Presently, active sludge method, or the establishment of secondary sewage treatment plants has been mainly adopted to treat urban sewage.

目前，对城市污水的处理主要采用活性污泥法即兴建城市二级污水处理厂。

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Sewage treatment plants yield lots of sewage sludge. The sludge has complex composition, containing a variety of bacteria, and then the improper disposal is bound to cause secondary pollution.

来自污水处理厂的污泥产量大，成分复杂，含有多种病菌，处置不当势必造成二次污染。

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