Which of the following factors would increase oxygen demand in an aeration tank?

An increase in microorganisms in an aeration tank directly contributes to higher oxygen demand. Microorganisms, particularly bacteria, are responsible for breaking down organic matter through aerobic respiration, a process that consumes oxygen. As the population of microorganisms grows, their metabolic activity increases, leading to a higher demand for oxygen in order to support their growth and the degradation of waste materials.

In contrast, an increase in pH typically affects microbial activity and could lead to less efficient respiration, potentially lowering oxygen demand under certain conditions, depending on the specific microorganisms involved. The presence of inert wastes does not contribute to oxygen demand since these substances are not broken down by microorganisms. An increase in toxic substances can inhibit microbial activity and thus decrease the overall oxygen demand, as the affected microorganisms cannot perform their metabolic processes efficiently.

Therefore, the factor that most significantly raises oxygen demand in an aeration tank is the increase in microorganisms due to their enhanced activity in breaking down organic materials.