廢水處理系統(tǒng)就是利用物理、化學(xué)和生物的方法對廢水進(jìn)行處理，使廢水凈化，減少污染，以至達(dá)到廢水回收、復(fù)用，充分利用水資源。

按處理程度，廢水處理(主要是城市生活污水和某些工業(yè)廢水)一般可分為三級：
一級處理：
任務(wù)是從廢水中去除呈懸浮狀態(tài)的固體污染物。為此，多采用物理處理法。一般經(jīng)過一級處理后，懸浮固體的去除率為70%~80%，而生化需氧量(BOD)的去除率只有25%~40%左右，廢水的凈化程度不高。

二級處理：
任務(wù)是大幅度地去除廢水中的有機(jī)污染物，以BOD為例，一般通過二級處理后，廢水中的BOD可去除80%~90%，如城市污水處理后水中的BOD含量可低于30毫克/升。需氧生物處理法的各種處理單元大多能夠達(dá)到這種要求。

三級處理：
任務(wù)是進(jìn)一步去除二級處理未能去除的污染物，其中包括微生物未能降解的有機(jī)物、磷、氮和可溶性無機(jī)物。

三級處理是高級處理的同義語，但兩者又不完全一致。三級處理是經(jīng)二級處理后，為了從廢水中去除某種特定的污染物，如磷、氮等，而補(bǔ)充增加的一項或幾項處理單元;高級處理則往往是以廢水回收、復(fù)用為目的，在二級處理后所增設(shè)的處理單元或系統(tǒng)。三級處理耗資較大，管理也較復(fù)雜，但能充分利用水資源。有少數(shù)國家建成了一些污水三級處理廠。


Waste water treatment system is to use physical, chemical and biological methods to treat waste water, make waste water purification, reduce pollution, to achieve waste water recovery, reuse, full use of water resources. According to the degree of treatment, wastewater treatment, mainly urban domestic sewage and some industrial wastewater, can be generally divided into three levels: the first level of treatment: The task is to remove suspended solid pollutants from wastewater. To this end, the use of physical processing. Generally, after primary treatment, the removal rate of suspended solids is 70% ~ 80% , while the removal rate of biochemical oxygen demand is only about 25% ~ 40% . Secondary Treatment: The task is to substantially remove the organic pollutants in the waste water. In the case of Bod, generally after secondary treatment, the BOD in the waste water can be removed by 80 to 90 per cent, for example, the BOD content of the treated water can be less than 30 mg / l. Most of the treatment units of aerobic biological treatment can meet this requirement. Tertiary Treatment: The task is to further remove pollutants that can not be removed by secondary treatment, including organic matter, phosphorus, nitrogen and soluble inorganic matter that can not be degraded by microorganisms. Tertiary processing is a synonym for advanced processing, but they are not exactly the same. Tertiary treatment is the addition of one or more treatment units after secondary treatment in order to remove certain specific pollutants such as phosphorus and nitrogen from wastewater, while advanced treatment is often for the purpose of wastewater recovery and reuse A processing unit or system added after a secondary treatment. Three-stage treatment is expensive and complex, but it can make full use of water resources. A few countries have set up some tertiary treatment plants for sewage.