7.3 Storage Facility Cleaning & Disinfection

7.3 Storage Facility Cleaning & Disinfection

Quick Answer: A storage tank is taken offline, drained, cleaned of sediment, disinfected per AWWA C652, sampled for coliform, and returned to service — typically every 3–5 years or as inspection warrants. Entry into a tank is a confined space, so atmospheric testing, ventilation, an attendant, and a rescue plan are required.

Why Clean Storage Tanks?

Storage tanks accumulate sediment carried in from the source or from main breaks. Over time that sediment harbors biofilm, supports microbial growth, consumes disinfectant residual, and can be resuspended during high demand, leaving customers with cloudy, odorous water. Routine cleaning removes the sediment, restores the disinfectant residual in the stored water, and gives operators a chance to inspect the interior coating and appurtenances. Cleaning frequency is typically every 3–5 years or sooner if an inspection, water-quality sample, or customer complaint warrants it. Some states and primacy agencies set a maximum interval in regulation; the operator should confirm the local requirement.

The Cleaning Procedure

The standard procedure is:

1. Isolate the tank from the system and bypass or backfeed the served zone so customers keep water.
2. Drain the tank to a storm or sanitary sewer per local permit; dechlorinate the discharge if it is chlorinated.
3. Remove sediment from the floor using vacuum, squeegee, or washdown. Do not stir sediment into water that will go to the sewer without approval.
4. Wash down interior walls and floor with potable water.
5. Inspect the interior (see checklist below).
6. Disinfect the interior per AWWA C652.
7. Sample for coliform bacteria.
8. Flush and refill, then return to service.

Sediment Characterization

Before disposal, the sediment is often characterized. The character of the deposit tells the operator where it came from: reddish-brown sediment is usually iron and manganese from the source or from old cast-iron mains; gray-black sediment often indicates sulfate-reducing bacteria or anaerobic conditions; sand and grit point to a filter break or poor settling at the source. The finding drives follow-up: iron deposits may trigger a corrosion-control review, and sand may trigger a filter inspection at the plant.

Disinfection per AWWA C652

AWWA C652 defines three accepted disinfection methods for water-storage facilities:

The number to remember for Method 1 is the residual you must still have at the end — not the dose you started with. C652 states the requirement as ≥10 mg/L free chlorine in the full facility at the end of the 6-hr or 24-hr period, so the operator doses above that to allow for demand. After the hold, the residual is reduced (typically to about 2 mg/L) before the tank is put back in service.

After the contact period, the tank is flushed until the free chlorine residual drops back to the normal distribution range, and bacteriological samples are collected. The tank cannot be returned to service until the samples are coliform-free. If any sample is positive, the disinfection and sampling are repeated.

Bypassing the Served Zone

While the tank is offline, the customers it normally serves still need water. The crew arranges a bypass — a temporary connection around the tank — or backfeeds the zone from an adjacent pressure zone through a pressure-regulating valve. Either way, the goal is to keep residual pressure in the served area above the 20 psi minimum through the entire outage. If neither bypass nor backfeed is possible, the utility issues a precautionary boil-water notice or staged shutdown and notifies customers in advance.

Confined-Space Entry Precautions

A water tank is a permit-required confined space — it has limited entry/exit, is not designed for continuous occupancy, and can hold a hazardous atmosphere. Before anyone enters, the operator must:

• Test the atmosphere for oxygen (19.5–23.5%), combustible gases, and toxic gases (chlorine, hydrogen sulfide, carbon monoxide)
• Ventilate continuously with a blower
• Have an attendant outside at all times
• Maintain communication between entrant and attendant
• Prepare a rescue plan — non-entry retrieval (tripod, harness) preferred
• Lock out the inlet and outlet valves so the tank cannot refill while someone is inside

These are not suggestions; they are OSHA permit-required confined-space requirements and they are the single most important safety control in a tank-cleaning project.

Inspection While Offline

A tank outage is a rare chance to inspect the interior and the components that cannot be seen from outside. The inspection checklist includes:

• Interior coating — blistering, rust, holidays, adhesion failure
• Vents — screened, clear of obstructions, functioning
• Overflow screen — intact and screened against insects and animals
• Hatch — gasket condition, lock, lid seal
• Cathodic protection (if installed) — anodes, reference-cell readings
• Baffles and mixer (if present) — integrity, corrosion
• Ladder and safety climb — corroded rungs, loose bolts

Every finding is documented with photos and condition ratings, and any defect serious enough to threaten water quality or structural integrity is scheduled for repair before the tank is returned to service.

Returning to Service

After the tank is disinfected, flushed, sampled coliform-free, and inspected, it is refilled, the final disinfectant residual is confirmed, and the tank is rotated back into service. Operators bring the tank back online slowly to avoid a pressure surge and to let the system hydraulic grade stabilize. The cleaning is documented in the facility record with date, crew, disinfection method, sample results, and inspection findings. That record is the baseline for the next cleaning cycle and the basis for any future regulatory inquiry.