Today’s leather tanneries use skins which vary in type and quality and therefore require a range of tanning processes. This results in a wide range of both volume and content of tannery effluent streams which require clarification to meet discharge consent levels.

Case studies

The latest Krofta DAF clarifier to be installed in the leather tanning industry, a model SPC20 Supracell, was ordered by NCT leather, following extensive trials using pilot equipment.

Operating at the company’s Bridge of Weir tannery, it treats primary effluent consisting of mixed chrome and liming liquors from the process.

Clarified water from the Supracell is discharged for further biological treatment, the high throughput, short retention time of 2.5-3mins and minimal space requirement of the unit making it ideal for installation at tanneries where space is often limited. NCT also has a Krofta Supracell model SPC15 clarifier for tertiary polishing prior to final sewer discharge. With a capacity of 80m3/hr the unit reduces incoming suspended solids from 500mg/l to below 200mg/l.

At the Paisley plant of WJ & W Lang, Krofta has installed a model SPC12 skid mounted clarifier to treat the tannery’s tertiary effluent following a centrifuge. With an average suspended solids reduction from 1000mg/l to below 200mg/l the clarified water is clean enough for direct sewer discharge.

The COD requirement is reduced from 3500 to below 2500mg/l with chrome content being totally eliminated from a level of 6mg/l. Primary coagulant and secondary flocculant chemicals are employed in the process.

In Warrington, Cheshire, Eagle Ottawa Leather has a Supracell clarifier operating on its tannery effluent following aeration and balancing.

The model SPC20 unit was installed as a complete turnkey installation by Krofta and reduces the average suspended solids inlet from 10,000 mg/l down to below 500 mg/l.

Eagle Ottawa already had aeration and equalisation tanks which were incorporated into the system. The level control from the Supracell remains the equalisation tank level and an extra outlet was incorporated into the clarifier base for connection to a level control loop.

Floated sludge form the installation is handled by a centrifuge and the clarified water discharged direct to the sewer. A single shot chemical dosage of 20-30mg/l is used.

JJ Williamson & Sons uses a Krofta skid mounted model SP12 Supracell clarifier for final clarification of its tannery effluent following centrifuge treatment. This upgrading of the company’s effluent system is designed to improve the quality of final discharge and some re-work of existing tanks was completed to suit the application. The Supracell treats centrate, removing residual suspended solids whilst the sludge is returned to the centrifuge for additional thickening with clarfied water being discharged direct to the sewer.

The plant has a capacity of 60m3/hr with up to 50% recycle flow pressurisation to accommodate any sudden increases in solids loading. Average suspended solids inlet levels of between 2-3000 m/l are reduced to around 300 mg/l and BOD is reduced from 2500mg/l to 1000mg/l.

The latest Supracell order for clarifying tannery effluent is for an SPC20 at Holmes Halls Tanners in Hull, to be installed in January 2000. This unit will operate on primary effluent treatment and will replace existing rectangular flotation cells.

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