The Hydro Tank model showed that modifying the riser with a restrictor plate to a 2-foot diameter (the same
diameter as the pipe leading to the tank) would provide a good mixing pattern and that complete mixing should
be achieved in 8. 2 hours, which is a typical fill cycle for this tank. This is in comparison to over 20 hours required
for the 5-foot inlet. In addition, an evaluation of the historical pressure loss over a variety of operating conditions
indicated that a reduction in inlet diameter to 2 feet would not have much difference on headloss largely because
the pipe leading to the tank already had a 2-foot diameter.
The reduction of the inlet diameter was then included in a planned tank renovation project. This project also
included the installation of sampling taps at the 25, 50, and 75% elevations as well as the continuous monitor
on the common inlet/outlet. Figure 1 is a photo of the modification that was made to the tank inlet reducing the
opening from 5 feet to 2 feet in diameter.
In the three years subsequent to the inlet riser modification, the steep drops in chlorine levels that were once
commonplace do not occur anymore. The chlorine levels observed in the tank effluent are remarkably stable over
the operational cycle. In addition, water quality measurements taken at the newly installed sample taps show
there is very little variation in the water quality and temperature, thereby, indicating a well-mixed tank.
Additional software called Comp Tank was also included in Water Quality Modeling of Distribution System
Storage Facilities. GCWW compared the chlorine levels observed with those predicted from an ideally mixed
tank. The observed levels that GCWW sees is very close to those predicted by Comp Tank indicating that the
mixing achieved after the inlet modification is very close to the mixing predicted in an ideal tank.
In conclusion, tank mixing and turnover can be a significant area for improvement and should be considered by
any utility who wishes to reduce water age, improve disinfection residual, and reduce DBPs. Utilizing tools like
those developed for and included with Water Quality Modeling of Distribution System Storage Facilities
are extremely powerful, easy to use, and can be used by utility designers and engineers to evaluate the design of
existing and planned storage facilities to optimize mixing and reduce water age.
Figure 1: Photo of restrictor plate modification installed on riser pipe opening into bottom of tank bowl.