Adapting dissolved air flotation for the clarification of seawater

The high ionic strength of seawater affects the performance of the dissolved air flotation (DAF) clarification process in a number of ways. At a reference temperature of 20 °C, the density of seawater is 3% higher than freshwater; the dynamic viscosity 8% higher; and the surface tension 1% higher. These differences cause very small changes in the rates of movement of particles and bubbles in both the contact and separation zones of the DAF reactor, that can be ignored for practical design purposes. Much more important are the differences in the solubility of air in seawater (controlled by Henry's constant) and the air transfer efficiency in pressure saturators or other air saturation devices (largely controlled by the molecular diffusivities of the air gases in water). At 20 °C, a typical air saturator only transfers 74% of the air to seawater relative to freshwater. This shortfall can be corrected by either increasing the recycle rate, or by operating the saturator at a higher pressure. The paper presents design charts for both these options.

► We compare the performance of dissolved air flotation in seawater and freshwater.
► The contact between particles, bubbles and water is slightly impaired in seawater.
► Air solubility is less in seawater, necessitating substantial design changes.
► Two charts are provided to make design corrections for seawater.