Pros and Cons of the Biochemical Oxygen Demand Test

What's the fuss over the BOD test? It's always five days late, for starters. Read more about its history, its future and why it remains controversial.
Pros and Cons of the Biochemical Oxygen Demand Test
So what's the fuss over the BOD test? As long as NPDES permits include a BOD limit, operators will be doing a BOD test.

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The BOD test used at wastewater treatment plants is certainly controversial and borders on superfluous. At best, it’s something operators need to simply accept as a part of their lives, and at worst it can be downright stressful. What is to be done about this outdated, outmoded and perpetually tardy analysis?

A history of tests
The BOD test made sense many years ago because of its simple, direct correlation to wastewater treatment. Microbial action is the common factor. But why do we wait five days? Originally, the test was applied to wastewater treatment in England, where five days is the maximum timeframe for river water to reach the sea. The test has become a standard for how we determine wastewater strength, expressed in pounds of oxygen demand. A high BOD sample requires a large amount of oxygen for microbes to metabolize available food. Because oxygen depletion damages aquatic life, knowing the oxygen demand of a sample is critical for the health of receiving waters.

Through the years, many improvements have been made to the BOD test, including the standardization of organisms for seeding — creating repeatable, reliable results — and the creation of the glucose-glutamic acid check — a check of the purity of the dilution water, as G-G has a very repeatable BOD. Additives have also been developed to inhibit the BOD attributed to nitrification. And of course, technology has improved the instruments that read the dissolved oxygen at the start and end of the test. The probes now include easy-change membranes and even optical systems that eliminate semi-permeable headaches. The meters include scanners to keep track of large numbers of bottles, while software calculates, analyzes and records the results.

Critiquing the standard
So what’s the fuss? The BOD test is always five days late. Also, it is an empirical test with no concrete way to determine accuracy. According to Standard Methods for the Examination of Water and Wastewater (22nd edition), “There is no measurement for establishing bias of the BOD procedure.”

Now consider the chemical oxygen demand test. The COD test uses strong oxidizers under acidic conditions to oxidize organics in a sample. Because the COD test oxidizes all organics — even what microbes can’t readily metabolize — the results show a higher oxygen demand than the BOD test. The test generally takes about two hours, and the results are comparable to the BOD test. The COD test is less labor intensive than the BOD test, but tends to cost a bit more.

As long as NPDES permits include a BOD limit, operators will be doing the BOD test. That isn’t to say operators must continue using the BOD test for process control. Once we establish sound data to correlate BOD and COD tests, we can determine a conversion factor that allows calculations based on COD test results. Depending on circumstances, the prompt information provided by the COD test can make or break a wastewater treatment process.

I ran an anaerobic digester facility for a juice manufacturer where the COD of the influent could easily increase by 10 or even 100 fold in a matter of minutes. In the time it would take to set up BOD dilutions, the digester would be dead.

Embracing the positive
Because we are stuck with it, we might as well embrace the BOD test.

One of the biggest complaints of the BOD test is blank depletion. According to Standard Methods for the Examination of Water and Wastewater (21st edition), “The DO uptake in 5 d must not be more that 0.20 mg/L and preferably not more than 0.10 mg/L.” This is all about quality of the dilution water and cleanliness of bottles.

If you’ve ever cleaned BOD bottles with chromic acid, you probably don’t mind that Standard Methods removed this cleaning method from the procedure. Chromic acid is not nice to work with, but it certainly does prepare bottles well. Although thorough scrubbing with detergents followed by complete rinsing should do the trick, consider trying chromic acid wash if you are having problems with blank depletion.

Also, focus on your source of dilution water. I’ve seen lab techs frantic about finding the source of dilution water contamination. From the distiller to the deionizer to the storage carboy, the techs will clean, rinse and retest. All the while, nobody tried raw tap water, which was later found to be suitable as dilution water. I prefer to make dilution water in the lab, although distilled water, which is widely available, is perfectly suitable.

I’m sure most operators would just as soon substitute the COD test for the BOD test. The only drawback is cost. If your plant can absorb the expense, the COD test can make your life easier. Just don’t be too quick to mothball your BOD apparatus — the BOD test will likely be with us for a long time.


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