Complete four-level digital study guide (2nd edition, January 25, 2025) for wastewater treatment operator certification. Includes 24 complete practice exams (100 questions each, 2,400 questions total), six original exams for each level (1–4), integrated math questions plus 100 bonus math problems, comprehensive solutions and explanations, searchable digital content, adjustable text size, and enhanced typesetting. Designed as an all-in-one career preparation resource for operators at all certification levels.