The Basics:
Dissolved air flotation (DAF) systems have been in widespread use for over 50 years treating industrial wastewater streams. The process excels at removing suspended solids, oils and other contaminants that are difficult to separate through gravity separation alone. To achieve solids removal, air is dissolved into water at an elevated pressure and released from solution inside the separation chamber. Inside the separation chamber the small air bubbles interact with and attach to the solids, floating them to the surface of the water because the combined air-solids particle has a density that is less than the water. At the water's surface a skimming mechanism gently moves the "float" to a collection chamber. The clarified water free of suspended solids, oils and many other contaminants flows out of the separation chamber and is ready for further processing or discharge to a municipality.
Applications
DAFs have a wide range of industrial wastewater applications. Some characteristics that make for a good DAF installation are the following:
Industries where dissolved air flotation is applied frequently:
- Solids that settle slowly or not at all via gravity
- Small footprint available for wastewater treatment
- Sludge storage is limited and/or hauling is costly
- Wastewater characteristics and/or solids loading are variable
Industries where dissolved air flotation is applied frequently:
- Food Processing (Primary, Secondary and Tertiary)
- Industrial Rendering
- Dairy
- Oil and Gas (Dissolved Gas Flotation)
- Petrochemical (Dissolved Gas Flotation)
- Pulp and Paper
Components
A dissolved air flotation unit has a number of key components that ensure reliable performance.
- The contact chamber serves two purposes. The first is to allow the air and flocculated particles to interact with each other and begin the flotation process. The second is to evenly distribute the aerated wastewater as it enter the remainder of the DAF. In many applications well over 50% of the solids float to the surface of the water in this chamber.
- The Floatation Cell provides surface area and volume for the remaining solids to float. In many cases inclined plates can be used to improve performance and reduce the size of the DAF.
- The air saturation system is critical to the success of a DAF and a source of significant variability between manufacturers. Modern systems utilize a slip stream of the clarified water. A pump is used to pressurize the water and inject it into a pressure vessel where air is added to generate the air-water solution. Specialty pumps are available that can handle direct injection of air to generate an aerated water stream.
- An effluent discharge baffles hold back the float and sediment to ensure a clarified stream of water is discharged from the unit.
- A sloped bottom and auger allows for the removal of settled solids. Often times a wastewater stream contains some components that are unlikely or impossible to float (rocks or sand) so it is critical that a DAF have the ability to remove the solids that accumulate over time. The contact chamber often times has a drain valve as well to remove solids.
- Surface skimming gently removes the float that accumulates on the surface of the water.
Sizing
To properly size a DAF for industrial wastewater treatment it is important to have the following information:
In addition to these wastewater properties, the industry in which it will operate is critical to proper sizing. A DAF system at a poultry facility will be very different from a DAF that is installed at a cheese production facility even if they have similar wastewater flow rates.
This information allows the DAF Manufacturer to perform three process calculations that will inform the size of the DAF and amount of recycle required.
Solids Loading Rate (SLR) is used to estimate the amount of float that will need to be removed by the DAF and is typically expressed as lbs / (sqft * hr). SLR is important because if it is too high, solids accumulate on the surface of the DAF and build up. Over time the DAF will fill with solids which will carry over into the effluent.
- Wastewater flow rate
- Equalization volume available/required
- Total Suspended Solids (TSS)
- Fats, Oils and Grease (FOG)
In addition to these wastewater properties, the industry in which it will operate is critical to proper sizing. A DAF system at a poultry facility will be very different from a DAF that is installed at a cheese production facility even if they have similar wastewater flow rates.
This information allows the DAF Manufacturer to perform three process calculations that will inform the size of the DAF and amount of recycle required.
Solids Loading Rate (SLR) is used to estimate the amount of float that will need to be removed by the DAF and is typically expressed as lbs / (sqft * hr). SLR is important because if it is too high, solids accumulate on the surface of the DAF and build up. Over time the DAF will fill with solids which will carry over into the effluent.
Hydraulic Surface Loading Rate (HSLR) is used to relate the horizontal water flow and vertical suspended solids velocity and is typically expressed as gpm/sqft. In events where the HSLR is high, the solids do not have enough time to rise to the surface before the water exits the DAF.
The air to solids ratio relates the amount of air is being added to the wastewater stream and is typically expressed as lbs air / lbs solids. The required A:S ratio is a DAF specific measure and the minimum amount varies between manufacturers. A good rule of thumb is that the A:S should be between .005 and .06 lbs air / lbs solid.