Potential biomarker for denitrification of wastewaters: Effects of process variables and cadmium toxicity

Laboratory experiments have been conducted to characterize the sensitivity of a pure-cultured denitrifying bacterium from wastewater sludge, selected by virtue of its exhibiting promising qualities as a biomarker, to wastewater process variables and potential toxic inhibition by a heavy metal, cadmium. A pure culture method offers the possibility of development of a standardized toxicity test with reproducible results that are not site specific, and enables close examination of one specific reaction without interference from other organisms in the system. Denitrification by the selected bacterium, measured in terms of nitrite reduction, is rapid, reproducible, and conforms to a linear time-concentration profile, provided a select range of process conditions is employed. The denitrification rate is a maximum at about 30°C and in a pH range of 7.5 to 8.0. The rate per unit wet mass of cells is constant for varying cell concentrations and given conditions of pH and temperature. A noncompetitive inhibition model provided a good description of the relationship between denitrification rate and the concentration of a heavy metal, cadmium. The model constant, Ki, the concentration resulting in 50% inhibition, is 12 mg/l for cadmium for the test conditions used and represents a means for assessing the relative toxicity of inhibitors. The reproducibility of the test cases studied and straightforward analysis render the bacterium and test procedures promising for development of a rapid toxicity test for wastewater in a denitrifying environment. Ongoing studies are directed toward evaluating the extent to which the test organism represents the response of a denitrifying sludge population to potentially toxic compounds.