Dissolved Air Flotation (DAF) Thickeners

Dissolved Air Flotation (DAF) thickeners concentrate waste activated sludge by floating solids to the surface using pressurized air bubbles, creating a dense float layer that's mechanically scraped for collection. The process dissolves air under 60-80 psig pressure, then releases it at atmospheric pressure to generate microscopic bubbles that attach to sludge particles and lift them upward. Typical municipal installations achieve 4-6% total solids concentration from incoming 0.8-1.2% feed sludge, representing a 4-5x volume reduction. The primary trade-off is higher energy consumption compared to gravity thickening, requiring 15-25 kWh per dry ton of solids processed, making it most cost-effective for plants with limited footprint or poor-settling biological sludge.

• Primary Sludge Thickening (2-20 MGD plants): DAF thickeners concentrate primary clarifier underflow from 2-4% to 6-8% solids before anaerobic digestion. Selected over gravity thickeners for better polymer efficiency and smaller footprint. Feeds directly to digesters or intermediate storage.

• WAS Thickening (5-50 MGD plants): Thickens waste activated sludge from 0.8-1.2% to 4-6% solids before digestion or dewatering. Preferred over belt thickeners for consistent performance with variable sludge characteristics. Often combined with primary sludge streams.

• Biosolids Processing (10+ MGD plants): Pre-thickens mixed primary/secondary sludges before centrifuge or belt press dewatering. Reduces polymer consumption in downstream processes by 30-40%. Critical for plants with limited digester capacity.

• Scum/Float Concentration: Processes DAF skimmings from water treatment or industrial pretreatment, concentrating oils and floatable solids before disposal or further treatment.

Daily Operations: Monitor influent flow rates, sludge blanket depth, and float quality hourly. Adjust polymer dosage based on jar testing (typically 2-8 lb/ton solids). Check saturator pressure (maintain 60-80 psi) and air release valve operation. Record thickened sludge solids concentration and adjust skimmer speed accordingly.

​

Maintenance: Weekly bearing lubrication on rake mechanisms and skimmer drives. Monthly saturator cleaning and air release valve inspection. Quarterly polymer feed system calibration. Annual gearbox oil changes and tank cleaning. Requires confined space entry procedures and fall protection for tank access. Basic mechanical skills sufficient for routine maintenance.

​

Troubleshooting: Poor thickening indicates polymer overdose/underdose or saturator issues. Excessive carryover suggests high loading rates or inadequate detention time. Float breaking up indicates mechanical problems or improper air-to-solids ratio. Typical equipment life: 15-20 years for tanks, 8-12 years for mechanical components. Early warning signs include unusual noise, vibration, or declining performance despite proper

• Saturation System: Includes saturator tank (typically 50-150 gallon capacity), recycle pump (20-40% of feed flow), and pressure control valve. Dissolves air at 60-90 psi. Sizing based on A/S ratio requirements of 0.02-0.06 lb air/lb solids.

• Flotation Tank: Rectangular concrete or steel tanks with 10-20 minute detention time. Includes polymer feed system, slow-speed rake mechanism (1-3 fpm), and adjustable weirs. Typical loading rates: 2-6 gpm/sf for primary sludge, 1-3 gpm/sf for WAS.

• Skimming System: Chain-and-flight or helical screw collectors remove float at 6-12% solids concentration. Variable speed drives allow adjustment for sludge characteristics. Discharge typically 2-4 feet above tank bottom.

• Chemical Feed Equipment: Polymer preparation and feed systems with 0.1-0.5% active polymer dosing. Includes static mixers for rapid dispersion and contact chambers for flocculation before flotation.

• Hydraulic Loading Rate: 2-6 gpm/ft² (typical 3-4 gpm/ft²) based on sludge characteristics and target underflow concentration

• Solids Loading Rate: 15-40 lbs/day/ft² for waste activated sludge; 20-60 lbs/day/ft² for primary sludge

• Air-to-Solids Ratio: 0.02-0.06 lbs air/lbs solids (typically 0.03-0.04 for WAS)

• Pressurization System Pressure: 60-90 psig (75 psig standard)

• Recycle Flow Rate: 8-12% of feed flow for pressure dissolution systems

• Float Solids Concentration: 3-6% for WAS; 4-8% for primary sludge

• Underflow Clarity: <100 mg/L TSS (target <50 mg/L)

• Detention Time: 20-40 minutes in flotation zone

• Skimmer Speed: 2-6 ft/min surface velocity

• Unit Dimensions: Rectangular tanks 10-100 ft length, 8-40 ft width, 8-12 ft sidewater depth

• What air delivery method should be specified - pressure dissolution vs. induced air systems? Pressure dissolution systems require 8-12% recycle flow and 60-90 psig compressors but achieve better performance. Induced air systems have lower power requirements but struggle with fine bubble generation. Wrong choice impacts both CAPEX ($50-100K difference) and long-term reliability.

• What target float solids concentration is required for downstream processing? WAS applications typically achieve 3-5% while primary sludge reaches 5-8%. Underestimating impacts dewatering equipment sizing and polymer consumption. Requires pilot testing data or similar sludge performance records.

• Should rectangular or circular configuration be selected? Rectangular units (10:1-6:1 L:W ratio) provide better hydraulics for larger plants (>5 MGD) but require more complex skimming mechanisms. Circular units work well for smaller facilities but have geometric limitations above 40-50 ft diameter.

• What level of process redundancy is needed? Single units create operational risk during maintenance. Multiple smaller units (50-75% capacity each) provide flexibility but increase complexity and cost by 15-25%.

• Division 40-48: 40 05 00 - Common Work Results for Process Integration

• Primary: 40 30 00 - Process Clarification Equipment (DAF thickeners)

• Secondary: 40 20 00 - Process Air and Gas Systems (pressurization equipment)

• Material/Equipment Verification: Verify stainless steel grade (316SS minimum for municipal applications), Confirm air dissolution system capacity and redundancy, Check skimmer mechanism materials and drive specifications

• Installation Requirements: Requires substantial concrete work - plan 8-12 week curing time, Air compressor room ventilation and noise considerations, Electrical coordination for VFD drives and control systems

• Field Challenges: Levelness critical - 1/8" tolerance typical, Polymer feed system integration often problematic, Weather protection during startup essential

• Coordination Issues: Lead times 16-20 weeks for municipal units, Early coordination with structural engineer for loadings

• Veolia - Supracell circular units (10-100 ft diameter) dominate municipal market with proven track record at facilities like Denver Metro

• Evoqua - Microcel rectangular DAF systems popular for smaller plants (0.5-10 MGD)

• Xylem - Sanitaire brand offers packaged DAF units for biosolids thickening

• WesTech - Provides both circular and rectangular configurations with strong municipal references including Phoenix 91st Avenue

• Gravity Belt Thickeners - Lower capital cost ($200K vs $400K for DAF), simpler operation, but limited to 4-6% solids. Preferred for smaller plants under 5 MGD.

• Rotary Drum Thickeners - Compact footprint, good for retrofits, but higher polymer consumption.

• Centrifugal Thickeners - Higher solids capture (8-10%), but significantly higher power costs ($50K+ annually) and maintenance complexity. DAF offers best balance of performance and operating simplicity for most municipal applications.

Work closely with manufacturer's field service during startup - their experience prevents costly commissioning delays. Consider split-case centrifugal pumps instead of positive displacement for recycle flow - lower maintenance and energy costs. Specify redundant air dissolution tanks; single-point failures shut down entire system. Budget 15-20% above base bid for ancillaries like polymer systems, instrumentation, and electrical integration that manufacturers often exclude.