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 contained high concentrations of nitro-aromatic compounds and amino-aromatic compounds and several studies have reported the alteration of nitroaromatics to more potent cytotoxic and genotoxic compounds after degradation.
The aim of this study was to follow the change in toxicity (both general and mechanism-specific) of three different sludge treatment methods: aerobic composting; anaerobic digestion and constructed wetland. We used DR-Calux assay to detect the persistent lipophilic compounds causing dioxin-like activity; the umu-C assay to detect genotoxicity and the fish egg assay to detect embryotoxicty.
The results showed that anaerobic treatment is the least suitable for these sludges; showing high induction in all toxicity tests compared to the aerobic treatment. The anaerobic treatment also transformed the nitroaromatic compounds to more lipophilic and persistent forms.
The constructed wetland left a non-toxic effluent and a bed material with lower toxicity than expected considering the concentrations and loading period.