Measure recycle and define the air-to-solids basis
In a recycle DAF system, part of the clarified water passes through pressurisation and returns to the inlet. Qr / Qf expresses recycled flow relative to external feed flow. State that definition: a percentage based on total tank flow would give a different number.
The air-to-solids ratio must identify the mass of dissolved air released and the solids mass being counted. It is not simply compressor air divided by feed TSS. Gas losses, incomplete saturation and temperature changes can matter. EPA documents describe pressure, recycle, hydraulic loading, solids and chemicals as interacting variables rather than independent adjustments.
Specify whether the denominator includes only incoming suspended solids or also solids introduced or precipitated by coagulation. Two trials reporting the same apparent ratio may be using different calculation bases.
A worked calculation, rather than an operating setpoint
Consider an original example: feed flow is 60 m³/h with 500 mg/L TSS, with no chemical solids added to the balance. Solids loading is 60 × 500 / 1000 = 30 kg/h. A 20 % recycle provides 12 m³/h. Assume an evaluation of the saturator estimates 35 mg/L of effectively releasable air in that stream under the tested conditions.
Available air is then 12 × 35 / 1000 = 0.42 kg/h, and the air-to-solids ratio is 0.42 / 30 = 0.014 kg/kg. This is an arithmetic result based on stated assumptions, not a recommended ratio or a removal prediction.
If effective flotation area is 20 m², hydraulic loading including feed and return is (60 + 12) / 20 = 3.6 m³/(m²·h). Increasing recycle would raise available air only if the releasable concentration remains unchanged; it would also increase hydraulic loading. The equipment must accommodate both effects.
Check faults that more chemical cannot correct
Inspect the pressurised-water outlet, pressure losses, valves, nozzles, saturator level and distribution between injection points. A correct pressure reading at one location does not prove that gas is distributed effectively. Verify flowmeters before comparing nominal recycle percentages.
Separate floc formation from separation performance. Suitable flocs may break at the flotation inlet, or be carried over because of overloading or poor float removal. Record float appearance and treated-water quality using consistent analytical methods.
Check whether a pollution peak changes TSS, oil, pH or temperature. Holding recycle constant while solids loading rises reduces available air per unit of solids, even if pressure does not change.
| Observation | Useful check |
|---|---|
| Uneven flotation across the tank | Return distribution, injection and short-circuiting |
| Good upstream floc with downstream carryover | Inlet, hydraulic loading and float removal |
| Failure during load peaks | Solids load, releasable air and saturator capacity |
Test gas, hydraulics and chemistry on a common basis
Keep a reference condition and change one variable at a time within equipment limits. Compare treated water, float mass, active chemical dose, electricity and stability. Repeat with representative wastewater and allow operating conditions to stabilise. A jar test alone does not reproduce the hydraulics of a DAF unit.
Use historical EPA studies to understand the measurements needed, rather than transplanting their operating settings into another industry. If a nanobubble generator is proposed as an addition, assess it separately: its gas, size distribution and behaviour cannot be assumed equivalent to air released by depressurisation.
A project decision should demonstrate the combined result and explain what changed. An isolated improvement does not establish chemical savings or justify replacing a saturator without a comparable trial and complete balance.
Common questions
Does more recycle always improve flotation?
No. It may increase available air, but also hydraulic loading, electricity use and disturbance. Results must be checked on the actual equipment and wastewater.
Can compressor air directly determine air/solids?
No. The comparison needs the fraction effectively dissolved and released, with the measurement basis and losses documented.