Leaching of ashes from co-combustion of sewage sludge and wood-Part II: The mobility of metals during phosphorus extraction

Artikel i vetenskaplig tidskrift, 2008

Sewage sludge and its ashes after combustion are contaminated with metals in various concentrations. In the work described in this paper, the mobility of metals during recovery of phosphorus by acid leaching of fly ashes from co-combustion of sewage sludge with wood was investigated. The metal concentrations in two sewage sludges, fly ashes and leachates from acid phosphorus extraction, were compared with phosphorus rock and different fertilisers used in agriculture. The secondary cyclone ashes were found to be less contaminated with trace elements than the bag-filter ashes. The largest problem is cadmium, which in all ashes studied has a too high level to meet the legislative limits for the maximum dosage in Sweden and the proposed limits in the European Union. The legislation includes limits for cadmium (Cd), mercury (Hg), chromium (Cr), copper (Cu), nickel (Ni), lead (Pb) and zinc (Zn) calculated for an amendment of X kg of fertiliser per year and hectare of land. The solubility of the trace metals in the cyclone ash, with Cd as an exception, is in general much lower than the solubility of phosphorus. A decrease in pH results in an increased release of Cd but has just a slight influence on the other metals analysed in this study. The Cd yield increases by 30% when pH is lowered from 2.5 to 1.0, whereas the Hg release is not affected at all. The trace element concentrations in the leachates are far below the European and Swedish limitations of metal concentrations in fertilisers. The leachates thus fit as fertilisers and also as raw material to the industry, if Cd is removed. In Part I of the project [Pettersson A, Åmand L-E, Steenari B-M. Leaching of ashes from co-combustion of sewage sludge and wood. Part I: recovery of phosphorus. Biomass and Bioenergy, 2007, accepted for publication [1]] focus was on the phosphorus recovery only. © 2007 Elsevier Ltd. All rights reserved.