9.1 Storage Tank and Booster Pump Station Operation, Inspection and Disinfection
Key Takeaways
OHA treats an unsecured or leaky roof or hatch, an unscreened vent, or a drain or overflow without a flap valve or screen as a significant deficiency at storage facilities.
AWWA C652 tank disinfection options are 10 mg/L free chlorine held 6 or 24 hours, 50 mg/L held 6 hours, or a 200 mg/L spray of all wetted surfaces.
After routine maintenance, an Oregon reservoir may return to service before bacteriological results if the water leaving it has at least 0.4 mg/L free or 2.0 mg/L combined chlorine.
Divers in drinking water reservoirs must wear a full-body dry suit and hardhat with an external air supply and be sprayed with 200 mg/L chlorine solution.
Booster pumps taking suction from mains need a low-pressure cut-off set at no less than 20 psi, and pressure tanks must be rated at least 50 percent above maximum system pressure.
Why Storage Gets Special Attention
Reservoirs hold large volumes of finished water that is open to the outside world through vents, hatches, overflows and drains. Birds, rodents, insects and stormwater entering through a failed screen or hatch have caused waterborne outbreaks. That is why OAR 333-061-0076 lists these storage problems as significant deficiencies:
- roof and access hatch not adequately secured;
- roof and access hatch not watertight;
- no flap valve, screen or equivalent over the drain and overflow; and
- no screened vent.
Routine Inspection Checklist
| Item | What to look for |
|---|---|
| Hatches | Locked, gasketed, curbed and watertight; intrusion alarms working |
| Vents | Intact screens; downward-facing; no ice or debris blockage that could pull a vacuum |
| Overflow and drain | Flap valve or screen in place; discharge through an air gap to a splash pad or ditch, never directly to a sewer or storm main |
| Exterior | Coating failures, rust streaks, leaks, cracked concrete, ladder and fall-protection condition, vegetation and site security |
| Cathodic protection | Rectifier output and pipe-to-water potential readings, on schedule |
| Water quality | Disinfectant residual, temperature stratification, water age, sediment depth |
| Controls | Level transmitter, altitude valve, high- and low-level alarms |
Periodic interior inspections, every few years and more often for older tanks, use washouts or remotely operated vehicles. Divers can inspect while the tank stays in service, under strict sanitary rules. They check sediment, coating condition, corrosion, roof structure and inlet and outlet piping. AWWA M42 (steel tanks) and D100-series standards guide steel tank work, and coatings must be NSF/ANSI 61-certified.
Disinfecting Reservoirs (OAR 333-061-0050(10) and AWWA C652)
| Method | Procedure | Before service |
|---|---|---|
| 1 | Fill the tank and hold a free chlorine residual of at least 10 mg/L for the required 6- or 24-hour period | Drain, refill with potable water and collect a coliform sample |
| 2 | Fill the tank with a 50 mg/L solution and hold 6 hours | Drain, refill and sample |
| 3 | Spray or brush a 200 mg/L solution on every surface that will contact water when full | Fill without flushing, then sample |
The tank goes into service when the coliform sample is negative. Oregon allows an exception after routine maintenance, inspection or repair: the reservoir may return to service before the lab report if the water leaving it has at least 0.4 mg/L free chlorine or 2.0 mg/L combined chlorine (OAR 333-061-0050(10)(h)).
Divers must wear a full-body dry suit with a hardhat and an external air supply. They are disinfected by spraying a 200 mg/L chlorine solution on every surface that will contact drinking water (OAR 333-061-0050(10)(i)).
Super-chlorinated water drained from a tank must be dechlorinated before it reaches a ditch, storm drain or stream.
Operating Storage for Quality and Reliability
- Cycle levels. Daily drawdown and refill limit water age and stratification. Many utilities target turning over the volume every few days, balanced against fire and emergency reserves.
- Mixing. Separate inlet and outlet locations, mixing nozzles or active mixers prevent short-circuiting and thermal layering.
- Freeze protection. Ice can crush vents and hatches. Keep water moving and inspect after cold snaps.
- Altitude valves close to prevent overflow when a tank fills from a higher pressure zone. A stuck valve causes overflow; a closed one starves the zone.
Booster Pump Stations
OHA's pumping facility standards (OAR 333-061-0050(7)) include:
| Standard | Reason |
|---|---|
| Take suction from tanks or reservoirs where possible | Avoids pulling the suction main below atmospheric pressure |
| Pumps drawing from mains have a low-pressure cut-off on the suction side set at no less than 20 psi | Keeps the upstream zone above 20 psi; missing cut-offs are a significant deficiency |
| Avoid suction lift; install pumps under positive head, or prime with water that meets MCLs | Prevents loss of prime and contamination |
| Locate above the 100-year flood level and grade drainage away | Protects electrical equipment |
| Durable, lockable, heated and lighted building with sloped floor drains | Security and reliable operation |
Hydropneumatic tanks in small systems are pressure tanks. They must be steel or another NSF/ANSI 61-certified material and designed for a pressure at least 50 percent greater than the maximum system pressure. They maintain pressure between pump cycles but provide almost no fire storage. Operators check the air-to-water balance (waterlogged tanks cause rapid pump cycling), the pressure switch settings, the relief valve and the sight glass.
Common Booster Station Problems
- Short cycling: an undersized or waterlogged pressure tank, or switch differential set too close.
- Low suction pressure: upstream demand or a closed valve; the cut-off trips the pumps.
- Surges at pump start and stop: check valves slamming. Use soft starters, VFD ramps or surge-anticipating valves.
- Overheating motors: blocked ventilation or failed thermostatic controls.
- Security: unlocked doors and unmonitored intrusion alarms are both sanitary and safety risks.
Divers will inspect an in-service steel reservoir. What does OAR 333-061-0050 require of them?
No special requirements, provided the tank is first drained to half its level
A 50 mg/L chlorine bath and a negative coliform sample taken from each diver's suit
A full-body dry suit with hardhat and surface air, sprayed with 200 mg/L chlorine
Wet suits and scuba tanks, with a shower in potable water before entering the tank
After a planned recoating repair, an operator disinfects a reservoir and refills it. The water leaving it has a 0.6 mg/L free chlorine residual. Under Oregon rules, may it return to service before the coliform result arrives?
No; every reservoir must wait for a negative coliform result before service
Yes, but only if the leaving water's residual is above 2.0 mg/L free chlorine
Yes; after maintenance or repair, leaving water needs at least 0.4 mg/L free chlorine
No; the reservoir surfaces must first be sprayed again with 200 mg/L chlorine
A booster station pumps directly from a distribution main to serve a hillside zone. What device does OHA require on the suction side?
A low-pressure cut-off set at no less than 20 psi
A reduced pressure backflow assembly on the suction
A sodium hypochlorite booster feed ahead of the pumps
An altitude valve on the pump suction header
Sections you finish are checked off in the contents.