New DAF Nozzle Design

The objectives for potable water treatment include the removal of suspended and dissolved impurities as well as the removal of Cryptosporidium, Giardia, E. coli and other parasites which pose serious health risks. Dissolved air flotation (DAF) is now extensively used for potable water treatment due to its effectiveness over a wide range of raw water quality. It has the advantage over the traditional process of separation via sedimentation of being able to remove light or neutrally buoyant particulates from the water. DAF is a gravity assisted separation process in which microbubbles are used to remove suspended solid particles from water. While DAF has been employed on a worldwide basis for potable water treatment since the 1960s, there is still considerable scope for improving the efficiency of the process.

The fundamental act in flotation is the successful formation of bubble/floc agglomerates which are capable of rising to the top of the flotation tank. Observations of the interaction of bubbles and floc particles within a full-scale DAF tank have recently been recorded. The authors summarise the key findings from this observational study and then report on a laboratory-scale experimental study of microbubble formation.

Since it has been observed that bubble clustering is the primary mechanism for floc removal within a DAF system, the authors ask: "What can be done to improve the efficiency of a DAF system?" The obvious answer, of course, is to promote bubble clustering. The less obvious answer is to reduce or eliminate factors which are adverse to bubble clustering. The conclusion from the reported field study states that a DAF nozzle should be designed to (1) minimise the level of turbulence at the outlet of the DAF nozzle while (2) ensuring that the bubbles generated by the nozzle are small enough (maximum size less than 200 mm, but preferably smaller) that they do not lead to floc fragmentation or the destruction of bubble clusters.

The authors arrive at two design criteria from these conclusions: the nozzle should be designed to produce relatively small bubbles and to minimise the shear associated with injecting the bubbly recycle stream into the flotation tank. A series of laboratory experiments into the formation of microbubbles were performed in order to design a nozzle based on these design criterion.

Experimental parameters such as the effect of varying the diameter and the length of the constriction in the primary pressure reduction nozzle were examined both with and without the use of a cylindrical shroud. Median bubble diameter was found to be unacceptably high for a constriction diameter of 0.5mm. Experiments with constriction diameters of 1mm indicated that the shorter the length of constriction, the smaller the bubbles. The effect of the cylindrical shroud, which also has an optimum length, is to influence and localise the dissipation of turbulent kinetic energy.

Finally, the authors also decided to incorporate an external shroud with the purpose of reducing the level of turbulence at the DAF nozzle exit. The resultant jet minimised the amount of floc fragmentation in the contact zone of a DAF tank while promoting the formation of bubble clusters.

The nozzle design has been granted a UK Patent and has been successfully deployed at various DAF facilities within Yorkshire Water.