DAF saturator and recycle system
The flotation tank gets the attention, but the saturator is where the process actually happens. Everything a DAF does depends on how much air is dissolved, how completely it comes out of solution, and what size the bubbles are when it does.
The saturator is a pressure vessel operating at 4–6 bar in which a recycled portion of clarified effluent — typically 6–120 % of forward flow depending on air demand — is brought into contact with compressed air until it approaches saturation. When this stream is released through a nozzle into the flotation cell at atmospheric pressure, the dissolved air comes out of solution as 20–50 µm bubbles that attach to coagulated floc. Saturator efficiency is 60–90 % depending on whether the vessel is packed, and the achievable air release is governed by Henry’s law, so it falls sharply as water temperature rises.
How much air actually dissolves
Air solubility follows Henry’s law: it rises with pressure and falls with temperature. This table is the one that governs every DAF sizing calculation, and the one most often skipped.
| Water temp | Solubility at 1 atm | At 4 bar (abs 5) | At 5 bar (abs 6) | At 6 bar (abs 7) |
|---|---|---|---|---|
| 10 °C | 29.2 mg/L | 146 mg/L | 175 mg/L | 204 mg/L |
| 15 °C | 26.6 mg/L | 133 mg/L | 160 mg/L | 186 mg/L |
| 20 °C | 24.3 mg/L | 122 mg/L | 146 mg/L | 170 mg/L |
| 25 °C | 22.4 mg/L | 112 mg/L | 134 mg/L | 157 mg/L |
| 30 °C | 20.8 mg/L | 104 mg/L | 125 mg/L | 146 mg/L |
| 35 °C | 19.5 mg/L | 98 mg/L | 117 mg/L | 137 mg/L |
| 40 °C | 18.4 mg/L | 92 mg/L | 110 mg/L | 129 mg/L |
Calculating recycle flow
Recycle ratio is a result, not an input. Anyone quoting “10 % recycle” before knowing your solids load and temperature is guessing.
- Solids load (kg/h) = design flow (m³/h) × influent TSS (kg/m³), plus chemical floc generated by the coagulant dose.
- Air demand (kg/h) = solids load × A/S ratio. Use 0.005–0.02 for municipal and fibre, 0.02–0.04 for food and protein, 0.04–0.06 for oily and emulsified streams.
- Available air (kg/m³) = [saturation at working pressure and temperature × efficiency − saturation at 1 atm] ÷ 1,000.
- Recycle flow (m³/h) = air demand ÷ available air.
- Recycle ratio (%) = recycle flow ÷ design flow × 100.
Worked: 20 m³/h at 1,200 mg/L TSS = 24 kg/h solids. At A/S 0.04 (oily stream) air demand is 0.96 kg/h. At 5 bar, 32 °C, 80 % efficient packed vessel: (121 × 0.80 − 20.2) ÷ 1,000 = 0.0765 kg/m³. Recycle = 0.96 ÷ 0.0765 = 12.5 m³/h, a 63 % recycle ratio. Run it at 20 °C instead and the answer is 10.0 m³/h — a unit sized on the cooler figure delivers a fifth less air than the process needs. Use the sizing calculator to avoid doing this by hand.
Packed versus unpacked saturators
| Unpacked (sparged) | Packed | |
|---|---|---|
| Saturation efficiency | 50 – 70 % | 80 – 95 % |
| Vessel size for same air | Larger | Smaller |
| Recycle flow required | Higher, so bigger pump and more energy | Lower |
| Fouling risk | Low | Packing fouls on scaling or biological water |
| Maintenance | Minimal | Periodic packing clean or replacement |
| Best for | Hard, scaling or biologically active water | Clean recycle water, energy-sensitive duty |
Release nozzles: the component nobody specifies
Bubble size is set almost entirely at the release point. A well-designed nozzle produces a tight distribution centred on 30–40 µm. A worn or badly designed one produces coarse bubbles above 100 µm, which rise too fast to attach to floc and simply pass through. Symptoms of nozzle problems — visible large bubbles, boiling at the surface, a thin float despite adequate air — are frequently misdiagnosed as chemical problems.
- Specify accessible nozzles. If cleaning them means draining the cell, they will not be cleaned.
- Maintain the pressure differential. Bubble formation depends on a sharp pressure drop. A partly closed valve upstream of the nozzle ruins it.
- Watch for wear. Nozzle orifices erode, particularly where the recycle stream carries any solids. Check annually, replace on a schedule rather than on failure.
- Never take recycle from the raw feed. Recycle must be clarified effluent. Solids in the saturator foul the vessel, block nozzles and destroy bubble quality.
Frequently asked questions
What pressure should a DAF saturator run at?
4 to 6 bar gauge, with 5 bar the usual design point. Higher pressure dissolves more air per unit of recycle, reducing recycle flow and pump size, but increases compressor energy and vessel cost, and above about 6 bar the marginal air gain no longer justifies it. Lower pressure needs proportionally more recycle flow, which increases hydraulic loading on the flotation cell — a real constraint, since recycle flow adds to the flow the cell must handle.
What is a typical DAF recycle ratio?
It depends entirely on air demand, so it ranges from about 6 % on a municipal stream to over 100 % on a heavily dosed oily industrial stream. Any single figure quoted without knowing your solids load, A/S ratio and temperature is a guess. Municipal 6–15 %, paper 15–30 %, food and dairy 20–40 %, edible oil and tannery 50–120 % are reasonable expectations by sector.
Why is my float thin even though the saturator is at pressure?
Check in this order: recycle flow actually delivered (not the setpoint — measure it); water temperature against the design figure; nozzle condition; air supply to the saturator, since a compressor that cannot keep up leaves the vessel at pressure but under-aerated; then coagulant dose and pH. Temperature and nozzle wear account for most cases we are called to. See the troubleshooting guide.
Can I use plant air for the saturator?
Only if it is oil-free and dry. Compressor oil carried into the saturator coats the packing, contaminates the recycle water and interferes with bubble formation and floc attachment. Use a dedicated oil-free compressor or a properly maintained filtration train. This is a common and expensive shortcut on sites where a general-purpose compressor is already available.