Biochemical Oxygen Demand

In a few recent blogs, I have lamented that some of our water quality requirements (Disinfection By-Products, Chloride, etc.) seem disproportionately strict. So what is important, and what should we be strict about? On top of the list is Biochemical Oxygen Demand – BOD.
The first and most important law in the science of ecology is this: “If you leave it, they will come.” (With all due respect to the movie “Field of Dreams.”) What we mean by this is that some population — usually microscopic — will always arise to take advantage of an available food supply.
If you don’t believe it, think about “road kill.” What government agency or charitable organization cleans that up? It’s usually birds, insects, and other animals that won’t let a good meal go to waste.
The same is true for human wastes: they are a food supply for other life forms. In short, BOD measures how much “food” is present. What is the connection between “food” and “oxygen demand”? Let’s start by why we eat food in the first place: to derive energy, through a process known as respiration. There are two forms of respiration: aerobic and anaerobic — with and without oxygen. For humans — and other “macroorganisms,” anaerobic respiration is pretty limited; we need oxygen to survive. But many microorganisms can do just fine in the absence of oxygen. They do pay a price, however: anaerobic respiration is about 30 times less efficient than aerobic. So, in nature, aerobic organisms have a decided competitive advantage — as long as oxygen is present.
The oxygen required to fully decompose a food supply aerobically is the BOD. When the food is gone, it has been transformed into carbon dioxide and water, so the end products of the respiration are essentially invisible.
But what happens if the oxygen runs out before the food does? That’s when the less competitive anaerobic respiration takes over. The end products of this include carbon dioxide and water, too, but also include such obnoxious products as methane, hydrogen sulfide, and ammonia. The rotting food now stinks — and that’s how we know there is too much food for the available oxygen.
If human wastes are discharged into a river or stream — as most sewage is — the wastes will be consumed by populations of microorganisms that are all too happy to eat them. But if the food supply outstrips the available oxygen, that river will begin to smell. Not only that, but the fish and other life forms that require oxygen will either die or migrate away from the “dead zone.”
The BOD test estimates the impact our waste discharges will have on a receiving stream. The lower the BOD, the better the job we have done in treating the waste prior to discharge. So BOD is on the top of my list for the most important water quality characteristics.

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