Treatment Of The Coking Wastewater By New Microbial Immobilization Process

Coking wastewater is difficult to tackle due to its being complex, toxic and containing a large number of refractory organic matters. The traditional biological treatment or a single wastewater treatment technology is hard to meet the national effluent discharge standard for COD and ammonia-nitrogen in the coking wastewater.Based on the previous works, the immobilization of microbial technology combined with A-A-O process was introduced into this project. The A-A-0 process is a relatively wide range of coking wastewater treatment process, but the disadvantages of this process are microbial survival environments on the demanding, lower microbial activity and a large quantity of residual sludge. Immobilized Microbial Technology can provide strong adaptability and less residual sludge, which will hopefully make up the shortage of the A-A-0 process.In this paper, the improved microbial immobilization technology with the A-A-O process was investigated to remove the COD and ammonia - nitrogen in the coking plant wastewater. It contains the following contents:The activity of immobilized beads, which is produced by optimized PVA-H₃BO₃ embedding without domesticated activated sludge, can be recovered in about 10 days. As was shown in the experiments, the water-soluble expansion of the immobilized beads was improved comparing with the beads obtained by common method. The short lifespan of immobilized beads was also disappeared by the adoption of optimized technology. After cultivated three months, immobilized beads could be in good shape. The mechanical strength of the immobilized beads is shown as follow: the new beads prepared with ordinary methods < the new frozen beads < the common beads cultivated in 30 days < the frozen beads cultivated in 30 days.Under the same conditions, the cooking wastewater was treated with the activated sludge and the immobilized beads, respectively. The immobilized beasds had a better capability of organic degradation than activated sludge, particularly in dealing with high concentration of wastewater. Immobilized beads remained a relatively high biological activity, the COD and ammonia-nitrogen removal rate were higher. When the influent COD concentration range from 463.5 to 2654.9 mg/L, ammonia-nitrogen concentration from 75.16 to 388.0 mg/L, the removal rate of COD ammonia-nitrogen were stable at between 90% and 98%, around 90%, respectively, and the highest ammonia-nitrogen removal efficiency was 95.4%. The results shows that the frozen beads have a higher tolerance to the coking wastewater.The optimum operating parameters for the new microbial immobilization process were obtained in this paper: temperature 25 to 35°C, reflux ratio 4.0, hydraulic loading 1.8 m³/(m³d), pH 8.0 in aerobic pond. When the system was operated normally, the qulity of effluent meets the national sencond effluent discharge standard, and the average removal rate of COD and ammonia-nitrogen were 93.6% and 94.4% accordingly. The strong tolerance of this system is suggested as the reason for high removal efficiency. And NH₄⁺-N removal only occurs in aerobic stage, COD removal efficiency in the anaerobic, anoxic and aerobic stages were of the 16.9%, 57.1% and 20.3% correspondingly. Removal of organicals in anaerobic stage was low, but it greatly improved the biodegradability of organic compounds, and provided a carbon source and reduced the cost of wastewater treatment. Anoxic stage contributes greatly to the total removal of COD. Most of organic compounds have been degraded after anaerobic, anoxic treatment, when the wastewater flowed to the aerobic tank, the COD concentration is about 200-300 mg/L, which is benefit for nitrification.The comparative experiments were conducted between the immobilized microorganisms in the new technology and the A-A-0 process. COD removal from the A-A-O in the influent concentration of 875.0 mg/L was up to 89%, with the increase of the influent COD concentration, COD removal efficiency would be gradually declined, and it comes to below 60% when the influent COD concentration was 2340.2 mg/L. When the organic loading rose suddenly, the A-A-0 process can be recoverd to nomal operating conditions in 5-6 days while new technology is only 3 days. The A-A-0 process without sedimentation tank has more free suspended sludge in the effluent. The COD removal for immobilized microorganisms of new technology maintained at more than 90%, also the effluent was more than clarify without the muddy separation, which can save the sedimentation tank and the sludge treatment follow-up. In HRT 13.3 h, influent COD and nitrogenous compounds concentration of 2316.1 mg/L and 300.5 mg/L, the A-A-O process is difficult to meet the requirment of national effluent discharge standard, but the new technology still maintained a good effect, it is a promising way to treat coke plant wastewater.