OXIDANTS
A wide variety of oxidants are used for treating treating water. They have been used mostly for disinfection, oxidizing ferrous ions to ferric ions and more recently to attempt to eliminate or reduce BOD so discharge permits could be met or avoid installing installing a biological treatment plant. The most common oxidants are: Hydrogen peroxide Ozone Chlorine Chlorine Dioxide Potassium Permanganate Ferrate
H2O2 O3 Cl2 ClO2 KMnO4 FeO4-2
To oxidize organic compounds to carbon dioxide and water, several reactions must take place. First, the oxidant and organic compound react to form form reaction products. These reaction products might be simply a cleaving of the molecule into two two pieces. This reaction does not significantly alter the BOD or COD content of the water. To make a significant change in BOD and COD, the organic or inorganic compound must be further oxidized to completion completion (second step). For an organic molecule, complete oxidation would generate carbon carbon dioxide and water. To achieve this, additional oxidant must be added. Since the oxidants are not specific for organic organic and inorganic compounds, they will oxidize all of the compounds available. To calculate the oxidant dosage required to treat a water stream, we need to know how many reactive oxygens [O] are available from the oxidant. Chlorine Hypochlorite Chlorine Dioxide Ozone Peroxide Permanganate Ferrate
Cl2 + H2O --> [O] + 2Cl - + 2H+ HOCl --> [O] + Cl- + H 2ClO2 + H2O --> 5[O] + 2Cl- + 2H O3 ---> [O] + O 2 H2O2 ---> [O] + H 2O MnO4 +H2O ---> 3[O] + MnO 2 + 2OH 2FeO4-2 + H2O ---> 3[O] + Fe 2O3 + 4OH-
Thus, chlorine dioxide is a much more efficient oxidant than peroxide or ozone since it generates five (5) reactive oxygens compared to peroxide and ozone, which generates one (1). The calculation used to estimate dosage in a water system is: Oxidant Dosage (ppm) = ((Mw of Oxidant)/16n) x COD
Where Mw is the molecular weight of the oxidant, n is the number of reactive oxygens generated and COD is the Chemical Oxygen Demand of the water. Thus, for a system with 500 ppm COD, it will take a calculated 419 41 9 ppm chlorine dioxide to convert the COD to non-oxidizable species, CO2 and H2O. Calculation: Dosage = ((67/16x5)) x 500 Dosage = (0.83)(500)
Dosage = 419 ppm Using BOD instead of COD can be okay, as long as there is not much difference between these values. Remember, the BOD value is ALWAYS lower than the COD value. Oxidants are not specific for organics, so the inorganics that can consume oxidants will do so. Always use the COD value when calculating, but if only a BOD value is handy, the calculated dosage will be the minimum oxidant dosage required. The oxidants must consume essentially all oxidizable material and convert the organics to carbon dioxide and water before a significant reduction in BOD can be measured. This is why simply adding a small amount of oxidant to a water stream usually has very little or no impact on measured BOD. In practice, high dosages are required and these high dosages makes oxidation an impractical process for reducing BOD in water. If you need more convincing on how impractical simple oxidation is, here is an example. For a given wastewater containing 5 x 10-3 M of phenol, a flow of 1 MGD and using a conversion factor of 8.34 LB/mg/L/MG, here is the calculated dosage and cost of feeding peroxide. A. Stoichiometric O2 Dosage (LB/D) = (3.5x10-2M)(34,000mg/mole)(8.34) = 9,341 LB O2 /Day B. Stoichiometric H2O2 dosage (LB/D) = (7.0x10-2M)(34,000mg/mole)(8.34) = 19,849 LB H2O2 /Day C. At a cost of $0.5 per pound for peroxide, that calculates out to $9,925 per day or $3,622,625 per year!! - For this kind of money, a bioplant is much more cost effective.
