Que 1 :
I am performed microbial analysis of RO water.
As per my experiment, the fresh sample of water has 1 x 103 CFU/mL from plate count method but after 4 to 6 days later when I am performing same analysis for the same sample there should be 0 count in the sample.
After 5 days there is 0 count so can we suggest it as a suitable for drinking water ?
Can you please help me out for understanding this stuff?
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Que 2 :
In BIS standard there is permissible limit of
Dear Rahul,
Que 1 : One sample is not enough to get idea about your experiment and do this for more samples with replicates to prove what you mentioned here.
If there is no growth of microbes in treated water that's suitable for drinking when concerning only the microbial quality of water.
Que 2 : According to the standard methods, You cant count one colony on plate for diluted samples. if there is less colony count (< 30 CFU/ml) you have to perform test without diluting the sample.
If that standards indicating permissible level
I am not in my computer. There is a collection of papers edited by George Dissmeyer USFS on Drinking Water from Forests and Grasslands that might interest you, and discusses many of the potential contamination sources. I agree, one or two samples are insufficient, especially if connected to directly or even intermittently connected to surface water sources. Water quality can vary widely in surface sources, and subsurface sources may also experience intermittent contamination from surface. Stormwater can cause runoff and contamination. Bacterial indicators such as fecal coliform are just that, indicators. When in doubt, boiling water, chlorinating, UV treatment, RO filters are some common methods used. Time such as a week for water to sit before drinking is not a standard or suitable treatment practice.
On dilution for bacterial testing of unknown sample, it is not uncommon to have to filter various amounts to stay within count range desired after incubating numerous tests to stay within desired test range. For low contamination levels, one might filter several liters if the desire is to report a value. Since the sample needs to be stored and tested in a timely standard for research reporting and community safety, one might run 5 levels, ranging from full strength to maybe 50 or 100 times dilution, perhaps even greater dilution if runoff or potential sewage contamination. If you sample enough conditions, you may begin to know if and what dilution May be needed. Millipore Company is the one I often used for supplies, and Standard Methods for Analysis of Water and Wastewater.
Q 1: You are working with RO water, the process remove all the minerals from treated water. In initial stage the water contain bacteria, but after few days bacteria can die to lack of food, or eaten by same of others species bacteria. Like the process of bacterial death in bio-layer of bio-sand filter.
As Yohan Mahagamage has mentioned, a single test is not sufficient to test definitively of your results. I would recommend doing a minimum of a triplicate. Also did you do an experimental blank? i would also recommend that there might be the case whereby the first sample was contaminated. As a good practice, i would leave on non-selective media in the BIO safety cabinet (BSC) in whilst plating so that i know that the BSC is clean and not contaminated.
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