Polymer is a coagulent that is added to a flow to help separate out components, typically solids from liquids. In dewatering it assists with centrifuging sludge, and in filtering water assists with...
the sewage sets of carbon minoxide which travels through the pipes over ground into the air. also if sewage is left to long "mellowing" it will create an acid which will destroy stuuf
Chlorine gas may be added to the treated effluent to disinfect it (kill pathogens) prior to discharge. This is usually a tertiary or quaternary treatment.