Floculators for reverse osmosis desalination plant

In reverse osmosis desalination processes, it is essential to pretreat the water captured from the sea. In this process, semi-permeable membranes through which water flows are required, trapping salt particles on one side of the membrane and decreasing the concentration of salts in the water that passes through the membrane.

The lifespan and efficiency of these membranes as well as other elements will depend to a large extent on the correct removal of biological matter or colloidal particles that are suspended in sea-caught water.

One of the most commonly used processes in desalination plants is the so-called DAF, Dissolved Air Flotation.  This process involves introducing air bubbles that adhere to the suspended matter in the water, temporarily reducing its density and causing the particles to rise upwards, floating on the surface. In this way, the clarified water accumulates at the bottom of the tank and through a piping system is extracted for further storage in another tank.

Before the DAF process itself, the coagulation and flocculation phases are required.

In the coagulation phase, the destabilization of colloidal particles suspended in sea-caught water is achieved by the addition of reagents (usually aluminum or iron metal salts) and stirring mixing by agitation.

Subsequently, in the flocculation phase, the agglomeration of the colloidal particles is achieved by forming floccules by moving the water by flocculating agitators. These floccules are transported to the DAF chamber for separation by the flotation achieved by adhering to the air bubbles.

In the process of pretreatment of water, the correct sizing of the agitators of the coagulation and flocculation chambers is paramount for which different design parameters should be considered for each phase:

• In coagulation, very short mixing times and very high stirring intensities (speed gradient between 400 and 800 s-1) are required so it is important to avoid oversize as it can lead to excessive energy consumption. On the other hand, insufficient mixing due to poor stirrer design can lead to excessive coagulant consumption and poor DAF system performance.
• In flocculation, a slow and smooth mixture is required in order for the particles to contact each other and clump together forming floccules. For this purpose, Fluidmix has specially designed propellers with high pumping capacity and low shear, essential conditions to facilitate the process of formation of flocs and prevent breakage of them.

Fluidmix has extensive experience in the selection of agitators for DAF processes endorsed by a multitude of references worldwide. Specifically, in the Middle East, much of the plants developed in recent years are equipped with our agitators.   In the images, you can see the recent supply of these equipments to important Spanish multinational engineering for the DAF process flocculation stages at one of the largest desalination plants in the Middle East.