Disinfection is an important step in ensuring that water is safe to drink. Water systems add disinfectants to destroy microorganisms that can cause disease in humans. Disinfection kills or inactivates disease-causing organisms in a water supply and must provide a 99.9 percent inactivation of Giardia lamblia cysts and enteric viruses to protect health.

There are two kinds of disinfection: primary disinfection achieves the desired level of microorganism kill or inactivation, while secondary disinfection maintains a disinfectant residual in the finished water that prevents the regrowth of microorganisms.

Our natural environment contains numerous microorganisms. Most of these present no concerns. However, some—such as Giardia lamblia and various viruses, which can be present in water supplies—are extremely harmful and can cause disease in humans. These disease-causing organisms are known as pathogens.

Because pathogens can be present in drinking water supplies, disinfection is very important for surface water and groundwater under the influence of surface water. Disinfection treatment methods include chlorination, chlorine dioxide, chloramines, ozone, and ultraviolet light. When combined with conventional treatment, such as coagulation, flocculation, sedimentation, and filtration, good results have been obtained. Direct filtration, slow sand filtration, and diatomaceous earth filtration, along with disinfection, have been just as successful.

Groundwater systems that disinfect may have to add filtration if the water contains iron and manganese. In fact, insoluble oxides form when chlorine, chlorine dioxide, or ozone are added to these systems. Both ozonation and chlorination may cause flocculation of dissolved organics, thus increasing turbidity and necessitating filtration.

The effectiveness of disinfection is judged by analyzing for an indicator organism (total coliform bacteria). This organism is considered harmless, but its presence indicates that pathogens may also have survived.

Historically, boiling of water or use of copper and silver vessels for storing water which effect some measure of disinfection have been employed. Modern disinfection processses include use of

  • Physical methods such as thermal treatment and ultrasonic waves
  • Chemicals including oxidizing agents such as chlorine & its compounds, Bromine, Iodine, Potassium Permagnate, Ozone and metals like silver
  • Radiation

Criteria for good disinfectant

To be a good disinfection agent, it should satisfy following criteria;

  • It shall be capable of destroying the pathogenic organism present within the contact period available and not unduly influenced by the range of chemical & physical properties of water
  • Should not leave products of reaction which render he water toxic or impart colour or otherwise make it unpotable.
  • Possess the property of leaving residual concentration to deal with possible recontamination
  • Be amenable to detection by practical, rapid and simple analytical techniques in the small concentration ranges to permit control of disinfection processes.

 Factors affecting efficiency of disinfection

The efficiency of chemical disinfection is influenced by following factors;

  • Type, condition concentration and distribution of organisms to be destroyed
  • Type & concentration of disinfectant
  • Chemical & Physical properites of water to be treated
  • Contact Time available for disinfection
  • Temperature of water

Contact Time

Under ideal conditions and at constant temperature, the number of organism (Nt) surviving after a period of time t is related to the initial number (No) by Chicks Law

log(No/Nt) = k.t

Where k is a constant with dimension 1/T
Departures from Chicks law are not uncommon. Rates of kill have been experimentally observed to increase with time in some cases and decrease with time in other cases. To account for these departures from Chicks law, the following modified equation has been suggested

log(No/Nt) = k.t^m

Where m is a constant. If m is greater than 1 then the rate of kill increases with time and if m is less than 1, the rate of kill decreases with time.

Concentration of disinfectant

Rate of disinfection is affected, within limits, by changes in concentration of disinfectant. The relation between disinfectant concentration and time required for killing a desired percentage of organism is generally expressed by following equation;

C^n . tp = Constant

Where C is the concentration of disinfectant, n is a coefficient of dilution and t is the time required for a constant percentage kill of the organisms. Value of n greater than 1 indicates rapid decrease in efficiency of disinfectant as its concentration is reduced, if n is less than 1, contact time is more important than concentration and if n is equal to 1, both affect the efficiency of disinfection to some extent.

Temperature of Water

The effect of temperature on rate of kill is usually expressed by the Vant Hoff-Arrhenius relationship assuming that the rate of disinfection is controlled by either rate of diffusion of disinfectant through the cell wall or by rate of chemical reaction with cellular enzymes.

Log t1/t2 = E(T2-T1)/(2.303R.T1.T2)

where t1, t2 are time given for given percentage of kill at Temperature T1 and T2 in Degree Kelvin, E is the activation energy in J/mol or cals/mol and R is gas constant equal to 8.314 J/molDegree Kelvin or 1.99 cals/mol.Degree Kelvin. Typical values of E at pH 7.0 for aqueous chlorine and chloramines are 34332 and 50242 J/mol respectively. This type of relationship indicates that at lower temperature of water i.e. in winter season, the time required for achieving the same percentage of kill for the same concentration of disinfectant would be higher than those for higher temperature i.e. in summer season.