Tannery Wastewater Treatment
Chromium, sulfides and high salinity make tannery effluent one of the toughest to treat. A technical guide to physicochemical + biological treatment for MADES compliance.
Tannery effluent is one of the most complex wastewaters in industry: it combines trivalent chromium from tanning, sulfides from unhairing/liming, high salinity (chlorides and sulfates), heavy organic load (COD from 3,000 to 8,000 mg/L and sometimes higher) and wide pH swings. That is why no single technology handles it: it needs a physicochemical line to capture metals and solids, followed by a biological stage to cut organic matter, before meeting MADES discharge limits.
The tannery effluent challenge
Tanning a hide means soaking it through several baths. Each stage adds a different contaminant, and blending them blindly makes treatment worse. The three main challenges are:
- Chromium (Cr III): comes from the tanning bath. It is a regulated metal; mixed with unhairing sulfides it can form compounds that are hard to separate. Best practice is to segregate the chrome bath and precipitate it on its own.
- Sulfides (S²⁻): from unhairing/liming. They are toxic to biomass and, as pH drops, release hydrogen sulfide (H₂S), a hazardous, foul-smelling gas. They are oxidized or precipitated before biological treatment.
- Salinity and solids: hide salting and the baths leave high chlorides and sulfates, plus fats, hair and fleshings that must be removed physically.
The Tech Tank solution: physicochemical + biological line
Tech Tank delivers turnkey wastewater treatment plants (WWTP) sized for each tannery's real load. A typical setup combines stream segregation, physicochemical treatment and a biological stage housed in bolted steel tanks with a corrosion-resistant coating (glass-fused-to-steel or epoxy) that stands up to the chemical and saline aggressiveness of the effluent.
1. Segregation and pretreatment
The chrome and sulfide streams are separated from the rest. Bar screens, sieves and grease traps remove hair, fleshings and coarse solids. An equalization tank buffers flow and pH peaks, a prerequisite for the downstream stages to run steadily.
2. Chromium recovery
The chrome bath is treated on its own: pH is raised with alkali to precipitate chromium as hydroxide, which settles and can be recycled back to tanning. This cuts chromium consumption and lowers the metal load of the combined effluent.
3. Sulfide oxidation
The unhairing stream is oxidized (by catalyzed aeration or chemical dosing) to convert sulfides into sulfates, preventing H₂S generation downstream.
4. Physicochemical (coagulation-flocculation-DAF)
Coagulants and flocculants aggregate colloids, fats and residual metals; a dissolved air flotation (DAF) unit or a clarifier separates them as sludge. This removes a large share of COD and nearly all suspended solids.
5. Biological treatment
The clarified effluent moves to a biological process —activated sludge or, where load and biodegradability allow, an anaerobic reactor first— to degrade soluble organic matter. Tech Tank's bolted tanks install fast and offer long service life against corrosion.
Benefits with data
A well-designed line achieves high, consistent removals. Typical ranges a physicochemical + biological train can reach:
| Parameter | Raw effluent (order of magnitude) | Expected removal |
|---|---|---|
| COD | 3,000–8,000 mg/L | 85–95% |
| BOD₅ | 1,000–3,000 mg/L | 90–97% |
| Suspended solids (TSS) | 2,000–4,000 mg/L | 90–98% |
| Total chromium | 50–200 mg/L | >98% (with recovery) |
| Sulfides | 50–300 mg/L | >95% (oxidation) |
Inlet values vary with the type of tanning (chrome vs. vegetable), the bath-to-hide ratio and plant practices. The design is tuned to the real effluent characterization, not to generic averages.
Discharge parameters and regulation
In Paraguay, effluent discharge is governed by MADES under Law 294/93 (Environmental Impact Assessment) and Law 3239/07 (Water Resources). Limits for chromium, sulfides, BOD, COD and solids depend on the receiving water body and each facility's environmental license. We recommend verifying current regulations and license conditions directly with MADES before setting the treatment quality target.
Why a turnkey approach
Tannery effluent punishes improvised designs: a poorly segregated system releases H₂S, drags chromium through or collapses the biomass. Tech Tank delivers the integrated project —engineering, tanks, equipment, installation and commissioning— so the plant runs within spec from start-up. See the full approach in our wastewater treatment (WWTP) solution and in the guide to turnkey wastewater treatment plants. To set the discharge target, review the wastewater discharge regulations in Paraguay (MADES) first.
Run a tannery and need to comply with MADES? Contact our team with your effluent characterization and we will propose a tailored treatment line.
Frequently Asked Questions
Why must the chrome bath be separated from the rest of the effluent?
Because segregating chromium lets you precipitate and recover it efficiently, recycling it back to tanning and sharply reducing the metal load of the combined effluent. Mixed with unhairing sulfides, separation becomes harder and more expensive.
Is H₂S a real risk in a tannery?
Yes. Acidifying sulfide-bearing streams releases hydrogen sulfide, a toxic, foul-smelling gas that is dangerous in confined spaces. That is why sulfides are oxidized or precipitated in a controlled way before the effluent pH is lowered.
Is biological treatment alone enough?
No. High salinity, metals and sulfides inhibit the biomass if the effluent enters raw. A physicochemical stage (segregation, oxidation, coagulation-flocculation) must come first; only then does biological treatment degrade the soluble organic matter.
What kind of tank suits this effluent?
Bolted steel tanks with a corrosion-resistant coating, such as glass-fused-to-steel or epoxy, that withstand the chemical and saline aggressiveness. They install fast, are modular and offer long service life compared with concrete attacked by the effluent.
How much COD can be removed?
A well-designed physicochemical plus biological train can remove 85% to 95% of COD, and over 90% of BOD and suspended solids. The exact result depends on the effluent characterization and the discharge target set with MADES.