코크스폐수 처리를 위한 사전탈질공정 내에서 시안 및 황화시안 농도변화가 미생물 군집 및 활성에 미치는 영향
- 코크스폐수 처리를 위한 사전탈질공정 내에서 시안 및 황화시안 농도변화가 미생물 군집 및 활성에 미치는 영향
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- The steel industries generate large amount of wastewater during the manufacture and processing of iron. Particularly, coke plant wastewater contains high concentration of toxic pollutants such as phenols, thiocyanate, ammonia and cyanide, thus it must be treated before discharging into environments. Traditional treatment of this wastewater utilizes expensive caustic treatment and steam stripping to reduce the contaminant load, followed by conventional biological treatment. Although the activated sludge processes of coke plant wastewater are adapted for the treatment of this wastewater, the high concentration of pollutants is known to be toxic to the activated sludge. Therefore, many researchers have studied how to efficiently treat the coke plant wastewater biologically. Among many proposed processes, the pre-denitrification process has been preferred in Korea due to its simplicity and cost efficiency.
Although the full-scale pre-denitrification process has been successfully operated for a long time without any supply of nutrients, sudden failures in nitrogen and carbon removal have occasionally occurred. To clarify the main cause of these sudden failures of the process, we conducted comprehensive studies on the pre-denitrification process using a lab-scale reactor system with real cokes wastewater and investigated bacterial populations relevant to nitrification in a full-scale cokes wastewater treatment plants according the variation of seasons.
Firstly, the shock loading effects of three major pollutants were investigated individually. As the loading amount of phenol increased to 600 mg/L, more COD, TOC and phenol itself were flowed into the aerobic reactor, but phenol itself did not inhibit nitrification and denitrification, owing to the effect of dilution and its rapid biodegradation. Higher loading of ammonia or thiocyanate slightly enhanced the removal efficiency of organic matter, but caused the final discharge concentration of total nitrogen to be above its legal limit of 60 mg-N/L. Meanwhile, continuous inflow of abnormal wastewater collected during unstable operation of the full-scale pre-denitrification process, caused a sudden failure of nitrogen removal in the lab-scale process, like the removal pattern of the full-scale one. This was discovered to be due to the lack of inorganic carbon in the aerobic reactor where autotrophic nitrification occurs.
Cokes wastewater generated through coal coking in the iron and steel manufacturing industries typically has high concentrations of SCN-. Although significant information has been published about the effects of biological treatment using SCN-, no study of the effect of a SCN- shock loading occurrence on the process performance and microbial population has yet been conducted Therefore, changes in process performance and the nitrifying bacterial community associated with an increase of thiocyanate loading were investigated in a pre-denitrification process treating coeks wastewater. The increased SCN- loading led to the concentration of TN in the final effluent to increase to around twice the legal discharge level. However, increasing the internal recycling ratio as an operation parameter from 2 to 5 resulted in a 21% increase in TN removal efficiency, reducing discharge to below the regulation level of 60 mg-N/L. This improvement was achieved by decreasing the nitrate ion fraction in the effluent. The ammonia oxidizing bacterial (AOB) and nitrite oxidizing bacterial (NOB) communities in the aerobic reactor were analyzed by terminal restriction fragment length (T-RFLP) and quantitative real-time PCR (qPCR). We found that the Nitrosomonas europaea lineage was the predominant AOB and the Nitrosomonas nitrosa lineage was a minor population. The percentages of the AOB population within the total bacteria increased from about 4.0 to 17% in the aerobic reactor with increased SCN- concentration. We also observed coexisting Nitrospira and Nitrobacter genera for NOB. The increase of nitrite loading seemed to change the balance between Nitrospira and Nitrobacter, resulting in the high dominance of Nitrospira over Nitrobacter. Meanwhile, a Thiobacillus thioparus detected from a denaturing gradient gel electrophoresis (DGGE) band was suggested to be the main microorganism responsible for the SCN- biodegradation observed in the system.
The presence of cyanide in cokes wastewater leads to severe problems for biological wastewater treatment. It is known to inhibit nitrification and denitrification in activated sludge systems, with only 1−
2 mg/L of free cyanide. Therefore, the changes in process performance and microbial communities under free cyanide (CN-) were investigated in a lab-scale activated sludge process. The performance of phenol degradation did not appear to be adversely affected by increases in CN- concentrations. In contrast, CN- has been found to have an inhibitory effect on SCN- biodegradation, resulting in the increase of TOC and COD concentrations. Nitrification also appeared to be inhibited at CN- concentrations in excess of 1.0 mg/L, confirming that NOB is more sensitive to the CN- toxicity than AOB. After CN- loads were stopped, SCN- removal, denitrification, and nitrification inhibited by CN- were recovered to normal performance. The AOB and NOB communities in the aerobic reactor were analyzed by T-RFLP and qPCR. The Nitrosomonas europaea lineage was the predominant AOB at all samples during the operation, but an obvious change was observed in the diversity of AOB at the shock loading of 30 and 50 mg/L CN-, resulting in Nitrosospira becoming dominant. The increase of CN- loading seemed to change the balance between Nitrospira and Nitrobacter, resulting in the high dominance of Nitrobacter over Nitrospira. Meanwhile, through using the qPCR, we observed that the nitrite-reducing functional genes (i.e., nirS) were dominant in the activated sludge of the anoxic reactor, regardless of CN- loads.
Meanwhile, surveys of microbial community in full-scale cokes wastewater treatment plants were conducted to further understand nitrifying processes under stress. The increase of phenol concentration in the raw wastewater might result in the poor nitrification performance. The number of AOB population at the inhibited nitrification point was much smaller, compared to the population number at the normal performance. However, the number of nitrifying bacteria did not directly correlate with nitrification activity. The number of AOB population increased at higher thiocyanate loading, and the increase of the Nitrospira population was observed when the full-scale process showed good nitrification performance. Meanwhile, the presence of the members of the thiocyanate-degrading Thiobacillus gene was proved in the activated sludge.
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