Procedure to Screen Illicit Discharge of Toxic Substances in Septic Sludge Received at a Wastewater Treatment Plant

This paper presents an integrative approach, using toxicological and chemical analyses, to screen toxic and illegal substances that could be added to the septic sludge transported by a tanker truck to the wastewater treatment plant of the Montreal Urban Community (MUC). Microtox, lettuce root elongation, and a bacterial respiration test were used to establish the toxicity range of a normal sludge and the determination of threshold limit criteria. Septic sludge samples were spiked with different types and amounts of contaminants (copper, zinc, phenol, industrial sludge). Conservative criteria were applied to detect abnormal toxicity with great reliability while avoiding false positives (i.e., detecting abnormal toxicity in nonspiked sludge). Taken individually, toxicity tests using Microtox were revealed to be the least discriminating toxicological method (efficiency of 45% when the ratio of the IC50values is considered), whereas lettuce root elongation was relatively the most efficient (80% of spiked samples). As a whole, the battery of toxicity tests detected at least 93% of the spiked sludge samples. This procedure is also very efficient, i.e., easy to apply, cost effective, and rapid. In certain cases, an abnormal toxicity level can be determined within a few hours, whereas a septic sludge can be classified as normal within 5 days.