The fly ash from sewage sludge thermal treatment recovery as LECA for constructed wetlands

摘要

Thermal treatment of sewage sludge produces another kind of waste - fly ash. The fly ash properties - high content of Ca~(+2), Mg~(+2), P2O5, and alkaline reaction induce to put the hypothesis that light expanded clay aggregate (LECA) prepared with high content of fly ash may be a product with good sorption capacity for such elements like heavy metals, phosphorus, and ammonium nitrogen. Five kinds of LECA have been made, with increasing content of fly ash 0% (bentonite), 12.5%, 25%, 50%, and 100%. The aim of the study was to determine the optimal composition of LECA for potential use as a fulfillment in constructed wetlands (CW) reed beds. In laboratory tests, the following parameters of obtained LECA have been determined: density, bulk density, squeezing strength, hydraulic conductivity, sorption capacity for ions: Zn~(+2), NH4_4~+, PO_4~(-3) . Conducted tests determined that all kinds of LECA had good mechanical and hydraulic properties, and might be used in CW without problems with squeezing and colmatation. Phosphates and zinc ions have been adsorbed with high efficiency (60-80%) especially by LECA with 25% to 100 % of fly ash content. Efficiency of ammonium nitrogen removal was low, less than 15%. Sorption capacity measurements in dynamic conditions, showed increasing removal efficiency with decrease of hydraulic retention time of water solution of Zn~(+2), PO_4~~(-3).