Well,

In the two cases, and at the same discharge in the outlets and equal tanks volume, the retention time is equal.

Regards

Thanks Bayan

The theory indicates that is the case but we have found in practice that tanks configure in parallel can often have preferred flow paths meaning that the water age in one tank is much older than the other tank. When the two paths combined the water quality takes on the characteristics of the older water i.e. more chlorine decay or increased disinfection by-product formation.

Even the tanks and pipework are identical, the water seems to take preferential flow paths, which I don't understand.

Well,

In the area where water enters the reservoir, flow is usually of laminar type at low speed and maybe in the form of turbulent flow at high speed, creating water currents in the form of swirls due to the movement of rapid water currents within a calm water mass, this rapid current captures additional water and displaces calm water on its path length creates a moving waterway separated from the original stream in the form of water swirls. These pathways are called the preferred pathways for the movement of the water mass and this phenomenon is created by the nature and properties of water molecules and their interconnectedness due to the polarizing property of water molecules and their ability to form and take the shape of the space that enters or leaves it (the water molecule in the form of a Mickey Mouse head - hydrogen bonding to oxygen).

This condition is similar to the mechanical movement of the Gulf Stream in the Atlantic Ocean - see attached figure.

Regards