How Do I Measure Organic Load To Or From My Wastewater Treatment Plant?

By Ed Helmig


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Background and Perspective

Instantaneous measurement of organic loads (as oxygen demand) to or from a wastewater treatment system can be extremely valuable. The traditional method of running a 5-day biochemical oxygen demand (BOD5) test is not a practical option. In addition to the fact that the test takes 5 days to complete, it is unlikely to accurately represent the load for a variety of scientific and technical reasons. 

The International Water Association (IWA) Task Group on Mathematical Modeling for Design and Operation of Biological Wastewater Treatment (1) models treatment plants based on chemical oxygen demand (COD) not BOD5  and/or total organic carbon (TOC). COD provides a link between electron equivalents in the organic substrate, the biomass, and the oxygen used (2). In addition accurate mass balances are made using COD; therefore, COD has become the key parameter in the understanding and modeling of all wastewater treatment plants worldwide. COD is so relevant that it is even used to represent substrate (or organic load) in anaerobic models.

Many practitioners choose an on-line TOC meter over and on-line total oxygen demand (TOD) or COD meter because they think they are saving money and will get the same or similar result; this is a false assumption and can be a costly mistake. Extrapolating organic carbon measurements to oxygen demand is also a flawed approach as we will clearly show in this article.

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