Reduction of chemical oxygen demand in a conventional activated sludge system treating coke oven wastewater

Abstract

The best available techniques reference document for iron and steel production sets a strict limit value for the treated effluent of coke oven wastewater treatment plants regarding Chemical Oxygen Demand (COD). The objective was to find options to decrease residual COD, in order to safely meet the threshold for a specific plant. Membrane filtration experiments showed that the remaining COD can only be reduced by 6–27%, indicating that the majority of compounds are dissolved, not suspended. Inert COD measurements of the effluent showed that 82–96% of the residual COD is non-biodegradable. Of the approximately 30 different organic substances found in the influent, Gas Chromatography - Mass Spectrometry (GC-MS) analysis detected, two-thirds were also present in the biologically treated wastewater. These results proved that the activated sludge treatment step is operating at its maximum achievable efficiency, thus further polishing steps had to be investigated to achieve the required COD. A combined physico-chemical post-treatment of the biologically treated effluent showed outstanding results and managed to decrease the COD to below 220 mg/l in all tested samples. This solution could be used universally in those coke oven wastewater treatment plants that are experiencing issues with levels of residual COD in their discharge.