Optimizing dissolved air flotation design and saturation

Dissolved air flotation (DAF) of iron hydroxide precipitates at working pre ssures lower than 3 atm, using modified flotation units to improve the coll ection of fragile coagula, was studied. Conventional DAF flotation was stud ied as a function of saturation pressure in the absence and presence of sur factants in the saturator. Without surfactants, the minimum saturation pres sure required for DAF to occur was found to be 3 atm. But, by lowering the air/water surface tension in the saturator, DAF was possible at a saturatio n pressure of 2 atm. This behavior was found to occur in both batch and pil ot DAF operation tests and almost complete recovery of the precipitates was attained. Results are explained in terms of the minimum "energy" which has to be transferred to the liquid phase to form bubbles by a cavity phenomen on. Further, studies were conducted changing equipment design and feed bubb les size distribution (mixing micro and "midsized" bubbles). Thus, bubbles entrance position in the collision-adhesion zone ("capture" zone) was compa red to bubble entrance position in the water flow inlet below the floating bed. A "mushroom" type diffuser was used for the "capture zone" experiment and better performance was obtained. Results are explained in terms of diff erent mass transfer phenomena in the collection zone and in the separation zone. Finally, results obtained with the use of a column flotation cell wor king as normal DAF and with a wide bubble size range are presented. Results indicate good performance and some gains in process kinetics with middle s ize bubbles.