In recent years, in order to treat urban sewage, the construction of sewage treatment plants has been heating up  The large-scale installation of sewage treatment plants can indeed reduce COD emissions from urban domestic sewage in a short period of time
However, in some places, the COD concentration can also be greatly reduced after the ecological purification of fish ponds and natural wetlands for domestic sewage in cities and towns
Pollution control is to pay the price of consuming energy and funds.