Guest Column | March 23, 2015

Math Solutions: Controlling Secondary Treatment

Wastewater Dan Theobald

By Daniel Theobald, "Wastewater Dan"


Water Online’s “Math Solutions” series, presented by wastewater consultant and trainer Dan Theobald (“Wastewater Dan”), instructs operators on specific math solutions for operator certification exam preparation.

Wastewater sidestreams, per the U.S. EPA (EPA Publication 832R85102), constitute the flows generated within a wastewater treatment plant that are in addition to the plant influent — including supernatants, backwash waters, rinse waters, plant drainage water, etc. They can be recycled either to the head end of the WWTP or into secondary treatment units, and feature characteristics that are highly variable depending on the plant's treatment process and operation practices. Wastewater Dan presents math solutions to help operators successfully calculate sidestreams and control the secondary treatment process, as well as solve related math problems when posed on certification exams.

The following is a hypothetical question/wastewater scenario presented with a three-step instructional method to controlling secondary treatment:

The influent flow to a secondary treatment plant has a primary effluent of 4,791,667 gallons/hour (GPH) with 123 mg/L suspended solids and a sidestream of .97 MGD with 1,458 parts per million (PPM) suspended solids. What percentage is the sidestream suspended solids to the influent load of the secondary treatment plant?

Calculate primary effluent solids (lbs.):

Calculate sidestream solids (lbs.):

Calculate percentage of sidestream solids to total influent:

Complete playlist of Wastewater Dan’s Math Solutions      

This is the Simulating Jar Test Results 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: "Nachklärbecken - secondary sedimentation," © 2011 Sustainable sanitation , used under an Attribution 2.0 Generic license: