7.13 Storage Facility Inspection, Sanitary Defects, Water Age Control, Cleaning & AWWA C652 Disinfection
Key Takeaways
- Tank vents must face downward and carry 24-mesh corrosion-resistant screen, and overflows and drains must terminate with an air gap above grade.
- An overflow piped directly into a sewer or storm drain without an air gap is a direct cross connection to the finished water supply.
- Increasing daily turnover by widening the tank's operating level band is the single most effective control for excessive water age and residual loss.
- A drained storage tank is a permit-required confined space, and diver or remotely operated vehicle inspection avoids taking the tank out of service.
- AWWA C652 offers three disinfection methods: 10 mg/L held 24 hours, a 200 mg/L spray with 30 minutes contact, or 50 mg/L in 5 percent of volume for 6 hours followed by a 24-hour hold, each requiring satisfactory bacteriological samples before return to service.
Storage is a treatment barrier that people walk on top of
WPI lists "clean and disinfect the finished water storage facilities" and "inspect finished water storage facilities (observe discrepancies, drains, screens, corrosion control, structural issues)" as distribution job tasks, and Regulation 11 counts finished water storage among the eight sanitary survey elements. A storage tank sits between the treated water and the customer with nothing but its own integrity protecting the supply.
The sanitary defects that matter
Every one of these is a documented cause of a waterborne outbreak or a boil order somewhere:
| Component | Requirement | Common defect |
|---|---|---|
| Roof and hatches | Watertight; hatch curbed at least 4 inches, with an overlapping shoebox-style cover, gasketed and locked | Unlocked or flat hatch, missing gasket, ponded water on the roof draining into the hatch |
| Vent | Downward facing, screened with 24-mesh corrosion-resistant screen, sized for maximum fill and draw rates | Missing or torn screen, upward-facing vent, undersized vent that collapses the roof during a fast drain |
| Overflow | Terminates with an air gap above grade, screened with 24-mesh, with a splash pad | Overflow piped directly into a storm sewer or submerged, creating a cross connection |
| Drain | Discharges with an air gap | Drain valved directly to a sewer |
| Access ladder and manway | Secured against unauthorized entry | Unlocked access, no security |
| Coating | Interior coating rated for potable contact, in sound condition | Blistering, holidays, exposed steel, corrosion products in the water |
| Cathodic protection | Anodes intact where installed | Depleted anodes; corrosion accelerating |
| Sample tap and level instrumentation | Functional and accessible | Broken transducer causing overflow or drain-down |
| Structural | No cracks, settlement, or seam leakage | Leaking seams, concrete spalling, freeze damage |
Roof drainage is a Colorado-specific concern: a flat or poorly drained tank roof holding snowmelt and bird droppings directly above an inadequately curbed hatch is an obvious contamination path.
Water age and mixing
Storage buys reliability at the cost of detention time. Excessive water age consumes disinfectant residual, allows nitrification in chloraminated systems, produces taste and odor complaints, and increases disinfection byproduct formation. Poor mixing makes it worse: many tanks that fill and draw through a single common inlet-outlet pipe develop a short-circuited zone near the pipe and a stagnant zone elsewhere. Thermal stratification in a tall standpipe leaves cold, old water at the bottom in winter and warm, old water at the top in summer.
Fixes an operator can apply:
- Deeper daily turnover. Cycling more of the tank volume each day is the single most effective control; a tank that only fluctuates a few feet is largely stagnant.
- Active mixing systems — submerged jet mixers or circulators.
- Separate inlet and outlet piping to force flow through the volume.
- Seasonal setpoint changes so more of the tank is used in the low-demand season.
- Booster chlorination at the tank outlet where residual cannot otherwise be maintained. Note that Regulation 100 raises a distribution system to Class 2 when booster chlorine stations exist within the distribution system.
Inspection and cleaning
- Routine exterior inspection — at least annually, and after storms and any known intrusion. Check hatches, vents, screens, overflow, ladder security, coating condition, and site conditions.
- Comprehensive inspection — commonly every 3 to 5 years, either by draining and entering, or by diver or remotely operated vehicle (ROV) inspection which avoids taking the tank out of service. Divers and ROV equipment used in potable tanks must be dedicated to potable service and disinfected before entry.
- Cleaning removes accumulated sediment, which is where biofilm and nitrifiers live. Sediment depth is measured and recorded.
- Confined space. A drained storage tank is a permit-required confined space. Entry requires atmospheric testing, ventilation, attendant, retrieval equipment, and a permit. Freshly coated tanks add solvent vapor hazards and require specific ventilation and monitoring.
Disinfection after work
Any time the interior is entered, cleaned, repaired, or recoated, the tank is disinfected before return to service under AWWA C652, which provides three methods:
- Method 1: Fill the tank with water dosed to about 10 mg/L free chlorine and hold for 24 hours, then drain or dilute.
- Method 2: Spray or brush all interior surfaces with a 200 mg/L solution, then allow at least 30 minutes before filling.
- Method 3: Fill about 5 percent of the tank volume with water dosed to about 50 mg/L, hold 6 hours, then fill the remainder and hold an additional 24 hours.
After the holding period the tank is filled with potable water and bacteriological samples are collected and must be satisfactory before the tank returns to service. Highly chlorinated water drained from the tank must be dechlorinated before discharge to a storm drain or watercourse.
Which finished water storage defect creates a direct cross connection to the distribution system?
Under AWWA C652 Method 1, what chlorine concentration and holding time are used to disinfect a storage tank before returning it to service?
A tall standpipe shows low chlorine residual and taste and odor complaints in the neighborhood it serves, and its level fluctuates only about three feet per day. What is the most effective first correction?