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

Differential filtration was used to measure silver (>2nm) entering and leaving nine sewage treatment plants (STPs). The mean concentration of colloidal (2–450nm) silver, which includes nanosilver, was found to be 12ngL−1 in the influent and 6ngL−1 in the effluent. For particulate silver (>450nm) the mean values were 3.3μgL−1 for influent and 0.08μgL−1 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 3–14mgkg−1 DW in the sludge (median 3.6), which if the sludge were added at the recommended rate to soil, would add 11μgkg−1yr−1 to the top 20cm soil layer. Predicted concentrations using the LF2000-WQX model for all the rivers of England and Wales for nanosilver were typically in the 0–1ngL−1 range but levels up to 4ngL−1 are possible in a high discharge and low flow scenario. Predicted concentrations for the total particulate forms were mostly below 50ngL−1 except for a high discharge and low flow scenario where concentrations could reach 135ngL−1.