why is chemical treatment (alum, Cl2) of wastewater done at the endas opposed to the begininng?

Answer 1

For wastewater these are general used for two different processes and one common process(color removal). The reason they are used at the end is that most treatment systems have a biological component that removes 80-95% of the material in the wastewater. By using these at the end you reduce the costs greatly.

Alum - this is also used to help settle the bacteria or other solids resulting from the biological step in a clarifier. If you used it at the beginning, you may need to do it twice. Alum is also used for color removal.

Cl2 - this is used for disinfection and color removal. If used at the beginning, it would also react with that 80-95% that will be removed in the biological stage, thus creating a great consumption.

Special Cases:

Alum - It can be used at the beginning if you are doing a chemical treatment process only. In other words, no biological step. This is not as common. Also, if you have a special wastewater with large amount of solids, you can do a prelim alum step just to remove those solids, but not enough to treat the waste.

Chlorine - I am involved with plants where we do use the chlorine first. In these special cases, we have industrial plants that are newly built and the industrial waste and sanitary waste is piped separately. The sanitary is disinfected and then added to the industrial before going to the biological system. Because the sanitary waste volume is very small in comparison, this actual saves money over treating all the wastewater at the end. It also removes in the biological step any chloro-organics that might form in minute quantities.

Answer 2

Because it's much cheaper to filter off what you can first. Chemical treatment will not then be wasted on large particles.