Seasonal and time variability of heavy metal content and of its chemical forms in sewage sludges from different wastewater treatment plants

Sewage sludges obtained from seven wastewater treatment plants from the province of Salamanca, Spain, were periodically sampled to determine seasonal and time variation of their elemental composition over 2000 to 2002. The aim of this paper was to provide additional insight to evaluate the potential environmental impact following soil incorporation of these materials as amendments. Aqua regia extractable metals (pseudo total content) of Cd, Cr, Cu, Ni, Pb and Zn were determined and furthermore, the main chemical forms of metals within the sludge were evaluated using a five-step fractionation procedure. All the studied sludges displayed high fertility properties due to their richness of OC, P and K. Total mean concentrations of Cd, Cr, Cu, Ni, Pb and Zn in the sludges were within the regulation of the Spanish legislation. Using an multifactor analysis of variance, significant differences between Cr, Cu, Ni, Pb and Zn pseudo total contents (p < 0.01) of sludges at different sites were found while the Cd content was statistically similar. Also significant differences were found between these pseudo total contents of heavy metals in samples collected along the time after three years (0.001 < p < 0.08) and in samples taken at different seasonal periods (summer and winter) for Cr, Cu, Ni and Zn (0.001 < p < 0.07). The fractionated metals Cd, Ni and Zn were observed to be in mobile or bioavailable fraction of the sludge up to 35% of total metal content. Cu and Pb distributed in the organically bound fraction up to 25% and 60%, respectively. The Cd, Cr, Cu, Ni, Zn in sludges were predominantly bound within the oxide or silicate components. Significant differences between distribution fractions of metals considered together (p < 0.001) were found at different years, and for each individual metal, significant differences can be observed between distribution fractions of sludges collected at different sites, times and seasonal periods. The results showed that the studied sludges can potentially enhance soil agronomic properties. The fractionation data indicated that most metals occur in weakly mobile, non-bioavailable form, and only Cd, could have potential moderate mobility in soil–water system.