Tannery and leather wastewater treatment with DAF
Tannery effluent is the most chemically hostile stream a DAF will ever see: sulphide, chromium, lime, salt and protein, at pH 3 to 12 depending on which drum just discharged. Get the segregation and the metallurgy right and flotation works well. Get either wrong and the unit will not last five years.
A dissolved air flotation unit on segregated, pretreated tannery effluent removes 80–95 % of suspended solids, 85–95 % of fats and grease, 50–70 % of COD and better than 95–99 % of chromium once it has been precipitated as hydroxide, dosed with 200–800 mg/L ferric chloride plus 2–6 mg/L anionic polymer at 5–8 m/h. Two things must happen before the DAF: beamhouse sulphide must be catalytically oxidised, and chrome liquor must be segregated and precipitated separately. Chloride from pickling routinely exceeds 10,000 mg/L, which makes SS316L the minimum sensible material and duplex 2205 the safe one.
Segregate first. Everything else is secondary.
The single most common failure in African tannery effluent plants is a combined drain. Mixing sulphide-bearing beamhouse liquor with acidic pickle or chrome liquor releases hydrogen sulphide — lethal at concentrations that are reached in minutes in a covered sump — and simultaneously destroys any chance of recovering chromium. Three streams must be kept apart:
| Stream | Origin | Key contaminants | Required pretreatment |
|---|---|---|---|
| Beamhouse / unhairing | Soaking, liming, unhairing | Sulphide 200 – 3,000 mg/L, lime, hair, protein, pH 11 – 12.5 | Catalytic oxidation with air + Mn²⁺ catalyst to <2 mg/L sulphide |
| Tanyard / chrome | Pickling, chrome tanning | Cr(III) 1,500 – 5,000 mg/L, chloride 10,000 – 30,000 mg/L, pH 2.5 – 4 | Segregate; precipitate Cr as hydroxide at pH 8 – 9 and recover, or settle separately |
| Post-tanning & finishing | Retanning, dyeing, fatliquoring | Fats 500 – 2,000 mg/L, dyes, syntans, pH 3.5 – 6 | To balance tank |
| Combined after pretreatment | — | TSS 1,000 – 4,000, COD 2,000 – 8,000, FOG 100 – 800 mg/L | pH to 7 – 8.5, then coagulation and DAF |
Process sequence that works
- Coarse and fine screening on each stream, 6 mm then 1–2 mm. Hair, fleshings and trimmings must never reach a pump.
- Catalytic sulphide oxidation of beamhouse liquor: aerate with manganese sulphate catalyst at 20–100 mg/L, 4–8 hours, to below 2 mg/L sulphide.
- Chrome precipitation on the segregated tanyard stream: raise to pH 8.5–9.0 with magnesium oxide or caustic, settle, and either recover the chrome cake for re-dissolution and reuse or dispose of it as a separate hazardous fraction.
- Combine and balance the treated streams, 10–16 h HRT, mixed.
- pH correction to 7.0–8.5, coagulate with ferric chloride, flocculate with anionic polymer.
- Dissolved air flotation at 5–8 m/h.
- Biological treatment for the remaining soluble COD, which will still be 800–3,000 mg/L.
Chrome recovery deserves separate attention: a tannery running 5 t hide/day discharges roughly 100–200 kg of chromium salts a year to sludge if it is not recovered. Recovery pays back quickly and converts a hazardous waste liability into a raw material.
Why ferric, not aluminium
Ferric chloride is the standard coagulant for tannery effluent for three reasons: it works across a wider pH band than aluminium salts in the presence of high chloride and sulphate; it scavenges any residual sulphide as insoluble iron sulphide; and it produces a denser, more shear-resistant floc that survives the recycle nozzle. Doses of 200–800 mg/L are normal — considerably higher than most industries, because the organic load and the ionic strength both work against coagulation.
Expect high sludge volumes. A tannery DAF generates 1.5–4 % of influent volume as float at 3–6 % dry solids. Sludge handling and disposal is often the largest operating cost in the plant, and it must be designed in, not discovered.
