As indicated by these examples, H2O2 can be used as a stand-alone treatment or as an enhancement to existing physical or biological treatment processes, depending on the situation.
Hydrogen peroxide can be used alone or with catalysts – such as iron (Fe2+ or Fe3+), UV light, ozone (O3) and alkali – to oxidize BOD/COD contributing compounds in wastewaters. The type of oxidation needed depends on the type of BOD/COD present. This relationship is present in the figure below.
Chemical Oxygen Demand
|Oxidant System||Type A||Type B||Type C|
|(Sulfide, Thiosulfate, Sulfite)||(Phenols, Cyanides, Amines)||(BTEX, TOCl, Paraffins)|
|H2O2 / OH-||X||X|
|H2O2 / M+||X||X|
|H2O2 / M+||X||X|
|H2O2 / H+||X||X|
|H2O2 / Fe||X||X||X|
|H2O2 / O3||X||X||X|
|H2O2 / UV||X||X||X|
Note: Whether an oxidant system will degrade a specific pollutant (i.e., affect its COD) will depend on the oxidant system and the pollutant. Type A oxidants react only with Type A pollutants; whereas, Type C oxidants, being more reactive, react with most any pollutant. However, Type C oxidants generally react preferentially with Type A pollutants.
If a large fraction of the BOD and COD is contributed by inorganic reduced sulfur compounds such as sulfides, sulfides, or thiosulfate, then hydrogen peroxide alone is typically effective. Depending on the wastewater pH, the oxidation of these compounds by H2O2 yields sulfate or colloidal sulfur, neither which contribute to BOD and COD. If the primary contributors to BOD and COD are dissolved organics, then a more reactive oxidation system is needed. Moderate activation of hydrogen peroxide can be achieved by: 1) alkali (generating the perhydroxyl ion , OOH- – the active agent in peroxide bleaching systems); 2) certain transition metals (e.g., tungstate, vanadate, molybdate) which form reactive peroxometal complexes in-situ; and 3) certain mineral acids (e.g., sulfuric) which form reactive peroxyacid derivatives such as peroxymonosulfuric acid (Caro’s Acid) ex-situ. For the more recalcitrant organics such as chlorinated solvents, extremely reactive free radical systems (termed Advanced Oxidation Processes) are needed. A generalized reaction using Fenton’s Reagent for reducing BOD and COD can be expressed as follows:
Step-1: BOD/COD + H2O2 —> partially oxidized species
Step-1: partially oxidized species + H2O2 —> CO2 + H2O + inorganic salts
The extent of oxidation (and therefore the degree of direct BOD/COD reduction) typically depends on the amount of hydrogen peroxide used. The theoretical hydrogen peroxide requirement is about 2.1 lbs (as 100%) per lb-BOD and COD oxidized. In many cases, however, complete digestion of the organic compounds to carbon dioxide and water is not needed. Partial oxidation to intermediate compounds minimizes chemical consumption and often results in substantial reductions in BOD and COD and toxicity.
Enhanced physical separation of BOD and COD with hydrogen peroxide may occur is two ways. In the first case, partial oxidation of organic contaminants results in more polar (charged) substances which are more amenable to adsorption onto coagulants and flocculants. As illustrated in the example below, this allows BOD and COD removal efficiencies with less than stoichiometric hydrogen peroxide doses.
In the second case, enhanced physical separation (flotation) of fats, oils and greases (FOG) is provided by H2O2. This occurs by the natural decomposition of hydrogen peroxide to oxygen and water, i.e., hydrogen peroxide will supersaturate the wastewater with oxygen, which results in the formation of evenly dispersed microbubbles which scavenge FOG constitutents as they rise to the surface of the water. In some cases, this can increase BOD removal through dissolved air flotation cells from e.g., 50% to 90-95%. Typical doses are 25-100 mg/L H2O2, the cost for which can often be offset against savings in coagulant use – a polyelectrolyte polymer is generally still needed.
The BOD removal efficiency of aerobic biological treatment processes depends on a number of factors including (but not limited to): influent BOD loading, F:M ratio, temperature, nutrient levels, and dissolved oxygen (DO) concentrations. Many biological treatment facilities use hydrogen peroxide to supplement DO levels when oxygen limited conditions in aeration basins or lagoons result in poor BOD removal. These conditions can be brought about by unexpected peaks in influent BOD loading; seasonal variations in BOD loading (e.g., fruit and vegetable processing); and hot weather – which reduces the efficiency of oxygen transfer by mechanical aeration equipment (i.e., O2 solubility decreases as temperature increases). These conditions may or may not be accompanied by filamentous bulking (see Municipal Wastewater Applications : Filamentous Bulking Control).
When hydrogen peroxide is used to supplement DO, it is metered directly into the aeration basin of a biological treatment system to provide an immediate source of DO. The conversion of hydrogen peroxide to DO in an activated sludge mixed liquor proceeds according to the following reaction:
2 H2O2 —> O2 + 2 H2O
Theoretical hydrogen peroxide requirement: 0.48 lbs H2O2 (100%) per mg/L DO
Catalase enzyme is a natural decomposition catalyst for hydrogen peroxide, and is found in all activated sludge mixed liquors, being produced by most aerobic organisms. Because this enzymatic decomposition of hydrogen peroxide is very rapid, the oxygen supplied by hydrogen peroxide is immediately available for uptake by the aerobic organisms.
The above reaction shows that two parts of hydrogen peroxide will yield one part of DO. Therefore, the amount of hydrogen peroxide required to oxygenate the wastewater is surprisingly small. For example, the theoretical amount of hydrogen peroxide required to increase the DO by 1 mg/L in a treatment plant that averages 5 MGD flow is about 17 gpd-50%. In actual practice, the requirement may be higher due to inefficiencies in oxygen uptake and side reactions with oxidizable compounds.
Note: When measuring the BOD or COD of hydrogen peroxide treated wastewaters, it is important to determine the residual hydrogen peroxide concentration (if any) prior to analysis. This is because H2O2 will interfere with both of these analytical methods. In the standard BOD test, residual hydrogen peroxide in the sample will liberate oxygen over the test period, resulting in a “false low” BOD value (1 mg/L H2O2 = 0.5 mg/L DO). In the standard COD test, residual hydrogen peroxide will react with the potassium dichromate reagent, resulting in a “false high” COD value. For methods to remove residual hydrogen peroxide prior to BOD and COD analysis, or to mathematically account for the residual H2O2, see Interferences with Analytical Methods.
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