Three different sized laboratory scale Rotating Biological Contactor (RBC) units were constructed to study the controlling factors of physical oxygen transfer. Physical parameters were identified and evaluated by the volumetric oxygen transfer coefficient (KLa) from non-steady state clean water tests. The controlling factor was found to be the liquid film flow rate per unit volume and the function could be practically expressed as w1.5 D0.5 S−1. The turbulene effect on the free water surface and subsurface volume becomes comparatively less significant as the disk size increases. The data indicated that the physical oxygen transfer contribution to the biological treatment oxygen requirement became less significant than oxygen transfer through other mechanisms as the disk size increases.

Richard W. Schneeman--------A contribution to the JOURNAL by Gail P. Edwards, Prof. Emeritus of San. Chemistry, Alan H. Moloj, Assoc. Prof. of Civ. Eng., and Richard W. Schneeman, Asst. Research Scientist, all of New York Univ., Bronx, N.Y.

Abstract-Sediments are evaluated for toxicity by measuring mortality in a single cohort of amphipods in either acute (10-d) or chronic (28-d) bioassays. This investigation differed from conventional bioassays in four ways: Sublethal effects (fecundity) were estimated; the testing period was 60ϩ d; two successive generations were examined; and Elasmopus laevis Smith, 1873, amphipods were employed. Four test sediments were created between 58 and 424 g/g of lead using the 30-g/g whole-sediment as the control. Bioaccumulated lead at 60 d varied as a linear function of lead concentration in the sediments. Fecundity, as estimated by offspringper-chamber and/or percent reproductive success, was reduced as sediment lead concentrations increased and reproduction was delayed compared with the control. The reduction in offspring production per test chamber varied significantly as an inverse function of lead sediment concentration, best described by a curvilinear exponential equation. It was concluded that E. laevis exposed to 118 g/g and higher could not maintain a population as large as that in the control. Although the current sediment quality guideline for lead stipulates that adverse biological effects likely will occur above 218 g/g, this study revealed a statistically significant negative reproductive response at 118 g/g lead, and suggests that the current regulatory guideline for lead, based on lethality, should be reconsidered.

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