Starting from the necessity of urban sewage treatment plant construction, the paper presents the mechanism, work conditions and selecting principle of biological nitrogen and phosphorus removal, and puts forward the some technologies of nitrogen and phosphorus removal which are suitable for the urban sewage treatment plant. And then, it analyzes in terms of effluent quality, the total investment, land occupation and condition, and compares applied conditions, advantages and disadvantages of each process, and finally gives the selection of nitrogen and phosphorus removal process and gets conclusion.% 本文在说明当前中小城镇污水处理厂建设的必要性的基础上，阐述生物脱氮除磷工艺机理、作用条件和选择总则，并提出几种最适合中小城镇污水处理厂建设的脱氮除磷工艺，然后从出水水质、总投资、占地和现场条件影响等方面进行分析，说明每种工艺的适用条件和优缺点，最后通过实例解析脱氮除磷工艺选择的方法并得出结论。
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