Editor's Note: This article is part of our "Water Treatment 101" series, which covers basic treatment processes. To read the first article in this series, see "6 Tips for Improving Coagulation, Flocculation and Clarification." 

Disinfection is a critical factor in all forms of water treatment. However, deciding which process to use is almost always driven by cost, regulations, local process requirements and concerns for safety.

Daryl Weatherup, the global product manager for disinfection at Evoqua Water Technologies, breaks down the topic.

“There are two basic types of disinfection,” he says, “primary disinfection and secondary [also referred to as residual] disinfection.”

Drinking water plants practice both types: Primary disinfection destroys harmful bacteria, viruses and other microorganisms, and secondary disinfection helps plants meet disinfection byproduct requirements. A plant might also use disinfectants in a pre-oxidation step before coagulation, flocculation, sedimentation and filtration.

 “There’s always going to be some form of chlorine used in treated drinking water systems,” Weatherup says. “Free chlorine, chloramines and chlorine dioxide are the three forms of secondary disinfection allowed to meet EPA requirements. Chlorine is by far the most commonly used method of primary disinfection municipally. What is changing is the form in which it is available.”

In wastewater treatment, disinfection is used at the end of the process to purify treated wastewater before it’s discharged. A chlorine residual is not required, however, and might not be desirable except for water reuse where further treatment occurs. Most wastewater plants that discharge to the environment include a dechlorination step or use ultraviolet disinfection, which does not provide residual.

Disinfection chemicals and methods vary:

• Gaseous chlorine delivered and stored in cylinders
• Sodium hypochlorite produced onsite by electrolyzing a brine solution
• Sodium hypochlorite sourced in bulk container
• Chlorine dioxide, which is generated onsite
• Chloramines, which are formed in situ by the combination of chlorine and ammonia

Chlorine does have drawbacks, however. It reacts with natural organic matter to form halogenated byproducts, which are health hazards and are closely regulated in public drinking water systems.

Chloramines are used to reduce the formation of disinfection byproducts (DBPs) and trihalomethane (THM) in the distribution system over extended periods of time.

Chlorine dioxide is also used in DBP-reduction strategies, primarily in a pre-oxidation step to oxidize organic matter and improve clarification and filtration.

Physical or non-chemical disinfection technologies include:

• Ultraviolet light
• Ozone
• Advanced oxidation processes (AOP)

Although these methods might require increased electrical power, they reduce the cost and handling of chemicals. In the case of ozone, disinfection of pollutants produces no disinfection byproducts. Ultraviolet light is widely used to complement chlorine in primary disinfection for the inactivation of pathogenic microorganisms — particularly Cryptosporidium and Giardia — without the potential to form DBPs.

Weatherup’s firm, Evoqua Water Technologies, offers a full range of disinfection processes through the Wallace & Tiernan product line — the group that invented and introduced chlorination for disinfection and successfully countered waterborne diseases in the early 1900s.

He says a plant’s disinfection technology choice is site-specific, depending primarily on local costs for chemicals and electricity. Safety is also a factor, as many treatment plants move away from delivery, storage and handling of gaseous chlorine cylinders.

“To maximize the efficiency and effectiveness of their disinfection process, many plants are moving toward using a combination of disinfectants or a multibarrier approach to disinfection,” Weatherup says. “Adding the best disinfectant — at the right step in the process — and controlling it well leads to best results.”

Weatherup points out two other critical aspects of an effective disinfection system: dosing control and measurement, which are closely linked.

“You can’t control what you can’t measure,” he says.

An accurate and easy-to-manage monitoring system ensures proper dosages and measures and records the level of disinfection from point to point throughout the system. As a result, the treatment operation meets its regulatory requirements for clean water for public use or return to the environment.