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。
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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