OCWA Water Quality Series – Disinfection

The two main processes that OCWA—Central New York’s Water Authority—utilizes to treat the drinking water it delivers to its 500,000 consumers are filtration and disinfection. Today’s article will focus on the disinfection process.

Background on Disinfection

Disinfection is the process of treating water for public use to remove or inactivate possible harmful organisms.   Microbial contaminants exist in many forms—amoebic cysts, protozoa, viruses, or bacteria—and are in part the result of human and/or animal waste.

There are many ways to disinfect water, but the most common method of disinfecting minimally contaminated raw water (such as that which OCWA draws from Lake Ontario and Otisco Lake) is chemical treatment using chlorine.  Chemicals (or oxidants) can also serve other purposes during the disinfection process, including controlling biological growth in pipelines; controlling tastes and odors; and reducing some potentially harmful organic compounds.

Chlorine is very effective at inactivating most pathogens, and its ease of use has made it the disinfectant of choice for many water systems, including OCWA.  The process of adding chlorine to water is called chlorination, and its effectiveness depends on two primary factors—the disinfectant concentration and the contact time.

The Disinfection Process

There are multiple forms that chlorine can take when used as part of the drinking water disinfection process.  OCWA uses liquid sodium hypochlorite because it is safer than the other widely-used formulation, chlorine gas, which can be volatile and tricky to handle.

OCWA adds very small doses of sodium hypochlorite (1 part per million) using a mechanical feed system at its two water treatment plants (Lake Ontario and Otisco Lake).  To put that in context, one part per million is the equivalent of putting ONE drop of water from an eyedropper into 10 gallons of water.

Once added, OCWA continuously monitors the water in its system to ensure that there is sufficient disinfectant residual throughout its five-county (Onondaga, Oswego, Madison, Oneida and Cayuga) distribution network.  It has chlorine booster stations (both permanent and seasonal) strategically located throughout its system to add hypochlorite as necessary.

Monitoring for Disinfection By-Products

Although chlorine disinfection provides many benefits, it forms a variety of unwanted disinfection by-products (DBPs).  DBPs form when chlorine reacts with organic matter present in the water.  Several factors influence DBP formation, including disinfectant concentration and reaction time; the amount of natural organic matter in the water; and water temperature.  For example, the rate of chemical reactions is likely to increase with increasing temperatures, leading to increased DBP formation in warm-weather months.

As disinfectant concentration can affect the amount of DBPs, as well as the taste and odor of water, OCWA is careful not to add too much chlorine.  Our operators and water quality specialists constantly electronically monitor and manually sample our water to confirm that we have the correct levels.

OCWA regularly monitors, tracks and evaluates trends in DBP concentrations throughout its water system.  We have optimized sampling locations based on sites with increased potential for DBP formation.  We also look to control DBP formation via a combination of treatment plant and distribution system operational strategies.  These include frequent distribution system flushing; limiting water age and hydraulic resistance time; and utilizing activated carbon filtration.