IS BIOMASS CONCENTRATION A FACTOR DETERMINING THE SENSITIVITY OF ACTIVATED SLUDGE TO TOXIC SHOCKS?

Fresh mixed liquor from a full-scale facility was used to test if biomass concentration is a determining factor in the toxicity response of activated sludge to xenobiotic toxins. The mixed liquor was diluted to 50% and 10% of its original concentration with filtered supernatant, and the respiration inhibition induced by cadmium, 2,4-Dinitrophenol (DNP) and N-methylmaleimide (NEM) was measured through specific oxygen uptake rate (SOUR) assays with all three biomass concentrations. The results indicate that, on a mg contaminant/L mixed liquor basis, the less concentrated biomasses were more sensitive to both cadmium and NEM than the mixed liquor in its original concentration. For DNP, the toxicity response was the same for all the biomass concentrations tested (on a mg/L basis). If the data is interpreted on a mg contaminant/g VSS basis, though, opposite results are observed, that is sensitivity to the chemical increases as the biomass concentration increases, for all three toxins tested. These trends were observed both when the respiration inhibition was measured immediately after blending the toxin with the biomass, and when a reaction time of 2 hours was allowed, indicating that the contact time between the toxin and the mixed liquor does not have an impact on the relative trends observed. These results do not allow for a definitive conclusion on the role that cell density-dependent mechanisms may have on the resistance of activated sludge to chemical stress. On-going experiments will monitor for the production of signal molecules upon exposure of activated sludge to stress conditions.