Potable Water Disinfection: Chlorine Treatment and Break Point Chlorination

Disinfection

Disinfection of potable water is the specialized treatment for destruction or removal of organisms capable of causing disease. Disinfectants widely used in water treatment are oxidizing agents such as chlorine, ozone and ultraviolet. Being a strong oxidant chlorine will react with the oxidizable materials present in water before it has a chance to act as a disinfectant.

Chlorine in water: Chlorine has been widely used in disinfecting water. It is cheap reliable and easy to handle. Chlorine and water react according to the following equation. Cl2+ H2O HOCl (hypochlorous acid)+HCl H++ OCl (hypochlorite ion) HOCl

Hypochlorous acid is a strong disinfectant because of its ability to diffuse easily through the cell walls of a microorganisms and disrupt the life function. The relative concentration of HOCl and OCl vary with pH, water temperature and the concentration of chlorine in solution. HOCl is many times stronger as an oxidant than OCl . The predominant concentration of HOCl and OCl are below pH=6 and above pH=7.5 respectively. Therefor, the disinfection power of chlorine decreases with increase in pH.

The chlorine existing in water as hypochlorius acid, hypochlorite ion and molecular chlorine is defined as free available chlorine. Free available chlorine react with ammonia and many organic amines to form chloramine which is less active. This chloramine is known as combined available chlorine. NH3+ HOCl NH2Cl (monochloramine) + H2O NH2Cl+ HOCl NHCl2(dichloramine) + H2O NHCl2+ HOCl NCl3(trichloramine) + H2O

Break point Chlorination: When chlorination is added to water, it is consumed in oxidizing a wide variety of compounds present in water (organic matters and other compounds). No chlorine can be measured until the initial chlorine demand is satisfied (point A B) In the following figure. After meeting the initial demand, the added chlorine reacts with the ammonia to produce combined chlorine residual (chloramines) which increases with additional dosage until a maximum combined residual is reached (Point B C).

Further addition of chlorine causes a decrease in combined residual, this chlorination. At this point, some of the chloramines are oxidized to nitrous oxide or other gases (Point C D). is called breakpoint After breakpoint is reached, free chlorine residual (unreacted hypochlorite) develops at the same rate as that of the applied dose (Point D E).

. Chlorine residual (mg/L) Chlorine dosage (mg/L)

Combined residual as indicated by bacterial results, are generally adequate. For grater safety, free residual chlorine can be used by adding sufficient chlorine to destroy the ammonia. Ex The following data was obtained in a chlorination experiment. Plot the data and determine the break point dosage. What dose is required to provide a free residual of 1 mg/L. Dosage (mg/L) 1 2 3 4 5 6 7 Residual (mg/L) 0.8 1.55 1.95 1.25 0.5 0.85 1.95

Solution: Plot the data in the table 2.5 Residual (mg/L) 2 1.5 1 0.5 0 1 2 3 4 5 6 7 Chlorine dosage (mg/L) Break point dosage is 3 mg/L. 6.2 mg/L is the dose required to provide 1 mg/L of free residual.

Mechanism of disinfectant: The five principal mechanisms that have been proposed to explain the action of disinfectants are: 1. Damage to the cell wall: it will result in cell lysis and death. 2. Alteration of cell permeability: certain disinfectants destroy the selective permeability of the membrane and allow the vital nutrients, such as nitrogen and phosphorous to escape. 3. Alteration of the colloidal nature of the protoplasm: heat, for example, will coagulate the cell protein. While, acids or base will denature protein. 4. Alteration of the organism DNA: UV radiation can rupture some DNA stands. 5. Inhibition of enzyme activity: oxidizing agents such as chlorine can alter the chemical arrangement of enzymes and inactivate the enzymes.

Characteristics of an ideal disinfectant: Availability: should be available in large quantities and reasonably priced. Deodorizing ability: should deodorize while disinfecting. Homogeneity: Solution must be uniform in composition. Interaction with extraneous material: should not be absorbed by organic matter other than bacterial cell. Noncorrosive and nonstaining: should no disfigure metals or stain clothing. Nontoxic to higher form of life: should be toxic to microorganisms and non toxic to humans and other animals. Penetration: should have the capacity to penetrate through surface.

Safety: should be safe to transport, store, handle and use. Solubility: must be soluble in water or cell tissue. Stability: should have low loss of germicidal action with time on standing. Toxicity to microorganisms: should be effective at high dilutions Toxicity at ambient temperatures: should be effective in ambient temperature range.

Factors influencing the action of disinfectants: Contact time: the longer the contact time, the greater the kill. Concentration of disinfectant: usually, higher concentration achieve better disinfection process. Intensity and nature of physical agent or means: such as light and heat. It has been found that there effectiveness is a function of there intensity. Temperature: Generally, increasing the temperature results in a more rapid kill with chemical disinfectants . Nature of suspending liquid: for example, extraneous organic material will react with most oxidizing disinfectants. The presence of suspended matter will reduce the effectiveness of disinfectants by absorption of the disinfectant and by shielding the bacteria.

Typical chlorination tank The baffling is generally utilized to reduce short-circuiting by reducing dead space area

Typical chlorination tank The baffling is generally utilized to reduce short-circuiting by reducing dead space area

Typical chlorination tank The baffling is generally utilized to reduce short-circuiting by reducing dead space area

Other Disinfection agents: Ozone: Ozone has been used for drinking water treatment on a municipal scale long time ago. More than 2,000 water treatment works in Europe countries use ozone for disinfection. In Canada, and the United States ozone has been used extensively. Generally, ozone is far more rapid in its action than other oxidizing agents. Ozone demand is usually satisfied in 30 seconds or less.

ozone is transferred into the water by bubbling it through the bulk solution. The decay phenomena (which requires on-site generation) prevents maintenance of a residual ozone concentration which could guard against contamination of the distribution system. Once ozone is injected into water, it immediately starts oxidizing organic material in the membranes of bacteria, viruses, and parasites. This weakens, ruptures, and kills their cells.

Ultraviolet radiation: Depending on the intensity of the UV radiation and the degree of dispersion in the flow, the contact time necessary to inactivate microorganisms range from a few seconds to a few minutes. UV light disinfects by penetrating microorganisms and destroying their DNA in UV water disinfection technology. Because DNA is essential for organisms' functions and reproduction, destroying it prevents the organism from being active and multiplying. No harmful byproducts are formed.