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Influent toxicity can be a critical problem for publicly owned treatment works that use the activated sludge process as part of their treatment regime. If an undetected contaminant enters the plant through a failure of the pretreatment system, illegal dumping, etc., the microbial population in the activated sludge can be lost. This can lead to weeks' worth of permit violations as the effluent leaves the plant following only primary treatment and the activated sludge system is reseeded and restored to equilibrium. This in turn can lead to public health issues and concerns for contamination of drinking water plants that get their raw water from the same source.

In this project, the researchers developed two protocols for the screening of wastewater treatment plant influent for toxicity. Both protocols are based on a genetically engineered bioluminescent bacterium designated Shk1. Scientists at the University of Tennessee's Center for Environmental Biotechnology constructed Shk1 from a host Pseudomonas strain isolated from an industrial wastewater treatment plant (WWTP).

The first of the two Shk1-based assays that were developed utilizes a batch-wise sampling technique for analyzing grab-samples from industrial effluent, WWTP influent, and the various operations in an activated sludge WWTP (aeration basin, clarifier, etc.). The second method utilizes a continuous sampling technique and is designed for continuous monitoring of the wastewater treatment plant influent upstream of the activated sludge process.

The researchers used the batch Shk1 assay to test the influent, activated sludge, and clarifier supernatant in a bench-scale wastewater treatment plant subjected to shock loads of metals (zinc, copper, nickel, and cadmium) for toxicity. They compared data on the repression of bioluminescence to activated sludge respirometry and conventional measures of plant performance (effluent ammonia and chemical oxygen demand, COD). In general, they found that the Shk1 assay indicated toxicity at levels similar to that indicated by activated sludge respirometry. However, no quantitative relationship could be established between increasing Shk1 toxicity response and effluent quality. Effluent COD and NH3 data (when available) showed little or no significant effect or were highly variable.

The researchers used the Shk1-based continuous toxicity screening method to generate toxicity data for a large suite of metals and synthetic organic compounds. They compared these results to literature data for toxicity as indicated by activated sludge respirometry and by the P. phosphoreum-based assay. In general, the Shk1 system gave EC50 values similar to those found in the literature for activated sludge respirometry for 102 organic compounds and to concentrations found to affect activated sludge for seven metals.

The researchers adapted the continuous monitoring system for field application and installed it immediately downstream of the effluent from the primary clarifier in a municipal wastewater treatment system. They compared data from the Shk1-based system to plant performance data. During the time of the field study, no significant event occurred during which the operation of the plant was seriously impaired. Therefore, the researchers compared the Shk1 signal to the operations data provided by the plant personnel to determine if any correlation existed between the signal from Shk1 and minor fluctuations in the operations data. They found no simple quantitative relationship between the signal from the toxicity monitoring system and the plant performance data. They applied principal component and factor analysis to the Shk1 data and 20 additional plant variables. The results of these analyses showed that 10 principal components were needed to account for 90% of the variability of the data and that the signal from Shk1 was therefore not sufficient to predict the system state in the absence of a major toxic event without knowledge of the values of other operating variables. In summary, these analyses indicated that the Shk1 signal would be a valuable addition to models to predict the future system state from the influent, operating, and effluent variables but it is not a sufficient variable by itself.
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Document Type: Research Article

Publication date: 01 January 2002

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