DAF troubleshooting guide
Almost every underperforming flotation unit has one of about a dozen causes. This is the order to check them in, and why that order matters.
When a DAF underperforms, check in this order: pH at the flotation inlet, actual delivered coagulant and polymer dose, water temperature against the design figure, measured recycle flow, saturator pressure and air supply, and nozzle condition. The first three account for the majority of cases. Changing the coagulant before checking pH, or replacing equipment before measuring recycle flow, wastes money on a symptom. Almost all diagnosis can be done with a pH meter, a bucket and a stopwatch, and a jar test kit.
The diagnostic order
- pH at the flotation inlet, logged continuously for 24 hours — not spot-checked, and not read from the balance tank probe. Most intermittent failures are pH excursions nobody was watching for.
- Delivered chemical dose. Measure with a calibration column. Setpoints lie: pumps lose prime, diaphragms perish, suction lines airlock, and stock strength varies between deliveries.
- Water temperature against the temperature the unit was designed for. Every 10 °C above design costs roughly 8–10 % of available air.
- Recycle flow, measured. Bucket and stopwatch on the recycle line if there is no meter. A recycle pump that has lost 30 % of its duty to wear looks entirely normal from the control room.
- Saturator pressure and air supply. Pressure alone is not sufficient — a vessel can sit at 5 bar while under-aerated if the compressor cannot keep up or the air valve is throttled.
- Nozzle condition. Look at the bubbles. A healthy DAF produces a fine white milkiness. Visible individual bubbles mean the nozzles are worn, blocked or the pressure differential has been lost.
- Hydraulic loading. Confirm actual flow, including recycle, against design. Plants creep upward in throughput over the years without anyone recalculating.
- Flocculation. Look into the flocculation chamber. You should see visible, discrete floc entering the cell. If the water looks smooth, no floc is forming and nothing downstream will work.
Symptom table
| Symptom | Likely causes, most common first | Check |
|---|---|---|
| Thin, grey, watery float | Insufficient air; water hotter than design; recycle flow low; nozzles worn | Measure recycle flow and temperature; inspect bubble appearance |
| No float at all, water looks clear-ish but TSS high | Coagulation failing — pH out of window, or dose lost | pH at inlet; calibration column on coagulant pump |
| Float forms then sinks | Coagulant overdose making floc too dense; insufficient air; long detention letting bubbles detach | Jar test at reduced coagulant dose; check A/S ratio |
| Slimy float that will not skim | Polymer overdose, re-stabilisation | Halve polymer dose and observe |
| Solids carry-over into effluent | Hydraulic overload; short-circuiting; weir out of level; floc sheared in pump | Confirm actual flow incl. recycle; check weir level with a hose level |
| Large visible bubbles, surface boiling | Nozzle worn or missing; pressure differential lost; air injected downstream of saturator | Inspect nozzles; verify pressure immediately before nozzle |
| Saturator level rising / flooding | Air supply insufficient or air valve closed; level control failed | Compressor output; air control valve; level probe |
| Performance good in morning, poor later | CIP or batch discharge changing pH or load; chemical tank running out | 24 h pH log; chemical consumption vs stock |
| Gradual decline over months | Nozzle erosion; recycle pump wear; saturator packing fouled; creeping throughput | Measure recycle flow against commissioning record |
| Sudden failure after chemical delivery | Different product strength or grade supplied; wrong polymer charge | Check delivery note against specification; jar test the new batch |
| Black or rotten-egg smelling float | Septic balance tank; sulphide generation | Balance tank retention and mixing; consider aeration or nitrate dosing |
| Effluent COD high, TSS fine | COD is soluble — not a DAF fault | Filtered COD test; biological stage required |
The temperature trap
This deserves its own section because it is the most common design-stage error on African installations, and because it presents as a chemical problem.
A unit specified in Europe on 20 °C data and installed on 32 °C effluent dissolves roughly 20 % less air per cubic metre of recycle. The operator sees a thin float, assumes the coagulant is failing, and increases the dose. More coagulant makes more floc, which needs more air, which is exactly what is missing — so the float gets worse, the chemical bill rises, and the diagnosis moves further from the cause.
The fix is usually available without new equipment: raise the saturator pressure toward 6 bar, and increase the recycle ratio if the pump has the capacity. If it does not, a larger recycle pump costs a fraction of a new DAF. Verify the arithmetic in the sizing calculator at your real operating temperature before ordering anything.
What good looks like
- Contact zone: uniform white milkiness, no individual bubbles distinguishable, no boiling.
- Flocculation chamber: visible discrete floc, 1–3 mm, moving slowly without being torn apart.
- Float layer: 30–80 mm thick, firm enough to hold a shape when the surface is touched, uniform across the cell width.
- Subnatant: visually clear, no rising fines, no sinking floc on the tank floor.
- Effluent weir: even flow across the full width — an uneven curtain means the weir is out of level and the cell is short-circuiting.
- Sludge: discharging at 3–6 % dry solids, not as thin grey water.
Frequently asked questions
Why is my DAF float thin and watery?
Almost always insufficient air reaching the flotation zone. Check, in order: measured recycle flow against design, actual water temperature against design temperature, saturator pressure, air supply to the saturator, and nozzle condition. Adding more coagulant makes it worse, because more floc requires more air. Resist that instinct until air delivery has been verified.
Why does my DAF sludge sink instead of float?
Three common causes. Metal coagulant overdose produces dense floc that a bubble cannot lift — jar test at a lower dose. Insufficient air means the air-to-solids ratio is below what the stream needs. Or the floc is being sheared after flocculation, so the bubbles have nothing to attach to; check for a pump, a sharp bend or a partly closed valve between the flocculation chamber and the cell.
My DAF worked fine and then stopped. What changed?
Something did. The usual candidates: a new chemical delivery at different strength or grade; a change upstream in the process or in cleaning practice; a dosing pump that has lost prime or a perished diaphragm; a seasonal temperature rise; or a blocked or eroded nozzle. Start with a 24-hour pH log at the DAF inlet and a calibration-column check on both dosing pumps. Those two tests find most of these.
Can an undersized DAF be uprated without replacement?
Often, yes. Raising saturator pressure from 4 to 6 bar increases dissolved air by roughly 40 %. A larger recycle pump increases air delivery proportionally. Adding a flocculation chamber upstream improves floc quality and effective capacity. Improving the balance tank stabilises everything. In combination these can add 20–40 % of effective capacity for a small fraction of replacement cost — worth investigating properly before accepting a quotation for a new unit.