Assessment of metal speciation evolution in sewage sludge dewatered in vertical flow reed beds using a sequential extraction scheme

An investigation was carried out into the speciation of metals contained in an industrial sewage sludge dewatered in vertical flow reed beds as compar ed to conventional sand drying beds. The highly contaminated sludge was loa ded for two consecutive months and the dewatered sludge layers were monitor ed periodically du ring an ageing period of 15 months. Metal speciation was assessed by means of sequential extraction. The raw sludge was characteris ed by high metal mobility. 445 mg.kg(-1) dry matter (DM) Mn (70% of total M n), 632 mg kg-l DM Ni (42% of total Ni), 11,246 mg kg(-1) DM Fe (32% of tot al Fe), 2,315 mg kg(-1) DM Zn (19% of total Zn) and 258 mg kg(-1) DM Pb (14 % of total Pb) were retrieved in the exchangeable and acid extractable frac tions. Cu and Cr were less mobile, with 24 mg kg(-1) DM Cu (1.2% of total C u) and 7 mg kg(-1) DM Cr (0.8% of total Cr) occuring in these fractions. Upon sludge ageing, all metals except copper, were greatly immobilised in t he oxidisable, reducible and residual fractions. However, considerable amou nts remained mobile, posing environmental threats upon final disposal of th e dewatered sludge.