Change of toxicity during secondary treatment of industrial sludge containing nitroaromatics

Sammanfattning: Wastewater treatment plants in Sweden are facing a great challenge due to the prohibition of depositing organic waste which gained legal force in January 2005. Biological and commercial available alternatives to disposal are composting and anaerobic digestion. A promising technique for wastewater and sludge treatment is the use of constructed wetland.In this thesis we have looked at the sludge from a wastewater treatment plant receiving wastewater from industries manufacturing pharmaceutical substances, chemical intermediates and explosives. The wastewater and sludge contains high concentrations of nitro-aromatic compounds and amino-aromatic compounds, both original compounds and degradation products. The degradation of nitroaromatics under different oxygen conditions is well examined and several studies have reported the alteration of nitroaromatics to more potent cytotoxic and genotoxic compounds after degradation. The use of bioassays is a practicable approach to estimate the toxic potency of complex samples such as sludge, since it is difficult to analyse all toxic compounds in a sample by using chemical analysis.The aim of this study was to follow change in toxicity in three different sludge treatment methods; aerobic composting and anaerobic digestion and constructed wetland to follow change in general and mechanism-specific toxicity.In order to detect both lipo- and hydrophilic compounds, different extraction and bioassays are necessary. We used organic solvents and multi-layer cleaned-up samples in the DR-Calux assay to detect the persistent lipophilic compounds causing dioxin-like activity. We also used both water and acetone extracts in the umu-C assay to detect genotoxicity and in the fishegg assay to detect embryotoxicty of water and semi-water soluble compounds in low concentrations.The results showed that anaerobic treatment is less suitable for this particular sludge, since the anaerobic residues showed high induction in all toxicity tests compared to the residues of the aerobic treatment. The anaerobic treatment also transformed the toxicants in the sludge to more lipophilic and persistent forms as well as to more water soluble, genotoxic compounds. This makes the treated material difficult to handle due to the toxic potential of the material as well as of the leachate. The aerobic treated material also showed an increased toxic potential of the material but the leachate showed no toxicity. However, high concentrations of nitrite were formed during both aerobic and anaerobic treatment, which is of environmental concern.The constructed wetlands eluated a non-toxic effluent and had a bed material with lower toxicity than expected considering the sludge concentrations and loaded sludge amount. The bed material contained concentration of TEQ considered as acceptable levels for sensitive ground use such as parks, lawns and other grounds without groundwater protection.However, all three treatment methods need to be optimised and the residual needs further evaluation in order to establish a suitable biodegradation method.