Sensitivity Of Thiocyanate Acting On Compliance Control Of Coking Wastewater

Coking wastewater is a typical hard-biodegradable and high strength industrial organic wastewater with a considerable discharge amount. Current biological treatment systems are facing problems with low efficiency, high cost and undesirable effluent qualities of COD, NH₄⁺and chroma. Based on the operational data in A/O₁/H/O₂ coking wastewater treatment plants, thiocyanate was selected as an entry points to analysis its degradation reaction kinetics using activated sludge from real coking wastewater system. The effects of phenol on SCN^- degradation and SCN^- on nitrification were also studied in order to assess the interaction effect between SCN^- and other pollutants, so as to investigate the impact of the interactive reactions on the control of stable coking effluent. All of the efforts help to provide a theoretical basis for selection and optimization of coking wastewater treatment process and form an economic, efficient and stable biological treatment technology.Thiocyanate (SCN^-) is one if the highest amount of inorganic contamination in coking wastewater and has non-neglectable contribution to COD, chroma and NH₄⁺-N value, which also has an adverse effect on aquatic flora and fauna. Test results showed that 1.0 mg SCN^- represents 1.10 mg COD, and it is equivalent to 15% of COD in coking wastewater, which is regarded as the second source of COD just after phenal. Therefore, SCN^- must be included in the control of coking wastewater as key elements on COD removal.Using real coking wastewater and activated sludge from coking wastewater treatment plant to study thiocyanate biodegradation feature in complex water environment. Results showed SCN^- could be used by microorganism in aerobic conditions. It has been found that under specified culture condition, the degradation rate of SCN^- reached up to 20.15 mgSCN^-·gMLSS^-1·h^-1 and the sludge activity was not significantly affected by initial concentration of SCN^-. The SCN^- degradation process follows Michaelis–Menten model. Despite of its biodegradable characteristic substance, biodegradation of thiocyanate was lower and more sensitive than that of phenol. That is why SCN^- degradation is the rate-limiting step in coking wastewater treatment.Apply active sludge from decarbon part and denitrification part of coking wastewater treatment plant to study the inhibition effects of phenol on SCN^- degradation and SCN^- on nitrification. The experiment results indicated phenol has toxic inhibition on thiocyanate degradation which prolong the biodegradation time of thicyanate. If the concentration of phenol was high enough, the SCN^- degrading bacteria even could not acclimate in the system. Moreover, SCN^- inhibited the nitrification of NH₄⁺, especially for the conversion of nitrite to nitrate which resulted in the accumulation of NO₂^-.It would prolong nitrification time and result in accumulation of NO2- which causes trouble in nitrification construction and optimization.Due to inhibitory interaction among thiocyanate, phenol and nitrification, coking wastewater treatment became more difficult and complicated. When SCN^- is exposed in initial coking wastewater containing high concentration of phenal and other toxic and inhibition pollution components, the whole treatment systrem would be difficult to control and unstable, and the HRT would be prolonged, which result in higher project investment and operating costs. As a conclusion, the pollution control of coking wastewater should identify the interaction effects among main contaminants, consider about the co-substrate effect or poisonous effect between the adaptation of sludge activity and pollutants concentrations,to find a right process to avoid these inhibitions so as to establishing a stable efficiently treatment system for coke-ovens wastewater.