Oxidation of aged raw landfill leachate with O3 only and O3/H2O2:: Treatment efficiency and molecular size distribution analysis

An aged raw landfill leachate was taken from the equalization storage tank at Clover Bar Landfill Leachate Treatment Plant, Edmonton, Alberta, Canada. The average quality parameters of this leachate were: COD=1,090mg/L, BOD5=39mg/L, color=1,130 TCU, NH3-N=455mgN/L, alkalinity=4,030mg/L as CaCO3 and pH=8.30. The major fraction of this leachate was large refractory organic compounds. Ozone (O-3) only and O-3 combined with hydrogen peroxide (O-3/H2O2) were applied to treat this leachate, aiming at enhancing COD and color reduction and increasing its biodegradability (i.e., the ratio of BOD5/COD). All of the O-3 only and O-3/H2O2 oxidation experiments were performed in a gas washing bottle equipped with a fine bubble diffuser. The used ozone dose ranged from 1.2 to 12.5g O-3/L leachate for O-3 only treatment, and 1.8 to 13.8g O-3/L leachate for O-3/H2O2 treatment. H2O2 dose for O-3/H2O2 treatment was 0.63g H2O2/L leachate. COD, BOD5, color, NH3-N, nitrite+nitrate, and alkalinity were measured before and after treatment. Meanwhile the molecular size distribution of the leachate, before and after treatment, was analyzed by using a high-performance liquid chromatograph (HPLC) with gel filtration column and UV detector at 254 nm. The addition of H2O2 had an insignificant effect (at 5% significance level) on enhancing COD and color reduction. After oxidation, the maximum BOD5 increase was about 110% for O-3/H2O2 treatment and about 141% for O-3 only treatment at a used ozone dose of 3.6g O-3/L leachate and 2.6g O-3/L leachate for O-3/H2O2 and O-3 only, respectively. As the used ozone dose increased, NH3-N and alkalinity decreased considerably, and nitrite+nitrate increased accordingly. Treatment efficiency models, which describe the changes in COD, BOD5/COD, NH3-N, nitrite+nitrate, and alkalinity as functions of the used ozone dose, were developed. Statistically (at 5% significance level), the treatment efficiency models for both treatments are not different. According to the results of molecular size distribution analysis, no correlation was observed between the BOD5 increase and oxidation by-products' formation.