Turnover days
How many days a tank takes to exchange its whole volume, compared with the 3 to 5 day starting point MassDEP recommends.
Result
Above the 3 to 5 day starting point of MassDEP Guidelines 8.1.6.
The formula
Turnover days = tank volume ÷ volume exchanged per day. The exchange is the gallons a day, or the daily level swing in feet × gallons per foot of height.
A starting point
MassDEP recommends a 3 to 5 day complete turnover as a starting point and asks that each tank be evaluated for its own goal. The made-up Main Street Tower, 500,000 gallons with 110,000 a day, turns over in 4.5 days.
Mixing matters too
Turnover is a planning figure. How water ages inside a tank also depends on how it mixes; stratified layers can stay longer.
Sources: MassDEP Guidelines for Public Water Systems, Chapter 8 Finished Water Storage (Rev 5-11); EPA Distribution System Issue Paper: Finished Water Storage Facilities (2002). Nothing typed here leaves the page.