Tanneries for leather processing have high levels of water consumption. Some estimates are that to make good quality leather from one tonne of raw material, up to 60 m³ of water is required. This volume of wastewater contains high levels of toxic chemicals and organic substances that cannot be directly discharged into the local sewer treatment plant (STP).
Colourful Leather Produced In A Tannery
Tanneries discharge three quite distinct wastewater streams:
- sulphide stream
- chrome stream
- general wastewater
Environmental protection regulations stipulate that industries are not allowed to discharge sulphide or chromium in the wastewater. Thus, removal of these substances from the wastewater is very important.
Sulphides are used in tanneries for dehairing the hides, but it causes the irritating, rotten-egg smell. It also causes headaches, nausea and effects the central nervous system of humans. It is known to be corrosive to the local STP pipelines and can disrupt downstream biological treatment process. Chromium salts are used during the tanning process and is toxic to living organisms and for the anaerobic digestion process.
Conventional treatment for a sulphide stream is oxidisation using a catalyst and oxygen. It is common to use manganese oxide as the catalyst and air is introduced into a series of oxidation tanks. In the presence of this catalyst, sulphide is removed in a catalytic chemical reaction producing elemental sulphur and water. Elemental sulphur is a non-toxic and non-corrosive solid that can be used for feedstock for the chemical, fertiliser and materials manufacturing industries.
The treatment of wastewater containing chromium relies on the solubility and precipitation of chromium hydroxide at a certain pH. The relative solubility of chromium in water varies over a range of pH values. The maximum insolubility of chrome is at a pH of 7-8 where the soluble concentration is <0.1 mg/L. There are however complex compounds present in wastewater that affect the chromium solubility.
Chrome Sludge Being Scraped Off The Surface Of The DAF
After undergoing dedicated treatment, the wastewater will be combined with the general waste streams in a balance tank and treated in a DAF. Treatment for suspended solids (TSS), sulphates, ammonia, oil and grease (O&G), and Chemical Oxygen Demand (COD) will occur. Removal of TSS in a wastewater treatment plant will by default reduce the total COD, O&G and N due to the associated insoluble component of these substances.
About the Hydroflux Group
The Hydroflux Group comprises eleven companies based in Australia, New Zealand, the Pacific, and the UK, providing design and build, equipment, processes and operational services in water and wastewater treatment. Hydroflux Industrial specialises in industrial wastewater treatment including designing and constructing plants and supplying equipment across all sectors. Hydroflux Industrial is a member of the Hydroflux Group.
Hydroflux Industrial Pty Ltd
Level 26, 44 Market St. Sydney NSW 2000 Australia
P +61 2 9089 8833 F +61 2 9089 8830