Flotation may be used instead of the normal clarification by solids-downward-flow sedimentation basins as well as thickening the sludge instead of the normal sludge gravity thickening.

Water containing solids is clarified and sludges are thickened because of the solids adhering to the rising bubbles of air.

The breaking of the bubbles as they emerge at the surface leaves the sludge in a thickened condition.

In a flotation system for solid/liquid separation, there are at least two methods by which gas bubbles can be used to increase the buoyancy of suspended solids:

(a) entrapment of the bubbles in the particle structure and

(b) adhesion of the bubbles to the particle surface.

In the former case, as the gas bubbles rise toward the surface, the controlled turbulence in the inlet compartment causes contact between the solids.

The floc, formed by the natural floc-forming properties of the materials or by the chemicals that have been added, increases in size because of more contact with other solids.

Eventually, a structure is formed that does not permit rising gas bubbles to pass through or around it.

What is a DAF?

Dissolved air flotation (DAF) is a method of water treatment that clarifies wastewater by eliminating suspended particulates.

The air is removed by dissolving it under pressure in the water or wastewater and then releasing it at atmospheric pressure in a flotation tank.

The released air creates tiny bubbles that stick to the suspended matter, causing it to float to the top of the water and be retrieved by skimming equipment.

To improve solids removal, chemicals can be added to the feed water.

Where and why are DAFs used?

DAFs are commonly employed in the treatment of industrial wastewater effluents ranging from oil refineries, chemical plants and paper mills to the food and beverage industries.

DAFs are used to remove suspended particles from wastewater, such as Total Suspended Solids (TSS), Fats, Oil & Grease (FOG) and other pollutants so that the water can be reused or discharged to a wastewater treatment plant.

How does it work?

Wastewater is injected into a DAF system and then struck with a stream of “whitewater,” which is recirculated cleared DAF water that is hyper-saturated with dissolved air.

As these two liquids combine, small bubbles cling to solid particles, allowing them to float to the surface of the DAF tank.

A skimmer gently nudges the sludge toward a discharge hopper as solids gather in a floating layer on the top of the DAF tank.

Any solids that do not float will sink to the DAF tank’s “V” bottom.

An automatically controlled pneumatic drain valve concentrates and discharges settled sediments.

The cleared water exits the DAF unit via an under-over weir on either side.

Some of this water is used in the recirculation loop, while the remainder is discharged from the vessel.

What are common types of DAFs?

Open tank

Open Tank DAFs are ideal for treating wastewater that is heavily loaded with solids.

This type of wastewater requires significant amounts of free surface area for flotation and separation.

Open tank DAFs are normally rectangular and are built wider and longer as the free separation area requirement increases.

Plate pack

Plate Pack DAFs are distinguished by tall (high-built) tanks with inclined, corrugated plate packs.

In a cross-flow design, water is supplied into the plate packs, lowering the distance particles must float to be efficiently separated.

As they collide with an angled plate, light solids accelerate upward and heavy particles settle.

Wastewater enters the plate pack heavily laden with flocculated contaminants and exits devoid of suspended and colloidal solids.

Which DAF type is right for my application?

A plate pack DAF unit is best suited for high hydraulic and low solids loading rates.

DAF tanks with an open design are more suitable for large solids loading rates.

But, before we go ahead and position plate pack DAFs and open tank DAFs in specific applications or sectors, let’s understand one concept: everywhere a plate pack DAF unit may be used, an open tank DAF unit cannot.

A law similar to “a square is a rectangle, but a rectangle is not a square.”

The drawback of constantly using an open-style tank is the amount of floor space they take up and the overall cost, especially when flow rates exceed a few hundred gallons per minute.

What are the Common Components of a DAF?

Flotation Tank

Water is separated from contaminants in the Flotation Tank.

The water flows slowly through the flotation tank, allowing heavy objects to settle and lighter objects to float to the top with the help of bubbles and flocculation.

The depth, width and length of the flotation tank can vary depending on the application and the time required to remove contaminants.

Dewatering Grid

The Dewatering Grid is a rectangular structure of angular steel plates that keep sludge from rising to the surface.

The sludge can only be skimmed and transferred to the float hopper after it has thickened enough to rise above the top edge of the grid.

The Dewatering Grid assists operators in the following ways: controlling sludge thickness; eliminating premature solids removal; reducing sludge build-up and/or re-entrainment and producing drier sludge.

Skimmers

While typical DAF system designs force sludge over the whole length of the tank in the same direction as the wastewater flow, a Counter-Current Skimmer uses a skimmer unit that rotates against the hydraulic flow of the water.

The Counter-Current Skimmer reduces sludge skimming distance while eliminating solids carry-over.

Flocculation

Flocculation is designed to provide the mixing action and retention time required to adequately coagulate and flocculate solids in wastewater to improve solids removal.

Recycle Pump

Standard ANSI pump

DAF pumps are an essential part of all DAF systems.

It carries some of the highest capital, operating and maintenance costs associated with wastewater pre-treatment systems.

DAF manufacturers approach recycle pumps in two ways.

The first method is to supply a specialized white-water pump.

This pump not only pumps water but also dissolves air in it.

These pumps are frequently more difficult to find and more expensive.

Furthermore, when air is introduced into the pump, there is always the potential for cavitation, which causes internal damage and necessitates more frequent part replacement than intended.

The second method is more efficient and cost-effective. It will consist of a regular ANSI pump and an angled air-dissolving tube.

In the second situation, the pump does not dissolve air; instead, it just pumps water.

Because there is no air in the pump, cavitation is extremely unlikely.

In this approach, ordinary ANSI recycles pumps do not dissolve air; they simply pump water.

This means we can offer better solids tolerances, use stronger pump materials, operate at lower pressures and do it at a considerably lesser cost than a dedicated whitewater pump.

Angled Air Dissolving Tube

Many DAF system manufacturers employ a mechanical technique to dissolve air into water, such as a specialized “whitewater pump,” but there is a more efficient and cost-effective approach.

Whitewater is produced in the air-dissolving tube.

The recirculation piping’s short expansion allows cleared effluent and a tiny volume of compressed air to mix until saturation is attained.

Because of the angled configuration’s enhanced water and air interaction, saturation occurs practically rapidly.

This design is so effective that many customers will switch from a specialist whitewater pump to an ANSI pump and angled air-dissolving tube.

References

[1] Nicholas P. Cheremisinoff, Handbook of Water and Wastewater Treatment Technologies, Butterworth-Heinemann,2002,p 62.

[2] Lawrence K. Wang, Physicochemical treatment processes, Humana Press Inc, Totowa, New Jersey 0751,2004.