Dissolved Air Flotation (DAF)

Dissolved Air Flotation (DAF) removes suspended solids and algae from water by dissolving air under pressure and releasing it at atmospheric pressure to create microscopic bubbles that attach to particles, causing them to float for surface removal. DAF systems typically achieve 85-95% removal of suspended solids in municipal applications, with surface loading rates of 4-8 gpm/ft². The primary trade-off is higher energy consumption compared to sedimentation, requiring 15-25 kWh per million gallons treated for air dissolution and recycle pumping, making it most cost-effective for plants with challenging raw water quality or space constraints.

• Water Treatment Clarification (5-50 MGD): DAF replaces conventional sedimentation for low-turbidity source waters (<50 NTU). Located between coagulation/flocculation and filtration, DAF achieves 85-95% turbidity removal with 2-4 hour detention versus 6-8 hours for sedimentation
• Thickening Primary Sludge (1-20 MGD plants): DAF thickens waste activated sludge from 0.5-1.0% to 3-5% solids before digestion. Positioned after secondary clarifiers, upstream of digesters. Reduces digester volume requirements by 60-70% compared to gravity thickening
• Industrial Pretreatment: DAF removes fats, oils, and grease (FOG) from food processing discharges before biological treatment. Achieves 90-98% FOG removal when conventional primary clarification fails with emulsified wastes

Daily Operations: Monitor influent turbidity, chemical feed rates, and float blanket thickness (typically 6-12 inches). Adjust air-to-solids ratio (0.02-0.05 lb air/lb solids) based on float quality. Check pressure vessel gauge (should maintain 80-90 psig) and recycle flow rate. Log skimmer runtime and float removal frequency.

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Maintenance: Weekly bearing lubrication on skimmer drives and compressor maintenance per manufacturer schedule. Monthly pressure vessel inspection and safety valve testing. Quarterly chain tension adjustment and wear plate inspection. Replace air diffusers annually. Requires confined space entry procedures for tank maintenance and basic mechanical skills for drive components.

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Troubleshooting: Poor float formation indicates inadequate air supply, improper chemical dosing, or temperature effects. White foam suggests over-aeration; adjust air flow or recycle rate. Excessive carryover points to high surface loading or skimmer malfunction. Typical equipment life: skimmer chains 5-7 years, compressors 10-15 years with proper maintenance, concrete tanks 30+ years.

• Pressurization System: Includes air compressor (5-15 HP), pressure vessel (80-100 psig), and recycle pump (20-35% of influent flow). Dissolves air into recycled effluent at 4-6 times atmospheric saturation. Vessel sized for 1-3 minute detention
• Flotation Tank: Rectangular concrete basin with 10-20 minute detention time. Surface loading rates of 2-6 gpm/ft² for water treatment, 0.5-2 gpm/ft² for sludge thickening. Includes inlet distribution, skimmer mechanism, and sludge collection hopper
• Release System: Pressure reduction valve or needle valve creates 20-40 micron air bubbles. Critical for bubble size distribution - oversized bubbles reduce efficiency, undersized bubbles lack lifting capacity
• Skimmer Mechanism: Chain-and-flight or rotating arm system removes floated solids. Travel speed 1-5 ft/min with adjustable blade height. Stainless steel construction for corrosion resistance

• Surface Loading Rate: 2-8 gpm/ft² for clarification, 1-4 gpm/ft² for thickening applications. Municipal plants typically design at 4-6 gpm/ft² with peak capacity to 8 gpm/ft²
• Air-to-Solids Ratio: 0.02-0.06 lb air/lb solids removed. Typical municipal design: 0.03-0.04 lb/lb for conventional coagulation, up to 0.05 lb/lb for high-strength applications
• Recycle Rate: 8-12% of influent flow for pressure saturation systems. Higher ratios (10-15%) needed for difficult-to-float solids or cold water conditions
• Saturator Pressure: 60-80 psig typical, with 75 psig most common. Higher pressures (80-90 psig) for enhanced flotation but increase energy costs
• Detention Time: 10-20 minutes in flotation zone, 2-4 minutes in contact zone. Municipal designs typically use 15-minute total detention
• Skimmer Speed: 3-10 ft/min surface velocity. Slower speeds (3-5 ft/min) for fragile floc, faster (8-10 ft/min) for dense sludge applications
• Underflow Rate: 0.5-2.0 gpm/ft² for clarification applications. Thickening applications: 0.2-0.8 gpm/ft²

• What surface loading rate should be used based on raw water characteristics? Need TSS levels, particle size distribution, and seasonal temperature variations. Conservative 4 gpm/ft² for variable raw water quality vs. aggressive 6-8 gpm/ft² for consistent conditions
• Should the system use pressure saturation or induced air flotation? Pressure systems handle 2-50 MGD efficiently but require high-pressure pumps and saturators. Induced air works for <5 MGD but has higher maintenance
• What air-to-solids ratio is required for target removal efficiency? Need pilot testing or similar water experience. Typical 0.03 lb/lb achieves 85-90% TSS removal, but difficult waters may need 0.05+ lb/lb
• How should seasonal temperature variations be accommodated? Cold water (<40°F) reduces air solubility and bubble formation efficiency. May require 20-30% higher recycle rates or pressure

• 46 23 53 - Dissolved Air Flotation Water Treatment Equipment: Primary section covering DAF clarifiers, saturators, and associated equipment. May reference 40 30 00 for process controls and 46 21 00 for chemical feed systems integration

• Material/Equipment Verification: Verify stainless steel grades (316SS minimum for contact zones), confirm polymer feed system compatibility, check saturator vessel ASME certification
• Installation Requirements: Level concrete pads within 1/8" over 10 feet, overhead crane access for saturator maintenance, electrical classification for pump/motor areas
• Field Challenges: Skimmer alignment critical for performance, recycle pump cavitation during startup, polymer feed point optimization requires field adjustment
• Coordination Issues: 16-20 week lead times typical, coordinate with structural for loading requirements

• Xylem: Sanitaire AquaDAF systems, proven in 1-50 MGD applications
• Evoqua: Microflot units, strong municipal track record
• WesTech: Supracell DAF systems, popular for smaller plants (0.5-10 MGD)
• World Water Works: Dissolved air flotation clarifiers, competitive pricing for mid-size installations

• Conventional Sedimentation: 30-40% lower capital cost, better for high-solids applications, requires larger footprint
• Ballasted Flocculation (Actiflo): 50% higher cost but 75% smaller footprint, excellent for peak flow handling
• Membrane Bioreactors: 2-3x DAF cost but eliminates secondary treatment, preferred for tight discharge limits

Manufacturer Relationships: Establish service agreements early - DAF systems require more frequent maintenance than conventional clarifiers. Xylem and Evoqua offer comprehensive service packages. Cost-Saving: Specify standard saturator sizes (avoid custom vessels) and consider packaged polymer systems. Size recycle pumps for 20% capacity margin - undersized pumps cause immediate performance issues. Field-verify A/S ratios during commissioning; manufacturer calculations often need adjustment.