6.5 Storage Tanks & Reservoirs
Key Takeaways
- Distribution storage serves three purposes: equalizing demand, providing fire flow reserve, and supplying emergency reserve during outages.
- Elevated tanks and standpipes float on the system and set the hydraulic grade line, while ground storage usually requires booster pumping.
- Inadequate turnover is the leading water quality problem in storage, producing residual loss, nitrification, and thermal stratification.
- Tank vents and overflows must be screened and the overflow must discharge through an air gap, never submerged or directly connected to a sewer.
- Tanks must be inspected on a regular cycle and disinfected after cleaning or repair before returning to service.
6.5 Storage Tanks & Reservoirs
Storage is where the distribution system's water sits still — and standing water is where water quality goes to die. Operators are tested both on why storage exists and on the specific sanitary defects that turn a tank into a contamination pathway.
1. Why Storage Exists
| Purpose | What it does |
|---|---|
| Equalization | Lets the plant and wells run at a steady average rate while the tank absorbs peak-hour demand. Without it, source and pumping capacity would have to match peak instantaneous demand |
| Fire flow reserve | Holds a dedicated volume for firefighting, usually the controlling factor in sizing storage for small systems |
| Emergency reserve | Keeps water flowing during power failure, main break, pump failure, or a plant shutdown |
| Pressure stabilization | Elevated storage floats on the system and holds the hydraulic grade line steady, damping pressure fluctuation |
Storage also reduces energy cost by letting pumps run during off-peak power periods.
2. Storage Types
| Type | Description | Hydraulics |
|---|---|---|
| Elevated tank | Bowl on a tower or pedestal | Floats on the system; sets the hydraulic grade line directly. All usable volume is at useful pressure |
| Standpipe | Tall cylindrical ground-supported tank | Only the upper portion is at useful pressure; the lower portion is effectively dead storage requiring pumping |
| Ground storage reservoir | Large-volume tank at grade | Requires booster pumping to serve the system |
| Hydropneumatic tank | Pressure vessel with an air cushion | Small systems; very little usable volume, primarily controls pump cycling |
| Clearwell | At the plant, downstream of filters | Provides disinfection contact time for CT credit and buffers between filtration and high-service pumping |
Note the exam distinction: a clearwell's primary job is contact time, not distribution storage.
3. Turnover — The Central Water Quality Issue
Water sitting in a tank for days loses disinfectant residual, warms, stratifies, and grows biofilm. Good practice targets complete turnover in roughly 3 to 5 days, achieved by cycling the level meaningfully rather than holding the tank nearly full.
Symptoms of poor turnover
- Disinfectant residual falls at the tank outlet compared with the inlet.
- Heterotrophic plate counts rise; coliform positives appear near the tank.
- Nitrification in chloraminated systems — falling chloramine residual with rising nitrite and nitrate, as described in Section 5.3.
- Thermal stratification — warm water layers on top and never mixes, so a fraction of the tank is effectively stagnant year-round.
- Taste, odor, and color complaints concentrated in the tank's service area.
Corrective measures
| Measure | Effect |
|---|---|
| Increase level cycling | Deeper daily draw-down forces exchange of the stored volume |
| Separate inlet and outlet piping | A single common inlet/outlet pipe lets fresh water enter and immediately leave, leaving the bulk stagnant — this is the classic design defect |
| Active mixing systems | Mechanical or passive mixers break thermal stratification |
| Deep-cycle then refill | Periodic aggressive draw-down to exchange the full volume |
| Booster disinfection at the tank | Restores residual where turnover alone cannot |
| Reduce oversized storage | Some systems simply hold more storage than demand can turn over |
4. Sanitary Defects — The Inspection Checklist
These are exactly the items a sanitary survey looks for, and they are prime exam material:
| Component | Requirement |
|---|---|
| Hatches and access openings | Locked, with a raised curb at least 4 inches and an overlapping, downward-shielded cover with a gasket |
| Vents | Must be present and sized for maximum flow, downturned or otherwise shielded, and screened (commonly 24-mesh corrosion-resistant) to exclude insects, birds, and debris |
| Overflow | Screened, downturned, and discharging over an air gap — never submerged, never directly connected to a sewer or storm drain |
| Drain | Discharged through an air gap |
| Roof and seams | Watertight; no ponding, no openings, no bird access |
| Coating and cathodic protection | Interior coating intact and NSF/ANSI 61 certified; cathodic protection functioning |
| Site | Fenced, secured, graded to drain away from the structure |
The single most-tested defect: a submerged overflow or an overflow piped directly into a sewer creates a direct cross-connection between the finished water storage and the sewer. The overflow must always terminate above grade with a visible air gap and a screened, downturned opening.
5. Inspection, Cleaning, and Return to Service
Inspection frequency: exterior inspections should be frequent and routine; interior inspections are commonly performed on a 3-to-5-year cycle, by draining and entering, by diver, or by remotely operated vehicle. Sediment accumulation, coating failure, corrosion, and structural defects are the findings that drive maintenance.
Confined space: entering a drained tank is a permit-required confined space entry. Every requirement in Section 13.1 applies — atmospheric testing, ventilation, attendant, retrieval equipment, and permit.
Disinfection before return to service
Any tank that has been drained, cleaned, coated, or repaired must be disinfected before it is returned to service, following AWWA C652. Three accepted methods:
| Method | Approach |
|---|---|
| Chlorination Method 1 | Fill the tank with water dosed to about 10 mg/L free chlorine and hold for at least 6 hours |
| Chlorination Method 2 | Spray or brush all interior surfaces with a strong solution of roughly 200 mg/L and hold at least 30 minutes |
| Chlorination Method 3 | Fill about 5% of the tank volume with water dosed to about 50 mg/L, hold 6 hours, then fill to overflow and hold a further period |
After disinfection, the tank is drained or the water is neutralized and disposed of appropriately, then refilled and sampled for bacteriological analysis. Two consecutive satisfactory samples taken 24 hours apart are required before the tank returns to service. Highly chlorinated water must be dechlorinated before discharge to protect receiving waters — see Section 9.1.
A storage tank shows a much lower disinfectant residual at its outlet than at its inlet, and nearby customers report taste complaints. What is the most likely cause?
How must a finished water storage tank overflow terminate?
After a storage tank is drained, cleaned, and recoated, what must occur before it returns to service?
Which storage type floats on the distribution system and directly establishes the hydraulic grade line?