The influence of biosolids treatment files on the mobility of metal trace elements

The production of sludge in France is estimated to be about 900,000 metric tons dry matter per year and 60% of this is recycled onto agricultural land . At present, the long term future of this procedure is open to question an d among the different arguments being put forward are the levels of metalli c trace elements and the risk of accumulation in soils. This study presents the behaviour of metallic trace elements in sludges fro m three different treatment procedures: thickened liquid sludges, dewatered sludges and dried sludges. These biosolids are mixed with a clay soil and then placed in a temperature and humidity controlled glasshouse. Several co ntainers are seeded with ryegrass and compared with controls. For the three harvests, covering all the amendments studied (including non-amended soil) , the differences are not really representative. Absorption by the ryegrass is low in all cases. For the cadmium, the chromium, the nickel and the lea d, the roots are 5 to 10 times more concentrated than the leaves. The major ity of these elements stay absorbed in the roots, regardless of the amendme nt used. The addition of the sludges has considerably reduced the uptake of water in ryegrass throughout its growth cycle. Quite apart from their fertilizing q ualities, wastewater treatment plant sludges could offer important implicat ions for irrigation.