Wednesday, February 07, 2018

If you only want to test one drinking water parameter, this is the one... and how to do it

If you have been reading my blog, you will no doubt find that I have touched upon quite a number of water quality parameters e.g. COD (chemical oxygen demand), pH, electrical conductivity. Yet, when it comes to drinking water, the number one parameter to check is:

Bacterial count

And not just any bacteria, the favourite in the water industry seems to be E. coli though Enterococcus seems to be a strong contender. Essentially, the idea is to find an indicator microbe to "indicate" recent faecal pollution which implies possible presence of human pathogens. In addition, this microbe has to be:
  • Always present in faeces of humans 
  • Present in high numbers
  • Easy to detect by simple and inexpensive methods
  • Unable to multiply after they have left the body and entered the water supply
  • Not a pathogen itself
Obviously, no microbe can satisfy all the above criteria perfectly but E. coli (and Enterococcus) comes as close as you can get.

How to measure it?

In 1 of my previous posts - How do I test my own tap water: a DIY guide (part 2) , I recommended this product for E. coli.
Industrial Test Systems 487197 WaterWorks EZ Cult Bacteria Test. USD9.77
Thanks to Victor who provided 2 test bottles, I finally have a chance to test it out. Personally, I still find this product rather pricey, especially when you factored in the shipping costs. But as mentioned in my previous post, this product is as idiot-proof as you can get.

Anyway, with Victor's help, we set up 1 bottle to be the sample with live faecal pollution and the 2nd bottle to be the blank with deionised water.

I also took the chance to try out Aquagenx CBT (compartment bag test) specifically to quantify E. coli in MPN (most probable number). The setup is similar to the above - 1 live sample and 1 blank.

Important: WaterWorks EZ Test is qualitative - either YES or NO for the presence of coliform (and E. coli under UV light) while Aquagenx CBT is a semi-quantitative test that provides a numeral in MPN for E. coli count.

Though Aquagenx CBT allows for "incubation" at ambient temperature provided it does not fluctuate too much, we nevertheless incubate all tests in a mini-incubator that I dug out. (Personally, I find the requirement for incubation at a steady temperature e.g. 35C to be the most formidable technical challenge for anyone engaging in DIY bacterail testing, more so if under field conditions.)


Figure: Before incubation. Left - live sample, right - blank.

Figure: Preparing for incubationl Notice the leftmost bottle for live sample has already turned green, indicating the presence of coliform. And this is only at most half an hour after inoculation. The Aquagenx CBT (rightmost) is the live sample.

Figure: After 1 day of incubation. The CBT blank (white clip) still remains brownish while the CBT sample (red clip) has fully turned green, indicating E. coli and also translating to > 1000 MPN/100mL or very high risk. Unfortunately, my UV light was not working so I could not confirm the presence of E. coli in the EZ Test sample as under UV light, E. coli will display fluorescence.

Ok, that's it, folks! Hope the discussion above is helpful to those doing testing of drinking water, especially in the field. Oh, the price of Aquagenx CBT: including shipping cost --> ~USD150 for 10 tests.