This question is regarding the population and growth of microbes or microorganisms in the light and dark grey water. is there an accepted techniques for the separation and range of microbes?
Yes, it is possible to check the growth of microbes in light and dark grey water.
You can take 500uL broth sample and centrifuge at 12000rpm for 3-5 mins. Remove the supernatant and add 500uL Phosphate buffer saline (PBS - pH 7.0) mix it and centrifuge it again and remove the supernatant. Now add the 500uL of PBS again and mix the cell pellets and you can take OD at 600nm.This will work if you have only one microbial strain growing in water.
If you have different strains growing in water sample then you can go for serial dilution technique. Paper attached for your reference:
Article Estimation method for serial dilution experiments