How to Recover Microorganisms From Water by Filtration
When considering a water source, such as a pond or stream, for private use, the owner may wish to search for the presence of specific microorganisms in the water. Two of the microorganisms referenced in the Safe Drinking Water Act are cryptosporidium and giardia.
A cost-effective way to recover microorganisms from water is by filtering the sample through commercially available water filters (Millipore, USFilters and others) which are rated by the size of particles (and above) the filter will block. This filter rating is in microns, and most microorganism sizes can be found expressed in a range of microns as well.
Recovering microorganisms from water is a straightforward process when using filtration. When you know the size of the organism, you can choose the correct filters. Simply choosing the finest type of filter or reverse osmosis will net all microorganisms, as well as colloidal metals, viruses and dissolved organics.
This discussion will center on a small-scale filtering process, using a water sample gravity-fed through two filters.
- Difficulty:
- Moderate
Instructions
Things You'll Need
- Pre-filter and housing Collection filter and housing Connecting hoses
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1
Determine the size of the microorganism you wish to recover. For information on specific water-borne microorganisms in your area, contact your local health department or the nearest municipal water utility. (Ex: Cryptosporidium is in the 1-to-10-micron range.)
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2
Choose a pre-filter that will pass the microorganism you wish to recover, but will screen out larger particles. Choose a pre-filter that is slightly larger than the upper end of your target microorganism's size range.
Ex: Since crptosporidium is in the 1-to-10-micron range, to recover cryptosporidium, the best pre-filter would be 12 to 20 microns.
Choose your pre-filter to screen out particles slightly larger than the upper end of your target microorganism's size range.
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3
Select a collection filter that will catch the lower end of your chosen sample range, but will pass materials smaller than you wish to collect.
Ex: Since the low end of crptosporidium's size range is 1 micron, collecting cryptosporidium should be done with a 1.0 micron filter.
Choose your collection filter to catch the low end of your target microorganism's size range, but no smaller. -
4
Connect your sample source by sterile hose to the pre-filter housing (with filter inside). Connect the pre-filter to the recovery filter using a second hose. Connect a third hose to the collection filter's exit port and into a drain or container for later disposal.
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5
Pour the source water into the pre-filter, which will pass to the collection filter and to the drain.
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6
Remove the collection filter from its housing after all of the sample has passed through both filters. The sample filter now has all of the cryptosporidium, as well as some of the larger bacteria, smaller giardia and traces of other materials.
How you separate the specific microorganism from the rest will depend on the microorganism you are attempting to recover.
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1
Tips & Warnings
Most filters can be reused by back-flushing (running pure water backward through the filter under low pressure). When you choose a filter, note if it must be pre-wetted (which entails running water or a solution through the filter before use).
Exercise caution when handling recovered microorganisms. Depending on your sample choice, there may be a biological hazard.
