AWWA M7 : 2025

The drinking water community shares its water systems with a world of microscopic living creatures. Water professionals know that these organisms inhabit, grow, and potentially reproduce in source waters and throughout the treatment plant and distribution system. These creatures may contribute to numerous operational challenges, from problems in source water through finished water, including encrustation or corrosion within the water system and water quality problems at the customer’s tap. This organism diversity makes it difficult for water treatment operators and water quality managers to be familiar with every organism. When problems arise, there is often no place for an operator and water quality manager to turn to for help. To address this gap, this manual provides information regarding nuisance organisms, the problems they cause, treatment options, and references to literature resources. It is intended to be a key reference for finding answers. The third edition was a complete revision of AWWA Manual M7, which stemmed from the results of the 1989 AWWA Survey of Nuisance Organisms that demonstrated a need for information to help identify and control nuisance organisms. The difficulties identified in the survey were related to microorganisms that cause problems with plant operations or for customers because of undesirable tastes, odors, and color or their mere presence in what is supposedly a clean glass of water. This fourth edition includes two new chapters, “Free-Living Protozoa” and “Invasive Aquatic Plant Species,” as well as a new Taxonomy Overview. The protozoa chapter in previous editions focused on pathogenic protozoa. Because pathogenic protozoa are covered in AWWA’s M48 Waterborne Pathogens manual, the chapter in Manual M7 was revised to focus on non-pathogenic or free-living protozoa. The addition of the “Invasive Aquatic Plant Species” chapter increases the Manual’s applicability to source water protection and improvement. The Taxonomy Overview is designed to help operators and other water professionals recognize the relationships of organisms, which can provide a better understanding of the significance and control of the organisms in water supplies. This edition also clarifies “algae” by refocusing and expanding Chapter 10 to “Algae and Cyanobacteria.” However, Chapter 10 is an introduction to the biology, ecology, and water treatment problems potentially caused by algae and cyanobacteria; readers are referred to M57 Algae: Source to Tap, which provides in-depth information and a comprehensive review of algae and cyanobacteria with respect to water treatmen