Studying various microbes and analyzing the total microbial count of a sample from your secondary wastewater treatment unit can give you an in-depth view to the health of your biological wastewater treatment plant. But in absence of a microscope or a testing facility, physical characteristics such as odour, colour, turbidity, foam etc of effluent can indicate health of wastewater treatment plant. Estimation of the sludge volume index is an important way to understand the condition of your wastewater treatment plant. Here, we understand the sludge development in an aeration tank and the settling ability of the sludge, both of which are positive indicators of the biological health of your effluent.