DAF Recycle System Design: Recycle Ratio and Pump Sizing

What does the recycle system do in a DAF?

In a dissolved air flotation unit the raw feed is never pressurised. Instead a fraction of the clarified effluent is drawn off, pumped up to the saturator pressure (typically 400–600 kPa gauge), and saturated with air in a packed or unpacked pressure vessel. When this air-charged stream is injected back into the flotation tank through release valves or needle nozzles, the sudden pressure drop to atmospheric supersaturates the water and precipitates a cloud of 20–50 µm bubbles that attach to conditioned flocs and lift them to the surface.

The recycle is taken from clean, clarified water for two reasons. First, low-turbidity water dissolves and holds air far more predictably — suspended solids and FOG in the raw feed would foul the packing and blind the saturator nozzles. Second, saturating the raw feed directly (full-flow pressurisation) would shear the flocs you have just built. Drawing from the clarified DAF effluent keeps the saturator clean and the flocs intact.

The physics is governed by Henry's law: the equilibrium mass of dissolved air is proportional to its partial pressure, so raising the saturator to 5–6 bar dissolves five to six times the air that atmospheric water holds. On release, Henry's law reverses — the water is supersaturated, and dissolved air comes out of solution by heterogeneous nucleation on floc surfaces and nozzle turbulence. The bubble-floc bond itself is a surface-chemistry problem: coagulation must first collapse the electrical double layer (a DLVO effect) so that hydrophobic bubbles can attach to the destabilised floc rather than bounce off it. A well-run recycle system therefore only performs as well as the upstream coagulation allows.

How is the recycle ratio set by the air-to-solids target?

The recycle ratio R = Q_r/Q (recycle flow over feed flow) is not chosen arbitrarily — it is back-calculated from the required air-to-solids (A/S) ratio, the single parameter that governs whether there is enough bubble mass to float the solids load. The governing relationship for a pressurised-recycle DAF is:

Air solubility s_a is strongly temperature-dependent (roughly 29.2, 22.8, 18.7 and 15.7 mL/L at 0, 10, 20 and 30 °C). Because warm water holds less air, R always has to rise in summer to hold the same A/S. The saturation fraction f captures how completely the vessel reaches equilibrium: an unpacked saturator with a spray header manages f ≈ 0.5–0.6, while a packed column with adequate residence time reaches f ≈ 0.8–0.9. Because f multiplies P directly in the air-release term, a poorly performing saturator is indistinguishable, in the A/S equation, from running at a lower pressure — and is corrected the same way, by adding recycle flow. Rearranging for the recycle ratio gives the design equation below.

Worked example 1: recycle flow and ratio from a target A/S

Take a feed of Q = 100 m³/h at S_a = 800 mg/L TSS, a bench-test target of A/S = 0.04, water at 20 °C (s_a = 18.7 mL/L), a saturator held at 500 kPa gauge and saturation fraction f = 0.8.

• Absolute pressure: P = (500 + 101.35) / 101.35 = 5.93 atm.
• Air-release term: f·P − 1 = 0.8 × 5.93 − 1 = 3.74.
• Solve for R: R = (A/S · S_a) / [1.3 · s_a · (f·P−1)] = (0.04 × 800) / (1.3 × 18.7 × 3.74) = 32 / 91.0 = 0.35.
• Recycle flow: Q_r = R × Q = 0.35 × 100 = 35 m³/h.

So a 35% recycle delivers the target A/S at this temperature and pressure. Note the sensitivity: repeat the calculation at 30 °C (s_a = 15.7) and R climbs to 0.42 (42 m³/h) for the same duty — a 20% larger recycle pump just to cope with warmer water.

How do you size the recycle pump?

The recycle pump must deliver Q_r against a discharge pressure equal to the saturator pressure plus system losses (pipe friction, the saturator internals and the injection nozzles). Convert that pressure to head and apply the hydraulic power equation:

Worked example 2. Using Q_r = 35 m³/h = 0.00972 m³/s, a saturator at 500 kPa plus 50 kPa of piping/nozzle losses gives Δp = 550 kPa.

