Particulate and colloidal silver in sewage effluent and sludge discharged from British wastewater treatment plants

Differential filtration was used to measure silver (>2 nm) entering and leaving nine sewage treatment plants (STPs). The mean concentration of colloidal (2-450 nm) silver, which includes nanosilver, was found to be 12 ng/L in the influent and 6 ng/L in the effluent. For particulate silver (>450 nm) the mean values were 3.3 µg/L for influent and 0.08 µg/L for effluent. Thus, removal was around 50% and 98% for colloidal and particulate silver respectively. There was no significant difference in performance between the different types of STP investigated (three examples each of activated sludge, biological filter and biological filter with tertiary treatment located across England, UK). In addition, treated sewage sludge samples (biosolids) were taken from several STPs to measure the total silver likely to be discharged to soils. Total silver was 3-14 mg/kg DW in the sludge (median 3.6), which if the sludge were added at the recommended rate to soil, would add 11 µg/kg/yr to the top 20 cm soil layer. Predicted concentrations using the LF2000-WQX model for all the rivers of England and Wales for nanosilver were typically in the 0-1 ng/L range but levels up to 4 ng/L are possible in a high discharge and low flow scenario. Predicted concentrations for the total particulate forms were mostly below 50 ng/L except for a high discharge and low flow scenario where concentrations could reach 135 ng/L.