Heavy metal fractionation and organic matter stabilization in sewage sludge treatment wetlands

The presence of heavy metals in sludge stabilized in a reed bed system may affect its use for agricultural purposes. However, the environmental impact of sludge depends on the availability and phytotoxicity of these heavy metals. The aim of this research was to determine the effectiveness of a reed bed (Phragmites australis) sludge treatment system in two urban wastewater treatment plants in Italy after a three-year period of operation: (i) by estimating the process of sludge stabilization, following conventional and nonconventional parameters related to the evolution of organic matter quality (water soluble carbon, dehydrogenase activity, pyrolytic fragments): (ii) by following the heavy metal bioavailability in the sludge through their fractionation. For heavy metal fractionation, the Community Bureau of Reference (BCR) was followed. The results showed that there was mineralization and stabilization of sludge over time, suggested by the decrease of about 35% in water soluble carbon and of about 60-80% of dehydrogenase activity. Moreover, significant values of benzene (17%), toluene (31%) and phenol (9%) were found at the end of experimentation in both treatment wetlands, highlighting the re-synthesis of humic-like matter. The results also showed that the content of heavy metals after 30 months was associated with the less mobile fractions of the sludge (more than 60% of total heavy metal content for almost metal), in particular, the fraction linked to the organic matter.