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Defining Wastewater: Flows And Elements Of Wastewater Treatment Systems

Posted on by Aaron Burke, P.E.

A few weeks back I had the opportunity to present a webinar covering some wastewater basics, Wastewater 101. This one-hour session covered a wide range of wastewater topics starting with the reason we started to treat wastewater in the first place all the way through where we are today with what we know about wastewater and how we design effective treatment. Today, I would like to dive a bit deeper into terms surrounding wastewater system design. In order to design a wastewater treatment system it is important to first understand flow and what elements are required to be treated.


Flow is a very fluid term in wastewater and can be defined a number of ways based on what is being measured. Flow is typically measured using a parshall flume (see image to the right).     

  • Average Daily Flow (ADF): Average Daily Flow is the average of 24-hour volumes to be received by the wastewater system for a continuous 12-month period. If you’re working on a design for a seasonal facility then the ADF is based on the average flows observed for each specific seasonal period, and the system design may need to consider multiple design bases to ensure effluent water quality is met year round.
  • Peak Daily Flow (PDF): Peak Daily Flow is the largest volume of flow during a continuous 24-hour period.
  • Peak Hourly Flow (PHF): Peak Hourly Flow is the largest volume of flow during a one-hour period.
  • Peak Instantaneous Flow (PIF): the Peak Instantaneous Flow is the maximum flow rate measured at any moment in time.
  • Diurnial Flow Pattern: The Diurnial Flow Pattern is the daily cycle of instantaneous flows over the course of a day, and it is often observed in one-hour increments.

These flow rates will typically be measured in gallons per day (GPD), gallons per minute (GPM), or million gallons per day (MGD) for larger wastewater systems.

Waste Constituents: 

There are many waste constituents that can be found within wastewater. The following list describes some of the most common parameters considered when designing a wastewater treatment system.

  • Chemical Oxygen Demand (COD): A measure of the oxygen equivalent of the organic material in wastewater.
  • Biochemical Oxygen Demand (BOD): A measure of the aerobic, biological decomposition of an organic waste.
  • Total/Volatile Suspended Solids (TSS/VSS): TSS is a portion of the solids retained on a filter with a specified pore size, measured after drying. VSS is the portion of TSS that burns off after ignition above 500 Celsius.
  • Nitrogen: A macro-nutrient that is essential to the growth of microorganisms, plants, and animals. In raw wastewater it is found in many different forms, but most commonly measured as Ammonia Nitrogen (NH3-N) or Total Kjeldahl Nitrogen (TKN). In treated effluents it is typically measured as Total Nitrogen (TN), which is typically the sum of effluent of Nirite (NO2), Nitrate (NO3), and TKN.
  • Phosphorus: Also a macro-nutrient essential for plant and biological growth. Raw wastewaters typically measure Total Phosphorus at the influent of the treatment plant.
  • Dissolved Oxygen (DO): A measure of the available O2(g) in a water sample that is available for respiration of aerobic microorganisms and other life forms. It is an important tool in process control and is often specified as an surface discharge effluent limit.

This just touches the surface on terms within the wastewater industry. Take a few minutes to watch the webinar that I mentioned at the beginning of this post, Wastewater 101, to gain more insight these and more terms that are used within the wastewater industry. If you’re short on time, the segment on wastewater terms begins 15 minutes into the video.  

About the Author:
Aaron Burke, P.E.
Wastewater Engineer

Aaron brings 17 years of experience to the Technical Services team at Infiltrator Water Technologies. He has worked in administrative project management, construction, engineering design, and has started two wholly owned companies. His expertise is in wastewater treatment process design and treatment plant operations. Aaron holds a Bachelor’s degree in Civil Engineering from Rose-Hulman Institute of Technology and a Master’s of Engineering in Civil Engineering from Norwich University, and is a licensed professional engineer, LEED accredited professional, and certified wastewater operator.