| The oxidation ditch, as a modified activated sludge treatment process, has been widely applied in municipal wastewater treatment plants, due to its outstanding advantages, including simple process, stable operation, strong impulsion load resistance and superior effluent quality, etc. The surface aerators were used in the traditional oxidation ditch, and alternation of aerobic and anoxic zones were formed along the channel, which was called point aeration. Nitrogen removal performance in oxidation ditch under point aeration mode is greatly affected by dissolved oxygen and anoxic-oxic zoning proportion. The problem of high energy was also found in oxidation ditch, due to the equal flow velocity in the all channels. Furthermore, the insufficient carbon source has become the dominant factor limiting the efficiency of biological denitrification and phosphorus removal in oxidation ditch process in China. Thus, the problem of carbon source shortages needs to be solved urgently. Therefore, it is essential to study the aeration mode and carbon source utilization in oxidation ditch, in order to establish theoretical and technical fundamentals for enhancing nitrogen removal and reducing energy in oxidation ditch.This study is part of project of the National Program of Water Pollution Control. In order to enhance the nitrogen removal, the study on optimization of carbon source utilization and aeration mode in oxidation ditch was conducted. The transformation characteristics of particulate carbon source was analysed, and the study on nitrogen removal performance and mechanism analysis under different aeration modes in a full-scale carrousel oxidation ditch was carried out. Additionally, the flow field in a variable-velocity oxidation ditch by trepanning was analysed. The theory and technology developed in this study was utilized in three wastewater treatment plants to verify the technology’s feasibility and good outcomes. The main work and achievements are as follows:(1) The carbon source composition of sewage was analysed, the high content of particulate carbon source and poor biodegradability was found. A large amount of particulate carbon source is removed by sedimentation in traditional primary settling tanks, which decreased carbon source utilization efficiency. A novel activated primary tank process(APT) is developed comprising: 1) allowing settled raw sludge solids to accumulate in the bottom of the primary settler tanks; 2) installing a mechanical elutriation unit; 3) partially recycling this sludge to elutriate the fermentation products out of the sludge. The effects of solids retention time(SRT), elutriation intensity(G) and return sludge ratio(RSR) on particulate carbon source transformation were evaluated in a pilot scale reactor. The results indicated that the APT was beneficial for the breeding of fermentative bacteria, such as Paludibacter, Macellibacteroides, Ruminococcaceae etc. and lots of fermentative bacteria were enrichment in the APT to convert the primary sludge into soluble COD. Mechanical elutriation significantly promoted the carbon source conversion by disintegrating sludge and releasing fermentation products out of the sludge. The optimal conditions of APT were SRT =5 d, G=152 s-1 and RSR= 10%, SCOD and VFA yields were 57 mg/L and 22 mg/L. The composition of carbon source in sewage was transformed by the APT, and the content of the readily biodegradable organic matter was increased, which improved the performance of biological nitrogen and phosphorus removal.(2) The effect of aeration mode on nitrogen removal in oxidation ditch was analysed, in order to enhance the nitrogen removal performance. The results showed that with the same total aeration input, step aeration and point aeration demonstrated good nitrification outcomes with the average efficiency in removing NH4+-N of more than 98%. However, TN concentration in effluent was 5.3mg/L and 11.1mg/L under step aeration and point aeration, respectively. The removal efficiency of total nitrogen under step aeration was much higher than that under point aeration. The results indicated that an extended aerobic zone followed the aeration zones could affect the proportion of anoxic and oxic zones. With the same total aeration input and aeration zones, the step aeration, in which aeration zones were gathered continuously with fewer extended aerobic zone, showed larger anoxic zones, and enhanced the nitrogen removal. More importantly, the microbial community was influenced by different aeration modes, the results from high throughput sequencing showed that there were significant differences in the microbial community structure under two aeration modes. Compared with the point aeration, step aeration provided the suitable environment for denitrifying bacteria. The effects of frequently alternating anoxic-oxic environment under point aeration on microorganism were reduced. The diversity and relative abundance of denitrifying bacteria under the step aeration(1.55%) was higher than that under the point aeration(1.12%), which improved the performance of nitrogen removal.(3) A novel variable-velocity oxidation ditch process was developed by trepanning on the anoxic zone and installing the retaining walls and guide wall. The flow field of variable-velocity oxidation ditch was analysed by Computational Fluid Dynamics(CFD) simulation and velocity measurement. The results indicated that the velocity distribution of two means could coincide with each other, and the flow velocity of oxidation ditch was changed by the trepanning scheme. The flow velocity in the channel with trepanning increased to 0.25 m/s, when the flow velocity in oxidation ditch was 0.15m/s. The variable-velocity oxidation ditch could reduce the energy of propeller by 21.6%.(4) The technology developed in the study, as such activated primary tank process, step aeration mode and variable-velocity oxidation ditch process, was applied in three wastewater treatment plants including Xi’an No. 4 wastewater treatment plant, Kunming No. 1 wastewater treatment plant and Xi’an No. 3 wastewater treatment plant. The problems of insufficient carbon source, low nitrogen removal efficiencies and high energy were solved after the application. Nitrogen removal efficiencies in oxidation ditches were improved and the energy was reduced significantly. |
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