Vishuddhi Enviro
An Environmental Company

Dissolved Air Floatation

The DAF process consists of super saturation of discharge water from the effluent end of the DAF tank with air. The super saturated water stream is then mixed with the wastestream. As pressure is removed from the saturated stream millions of microscopic bubbles form and attach themselves to the contaminants in the wastewater, thereby changing their bouyancy and floating them to the water surface where they can be skimmed and removed from the water. We offer our unique “Dinky DAF” design for small DAF applications up to 36 sq. ft. Chemical pre-treatment systems can be provided where required.

Chemical pretreatment often improves DAF solids removal efficiencies. The use of chemical flocculants with DAF is based on system efficiency, application (use of DAF) contaminant characteristics and cost. Commonly used chemicals include trivalent metallic salts of iron, such as FeCI2 or FeSO4 or aluminum, such as AISO4. Organic and inorganic polymers (cationic or anionic) are often used to enhance the flotation process. The most commonly used inorganic polymers are the polyacrylamides.

Attachment of most of the bubbles to solid particles can be effected through surface energies while others are trapped by the solids or by hydrous oxide flocs as the floc spreads out in the water column. Colloidal solids are normally too small to allow formation of sufficient air-particle bonding. They must first be coagulated by a chemical such as the aluminum or iron compounds mentioned above and then absorbed by the hydrous metal oxide floc generated by these compounds. Frequently the coagulant is required in combination with the flocculant.

Dissolved Air Flotation (DAF) is a clarification process for the separation of solids, grease and oils. It works by producing a stream of micro-fine air bubbles that attach to solids and float them to the surface where they can be removed by a surface scraping mechanism. DAF is ideally suited to particles and flocs that are slow settling, neutral density or buoyant. This is particularly relevant to food and other industries where solids are light and voluminous and so not suited to settling. DAF also takes up considerably less space than settling methods, generally less than 25% equivalent surface area needed.

In wastewater treatment, the advantage of flotation over sedimentation includes:
  • When the stream is variable and there is a tendency for some of the particles to rise or oil is present, a sedimentation process could be compromised as some of the waste naturally floats to the surface. Flotation will cause everything to go in the same direction, to the surface.
  • Very small or light particles that otherwise would settle slowly can be removed more completely and in a shorter period of time.
Four primary ways of introducing the bubbles are:
  1. Injection of the air while the liquid is under pressure, followed by the release of the pressure. This is called dissolved air flotation (DAF) because the air actually dissolves into the water at the increased pressure.
  2. Aeration at atmospheric pressure, which is just called air flotation. Air is simply introduced in the gas phase directly to the liquid through diffusers. This method is not particularly effective at floating the particles.
  3. Induced air flotation (IAF) involves saturating the wastewater with air either directly in an aeration tank or by permitting air to enter on the suction side of a pump or with a venturi. The partial vacuum, which is applied, causes the dissolved air to come out of solution as minute bubbles.
  4. Suspended air flotation (SAF) is a newer process where a bubble generator makes bubbles with the use of a surfactant. The bubble generator operates at a relatively low pressure of 15 – 25 psig and it is reported that the minute bubbles that are made are smaller than those made by DAF’s, are more numerous, and rise as a mass faster.
Float Dewatering : DAF float often contains 2 to l0 percent solids. The solids may need to be dewatered before disposal to reduce the sludge volume by reducing water content. Float dewatering is usually performed by using one of the following technologies:
  • Filter press
  • Belt filter press
  • Centrifuge
  • Drying bed
  • Vacuum precoat filter