Septic tank sludge is regularly hauled to the Montreal Urban Community (MUC) wastewater treatment plant. It is then discharged and mixed with the wastewater inflow before entering the primary chemical treatment process. An ecotoxicological procedure integrating chemical and toxicological analyses has been recently developed and applied to screen for the illicit discharge of toxic substances in septic sludge. The toxicity tests used were the Microtox, the bacterial-respiration, and the lettuce (Lactuca sativa) root elongation tests. In order to validate the applicability of the proposed procedure, a two-year interlaboratory study was carried out. In general, the results obtained by two independent laboratories (MUC and the Centre d'expertise en analyse environnementale du Quebec) were comparable and reproducible. Some differences were found using the Microtox test. Organic (e.g., phenol and formaldehyde) and inorganic (e.g., nickel and cyanide) spiked septic sludge were detected with good reliability and high efficiency. The relative efficiency to detect spiked substances was > 70% and confirms the results of previous studies. In addition, the respiration test was the most efficient toxicological tool to detect spiked substances, whereas the Microtox was the least efficient (< 15%). Efficiencies to detect spiked contaminants were also similar for both laboratories. These results support previous data presented earlier and contribute to the validation of the ecotoxicological procedure used by the MUC to screen toxicity in septic sludge.
This article presents an integrative approach, using toxicological and chemical analyses, to screen for toxic substances that could be illegally added to the chemical-toilet sludge received at the wastewater treatment plant of the Montreal urban community. Four toxicity tests (Microtox, bacterial-respiration, root-elongation, and seed-germination tests) were used to establish the toxicity range of a "normal" sludge and the determination of threshold limits criteria. Chemical-toilet sludge samples were spiked with two types and amounts of contaminants (zinc, phenol). Conservative criteria were used to detect abnormal toxicity with great reliability and avoid false positives (i.e., detecting abnormal toxicity in nonspiked sludge). Taken individually, the seed-germination test was the least discriminating toxicological method (detecting only 10% of the spiked samples); the bacterial-respiration test was relatively better (detecting 72% of the spiked samples). Using a limited battery of two toxicity tests (Microtox and respiration test), the identification of contaminated chemical-toilet sludge can be detected with good efficiency and possibly great reliability (more than 80% of the spiked samples). This proposed procedure is efficient, easy to apply, cost-effective, and very fast (an abnormal toxicity level can be determined within a few hours).
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