We manufacture an ozone based air and water purification system.  We need to determine the concentration of any bacteria that remain after the purification process.  We have been using a luminometer to determine the bacterial count, and verify the effectiveness of our system on solid surfaces, as well as in water.  However, it is very difficult to determine the concentration of bacteria present in the air.

Bacterial respiration produces a mixture of methane, CO2 and volatile organic gases. Is it possible to measure the waste products produced by bacterial respiration, and use this as an indicator?

We were wondering whether we might be able to use a photoionization detector (PID) for this purpose.  When we spoke with one of the other leading PID manufacturers we were told this probably would not work.  What do you think?


I agree with the answer you received from that “other” PID manufacturer.  Direct reading instruments can be used to measure by-products of microbial respiration, but it takes a lot of microbes to produce a meaningful change in concentration.  In order to produce contaminants microbes have to be actively metabolizing.  Most of the bacteria present in the air are in the form of dormant spores, and are not actively respiring.  Spores are smaller, lighter, and remain suspended in the air for a much longer period.  You can easily culture any viable spores that remain after sterilization, but measuring atmospheric contaminants produced by the microbes while they are still in the air is next to impossible. Even when they are present on solid surfaces or in water, the bacteria have to be actively respiring in order to produce detectable metabolic by-products.

In a confined space, where there is no mixing with fresh air, microbial decomposition can easily create hazardous atmospheric conditions.

There are many different types of bacteria and microbes involved in this process.  Some types of “aerobic” microbes use oxygen, and produce carbon dioxide.  Other types of “anaerobic” bacteria that do not use oxygen produce methane and hydrogen sulfide. Which types of bacteria are active at any moment depends on the type of organic material that is present in the confined space, the oxygen concentration in the space at that time, and other environmental conditions such as humidity and temperature.

The effects of microbial decomposition on the atmosphere in the space often (but not always) follow the same sequence. Aerobic respiration, which utilizes oxygen, is the most efficient way to convert organic material into energy.  That’s why human beings are aerobic organisms that require oxygen.  When not actively metabolizing, bacteria and microbes are present in the form of dormant spores. The still atmosphere in a confined space initially contains plenty of oxygen. These early conditions are good for aerobic decomposition.  Oxygen using bacteria and microbes become active, and begin to proliferate.  Aerobic bacteria deplete the oxygen, and generate CO2. Being much heavier than fresh air, the CO2 tends to accumulate in the bottom of the space, creating locally anaerobic conditions. Anaerobic bacteria remain in the form of inactive spores until conditions become agreeable for their metabolism.  As the atmosphere becomes increasingly oxygen deficient, anaerobic microbes germinate and begin to metabolize.

Anaerobic microbes do not require oxygen.  Anaerobic decomposition is less efficient, and proceeds more slowly than aerobic decomposition.  The metabolic byproducts of anaerobic respiration include methane (CH4) and hydrogen sulfide (if the organic material in the space includes sulfur).  The more sulfur the organic material in the space contains, the greater the concentration of H2S that is likely to be produced by anaerobic bacterial action.  Being heavier than air, the H2S also tends to accumulate near the bottom of the space.  Methane, being lighter than air, tends to rise, and accumulates near the top of the space, or escapes from the space, if there are any openings.

I would suggest continuing to assess the bacterial count on solid surfaces and in the water by means of the luminometer.  Unless you leave the sterilized area alone for a lengthy period of time, you are unlikely to see anything going on in the air, even if the purified armosphere includes viable spores. The spores have to germinate to have an effect on the atmosphere.

On the other hand, a prime application for the G450 and G460 is to monitor the atmosphere where microbial action can cause dangerous conditions.  Anaerobic fermentation is used to produce alcohol, wine, distilled spirits and beer.  It is highly associated with the presence of dangerous levels of CO2, as well as oxygen deficiency.  Hydrogen sulfide is highly associated with sewage and wastewater treatment, oil production and refining, commercial fish and meat processing, and many other industrial applications.  Methane produced by microbial action is highly associated with many types of confined spaces, including sewers, manholes, digesters, vaults and tunnels.

So, while a few dormant spores may not cause a measurable change in the atmosphere, large numbers of actively metabolizing bacteria can rapidly produce deadly conditions!

Thank you for the question.

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