Removal of THM Precursors by Adsorption, Coagulation and UV Irradation

Abstract

The general characteristics of humic substances (HS) and their negative effect on water quality and its treatment are described. The paper presents the results from the removal of humic substances from the Hriňová (Slovakia) water reservoir using 1) granular activated carbon (GAC) from two producers (Chemviron, Cabot) at three different pH levels, 2) by coagulation with two coagulants (aluminum sulfate and ferric sulfate), and 3) by an advanced oxidation process using UV radiation with the addition of the oxidizing agent hydrogen peroxide. The effectiveness of the removal of humic substances (expressed by the TOC parameter) from water with adsorption by GAC, depending on the contact time with the water, ranged from 14 % for one hour to 50% for eight hours of contact time. The calculated GAC adsorption capacity of the humic substances ranged from 0.05 mg/g for one hour to 0.19 mg/g for eight hours of contact time. The pH of the water had no significant effect on the level of efficiency. A high level of efficiency (50%) and the lowest value of TOC are obtained at a pH of 6.5. The humic substances remain in the water though the optimal dose of the coagulant was used. The ferric sulfate coagulant is more suitable for removing of humic substances from water as aluminum sulfate. A dose of 16 mg/L of 1% solution Fe2(SO4)3, which is the equivalent of 4.47 mg/L Fe3+, is sufficient for the reduction of humic substances below 1 mg/L. The effectiveness of the removal of humic substances with coagulation using iron sulfate was 83% for a parameter of HS and 50% for TOC or CODMn. The iron sulfate has the influence on a reduction in pH of water, therefore pH of water should be adjusted after coagulation. A laboratory study of removal of humic acids from surface water by UV radiation and addition of hydrogen peroxide in order to increase the oxidation of organics are presented. The results showed that UV radiation and addition of H2O2 during the first few minutes of irradiation increases the concentration of the humic substances, CODMn and TOC in water. After 3 minutes of exposure to UV radiation, these values were reduced to baseline, and in the next irradiation phase the values of the monitored parameters decreased. The highly reactive OH radicals with a high oxidation potential oxidize humic substances to various intermediates (a lower molecular weight substances). After 20 minutes of irradiation, a decrease in the parameters of approx. 39% for TOC, 62% for CODMn and 37% for humic substances was observed.