Septic Tank: The BOD5 Measurement

As we have talked about many times on this blog and website, the septic tank has two primary functions with regards to organic material in the wastewater:

  • It provides holding time for the material to settle at the bottom of the tank
  • It provides an environment for bacteria to break down the material into gas.

In this blog, we, again, ask the question: “How does a homeowner know how effectively the septic tank is actually performing these functions?”

Search the internet with the terms “onsite wastewater treatment systems” and many websites and documents show up describing how the performance of a septic tank and system can be measured. One of the best and most important documents in the septic system industry, however, shows up at the top of the results: the EPA’s Onsite Wastewater Treatment Systems Manual, EPA/625/R-00/008.

In EPA/625/R-00/008, the EPA defines several standard measurements for septic tank performance. One of these measurements is BOD5, or biological oxygen demand. The “5” represents the time period that the biochemical oxygen demand is measured over which, in this case, is 5 days. Briefly, BOD5 quantifies the strength of the organic material in the wastewater by measuring how much oxygen is consumed by that material in a 5 day period. The higher the number is, the dirtier the wastewater. Note: The BOD5 measurement loosely correlates to the TSS measurement, which was discussed in an earlier blog.

EPA/625/R-00/008 Table 3-7 state that the typical residential influent (incoming wastewater into the septic tank) contains anywhere from 155 to 286 milligrams per liter of wastewater. Note: the BOD5 test measures oxygen demand and the results are correlated to a biological mass per liter reading; BOD5 is an indirect measurement of biological mass density.

EPA/625/R-00/008 Table 3-19 states that the effluent (outgoing wastewater) from the typical anaerobic septic tank contains BOD5 concentrations in the 140 to 200 milligrams per liter range. This is not a dramatic reduction in the BOD5 measurement and implies a significant amount of biological mass leaves the anaerobic septic tank and enters the drainfield. The long term effect of this process is the failure of the drainfield to pass water back to the ground which happens, on average, within 15 years of operation.

The BOD5 measurements for aerobic septic tanks, however, are dramatically lower than the anaerobic tanks because the aerobic bacteria have a more active and efficient metabolism; they consume far more of the organic material in the septic tank, preventing it from entering the drainfield . These lower BOD5 readings, when combined with the dissolved oxygen and aerobic bacteria, extend the life of the drainfield indefinitely.

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