A fundamental study of dissolved air flotation for treatment of low turbidity waters containing natural organic matter

Dissolved air flotation (DAF) is an attractive solid/liquid separation process for highly colored, low turbidity water supplies. Low density floc is produced when these waters are coagulated, therefore DAF may be more effective than conventional gravity settling (CGS). Communities required to filter their supplies should consider DAF as an alternative pretreatment prior to filtration. Although DAF is a promising alternative to CGS, no rational basis has been developed for selection, design and operation of DAF facilities. The research objectives were: (1) to develop a fundamental basis for the application of DAF to water treatment, (2) to determine via experiment the key variables which affect DAF performance for the removal of clay turbidity and natural organic matter (NOM), and (3) to compare DAF to CGS. A conceptual model was developed based on colloidal stability and particle deposition and used to identify the variables which affect DAF. The variables which included pH, coagulant type and dose, temperature, DOC and clay concentrations, flocculation time, DAF detention time, and bubble concentration were studied using synthetic waters. Synthetic waters were used to compare DAF to CGS. Synthetic waters were prepared from extracted aquatic fulvic acid and research grade montmorillonite clay--their use allowed water quality variables to be carefully controlled. Synthetic water results were then verified using two natural waters. Modelling predicted that particle stability and size, bubble size and rise rate, bubble volume concentration, and detention time would affect DAF performance. Experiments indicated particles must be coagulated for successful flotation. Temperature and flocculation time affected DAF performance in experiments using alum. It is hypothesized that the effects of cold temperature on DAF performance are due to changes in coagulation mechanism and the physical and chemical stability of the particles as it relates to charge and bound water at the surface. In addition, bubble volume affected DAF performance for high DOC waters and waters containing clay. Comparisons of DAF to CGS indicated that the DOC, organic halide precursors, and dissolved aluminum after treatment were comparable for both. However, DAF produced significantly lower turbidities than CGS, particularly at colder temperatures