Guest Column | July 10, 2014

Math Solutions: Waste Activated Sludge (WAS) Calculations

Wastewater Dan Theobald

By Daniel Theobald, "Wastewater Dan"

Water Online’s “Math Solutions” series, presented by wastewater consultant and trainer Dan Theobald (“Wastewater Dan”), provides an understanding of the sometimes difficult calculations involved in achieving wastewater operator certification.

Waste activated sludge (WAS) calculations in your plant may be based on data similar to the following:

An activated sludge process has three aeration basins. Each basin is 90 feet long, 45 feet wide, and 12 feet deep. The mixed liquor suspended solids (MLSS) concentration is 2,750 mg/L. There are two clarifiers. Each clarifier is 36 feet in diameter and 11 feet deep. A clarifier core sample indicates that the solids concentration in the clarifier is equivalent to 500 mg/L. If the return activated sludge (RAS)/WAS concentration is 5,950 mg/L, find the WAS pumping rate in gallons per minute (GPM) if the pump operates 17 minutes out of each hour to maintain a sludge age of 15 days.

Math Solutions to solve WAS pumping rate in GPM include calculations of tank volumes, pounds in activated sludge process, WAS pounds, and WAS pumping rate in GPM.

Calculate Volumes In Million Gallons (MG)

Aeration basins:




Calculate Pounds (Lbs.) In Activated Sludge Process

Aeration basins:



Calculate WAS

WAS pounds (lbs.) per day:  




If you have specific wastewater math queries that you would like to see addressed in “Math Solutions,” please submit a question.

About Dan Theobald:

Known in the industry as “Wastewater Dan,” Daniel L. Theobald, proprietor of Environmental Services (, is a professional wastewater and safety consultant/trainer. He has more than 24 years of hands-on industry experience operating many variants of wastewater treatment processing units and is eager to share with others his knowledge about water conservation.

Theobald serves as an active consultant for industries looking to achieve and maintain improved wastewater treatment at reduced cost. He is a Lifetime Member of the Who’s Who Registry of Professionals and holds numerous certifications from wastewater management regulatory boards and professional organizations. Theobald contributed one chapter to the Water Environment Federation’s ( Manual of Practice # 37 (MOP-37), a technical manual resource guide for biological nutrient removal, published in 2013.