Flocculation, or the mixing of flocculants with the wastewater, is carried in tanks fitted with agitators, as shown in illustration 8.
In small installations the sedimentation tanks can be made of brick square with the bottom of a truncated pyramid, or in metal or fiberglass with vertical cylindrical shape with a conical base; in larger installations the basins are usually made of masonry rectangular or circular, and equipped with mechanical systems of conveying and collecting the sludge (scraper bridges with central or peripheral traction, or to-and-fro bridges).
TREATMENT OFTANNERY WASTEWATER 1 Presented by SUSAN ANN JAYAN II M.SC., BiotechnologySchool of Biotechnology & Genetic Engineering Bharathiar University
An aerial view..medium-sized tannery wastewater treatment plant 2
Tanning industry 3What is tanning? Tanning is the process that converts a part of a once living animal (i.e., hides and skins) into a non-rotting, soft and durable textile called leather Two types of tanning, broadly : Vegetable tanning Chrome tanning Procedures involved.. After removal of hairs and flesh, the involved processes are curing, receiving and storing, soaking, unhairing, bating, pickling, tanning, wringing, sorting, splitting and shaving.
Chemicals used in a tannery 4LEATHER PROCESS CHEMICALS USEDCuring & Preservation Sodium chloride, pentachlorophenolSoaking, Liming & Unhairing Sodium sulphide, caustic soda, surfactants, milk of limeDeliming Ammonium chloride, ammonium sulphateDegreasing Alkyl phenyl ethoxylatePre-tanning Basic chrome sulphateTanning/retanning Mineral tannages/retannagesFat liquoring Synthetic at liquors, surfactants from petrochemicalsFinishing Cadmium and lead chrome pigment, nitrocellulose and liquor emulsion water-proofing agent
TREATMENT REGIME 7 Combination of physical, chemical and biological systems 1. Separation of effluents within the tannery Wastewater collection within departments to maintain separate beamhouse effluents By means of gutters and sluice gates 2. Hair recovery from liming baths To reduce pollution load considerably Special passages of very fine screens Recycling outside the drum Mechanical separation effectively reduces concentration of suspended solids 30-40% & COD 20-30%
3. Chrome recovery from Tanning baths Drastic reduction of heavy metal load 9 Sludge containing high amounts of chromium is usually disposed off in landfills Re-use of expensive chromium – Add alkaline lime to acidic tanning bath and adjust pH to slightly above neutrality – bivalent chromium will completely settle 4. Screening operations Fundamental Coarse grid bars Filtering at 10mm Prevents clogging by coarse pieces of leather, wood, etc. Rotating brush Filtering at 2-3mm diameter
12 5. Equalization & Sulfide oxidation Necessary due to unequal discharge of effluents Sulfide oxidation carried out and the contents are kept mixed Manganese sulfate may be added to speed up, acting as catalyst
14 6. Flocculation Chemical flocculation for suspended solids & unyielding colloidal organic matter Flocculants – metal salts - ammonium sulfate, most effective organic coagulants (polyelectrolytes) – increase size of sludge flakes
Flocculation in tanks fitted with agitators 15
16 7. Sedimentation Gravitational settling of suspended solids, then disposal of clarified supernatant into sewers Sedimented sludge is collected by pumps for dehydration 8. Sludge treatment Natural drying beds Sludge is distributed in layers a few cm high and loses water by filtration on a bed of sand and gravel and natural evaporation Frequently used in geographical zones with dry hot climate Mechanized systems, e.g., filter presses, continuous belt presses, centrifugal decanters Used when the quantity of sludge produced each day is high, because the area needed would be excessive
17 9. Biological treatments To reduce the soluble and suspended or colloidal organic matter in effluents into carbon dioxide and minerals Bubble aeration systems Lagoon systems Sequencing batch biofilm reactor (SBBR) Combined with chemical oxidation by ozone. The treatment was carried out at laboratory scale on a real primary effluent coming from a centralized plant treating the wastewater of a large tannery district in Northern Italy  Microbial consortia Fungal and bacterial biosorptive degradation Bacillus, Rhizopus, Aspergillus, Streptovertcillum, Saccharomyces
18 10. Other innovative methods Use of activated clays in the removal of dyes and surfactants The capacity of sepiolite and acid-activated bentonite to adsorb anionic dyes normally used in the tannery was compared with conventional adsorbents Natural bentonite activated with 0.5 M H2SO4 was the most effective adsorbent for ethoxylated nonylphenol. Use of electrochemical processes as alternative to biological nitrification/denitrification processes, for removal of a variety of pollutants And their combinations are still being studied Quicker and reduction in global plant volume (considering both biological and electrochemical sections) and sludge generated for disposal
References 191. http://www.fgmengineering.it >wastewater treatment>tannery2. Thakur S. T., Environmental Biotechnology – Basic Concepts and Applications. I.K. International Pvt. Ltd; 398-4083. C. Di Iaconi , A. Lopez , R. Ramadori , and R. Passino. Tannery Wastewater Treatment by Sequencing Batch Biofilm Reactor. Environ. Sci. Technol., 2003, 37 (14), pp 3199–32054. A.G Espantaleón et al.,(2003). Use of activated clays in the removal of dyes and surfactants from tannery waste waters. Applied Clay Science, 2003,24 (1-2), pp 105-1105. Lidia Szpyrkowicz, Santosh N. Kaul, Rao N. Neti. Tannery wastewater treatment by electro-oxidation coupled with a biological process. Journal of Applied Electrochemistry (2005) 35: 381-390