Laboratory Techniques for Evaluating Batch Flotation of Activated Sludge
Authors: Gehr R. , Henry J.G. 
Published: March 2019

A crucial factor with the operation of flotation units for sludge thickening is the amount of air available to effect flotation. Usually assumptions are made regarding efficiency of the saturator and only during the past two years have methods been reported which measure this quantity directly, but on a batch basis. The present study used a modified dissolved oxygen meter to measure this quantity directly and hence calculate the total air available. The meter was also connected to a full-scale unit at the City of Hamilton Sewage Treatment Plant. Approximately 80–90% saturation was achieved in the laboratory unit, but only 40–50% was possible in the full-scale plant.

Using the three-cylinder batch flotation apparatus developed by Wood and Dick (1973), tests were carried out at the Hamilton Plant, the Humber Plant of Metropolitan Toronto and in the University of Toronto laboratory. In all cases activated sludge obtained directly from the return sludge pumps was used for the tests.

The tests at Hamilton revealed that optimum performance of the laboratory flotation unit could be expected with an influent suspended solids concentration of 1.7%. In order to assess the performance and take into account the variation of the influent sludge concentration, a dimensionless parameter was developed which, it is believed, can be used both as a performance gauge and a design parameter. This parameter is the ratio of the effective times thickening and the air to solids ratio. The effect of polymers on flotation performance was also studied. The conclusion was that sludge concentration was not improved, but that process stability was, and that the subnatant suspended solids were obviously reduced. The full-scale unit gave a higher sludge concentration and lower subnatant concentration than the laboratory unit, but could not operate for more than a few minutes without polymer addition.

Tests at the Humber Plant confirmed that a constant relationship existed between the effective times thickening and the air to solids ratio, but they also showed that this value would differ with the source or type of sludge being investigated. The effect of using tap water instead of clarified effluent as the liquid to be pressurized was evaluated. Little difference could be observed when polymer was not used, but the required polymer dosage was reduced by as much as 75% when tap water was used in place of effluent. It was concluded that the polymer was first used up by the solids in the effluent and that only the remainder was available to improve flotation. While this problem would not be so severe in full-scale units, because polymer is introduced to the flotation tank and is not pressurized along with the recycle liquid, the result does show that the type of liquid being used to supply the air bubbles should be investigated further. In practice, subnatant recycle from the flotation unit itself is normally the pressurized liquid. However, this practice is not recommended because in the event of a process failure, the saturator becomes clogged.

It is often inconvenient to set up equipment at the sludge source and frequent collection of samples may also be impractical. Tests conducted at the University laboratory were done to investigate the effect of sludge storage on flotation behaviour. Initial storage involved refrigeration of some samples and continuous aeration of others. The former method was abandoned because results were erratic. The aerated samples showed an improvement in flotation performance as far as the dimensionless parameter, mentioned previously, was concerned but the subnatant solids concentration rose by as much as 100%.