Degree Name

Master of Science (MS)

Semester of Degree Completion


Thesis Director

William A. Weiler


Inhibition of reaerated activated sludge was assessed using reduction of dissolved oxygen uptake by the microbial population as a criterion. Oxygen uptake is essential to biodegradation of the organic components in the activated sludge process; therefore, a decrease in dissolved oxygen uptake is an indication of reduced efficiency in wastewater treatment.

The irihibitors tested were cadmium, copper, lead, mercury, silver, 2,4-Dichlorophenoxyacetic acid (2,4-D), Terbacil and Zectran. The IIC50, determined by extrapolation of plotted data, for each inhibitor in mg/l was Hg (20), Cu (50), Ag (43), Cd (115), Pb (550), and 2,4-D (430). The IIC50 was determined by testing the effect on oxygen uptake of concentrations ranging from 0.001 to 1000 mg/l of the inhibitor. With the exception of Terbacil and Zectran, whose inhibitory effects were negligible and were deleted from further study, the IIC50 determined for each inhibitor was utilized in subsequent short-term and long-term studies.

Short-term studies measured the effect of the inhibitors on dissolved oxygen uptake over a 4-hour period. These experiments were intended to simulate the immediate effect upon the contact stabilization unit of a sewage treatment facility. In general, the short-term effects of the inhibitors followed a similar pattern; an initial marked inhibition of dissolved oxygen uptake was followed by small increases until maximal inhibition, which occurred after 3 or 4 hours in the presence of the IIC50. The observed variability in the percent inhibition of dissolved oxygen uptake was attributed in part to the inherent variability of different sludge samples.

Long- term studies, extending over a 2-week period, were intended to indicate possible inhibitory effects on dissolved oxygen uptake by an agent which remains within or is continually introduced into the sewage treatment facility. A periodicity of inhibition followed by semi-recovery was exhibited in studies of several inhibitors. Stimulation of dissolved oxygen uptake occurred during certain phases of the long-term study for both mercury and 2,4-D.

It was concluded that the inhibitory effects of the heavy metals Hg, Cu, Ag, Cd, and Pb, and of the herbicide 2,4-D are significant and will therefore affect the efficiency of wastewater treatment plant operation.