15.2 Disinfecting Storage Reservoirs (C652) & Wells (C654)

Two Standards, Two Triggers

[!IMPORTANT] The trigger logic is reversed between mains and wells. A main is always disinfected before use. A well is sampled first, and disinfection is required only if coliform is detected. Reservoirs follow the main pattern - disinfect, then sample. The three-month out-of-service threshold applies to wells and is a frequent exam distractor.

---

AWWA C652 Reservoir Disinfection Methods

Method 1 - Fill and Hold

The tank is filled to overflowing with potable water chlorinated to a free chlorine residual of at least 10 mg/L, held for at least 24 hours, and must retain a residual of at least 10 mg/L at the end of the period. The water may then be flushed to waste (with neutralization) or, if the residual has been reduced to normal distribution levels and quality is acceptable, may be usable.

• Simple and thorough - every surface below the water line is contacted
• Uses a full tank volume of heavily chlorinated water, which must then be neutralized and disposed of

Method 2 - Spray or Brush

All interior surfaces that will contact stored water are sprayed or brushed with a solution of at least 200 mg/L available chlorine, with the surfaces kept wet for at least 30 minutes. The tank is then filled with potable water, and the water may be placed in service after bacteriological clearance.

• Uses far less chlorine and produces far less waste
• Requires people inside the tank, which makes it a permit-required confined space entry with all that entails
• Requires thoroughness; any missed surface is undisinfected

Method 3 - Partial Fill and Hold

A portion of the tank is filled with water chlorinated to a high concentration, held for a specified retention period so that the chlorinated water contacts the lower interior surfaces, and the facility is then filled to the operating level with potable water and held again so that the final mixed water carries a specified residual. The exact fill fraction, dose, retention times, and final residual are set by the edition of C652 in force - verify them in the standard your specification cites rather than from memory.

• A middle ground between Methods 1 and 2 in chlorine use and waste volume

Bacteriological Clearance

After disinfection and refilling, samples are collected and must be coliform-negative. If coliform is detected, 22 CCR 64582 requires that the reservoir be resampled and that the results be submitted to the State Board for review and approval before the reservoir is placed in service. Sample bottles must contain sodium thiosulfate, and analysis must be at an ELAP-certified laboratory.

Tank Entry Safety

Tank interiors are permit-required confined spaces, and tank work has killed operators:

• Atmospheric testing before and continuously during entry - oxygen, flammability, toxics. Chlorine vapor accumulates in the headspace of a tank being disinfected and in a tank recently coated
• Coating solvent vapors are both toxic and flammable; a recoated tank can hold an explosive atmosphere for days
• Forced-air ventilation throughout the entry
• Attendant stationed outside, in continuous communication, never entering to attempt rescue
• Retrieval system - full body harness and mechanical retrieval for vertical entries
• Lockout/tagout of fill valves, drains, mixers, and cathodic protection rectifiers
• Fall protection on roofs and ladders; slippery interior surfaces
• Never enter a tank alone, and never enter on the strength of a single "it smells fine" judgment

---

AWWA C654 Well Disinfection

When It Applies

Under 22 CCR 64583, a new or repaired well, or a well that has not been in operation for more than three months, shall be sampled for bacteriological quality prior to use. If coliform is detected, the well is disinfected per AWWA C654-03 and resampled, with results submitted to the State Board for review and approval before the well is placed in operation.

Calculating the Chlorine Charge

Well disinfection requires knowing the volume of water standing in the casing plus an allowance for the gravel pack and the formation.

$\text{Gallons per foot} = 0.0408 \times D^{2}_{(\text{in})}$

Worked example. A 12-inch well has 240 feet of water standing in the casing and will be dosed to 100 mg/L using 65 percent calcium hypochlorite.

Volume = 5.88 x 240 = 1,411 gallons = 0.001411 MG Chlorine required = 0.001411 x 100 x 8.34 = 1.18 lb Calcium hypochlorite = 1.18 / 0.65 = 1.81 lb

In practice the dose is increased to account for the gravel pack and the formation immediately around the screen, and the standard also addresses higher doses (often 100 to 500 mg/L) depending on the situation and whether biofouling is being addressed.

Procedure

1. Clean the well - remove oil, grease, drilling mud, and debris; the presence of organic material consumes chlorine and shields organisms
2. Introduce the chlorine solution so it contacts the entire water column, using a tremie pipe or by circulating, not by simply pouring it down the casing where it stratifies at the bottom
3. Agitate and circulate - surge the well, or recirculate with the pump, so the solution enters the gravel pack and the near-well formation
4. Hold for the specified contact period, commonly at least 2 to 24 hours depending on method and dose
5. Pump to waste until the residual is at or below the distribution system level; neutralize the discharge before it reaches a storm drain or watercourse, and be aware that the discharge may also carry turbidity and sediment
6. Sample for bacteriological quality, and repeat until the results are acceptable
7. Submit results to the State Board if coliform had been detected, and obtain approval before returning the well to service

Disinfecting the Pump and Appurtenances

The pump, column, discharge head, sounding tube, and any tools that entered the well must also be disinfected - normally by swabbing or spraying with a 200 mg/L solution or dipping before installation. A newly rebuilt pump lowered into a disinfected well with grease and shop dirt on it recontaminates the well immediately.