Evaluation of a Turbidity Meter for Use at the Defense Waste Processing Facility
Savannah River Remediation’s (SRR’s) Defense Waste Processing Facility (DWPF) Laboratory currently tests for sludge carry-over into the Recycle Collection Tank (RCT) by evaluating the iron concentration in the Slurry Mix Evaporator Condensate Tank (SMECT) and relating this iron concentration to the amount of sludge solids present. A new method was proposed for detecting the amount of sludge in the SMECT that involves the use of an Optek turbidity sensor. Waste Services Laboratory (WSL) personnel conducted testing on two of these units following a test plan developed by Waste Solidification Engineering (WSE). Both Optek units (SN64217 and SN65164) use sensor model AF16-N and signal converter model series C4000. The sensor body of each unit was modified to hold a standard DWPF 12 cc sample vial, also known as a “peanut” vial. The purpose of this testing was to evaluate the use of this model of turbidity sensor, or meter, to provide a measurement of the sludge solids present in the SMECT based upon samples from that tank. During discussions of the results from this study by WSE, WSL, and Savannah River National Laboratory (SRNL) personnel, an upper limit on the acceptable level of solids in SMECT samples was set at 0.14 weight percent (wt%). A “go/no-go” decision criterion was to be developed for the critical turbidity response, which is expressed in concentration units (CUs), for each Optek unit based upon the 0.14 wt% solids value. An acceptable or a “go” decision for the SMECT should reflect the situation that there is an identified risk (e.g. 5%) for a CU response from the Optek unit to be less than the critical CU value when the solids content of the SMECT is actually 0.14 wt% or greater, while a “no-go” determination (i.e., an Optek CU response above the critical CU value, a conservative decision relative to risk) would lead to additional evaluations of the SMECT to better quantify the possible solids content of the tank. Subsequent to the issuance of the initial version of this report but under the scope of the original request for technical assistance, WSE asked for this report to be revised to include the “go/no-go” CU value corresponding to 0.28 wt% solids. It was this request that led to the preparation of Revision 1 of the report. The results for the 0.28 wt% solids value were developed following the same approach as that utilized for the 0.14 wt% solids value. A sludge simulant was used to develop standards for testing both Optek units and to determine the viability of a “go/no-go” CU response for each of the units. Statistical methods were used by SRNL to develop the critical CU value for the “go/no-go” decision for these standards for each Optek unit. Since only one sludge simulant was available for this testing, the sensitivity of these results to other simulants and to actual sludge material is not known. However, limited testing with samples from the actual DWPF process (both SRAT product samples and SMECT samples) demonstrated that the use of the “go/no-go” criteria developed from the sludge simulant testing was conservative for these samples taken from the sludge batch, Sludge Batch 7b, being processed at the time of this testing. While both of the Optek units performed very reliably during this testing, there were statistically significant differences (although small on a practical scale) between the two units. Thus, testing should be conducted on any new unit of this Optek model to qualify it before it is used to support the DWPF operation.