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Dairy wastewater treatment with DAF

Milk fat and casein float readily once coagulated, which makes dairy effluent an excellent flotation duty. What ruins dairy DAF installations is not the fat — it is the caustic and acid from clean-in-place, arriving in slugs that destroy the coagulation chemistry three times a shift.

What is in dairy effluent

A dairy generates 1.3 to 3.5 litres of effluent per litre of milk processed, depending on product mix and how disciplined the plant is about dry cleaning before wet cleaning. A liquid-milk and yoghurt plant sits at the low end; a cheese and butter plant with whey handling sits at the high end and carries far more organic load.

Parameter	Liquid milk	Cheese & whey	Milk powder	Removed by DAF
COD	1,000 – 2,500 mg/L	3,000 – 8,000 mg/L	1,500 – 4,000 mg/L	40 – 60 %
BOD₅	600 – 1,500 mg/L	1,800 – 5,000 mg/L	900 – 2,400 mg/L	35 – 55 %
TSS	300 – 900 mg/L	500 – 1,500 mg/L	400 – 1,200 mg/L	85 – 95 %
FOG	100 – 400 mg/L	300 – 800 mg/L	150 – 500 mg/L	90 – 97 %
Total N	30 – 80 mg/L	60 – 180 mg/L	40 – 120 mg/L	15 – 30 %
Total P	10 – 40 mg/L	20 – 80 mg/L	15 – 50 mg/L	60 – 85 %
pH (unbalanced)	4.0 – 11.0	3.5 – 11.5	4.5 – 11.0	n/a

The CIP problem, and why balancing comes first

Clean-in-place cycles push 1–2 % caustic and 0.5–1 % nitric or phosphoric acid into the drain. Instantaneous pH at the effluent sump swings between 3.5 and 11.5. Every coagulant has a working pH window — polyaluminium chloride is effective at roughly pH 6.0–8.5, alum at 5.8–7.5, ferric chloride at 4.5–6.5 for this duty. Outside the window, coagulation fails, the float goes thin and grey, and solids pass straight through.

The fix is not a bigger DAF. It is a balance tank with a hydraulic retention time of 8 to 12 hours, mechanically or coarse-bubble mixed, with automatic pH correction on the outlet. Undersize the balance tank and you will spend the plant’s life chasing dose setpoints that never settle. Size it correctly and the DAF becomes a stable, almost boring piece of equipment — which is what you want.

• Balance tank: 8–12 h HRT at average flow, sized on the largest single CIP discharge plus one shift’s production flow.
• pH correction: inline on the balance tank outlet, target 6.5–7.5, with dead-band control to avoid chasing.
• Fine screening: 1–2 mm before the balance tank, to keep curd fines and packaging debris out of the pumps.
• Temperature: dairy effluent is often 30–40 °C. Air solubility at 35 °C is about 24 % lower than at 20 °C, so recycle ratio must be corrected. See the design parameters reference.

Chemical regime

Milk fat is emulsified and protein-stabilised, so it will not separate by gravity in any useful time. Coagulation has to break the emulsion by charge neutralisation before flotation can lift it.

Chemical	Typical dose	Purpose	Notes
Polyaluminium chloride (PAC)	80 – 300 mg/L	Charge neutralisation, emulsion break	Widest pH tolerance; first choice for most dairies
Alum	150 – 400 mg/L	Alternative coagulant	Cheaper where available; narrower pH window, more sludge
Ferric chloride	100 – 250 mg/L	Where phosphorus removal is also required	Colours the float; avoid if float is destined for animal feed
Anionic polyacrylamide	1 – 3 mg/L	Floc bridging and strengthening	Dosed after coagulant with 30 – 60 s of gentle mixing
NaOH / H₂SO₄	As required	pH correction	On balance tank outlet, not in the flocculation stage

Dose ranges are starting points for jar testing. See coagulants, polymers and jar testing for the test protocol that actually predicts full-scale behaviour.

Worked sizing example: 200,000 L/day dairy

A liquid-milk and yoghurt plant processing 200,000 L/day of raw milk over two shifts, in a location with 30 °C ambient effluent temperature.

1. Effluent volume. At 2.2 L effluent per L milk: 440 m³/day. Over 16 operating hours plus balancing to 20 h: design flow 22 m³/h, peak 35 m³/h.
2. Load. TSS 600 mg/L, FOG 250 mg/L, COD 1,800 mg/L. Solids to the DAF: 22 × 0.600 = 13.2 kg/h TSS, plus coagulant solids.
3. Flotation area. At 8 m/h surface loading: 22 ÷ 8 = 2.75 m². Select a 3.0 m² unit (approximately 2.4 m × 1.25 m effective flotation zone).
4. Air demand. A/S ratio 0.03 kg air/kg solids on a fat-rich stream: 13.2 × 0.03 = 0.40 kg air/h.
5. Recycle flow. At 5 bar saturator pressure and 30 °C, usable dissolved air is roughly 90 mg/L at 80 % saturator efficiency. Recycle = 0.40 kg/h ÷ 0.090 kg/m³ = 4.4 m³/h, i.e. a recycle ratio of 20 %. Had this been calculated at 20 °C the answer would have been 3.6 m³/h — an 18 % shortfall in the field.
6. Float sludge. Roughly 13 kg/h dry solids at 4 % consistency ≈ 0.33 m³/h, or 6.6 m³ per 20 h day. Size the sludge holding tank for two days.
7. Expected effluent. TSS 30–70 mg/L, FOG 10–25 mg/L, COD 750–1,050 mg/L. If the consent is 400 mg/L COD, a biological stage is mandatory and the DAF is its protection, not its replacement.

Run your own numbers in the DAF sizing calculator, which applies the temperature correction automatically.

Materials and equipment selection

Dairy effluent is not chemically aggressive, but it is cleaned aggressively. The tank will see caustic and nitric acid in whatever concentration escapes the balance tank, plus hot water. SS304 is the sensible default. Carbon steel with epoxy is workable where the balance tank and pH control are genuinely reliable, and a false economy where they are not. SS316L is only justified where chloride-based sanitisers are in routine use.

Specify a variable-speed skimmer — dairy float varies in thickness through a shift far more than most streams — and a sludge hopper with a steep enough wall angle that fat does not bridge in it.

Frequently asked questions