Which type of wastewater treatment process primarily focuses on utilizing microorganisms in suspension?

The answer is focused on suspended growth systems, which are designed primarily to use microorganisms that are suspended in the wastewater. In this type of treatment process, the microorganisms are not attached to a surface but rather flourish throughout the liquid medium, where they break down organic matter and contaminants in the wastewater.

This method is commonly exemplified by activated sludge systems, where air is supplied to the mixed wastewater containing microorganisms, enabling them to thrive and effectively treat the water by biological decomposition.

In contrast, fixed film systems involve microorganisms attached to surfaces or media, which serves as a filter for the wastewater, making them distinctly different from the suspended growth approach. Primary treatment systems focus on physical processes such as sedimentation and screening to remove solids from wastewater without relying on microorganisms. Tertiary treatment systems involve additional treatment steps after primary and secondary processes, aimed at further purifying wastewater but do not primarily rely on microorganisms in suspension. Thus, suspended growth systems are indeed the right answer because they uniquely highlight the reliance on suspended microorganisms for wastewater treatment.