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NewsroomUnderstanding Biochemical Oxygen Demand
For every pound of BOD5 (5-day Biochemical Oxygen Demand), about two pounds of oxygen are required for complete digestion. This is because at five days, biological digestion is only about halfway to completion. The carbonaceous portion of BOD is quickly consumed, while nitrifying bacteria (which consume organic nitrogen and ammonia) take longer. If the testing period for a standard BOD test were several weeks instead of five days, it would result in a more meaningful number but of course it would be hard to correct problems with the pond in real time.
Aerating effluent immediately before the discharge box will not have a significant effect on BOD. The BOD test measures the change in dissolved oxygen, and effluent samples are diluted with oxygen-saturated water, so if the effluent is aerated at the point of sampling it will have no effect on the test. Aerobic bacteria need time to work, so aerating several days in advance is necessary.
Ozonating or otherwise sterilizing a sample will have no effect on BOD, and may even increase it. Labs typically add active seed bacteria when running a BOD test. Sterilizing the effluent can break up algal cells, making nutrients more available to the seed bacteria.
To meet the BOD limit, ponds should discharge more in the summer and fall, holding water in the winter and spring, if possible. The metabolism of aerobic bacteria decreases dramatically with lower temperatures, and comes to a near stop below 40°F. This results in less treatment, and a higher BOD in winter and spring effluent.
Pond turnover is another important factor to consider. Ice forming on the surface results in low oxygen levels and sluggish biological activity. When the aerobic bacteria wake up in the spring, they quickly start consuming oxygen and the whole pond can go anaerobic in a matter of days. Excessive algae growth, sudden die-off, and the lack of algae produced oxygen can cause turnover. Cold fall rains can cool the surface of an unmixed lagoon, while the sludge layer remains warm and starts to rise. In any case, turnover results in foul odors, complaints, and a rising decomposing sludge layer. A turned pond can take weeks to sort out, even with aeration or the addition of nitrate. The BOD and ammonia in the effluent will spike. This is why it’s important to occasionally check the sludge thickness in the ponds, test the composition of the sludge (is it mostly organic, or just sand and silt?), and to add year-round aeration if necessary.
Richard Ammel, PE
Richard Ammel, PE, EBH Engineer, is a licensed professional engineer who has designed and inspected the construction of sewers, pump stations, and wastewater treatment systems for several small towns in Kansas. He received his Bachelor of Science (B.S.), Civil Engineering degree from Colorado School of Mines. Contact Richard if you would like to discuss your municipality’s waste water treatment challenges at 620-793-8411.
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