Reengineering water treatment units for removal of Sr-90, I-129, Tc-99, and uranium from contaminated groundwater at the DOE's Savannah River Site
The 33 years of active operation of the F- and H-Area Seepage Basins to dispose of liquid low-level radioactive waste at the Department of Energy's Savannah River Site has resulted in the contamination of the groundwater underlying these basins with a wide variety of radionuclides and stable metals. The current Resource Conservation and Recovery Act (RCRA) Part B permit requires the operation of a pump-and-treat system capable of (1) maintaining hydraulic control of a specified contaminated groundwater plume, (2) treatment of the extracted groundwater, and (3) reinjection of the treated water hydraulically upgradient of the basins. Two multimillion-dollar water treatment units (WTUs) were designed and built in 1997 and the basic design consists of (1) reverse osmosis concentration, (2) chemical addition, neutralization, precipitation, polymer addition, flocculation, and clarification of the reverse osmosis concentrate, and (3) final polishing of the clarified water by ion exchange (IX) and sorption. During startup of these units numerous process optimizations were identified and, therefore, the WTUs have been recently reengineered. A systematic approach of: (1) developing a technical baseline through laboratory studies, (2) scale-up and plant testing, (3) plant modification, and (4) system performance monitoring was the basis for reengineering the WTUs. Laboratory experiments were conducted in order to establish a technical baseline for further scale-up/plant testing and system modifications. These studies focused on the following three areas of the process: (1) contaminant removal during chemical addition, neutralization and precipitation, (2) solids separation by flocculation, coagulation, clarification, and filtration, and (3) contaminant polishing of the clarified liquid by IX/sorption. Using standard laboratory-scale jar tests, the influences of pH and Fe concentration on contaminant removal during precipitation/neutralization were evaluated. The results of this work led to plant testing and operation modification that reduced the chemical dosing by a factor of 2 to 4 for one WTU and complete elimination of chemical addition at the other WTU. The solids separation process was also reengineered through polymer-dose optimization and plant modifications to control the mixing intensity and flocculation reaction time. The results were enhanced floc quality (floc size, settling rate, sludge volume, supernatant turbidity) at a polymer dose of one-half historical levels. IX/sorption optimization was initiated by evaluating 14 different IX/sorbent materials for their ability to remove specific nuclides in a series of column experiments. Analysis of time-dependent effluent activity/concentration data was used to determine the contaminant breakthrough and identify alternative resins for plant testing. Significant plant modifications, including the replacement of the natural zeolite with a commercial water softening IX resin that is one-third the cost and has twice the operational life for {sup 9}0Sr removal, were made to both WTUs. The result of this reengineering has been enhanced contaminant removal, dramatically increased system reliability, lower operational costs, reduced secondary waste volume, and decreased personnel exposure. The specifics of this reengineering process and the associated system changes are presented in this paper.
- Research Organization:
- Savannah River Site (US)
- Sponsoring Organization:
- US Department of Energy (US)
- DOE Contract Number:
- AC09-96SR18500
- OSTI ID:
- 751296
- Report Number(s):
- WSRC-MS--2000-00097
- Country of Publication:
- United States
- Language:
- English
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