Copper Removal to PPB Residuals via Iron Coagulants and Biosolids Storage Conditioning

Copper removal to residual parts-per-billion (ppb) levels was achieved in an activated sludge system by employing iron bioflocculation and biosolids storage conditioning. Low copper residuals correlated to an increase in extracellular polysaccharides that were produced by microorganisms within activated sludge. In batch bench-scale tests, supernatant copper levels between 15 and 35 ppb were achieved when employing a coagulant dosage of 15 mg/L (as Fe) and when allowing 18 to 26 h of biosolids storage conditioning to enhance the production of exopolymers. These copper levels were less than half as much as when neither coagulant nor biosolids storage conditioning was employed. Minimum copper residuals occurred in a pH range from 6.5 to 8.8, and when at least half of the biosolids had been stored for a day. Biosolids that were conditioned under a closed-to-oxygen environment yielded the lowest copper residuals among the bench-scale conditions that were tested, and it also produced the largest amounts of extracellular polysaccharides, as determined by a ruthenium red adsorption method and an india ink method.