Wastewater Treatment, BOD – Basic Facts About Measuring Wastewater Pollution

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For those involved in wastewater treatment, BOD facts comprise vital information they need to know about measuring wastewater pollution. In this article I explain these facts.

Biochemical Oxygen Demand (BOD):

In wastewater treatment, BOD is the amount of dissolved oxygen (DO) used by microorganisms in the biochemical oxidation of organic and inorganic matter in wastewater. It is measured by incubating a sample at a standard temperature (usually 20 C) for a set period of time (commonly 5 days). The sample must be diluted if the BOD is high.

BOD (mg/l) = (D1 – D2) x D.F

Where:

D – initial DO 1

D – final day DO 1 and 2

D.F – dilution factor.

I. Typically, as seen during wastewater treatment, BOD values in domestic wastewater vary between 110 and 400 mg/l (Metcalf and Eddy, 1979).

II. For typical untreated domestic wastewater, the BOD / 5 COD ratio varies from 0.4 to 0.8, and the BOD / TOC ratio 5 varies from 1.0 to 1.6.

In wastewater treatment, BOD value ultimately depends on the carbonaceous phase and nitrogeneous phase.

(Ref: Metcalf and Eddy, inc. 1979, Wastewater Engineering: treatment, disposal and reuse, 2nd Ed., Mcgraw-hill, New York).

Carbonaceous Biochemical Oxygen Demand (CBOD):

CBOD is the amount of dissolved oxygen used by microorganisms in the biochemical oxidation of organic matter only. It is measured by preventing nitrification during the incubation period of the BOD test. Methods for preventing nitrification include pre-treating the sample by adding inhibitory agents such as ammonia, methylene blue, thiourea, allylthiourea, 2-chlor-6 (trichloromethyl) pyridine (TCMP), or proprietary products.

Nitrogeneous Biochemical Oxygen Demand (NBOD):

Autrotrophic bacteria such as nitrifying bacteria require oxygen + to oxidize NH to NO. The oxygen demand exerted by nitrifiers is called autrotrophic BOD in wastewater or nitrogeneous biochemical oxygen demand (NBOD).

NBOD = BOD – CBOD

Where:

NBOD – nitrogeneous biochemical oxygen demand

BOD – biochemical oxygen demand

CBOD – carbonaceous biochemical oxygen demand

The theoretical nitrogeneous oxygen demand is 4.57 g oxygen used per gram of ammonium oxidized to nitrate. However, this value is actually lower and must be corrected due to incorporation of some of the nitrogen into the microbial cells.

Thus NBOD is as follows:

NBOD (mg/l) = (available N – assimilated N) x 4.33

(Verstraete, W., and E. VanVvaerenbergh, 1986):

During wastewater treatment BOD tests are needed to be carried out to distinguish between carbonaceous and nitrogeneous BODs. It is recommended to add 2-chloro-6 (trichloromethyl) pyridine at a final concentration of 10 mg/l for nitrification inhibition.

Ref: Verstraete, W., and E. Van Vaerenbergh, 1986. Aerobic activated sludge, pp. 43-112. In: Biotechnology, vol.8, Microbial Degradations, W.Schonborn, Ed., Vch, Weinheim, Germany)

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