Temporal variability of sewage sludge heavy metal content from Greek wastewater treatment plants

Abstract

Abstract:The aim of this study was to evaluate the temporal variations of selected heavy metals level in anaerobic fermented and dewatered sewage sludge. Sewage sludge samples were collected in different seasons and years from three municipal wastewater treatment plants (WWTPs) located in Northern Greece, in Kavala (Kavala and Palio localities) and Drama (Drama locality) Prefectures. An investigation of the potential of sludge utilization in agriculture was performed, based on the comparison of average total heavy metal concentrations and of chromium species (hexavalent, trivalent) concentrations with the allowed values according to the Council Directive 86/278/EEC and Greek national legislation (Joint Cabinet Decision 80568/4225/91) guidelines. In this regard, all the investigated heavy metals (Cd, Cr, Cu, Ni, Pb, Zn, Hg) and chromium species Cr(VI) and Cr(III) have average concentrations (dry matter weight) well below the legislated thresholds for soil application, as following: 2.12 mg kg -1 Cd; 103.7 mg kg -1 Cr; 136.4 mg kg -1 Cu; < 0.2 mg kg -1 Hg; 29.1 mg kg -1 Ni; 62.0 mg kg -1 Pb; 1253.2 mg kg -1 Zn; 1.56 mg kg -1 Cr(VI) and 115.7 mg kg -1 Cr(III). Values of relative standard deviation (RSD) indicate a low or moderate temporal variability for domestic-related metals Zn (10.3-14.7%), Pb (27.9-44.5%) and Cu (33.5-34.2%), and high variability for the metals of mixed origin or predominantly resulted from commercial activities, such as Ni (42.4-50.7%), Cd (44.3-85.5%) and Cr (58.2-102.0%). For some elements the seasonal occurrence pattern is the same for Kavala and Palio sludge, as following: a) Cd and Cr: spring>summer>winter; b) Cu, Ni and Pb: winter>spring>summer. On average, in summer months (dry season) metal concentrations are lower than in spring and winter (wet seasons), with the exception of Zn. For Kavala and Palio the results demonstrate that the increased number of inhabitants (almost doubled) in summer time due to tourism does not influence the metal levels in sludge. Comparing the results obtained for similar spring-summerwinter sequences in 2007 and 2010/11 and for the spring season in 2007, 2008 and 2010, it can be noticed that, in general, the average heavy metal contents show an increasing tendency towards the last year. In all the measurement periods, the Palio sludge had the highest metal contents and Kavala sludge the lowest, leading to the conclusion that the WWTP operating process rather than population has a significant effect upon the heavy metal content of sludge. Cr(VI)/Cr(total) concentration ratios are higher for Kavala sludge in the majority of sampling campaigns, followed by Drama and Palio sludge. The metals which present moderate to strong positive correlation have common origin, which could be a domestic-commercial mixed source.

Identification of the Chemical Forms of Heavy Metals in Municipal Sewage Sludge as a Critical Element of Ecological Risk Assessment in Terms of Its Agricultural or Natural Use

“…Moreover, significant correlations between Cd, Pb, Zn and DM and OM have also been noted. The obtained results are in good agreement with the experimental data presented by other researchers [ 9 , 40 ]. Minor differences in correlations between individual variables are probably related to the processes to which the sewage sludge was subjected.…”

Digestion Procedure and Determination of Heavy Metals in Sewage Sludge—An Analytical Problem

“…Despite the fact that samples of sludge were collected in short intervals of time from a small sewage treatment plant, a certain temporal variability of the metal contents is observed. Similar relations span over longer time were found in other works [15,43,44]. The content of metals (except for Cr) in sample No.…”

“…Regardless of the decomposition method applied, the metal contents occur in the following order: Zn > Cu > Pb > Ni > Cr > Cd either in raw or lime-stabilized sludge. This order is slightly different from those previously reported [24], but it is very similar to that described by Spanos et al [43] and Sánchez et al [44]. Depending on the source of sewage, differences in Cr, Ni and Pb content are observed.…”

“…Pearson correlation coefficients (R) indicate the relationship between copper and zinc (R = +0.771) in the raw sludge. A high positive correlation could be related to domestic-commercial origin of these elements [43]. In the stabilized sludge, the correlation coefficients are high or moderate and amount to R = +0.945 (Cu/Zn), R = +0.879 (Pb/Ni), R = +0.872 (Pb/Cd), R = +0.677 (Ni/Cd), R = −0.768 (Cd/Cu), R = −0.762 (Pb/Cu), R = −0.754 (Cd/Zn) R = −0.657 (Pb/Zn), R = −0.606 (Ni/Cu), R = −0.547 (Ni/Zn).…”

“…Higher correlations in stabilized sludge are due to complex processes occurring in the sludge, e.g., gradual decomposition of the organic matrix, sorption of metal cations from the environment by an alkaline sludge or leaching of easily soluble metal compounds with organic substances [1,4]. The negative correlation between metals may be explained by their origin from multiple sources (e.g., commercial, micro-industrial units, domestic) and greater variation in the pollution load [27,43].…”