HESCO ProactiView Comparing Chlorine Feed Systems

Key Words: Feed Systems, Disinfection; Chlorine Gas Safety; Prevention of Air-Bound Sodium Hypochlorite Pumps

Disinfection is a crucial process in virtually all water and waste water treatment facilities. Chlorine gas is inexpensive to purchase, and highly effective, but the safety precautions are extensive and a leak can be devastating. Liquid Sodium Hypochlorite is safer and effective, but stills needs careful handling, and off-gassing can bind pumps if not properly vented. This ProactiView addresses all the pros, cons, benefits and safety concerns of both gas and liquid chlorine, so you can make an informed decision.

Contents

Chlorine Disinfection in Water and Wastewater Treatment
Chlorine Gas Benefits
Safety Concerns for Chlorine Gas
Sodium Hypochlorite Liquid Benefits
Safety Concerns for Sodium Hypochlorite
Sodium Hypochlorite Off-gassing and Associated Binding of Pumps
Guidelines for Selecting One or the Other
Head-to-Head Comparison Matrix
Follow Up with HESCO Regarding Chlorine Feed Systems

Chlorine Disinfection in Water and Wastewater Treatment

Design Engineers must design a disinfection system that is safe, reliable, and easy to operate. Operators must maintain the systems, ensure they are performing, and keep operating costs under control, all while ultimately safeguarding public health. Together they must determine and choose the disinfection system that is right for their specific site and application. The options for achieving disinfection typically include chlorine, UV, and ozone. Each approach has its advantages and disadvantages. In this ProactiView, the focus will be on disinfection via Chlorination, and addressing the questions associated with choosing between Liquid Sodium Hypochlorite versus Chlorine Gas.

In water treatment plants, disinfection is sometimes achieved in stages, with a preliminary dose at the influent to the plant, and always a final dose leaving the plant to maintain a disinfectant residual within the distribution system. In a waste water treatment facility, disinfection typically takes place at the last step of the treatment process just prior to discharge, to ensure the receiving water and its uses are not detrimentally impacted by high levels of pathogens that could otherwise present a public health risk.

When considering Chlorine as a disinfectant, one must choose between a gas or liquid feed system. Chlorine Gas has been used for disinfection since the early 1900s. It is the least expensive disinfection method and therefore the most common. However, chlorine gas is toxic, corrosive and displaces oxygen. Because of these factors, other disinfection alternatives such as sodium hypochlorite, UV and Ozone have become more popular and in recent years are often replacing chlorine gas systems despite their higher costs.

Chlorine Gas Benefits

Chlorine gas is an excellent oxidizer, and with the right amount of concentration, and time (dose), is one of the most effective agents for neutralizing harmful pathogens. Typically a gas feed system consists of a source (either a 150lb or 1 ton cylinder), a vacuum regulator, a gas flow meter and rate control valve, and an ejector assembly. The ejector assembly creates a vacuum, draws gas from the regulator under this vacuum pressure, and injects it into the process stream.

The benefits of gas feed systems are that they are very simple and provide very reliable operation. Gas feed systems have a small footprint, both in terms of storage and feed equipment. Capital equipment costs are relatively low, aside from emergency scrubbing equipment. Chlorine Gas is also the most economical form (pound for pound) in which to purchase chlorine. However, chlorine gas has some important inherent risks that must be weighed when considering disinfection systems. It can be lethal if inhaled and therefore must only be handled by personnel that have been extensively trained with the proper safety equipment.

Safety Concerns for Chlorine Gas

Chlorine gas is primarily a respiratory irritant. In extreme cases, breathing chlorine may kill from respiratory collapse or lung failure. A Chlorine Gas Feed System therefore, must address the safety concerns associated with the potential for gas leaks. The nature of a vacuum regulation type feed system helps to prevent leaks. Additional safety is sometimes designed into the system to address the potential accidental release of gas. This is commonly done in one of two ways. Gas Scrubbers remove the toxic gas from the discharge air until the leak can be shut down. Automatic Shut-off Systems shut down the source of the gas upon any emergency condition such as fire, earthquake or leak detection.

Sodium Hypochlorite Liquid Benefits

In light of the factors associated with Chlorine gas discussed above, Sodium Hypochlorite has become a widely used approach for disinfection applications. Also referred to as NaOCl, bleach, hypo, or liquid chlorine, Sodium Hypochlorite is typically supplied as a liquid and delivered in 5 to 20% concentrations by weight, with the higher concentrations being progressively less stable in terms of strength over time. Typically, sodium hypochlorite feed systems include a bulk storage tank with a transfer pump to a day tank, from which chemical feed pumps draw and inject the liquid into the process stream. The biggest benefit of liquid sodium hypochlorite feed systems is that the liquid is fairly easy to handle and far less lethal than chlorine gas. These benefits are somewhat offset by the fact that liquid hypochlorite is less effective and more costly than chlorine gas.

