First Comprehensive Analysis of Potential Ecological Risk and Factors Influencing Heavy Metals Binding in Sewage Sludge from WWTPs Using the Ultrasonic Disintegration Process

Abstract

In this study, the occurrence, fractionation, and potential ecological risk associated with seven heavy metals (HMs), i.e., Cd, Cr, Cu, Ni, Pb, Zn and Hg in sewage sludge (SS) were investigated. The main aim of the study was to conduct the first comprehensive analysis of the potential ecological risk of HMs in SS collected from two municipal wastewater treatment plants (WWTPs) using ultrasonic disintegration (UD) of thickened excess sludge aimed at improving the effects of anaerobic digestion (AD). In order to assess the level of potential ecological risk, two groups of indices related to the total content of HMs and their chemical forms were used. For this purpose, a modified BCR sequential extraction was conducted. The obtained results revealed that according to the values of total indices, the highest potential ecological risk was posed by Cd, Cu and Zn (and to a lesser extent by Ni and Cr), while in relation to speciation indices by Ni, Zn, Cd (and in some cases by Cr). In general, the highest risk was indicated at the beginning (primary and excess SS) and the two final stages of sludge processing (digested and dewatered SS). This means that the level of ecological risk may strongly depend on the processes used in WWTPs and especially on AD, dehydration and the activated sludge process, as well as on the characteristics of raw wastewater. The results of the statistical analysis and balance sheets revealed that the key factors which may influence the way that HMs bind in SS are: pH, TOC, OM and Eh. The obtained results showed that UD does not practically cause changes in the total HMs concentrations in SS and their release into supernatant in the mobile forms. This proves that UD is a safe and environmentally friendly method of sewage sludge pretreatment.

Ecological and human health risk assessment of heavy metals in sewage sludge produced in Silesian Voivodeship, Poland: a case study

“…In turn, during anaerobic digestion, where the organic matter content undergoes reduction, metals bound to OM are released. This may increase the share of these elements in immobile fractions, especially in fraction F3 which was observed in previous work (Tytła et al, 2023 ). We observed the above-mentioned tendency only in relation to Cd, Cu, Ni, and Zn (WWTP1) and to Cd, Ni, and Zn (WWTP2).…”

“…However, in this research, the highest concentrations of metals were found at sampling points S6 (digested sewage sludge) and S7 (dewatered sewage sludge). In the case of digested SS, the probable cause of the increase of HMs concentrations was their adsorption on the flocs surface, whereas, for the dewatered one, the main cause was the increase of DM content, as a result of the dehydration process (Tytła et al, 2023 ). It was also revealed that the concentrations of HMs in the dewatered sludge (S7) from the studied WWTPs are higher than those from WWTPs, located in other regions of the country, e.g., in the Pomeranian Voivodeship (north Poland) (Milik et al, 2017 ), Warmian-Masurian Voivodeship (north-east Poland) (Gusiatin et al, 2018 ), and Świętokrzyskie Voivodeship (southern part of central Poland) (Kowalik et al, 2021 ).…”

“…It was also found that the main processes that can influence the binding of heavy metals and consequently the level of potential ecological risk that these contaminants pose to the natural environment are the activated sludge process (S3) and anaerobic digestion (S6). As it is commonly known, the activated sludge process is carried out in aerated biological reactors, where the dissolved oxygen concentration undergoes substantial fluctuations (Tytła et al, 2023 ). In the WWTP1 and WWTP2, the excess sludge samples (S3), which had previously undergone the activated sludge process, displayed an elevation in both pH and redox potential values.…”

“…The values of the relative standard deviation (RSD) for analyzed HMs did not exceed 3%. The operational parameters for ICP-OES were described in detail, in the previous study (Tytła et al, 2023 ). All chemical reagents used in this study, except reference materials, were manufactured by Avantor Performance Materials Poland S.A. (formerly POCH; Poland).…”

“…In the scientific literature, there are several articles that focus on evaluating the potential ecological risk associated with HMs in SS used for agricultural purposes. However, there is a scarcity of research that would focus on binding these elements in sludge and its influence on the level of the final ecological risks (i.e., Wang et al, 2006 ; Gwebu et al, 2017 ; Tytła et al, 2023 ). There are also very few studies that specifically focus on the assessment of human health risks of HMs in sewage sludge (i.e., Kendir et al, 2015 ; Duan et al, 2017 ; Duan & Feng, 2021 ; Yakamercan et al, 2021 ; Espinoza-Guillen et al, 2022 ; Qayoom et al, 2022 ; Yakamercan & Aygün 2023 ); however, none of them deals with SS produced in the municipal WWTPs located in Poland.…”