Secondary treatment is the biological treatment that controls the environment for growing enough quantity of microorganisms to degrade organic substances in the wastewater followed by separating the microorganisms from the treated liquid to achieve the objective of purification . the biological treatment can be either aerobic or anaerobic 二级处理通常系指生物处理而言，主要原理在于控制及培养一定数量之微生物，利用微生物生长代谢作用，来分解去除废水中的有机物，再将处理水和微生物分离，并废弃增殖过剩之微生物量，而达到净化水质之目的。