Safety Concerns for Sodium Hypochlorite

Liquid Sodium Hypochlorite is generally considered safer than Chlorine gas, however certain risks still remain. Typically the strength of Sodium Hypochlorite used for disinfection is in the range of 10-15%. At this strength, the liquid can cause severe irritations of the skin, and liquid contact with the eyes can cause permanent damage. As such, protective clothing and eyewear must be used whenever working around a liquid feed system. A small leak under pressure, can easily drip or spray onto unsuspecting, or unprotected personnel. It is caustic, and can make surfaces slippery, so extreme care must be used to ensure it does not get on floors or traffic areas, and any that does must be thoroughly cleaned. Generally, an isolated containment area is curbed off to contain any spills, simplify containment and proper clean-up and protect adjacent equipment. Designers can help address these concerns by taking safety into serious consideration during the system layout and design. Operators must train personnel and institute strict safety procedures, for these areas.

Sodium Hypochlorite Off-gassing and Associated Binding of Pumps

The biggest problem presented to the operators, and therefore the challenge to design engineers, is reliable operation of feed pumps, which can lose prime or become “air-bound” by the off-gas continuously released from the sodium hypochlorite. If the off-gassing is extensive enough, the gas formed in the liquid can accumulate in the piping and move into the pump head. When a reciprocating positive displacement pump becomes filled with gas, the displacement of the pump head merely compresses and expands the gas, and ceases to pump liquid. This situation requires manual intervention, and an operator must manually re-prime the pump as soon as possible, to prevent a threat to public health.

Proper system design requires suction piping sloped back towards the storage; minimizing suction piping runs; minimizing suction lift conditions which may exacerbate off-gassing by maintaining a flooded suction to the pumps; and finally equipping the pump head with a gas relief system to prevent it from becoming “air bound” or losing prime. There are two methods of this. The older method is a “bleed” valve that continuously returns a small liquid stream back to the storage tank. The concept is that the off-gas is prevented from accumulating in the pump head and is released and routed in the bleed line along with the by-pass liquid back to the storage tank. The drawbacks to this are 1) it does not bleed if the pump is not running, therefore the head can gas up and lose prime while the pump is off, and 2) the return line can easily become clogged by crystallizing liquid, and the system then fails to release gas as intended.

The more recent method for preventing the loss of prime is via use of a specific pump head designed to continuously release the off-gas to the discharge piping whether or not the pump is running. This method has proved orders of magnitude better than the previously mentioned method. There is no clogging, and gas accumulations are released whether or not the pump is running.

Reliability can also be affected by grit, sediment, or solids becoming lodged in the pump check valves, preventing them from seating, essentially disabling the pump or decreasing its accuracy. Strainers are therefore required on suction lines to protect the pump check valves by removing grit & impurities that may be contained in the hypochlorite shipment.

Guidelines for Selecting One or the Other

Selecting one approach over the other (gas versus liquid feed) involves many considerations, and each approach has its inherent challenges. By looking at the following criteria and quantifying their importance to each specific application, the choice can become more clear.

• Budget - How much is available for capital equipment?
• Operating Costs - How important is low operating cost? How does that balance against worker and public safety?
• Safety - In terms of Occupational Health & Safety, do the operating personnel have the capability, expertise and training available to properly manage the risk associated with a gas feed systems? In terms of public safety, is the chemical feed system in proximity to public exposure in the event of a leak?
• Space - Is space limited for chemical storage and feed equipment?
• System Reliability - Will the system be in a staffed facility therefore providing a quick response time for manual intervention to system upsets? Will it be located in a remote unmanned facility and therefore demand high reliability, low maintenance and low attention?

Head-to-Head Comparison Matrix

The matrix below compares gas and liquid feed systems head-to-head. Please note that it is intended only as a starting point, and should not be used as a single means of decision.

Follow up with HESCO regarding Chlorine Feed Systems

Whether the system is gas or liquid, design criteria must be reliably attained, employee safety and public health must be ensured, and operating costs must be kept under control. It is the role of the designer to ensure the planned system will meet these demands. It is the role of the operator to ensure that the system performs to the desired disinfection level, and is cost effectively maintained to ensure reliable and safe operation.

Those that are interested in either designing new or improving existing gas or liquid systems, to increase their safety and improve their reliability are invited to contact HESCO for expert advice and support.

Please visit our web-based ProactiView comparing Chlorine Feed Systems for a FREE Handbook ($39.95 value),and an expanded collection of links, references, and downloadable resources.