Disinfection

Disinfection : It is the process of killing pathogenic bacteria.

Methods:

Boiling of water:

• Most effective method of disinfection of water
• Cannot be used for huge quantities of public water supplies
• Cannot take care of the future possible contaminations 
• Used only for Domestic purposes in emergencies

Treatment with excess limes :

• Effectively kills the bacteria, but cannot safe guard the future pollution.
• Enough lime is added to water to raise its pH value to 9.5 or more, thereby most of the bacteria will be killed.
• Recarbonaiton is necessary. The process of removal of excess lime before it is supplied to public is known as Recarbonation

Ozone :

• Ozone is a more powerful disinfectant than chlorine.
• Ozone is more costly than chlorine.
• Does not provide residual protection against recontamination.
• Cannot be stored.
• Used for disinfection of water on a small scale such as for swimming pool waters.

Iodine and Bromine pills:

• Compared lo chlorine , iodine provides longer lasting protection against pathogens and reduced offensive tastes and odours.
• Used for water supply for army troops, private plants, swimming pools.
• Cannot be used for public water supply us it is costly.

Ultra violet rays :

• Very costly technique and is an effective method for the disinfection of water
• No residual effect is available for protection against recontamination.
• Adopted for water supply installations of private Buildings, Institutions, Treatinsmall quantities of water in hospitals, dispensaries for surgical use. Swimming pools

Potassium Permanganate:

• For disinfecting well water supplies are generally contaminated with amounts of bacteria.
• Can remove 100 % organisms causing 
• It also oxidizes organic matter

Silver or EIectro-Katadyn 

• Removes algae also and takes care of future pollution. It is costly.

Chlorination :

• Universally adopted for public waters supplies because ¡us capable of providing residual disinfecting effects for long periods. it satisfies all the requirements of an ideal disinfectant.
• It takes care of any possible future contamination.(i.e it prevents recontamination) only disadvantage is it imparts bad taste.
• The addition of chlorine does not produce any significant change in the pH
• value of the natural waters
• Chlorination is not effective if the water is alkaline.

Action of chlorine: When chlorine is added to water, the following reaction takes place:

Hypochlorous acid (HOCl) and the hypochlorite ions (OCI) accomplish disinfection of water Thus Chlorine reacts with water to produce hypo chlorous acid (HOCI) and hypochlorite (OCI) which are together known as “free available chlorine.” HOCI is 80 to 100 times more powerful than OCI.

The free Cl2 reacts with compounds such as ammonia, proteins, amino acids and phenol that may generally be present in water to form Chloromines and Chloro derivatives which are called “Combined Chlorine”. It possesses some disinfecting properties but less effective compared to free available chlorine .Some free chlorine also exists along with combined available chlorine.

Free chlorine is about 25 times more powerful than combined chlorine.

These compounds either in free or combined form of chlorine, interfere with enzymes in the bacteria forming toxic chloro compounds and thus destroying the bacteria.

Residual chlorine = Chlorine dosage - Chlorine Demand

Chloramines:

• The Monochloramines and Dichloramines have got disinfecting properties
• The Trichloramine has not negligibal disinfecting property
• The relative distribution of chloramines depends on pH value
• If pH<4.4, only trychloramine exist 
• For pH : 4.4 to 5.5 ,only dichloramine exist 
• For pH : 5.5 to 8.4 , both mono and dichloramine exist 
• If pH > 8.4,only monochloramine exist 

M = Destruction of chlorine residual by reducing compounds(Fe, Mn, H2S)

N = Formation of chloro-organic compounds and chloramines

O = Destruction of chloramines and chloro organic compounds

P = Formation of free chlorine and presence of chloro-organic compounds not destroyed.

When chlorine is added to water, some of it is consumed in killing bacteria and oxidizing organic matter to form its compounds (Chloramines) and the remaining chlorine is available as residual chlorine. This is indicated by line Q. The fall of line R indicates oxidation of chloro-organic compounds and chloramines. indicated by bad odour and taste in water. Any further increase in chlorine dose beyond point B will appear as residual chlorine only and this i shown by line S.

