I'm not aware of a ...

I'm not aware of a particular tool and nutrients are important.  You have to start in reverse.  You have to know what are the discharge limits you are allowed to discharge at and is it based on an absolute or a centile discharge.  Once that has been established, you have to establish the space, which is available, the capital and the peak flows, which your plant is able to deal with.  The nutrient level could also be misleading because the nitrogen needs to be split into ammonia nitrogen and total nitrogen, which is the ammonia nitrogen and other nitrogen and is the phosphate in a combined form or free phosphate.  The other main question is what is the source of your effluent?  A simple plant for a hotel or housing estate could be different from a plant with industrial or food wastes.  Once that has been done, then you can start talking to specialist designers of effluent plants.  So the COD or BOD of the waste will need to be known.  Another need to know is whether there is a fat oil & grease (FOG) content.  

The basic principles are screening first then look at other processes.