The major losses will be across the racks/screens and any sudden changes of flow area. Metcalf & Eddy WW Engineering have the equations. That book is extremely useful for resolving other issues as well.
Engineering is the practice of the art of science - Steve
The trickiest bit of the works is usually the interinked weirs, flumes and channels associated with the inlet and the sidestream to storm tanks. Metcalf and Eddy doesn't cover how to do that bit AFAIK.
I'm sure there are plenty of others like myself with spreadsheets for this. Personally I'd want to write my own spreadsheet, I wouldn't trust soemone else's.
So many errors occur in spreadsheets. You would be better off using Mathcad or TK Solver. At least then youcan see your equations when it comes to error checking.
Flowpro 2 is a good freeware software program for losses in channels, weirs etc. You could use this to solve for nodes in your process.
There was a program called Arc that was designed to WWTPs but I havent used it.