Conventional drinking water treatment consists of a coagulation, flocculation, gravity separation, filtration and disinfection processes each working individually but also as an interdependent system. One of the main reagents used for drinking water treatment are coagulants that destabilise the suspended particles which results in the formation of flocs. For many years, the coagulant of choice was aluminum sulphate, also know as alum. Alum has slowly been replaced by new coagulants, such as polyalumium sulphates and polyaluminum chlorides, because they yield more consistent plant performance than with alum over the wide temperature range experienced by Canadian treatment plants. Recent research has determined that the alum solubility envelop varied significantly in terms of pH range with temperature, thus cold temperature performance may be improved by adjusting the coagulation pH. Dissolved air flotation (DAF) is now used at some water treatment plants to replace sedimentation because it is much more compact than gravity settling, and it is somewhat better than sedimentation for the removal of algae, organics and operation in cold temperatures.
The objective of this thesis is to help operators and managers of drinking water treatment plants incorporating DAF by: a) investigating the cold water turbidity removals of DAF systems using alum, the most economical coagulant; and b) investigating the impact of DAF saturator pressure on the bubble sizes produced and floc removal. This first initiative is based on fairly recent research on the impact of pH on the cold-temperature aluminum solubility. It uses this knowledge about the impact of pH to evaluate DAF treatment of Ottawa River water in cold-water conditions using DAF batch tests. The effect of pH against final turbidity at cold temperatures was first evaluated by increasing the pH of the coagulated water, the higher pH helped attain good turbidity removals. For the coagulant dose tested, good turbidity removals were observed for both warm and cold waters at nearly the same pOH conditions. At room temperature the turbidity removals increase with both increasing flocculation G and flocculation time. While at cold temperatures, when aluminum flocs are known to be much more fragile, the turbidity removals appear to be independent of G and GT.
The second initiative studied the relationship between floc size and bubble size in DAF systems by changing the DAF saturator pressure. Increasing the saturator pressure did not significantly decrease the mean bubble size. The flocs attach to bubbles that were significantly larger than the bubbles. The assessment of DAF efficiency based on the unitized effluent floc distribution proved inconclusive, it may be possible that the conditions resulting with the larger mean effluent floc size has a greater removal efficiency since it began with a smaller fraction of small flocs entering the flotation stage.
| Identifer | oai:union.ndltd.org:uottawa.ca/oai:ruor.uottawa.ca:10393/45715 |
| Date | 07 December 2023 |
| Creators | Hérard, Richard |
| Contributors | Narbaitz, Roberto M. |
| Publisher | Université d'Ottawa / University of Ottawa |
| Source Sets | Université d’Ottawa |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | application/pdf |
| Rights | Attribution-NonCommercial-NoDerivatives 4.0 International, http://creativecommons.org/licenses/by-nc-nd/4.0/ |
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