Effect of Zinc and Copper on Anaerobic Stabilization of Sewage Sludge
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The most frequently found metals in municipal sewage sludge are copper and zinc. The aim of this study was to the evaluate effect of these metals on anaerobic microorganisms during sewage sludge stabilization. Anaerobic fermentation tests were carried out in 24 batch fermenters with hydraulic retention time 21 days at mesophilic temperature conditions 38 oC +- 0.2 oC. Five different concentrations of metal ionts (100-1000 mg.l-1) were tested. Cumulative biogas and methane production were used as the comparative parameters of inhibition. Hypothesis, which predicted presence of inhibitory effect of zinc and copper on anaerobic microorganisms, mainly on methanogenic Archaea, was confirmed. The lowest concentration of zinc and copper which cause significant inhibition of biogas production was 400 mg Zn2+.l- and 1000 mg Cu2+.l- 1, which cause reduction of 10.3 +- 2.0 % and 82.8 +- 1.1 %, respectively. The lowest concentration of zinc and copper which lead to significant inhibition of methane production is 400 mg Zn2+.l- and 600 mg Cu2+.l- 1, which caused to reduction of 16.1 +- 3.2 % and 17.4 +- 2.2 %, respectively. The reduction in methane production is higher than in biogas production.