Since it's come up a few times lately, here's a list of some of the places I've found to go for information online:

A few online resources I've collected:

Wright's 'operators notebook' is one place to get started. Whether you agree with him 100% or not, he has a lot of experience and he's taken the time to write it down.

Mountain Empire Community College has their class lectures posted for a whole bunch of classes, mostly aimed at the technician level. A few of them:

Intro to Water & Wastewater Technology

Water Purification

Environmental Microbiology

Wastewater Treatment Plant Operation

Lenntech's Calculators are similar to the Engineering Toolbox, but focused on wastewater. Before you start plugging stuff in though, make sure you know what the numbers mean and when you're going to be dealing with an exception to the rule!

SNF Polydyne has some good sludge dewatering polymer information, but they're missing screw presses, and I have three words to say about " the only difference (between screw & drum thickeners) is that the sludge conveying is done with an Archimedean screw":

Thickened. Sludge. Fountain.

Anyone have others that you use? If so, post 'em below, and I'll get a sidebar going to keep this stuff handy for people.