17.1 Distribution Chemical Feed, Dosage Adjustment & Booster Chlorination

17.1 Distribution Chemical Feed, Dosage Adjustment & Booster Chlorination

Maintaining a detectable disinfectant residual throughout the distribution system is a regulatory requirement and the last barrier before the tap. In a hot, sprawling Arizona system with long transmission runs and low-turnover storage, that is genuinely difficult.

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Why Residual Disappears

Chlorine decay has two distinct components, and distinguishing them determines the correct response.

Temperature accelerates both. Chlorine decay roughly doubles for each 10°C rise, so an Arizona system holding 0.8 mg/L at a given point in January may hold 0.2 mg/L at the same point in July with identical operation.

Water age is the other master variable. Residual decays with time, so the extremities of the system, dead-end mains, oversized mains in low-demand areas, and slow-turnover storage tanks are where residual fails.

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Booster Chlorination

A booster chlorination station re-doses water at a point in the distribution system rather than at the plant.

Typical Locations

• At or downstream of storage tanks, where long detention consumes residual
• At pressure zone boundaries and pump stations
• At the entry to a long transmission main or a low-demand extremity
• At interconnections where water from another system enters

Control Strategies

Booster stations are usually remote and unattended, which raises the stakes on reliability: they need residual analyzer alarms, low-tank-level alarms on the chemical day tank, telemetry to SCADA, and failsafe interlocks that stop the feed pump when there is no flow. A feed pump running into a static main is the classic remote-site overfeed.

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Dosage Calculations in Distribution

The pounds formula does not change:

$$\text{lb/day} = \text{Dose (mg/L)} \times \text{Flow (MGD)} \times 8.34$$

Worked example — booster station. A booster station passes 0.85 MGD and must raise the residual from 0.3 mg/L to 1.0 mg/L, with a measured chlorine demand of 0.2 mg/L through the station.

Dose required = (1.0 − 0.3) + 0.2 = 0.9 mg/L

$$0.9 \times 0.85 \times 8.34 = 6.4\text{ lb/day of chlorine}$$

Using 12.5 percent sodium hypochlorite at about 1.043 lb available chlorine per gallon:

$$\frac{6.4}{1.043} = 6.1\text{ gal/day}$$

Worked example — batch dosing a tank. A 750,000-gallon reservoir must be raised by 1.5 mg/L.

$$\text{lb} = 1.5 \times 0.75\text{ MG} \times 8.34 = 9.4\text{ lb of chlorine}$$ $$\frac{9.4}{1.043} = 9.0\text{ gal of 12.5\% hypochlorite}$$

Note the difference between the two calculations: the first uses flow per day and yields a feed rate, the second uses a fixed volume and yields a one-time quantity. Reading which the question asks for is half the work.

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Chloramine Systems

Many large Arizona systems chloraminate because monochloramine is far more persistent than free chlorine and forms substantially fewer regulated disinfection byproducts — both valuable in a hot system with long detention.

Chloramination requires feeding both chlorine and ammonia and holding the ratio.

$$\text{Target Cl}_2 : \text{NH}_3\text{-N ratio} \approx \textbf{4:1 to 5:1 by weight}$$

Ammonia sources are aqueous ammonia (ammonium hydroxide), ammonium sulfate, and anhydrous ammonia. Anhydrous ammonia is a severe hazard and most utilities have moved away from it.

Application order matters: chlorine is normally added first and ammonia second, after the chlorine has mixed, so that monochloramine forms preferentially rather than the water passing through poorly mixed zones of high chlorine-to-ammonia ratio.

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Feed Equipment at Remote Sites

The equipment is the same as in the plant — positive displacement metering pumps, day tanks, calibration cylinders, back pressure and anti-siphon valves — but the operating context differs.