Florida Wastewater Practice Test

What growth phase is an aerobic digester operated in?

1. Logarithmic phase

2. Endogenous respiration

3. Stationary phase

4. Decline phase

The correct answer is: Endogenous respiration

The correct answer is the endogenous respiration phase, which is pertinent in the context of aerobic digestion processes. In aerobic digesters, microorganisms break down organic matter in a controlled environment that promotes the use of oxygen. During this digestion process, as the organic material decreases, the microbes start to consume their own stored energy reserves and begin utilizing the metabolic byproducts of their own decay. This phase is crucial because it allows the microorganisms to survive even when nutrients are limited. This self-sustaining behavior leads to a slowdown in growth rates, as the conditions shift towards lower substrate availability. The focus here, therefore, is on the balance between microbial maintenance and decay rather than active reproduction. In contrast, the logarithmic phase is characterized by rapid growth where microorganisms multiply quickly due to ample nutrients. The stationary phase occurs when the growth rate equals the death rate, leading to a stable population, but this is not the primary operational phase of an aerobic digester. The decline phase denotes a period of significant mortality among microorganisms due to a lack of nutrients or unfavorable conditions, which is not indicative of the controlled environment maintained in aerobic digestion.