Optimazation Of Biological Nitrogen And Phosphorus Removal In A~2/O Wastewater Treatment Process

Anaerobic-anoxic-oxic(A²/O)process is one of the widely used techniques in municipal wastewater treatment plants for simultaneous biological phosphorus and nitrogen removal. The technology has many advantages, such as simple configuration and short hydraulic retention time (HRT). In addition, it is easy to operate. Therefore, A²/O will be continuously taken as the main process in all kinds of newly designed and built wastewater treatment plants before some better biological phosphorus and nitrogen removal processes are invented.Though A²/O process has been used widely, it has some inherent contradictions which are difficult to overcome, for example, the contradiction between substrate competition and SRT makes the good nitrogen and phosphorus removal efficiency can not be achieved at the same time, as a result, the removal efficiency of the system can not be improved. As the standard of living improved and the living habits changed, the wastewater quality was influenced. At present, low influent C/N ratio is rather common in our country. The lack of carbon source will make the inherent contradictions of A²/O process more serious. Therefore, the conventional design parameters of A²/O process are relatively dated. According to this, the thesis systematically investigated how to improve the nitrogen and phosphorus removal efficiency of domestic wastewater with low C/N ratio, and utilize the carbon resource in influent wastewater effectively based on the employment of continuous-flow and batch experiments.The effect of DO on the performance of A²/O process was studied by setting the concentration of DO at 4.0, 3.0, 2.0, 1.0 mg/L. It was observed that the effect of DO on COD removal was invisible, whereas the removal rate of TN increased with the decrease of DO, and the efficiency of SND of the system gradually improved. The A²/O system could run in low DO condition when high nitrogen removal efficiency is required only. The concentration of DO has a great influence on phosphorus removal, the system keep high biological activity on phosphorus removal when the concentration of DO was not less than 1.0 mg/L. While when the concentration of DO was approximately 1.0 mg/L, the efficiency of phosphorus removal deteriorated gradually, chemical phosphorus removal was needed to complete the phosphorus removal performance of the system. On the whole, in order to achieve excellent nitrogen and phosphorus removal efficiency, the mean concentration of DO should be 2.0 mg/L at least.The carbon sources has a significant influence on the efficiency of nitrogen and phosphorus removal and the metabolism process of A²/O. Acetate and propionate were selected as sole carbon sources and the result showed that the anaerobic step fit the selective condensed theory. When acetate was used as the carbon source, the concentration and variation of glycogen were larger. The supposed metabolic pathway of this process was the combination of ED and EMP. The influence of carbon sources on nitrogen removal was insignificant, while it plays an important role in phosphorus removal. Compared with acetate, when propionate was the carbon source, there was less aeration needed (saving energy), the amount of energy cycled and needed was less as well as the transformation of intracellular substance. In addition, the reaction process was more stable.The simulation of toxic substance shock on the performance of the A²/O process showed that short-term (3 d) atrazine impact (15, 10, 5 mg/L) did not affect the removal efficiency of organics, nitrogen and phosphorus obviously. The SOUR results demonstrated that different microorganisms had different resistant capacity of atrazine. The heterotrophic bacteria activity was insignificantly influenced by atrazine addition, and the SOUR achieved were 5.74 mgO₂/g-SS h and 5.56 mgO₂/g-SS h respectively before and after the Atrazine impact. However, the nitrobacteria was greatly influenced, the SOUR of AOB and NOB were declined by 66.0% and 12.3% respectively during the atrazine addition.The variation of pH and ORP could be employed to indicate the A²/O dynamic process. pH can be used to indicate the reaction process and to preliminarily determine the result; ORP can work as the indicator of influent COD loading rate, at the same time , it corresponds to the concentration of NO_x^--N (NO+3^--N⁺NO₂^--N) in the anoxic effluent.There are phosphorus uptake phenomena in anoxic zone in A²/O process. It was found that when the anoxic phosphorus uptake ratio was approximately 50%, the integrated nitrogen and phosphorus removal efficiency of the system was relatively high. Keeping proper high recycle ratio, enlarging anoxic volume properly could enhance the anoxic phosphorus uptake, saving carbon source, consequently improved the performance of nitrogen and phosphorus removal. This provided a feasible way to treat wastewater with low C/N ratio in A²/O process. Furthermore, it would be favorable to amend the operational parameters. The microbial population variation was investigated by polymerase chain reaction-denaturing gradient gel electrophoresis (PCR-DGGE). The result showed that the microbial community structure and dominant bacteria change was consistent with the performance of the system and the anoxic phosphorus uptake ratio, while the variation was gradual transition. It was also found that Uncultured Chlorobi bacterium might be the major denitrifying phosphate accumulating organisms in the system.Based on synthetic wastewater, the sludge bulking caused by improper operation was investigated in A²/O system. It was found that the control of loading rate was more effective than DO concentration to inhibit sludge bulking in A²/O process.When the operation mode of the system changed from nitrogen removal to phosphorus removal in SBR, the activated sludge flocs in the system tended to become granular sludge gradually as the phosphorus removal performance became better. The granules formed in enhanced biological phosphorus removal system based on SBR configuration had a large amount of micropores. With the increase of the diameter of granules, the average pore width, total volume of the pores and specific area decreased gradually. Smaller granules had more complex microstructure, which favored the transition of substrates from the surface to the center of the granules, and the biological activity was greater. As the granules became bigger, the percentage of the depth of the biological activated zone in the granular sludge was decreased gradually, at the same time its activity and capacity of phosphorus removal declined correspondingly.