Optimization of Residuals Management at a Drinking Water Treatment Facility

Abstract

Drinking water treatment processes that use aluminum sulfate as a coagulant produce residuals streams with high levels of aluminum which must be treated before being discharged to the environment, and the treatment process produces a solids residuals stream that must subsequently be managed and disposed of. The goal of this thesis is to examine the effectiveness of a residuals treatment process currently in use at a WTP and explore options for optimizing treatment. Another objective was to characterize the solids stream residuals in order to identify potential options for use or disposal with consideration given to amendments that would improve any characteristics that may limit options for use or disposal and to identify the source or sources of elevated chromium in the solids residuals. A tracer study was conducted to evaluate the hydraulic retention time in the treatment lagoons and determined that short circuiting was occurring. A sampling program was conducted to determine the effectiveness of existing treatment and found that the aluminum in the effluent from the treatment lagoons was primarily in particulate form and the concentration often exceeded 100 µg/L. Bench-scale settling tests were conducted with polymer addition and cation ratio adjustment to determine if treatment could be optimized. The addition of cationic, anionic and non-ionic polymers were all found to increase aluminum removal from CFBW. Cation ratio adjustment was not found to increase aluminum removal from the CFBW. The solids stream residuals were characterized to assist with evaluating potential use or disposal options, and metals concentrations in the solids residuals exceeded CCME commercial and industrial soil quality guidelines for arsenic, chromium, and selenium. The solids stream residuals were blended with blast furnace slag to determine whether amending the residuals with slag could reduce the leachability of metals. It was found to reduce the level of manganese that was leached from the soil. A sampling program was conducted to identify the source or sources of chromium in the solids stream residuals. The aluminum sulfate was calculated to be an insignificant contributor to the chromium load in the sludge. As a significant volume of raw water is processed through the plant, an annual average chromium concentration as low as 0.86 µg/L could account for the remainder of the chromium load in the sludge.