Relaxation flotation
The relaxation flotation (also called pressure relaxation flotation ) is the most widely used flotation process in wastewater technology .
Basics
The relaxation flotation makes use of Henry-Dalton's law that the solubility of a gas in a liquid phase at constant temperature increases proportionally with the partial pressure of this gas above the liquid. So if you put water under pressure, it saturates with gas or air. If the water is then brought back to ambient pressure, a corresponding proportion of gas or air is released in the form of very fine bubbles.
The bubble size depends on the technology used and the operating conditions used, but is generally less than 100 μm . With the help of certain constructive and procedural measures, microbubbles of 30 μm can also be achieved.
Procedural principle
There are three process variants for generating the required fine gas bubbles :
- The full flow process , in which the entire inlet is saturated with air
- the partial flow method in which only part of the feed is saturated with air
- the recycling process , in which any amount of the purified water is recirculated and thereby saturated with air.
The amount of air introduced in the full and partial flow process is naturally limited and, accordingly, the separation efficiency that can be achieved. Another disadvantage is the risk of clogging and the high mechanical stress on the flakes in the saturation system.
These disadvantages do not exist in the recycling process . The amount of recirculation can be adapted so that the necessary number of fine gas bubbles is sufficient for all expected operating conditions.
Bubble size
The size of the bubbles after relaxation depends primarily on the design of the relaxation organ and, accordingly, the technology used. The pressure difference, the surface tension , the pH value , the salt concentration and the viscosity of the liquid also play a role. The constructive design of the air saturation part and expansion element differ between the individual providers of flotation systems.
The bubble size that arises during the expansion plays a decisive role for the flotation result, since the accumulation of air bubbles on solids takes place more easily, the smaller the bubbles are. Due to the better ability of small bubbles to accumulate, the reduction in bubble size leads, on the one hand, to improved utilization of the air introduced ( degree of saturation ) and, on the other hand, to more complete expulsion of the solid. This reduces the solids concentration in the clear flow of the relaxation flotation, and the stability and the dry matter content (TS) of the flotate cover increases.