Storage Tanks Impact on Water Quality and Assessing Performance Presentation 3 April 2015

Overview  Tank functions & designs  Tank operating parameters impacting water quality (chlorine residual)  Sampling at tanks and in their vicinity  Tank case histories  Assessing Performance  Approaches to improve water quality  Case study  Summary

Physical Characteristics  Storage tanks can vary by the following:  Size – diameter, height, volume  Shape – cylindrical (d>h), standpipe (h>d), rectangular, ellipsoidal, pedestal, etc.  Elevation – underground, ground, elevated  Inlet/Outlet – configuration (common or “flow-through”), diameter, location  Other – baffling, pillars, mixing systems (static or mechanical)

Function of Storage Tanks  Equalize water supply, so pumping doesn’t have to equal demand:  Fire flows  Main breaks  Off peak pumping  Maintain system pressure in specific range  Important to consider tank functions when assessing potential operational changes

Ground Tank Vent Hatch(Closed) Overflow Inlet/Outlet Courtesy of Jeff Swertfeger Greater Cincinnati Water Works

Underground Tank Vent/Overflow Hatch(Closed) Inlet Outlet Courtesy of Jeff Swertfeger Greater Cincinnati Water Works

Elevated Tank Vent Overflow Access Hatch Inlet/Outlet Monitor OperatingRange Courtesy of Jeff Swertfeger Greater Cincinnati Water Works

Factors Affecting Chlorine Residual in Tanks  Water age (tank turnover)  Tank Mixing  Fill period chlorine residual (chlorine in)  Chlorine demand in the tank  Bulk water  Tank surface  Sediment  Temperature  Higher temp = lower residual

Water Age in Tanks  Water age determined by average tank volume and volume added per day.  Water age will be the same throughout a tank only if it is well mixed.  Stagnant zones:  Stratification  Poor mixing  Short draw/fill cycle Operating Range? Courtesy of Jeff Swertfeger Greater Cincinnati Water Works

Tank Mixing Vent Hatch(Closed) Overflow Courtesy of Jeff Swertfeger Greater Cincinnati Water Works

Short Circuiting Vent Hatch(Closed) Overflow Inlet Outlet Courtesy of Jeff Swertfeger Greater Cincinnati Water Works

Inlet/Outlet Monitor Last in / First Out Courtesy of Jeff Swertfeger Greater Cincinnati Water Works

Tank Sampling Vent Hatch(Closed) Overflow Inlet Outlet Courtesy of Jeff Swertfeger Greater Cincinnati Water Works

Some “Problem Tanks” We Have Met Selected Case Histories

Case Study A

Case Study B

Case Study C

Case Study D

Case Study E

Case Study F

Assessing Storage Tank Performance  Objective: To develop an accurate “picture” of storage tank performance related to water quality Water Quality Data Operational Data Physical Character- istics

Tank Parameters Impacting Water Quality  Water quality in storage tanks is significantly influenced by both:  Turnover Time (Water Age) – Average length of time that water resides in a tank. Optimization Guideline: Turnover time should be less than 3-5 days. Each tank should be evaluated individually and given its own goal.  Tank Mixing – A function of the momentum of the inlet flow during a fill cycle.  Tank level (or pressure) data and physical characteristics are needed to calculate these parameters.

What Impacts Turnover Time?  Can be influenced by:  Tank volume  Location/elevation  Consumer demands  Pump schedules (i.e., duration, frequency, and rate of fill-and-draw cycles)  Operating range: Minimum emergency storage capacity System pressure requirements

Storage Tanks in Series WTP 2.0 Days 3.0 Days 4.0 Days 3.5 Days 4.5 Days 3.5 Days = 20.5 Days!

What Impacts Mixing?  Can be influenced by:  Tank shape  Temperature differentials: Inflow > Ambient = Turnover Inflow < Ambient = Stratification  Momentum of inlet flow (function of flow rate and diameter of inlet/outlet)  Inlet/Outlet – configuration (common or “flow through”), diameter, and location  Fill cycle duration  Engineering controls – baffling, static, or mechanical mixers

Temperature Effects on Tank Mixing Example Courtesy of Jeff Swertfeger Greater Cincinnati Water Works

Water Quality Monitoring  Common parameters used for tank assessment:  Free chlorine  Temperature  pH  DBPs (optional)

Water Quality Monitoring  Grab Sampling (In-Tank or DS):  Quick and easy assessment of water quality  Must consider whether tank is filling/draining  May not capture variability of data  Less expensive, but requires more time and labor  Online Monitoring:  Convenient and continuous assessment of water quality  Continuous readings capture variability of water quality throughout tank cycle  More expensive, but requires less time and labor

Online vs. Grab Sampling

Summary  Understanding how tanks operate is a critical aspect of assessing their performance.  Both adequate turnover and mixing are necessary.  Storage tanks are unique, and their performance should be assessed individually.  Consider secondary impacts and understand the extent of a problem when making operational changes.  Water quality data is the best indicator of tank performance.