Final answer:
The lowest portion of the storm drainage system is the building sewer.
Step-by-step explanation:
The lowest portion of the storm drainage system is known as the building sewer. This is where wastewater from various sources such as toilets, baths, kitchens, and industrial processes, which have been directed through the building's drainpipes, converge and are carried away from the building to the sewage treatment facilities or septic systems. The septic tank is a key component of a septic system, where solid waste settles and is decomposed by microorganisms anaerobically, but it is not the lowest part of the storm drainage system itself.