Purposes of Oxidation u Removal of iron, manganese, sulfides u Removal of Taste & Odor u Destruction of color u Removal of synthetic organics –chlorinated solvents –pesticides, etc. u Partial Degradation of natural organics –aids subsequent treatment

Oxidation: Types of Oxidants u Oxygen (O 2 ) u Chlorine (Cl 2, HOCl or OCl - ) u Chloramines (NH 2 Cl or NHCl 2 ) u Ozone (O 3 ) u Chlorine Dioxide (ClO 2 ) u Permanganate (MnO 4 - ) u Others: Peroxide, Bromine, UV light

Disinfection u Kill or inactivate pathogens –Bacteria, viruses protozoa u Disinfectants –Chlorine (Cl 2, HOCl or OCl - ) –Chloramines (NH 2 Cl or NHCl 2 ) –Ozone (O 3 ) –Chlorine Dioxide (ClO 2 ) –Others: Bromine, UV light u Primary purpose for drinking water treatment

Chlorine Chlorine or Cl 2 gas, has been used since the end of the last century as a disinfectant in the United States. It is an effective and inexpensive disinfectant. A problem with chlorine is the formation of trihalomethanes (THMs) -- chemicals formed when organic matter in the water reacts with the chlorine. Trichloromethane, a THM, is a carcinogen. The current limit for THMs is 100 ppb. Some of the alternative disinfectants discussed later produce lower amounts of THMs or do not produce THMs. When chlorine is injected into water it forms:

Chlorine Cont. The hypochlorous acid ionizes to hypochlorite. Although both hypochlorous acid and hypochlorite are disinfectants, hypochlorous acid is much more powerful. Both termed as free chlorine. The equilibrium reaction is:

Chlorine Reactions with Fe +2, Mn +2, S 2- H 2 S + 4 Cl H 2 O = H 2 SO 4 + 8HCL

Chlorine Reactions with Ammonia u NH 3 + HOCl = NH 2 Cl + H 2 O monochloroamine* NH 2 Cl + HOCl = NHCl 2 + H 2 O dichloroamine* u NHCl 2 + HOCl = NCl 3 + H 2 O Nitrogen trichloride Chloroamines are called combined chlorine residuals and have disinfecting power

Breakpoint Chlorination

Chick’s Law In the early 1900's Dr. Harriet Chick postulated a method of estimating the destruction of microorganisms by disinfectants as a function of time. Dr. Chick postulated that the death of the microorganisms was a first order process. So, for a given disinfectant and concentration: This can be separated and integrated (with N = N o at t = 0) to yield:

Chick’s Law cont. where, N o =initial concentration of microorganisms, no./mL N=concentration of microorganisms at time t, no./mL t=time of disinfection, [hr] k=an empirical constant descriptive of the particular microorganisms and disinfectant in use, [hr -1 ] Or:

Example 9.2 The following is actual data for the poliomyelitis virus exposed to an experimental disinfectant. Determine the k value for Chick's Law. Estimate the time required to obtain a reduction of 1/10,000th the original number of viruses. (Source: Floyd, Roger, et. al, "Inactivation of Single Poliovirus Particles in Water by Hypobromite Ion, Molecular Bromine, Dibromine, and Tribromine," Environmental Science and Technology, Vol. 12, No. 9, pp , September 1978.)

This data is plotted, along with a least squares regression line. The slope of the line is the disinfection constant k = 0.634/s.

Example 9.2 cont. The time required for a reduction of 1/10,000th is then:

DBPs (Disinfection By Products) u Reaction of Chlorine with Organics in water and wastewater u HOCl + NOM Oxidized NOM Chlorinated Organics TOX THM HAA Carcinogens