How to Size DAF for White Water Discharges in 2026

What 'White Water' Actually Means in a Factory Discharge

White water is the dilute fiber-and-filler suspension that drains through the paper machine wire and out of broke pulpers, carrying 800–3,000 mg/L of total suspended solids in a 30–55°C stream. Of that TSS, 40–60% is long fiber, 20–30% is filler (kaolin clay, ground calcium carbonate, TiO2), and 10–20% is fines and broke solids; the rest is dissolved colloidal material (Zhongsheng field data, 2026). The temperature window matters because saturation solubility of air in water falls from ~18 mg/L at 30°C to ~14 mg/L at 55°C, which directly reduces the air mass available for bubble-particle attachment. In food plants the same hydraulic framework applies to washwater carrying starch, pulp, and oil residues; in textile mills it covers desize baths loaded with size, fiber lint, and PVA. When pulped rejects carry surface sizing agents, AKD, rosin, or stickies, the DAF's job shifts from simple fiber clarification to simultaneous oil-plus-fiber capture, and the A/S setpoint must rise to overcome the depressed bubble attachment that surfactants cause.

Step 1: Characterize the Influent Before You Size Anything

A defensible DAF design starts with a 24-hour composite sample plus three 8-hour grabs, because paper-machine white water swings 1.5–2.5× between peak broke discharge and steady drainage (Zhongsheng field data, 2026). Record flow, TSS, COD, turbidity, temperature, pH, and oil & grease on every sample; the peak hour from this dataset — not the daily average — drives tank area. Upstream screening to ≤2 mm perforations is non-negotiable: long fibers mat across the saturator nozzle and throttle microbubble release within hours. Test surfactant and optical brightener content explicitly, because each 10 mg/L of residual surfactant in the feed shifts the required A/S ratio up by roughly 0.01 and can drop removal efficiency by 8–12% if the saturator pressure is left at the lower end of the band. Without this characterization, every downstream number is a guess.

Step 2: Set the Hydraulic Loading Rate and Retention Time

Hydraulic loading rate (HLR) is the metric that converts flow into tank surface area, and the working band for fiber-laden white water is 5–15 m/h, with 5–8 m/h preferred when effluent TSS must drop below 30 mg/L (Zhongsheng field data, 2026). The trade-off is square: halving HLR doubles the footprint, so most mills running to 50–80 mg/L effluent targets sit at 8–10 m/h. Flotation retention time runs 15–30 minutes; broke streams with high fines and ash content need the upper end because fine flocs rise more slowly. The two equations are: A = Qpeak / HLR and V = Q × t. Worked at Qpeak = 50 m³/h and HLR = 8 m/h, A = 6.25 m²; at the same flow with 20-minute retention, V = 16.7 m³, which implies an effective depth of 2.7 m. Depth outside 2.0–3.0 m usually signals either a poorly designed inlet distribution (too shallow) or excessive footprint (too deep) and is a flag to revisit the inlet header.

Step 3: Air-to-Solids Ratio and Recycle — The Two Knobs That Drive Removal

The air-to-solids ratio (A/S) is the mass of air released per mass of solids removed, and the operating band is 0.02–0.10, with 0.03–0.05 typical for white water (Zhongsheng field data, 2026). Mthembu et al. (DOI 10.51415/10321/3182) report optimum DAF performance at a 10% air-to-water ratio and 350 kPa saturator pressure for refinery oil/water separation, with removal holding above 85% in the 8–12% A/W band. The recycle stream equation is Qrecycle = (A/S × Sload) / (Cair × ρair), where Cair is dissolved air concentration at the saturator pressure. At A/S = 0.04, Sload = 75 kg/h (50 m³/h × 1,500 mg/L), and Cair = 12 mg/L, the math gives roughly 7.5 m³/h of recycle, or 15% of forward flow — the typical design point. Operating cost scales with recycle: each 5% increase in recycle flow adds ~0.2 kWh/m³ to power draw (Zhongsheng field data, 2026), so the recycle pump must be selected in parallel with the tank, not after it.

Step 4: Saturator Pressure, Microbubble Diameter, and Nozzle Sizing

Saturator pressure of 400–600 kPa (4–6 bar) is the standard band for white water; 300–425 kPa remains usable when paired with strong floc chemistry but yields 50–100 µm bubbles that capture fiber less efficiently than the 30–50 µm high-efficiency band. The Henry's-law-derived air mass balance at 40°C gives ~18 mg/L of dissolved air at 500 kPa versus ~6 mg/L at atmospheric pressure, releasing a usable 12 mg/L per litre of recycle when pressure is dropped to ambient through the nozzle (Zhongsheng field data, 2026). Microbubble diameter is the variable that ties saturator pressure to removal: 10–100 µm covers the working range, and the 30–50 µm window maximizes the collision-and-attachment frequency with 50–200 µm flocs. Nozzle velocity matters because velocities below 12 m/s allow bubble coalescence inside the manifold, while velocities above 20 m/s shear flocs apart. Specify stainless-steel needle orifices sized for 12–20 m/s, and verify differential pressure across the nozzle stays at 80–90% of saturator pressure.