Factors affecting bactericidal efficiency of chlorine:

1. Turbidity: Turbidity decreases the efficiency of chlorine as disinfectant.
2. Metallic compounds presence: Iron and manganese if present in water, they reduce the effectiveness of chlorine.
3. Ammonia Compounds: Reduces the efficiency
4. pH Value of water: Increased pH value reduces the effectiveness of chlorine as bactericide.
5. Temperature of water: At lower temperature, the efficiency will be less and hence requirement of residual chlorine will be more.
6. Contact Time: The killing of bacteria increases with increase of contact period.Viruses are more resistant than bacteria and hence require longer time of contact and higher doses of Cl2

Dosage:

The dose of chlorine which leaves a residual chlorine of about 0.2 mg/litre at the end of 10 minutes contact period is selected which gives the optimum dose of chlorine for the given water sample.

• Normal dosage 0.3 to 1.1 mgI 1
• Residual Chlorine 0.. I to 0.2 mg /1
• Chlorine dose must be generally increased during rainy season.
• Cl2 dosage = Cl2 demand - Cl2 residual

 Various forms of Chlorine:

1. As bleaching power or hypo chlorite
2. As chloramines
3. As chlorine gas or liquid chlorine
4. As chlorine dioxide gas

Bleaching powder.

The bleaching powder contains about 30 to 35 percent of available chlorine. Therefore, it can not be used for large public water supplies. This chlorination is known as hypochlorination.

Chloramines: Chloramines are the compounds formed by the reactions between ammonia and chlorine .chloramines do not cause bad taste and odour. Residual effects lasts for longer duration.

Chloramines are much weaker disinfectants as compared to free chlorine.

Chlorine gas or liquid chlorine: Chlorine in gaseous or liquid form is now-a-days

universally adopted(liquid chlorine) disinfectant for public water supplies. It is a powerful disinfectant and may remain in water as residual for sufficient time

No sludge is formed in its application, as may be formed by using hypochlorites or chloramines.

Chlorine dioxide: It has an oxidizing capacity 2 1/2 times that of chlorine. Further it is effective in the removal of tastes and odours, but due to its high cost of production it is not economical

Types of Chlorination :

Plain chlorination: Only chlorine treatment and no other treatment.

Helps to remove bacteria and colour from water and for also controls the growth of Algae. It is used in case of emergencies. when full fledged treatment cannot be given. Also adopted for clean water.

Prechlorination : Chlorine is added before raw water enters sedimentaion tanks. It helps in reducing the quantity of coagulants and to improve coagulation. It reduces taste and odour of water;

• controls the growth of algae .It reduces the bacterial load on filters
• It prevents putrefaction of sludge in settling tanks.

Post chlorination : This is the standard form of chlorination in which chlorine is added to water as it leaves filters and before it enters distribution system. The residual chlorine present in water is useful for its protection against contamination in the distribution system.

Residual 0.1 to 0.2 mg/l.

Double or multiple chlorination: ft consists of pre-chlorination and post chlorination. Adopted when raw water is highly contaminated and contains large amount of bacterial life

Break point chlorination (or free residual chlorination):

Break point chlorination gives an idea of chlorine to be added to water so as to ensure the desired amount of residual chlorine..

Advantages (or other actions) of Chlorine as disinfectant:

• It will remove taste and odour.
• It will have adequate bactericidal effect
• It will leave a desired chlorine residual
• It will remove manganese and iron.
• It will complete the oxidation of ammonia and other compounds. Organic matter.

Super chlorination :The application of chlorine to water beyond the stage of break point is known as super-chlorination. It is most commonly added at the end of filtration. Adopted whenever there is breakout of water borne diseases.

When super - chlorination  is practiced. it becomes necessary to remove the excess chlorine by any suitable method of dechlorination before water is supplied to the consumers

Dechlorination : Process of removing excess chlorine by dechlorinating agents. This is to avoid chlorinous taste from water. Chemicals used for dechlorination are:: sulphur dioxide, Sodium thiosulphate, sodium bio sulphate. Activated carbon, potassium permagnate ammonia.

Testing of chlorine Residuals:

1. Orthotolodine test
2. D. P. D test (Diethyl-p-Phenylene -Diamine)
3. Chlorotex test
4. Strach-lodide test

• The orthotolidine test is most commonly used to determine the amount of residual chlorine in water.
• The Strach-iodide test is more precise than the orthotolidine test, particularly when residual chlorine is greater than I p.p.m

KINETICS OF DISINFECTION:

Chick’s Law: The rate of kill of organisms is proportional to the number of organisms

remaining in water ai any time ‘t’.

 Increase in temperature results in a more rapid kill

Nt = N0e-kt

cn t= constant

k = Disinfection rate constant

C= concentration of disinfectant

n =dilution coefficient