DAF System Design & Engineering for Industrial Wastewater Treatment
Facing strict discharge limits? A properly engineered DAF system (Dissolved Air Flotation) removes 90-99% of Total Suspended Solids (TSS) and Fats, Oils, and Grease (FOG) from industrial wastewater. HEI Systems delivers high-performance DAF wastewater treatment through proven design criteria that maximize contaminant removal while reducing operating costs by up to 64%.
How DAF Systems Work: The Science Behind Superior Clarification
Five important factors must be carefully evaluated to ensure proper sizing and optimal performance of your DAF System in a wastewater treatment plant.A dissolved air flotation system clarifies wastewater by using microscopic air bubbles (30-50 microns) to float suspended particles, oils, and grease to the surface for removal. Unlike gravity clarifiers that wait hours for particles to settle, DAF systems actively float contaminants upward in just 3-5 minutes.
The Process: Pressurized water saturated with dissolved air (called “white water”) enters the flotation tank. As pressure drops to atmospheric levels, air precipitates as millions of micro-bubbles. These bubbles attach to chemically conditioned particles, reducing their density and causing them to float rapidly. A mechanical skimmer removes the concentrated sludge while clarified water flows from the bottom.
5 Critical Design Criteria for DAF Systems Wastewater Treatment
Generic calculations lead to underperforming systems and compliance failures. Site-specific engineering ensures your DAF system meets discharge requirements from day one.
Air to Solids Ratio in DAF Wastewater Cells
The A/S ratio determines how much air is needed to float your specific solids and FOG load.
HEI Standard: 0.03 lbs air/lb solids (Industry range: 0.01-0.10)
Technical Parameters: At 75 psig and 70°F, water achieves 9% air saturation. For waste streams with 1,000 mg/L TSS, you need 50% recycle. Streams with 2,000 mg/L TSS require 100% recycle.
Quick Calculation Example:
- Your flow: 100 GPM at 2,000 mg/L TSS
- Daily solids: 1,668 lb/day
- Air needed (0.03 ratio): 50 lb/day
- Required recycle: 100 GPM (100% of influent)
Hydraulic Loading in DAF Wastewater Treatment
This determines your tank size and prevents solids carryover.
HEI Standard: 2.5 gpm/ft² (Industry range: 1.5-5.0)
Sizing Formula:
- Total flow = Influent + Recycle (100 GPM + 100 GPM = 200 GPM)
- Required area = 200 GPM ÷ 2.5 = 80 ft²
Launder Weir Overflow Rate in DAF Wastewater Systems
Launder Weir Ring Overflow Rate in gpm per linear foot is a convenient-to-calculate value that can be assumed to reflect eddy current formation that would carry solids or FOG under the underflow baffle. It must not be so rapid as to entrain precipitated solids. The overflow must be completely uniform across the launder weir (and thus under the baffle).
HEI does not normally recommend the rectangular designed DAF unit because the high velocity at the inlet is never reduced. Hence, the expected separation is not as efficient in the rectangular design as can be expected in the circular design DAF’s. HEI utilizes the rectangular design for customers with limited available space.
Solids Loading in DAF Wastewater Treatment
Solids Loading is the relationship between the effective surface area of the DAF cell and the total amount of TSS & FOG entering the system. The average design load is 1.0 to 3.5 lbs./hr/sq. ft. HEI recommends not exceeding 2.0 lb./hr/sq. ft. The calculation should include not only the influent TSS but also all chemicals added to condition and flocculate the waste stream.
Solids Removal in Dissolved Air Flotation Systems
Solids Removal of both float and sediment (if any) must also be considered in the general design. In addition to the skimming rake to remove the float, HEI also has a bottom rake to sweep any sediment to the center bottom outlet.
Flocculation and Float Mechanism in Dissolved Air Flotation
HEI dissolved air flotation systems are very effective in the removal of suspended solids and free FOG. Emulsified oil and grease must be chemically conditioned before removal. Due to “van der Waals” charges, particles in industrial wastewater usually exist as a colloidal suspension. These charges must be neutralized before the solids can be flocculated and floated. The small, destabilized colloids collect and agglomerate with the long chained polyelectrolyte into large floccules which can be rapidly floated to the surface.
Influent Mixing Tubes for DAF System
The conditioned and flocculated influent is conveyed to the center distribution well through a large mixing tube (1). The pressurized recycle water saturated with air is added to the treated water through HEI’s proprietary mixing nozzle which induces mild turbulence throughout the mixing tube (2). The pressure of the recycle water is reduced to atmospheric at the pressure control valve (9). HEI’s mixing nozzle uniformly blends the air saturated recycled water with the conditioned influent as the dissolved air precipitates from solution.
Central Mixing and Distribution in DAF Wastewater Treatment Cell
The depressurized recycle water with the conditioned influent flows into the central distribution/mixing well (3). The distribution chamber is designed to retain the mixture and provide gentle hydraulically mixing for a short commingling period. Adequate time is permitted for the precipitated air molecules to seed the suspended solids and agglomerate to produce buoyancy. Other micro-bubbles get trapped inside the pores of the flocs, which certainly would not happen with larger bubbles. The wastewater flows out near the top of the mixing well through distribution orifices to uniformly distribute the treated water throughout the DAF cell.
Float Removal and Sediment Removal
Heavy solids, if any, settle on the bottom of the DAF tank. A bottom rake drive (4) prohibits solids from rat-holing (which is a common problem with cone-bottom tanks without rakes) by continually moving the collected solids to a center outlet. Heavy solids removal is controlled by valve #12 and may be either automatically or manually initiated. A surface skimmer moves the buoyant solids that float over a beach weir (5). The float then slides down the inclined floor of the trough to the outside collection tank (10).
DAF Cell Operation in Wastewater Treatment
The flow rate dramatically slows as it radically moves from the center of the DAF cell to its circumference. Adequate time is provided for the buoyant suspended solids to rise to the surface, forming a float or scum blanket, as the clarified liquid moves radically downward to the outlet baffle. The treated water flows under the outlet baffle (6), up through exit plenum and over the flow equalizing launder weir.
Outlet Chamber
The treated water flows over the launder ring, to the overflow weir, and then to the clear well (7). A predetermined portion of the treated water is recycled and the remainder gravity flows to the industrial drain or to further treatment (11).
Pressurization Unit in DAF Wastewater Treatment Systems
A portion of the treated water is utilized to provide the media to deliver the dissolved air to the DAF by pumping into a pressure tank (8) with compressed air at 75 psig. Drawing air into a pump suction is not recommended by HEI because the inherent pump cavitation dramatically shortens the life of the high pressure centrifugal pump. The liquid level in the pressure tank is controlled by HEI’s special level control that vents all undissolved air (large bubbles) to prevent their entering the DAF separation chamber. The pressurized water and compressed air are mixed and held long enough to create a saturated solution. The saturated recycle water passed through HEI’s unique pressure reduction valve (9) as it enters the mixing tube. When the pressure is relived, the saturated solution becomes super-saturated and the air begins to precipitate from solution.
Materials of Construction for DAF System Water Treatment Plant
HEI’s standard material of construction is ¼” A36 steel plate, sand-blasted to a SPC-6 finish and coated with immersion grade acid resistant epoxy paint. Stainless steel may be substituted as an option. A NEMA 12 central control panel is provided to control the entire operation.
HEI will provide the entire turn-key DAF system to include engineering, lift station, chemical conditioning, solids compaction, installation and start-up. HEI engineers and manufactures its own equipment and will provide individual items of equipment for complete turn-key plants.