.
- From Excel A, you can easily estimate the bleaching powder required knowing the chlorine dose required.
- From Excel B, the bleaching powder is estimated in detail including the factors of pH and temperature.
Excel A: Easy bleaching powder estimation.
Excel B: Accurate bleaching powder estimation based on temperature and pH of water
YouTube Video explaining the working design sheet
More Info on the effect of pH (Acidity/Alkalinity) on the dose of chlorine: Click Video