Chemical and ecotoxicological characterization of ashes obtained from sewage sludge combustion in a fluidised-bed reactor.

In 1999, the DEECA/INETI and the UBiA/FCT/UNL started a researching project on the partition of heavy metals during the combustion of stabilised sewage sludge (Biogran), in a fluidised-bed reactor, and on the quality of the bottom ashes and fly ashes produced. This project was entitled Bimetal and was funded by the Portuguese Foundation for Science and Technology. In this paper only the results on the combustion of Biogran are reported. The combustion process was performed in two different trials, in which different amounts of sewage sludge and time of combustion were applied. Several ash samples were collected from the bed (bottom ashes) and from two cyclones (first cyclone and second cyclone ashes). Sewage sludge, bed material (sand) and ash samples were submitted to the leaching process defined in the European leaching standard EN 12457-2. The eluates were characterized for a set of inorganic chemical species. The ecotoxicological levels of the eluates were determined for two biological indicators (Vibrio fischeri and Daphnia magna). The results were compared with the limit values of the CEMWE French Regulation. The samples were also ranked according to an index based on the chemical characterization of the eluates. It was observed an increase of the concentration of metals along the combustion system. The ashes trapped in the second cyclone, for both combustion trials, showed the highest concentration of metals in the eluates. Chemically, the ashes of the second cyclone were the most different ones. In the ecotoxicological point of view, the ecotoxicity levels of the eluates of the ashes, for both combustion cycles, did not follow the same pattern as observed for the chemical characterization. The ashes of the first cyclone showed the highest ecotoxicity levels for V. fischeri and D. magna. This difference on chemical and ecotoxicological results proves the need for performing both chemical and ecotoxicological characterizations of the sub-products of such type of thermal processes.

[1]  A. E. Greenberg,et al.  Standard methods for the examination of water and wastewater : supplement to the sixteenth edition , 1988 .

[2]  Joachim Werther,et al.  Gaseous emissions from co-combustion of sewage sludge and coal/wood in a fluidized bed , 2004 .

[3]  I. Lopes,et al.  Discriminating the ecotoxicity due to metals and to low pH in acid mine drainage. , 1999, Ecotoxicology and environmental safety.

[4]  G. Sakellaropoulos,et al.  Leaching and toxicity behavior of coal–biomass waste cocombustion ashes , 2006, Environmental toxicology.

[5]  Bernard Clément,et al.  Are percolates released from solid waste incineration bottom ashes safe for lentic ecosystems? A laboratory ecotoxicological approach based on 100 litre indoor microcosms , 2005 .

[6]  J. Seco,et al.  A study of the leachate toxicity of metal-containing solid wastes using Daphnia magna. , 2003, Ecotoxicology and environmental safety.

[7]  A Kungolos,et al.  Application of leaching tests for toxicity evaluation of coal fly ash , 2006, Environmental toxicology.

[8]  N Lapa,et al.  Ecotoxicological assessment of leachates from MSWI bottom ashes. , 2002, Waste management.

[9]  H. A. van der Sloot,et al.  Characteristics, treatment and utilization of residues from municipal waste incineration. , 2001, Waste management.

[10]  R. Murillo,et al.  Ecotoxicological assessment of ashes and particulate matter from fluidized bed combustion of coal. , 1998, Ecotoxicology and environmental safety.

[11]  H. Spliethoff,et al.  Legislative and environmental issues on the use of ash from coal and municipal sewage sludge co-firing as construction material. , 2001, Waste management.

[12]  Rafael Kandiyoti,et al.  Partitioning of trace elements during the combustion of coal and biomass in a suspension-firing reactor , 2002 .

[13]  J. Werther,et al.  Sewage sludge combustion , 1999 .