text: ATP is a molecule found only in and around living cells, and as such it gives a direct measure of biological concentration and health. ATP is quantified by measuring the light produced through its reaction with the naturally-occurring firefly enzyme luciferase using a luminometer. The amount of light produced is directly proportional to the amount of biological energy present in the sample.
  Within a water sample containing microorganisms, there are two types of ATP:
  * Intracellular ATP – ATP contained within living biological cells.
  * Extracellular ATP – ATP located outside of biological cells that has been released from dead or stressed organisms.
  Accurate measurement of these two types of ATP is critical to utilizing ATP-based measurements. Being able to accurately measure these different types of ATP offers the ability to assess biological health and activity, and subsequently control water and wastewater processes.