Operator Tips: Chemical Properties of Sodium Hypochlorite / Hypo, and Installation Design Issues

www.ablewright.com

By Doug Mitch / Able Wright, Inc.

Heightened chemical safety concerns have resulted in a growing demand for sodium hypochlorite, as a safer alternative to gaseous chlorine. Design & installation issues must be properly addressed to insure a reliable operating system…

General

Sodium hypochlorite can be used for disinfection, chloramination, odor control, filamentous bulking control in activated sludge, and ammonia removal. Sodium hypochlorite (NaOCl) is available commercially as a solution at 12.5% to 15% available NaOCl by weight (household bleach is 5.25% by wt), and contains 1.04 to 1.25 lbs available chlorine per gallon of solution. NaOCl is a highly reactive oxidizer. It is greenish-yellow in color with an odor similar to that of chlorine.

Properties

Although the commercial strength of NaOCl is 12.5% to 15%, the actual delivered product is usually weaker since hypochlorite is unstable and degrades as much as half every 100 days (at 70°F). This degradation accelerates in higher temperatures and in the presence of sunlight. Dilution greatly reduces degradation, especially for solutions delivered in concentrations less than 7% to 8%. The pH of sodium hypochlorite is high because sodium hydroxide is used in its manufacturing process.

Off-Gassing

NaOCl produces gas as a natural by-product during its decomposition, as much as 1% per day at room temperature. This gas is primarily oxygen; however chlorine gas can also be released at lower pHs. The gas by-product of decomposition can be hazardous if not properly vented off or kept moving through the piping system. If the gas becomes trapped in a piping system or in a pump liquid end, over time it can build up enough pressure to rupture the piping or pump head. Installing vent valves, keeping the liquid moving at high velocities and operating at cooler liquid and/or ambient temperatures can help reduce this problem. Commercial strength hypochlorite is often diluted prior to being injected into a water stream, in order to provide proper mixing and disinfection. When an insufficient amount of dilution water is used the hypochlorite can cause the pH to rise. If the dilution water is hard water, the rise in pH will result in calcium carbonate precipitation which will coat the inside of piping, valves and pumps. This scale deposit will tend to be greatest in areas of high turbulence, such as pump heads, valves and rotometers. In order to prevent scaling, use soft water. If soft water is not available use enough hardwater to keep the pH below 9.

Installation Design Issues

To minimize the undesirable effects of commercial bulk hypo, the following general recommendations are provided:

• When using solenoid or motor driven chemical metering pumps, select a true "Auto De-Gassing Liquid End" which will vent the accumulated gas from the top of the liquid end and prevent air binding & loss of prime

• Consider an all vacuum type liquid feed system

• Avoid piping system configurations which will trap hypo between two closed isolation valves or check valves

• Limit on-site storage supplies of hypo to 3 months or less to maximize the chlorine content and preserve freshness

• Keep the sodium hypochlorite solution as cool as possible and out of direct sunlight

• When diluting hypo use clean, mineral free, softened water

• Consider "On-Site Generation" of sodium hypochlorite using just water, salt and electricity to produce a fresh 0.8% solution on a daily basis, as you need it.

Engineers & Operators: If we can be of assistance to you during selection of equipment on your next disinfection or chemical feed application – give us a call. We offer field service and prompt delivery of spare parts kits & replacement components for all types of chemical feed equipment and systems.

Contact us at:

Able Wright, Inc.

PO Box 849

Plains, MT 59859

(406)250-8389

www.ablewright.com

AbleWright@blackfoot.net

Doug Mitch is the founder and President of Able Wright, Inc. – a provider of water treatment equipment and technical services, specializing in disinfection and chemical feed applications in Montana, Wyoming, Idaho, and Washington.

www.AbleWright.com