EVOLUTION OF CHEMICAL FORMS OF LEAD IN LIMED SEWAGE SLUDGE STORED IN CONTROLLED CONDITIONS
AARAB Tarik1,3, SMEYERS Mathias1, REMY Marc2, GODDEN Bernard3 and DELHAYE Jean-Paul1
1Laboratoire d’Agrotechnologies Végétales – Université Libre de Bruxelles, 2Lhoist Recherche et
Développement, 3Institut de Gestion de l’Environnement et de l’Aménagement des Territoires
(free)
Land application of sewage sludge is an excellent way of recycling both mineral elements and organic matter. However, beside pathogens and organic compounds, the sewage sludge could contain potentially harmful trace metals. In Wallonia (Belgium) during the year 2001, of the 18,514 tons of day matter produced, 56% were applied on agricultural land. The total production is expected to increase to 45,000 tons by the end of 2010 in accordance with the regional program on wastewater treatment.
Sewage sludges re-used in agriculture have to be stabilized and are often stored for several months before land spreading. Stabilization treatment may affect the behaviour of heavy metals such as Pb which is a toxic element for the environment. In the present study, the chemical forms of lead have been investigated in heaps of limed and non-limed sludge coming from the wastewater treatment plant of Roselies (Belgium). These limed and nonlimed aerobically digested sludges were stored during four months in controlled conditions close to field conditions. A sequential extraction procedure was used to determine the lead chemical forms of representative samples taken in the outer shell and in the depth of the heaps. The physico-chemical properties (pH, organic matter, dry matter and temperature) of these samples were also monitored.
In unlimed sewage sludge, lead is associated with the organic, exchangeable and residual fraction respectively to the amounts of metal present in the fraction. The exchangeable lead decrease in surface during the first two weeks of storage. A slower decrease in the oranicallybound lead is associated with an increase of the exchangeable fraction after a month of storage.
The liming process increases dry matter and reduces organic matter content and the proportion of lead associated with organic complexes. This decrease continues during two and a half months in surface while in depth, the organic fraction of lead remains constant. The exchangeable lead is much lower in sludge after the liming treatment and stays minoritary during storage. Residual lead is present in the beginning of storage in limed sludge but drops after two month of storage in surface and in depth of the heaps.
The minority fractions of the non-treated sludge, the carbonate and oxide fractions take an important place in limed sludge as high pH could lead to chemical insolubilisation of lead in oxide or carbonate forms. The oxide fraction drops after three months of storage in surface and in depth where the decrease is smaller and delayed. For both types of sludge, changes in depth are attenuated or delayed in respect to surface.
This study shows that liming has an effect on the lead distribution in sludge and that the time and condition of storage and the level of sampling have important effects on the chemical forms of lead. Immediate effects of liming indicated that lead passes to less soluble fractions (carbonate and oxide) but we demonstrate that these fractions remain quite stable during the first months of storage. As the exchangeable fraction is minoritary during storage, only the variations of the oxide fraction should be taken into account for the determination of the optimal period of storage that could be comprized between two and twelve weeks.
KEY WORDS Chemical forms, Lead, Liming, Sewage sludge, Storage
