BIOLOGICAL SOLUBILIZATION OF METALS IN ANAEROBIC SEWAGE-SLUDGE - FILTERABILITY, NEUTRALIZATION AND N AND P CONTENT OF PROCESSED SLUDGE

Biological solubilization of heavy metals in anaerobic sewage sludge was performed in a continuous reactor with two mean hydraulic residence times (0.5, 0.75 day). Following filtration of the processed sludge, the amount of solubilized Cu and Zn decreased from 92% and 94% to 78% and 77% respectively in the sludge. Most iron reprecipitates in filtered sludge. Processed sludge is easy to deshydrate. The filtrability index (r0.5) of processed sludge is 38 times smaller than anaerobic sludge, without adding polymers. It was found that the values obtained for r0.5 and inferior to 8 x 10(12) m/kg enable the sludge to be dehydrated in a plate filter press without adding polymers. Centrifugation can also be used. The content in nitrogen, phosphorus and hydrolysable phosphorus remain unchanged during processing. 47% of the ammonia is lost in the filtrate. The neutralization of processed sludge (pH = 7) and contaminated supernatant (pH = 10) require a small quantity of base. The selective precipitation of metals could allow the recovery of Cu in the generated metallic sludge. Finally, results show the preliminary feasability of the process. A pilot scale study could enable the scale-up of the process.