Tannery and Leather Wastewater Treatment: Soak, Lime and the Chrome Float

A tannery takes raw hides through soaking, liming, deliming and tanning to turn them into finished leather, and every one of those steps makes a distinctive waste stream. The soak float carries salt, blood and dung. The liming float is strongly alkaline with sulphide from the unhairing step. The chrome float carries trivalent chromium that must never reach the sewers mixed with sulphide. The effluent is high in organic load, lime, salt and, on chrome-tanned lines, a toxic metal that is easy to manage only if it is kept separate from the start. A tannery that segregates the streams treats a series of ordinary problems; one that mixes them treats a single impossible one.

Where the Water Comes From

Soak water is the first float and the saltiest. Raw hides are washed to remove blood, dung and preservative salt, and that water is high in chloride and dissolved organics from the start.

Liming and unhairing make the strongly alkaline float. Calcium hydroxide plus sodium sulphide loosens the hair, and the spent lime carries both the sulphide and the dissolved protein from the destroyed hair. For the reuse angle specifically, Chewing Gum Manufacturing effluent goes further than we can here.

Deliming and bating neutralise the lime and prepare the pelt. These streams are less caustic but still loaded with ammonium from the deliming salts and the enzymes used to bate.

Tanning is where the character splits. Vegetable tanning uses tannin liquors; chrome tanning uses chromium salts, and that float is recovered and reused rather than discharged wherever the plant can manage it.

The lime float is the most hazardous ordinary stream. Sodium sulphide is toxic to biology and to the operators, and it has to be destroyed or oxidised before the stream reaches a treatment plant.

Oxidation with air or peroxide converts sulphide to sulphate and thiosulphate. A well-run oxidation step removes the acute toxicity and leaves a stream the biology can handle.

The dissolved protein from unhairing is a strong organic load. It is biodegradable once the sulphide is gone, and it contributes most of the BOD of the liming float.

Lime also raises the hardness and the alkalinity of the combined stream. That has to be accounted for in the chemical dosing, because the precipitation step depends on the pH window being right.

The Chrome Float

Chrome tanning liquor is recovered wherever possible. The float is circulated and topped up, and only the bleed-off reaches the drain, which keeps the chromium load small.

What Our Project Data Actually Shows

Across 42 commissioned plants where we hold complete COD records, influent COD ranged from 200 to 172,000 mg/L and treated effluent from 50 to 5,000 mg/L. Average removal across that set is 87.1% — not a marketing figure, but the measured mean.

That average is worth pausing on. It sits well below the 95%-plus numbers most suppliers quote, because the set includes genuinely difficult streams. On some electroplating and municipal duties the installed configuration only reaches about half the influent COD, and saying that up front is more use to you than a number the plant will never hold.

Trivalent chromium is the form used in tanning, and it is far less hazardous than hexavalent — but it is still a metal that precipitates only in a narrow pH band and must be kept out of the sulphide stream.

Mixing chrome with sulphide is the classic mistake. Under the wrong conditions chromium reduces and the sulphide oxidises, and the result is a sludge no one wants to handle or discharge.

Segregated chrome is precipitated with alkali and the sludge is sent for metal recovery or licensed disposal. Treating it separately is cheaper and safer than diluting it into the main flow.

Salt, Chromium and Biology

After segregation and chrome recovery the main stream is an alkaline, high-organic, high-salt liquor. Biology handles the organic part provided the salt is not past its tolerance.

Salt is the ceiling. Above a certain chloride concentration the biomass is suppressed, and the design has to either dilute or select a halotolerant culture — pretreating the soak water helps here. Where several streams need joint treatment, glucose syrup production wastewater treatment is the more detailed reference.

A two-stage biological scheme with good equalisation absorbs the pH and load swings. The liming and deliming floats arrive at opposite ends of the scale and a mixed tank does much of the neutralising.

Sludge is the catch. Tannery sludges are often classified as hazardous because of the chromium content, and the disposal route has to be confirmed before commissioning, not after. If your site also runs this kind of duty, Lactose Production Wastewater Treatment system covers the cost side of the decision in more depth.

Operating Notes

Segregate the chrome float. Mixing it with sulphide creates a problem no ordinary plant can treat, and keeping it separate is the single biggest win.

Destroy the sulphide before biology. Oxidation removes the acute toxicity and turns a dangerous float into a treatable one.

Watch the salt ceiling. The soak water sets the chloride load, and the biomass has a limit that the design has to respect.

Where This Fits in the Wider Plant

Plants dealing with comparable loads face many of the same trade-offs covered in Casein Production Wastewater Treatment system. Most facilities do not run a single clean stream, and a shared equalisation and biological stage is usually the economical answer once the streams are chemically compatible.

Why Choose Baihuipu as Your Manufacturer

Picking a supplier for this kind of plant is mostly about who carries the risk when something goes wrong. We have built this equipment for more than twenty years, and we are the ones who fabricated it, programmed the controls and will still answer the phone three years from now.

Manufacturing Base in Guangdong

Owned production lines mean we control tolerances, materials and delivery. Standard modular units and bespoke skids are both built by our own people, so schedule commitments are ours to keep.

Experience Built the Hard Way

Since the early 2000s we have commissioned plants for food and beverage processors, chemical works, electroplaters, textile mills, mines and municipalities. The lessons that stick are scaling, fouling and sludge handling — those inform every design we issue.

Certification and Export Practice

CE marking is standard and we work to ISO 9001 procedures. Where a project calls for specific material grades, pressure vessel certification or ATEX-rated equipment, we build to that standard and ship the full documentation pack.

We Engineer the Whole Train

Screening, equalisation, biological or chemical treatment, membranes, evaporation, brine handling — the complete line comes from one organisation. Process design, mechanical, electrical and PLC programming sit under the same roof, so there is no gap for finger-pointing at handover.

After the Handover

Spares for everything we install are held in stock. Remote telemetry is available on most systems, and our service engineers commission, train your operators and turn out for emergencies.

Documentation You Can Actually Audit

Each system ships with as-built drawings, material certificates, welding records and a functional specification. If a client's own consultant wants to review the process calculation basis, we hand it over rather than treating it as proprietary.

Talk to Our Engineers

Send through your raw water data and we will advise on the viable process route and realistic operating cost before you commit to anything. We can be reached on WhatsApp: +86 13631765076 or through hkbhp.com.

WhatsApp: +86 13631765076

Frequently Asked Questions

Which discharge standard will the treated water meet?

We design against whatever consent applies at your boundary — GB 8978, local sewer limits, or a ZLD target with no liquid discharge at all. The design basis is agreed in writing before fabrication, so there is no argument later about what "compliant" meant.

Can you work with our existing equipment?

Often yes. We regularly integrate into existing civil structures where the concrete is sound, replacing internals and controls rather than starting over. We will tell you honestly when legacy kit is beyond economic repair.

Can the plant be modified later if our production changes?

Yes, if you tell us about likely expansion up front. Leaving room in the civil works and oversizing the equalisation tank costs far less than retrofitting later. Tannery and Leather Wastewater Treatment rarely fails because of bad chemistry — it usually fails because nobody planned for the second production line.

How do you handle sludge from this process?

Sludge handling is where most budgets quietly blow out. We size dewatering against realistic solids production, and we usually recommend a filter press for this duty. The disposal route — landfill, incineration, or a licensed contractor — should be confirmed before commissioning.

Do you handle installation and operator training?

Every unit we supply is commissioned by our own engineers. Training happens on site with your actual operators, and the O&M manual covers routine work plus troubleshooting. Remote diagnostics through the control system are included for the first year.