Different options for metal recovery after sludge decontamination at the Montreal Urban Community wastewater treatment plant N. Meunier*, J.F. Blais*, M. Lounès**, R.D. Tyagi* and J.L. Sasseville** * Institut National de la Recherche Scientifique (INRS-Eau), Université du Québec, 2700, rue Einstein, Sainte-Foy, Quebec, Canada, G1V 4C7 ** Biolix Corporation., Iberville II, 1175 Lavigerie, Bureau 50, Sainte-Foy, Quebec Canada, G1V 4P1 Abstract The MUC (Montreal Urban Community) treatment plant produces approximately 270 tons of dry sludge daily (270 tds/day) during the physico-chemical treatment of wastewater. Recently, this treatment plant endowed a system of drying and granulation of sludge for valorization as an agricultural fertilizer having a capacity of 70 tds/day (25% of the daily sludge production). However, the metal content (mainly Cu and Cd) of the sludge surpasses the norms for biosolids valorization. In order to solve this problem, a demonstration project, from the lab scale to the industrial pilot plant, was carried out to test the Metix-AC technology for the removal of metals. A strongly metal-loaded filtrate was generated during the sludge decontamination. Tests concerned the study of the metal recovery by total precipitation and selective precipitation, as well as the use of alternative products for the metal precipitation. Other works consisted to simulate the acid filtrate recirculation from the decontaminated sludge (25% of the total volume) in the untreated sludge (75% of the total volume) intended for the incineration. The total precipitation with hydrated lime appeared effective for the recovery of metals (87% Cd, 96% Cr, 97% Cu, 98% Fe, 71% Ni, 100% Pb, 98% Zn). However, this option entails the production of an important quantity of metallic residue, which should be disposed of expensively as dangerous material. The selective iron precipitation does not appear to be an interesting option because the iron in solution within the leached sludge was principally present in the form of ferrous iron, which cannot be precipitated at pH lower than five. On the other hand, the use of commercial precipitating agents (TMT-15, CP-33Z, CP-NB and CPX) without pH adjustment of filtrate gave good results for the recovery of Cu and, to a lesser degree for the recovery of Pb. However, the efficiency for the other metals’ (Cd, Cr, Fe, Ni and Zn) recovery was weaker (< 25%). Finally, the acid filtrate recirculation containing solubilised metals in untreated sludge destined for incineration appears to be the most interesting option. Metals in solution in the acid filtrate, precipitate or adsorb effectively (97% Cd, 97% Cr, 99% Cu, 82% Ni, 100% Pb and 87% Zn) on the solids of the untreated sludge. Moreover, TCLP tests were done on ashes produced during the incineration of sludge mixed with the acid filtrate produced during sludge decontamination. These tests showed that there were no significant differences, as regards the extractability of metals, between such ashes and those produced during the untreated sludge incineration without addition of filtrate. Therefore, it was predictable that this method can respect the current environmental standards required by the different governmental authorities. Keywords Biosolids; incineration; leaching; metal; precipitation; sewage sludge Introduction The MUC (Montreal Urban Community) wastewater treatment plant generates a large amount of sludge (approximately 270 tons of dry sludge daily), which should be treated and disposed of safely according to strict and environmental standards. At its arrival in the sludge treatment building, the sludge was sent to the homogenization tanks where it was mixed. From the homogenization tanks, the sludge was then pumped towards ten press fil- ters and five rotary presses for the dehydration stage. This dehydration allows us to increase the sludge concentration from 3.5% of solids to about 30% (w/w) and more. Following this, an important proportion of sludge (200 tds/d) was forwarded into incinerators. Recently, a part of sludge was sent by a series of conveyors to silos in order to be dried and granulated Water Science and Technology Vol 46 No 10 pp 33–41 © 2002 IWA Publishing and the authors 33 Downloaded from http://iwaponline.com/wst/article-pdf/46/10/33/425908/33.pdf by guest on 02 April 2021