What preferred substrate do PAOs take up when they release phosphorus?

Polyphosphate-accumulating organisms (PAOs) primarily utilize carbon as their preferred substrate when they release phosphorus. In wastewater treatment processes, particularly in enhanced biological phosphorus removal, PAOs take advantage of organic compounds, primarily carbohydrates, to store energy and accumulate phosphorus in the form of polyphosphate.

During periods of anaerobic conditions, PAOs uptake volatile fatty acids (VFAs)—which are a type of carbon compound—leading to the accumulation of intracellular polyphosphate. In return for this uptake of carbon, the PAOs release phosphorus into the surrounding environment. This phosphorus release is a crucial part of their metabolic process, allowing them to effectively remove phosphorus from the wastewater.

While ammonia, nitrate, and oxygen play roles in the overall biochemical processes in wastewater treatment, they are not the primary substrates that PAOs utilize for releasing phosphorus. Rather, ammonia is a nitrogen source, nitrate serves primarily in nitrification and denitrification processes, and oxygen is essential for aerobic conditions, but not specifically for phosphorus release by PAOs. Thus, the accurate understanding of PAOs' functionality highlights the importance of carbon as the fundamental substrate in this context.