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BRINGING DOWN COD LEVEL IN PICKLING WASTEWATER

2007

WE ARE HAVING A VERY HIGH COD LEVEL IN OUR WASTEWATER (PICKLING). tHE MAJOR ACIDS USED ARE HCl, NITRIC AND HYDROFLUORIC ACID. WE ARE NEUTRALISING THE DRAINED WEAK ACID AND RINSE WATER WITH caustic soda [affil links] LYE. WE ARE KEEPING CONTINUOUS AGITATION IN EFFLUENT TREATMENT TANKS AND DRAINING AFTER ATTAINING Ph 7. THE COD LEVEL IS IN THOUSANDS IN FILTER PRESSED WATER AND WE HAVE TO BRING IT BELOW 250. HOW CAN WE DRASTICALLY BRING DOWN THE COD LEVEL?

THANKS,
REGARDS,

VIJAYAN NAIR
PICKLING PLANT INCHARGE - MUMBAI, MAHARASTRA, India

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Not enough information on the waste stream and treatment. Also pH of 7 is extremely low for precipitation of metals; is this perhaps an adjusted value after filter pressing at pH 9.5?

L. Hartinger's book, Handbook of Effluent Treatment & Recycling for the Metal Finishing Industry [affil link on Amazon], 2nd Edn., pages 275-283 covers COD. The use of HF suggests the pickling of stainless steel, so perhaps an excess of sulfite or Fe(+2) was used for chemical reduction of Cr(+6). If so, perhaps use tighter ORP controls. Hydrogen peroxide is one of the 'cleanest' ways to raise ORP and lower the COD.

Besides, sulfite and Fe(+2), other sources of COD include thiosulphate, nitrite, Cu(+1), oils and greases removed during cleaning, thiocarbamate used for precipitation of metals, pickling inhibitors, surfactants, brighteners and polymer flocculant.

Ken Vlach [deceased]
- Goleta, California
Finishing.com honored Ken for his countless carefully researched responses. He passed away May 14, 2015.
Rest in peace, Ken. Thank you for your hard work which the finishing world, and we at finishing.com, continue to benefit from.
2007