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

Autotrophic Index (AI) In Wastewater

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

When the organic matter is high in wastewater, or it is increased due to additional influx of waste, it increases the quantity of the heterotrophic microorganisms in wastewater. These heterotropic organisms are mainly protozoa. But other examples of these organisms include fungi and bacteria. The increase in the quantity of these organisms causes an increase in the value obtained by the above mentioned ratio. 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 ratio it is necessary to ensure that the sample is tested within half an hour. Some of the microorganisms might die due to change in the water temperature and also due to the change in the quantity of oxygen and food in wastewater.

Given below are values for some of the microorganisms cultivated and tested in laboratory, that can affect the overall AI of wastewater.

An algal culture showed an AI of 40-96;

A sample of marine phytoplankton culture showed an AI of 76-200;

A sample of pond water culture showed an AI of 44-221;

A culture of marine seston showed an AI of 40-146;

A culture of lake seston showed an AI of 457;

Knowing the AI greatly helps the plant personnel in the treatment of wastewater.