2013 Water System Plan, Volume II - Seattle City Clerk's Office - City ...

5.9.3 Hydraulic Mixing for Water Age and Water
Quality Control
To prevent hydraulic dead zones and excessive water ages within a storage tank or reservoir,
there must be a means for complete hydraulic mixing throughout the entire volume of the
storage cell. The configuration and sizes of inlet and outlet pipes to the cell have a direct impact
on the degree of hydraulic mixing achievable.
Each storage cell should have a volume in which water age (hydraulic detention time) is not
more than 5 days at projected average water demands when the reservoir operates at full
capacity. The goal is to keep total water age through the reservoir to no more than a 5-to-7-day
range.
The Water Research Foundation publication, Maintaining Water Quality in Finished Water
Storage Facilities, describes design considerations and features for controlling water quality in
storage facilities.
5.9.3.1 Inlet and Outlet Pipes
Generally, a separation of the inlet and outlet points within a storage cell will enhance mixing
and help avoid water quality problems associated with dead zones and short circuiting. For
ground tanks, this is done by locating the inlet discharge near the perimeter of the cell with an
upward bend. The tank outlets are then placed into the center of the tank floor. In elevated
tanks, inlet an outlet points are separated one of two ways:
Bring separate inlet and outlet lines up through the tank
Split the line in the riser and use check valves to introduce water into the center of
the tank near the top of the water column. In this option, the outlet pipes are placed
at the tank perimeter with a vertical separation to the inlet elevation of not less than
about ½ the total cell height.
Proprietary pipe and valve systems for storage facilities can be specifically designed based on
the momentum mixing principle. The Red Valve Company system has gained widespread use.
For smaller tanks (less than 0.5 million gallons), the inlet and outlet may not need to be
separated. Smaller tanks have smaller volumes, which allow adequate momentum for mixing.
A. Inlet Pipe
The inlet pipe should be as small as practicable to maximize inlet velocity to provide for
adequate momentum mixing throughout the storage cell to preclude hydraulic shortcircuiting.
Reductions of inlet diameter have also been retrofitted on existing SPU tanks
during tank renovation by using a reducer on the discharge end of the inlet pipe.
B. Outlet Pipe
In ground level reservoirs, each storage cell should have two outlet pipes, one near the
mid-level and the other near the bottom of the cell. Both outlets should have isolation
valves. The mid-level and lower outlets remain open for normal operation. The lower
outlet should have a seismic valve that closes automatically during an earthquake to
prevent the cell from draining past the mid-level.
5-54 SPU Design Standards and Guidelines