Aerobic biodegradation of a photoprocessing wastewater (with a silver level attained after silver recovery) was assessed using fill-and-draw activated sludge reactors. The extractability of silver from the resulting waste activated sludge was also determined. Simulated, fixer-derived wastewater was fed to the activated sludge reactors at a volumetric loading level of 40% along with an organic mixture. The biodegradation of the organic mixture was not affected by the photoprocessing wastewater. Thiosulfate was completely oxidized to sulfate. Complete conversion of ammonia to nitrite was observed, but much lower levels of nitrate were detected in the fixer and fixer-plus-silver-amended reactors, as compared to the control reactor (fed only the organic mixture). Silver at an influent total silver concentration of 1.85 mg/L did not result in any adverse effects on the activated sludge process. Filtered (through a 0.2-mu m membrane Biter) effluent silver concentrations were below the detection limit of 0.01 mg/L. Practically, all silver was associated with the sludge solids and the mean steady-state silver content of the sludge was 1.84 mg Ag/g mixed liquor suspended solids. When fresh sludge and aerobically digested sludge solids were subjected to the toxicity characteristic leaching procedure, the resulting silver concentration was at least 40 times lower than the regulatory limit of 5 mg Ag/L.
