Investigation of the Mixing Energy Consumption Affecting Coagulation and Floc Aggregation

Abstract

Energy is an essential commodity invariably used for proper performance of major processes used in municipal water treatment. This research was primarily driven by the potential rise in energy demand in the water sector. In particular, the research focuses on rapid mix system, continuous-flow stirred tanks specifically designed for chemical dispersion in the coagulation process. Energy-intensive rapid mixing is often practiced in treatment plants for chemical-particle interactions, which is assumed essential for particle aggregation. The central hypothesis of this thesis was that removal of targeted impurities from drinking water supplies (e.g., natural dissolved organic matter) was achievable at a much lower mixing velocity during the rapid mix stage of coagulation. Insoluble precipitates (e.g., floc aggregates) resulting from coagulation can be effectively separated from drinking water in subsequent solid-liquid separation processes, including flocculation, clarification, and filtration.

This thesis integrates water quality and floc characterization techniques to evaluate the influence of mixing energy utilized during coagulation. Mixing energy was quantified from the root mean square velocity gradient (i.e., the G-value). A modified mixing arrangement was used to set the coagulation G-value ranging from 0 to 1450 s-1, keeping chemical (coagulant dose, pH) and physical conditions (temperature) similar. An in-line holographic microscopy technique was used for a non-destructive, direct measurement of floc characteristics (e.g., floc counts, sizes, relative floc velocities) on a laboratory workbench and in a full-scale flocculation tank. Major experimental findings from this present study confirmed that reduced coagulation mixing energy not only removed targeted impurities from the water but also produced enough flocs, which were effectively removed during settling. Specifically, the results presented offered a new range of reduced mixing intensities (110 s-1 < G < 450 s-1) during rapid mixing for a combined effective removal of total organic carbon, turbidity, and floc aggregates. Both water quality and macroscopic analyses support the contention that more mixing energy was expended than necessary in coagulation, which are typically designed at 600-1000 s-1 in treatment plants. It is anticipated that this research will contribute to a boarder conversation related to reducing energy footprints in the water sector.