Materials of construction: the decision that matters most
Pickling uses sodium chloride at 6–10 % on hide weight. The resulting effluent chloride concentration of 5,000–30,000 mg/L, combined with pH excursions and sulphate, is a severe pitting and crevice-corrosion environment.
| Material | Suitability | Expected life |
|---|---|---|
| Carbon steel + epoxy | Not suitable | 2 – 4 years, with failure at the liquid line and welds |
| SS304 | Not suitable above ~200 mg/L chloride | 3 – 6 years, pitting |
| SS316L | Minimum sensible choice | 8 – 15 years below ~5,000 mg/L chloride |
| Duplex 2205 | Correct choice for segregated tanyard duty | 15 – 25 years |
| GRP / FRP | Good chemical resistance | 10 – 20 years; check UV and mechanical loading |
| Concrete + chemical-resistant lining | Viable for large civil-built cells | Lining life 8 – 15 years, re-lineable |
Worked sizing example: 8 t hide/day tannery
- Effluent volume. 8 t/day at 35 m³/t = 280 m³/day. Balanced over 20 h: 14 m³/h design, 22 m³/h peak.
- Load after pretreatment. TSS 2,000 mg/L, FOG 400 mg/L. Solids to the DAF: 14 × 2.4 = 33.6 kg/h, plus roughly 12 kg/h of ferric hydroxide floc from a 400 mg/L dose.
- Flotation area. At 6 m/h: 14 ÷ 6 = 2.33 m². Select 2.5 m².
- Air demand. A/S of 0.035: (33.6 + 12) × 0.035 = 1.60 kg air/h.
- Recycle flow. At 5.5 bar and 30 °C, usable dissolved air ≈ 97 mg/L. Recycle = 1.60 ÷ 0.097 = 16.5 m³/h, a recycle ratio of 118 %. This is normal for heavily dosed, high-solids streams and it must be reflected in the saturator and recycle pump sizing.
- Float sludge. Approximately 46 kg/h dry at 4 % ≈ 1.15 m³/h, 23 m³/day. Plan for a filter press or centrifuge, not drying beds, unless land is abundant.
- Expected effluent. TSS 100–300 mg/L, COD 800–3,000 mg/L, chromium below 2 mg/L. Biological treatment follows.
Frequently asked questions
Can DAF remove chromium from tannery effluent?
Only after the chromium has been precipitated. Trivalent chromium in solution passes straight through a flotation unit. Raise the segregated tanyard stream to pH 8.5–9.0 with magnesium oxide or caustic and chromium precipitates as Cr(OH)₃, which a DAF then removes at better than 95–99 % efficiency. Precipitation and separation are two different steps and both are required.
Why must beamhouse and tanyard streams be separated?
Two reasons, one of them a safety case. Mixing alkaline sulphide liquor with acidic pickle liquor liberates hydrogen sulphide gas, which is lethal at concentrations reached within minutes in an enclosed sump. Second, once chrome liquor is diluted into the general stream, chromium recovery becomes uneconomic and the chromium reports to the sludge, turning ordinary sludge into a hazardous waste.
What material should a tannery DAF be made of?
SS316L as a minimum, duplex 2205 where the unit sees segregated pickling or tanyard liquor with chloride above roughly 5,000 mg/L. GRP and lined concrete are also viable. Carbon steel with epoxy and SS304 are not suitable: chloride from pickling at 5,000–30,000 mg/L causes pitting and crevice corrosion that will end the unit’s life well inside ten years.
How much sludge does a tannery DAF produce?
Between 1.5 and 4 % of influent volume as float at 3–6 % dry solids — substantially more than most industries, because coagulant doses are high and the influent solids load is high. For an 8 t hide/day tannery this is roughly 20–25 m³/day of float requiring dewatering. Sludge handling is frequently the largest single operating cost, and it should be designed and costed at the same time as the DAF.