*2.2.2. Limitations*

NaOCl can be commercially supplied or generated on-site, the latter being the safer of the two methods for handling reasons. In on-site generation, salt is dissolved with softened water to form a concentrated brine solution that is subsequently diluted and passed through an electrolytic cell to form sodium hypochlorite. Hydrogen is also produced during electrolysis, and it needs to be vented because of its explosive nature [7].

## *2.2.3. Process*

These dosing systems are mostly simple, but there might be a concern regarding the design. The design might influence the control of gas release from the bulk hypochlorite in dosing pumps and piping and scale formation. Gasification (mostly produces oxygen) can lead to vapor or gas bubbles' formation, specifically if sodium hypochlorite is below the atmospheric pressure, which leads to gas locking of the suction line in a diaphragm. Pumps should therefore be provided with flooded suction. Tanks must be properly vented out of all structures to the atmosphere.

Most commonly available dosing systems use diaphragm metering pumps. The pump action can lead to the development of vacuum. The vacuum causes the vaporization of the dissolved gases in the sodium hypochlorite, resulting in the pump losing its prime and a lower applied chlorine dose.

Consequently, to aid in the prevention of gasification, the dosing arrangements must have a positive head on the pump suction (with pump inlet always below the minimum tank liquid level). In addition, piping system configurations which will trap sodium NaOCl between two closed isolation valves or check valves should be avoided.

A calibration cylinder, a pulsation damper, a pressure relief valve, and a loading valve are the main components of a dosing system. Automatic auto-degas valves systems are also provided by some dosing pump suppliers. NaOCl is dosed either through a spreader bar submerged within an open channel or through an injection fitting (pressurized pipes). The pulsation damper and the dosing pump should be placed close to each other. Pulsation damping also helps in improving dispersion. A loading valve should also be provided in systems where the back pressure at the pump delivery side is not sufficient (<0.7–1.0 Bar), till a suction demand valve is installed on the suction side.

For protecting the diaphragm from rupturing, a PRV (pressure relief valve) should also be provided on the delivery side of the pump. PRV's operation must be detected and alarmed: e.g., the outlet of the valve could be directed to a small "catch-pot," equipped with a float switch. All the systems or pumps which are shut down should contain methods to relieve any pressure buildup.

As sodium hydroxide (NaOH) is used in its manufacturing, the pH of NaOCl is high. Extreme care is needed when using hard waters (or waters with CO<sup>2</sup> present), as the highly alkaline product can lead to lower flow rates, reduced pipe diameter, reduce pump capacities, and scale formation at dosing points [8].
