Coking wastewater contains toxic compounds such as ammonia, phenolics, cyanides, thiocyanate, and thiosulfate. In normal situation, these compounds can be 100% decomposed by activated sludge in the wastewater treatment plant. However, a new factor, high water temperature, caused by the covering of the basins, has been found to reduce the removal rate of the pollutants, especially the thiocyanate.
Several remediation measures were applied to improve the biological process performance. A series of batch and continuous-flow biodegradation tests have demonstrated that the relatively low temperature ranging from 20 to 37°C attained perfect removal efficiency (∼100%) of SCN− from the coke plant wastewater. When the temperature is increased to 38, 39°C, the SCN− removal rate decreased to 30 ∼ 50% after 48 hours of temperature shift. Moreover, when the temperature is increased to 40 to 45°C, SCN− biodegradation is significantly decayed one day after the temperature shock, and the SCN− removal efficiency dropped gradually to 0%, with microbial deterioration during day 1 to day 4. Meanwhile, the COD removal has also decreased from 85% down to 45% at 45°C. Both laboratory verifications confirmed that 40°C should be the critical water temperature for the SCN− degrading microbe to function well in the activated sludge process.
In order to stabilize the pollutant removing ability of the procedure, the water temperature of aeration basin is set to operate under 37°C. Four modifications were practiced on site to decrease the temperature of activated sludge. Only the last alternative measure worked well with the opened hand hole, 50% of total cover area, to discharge the water vapor and gas emission from the head space of aeration basin. The water temperature has dropped from 41 to 36°C in the early summer of June with ambient temperature of 37°C, suggesting that partially uncovering the basin does help maintain the water temperature.
Finally, we suggest some measures that can be applied to establish a standard process on site in order to ensure that the aeration basin temperature stays under 37°C and that our biotreatment system can maintain its function in the future.