Infiltration of combined sewer overflow and tertiary treated municipal wastewater: An integrated laboratory and field study on various metals

The heavy metal load of combined sewer overflow (CSO) and its retention during CSO infiltration was studied in a comparative field and laboratory experiment during a 2-year period, with simultaneous study of tertiary treated municipal wastewater (TMW). The concentrations of aluminum, chromium, copper, iron, manganese, and zinc in CSO significantly exceeded those of TMW by factors ranging from 2 (manganese) to 12 (aluminum). Correlation analysis suggests that aluminum, barium, iron, lead, and zinc originate from urban runoff, while chromium and most alkali and alkaline earth elements are likely provided by municipal wastewater. As copper correlates with boron and zinc, raw wastewater and surface runoff may contribute to the elevated copper concentration in CSO. Laboratory column and field data indicate that analyzed metals were well retained during soil passage under the oxidizing conditions and that groundwater quality under CSO infiltration is comparable to TMW infiltration. Laboratory experiments show that nitrification and mineralization produce more acidity than CSO can neutralize. Thus, co-infiltration of TMW with CSO or the addition of carbonate minerals to the soil is recommended to prevent soil acidification and maintain its retention potential.