3.5 Raw Water Storage, Clearwells & Reservoir Management

Two Named Blueprint Items

The treatment Expected Range of Knowledge lists Clear Well Storage and Raw Water Storage as separate sub-topics inside Source Water, along with Surface Water/Reservoirs. It also lists these abilities at T1-T4: calculate the volume of water in a storage facility, determine water level in a storage tank, reservoir, or well, and know storage tank disinfection procedures. Storage is treated as an operator competency, not an engineering afterthought.

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Raw Water Reservoirs and Thermal Stratification

A reservoir deeper than roughly 20 feet in a Mediterranean climate stratifies every summer:

Once the thermocline sets, the hypolimnion no longer receives oxygen. Bacterial decomposition of settled organic matter consumes the remaining dissolved oxygen and the bottom water becomes anoxic. Under reducing conditions:

• Insoluble ferric iron (Fe³⁺) is reduced to soluble ferrous iron (Fe²⁺)
• Insoluble manganese dioxide (Mn⁴⁺) is reduced to soluble manganous manganese (Mn²⁺)
• Hydrogen sulfide, ammonia, and phosphate are released from the sediments
• Sulfate-reducing and iron bacteria proliferate

In autumn the surface water cools toward the temperature of maximum water density (about 4 °C), the density difference collapses, and wind mixes the entire water column - fall turnover. Everything that accumulated in the hypolimnion is suddenly distributed lake-wide. The plant sees a step change: color, taste and odor, oxidant demand, iron and manganese, ammonia, and turbidity all jump at once.

Operational Responses

• Multi-level intakes. The single most valuable tool. Move the withdrawal port to the depth with the best combination of low turbidity, low iron and manganese, low algae, and adequate temperature.
• Hypolimnetic aeration or destratification. Diffused air or mechanical mixers keep the bottom oxic so iron and manganese stay insoluble. This is preventive; it must run through the season, not be started in October.
• Algae management. Monitor for cyanobacteria and their toxins (microcystin, anatoxin-a, cylindrospermopsin) plus the taste-and-odor compounds geosmin and 2-methylisoborneol (MIB), which are detectable at nanogram-per-liter levels. Respond by moving the intake, adding powdered activated carbon, or pre-oxidizing - but never chlorinate a heavy cyanobacteria bloom at the intake, because cell lysis releases intracellular toxin.
• Pre-position for turnover. Watch surface temperature weekly in September and October. When the epilimnion approaches hypolimnion temperature, be ready with permanganate or PAC feed, extra jar testing, and increased Fe/Mn monitoring.

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Clearwells: Two Jobs at Once

A clearwell is the finished water basin between filters and the high-service pumps. It does two very different things, and confusing them is a classic exam trap.

Job 1 - Disinfectant Contact Time (CT)

Chlorine needs residence time to inactivate pathogens. The clearwell provides most of it. But CT credit is not based on the theoretical detention time; it is based on T10, the time it takes for 10 percent of a tracer to appear at the outlet - the time the fastest-moving tenth of the water spent in the basin.

$T_{10} = \text{Theoretical Detention Time} \times \text{Baffling Factor}$

Worked example: a 750,000-gallon clearwell operating at 2,600 gpm with average baffling. Theoretical detention time = 750,000 / 2,600 = 288.5 minutes. T10 = 288.5 x 0.5 = 144.2 minutes. At a 1.1 mg/L free chlorine residual, CT achieved = 1.1 x 144.2 = 158.6 mg-min/L.

Adding baffle walls is usually far cheaper than adding storage volume. Going from poor (0.3) to superior (0.7) more than doubles CT credit with no new tank.

Job 2 - Equalization

The clearwell also absorbs the mismatch between a treatment plant that runs best at a steady rate and a distribution system whose demand swings hourly. Draw the clearwell down during morning and evening peaks, refill it overnight. But the two jobs compete: the lower the clearwell level, the shorter the contact time. Many plants run into a CT violation not because chlorine dropped but because level dropped during a peak. Set a minimum operating level that guarantees the required T10 at maximum flow, and alarm it.

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Finished Water Storage: Sanitary Integrity

Under 22 CCR 64585, every distribution reservoir must have vents and openings built to exclude rainwater, runoff, birds, insects, rodents, and other animals; at least one sampling tap; and no use that creates a contamination hazard. The recurring field defects are predictable:

Separate inlet and outlet arranged to minimize short-circuiting is a design requirement for new reservoirs precisely because a common inlet/outlet creates a dead volume that never exchanges. Where a common pipe already exists, operators manage turnover by deliberately cycling the tank through a wider level band and by seasonal or targeted deep-cycling.

Any reservoir that is newly installed or taken out of service for repair or inspection must be disinfected and sampled under AWWA C652-02 before return to service; if coliform is detected, the reservoir is resampled and the results go to the State Board for review and approval.