Thiocyanate degradation during activated sludge treatment of coke-ovens wastewater

The aim of this work was to determine the reaction pathway and kinetics of thiocyanate (SCN−) degradation during mixed culture (activated sludge) treatment of coke-ovens wastewater. The effect of phenol and cyanide, both present in coke-ovens wastewater, on thiocyanate degradation was also to be determined. Two 6 L perspex sequencing batch reactors (SBRs) were operated continuously for a period of 12 months, fed with coke-ovens wastewater from Port Kembla Steel Works. The results from 18 experimental periods are presented.The degradation of thiocyanate has been quantified by identification of specific rates of removal and oxygen uptake rate. The results indicate that thiocyanate is removed via microbial growth using thiocyanate as a substrate, and that SO42−, NH4+ and CO2 are the reaction products. Oxygen uptake rate data and sulphur and nitrogen balances indicate that this reaction was occurring.None of the results obtained showed inhibition of thiocyanate degradation due to phenol. However, phenol was degraded much quicker than thiocyanate, and thus phenol inhibition in coke-ovens wastewater treatment plants, even if it does occur, is not likely to be a significant issue. In contrast, cyanide has been found to have a significant inhibitory effect on the degradation of thiocyanate. This work suggests that thiocyanate removal may be totally inhibited in coke-ovens wastewater at concentrations of CN− in excess of 1 mg/L. Thiocyanate degradation is the slowest and most sensitive process, compared with the removal of phenol and cyanide, and will be the determining factor when identifying the hydraulic residence time required for treatment of coke-ovens wastewater (excluding nitrification).