| Currently, eutrophication caused by nitrogen and phosphorus becomes more and moreserious. Biological nitrogen and phosphorus removal processes and technologies are the keyand difficulty of study in wastewater treatment field all over the world. Cyclic ActivatedSludge Technology is a preferred process due to the capability of simultaneously removingnitrogen and phosphorus, as well as simple process and flexible operation, etc. However，itsapplication in China shows a low nitrogen and phosphorus removal efficiencies, especially forthe sewage of low COD/TN (COD/TN﹤8) real domestic wastewater. Shortcut biologicalnitrogen removal technology is favored by many researchers due to its low energyconsumption t, quick reaction and small sludge production. However, hard to maintainshortcut nitrification is a limited factor for its application. Ideally, it will be of significance inthe operation and management of the real wastewater treatment plant if shortcut nitrificationis achieved in CAST process. In the view of low nitrogen and phosphorus removal efficiencyin current wastewater treatment plants, the real domestic wastewater was employed in thisstudy to enhance the process performance of CAST based on the optimization of operationmode. The influence of operating parameters on nitrogen and phosphorus removalperformance of the enhancing process, such as low temperature, influent organic load, thesludge recycling rate and the type of carbon source were also investigated, based on whichshort-cut nitrification in the CAST was achieved and maintained.Experiments on the optimization of operation mode showed that the startup period of thetraditional CAST operated with feed/aeration mode was30d, whereas the modified CASToperated with step feed Anoxic/Oxic (A/O) mode needed a shorter time(18d) to startup thereactor and the effluent nutrients concentration could meet the national wastewatercomprehensive discharge standard level A. Moreover, sludge sedimentation performance wasgood and the aeration amount could save up to30~40%. Experiments of two operationalCAST at low temperature in winter showed that, total nitrogen (TN) in effluent of the twosystems increased because of the incomplete oxidation of ammonia nitrogen. Serious sludgebulking and massive sludge loss occurred, which resulted in a poor nutrient removalperformance. In the modified CAST system, however, slight sludge bulking was found. The removal of organic matter and phosphorus was not affected. As a result, the modified processwas proved to have a strong ability of resistance to temperature shock. Under the condition oflow temperature, extending aeration time could only recover nitrification performance andpartly improve the sludge settling performance. The combination increasing temperature andextending aeration time could improve both the total nitrogen removal and sludge settlingperformance quickly.The experimental results of influent organic loading showed that increasing influentorganic loading could improve nitrogen and phosphorus removal efficiency of the modifiedCAST in a short term and nitrogen removal efficiency could reach93.8%. Nevertheless,long-term feeding easily biodegradable carbon source could cause sludge bulking. Thispaper also investigated the effect of carbon source type (methanol, ethanol, acetate) andsludge load on nitrite accumulation and effluent pH value during denitrification process. Itwas found that the specific denitrification rate (rDN) and the effluent pH value of differenttypes of carbon source expressed as rDN, acetate>rDN, ethanol>rDN, methanolunder the same electronacceptor condition. With sodium acetate as carbon source, the specific denitrification rateincreased with the different electron acceptors dosing ratio (NO3--N/NO2--N). Moreover,nitrite accumulation was observed in each system. The study also found that, the properdosage of mixed carbon source favored eliminating the problem of high effluent pH valueand the slow reaction rate of the denitrification.Making use of temperature gradient caused by season variation and reducing aerationto startup short-cut nitrification provides a new idea to save energy and the enhancenitrogen removal of the real wastewater treatment operated with CAST process. In thispaper, increasing the reactor temperature from17℃to28±1℃and decreasing theaeration amount from0.6m3/h to0.4m3/h gradually, the modified CAST could performshortcut nitrification and denitrification and remain stable for132cycles with nitriteaccumulation rate up to86.08%and total nitrogen removal rate up to69.48%which was21.1%higher than that of complete nitrification stage. To study the influential factors ofshort-cut nitrification in CAST process, nitrite accumulation rate decreased to lower than30%when decreased influent total nitrogen concentration from61.44mg/L to45.45mg/Land then to35.46mg/L. Shortcut nitrification was destroyed. Increasing the influent total nitrogen concentration to59.0±1.0mg/L could improve nitrite accumulation rate to a certainextent but the rate was still less than50%. Even increasing total nitrogen concentration ofthe wastewater and the temperature to30±1℃, shortcut nitrification performance could notrecover.The phosphorus removal performance was also studied with the two CAST processesunder low temperature in winter. The results showed that phosphorus removal efficiency ofthe traditional CAST system was greatly improved because of incomplete nitrification ofammonia in the main react area. However, a serious sludge loss was observed and thephosphorus removal efficiency was greatly affected with the phosphorus removal efficiencyless than10%under long-term low temperature condition. Optimizing the operation modeand increasing temperature could improve the sludge settling and total phosphorus removalperformance. But, the total phosphorus removal performance of the modified CASTmaintained stable and the removal rate was more than88%. Experiments on enhancingphosphorus removal of modified CAST showed that decreasing recycling rate had a greatimpact on phosphorus removal. As the recycling rate decreased from25%to12.5%,phosphorus removal efficiency increased about50%. But, phosphorus removal disappeareddue to the biomass deficiency in the selector when recycling rate decreased to0. Increasingthe influent organic loading could enhance the performance of nitrogen and phosphorusremoval. Phosphorus was removed completely. In addition, the sludge of incompletenitrification system showed good aerobic phosphorus uptake and denitrifying phosphorusremoval using nitrate as the electron acceptor and phosphorus uptake rate under aerobiccondition was about2times of that under anoxic condition. The sludge of enhanced nutrientremoval system could use three electron acceptors (O2, nitrate and nitrite) to uptakephosphorus, and the phosphorus uptake rate using nitrate as the electron acceptor was thelowest while the sludge of shortcut nitrification system showed no phosphorus releasecapability. |
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