Masters Theses

Date of Award

5-1997

Degree Type

Thesis

Degree Name

Master of Science

Major

Environmental Engineering

Major Professor

Kevin G. Robinson

Committee Members

Gregory D. Reed, Terry L. Miller

Abstract

Wastewater collected from the City of Oak Ridge, TN monitoring station, located downstream in the collection system from the Department of Energy's (DOE) Y-12 Weapons Facility and the Union Valley Industrial Park, was studied to assess the extent of uranium accumulation on mixed liquor suspended solids (MLSS) and to determine biological treatability parameters in an activated sludge wastewater treatment environment. In order to evaluate the impact of MLSS composition on uranium uptake, preliminary batch experiments were performed using MLSS from four wastewater treatment plants (WWTP) in the Knoxville, TN area which treat different types of waste streams. These four different MLSS samples were contacted with daily discrete aliquots of the Y-12/Union Valley wastewater (uranium concentration = 0.003 µg/ml) over a period of seven days. The MLSS samples from Kuwahee WWTP (domestic and industrial flow) and Carter High School Package WWTP (domestic flow only) exhibited similar uranium uptake levels of approximately 2.5 µgU/g dry solids after seven days of contact. MLSS from the Powell Valley Foods WWTP (food processing waste) had the lowest specific uptake of uranium at approximately 1.5 µgU/g dry solids. It was hypothesized that residual oil and grease in this sample competed effectively with the MLSS for uranium binding. MLSS from the Forks of the River WWTP (industrial flow which contained high levels of ferric chloride) exhibited the highest uranium uptake on days one and seven of the study. (2.875 and 3.879 µgU/g dy solids) but dropped by two-thirds on day three (1.243 µgU/g dry solids). This drop was apparently related to a loss of oxygen in the sample between days two and three. The oxygen loss was indicated by a change in the color of the Forks of the River MLSS samples from reddish-brown to black as the Fe3+ content changed to Fe2+. Apparently the Fe2+ competed more effectively with uranium for binding sites than did the Fe3+.

In order to provide engineers with the biological parameters (kinetic coefficients) required to design a full scale wastewater treatment plant, a treatability study was performed on the subject wastewater. For this study, a total of six 9.0 liter, continuous flow, completely mixed, activated sludge, bench scale reactors with recycle were operated at two different temperatures. Three reactors were operated at 20°C at biological solids retention times (BSRT) of 6.7 days, 15.4 days, and 21.8 days. Three reactors were operated at 5°C at BSRT's of 6.4 days, 14.4 days, and 18.7 days. Kinetic coefficients for the reactors operating at 20°C were determined to be: substrate utilizations rates (q)=0.08 to 0.34d-1, substrate utilization rate constant (K)=0.041 L/mg-d, biomass yield (Y)=0.43, and decay rate (K)-0.025d-1. The treatability parameters for the 5°C reactors were outside the range of published parameters and were not considered reliable.

The effect of BSRT on uranium removal by the MLSS was evaluated in each of the reactors throughout the treatability study. Percentage uranium removal by the MLSS in the 20°C reactors increased slightly (from 19.4% to 22.0%) with increasing BSRT. The MLSS in the 5°C reactors exhibited a decrease in uranium removal percentage with increasing BSRT (from 14.3% to 6.1%). The MLSS in each of the reactors exhibited an increase in total uranium uptake with increasing MLSS concentrations. However, the specific uranium uptake by the (ugU/g dry solids) MLSS decreased with increasing BSRT and increasing MLSS. The maximum specific uranium uptake in the reactors operated at 20°C for BSRT's of 6.7, 15.4, and 21.8 days was 26.464, 17.623, and 18.482 µgU/g dry solids, respectively. The maximum specific uptake in the reactors operated at 5°C for BSRT's of 6.4, 14.4, and 18.7 days was 22.746, 15.518, and 11.268 µgU/g dry solids, respectively. Increasing BSRT and increasing temperature in the reactors apparently resulted in an increase in the biological production of extracellular polymers which bind uranium.

Following the treatability study, a 48-hour uranium sorption kinetic rate study was also performed to evaluate the rate at which equilibrium between aqueous and MLSS phase was achieved. Two of the 9.0 liter continuous flow reactors used in the treatability study were utilized for the kinetic study. One reactor was operated at 20°C and one reactor was operated at 5°C. Uranium accumulation in the 20°C reactor leveled off at approximately 3.5 µgU/g dry solids after 24 hours and then rose to approximately 6 µgU/g dry solids at the 48-hour point. The MLSS in the 5°C reactor reached an equilibrium uranium concentration of approximately 4.0µgU/g dry solids within 24 hours.

Determinations of U235 percentage of total uranium were made for each sample collected for uranium analysis because of the concern for build-up of the U235 isotope in the MLSS. The resulting data reveal no appreciable selective sorption of the U235 by the MLSS.

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