Metals removal from soil, fly ash and sewage sludge leachates by precipitation and dewatering properties of the generated sludge

This study concerns the treatment by precipitation of three acidic and metal-rich leachates by using various reagents. Two treatment modes (simple and combined precipitation) have been performed to evaluate the metals removal efficiency and the dewatering ability of the generated sludge. It appears that for the three leachates used, the Ca(OH)(2) addition gave better metals removal at pH 10.0 than the use of NaOH at the same pH. Moreover, the combination of NaOH and Na2S allows better removal for Pb2+ Zn2+, Cu2+ and Mn2+ ions than with NaOH/Na2CO3 or NaOH/Na2HPO4. The dewaterability (vacuum filtration) of precipitates produced during the treatment of soil leachate was established as follows on the basis of the specific resistance to filtration (SRF) values: sulphides (4.3 x 10(12) m kg(-1))< phosphates (6.4 x 10(12) m kg(-1)) <hydroxides [Ca(OH)(2)] (14.2 x 10(12) m kg(-1))< hydroxides [NaOH] (19.7 x 10(12) m kg(-1))< carbonates (26.5 x 10(12) m kg(-1)). For the fly ash leachate treatment, the obtained results were quite similar no matter the treatment mode used; the SRF values were in a range of 5-7 x 10(12) m kg(-1), while sludge volume index (SVI) ranged from 39 to 50 mLg(-1). The sludge generated during the treatment of sewage sludge filtrate using phosphate salt gave the best results regarding to SVI and SRF parameters (180 mLg(-1) and 7.6 x 10(12) m kg(-1), respectively). Experimental results show that whatever precipitating agent is used, vacuum filtration is more efficient in water elimination [total solids: 11-73% (ww(-1))] than in centrifugation [5-11% (ww(-1))] from sludge. (C) 2009 Elsevier B.V. All rights reserved.