Simultaneous sewage sludge digestion and metal leaching using an internal loop reactor

The objective of this work was to determine whether various sludge types (secondary, or aerobically digested), coming from different wastewater treatment plants, could be microbially leached to solubilize toxic metals and simultaneously digested (SSDML process). Indigenous sulfur-oxidizing microorganisms from acclimated sludge were used to generate sulfuric acid in situ by the oxidation of added sulfur. The experiments were conducted in internal loop tower type reactors (sludge volume 10 liters and inoculum volume 1.5 liters) with sulfur spheres of 25 mm diameter placed at the bottom of each reactor. Results obtained in batch experiments in bioreactors using sludges from four different wastewater treatment plants (secondary sludge from Sainte Claire (B1), secondary sludge from Beauceville (B2), secondary sludge from Black Lake (B3) and aerobically digested sludge from Beauceville (B4)), showed that the yield of metal solubilization (due to the pH decrease) was not affected by the type of sludge processed. The following metal solubilization yield was obtained: Cd, 83–100%; Cr, 34–46%; Cu, 61–100%; Fe, 17–49%; K, 32–69%; Mn, 91–100%; Ni, 57–84%; P, 33–69%; Pb, 17–44%; Zn, 83–96%. Cu and Ni, exceeding the norms in the fresh sludge, were reduced below the required levels at the end of the SSDML process. In conventional aerobic sludge digestion a volatile suspended solids reduction of 35% (B1), 17% (B2), 29% (B3) and 24% (B4) was observed in 16 days. An additional reduction of volatile suspended solids—22% (B1), 23% (B2), 15% (B3) and 17% (B4)—was observed after the addition of sulfur spheres and inoculation of sulfur-oxidizing microorganisms (the simultaneous sewage sludge digestion and metal leaching process) in 27 days. The use of sulfur in the form of spheres created anoxic zones in the bioreactor and thus favored denitrification (by autotrophic microorganisms and/or by the heterotrophs using nitrate as their terminal electron acceptor), leading to removal of the nitrates from the sludge. A substantial amount of ammonia was solubilized in the leachate produced, but the nitrogen, phosphorus and potassium values of the decontaminated sludge solids were only marginally affected. The sphere form of sulfur used in the experiments facilitated the recovery of unused sulfur at the end of the process and thus minimized the potential for sludge re-acidification.