• Head: H = Δp / ρg = 550,000 / (998 × 9.81) = 56.2 m.
• Hydraulic (water) power: P = Q_r · Δp = 0.00972 × 550,000 = 5,346 W ≈ 5.35 kW.
• Brake power at η_p = 0.65: 5.35 / 0.65 = 8.2 kW.
• Electrical input at η_m = 0.90: 8.2 / 0.90 = 9.1 kW.

A 11 kW motor would be selected to leave margin. The recycle pump is usually the single largest energy consumer in a DAF, which is why an oversized recycle ratio is expensive to run, not just to buy.

Why does NPSH matter for the recycle pump?

Because the recycle stream is warm, clarified water being lifted to a high discharge pressure, the pump suction is where cavitation risk lives. The available net positive suction head must comfortably exceed the pump's required NPSH:

Vapour pressure rises steeply with temperature (2.34 kPa at 20 °C to 4.25 kPa at 30 °C), eroding NPSH_a exactly when solubility is lowest and recycle flow is highest. For a flooded-suction draw off the clarified launder with h_s = +1.5 m and h_f = 0.6 m at 30 °C: NPSH_a = (101,350 − 4,250)/(996 × 9.81) + 1.5 − 0.6 = 9.94 + 0.9 = 10.8 m — ample. Cavitation problems almost always trace back to a suction lift, a throttled suction valve, or a blocked strainer rather than the theory.

How does recycle ratio interact with tank surface loading?

Here is the design tension. The recycle flow returns to the flotation tank, so it adds to the total hydraulic load. The surface (hydraulic) loading rate is the total flow over tank area:

So more recycle buys more dissolved air, but it also drives the float blanket upward faster and can push the rise rate past the point where fine bubbles are swept out with the clarified water. The table below holds Q = 100 m³/h and a fixed A = 12 m² tank, showing how A/S and SLR climb together (S_a = 800 mg/L, 20 °C, 500 kPa, f = 0.8):

Worked example 3. At R = 90% the A/S of 0.102 is generous, but the 15.8 m³/m²·h rise rate has already exceeded the 5–15 design envelope for this fixed tank — bubble carry-under becomes likely. The correct response is not to keep the small tank and crank the recycle, but to enlarge the tank area so that a modest R (35–50%) delivers the needed air at an SLR comfortably inside range. This is why recycle ratio and flotation tank sizing must be solved together, not in sequence.

What are the common recycle-system design mistakes?

• Sizing recycle before the tank. A high R papers over an undersized tank by adding air, but the extra flow overloads the surface — always co-optimise (see the trade-off above).
• Designing air for cool water only. Solubility falls ~15% from 20 to 30 °C; size the recycle pump and saturator for the warmest month or the summer float collapses.
• Ignoring nozzle losses. The pressure drop across release valves is what makes bubbles; account for it in pump head, not just the saturator set-point.
• Recycling raw or partially treated water. Solids foul the saturator packing and nozzles, degrading air transfer within weeks.
• No NPSH check at summer temperature. Vapour pressure and recycle flow both peak in summer — verify NPSH_a at the worst case, not the average.

For the neighbouring calculations, see the companion guides on DAF saturator design and the DAF air-to-solids ratio, and the broader DAF sizing workflow.

Sizing a DAF recycle system, step by step

1. Fix the A/S target. Set the air-to-solids ratio (0.01–0.06) from bench float tests on the real effluent and record the design TSS.
2. Choose saturator pressure and temperature. Select a saturator pressure (400–600 kPa) and design for the warmest operating temperature, where air solubility is lowest.
3. Solve for the recycle ratio. Rearrange the A/S equation to R = (A/S x Sa) / [1.3 x sa x (fP-1)] and compute the recycle flow Qr = R x Q.
4. Check surface loading. Confirm (Q + Qr)/A stays inside 5–15 m3/m2.h; if not, enlarge the tank rather than cutting recycle below the A/S requirement.
5. Size the recycle pump. Set discharge pressure = saturator pressure + losses, compute head, then P = Qr.dp / (eta_p x eta_m) for the motor.
6. Verify NPSH. Check NPSHa exceeds NPSHr with a 0.5–1.0 m margin at the summer temperature and highest recycle flow.