Alternate Disinfection Methods for Water and Wastewater Applications – Pros & Cons, by Able Wright

www.ablewright.com

By Doug Mitch / Able Wright, Inc.

There are several commonly accepted methods of disinfecting water and wastewater systems, each with its own Pros & Cons, as presented in the following summary. In some applications, more than one method may be employed. Most disinfection techniques can be divided into two primary groups:

A) Chlorine Based, and

B) Non-Chlorine Based…

Chlorine Based Disinfection Techniques:

Drinking water disinfection applications using chlorine based methods typically will apply a sufficient dose to overcome the chlorine demand, and maintain a 0.2 to 0.5 ppm chlorine residual in the water. A chlorine residual in the downstream water distribution system is desirable to provide a measure of sanitary protection. Chlorine destroys pathogens by oxidization.

Wastewater disinfection applications typically dose at a much higher chlorination feed rate than in drinking water, in order to destroy potentially harmful bacterial and viral pathogens before the effluent is released into the environment. As a secondary step, it is often necessary to "De-Chlorinate" wastewater effluent to minimize the effects of chlorine downstream.

1) Liquid Chlorination – using Sodium Hypochlorite, a strong form of bleach which can be purchased and delivered in bulk (typically 12.5% chlorine). It can also be safely generated on-site (0.8% chlorine) using just water, salt and electricity. The chlorine content of "Hypo" degrades at the rate of approximately 15% per month, while in storage. It must be accurately metered into the water or wastewater to provide a sufficient dose.

2) Gas Chlorination – Chlorine gas (100% pure) is withdrawn from a cylinder (150 Lb or 1 Ton size) and blended with water to form Hypochlorous Acid which is injected into water or wastewater in a controlled manner. Chlorine gas is very toxic, with safety issues which must be properly addressed. Similar to liquid chlorination, there is a long lasting residual effect, which may need to be de-chlorinated in wastewater applications.

3) Tablet Feeder Chlorination – Calcium Hypochlorite tablets (typically 65% available chlorine) are dissolved in water. The solution strength must be controlled and the mixture agitated to prevent solids settling. Cal Hypo tablets must be carefully stored to avoid potential spontaneous combustion. Again, similar to liquid chlorination, the residual effect of chlorine can be long lasting and may require de-chlorination.

Non-Chlorine Based Disinfection Techniques:

1) UV Disinfection – Water or wastewater is exposed to ultraviolet light lamps to "inactivate" bacteria and viruses. The UV dose must be controlled to provide a reduction in the number of viable pathogens. UV light exposure damages the DNA of micro-organisms, rendering them unable to reproduce. UV disinfection is chemical free, and does not have any residual effect after application.

2) Ozone Disinfection – Ozone gas is generated on-site, and blended with water to oxidize pathogens. Ozone is very unstable and cannot be stored – it must be used as soon as it is generated. Because it is such a strong oxidant and its unstable nature, ozone changes form quickly once it has been applied. Ozone disinfection does not have any residual effect after application.

Pros and Cons:

Each type of potable water disinfection has its own pros and cons. When selecting the best method for your application, it is wise to consider the following issues…

• Capital cost (original equipment purchase)

• Operating expenses – chemical costs, electricity, etc.

• Maintenance expenses and requirements

• Reliability and simplicity of the operation

• Technical skill of the operating personnel

• Ability to upgrade in capacity or dosage

• Safety issues

Engineers & Operators: If we can be of assistance to you during design, or upgrade, of a disinfection system – give us a call. We offer design assistance, field service and prompt delivery of spare parts kits & replacement components for all types of chemical feed equipment and disinfection 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