Everything you need to know about the impact of oxygen demand on septic systems


Biochemical oxygen demand (BOD5) is the most widely used parameter applied to wastewater. It’s a measure of the dissolved oxygen required by microorganisms to oxidize or decompose the organic matter in wastewater.

Types of BOD
There are several different procedures to evaluate the amount of oxygen demand in wastewater.

  1. BOD5: Biochemical oxygen demand is the quantity of dissolved oxygen consumed by microorganisms during the breakdown of organic matter in a wastewater sample during a five-day incubation period at 20 degrees C and measured in mg/L. It is used as a means to describe the amount of organic matter present in the water. Biodegradable organic matter is provided in terms of pounds of BOD5 per person (capita) per day by using the BOD5 concentration and daily flow. It is an indicator of the overall strength of the wastewater. Most designs assume that all residential sources generate a concentration of 300 to 400 mg/L of BOD5, and after pretreatment in a properly sized septic tank, the BOD5 is reduced by more than 50 percent.
  2. CBOD: Carbonaceous biochemical oxygen demand is the quantity of dissolved oxygen consumed by microorganisms during the breakdown of organic carbon in a wastewater sample during an incubation period of five days at 20 degrees C. An inhibitor is placed in the sample to prevent the growth of nitrogenous oxidizing microbial populations. It is used as a means to describe the amount of organic carbon present in the water that can be broken down with microbial processes. It is important to note that the system will need to provide additional oxygen to convert the ammonia to nitrate as shown below.

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  1. NBOD: Nitrogenous biochemical oxygen demand is the quantity of dissolved oxygen consumed by microorganisms during the oxidation of nitrogenous compounds such as protein and ammonium in a wastewater sample during an incubation period of five days at 20 degrees C. It is used as a means to describe the amount of organic nitrogen (such as urea and proteins) present in the water. It is not usually used in typical wastewater analysis.
  2. UBOD: Ultimate biochemical oxygen demand is the measure of the oxygen required to complete the breakdown of the organic matter. The UBOD consists of summing the oxygen demand required to oxidize the organic matter in the wastewater, synthesize the organic matter into new cell tissue, and the endogenous respiration where cell tissue is consumed by other microbes to obtain energy for cell maintenance. The UBOD is not typically a value measured in lab analysis.

Impact of BOD5 on septic systems
A typical BOD5 value for septic tank effluent is 150 mg/L. For conventional systems, many codes limit the BOD5 to less than 220 mg/L. This limit is in place to assure there will be sufficient oxygen available in the soil to break down the organic material present. After advanced treatment units (such as aerobic treatment units) and media filters that provide oxygen to the wastewater, the BOD5 levels are typically less than 25 mg/L.

A high BOD5 (greater than 220 mg/L) can cause the growth of excessive biomass that can clog and shorten the lifespan of the components in the system. High organic loading to the system causes high BOD5 levels. In a residential system, the number of people in the house could be greater than that for which the system was designed and originally constructed. In this situation, it is also possible that the concentration might not be elevated, but the overall organic mass loading could be significantly higher. An elevated BOD5 concentration could also be influenced by the activities that are happening at the source. In homes or restaurants, the presence of a garbage disposal, the types of foods prepared and methods to prepare them can increase the BOD5 levels. In a home, a large portion of BOD5 is produced from toilet water. Toilet water also produces a large part of the natural microorganisms.

High BOD5 in the effluent that's moving to the downstream components of the treatment train could be caused by reduced biological activity in the onsite wastewater treatment system. Chemicals used by the source may play a large role in inhibiting the reduction of the BOD5, therefore causing a high BOD5 concentration in the effluent. Onsite wastewater treatment systems use naturally existing microorganisms to reduce the contaminants and treat wastewater. During treatment, the microorganisms feed on constituents in the wastewater, reducing their concentration and resulting in cleaner wastewater. Harsh chemicals, such as bleach, detergents, cleaners, and disinfectants, can kill these microorganisms and reduce their ability to breakdown contaminants such as BOD5.

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Low BOD5 from a home may be due to a low occupancy or a low number of meals prepared at home. A low BOD5 concentration may also be created through dilution from higher than normal hydraulic flows into the wastewater treatment system. This dilution effect could be due to the extra use of appliances, such as a laundry machine, a Jacuzzi, or long showers. Leaking fixtures can also add extra water. If clear water sources such as water treatment systems or condensate drains are plumbed into the system, the increase in carriage water volume will dilute the constituents in the wastewater and decrease the concentration of food supply. Commercial systems may have a low BOD5 if a low percentage of the wastewater comes from the bathroom and the rest comes from sources with low BOD5 contributions with significant carriage water volume. In typical wastewater treatment trains, your senses may assist in estimating relative BOD5 concentrations.

Before collecting samples for analysis, be sure to first locate a certified lab in your area to discuss the required containers, storage and hold times. 

About the author
Sara Heger, Ph.D., is an engineer, researcher and instructor in the Onsite Sewage Treatment Program in the Water Resources Center at the University of Minnesota. She presents at many local and national training events regarding the design, installation and management of septic systems and related research. Heger is education chair of the Minnesota Onsite Wastewater Association and the National Onsite Wastewater Recycling Association, and she serves on the NSF International Committee on Wastewater Treatment Systems. Ask Heger questions about septic system maintenance and operation by sending an email to kim.peterson@colepublishing.com.

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