Wastewater biomass: how to use the autotrophic index to measure water quality

Microorganisms in the wastewater, along with other organic matter, increase the biomass of the wastewater. This increase is inversely related to the quality of the wastewater. In this article I discuss, in detail, a formula that is used to determine the quality of wastewater.

Autotrophic index (AI) in wastewater

To estimate the quality of the wastewater, factors such as the biomass content of the wastewater and the chlorophyll present in the wastewater should be considered. In fact, to determine the quality of the wastewater, the value of the ratio between the biomass of the wastewater (total) and the chlorophyll in the wastewater is used. This value is known as Autotrophic Index or AI in short.

When organic matter is high in wastewater, or increases due to additional waste input, the number of heterotrophic microorganisms in wastewater increases. These heterotropic organisms are mainly protozoa. But other examples of these organisms include fungi and bacteria. The increase in the amount of these organisms causes an increase in the value obtained by said relationship. It is written as:

total biomass (ash-free weight of organic matter) (mg/m^3)
————————————————– ——————— = AI
chlorophyll a (mg/m^3)

Note: When using this relationship, it is necessary to ensure that the sample is analyzed within half an hour. Some of the microorganisms may die due to the change in water temperature and also due to the change in the amount of oxygen and food in the wastewater.

Below are the values ​​of some of the microorganisms cultured and tested in the laboratory, which can affect the general AI of the wastewater.

A culture of algae showed an AI of 40-96;
A culture sample of marine phytoplankton showed an AI of 76-200;
A pond water culture sample showed an AI of 44-221;
A culture of sea seston showed an AI of 40-146;
A culture from Lake Seston showed an AI of 457;

Knowing AI greatly helps plant personnel in wastewater treatment.