Using microorganisms to break down organic matter (e.g., Activated Sludge Process, Trickling Filters, or Oxidation Ponds).

Proper disposal methods for solid waste. Plumbing: House drainage systems. The Value of Finding Quality Engineering Resources

If you'd like, I can (like Peavy & Rowe) to help you choose the best one for your studies. Let me know, and I can provide summaries.

Physical removal of grit, grease, and settleable solids.