1. Dissolved Air Flotation Introduction Dissolved air flotation Dissolved air flotation (DAF) is a process for the removal of particles/flocs by making the solid particles float rather than sink, as in clarification. The process works by attaching air bubbles to particles so as to reduce the density of the combined bubble–particle agglomerate.process works combined bubble–particle

2. The success of the process depends on two factors: (1) generation of enough bubbles and (2) conditions under which the bubbles and particles stick together.success

3. Schematic of a dissolved air flotation unit.

4. Process science DAF tanks have two zones. The purpose of the contact zone is to provide opportunities for bubbles and particles to collide and stick together, thereby forming the required bubble–particle agglomerate. In the separation zone, the bubble–particle agglomerates rise to the surface and are subsequently removed. 1. Contact zone The contact zone can be viewed as either a heterogeneous flocculation process (Fukushi et al., 1995) or by assuming that the rising bubble cloud acts like a filter bed such that the bubbles can be visualised as collector surfaces (Edzwald, 1995).

5. Schematic of contact and separation zones

6. The end expression relates the removal of particles to the total available surface area for contact and the residence time in the contact zone assuming plug flow conditions: where  pb is the attachment efficiency, T is the single collision collector efficiency, U b is the mean bubble rise velocity,  b (= c b /  air ) is the bubble volume concentration and t is the residence time in the contact zone.

7. The model demonstrates that the efficiency of removal can be enhanced by Maximising the single bubble capture efficiency Maximising the attachment efficiency Increasing the bubble volume concentration Increasing the contact time Minimising the bubble size. The model identifies three groups of variables that are crucial in the designand operation of DAF processes:  pb,  T and  b /d b.

8.  pb are variables related to pre-treatment and reflect the operation of the coagulation and flocculation stage of the process.  pb depends on the coagulation conditions (type, dose, pH) and is empirically determined. Values for  pb range from nearly 0 for poor attachment to 1 where all contacts lead to attachment. Typical values with good coagulation are between 0.1 and 0.5 meaning that an excess of bubbles is essential to ensure multiple contacts.

9. Two conditions are necessary for good attachment: minimisation of the particle charge and a sufficiently hydrophobic surface. Impact of zeta potential on removal in DAF.

10. The other requirement is that the surface of the particle exerts some degree of hydrophobicity.  T is the probability of a collision between a particle and an individual bubble and is controlled by the combination of the particle and bubble sizes.  b is controlled by the air pressure and flow rate (recycle ratio) through the saturator and informs on the total available surface area for contact through changing the number of bubbles in the contact zone.

11. 2. Separation zone Following successful collision and attachment, the bubble–particle agglomerate will rise to the surface. The impact of the attaching bubbles can be seen by using an adjusted Stokes’ law expression:

12. Technology options 1. Traditional Traditional DAF plants are either circular or rectangular. Circular tanks are normally used on small flotation plants, treating wastewater or sludge thickening applications, but some potable water plants exist with individual units of up to 20 m in diameter. Flotation tanks are typically designed with a depth of between 1.5 and 3 m at overflow rates of between 2 and 15 m 3 m−2 hr−1

13. Desludging is traditionally achieved mechanically whereby rubber or travel over the tank surface and push the float into a collection Channel. Mechanical desludging

14. 2. Combined flotation and filtration The process offers two main advantages over traditional systems: (1) disturbances in the flow which may cause the flocs to break are minimised. (2) the system can easily be switched to filtration-only operation with the DAF coming on line only when the water becomes excessively polluted.

15. 3. Counter current flotation (CoCoDAFF R ) CoCoDAFFr operates in a counter current direction with the flow maintained in a vertical direction enhancing both bubble–particle interactions and the time in the contact zone Schematic of CoCoDAFF

16. CoCoDAFF  distribution nozzles (with kind permission from Thames Water).

17. Schematic of DAFRapide  (with kind permission from Purac Ltd.). 4. Combined flotation and lamella plates

18. Applications The majority of applications of DAF processes in potable water treatment are in situations where low density particles with slow settling rates are encountered which restricts the effectiveness of clarification processes. Removal efficiencies of 90% and above are common for turbidity, natural organic matter (measured as DOC or UV254) and algae removal (Table5.3). In the case of algal and zooplankton (Daphnidae, Cyclopoida) performance is species dependant with free swimming species generally beingharder to remove, with removal efficiencies down to 40–50% in some case studies.

19. Table 5.3 Summary of DAF performance.