Sorption of Perfluorinated Compounds onto different types of sewage sludge and assessment of its importance during wastewater treatment

Abstract

The distribution coefficient (Kd) and the organic carbon distribution coefficient (KOC) were determined for four Perfluorinated Compounds (PFCs) to three different types of sludge taken from a conventional Sewage Treatment Plant (STP). Batch experiments were performed in six different environmental relevant concentrations (200ngL(-1)to 5μgL(-1)) containing 1gL(-1) sludge. Kd values ranged from 330 to 6015, 329 to 17432 and 162 to 11770Lkg(-1) for primary, secondary and digested sludge, respectively. The effects of solution's pH, ionic strength and cation types on PFCs sorption were also evaluated. Sorption capacities of PFCs significantly decreased with increased pH values from 6 to 8. Furthermore, the divalent cation (Ca(2+)) enhanced PFCs sorption to a higher degree in comparison with the monovalent cation (Na(+)) at the same ionic strength. The obtained Kd values were applied to estimate the sorbed fractions of each PFC in different stages of a typical STP and to calculate their removal through treated wastewater and sludge. In primary settling tank, the predicted sorbed fractions ranged from 3% for Perfluorooctanoic Acid (PFOA) to 55% for Perfluoroundecanoic acid (PFUdA), while in activated sludge tank and anaerobic digester sorption was more than 50% for all target compounds. Almost 86% of initial PFOA load is expected to be detected in treated wastewater; while Perfluorodecanoic acid (PFDA), PFUdA and Perfluorooctanesulfonate (PFOS) can be significantly removed (>49%) via sorption to primary and excess secondary sludge. In anaerobic digester, the major part (>76%) of target PFCs is expected to be sorbed to sludge, while almost 3% of initial PFOA load will be detected in sludge leachates.

Keywords: Distribution coefficient (K(d)); Fate; Perfluorochemicals; Sewage treatment; Sludge; Sorption.