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There are large quantities of microorganisms in wastewater belonging to different species. The treatment methods to be applied for these species as well as the quantities of disinfectants to be used depends on finding out the quantities of these organisms in wastewater. In this article I discuss a method that is used to estimate the number of microorganisms, which is a useful measurement during treatment.

Most Probable Number

The most probable number is often used by wastewater treatment plant personnel and laboratory analysts to estimate the number of microorganisms in wastewater. A sample taken from the treatment plant is required for this estimation. Probability tables and charts are used for reference during this estimation process.

Highly diluted samples of microorganisms in wastewater are incubated in a medium specifically created for their culture. Features such as their growth and the gas produced during their growth are noted down for measurement. These two features represent the ‘positive tubes’ used during calculations. Similarly negative values are also noted for use in calculation. The formula used here is called as the Thomas’s formula.

Under these formula, the most probable number is measured per 100 ml as –

[number of positive tubes x 100]

—————————————————————————– = MPN/100 ml

[ML sample in negative tubes x ML sample in all tubes]^1/2

As seen in the formula. the MPN is measured by dividing the result of the number of positive tubes (growth) multiplied by 100, by, the result of the multiplication of values of the milliliter sample (ML) of the negative tubes and of all the tubes, powered by half.

It is important to note here that the count of bacteria in separate samples will be different hence there will be different types of growth and in some cases, no growth. This will affect the count of positive tubes. Another condition possible is that one specie of microorganism may dominate a particular sample and will proliferate while other species die. In such cases too, the growth should be seen as positive. Sometimes as many as ten tubes containing highly diluted samples are required for an acceptable level of accuracy of the results.

Estimating the numbers of the microorganisms in wastewater is an essential precondition in applying the correct and appropriately scaled method during the treatment of organisms in wastewater.

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