Disinfection is the last step that follows the basic solids removal processes - coagulation/flocculation, sedimentation, filtration - and a primary disinfection method is chlorination.
Disinfecting water using the process of chlorination involves adding the chemical element chlorine (Cl) and other chlorine-containing compounds to remove as many pathogenic organisms as possible from the water. This step ensures that the water that gets distributed to households is free from any form of disease-causing microorganisms.
As a method of disinfection, chlorination has played a significant role in preventing the spread of many waterborne illnesses including typhoid fever, dysentery, cholera and Legionnaires' disease.
Chlorine in Water Treatment: A Brief History
While the chlorine compound sodium chloride has been known and used since ancient times,it was only in 1774 when chlorine was prepared in its pure form, and the credit goes to Swedish chemist Carl Wilhelm Scheele. Scheele was able to do this by heating manganese dioxide(commonly known as brown stone) with hydrochloric acid. The process causes the bonds between these elements to break, producing manganese chloride, water, and chlorine gas.
But even following its discovery, it wasn’t until 1835 that chlorine was found to be useful in eliminating foul odors from water, and 1890 that its potential as a water disinfectant also came to light. As chlorination gradually established itself as an effective tool in preventing the spread of common diseases transmitted through water, it became the primary disinfection method in the United Kingdom. By 1908, chlorination had found its way to the United States and by 1917, to Canada.
Given the fact that chlorine can be found almost everywhere, usually as part of a compound such as sodium chloride (salt), chlorination currently ranks as the most widely-used disinfection treatment across the globe. In the US today, over ninety percent of water utility plants use chlorine in that crucial disinfection stage.
The Properties of Chlorine
Together with fluorine, bromine, iodine, and astatine, the element chlorine is classified under the halogen series, and this explains its very reactive nature which is common among halogenous elements. When bonding with other elements,halogens can form large quantities of substances of compounds.
Under standard temperature and normal pressure, two chlorine atoms typically bond together to produce the molecule Cl2. In this gaseous form, chlorine is greenish-yellow in color, and gives off a characteristic penetrating and irritating odor. By itself, chlorine in gas or liquid form is non-explosive and non-flammable. However, because it readily reacts with other substances and can act as an oxidizing agent, it is capable of supporting combustion.
Chlorine is commonly occurring in nature. In fact, it is said that chlorine makes up about two percent of the earth’s surface material, although not as a free element. Itis available mostly in the form of sodium chloride in seawater and in natural deposits such as carnallite and sylvite. Chemical oxidation and/or electrolytic processes are required to produce chlorine in its pure state.
Chlorine in Water Disinfection
Chlorine works to kill pathogens by damaging the cell membrane of the microorganism.With the membrane rendered weak, the chlorine is now able to enter the cell and create a disruption in the two processes that are crucial for the survival of the cell - DNA activity and cell respiration.
Chlorination is not limited to the addition of pure chlorine and disinfecting chemicals may also include chlorine-containing substances. That said, there are five types of chlorine most widely used in the process:
Factors Affecting Chlorination
Regardless of the chlorine type or compound utilized, the efficiency and efficacy of chlorine disinfection can be impacted by several factors.
Chlorine contact time and concentration are two very important factors. Chlorine requires time to react with and inactivate the pathogens in the water. The contact time is measured from the time the chlorine is added to the water up to the time that water is consumed. A longer contact time assures a more efficient process. The same principle holds true for chlorine concentration. The greater the dosage or higher the concentration, the more effective the disinfection will likely be.
However, as the concentration of chlorine is increased, the water-chlorine contact time should be decreased accordingly. To get the right level of dosage and contact time that would yield the desired disinfecting action, the CT value is calculated,where C stands for the chlorine concentration and T represents time.
Another significant factor is the pH of the water. Water with a pH of 6.0 to 7.0 makes a more ideal medium for disinfecting with chlorine than water with higher pH values of about9.0 to 10.0. Chlorine dioxide is unaffected by changes in the pH, while monochloramines and dichloramines are created in the water at pH levels between4.5 and 8.5.
Turbidity,or the cloudiness of the water, also impacts the efficacy of the chorination process. If the water is cloudy, there’s a good chance that suspended solids are still present. These solids could attach themselves to the bacteria or virus and shield them from the action of the chlorine. Therefore, the more turbid the water, the less effective the chlorination would be.
Drawbacks of Chlorination
While there is no question of its disinfection capability, chlorination has also given rise to some very legitimate issues. The most serious concern is that the reactions between chlorine and other substances can create potentially hazardous chemical by-products including the compounds trihalomethanes (THMs) and haloacetic acids(HAAs). THMs have been associated with some cancer types, and at certain levels, a particular form of THM, chloroform, can cause dizziness, fatigue, and headaches. Chronic exposure to chloroform may even result to liver and kidney damage.
To mitigate thi srisk, the World Health Organization (WHO) has set a guideline value for chlorine in drinking water which is 5 mg/L. According to WHO, such concentration is considered acceptable and safe even for lifelong human consumption. Guideline values have also been established by WHO for several DBPs including trihalomethanes.