Treatment of Coke oven effluents – An overview of the effluent treatment plant for Coke oven plant, India

WABAG executed an Effluent Treatment Plant for DPL’s Coke oven plant located at Durgapur, West Bengal, India in 2010. The primary function of the Effluent Treatment Plant is to render the harsh coke oven effluents for discharging to the environment compliant with regulatory requirements. Coke oven effluents are one of many complex effluents that contain harsh pollutants in forms of cyanides, thiocyanates, phenols, sulphides and ammonia in addition to BOD and COD. These effluents demand a very efficient and robust treatment scheme before disposal to environment.

The production of coke involves the carbonization of bituminous coal by heating it in the absence of air at temperature range 900-1000 deg.C in an oven which drives off all the volatile portions in the coal. The gas which is evolved containing the volatile matters is collected through stand-pipes and is cooled in stages by spraying cold liquor over the gas, thereby producing mainly tar as the condensate. Further cooling processes produce condensate containing additional tar and ammonia liquor. These condensate liquors after the separation of tar in a tar-decanters, are recycled back. The excess liquor known as “ammonia liquor”, containing mainly ammonia and tar is sent for treatment. Principally the effluent streams generated from a Coke Oven Plant are from Ammonia scrubber, Tar separator, NO-Oxidizer plant and Quenching Tower.

Different trains of effluents totaling to 1000 cu.m/day comprising contaminated streams from scrubbers, coal tar decanters, etc. are treated in the effluent treatment plant employing a combination of physico-chemical treatment (clarification, stream stripping and alkaline chlorination) followed by a two stage biological treatment (Extended mode activated sludge process) is a robust and economic option for the treatment of effluents from coke-oven plants. This plant is an example of one of its kind for treatment of complex effluents. The very complex nitrogen compounds such as cyanide and thiocyanates in addition to high concentrations of ammonia can be reduced with appropriately designed stage wise treatments. The effluents are segregated and treated, viz., ammonia scrubber and tar separator effluents are equalized and clarified in tar clarifier for removal of floating tar, oils and grease, steam stripping of ammonia is carried out in Ammonia stripper, NOx gas plant effluents are added and treated in two-stage alkaline chlorination for removal of cyanide and thiocynates followed by oxidation of sulphides by hydrogen peroxide dosing, pH adjustment is done by adding quenching water and necessary acid/alkali. Then two-stage activated sludge biological treatment is adopted for biological degradation of pollutants such as BOD/COD and soluble phenols and Nitrification/Denitrification of balance ammonia. Lot of operational issues were kept in view at design and engineering stages of plant and ensured that the overall plant operation is not only economic but at technical ease.