How does adding primary treatment to an activated sludge process affect the food/microorganisms ratio?

Adding primary treatment to an activated sludge process lowers the food/microorganisms ratio by removing a significant portion of the organic material, or "food," before it enters the biological treatment stage.

In an activated sludge process, the food/microorganisms (F/M) ratio is a critical parameter that affects the efficiency of the biological treatment. The F/M ratio is calculated by dividing the mass of biochemical oxygen demand (BOD) or organic load that is available to the microorganisms by the mass of microorganisms present in the system.

When primary treatment is employed, large solids and a portion of the soluble organic matter are removed from the influent wastewater. This results in less organic material being fed to the activated sludge process compared to the situation where only secondary treatment is used. Consequently, this decrease in organic loading means that there is a lower amount of food available for the microorganisms relative to their biomass, thus reducing the F/M ratio.

A lower F/M ratio can lead to more efficient biological treatment processes, as microorganisms are encouraged to utilize the available organic matter more effectively and can help in preventing excessive growth of microorganisms, which may lead to operational issues like foaming or bulking. However, it's essential to balance this ratio correctly to ensure optimal microbial activity and