False High BOD Tests in Wastewater Lagoon Systems

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False High BOD Tests in Wastewater Lagoon Systems

In municipal wastewater lagoon systems in the United States, living algae comprises virtually all of the biochemical oxygen demand material and total suspended solids which is tested for regularly to ensure compliance with the city’s National Pollution Discharge Elimination System permit.

Both the BOD and TSS test are basically measuring the same thing — algae. The relationship between BOD and TSS for lagoon systems is well known in the industry and can be expressed mathematically as: BOD = (TSS x 0.5) + 7

This relationship is the reason that the typical NPDES permit for municipal lagoon systems throughout the USA have a TSS limit of 45 mg/L and a BOD limit of 30 mg/L. The TSS and BOD relationship formula holds true for those limits:

BOD = (45 mg/L of TSS x 0.5) + 7 = 30 mg/L                  

The BOD testing problem with lagoon water

The standard BOD five-day test is meant to be a test of carbon-based oxygen demand and for activated sludge systems, this standard works well; however, it was discovered in the 1980s that the BOD five-day test does not work well for lagoon systems.

In lagoon systems, nitrifying bacteria are quite often present in samples and will exert a nitrogen-based (ammonia reduction) oxygen demand during the BOD test. Labeling the entire oxygen demand during the five-day test as “BOD” (implying carbon-based demand) led to testing inaccuracies since some of the oxygen demand was actually nitrogen-based (thus giving a “false high BOD” result). 

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