In which growth phase does an aerobic digester primarily operate?

An aerobic digester primarily operates in the endogenous respiration phase, which is characterized by the microorganisms using stored energy reserves for maintenance and growth. In this phase, bacteria have utilized most of the readily available organic material and begin to rely on the energy stored within their own cells. This process helps in maintaining the population of microorganisms while also stabilizing the organic matter present in the digester.

During endogenous respiration, the environment is aerobic, meaning there is ample oxygen available, allowing the microorganisms to continue breaking down the organic material, albeit at a slower rate than in the log phase, where rapid growth occurs due to an abundance of nutrients. This phase is critical in treatment processes as it helps ensure that the digesting microorganisms can survive and continue their important work even when nutrient levels decrease.

The other phases mentioned, such as log, lag, and declining phase, represent different points in the growth cycle of microorganisms. The log phase sees rapid growth due to abundant nutrients, the lag phase is the initial slow adaptation period before growth spikes, and the declining phase indicates a decrease in microbial population and activity due to lack of nutrients or unfavorable conditions. However, the aerobic digester's primary function aligns closely with the processes that occur during endogenous respiration, making it the correct choice for