Design and Operational considerations when utilizing Sodium Hypochlorite:

The principle reasons for utilizing sodium hypochlorite rather than elemental chlorine for odor control, disinfection, etc., usually revolve around safety issues: Chlorine use involves routine air pac use and training, Leak Drills, Risk Management Programs (for storage/use of quantities greater than 2,500 pounds), Leak Scrubber System, Haz Whopper Training for leak containment and cleanup; chlorine is a lethal gas, etc. In addition, smaller, rural water and wastewater systems usually employ hypochlorite due to local availability at swimming pool suppliers, etc.

The use of sodium hypochlorite is about two to three times the cost of using chlorine purchased in ton containers. Small treatment facilities will find that the higher cost of using sodium hypochlorite begins to offset the cost of these greater safety measures. Larger facilities, when surrounded by homes and/or businesses, may justify the use of sodium hypochlorite on the reduction of potential legal costs and litigation should there be a sizeable release of chlorine off site, causing significant loss of property or life.

DESIGN and CONSTRUCTION:
The principle problems to be considered during design are: plugging due to scale formation; the formation of gas (predominately oxygen) in the pumps and piping system. The following items are intended to address these problems:

1. The sizing of the storage tank must be balanced with the fact that the solution loses strength over time. The rate of degradation increases with heat and sunlight. Tanks may be made of lined carbon steel, polyethylene, or fiberglass. The recommended storage limit is 28 days, with max storage life of 60 to 90 days. Due to off-gassing (the degradation of the NaOCl produces a gas which is mostly oxygen), tanks must be properly vented out of all structures to the atmosphere.

2. You must verify that all sodium hypochlorite tankage, piping, pumps, fittings, etc. are compatible with the chemical.

3. As with other chemical solutions, install strainer(s) to capture any impurities delivered with the hypochlorite solution.

4. Install a large diameter suction line vent/stand pipe (2x suction pipe diameter) return to the supply tank on an inclined plane, as close to each pump suction pipe as possible. This is to minimize the amount of gas in the suction piping reaching and then adding to the potential for gas binding in the pump or other feed system.

5. Valves: PVC diaphragm valves; and steel, PVC lined, plug valves are acceptable. Composite material diaphragm valves with Teflon diaphragms are acceptable use. Leaks are likely by weeping past the valve seal(s). To reduce potential gas build-up and potential rupture, use vented ball valves.

6. The pumps should be of the diaphragm metering style. The pump must be installed with a flooded suction, to reduce the potential to gas-bind the pump.

7. On the discharge piping, as close to the pump as possible, install a gas bleed-off equipment at a high point.

8. Consider the "double pipe" containment design and construction to contain leaks, when ever possible.

9. For each pump: install isolation valves; a calibration column on the pumps suction piping side; an individual discharge pressure gauge.

10. NOTE: Scaling has the potential to occur when hypochlorite is injected into a dilution stream, which is subsequently piped to the point(s) of application. The sodium hypochlorite solution is at a high pH, due to the use of caustic soda (sodium hydroxide, NaOH) in the manufacture of the sodium hypochlorite. The addition of this high pH hypochlorite solution to the dilution water, may increase the pH of the dilution water (if the source water pH and it’s corresponding alkalinity is not of sufficient strength to buffer the addition of the hypochlorite) to a level which promotes scale formation of calcium and magnesium based minerals. When the dilution water source has pH values above 9, with hard water characteristics, and moderate to low alkalinity values, scale will form on the inner aspects of the downstream pumps, piping, and valves. Passing the dilution water through water softeners prior to the addition of the hypochlorite solution, and/or pH adjustment to keep the pH below 8.9 to 9, and feeding the concentrated hypochlorite solution directly into an "open point (channels, etc)" of application have proven to be solutions to scale formation. Concentrated solutions of sodium hypochlorite pumped directly into "closed points of application (such as pipes) may experience plugging/scaling at the junction of the two pipes.

11. For those "long runs" of diluted concentrations of sodium hypochlorite, such as for chlorination of the Return Activated Sludge (RAS) and Raw Water Pre-chlorination, where the potential of scaling is further enhanced by intermittent use, consider in the design the use of flushing water to clear the piping system of the chemical solution when it is not required.

12. Install only hard piping systems. Flexible piping is not recommended. Schedule 80 PVC is usually installed.

OPERATIONAL:
SAFETY: First and foremost. While this chemical is not a lethal gas, such as chlorine it must be respected! Operators must wear all personal safety equipment, as specified in the chemical handling procedures, when working on this chemical feed system.

The principle problems are: plugging due to scale formation; the formation of gas in the pumps and piping system. The following items are intended to address these problems:

1. For freezing temperatures, it is best to maintain the weaker concentration sodium hypochlorite solution level high in the storage tank. (This must be balanced with the fact that the solution loses strength over time, but is negated to a large extent by the very low temperatures.) Remember, as the concentration of the hypochlorite solution increases, the temperature at which the solution will freeze is lowered. The lower concentration solutions will therefore freeze near pure water (32 degrees F). This may be somewhat offset by maintaining a higher level in the tank, conserving "mass heat". The higher concentrations, will freeze at a lower temperature, as in a 15 per cent solution freezing at about 6 degrees F below zero.

2. The solution loses strength over time. The rate of degradation increases with heat and sunlight. Storage time is recommended to be less than 28 days. A max. storage life is about 60 to 90 days, depending on the solution storage temperature.

3. After receiving a delivery of sodium hypochlorite, allow it to stand for a few hours or over night, before utilizing the chemical. This will allow it to liberate much of the gas contained within the liquid. (Better here than in the pump/piping system.)

4. Prior to placing a sodium hypochlorite pump on line, the operator should bleed any entrapped gas out of the chemical metering pump head(s) by opening the gas vent valve, and filling the pump head(s) with hypochlorite solution.

5. Gassification within the pump may be reduced by employing a combination of long strokes with lower pump speeds, as opposed to shorter strokes at a faster pump speed.

6. On a weekly basis, inspect all tanks, bulk-head fittings, chemical pumps, piping, pressure gauges, flow meters, and piping connections for leaks and potential failures. As with your other liquid chemical feed systems (such as sodium bisulfite), run the feed pump up to a full-flow status, for a few minutes, to a proper waste point, to clear gas, impurities in the piping, valves, etc. You may purge your system, by pumping at this high rate, to the point of application ONLY IF it will not produce adverse affects on your water quality!

7. Sodium hypochlorite forms hypochlorous acid, as when chlorine gas is applied. There is a difference in that the hypochlorite increases the number of hydroxyl ions ( - OH). This action results in a tendency to increase the pH value in poorly buffered waters (as stated above in dilution water scaling). Chlorine gas, when added to water increases the number of hydrogen ions, which tends to decrease the pH value in poorly buffered waters.

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