Following steps are involved in the design of wastewater treatment 

A) Assessment  of waste quality

B) Desired Treated  water quality 

C) Selection of Treatment  process

D) Treatment System  Design 

E) Engineering  of the Treatment Plant

F) Project Execution

A - There are about 36 to 38 parameters that need to be analysed to assess  the wastewater quality. These are called water quality  Parameters and indicate presence of pollutants  in the water.These parameters  are listed in State/Federal or Central pollution control board was site or a standard  water / wastewater  treatment hand book.

Once the complete analysis is available it can be compared with the desired water quality or the treated effluent quality desired.Normally each type of industry  has a typical effluent quality and can be accessed through  state ENVIRONMENTAL  POLLUTION  CONTROL AUTHORITIES. Similarly the effluent standard for each type of industry is pre scribed by the State/ Federal  Pollution  Control  Authority. 

Based on above data the treatment  specialist  decides the treatment  scheme.A typical treatment  scheme involves Physicochemical  treatment like coagulation settlement of  suspended solids. Biological Treatment  to remove dissolved organics. Filtration er for removal of residual pollutants. 

After this the Engineer  is carried out followed by project execution.

The whole process needs experienced  technologist , Hope this clarifies the concept.

To give specific reply specific data will be required 

Following steps are involved in the design of wastewater treatment 

A) Assessment  of waste quality

B) Desired Treated  water quality 

C) Selection of Treatment  process

D) Treatment System  Design 

E) Engineering  of the Treatment Plant

F) Project Execution

A - There are about 36 to 38 parameters that need to be analysed to assess  the wastewater quality. These are called water quality  Parameters and indicate presence of pollutants  in the water.These parameters  are listed in State/Federal or Central pollution control board was site or a standard  water / wastewater  treatment hand book.

Once the complete analysis is available it can be compared with the desired water quality or the treated effluent quality desired.Normally each type of industry  has a typical effluent quality and can be accessed through  state ENVIRONMENTAL  POLLUTION  CONTROL AUTHORITIES. Similarly the effluent standard for each type of industry is pre scribed by the State/ Federal  Pollution  Control  Authority. 

Based on above data the treatment  specialist  decides the treatment  scheme.A typical treatment  scheme involves Physicochemical  treatment like coagulation settlement of  suspended solids. Biological Treatment  to remove dissolved organics. Filtration er for removal of residual pollutants. 

After this the Engineer  is carried out followed by project execution.

The whole process needs experienced  technologist , Hope this clarifies the concept.

To give specific reply specific data will be required 

The design of a waste water treatment plant depends on what kind of waste water is is intended for. Is the water rich in nitrogen, phosphorous or organic matter? How large does it have to be, that is how much water per day needs to be treated? Is the water flow constant or can it vary over the week or year? How clean does the water have to be after the treatment? Does it only have to be clean enough to be released into nature, or is it intended to be potable/drinkable? What happens if something goes wrong and your process fails, what would the consequences be? 

If the water contains large amounts of organic materials, and/or you use an active sludge process, you will get a lot of sludge that can be used to make biogas, that can improve the economics of your plant.  

Is a biological process good enough, or do you need to remove metal ions et.c. with a chemical purification method? Is struvite build-up going to be a problem?

Does the water contain enough organic carbon to sustain the active sludge, or do you need to add a carbon source, methanol for instance?

For drinking water, how to treat parasites like cryptosporidium? Do you need UV-irradiation? Oxygen treatment? Chlorine? 

You need to take civil engineering bachelor's degree courses and focus on collection systems, hydraulics, construction methods, treatment  - it all depends on what the university offers you as it is different everywhere. With a bachelor's civil engineering degree you are not specialized but willing to do  everything from structural, foundations, drainage, collection systems, bridges, roads, dams, canals, railroads, airports, water, wastewater treatment, etc. - it depends on the firm you go to work. If they specialize in one or more fields like water and wastewater engineering - you hopefully learn by experience from your peers to be an expert after a few years and from training.

To really learn and specialize in treatment plant design quickly instead of learning and specializing from years of work experience, go for a Master's degree in civil engineering or it is a sub-department called environmental or water resources engineering. Depending on the university and what it can offer, you take the courses and become a specialist - but also take a project management class and  try to take an internship in a consulting engineering firm as pointed out before.

I would visit your engineering department and talk with an advisor. Below links will give an idea of what you will learn and do.:

http://uotechnology.edu.iq/dep-building/student-project/Sanitary&Environmental/2011/fatma_huseen.pdf

http://www.epd.gov.hk/epd/sites/default/files/epd/english/environmentinhk/water/guide_ref/files/guide_wpc_stp.pdf

https://www.unesco-ihe.org/wastewater-treatment-plants-design-and-engineering-1

Dear Le

Hope you have made  some progress

It depend on basically BOD, COD, TSS and TDS.