What is seed, and how is it used in the BOD dilution method?

Document ID

Document ID TE175

Published Date

Published Date 08/20/2019
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Question
What is seed, and how is it used in the BOD dilution method?
Summary
Using seed in BOD analysis
Answer
The term "seed" refers to microorganisms that consume the biodegradable organic matter in samples for measurement of Biochemical Oxygen Demand (BOD).

Domestic wastewater (influent) or effluent from biological water treatment plants (before disinfection) provide the best source of seed, and give the most reproducible results. Other sources, such as industrial wastewater, may not have enough microorganisms, or may contain toxins that prevent the organisms from growing. If wastewater is not available, prepare a seed solution from a freeze-dried capsule such as PolySeed®. Whatever source of seed is used, it will exert some demand. Therefore a seed control must be measured to correct for this demand. Older versions of the Standard Methods for the Examination of Water and Wastewater indicated that Dissolved Oxygen (DO) uptake of seeded dilution water should be between 0.6-1.0 mg/L, newer versions indicate a minimum of 2.0 mg/L. Refer to the current Standard Methods or your local regulatory official to verify this requirement.

When determining BOD, it is necessary to have a population of microorganisms that can oxidize (or consume) the biodegradable organic matter present in the sample. If there is too little seed present in the sample, complete consumption of biodegradable matter may not occur, resulting in inaccurate results. In samples such as influent samples and effluent waters prior to disinfection, this is not a problem as the sample will contain sufficient bacteria to do the job. However, in certain sample types, such as some industrial wastes, high temperature wastes, and treated effluent, there is not enough bacterial activity to consume the material that is present. In these cases, seed must be added. Seed is simply a solution that contains a sufficient population of bacteria. Hach offers PolySeed®, a seed capsule that can be added to samples.

Seed requires proper pH, temperature control, and nutrients such as phosphorus, calcium, and magnesium for proper growth. Hach nutrient buffer pillows provide the necessary nutrients and pH.

The Hach Technical Bluebook 'Introduction to Biochemical Oxygen Demand' provides more information on BOD.

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