Water Tank Chlorination: How It Works

Chlorination is the process of disinfecting water to prevent the spread of waterborne diseases, while maintaining the water as fit to use for its primary purpose. Waterborne diseases could include cholera, dysentery or typhoid, but chlorine is widely used for the deactivation of most microorganisms. It is one of the most common disinfectants for water, not only because it is cheap, but also because it is very effective. The process of chlorination can be hazardous, however, and you will need to ensure the safety of both yourself and others on site. Preparation for chlorination should involve obtaining permits, carrying out a risk assessment and preparing method statements, particularly as this is likely to be a recurring process. Contractors should also prepare contingency plans for the continuation of water supplies in order to minimise disruption for the organisation in question. What is chlorine? Chlorine is a reactive and corrosive gas, so appropriate precautions are required for transportation, storage and handling. It has been used for over 200 years, and is very effective for the deactivation of pathogenic microorganisms, which can be found in drinking water, swimming pools, waste water and of course water tanks. The principal of chlorination As chlorine is so reactive, it quickly forms compounds with other substances, and this is the key to how it disinfects water supplies. Chlorine kills bacteria and viruses by breaking down the chemical bonds in their molecules. Chlorine disinfectant contains chlorine compounds which exchange atoms with other compounds when they hit the water, in this case these compounds are enzymes in bacteria and other harmful cells. When the chlorine comes into contact with these enzymes, one or more hydrogen atoms in the molecule are replaced by the chlorine. As a result, the entire molecule falls apart and subsequently the cell or bacteria dies. The effectiveness of chlorine is dependent on the pH value of the water, and for optimum disinfection the water should ideally be between 5.5 and 7.5 pH. The dosing of chlorine then has to be sufficient to firstly react with the compound in the water, but also to then remain in the water and stay effective for a period of time. This is determined by a set frequency and concentration of dosage, but in general terms dosage is affected by the following factors:

  • Levels of organic matter in the water
  • pH value of the water
  • Contact time in the water
  • Temperature of the water
Chlorine gas can be obtained as a fluid gas and is highly water soluble, hence its popularity. Only small concentrations are required to kill bacteria, and in fact chlorine itself is only used for large municipal swimming pools and large water storage or re-circulation tanks. For small applications you can use sodium hypochlorite instead. Chlorination procedure The fresh chlorination process of a cold water tank typically requires it to be drained and then cleaned to remove residue, debris and scale. The tank should then be re-filled with fresh water and chlorine or sodium hypochlorite dosed in to the appropriate concentration. Engineers should then shut off the main inlet to the tank and test the water for the appropriate disinfection levels. At Water Hygiene Services we have qualified staff who can advise on your water chlorination requirements, including which disinfecting agent you should use and what dosing concentration and frequency is required. This will be a bespoke recommendation based entirely on your individual needs, so contact us today for further information.]]>