Step 5: Coagulant and Floc Chemistry — the Pre-DAF Step You Cannot Skip

Designing a DAF to do work the coagulant stage should already have done is the single most common sizing error. Dose a cationic coagulant — alum, ferric chloride, or polyaluminum chloride — at 30–50 mg/L with pH held between 5.5 and 7.0; Mthembu et al. (DOI 10.51415/10321/3182) report 85% removal at pH 5 and 50 mg/L coagulant in refinery service, with the 30–45 mg/L band performing comparably under Box-Behnken optimization. Follow with an anionic or cationic flocculant at 0.5–3 mg/L to build a 50–200 µm floc that the microbubbles can attach to. Allow 3–5 minutes of floc maturation in a separate pipe flocculator or stirred tank before the saturator nozzle; cutting maturation short reduces removal by 10–20% and overloads the downstream press (Zhongsheng field data, 2026). Holding pH and dose inside ±5% is the job of the automatic chemical dosing skid, which should be specified with a redundant pump and online pH probe for any line over 30 m³/h.

Step 6: Worked Example — Sizing a DAF for a 50 m³/h White Water Line

Inputs: Qavg = 50 m³/h, peak factor 1.6 → Qpeak = 80 m³/h, TSS = 1,500 mg/L, T = 40°C, pH 6.5. At HLR = 8 m/h the tank area is 10 m², and a square footprint of 3.2 m × 3.2 m gives the geometry. A 20-minute retention time yields 26.7 m³ of effective volume, which divides into 2.7 m of effective depth. Setting A/S = 0.04 and saturator pressure at 500 kPa gives Cair ≈ 12 mg/L; the recycle equation returns 12 m³/h, or 15% of Qavg. Utility loads: saturator recycle pump 1.5 kW, compressor 0.4 Nm³/min at 500 kPa, skimmer drive 0.75 kW, and a sludge withdrawal targeting 5% of inlet flow routed to a filter press. The full calculation chain is below.

Matching the Design to a Standard DAF Model

The worked example lands on the ZSQ dissolved air flotation system family, which covers 13 standard models from 4 m³/h to 300 m³/h with micro-bubble release heads and automatic skimming. The 50 m³/h average flow maps to the ZSQ-50, and the 80 m³/h peak hour sits inside its standard 15% oversize margin without requiring a derate. A complete DAF train also requires three companion items: a rotary mechanical bar screen upstream for fiber matting protection, the automatic dosing skid for chemistry, and a plate and frame filter press downstream to dewater the floated sludge to 30–35% dry solids for landfill or incineration. Mills running kraft or deinking reject streams should also review the kraft foul condensate pretreatment before DAF guide to confirm the upstream stripper is correctly sized.

Commissioning Checks and the Three Sizing Errors That Cost the Most

Commissioning should begin with jar tests to confirm the coagulant dose, followed by a 4-hour saturator flow test to verify the recycle pump hits 15% of forward flow at design pressure, and a bubble-size check using a column camera or graduated DAF cell (target: 80% of bubbles in the 30–50 µm band). The three sizing errors that cost the most are: (1) sizing on the daily average flow rather than the peak hour, which produces a 30–50% overload on the first upset and pushes sludge over the weir; (2) undersizing the recycle pump below 10% of forward flow, which collapses A/S and lets long fibers escape into the effluent; (3) omitting the 3–5 minute floc maturation tank, which drops removal by 10–20% and overloads the downstream press with under-flocced solids. Each error is recoverable during commissioning but expensive after handover.

Frequently Asked Questions

What hydraulic loading rate should I use for white water DAF sizing?

Use 5–15 m/h, with 5–8 m/h when effluent TSS must drop below 30 mg/L and 8–10 m/h for the more common 50–80 mg/L target (Zhongsheng field data, 2026). The lower the HLR, the larger the footprint, so most mills sit at 8 m/h to balance capital cost against clarity.

What air-to-solids ratio is typical for paper mill white water?

0.03–0.05 is typical, against a working band of 0.02–0.10. Mthembu et al. (DOI 10.51415/10321/3182) report optimum performance at 8–12% air-to-water and 300–425 kPa saturator pressure, with 85% removal at pH 5 and 30–50 mg/L coagulant dose in refinery service. White water streams carrying surfactant-laden broke or surface size should sit toward the upper end (0.06+).

Why is upstream screening mandatory before a white water DAF?

Long fibers mat across the saturator nozzle and throttle microbubble release within hours if screening is omitted. A deinking reject water pretreatment before DAF guide outlines the ≤2 mm perforation standard and the rotary bar screen sizing that holds it. The same screening protects the flocculator and the recycle pump impeller from rag and fiber wraps.

How does this method change for paint booth curtain water instead of fiber white water?

The hydraulic framework is identical but the chemistry shifts: paint overspray is emulsified oil plus resin, not fiber plus filler, so coagulant dose rises to 50–100 mg/L and A/S to 0.06–0.10. A dedicated DAF sizing guide for paint booth curtain water walks through the surfactant chemistry and the lower temperature window (20–30°C) that increases Cair by ~15% relative to hot paper-machine white water.

Further Reading

• deinking reject water pretreatment before DAF guide