A really, really summarised flowchart:
Primary Treatment
Removal of the solids
Secondary Treatment
Uses aerobic organisms to remove some of the nutrients
Tertiary Treatment
Ponding for 10 days as UV kills off organisms
and, my teacher said to include this one, but I'm not too sure,
Activated Sludge
Where alternating airation forces the organisms to remove phosphorous and nitrates.
And then you add boxes and pretty arrows and stuff like that and, whola!, you have yourself a really summarised flow chart!