Impacts of Sludge Volume and Sludge Age on Disinfection By-Product Formation in a Full-Scale Water Treatment Facility

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Virginia Tech

Impact of Sludge Volume and Water Quality on DBPs in a Full-Scale Water Works

The goal of this research was to determine the role of settled sludge on the formation of disinfection by-products in a full-scale water treatment plant. The occurrence of disinfection by-products in chlorinated drinking water has become a major concern to treatment facilities in their effort to comply with strict regulations set by the United States Environmental Protection Agency. Water samples were tested for trihalomethanes and haloacetic acids at both ends of the sedimentation process to evaluate formation over the length of the basin. Sludge volume and other important water quality parameters were also measured at the time of sample collection. Statistical analyses were used to analyze contributions from the sludge and to determine influential factors leading to disinfection by-product formation. The treatment plant incorporated chlorine dioxide into the treatment process seasonally, and effects were evaluated. Predictive models were developed from the data to be used under various treatment methods. The models created for trihalomethanes and haloacetic acids require measurements of chlorine dose, reaction time, total organic carbon, pH, water temperature, and sludge volume. The models performed well in predicting actual trihalomethane and haloacetic acid concentrations and could serve as a valuable tool in the control of disinfection by-products.

DBP Formation Potential of Settled Sludge in a Full-Scale Water Treatment Facility

It is still a common occurrence for water treatment facilities to store sludge in sedimentation basins for extended periods, rather than relying on mechanical collection equipment. The goal of this research was to characterize contributions from settled sludge to the formation of disinfection by-products (DBPs), and determine whether continuous removal is essential in the control of DBPs. Samples were taken from top and bottom sludge layers in the sedimentation basin and water was extracted either by draining or centrifugation. The water was analyzed for trihalomethanes and haloacetic acids and water quality measurements were recorded. Concentrations of both DBPs were very high in top-layer sludge; trihalomethanes ranged from 321.5 μg/L to 568 μg/L and haloacetic acids ranged from 74.6 μg/L to 409.8 μg/L. Evidence of biodegradation was observed in the bottom-layer sludge. The water samples were dosed with 4 mg/L chlorine, the United States Environmental Protection Agency's maximum residual disinfectant level, to determine if further DBP formation was possible. The extracted water from the bottom-layer sludge was shown to form high trihalomethane concentrations when chlorinated, and haloacetic acid concentrations were observed to increase when samples from the top-layer sludge were chlorinated.

trihalomethanes, haloacetic acids, disinfection byproducts, sludge