Here are some guidance ideas ...

Here are some guidance ideas to aid you in your search. 

For the US:  Consider the 10 States Standards which is pretty much applied and adopted by most states in the US.  The designs are conservative, but they do work.  The sponsoring organization is the  Great Lakes-Upper Mississippi River Board of State and Provincial Public Health and Environmental Managers (GLUMRB) or look up "10 states standards".  

For the EU you will need to look at their water quality directives which apply to specific regions.  These are not guidance documents for design, but guidance on effluent and water quality.

For a more complete explanation, see the 2nd Edition of Practical Wastewater Treatment by D. L. Russell-- published by John Wiley