Reducing the Heavy Metal Content of Sewage Sludge by Advanced Sludge Treatment Methods
ABSTRACT
Waste-activated sludge (WAS) processes are key technologies to treat wastewater: their effluents can meet stringent discharge standards, thus ensuring a minimum residual impact on the aquatic environment. The presence of heavy metals in the excess sludge poses, however, serious problems, and considerably hampers the final disposal alternatives, especially in the agricultural use (soil improvement/amendment). This article studies the effect of thermal hydrolysis and Fenton’s peroxidation on the heavy metal content of the dewatered sludge. Acid thermal hydrolysis reduces the heavy metal content in the filter cake except for Cu, Hg, and Pb. Alkaline thermal hydrolysis releases Cu, Pb, and Cr. Fenton’s peroxidation transfers Cd, Cu, and Ni from the filter cake into the filtrate. Land application of the residual cake can hence be reconsidered.
INTRODUCTION
WASTE-ACTIVATED SLUDGE (WAS) processes are key technologies to treat wastewater: their effluents can meet stringent discharge standards, thus ensuring a minimum residual impact on the aquatic environment. Through their microbial activity, these biological processes produce huge amounts of WAS, now commonly called biosolids. This excess sludge is an inevitable drawback inherent to the WAS process. Increasing amounts of WAS have to be dealt with due to (1) the higher wastewater collection rates, (2) the reliability and efficiency of wastewater treatment plants, and (3) the widespread application of denitrification and dephosphatation. The presence of heavy metals in the excess sludge poses, however, serious problems, since they are among the most important factors in assessing the final disposal alternatives of the sludge, for example, they will accumulate in the fly ash in the event of incineration, can be released through leaching in landfills, or may prevent land application.
Many countries have imposed legal restrictions concerning the heavy metal content, when using the sludge for land application (e.g., agricultural use as fertilizer or soil amendment), since high concentrations in the sludge may result in heavy metal accumulation in cultivate
d soils. In the European Union, the maximum acceptable concentrations are fixed by the Directive 86/278/EEC of the European Commission (Commission of the European Communities, 1986). Most countries apply, however, a more stringent legislation. In Flanders, the standards are described in VLAREA (Flemish Government, 1997). Both European and Flemish legal standards, as well as the average concentrations of heavy metals in municipal sewage sludge in Flanders (Aquafin, 2004), are presented in Table 1. Some metals present in the WAS do not comply with these legal standards, thus necessitating the application of less sustainable and more expensive disposal routes.
Heavy metals in sludge cannot be removed by common sludge treatment methods such as aerobic or anaerobic digestion.
Some advanced sludge treatment processes (AST) were recently confirmed as having the potential to enhance the dewaterability of the sludge. The most promising techniques include acid and alkaline thermal hydrolysis (Neyens and Baeyens, 2003a; Neyens et al., 2003a, 2003b) and the peroxidation of the sludge (Neyens and Baeyens, 2003b; 2003c; Ne
yens et al., 2002). In addition, Dewil et al. (2005) demonstrated an increase of the thermal conductivity of peroxidated sludge, hence improving its drying characteristics. Neyens et al. (2004) concluded that these techniques enhance the dewaterability of the sludge by degrading the extracellular polymeric substances (EPS) structures. Since a large part of the heavy metals are bound to these EPS, the AST processes are supposed to also release heavy metals from the sludge to the water phase, thus reducing the heavy metal concentration in the residual sludge cake after dewatering. In doing so, the water phase needs a traditional treatment for metal removal (settling, flotation, etc.) prior to being recycled to the plant influent. This article studies the effect of thermal hydrolysis and Fenton’s peroxidation on the heavy metal content of the dewatered sludge.
EXPERIMENTAL SETUP AND PROCEDURE
Sludges used
The experiments make use of thickened activated sludge samples, taken from the full-scale wastewater treatment plant (WWTP) of Leuven (located in Flanders, Belgium).
The WWTP of Leuven is a high-load WAS plant having a sludge ODS/DS (organic fraction of the total amount of dry solids) fraction of 67%. No primary sedimentation is present.
reactor pressure vesselThe activated sludge was collected and settled in the laboratory for about 4 h. The supernatant was thereafter poured off. The resulting dry solids content was about 3 wt.%.
Hydrolysis
Hydrolysis (acid or alkaline) of the sludge implies a treatment at moderate to high temperature. Acid thermal hydrolysis at low pH requires sulphuric acid (from a solution containing 1.75 kg H2SO4/L), whereas alkaline thermal hydrolysis occurs at high pH, obtained by adding Ca(OH)2 (from a suspension containing 50 g Ca(OH)2/L). After reaction, the sludge is neutralized by adding Ca(OH)2 or H2SO4 for, respectively, acid or alkaline thermal hydrolysis. Ciba® ZETAG 7878 FS40 polyelectrolyte is thereafter added before dewatering the sludge.

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