Treatment of Tannery Wastewater
Tanning industry is one of the oldest industries in the world. It is typically characterized as pollutants generated industries which produce wide varieties of high strength toxic chemicals. It is recognized as a serious environmental threat due to high chemical levels including salinity, organic load (chemical oxygen load or demand, biological oxygen demand), inorganic matter, dissolved, suspended solids, ammonia, total kjeldahl nitrogen (TKN), specific pollutants (sulfide, chromium, chloride, sodium and other salt residues) and heavy metals etc. Large quantity of water is used in tanning process of which 90% of the water is discharged as effluent. A part of the leather processing, solid and gaseous wastes are also discharged into the environment. During the chrome tanning process, 40% unused chromium salts are usually discharged in the final effluents, causing a serious threat to the environment. Exposure to chromium, pentachlorophenol and other toxic pollutants increase the risk of dermatitis, ulcer nasal septum perforation and lung cancer. Without any exceptions there is no effluents treatment plant (ETP) in leather tanning industries in the country and moreover, the owners of tannery industries are not much concerned about human health and environmental safety.
he wide range of toxic effects on the environment caused by untreated tannery effluents and sludges.
The following treatment steps are necessary and will be described in more detail afterwards;
- Mechanical treatment
- Effluent treatment
- Post- purification, sedimentation and sludge handling
- Chemical treatment (coagulation, flocculation)
In chemical treatment, involving coagulation–flocculation and sedimentation techniques.FeCl3 as coagulant to lower sensitivity to water temperature variation, high removing efficiency for color, organic matter and heavy metals, which are of particular benefit to industrial effluent treatment;
Coagulation–flocculation is one of the most important physicochemical treatment steps employed in industrial wastewater treatment to reduce the suspended and colloidal materials responsible for turbidity of the wastewater.
Chemicals are added in order to improve and accelerate the settling of suspended solids, especially of fine and colloidal matter.
In wastewater treatment operations, the processes of coagulation and flocculation are employed to separate suspended solids from water.
Flocculation is the action of polymers to form bridges between flocs and bind particles into large agglomerates or clumps. In this process it is essential that the flocculating agent be added by slow and gentle mixing to allow for contact between the small flocs and to agglomerate them into larger particles. The newly formed agglomerated particles are quite fragile and can be broken apart by shear forces during mixing. Care must also be taken not to overdose the polymer as doing so will cause settling/clarification problems.
Once suspended particles are flocculated into larger particles, they can usually be removed from the liquid by sedimentation, filtration, straining or floatation.
The inorganic coagulants are compounds that break colloidal suspensions and help floc forming. The most frequently used coagulants in tannery effluent treatment are:
- alum: industrial aluminium sulphate Al 2 (SO 4 ) 3 • 18H 2 O
- iron sulphate: industrial FeSO 4 • 7H 2 O
- iron chloride: industrial FeCl 3 • 6H 2 O
- lime: industrial calcium hydroxide Ca(OH) 2
Coagulant aid – flocculants – are water-soluble organic (anionic) polyelectrolytes that support agglomeration of colloidal and very fine suspended matter thus enhancing the impact of coagulation;
Comparison with other treatment processes
the used chemical coagulants were ammonium aluminium sulphate (NH4Al(SO4)2), aluminium sulphate (Al2(SO4)2, calcium carbonate (CaCO3), sodium citrate (Na3C6HsO7), FeCl3, alums, and TiO2 as coagulants, MgSO4 and NiSO4 as oxidizing agent, FeSO4, and H2O2 in Fenton-photo process for COD, Cr and other cations removal.
Moreover, FeCl3 is inexpensive compare to TiO2, alum, sulfate salts and other commonly used coagulants.Aluminium sulphate and ferric chloride were used as a coagulant in the process. . The COD and chromium were removed mainly through coagulation , Ferric chloride produced better results than aluminium sulphate. So this treatment process could be an efficient, cost effective and eco-friendly process for the removal of pollutants from leather industrial wastewater (effluents)
Although the leather tanning industry is known to be one of he leading economic sectors in many countries, there has been an increasing environmental concern regarding the release of various recalcitrant pollutants in tannery wastewater. It has been shown that biological processes are presently known as the most environmental friendly but inefficient for removal of recalcitrant organics and micro-pollutants in tannery wastewater. Hence emerging technologies such as advanced oxidation processes and membrane processes have been attempted as integrative to biological treatment for this sense.