Guest Column | October 4, 2016

Math Solutions: Mixed Liquor Volatile Suspended Solids (MLVSS) Calculations

Wastewater Dan Theobald

By Daniel Theobald, "Wastewater Dan"


Wastewater consultant and trainer Dan Theobald (“Wastewater Dan”) dives deeper into the activated sludge process with calculations focused on MLVSS.

We’ve learned calculations for sludge age process control and activated sludge process control in recent “Math Solutions” articles, but there are yet more problems to be solved when it comes to biological wastewater treatment.

MLVSS is part of mixed liquor suspended solids (MLSS), but the former represents the concentration of organic bacterial cells (biomass) in activated sludge (AS) while the latter indicates the total concentration of suspended solids in AS (including inorganic or non-biological solids in addition to biomass). Thus, MLVSS is more representative of microorganism concentration.

This tool for operators — represented through video tutorials — offers mathematical templates for performing accurate calculations. MLVSS calculations may have different requirements such as expressing the results as pounds, square feet, cubic feet, increment of millions, or even incorporating a time element in calculation solutions. Depending on the calculation used for MLVSS, calculations with the flow or concentrations require explicit formula use.

Exact step-by-step solutions to the word problem below are accessible via the videos that follow.

Determine the amount in milligrams per liter (mg/L or ppm) of MLVSS to be maintained in the aerator of a conventional activated sludge plant. Assume a food/microorganism (F/M) ratio of 0.65 lbs chemical oxygen demand (COD) per day per 1 lb of MLVSS under aeration for the plant. Average COD of untreated effluent is 800 mg/L, average primary treated COD is 645 mg/L, and  average daily flow is 102,000 gallons per hour (GPH). Aerator dimensions are 78.208556150 feet long, 40 feet wide, 17 feet side wall depth, and 2 feet free-board.

MLVSS Calculations

Primary effluent COD:

Flow (MG):

COD lbs/day added to aeration:

Aeration volume (MG):

MLVSS (lbs):

MLVSS concentration  (mg/L or ppm):

Playlist of 35-plus process control (including activated sludge) calculations

Playlist of 40-plus pounds-wastewater calculations

Playlist of all 145-plus Math Solutions

This is the MLVSS Calculations presentation in my series of “Math Solutions.” If you have specific wastewater math queries, 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.

Image credit: "Tenino WWTP Oxic Basin," Eric Shea © 2013, used under an Attribution 2.0 Generic license: