Modelling of floc-bubble aggregate rise rates in dissolved air flotation.

The use of relatively simple, but conceptually sound mathematical models, is a powerful tool to identify and understand parameters that are critical to a process. In this paper, a model is presented which addresses the rise rate of floc-bubble aggregates in the DAF separation zone. The model uses Stoke's Law as a point of departure, which is then progressively extended to incorporate the non-sphericity of the aggregates, the non-laminar nature of their movement, the fractal nature of the flocs, the physical constraints of attaching bubbles to a floc, and the limit on bubble numbers imposed by a typical air dosing system. The main findings are: There are two distinctly different DAF domains, namely a small floc domain and a large floc domain. In the small floc domain, the bubble size, the air volume, chemical dosing and the degree of flocculation have to be optimized and accurately controlled, while the large floc domain is less sensitive to these parameters. The calculated rise rates are significantly lower than some of the latest pilot testing results reported from the USA; a surprising finding at first. There are, however, numerous modelling simplifications that may explain this discrepancy. The most probable reasons are the further agglomeration of the aggregates in the separation zone due to differential rise rates or the recirculating flow within the white-water blanket, and the complex and poorly understood flow patterns within the separation zone.

An integrated approach to dissolved air flotation.

DAFRapide techniques utilising reduced flocculation times, of the order of 5 minutes, together with flotation loading rates of up to 40 m/h, can be incorporated with conventional filtration systems operating at up to 20 m/h. Product quality of typically < 1 NTU and < 0.1 NTU after flotation and filtration respectively can be achieved.

Removal and fate of Cryptosporidium in dissolved air drinking water treatment plants.

In the first part of the paper, data from pilot plant studies are used to evaluate Cryptosporidium removal by dissolved air flotation (DAF) clarification and dual media filters under challenge conditions. Oocyst removals were investigated for design detention times and hydraulic loadings for winter and spring seasons. Coagulation was optimized for turbidity and removal of natural organic matter. DAF performance was better for spring water temperatures achieving 2.5 +/- 0.3 log removal of oocysts compared to 1.7 +/- 0.3 log removal in the winter. Cumulative log removal across DAF and filtration exceeded 5.4, and was not affected by water temperature. Low turbidities and particle counts are indicators of good treatment and good removals of Cryptosporidium. The second part of the paper uses a mathematical model to predict the fate of Cryptosporidium through a DAF plant and the impact of filter backwash recycle on oocyst build-up in the plant influent. Model predictions show that the fate of Cryptosporidium and the build-up of oocysts in the plant influent depend on: DAF performance, the percent of filtered water production used for backwashing, and the percent of filter backwash recycle flow. A DAF plant with 2.5% filtered water production for backwashing and that achieves 1.6 log removal or greater of oocysts by DAF clarification will not have a build-up of oocysts in the plant influent regardless of the recycle rate. This is because the oocysts are concentrated in the DAF floated sludge and not within granular filters.

Effect of ozone on algae as precursors for trihalomethane and haloacetic acid production.

The effect of ozone on the trihalomethane (THM) and haloacetic acid (HAA) formation of two algae species was investigated. Scenedesmus quadricauda (green alga) and Cyclotella sp. (diatom) were cultured under controlled conditions and harvested in the log or late log growth phase. Experiments examined the formation of disinfection byproducts (DBPs) from the algal suspensions with and without preozonation. Preozonation with 1 mg/L increased chloroform formation from Scenedesmus by 17-44%. For Cyclotella, chloroform production increased by 5-26% with 1 mg/L ozone and by 39-109% with 3 mg/L ozone. Chlorinated HAA yields were not significantly increased after 1 mg/L ozone but increased by 38-76% for Cyclotella after 3 mg/L ozone. As compared to other sources of organic matter, algae under bloom conditions may contribute significantly to the DBP precursor pool. However, the majority of the DBP precursors (70%) were attributable to the cellular material, and thus removal of algae cells from a drinking water supply priorto oxidation will substantially reduce algal precursor concentrations.