Application of chemical oxidation processes for the removal of pharmaceuticals in biologically treated wastewater

Abstract: The discharge of effluents from wastewater treatment plants (WWTPs) is considered to be the major source of residual pharmaceuticals frequently found in aquatic environments. The complex nature of such compounds tends to make conventional biological treatments aimed at their removal ineffective. The present thesis concerns the removal of 62 different active pharmaceutical ingredients commonly detected in Swedish wastewater effluents by means of chemical oxidation techniques. Techniques with potential to be effective are in particular peracetic acid (PAA), chlorine dioxide (ClO2), ozone (O3) and a combination of ozone and hydrogen peroxide (O3/H2O2), which all were included in this study. The performance of a given treatment was evaluated in terms of the degree of pharmaceutical removal achieved and the oxidant demand of the wastewater. The effects of the characteristics of the wastewater have on the degree of removal efficiency of different pharmaceuticals were also evaluated. Ozone is considered to be the most efficient chemical oxidant for reducing the concentrations of a large number of different pharmaceuticals, the ozone dose required for this being fairly low (5-10 mg/L), depending upon the characteristics of the effluent. Over 90% of the pharmaceuticals investigated in most of the effluents could be eliminated to 90-100% by use of ozone, while several of the pharmaceuticals being observed to be recalcitrant to chlorine dioxide treatment. The addition of small amounts of hydrogen peroxide during wastewater ozonation, although not enhancing the removal of pharmaceuticals, was found to increase ozone decomposition, presumably resulting in the formation of hydroxyl (OH) radicals as secondary oxidants. The addition of small amounts of H2O2 in this way is seen as being advantageous in terms of its reducing both the treatment time and the reaction tank volume which is needed. Of the various water quality parameters investigated, the organic carbon content was found to have a particularly strong effect on the removal of pharmaceuticals, due to its competitive behavior towards the oxidant. PAA appears to have the lowest degree of pharmaceutical removal, making it not a suitable treatment option for removing pharmaceuticals in the effluents. Although chlorine dioxide and ozone appeared quite similar in their manner of removing pharmaceuticals, both of them reacting with electron-rich functional groups such as those of the phenolic and amino type, some of the pharmaceuticals reacted more slowly with chlorine dioxide than with ozone, given the same reactive substituent and structural similarities. Thus, a decision regarding the possible use of chlorine dioxide for tertiary treatment should take account of how strongly the pollutant or pollutants in question are affected by it. The use of chlorine dioxide appeared to be particularly beneficial when a small-scale WWTP is involved or when treatment is required for only a limited period of time. Although ClO2 is slightly more expensive to produce than ozone, the preparation system and the reaction chamber for treatment that are required are far simpler and less expensive to build than those needed for ozone treatment. It was noted that energy costs connected with ozonation are a function both of the ozone demand of the wastewater and the contaminant or contaminants to be removed. It appeared that, in view of the high degree of reactivity of ozone to a broad range of the pharmaceuticals that were investigated, ozonation of secondary effluent is the most suitable alternative for most WWTPs.