Biochemical Oxygen Demand, Part Deux

I reported to you about a week ago that we passed the 10,000 visits mark on this site. Thanks again to all. We just passed the 11,000 mark two days ago, so the site remains much more popular than I ever expected, with an average of over 100 visits per day lately.

Also unexpected is the most visited blog of the 230-plus postings of the past 16 months. It was the June 15 posting from last year on Biochemical Oxygen Demand. See:

http://mcleanwatertraining.blogspot.com/2010/06/biochemical-oxygen-demand.html

I didn’t see that one coming, especially on a site that is focused mostly on drinking water; BOD is an important wastewater quality indicator. And as I pointed out in that previous blog, I think it is the most important such indicator. But if it’s important to the folks who visit this site, I suppose a follow-up is in order!

As we noted in the previous blog, BOD represents the food supply that is available to microorganisms. Generally, this is carbon-based molecules, which are the easiest for most microorganisms to convert into the energy that fuels their growth, movement, and reproduction. This is sometimes referred to as “Carbonaceous” BOD, or CBOD — or just our usual BOD.

However, their are also nitrogen-containing compounds that are a food supply to certain microorganisms. Most notable among these compounds is ammonia. Nitrogen is present in all proteins, and some excess nitrogen leaves the body in fecal matter. However, a lot of nitrogen is rejected from the blood stream through the kidneys, and thus leaves the body — and enters our sanitary sewer systems — through urine.

Unless there are a lot of microorganisms that really like ammonia — we call them Ammonia Oxidizing Bacteria, or AOB — the CBOD is the first “food” consumed, and the “Nitrogenous” BOD consumption waits until after most of the CBOD is gone. That is what is reflected in today’s graph, which comes from the University of Maryland.

Because the standard laboratory analysis for BOD is five days in length, NBOD was overlooked for many years. In more recent years, wastewater treatment plants are often required to “nitrify” — that is to convert the ammonia into nitrate within the treatment plant. Doing so greatly reduces the NBOD — and the food supply that gets into the river or stream that receives the treated wastewater flow. It is just one more way that modern wastewater treatment plants and their Operators are protecting our environment from the impact of human wastes.

Nitrification is old hat to our wastewater cousins, but it is a new topic of interest for drinking water folks. The reason is that this same effect takes place in water distribution systems that use chloramines. The reason for this is that we are adding — on purpose! — ammonia to our drinking water to form chloramines. But if we don’t add precisely the right combination of chlorine to ammonia, some excess ammonia will wind up in our system. Another source of ammonia is from the decay over time of chloramines in the system. In either case, ammonia is present in our distribution system. And then we refer back to “Rule No. 1” of the science of ecology: if there is food, there will be microorganisms.

So water folks would be smart to learn more about the trials and tribulations of our wastewater cousins, and to apply their lessons to our water quality challenges. This definitely includes this concept of nitrification that is giving fits to us water folks using chloramines.

And maybe BOD is something all drinking water folks should comprehend and appreciate as well.

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