Frank R. Spellman 
Spellman’s Standard Handbook for Wastewater Operators (3 Volume Set) [PDF ebook] 

Hailed on first publication as a straightforward, practical, and to-the-point account of wastewater principles, practices, and operations for general readers, students, and wastewater operators in training and for all levels of operators at any level of licensure, Spellman’s Standard Handbook for Wastewater Operators, Volumes I, II, and III almost