单项选择题
A.a new bacteria colony should be put into the plant in advance
B.the disinfection chemicals should be always used
C.the air blower should be started and keep running in advance
D.the nutrient for bacteria should put into plant in advance
单项选择题 （）are used to digest the sewage to produce innocuous sludge in a biological sewage treatment plant.
单项选择题 The extended aeration process consists of （） the liquor either by bubbling air through it or by agitating the surface.
单项选择题 A number of biological sewage treatments plants are in use at sea but nearly all work on the principle called （）