Studying some of the qualitative properties and concentration of heavy metals in dried sewage sludge from Shahin-Shahr WWTP in Isfahan

Land application of sewage sludge is one of the most important disposal methods, which allows the sludge nutrients to be used beneficially. However, it should be noted that its pollutants and pathogenic organisms can pose a high risk to the public health and it should be monitored before land application. The aim of this study was to investigate the dried sewage sludge quality from Shahin Shahr WWTPS for its heavy metals concentration. Samples were collected seasonally from Shahin-Shahr sewage sludge (in three replicates) within 2011-2012, and the concentration of some heavy metals along with microbial and chemical parameters of sludge were measured using DTPA, atomic absorption and standard methods. The results were then compared with the standard levels (EPA) using t-test, and seasonal variations were studied by one way ANOVA and Duncan’s post-hoc test. The total and fecal coli forms values did not fall within any standard ranges, so re application of the sewage in the given conditions was restricted due to hygienic precaution. The pH, total solids, organic matter and moisture values were in normal ranges. Seasonal variations of sludge structure showed that the best conditions in terms of absorbable heavy metals concentration lowness belonged to winter. The maximum and minimum annual loads of absorbable concentration belonged to Fe and Cr respectively. Considering the total concentrations, most of the measured elements had the minimum concentration in winter (except for Zn, Pb and Cu) and the maximum concentration in spring and summer (except for As, Zn and Cu). All of the studied metals, except as, and their annual loads (kg/ha/year) had a concentration below the acceptable level. The annual load of as was estimated to be 1.6 kg/ha/year for. Although the annual load of as was below the standard level, it should be taken into account due to being close to this level.