Deep Dive into Water Disinfection: Keeping Water Safe

Colorado requires all public drinking water systems to have continuous chemical disinfection, except for two suppliers that meet strict standards to retain disinfection waivers and hand-pumped wells at campgrounds. Continuous disinfection is part of a multi-barrier approach to ensuring safe drinking water is available to your consumers.  

Disinfection of drinking water inactivates (i.e., kills or prevents pathogens from replicating) waterborne pathogens, such as bacteria and viruses. The amount of microorganisms needed to cause an infection varies widely between pathogens. The median infectious dose for Salmonella typhosa is 1,000,000 organisms (but can be much less for infants), Hepatitis A virus is between 10-100, and Giardia lamblia and Cryptosporidium are less than 10 organisms. Disinfection does not completely eliminate all living organisms in the water, but does significantly reduce potential acute waterborne disease risk. The pathogen risk reduction is expressed in terms of “log inactivation”. For example, a 3.0 log inactivation value means that 99.9% of microorganisms of interest are inactivated. 

Disinfection can be accomplished using either chemical oxidants, such as free chlorine, chloramines, or ozone, or by photo-inactivation with ultraviolet light. Most Colorado water systems use free chlorine as their main disinfectant. Disinfection with chlorine or chloramines provides a persistent disinfectant residual in the distribution system. This disinfectant residual can provide defense against harmful organisms that enter the distribution system through backflow events, pipeline leaks, low pressure events, or other contamination pathways. 

Regulation 11: Colorado Primary Drinking Water Regulations requires that systems maintain a minimum disinfectant residual concentration of 0.2 mg/L at the entry point and 0.2 mg/L in the distribution system. These two requirements are the same for groundwater (GW), groundwater under the direct influence of surface water (GWUDI), and surface water (SW) systems. Each source type has additional disinfection requirements outlined below: 

• GW sources are considered protected groundwater and are expected to have limited or no pathogens. GW systems must comply with the Groundwater Rule through either triggered source water monitoring in the event of a total coliform positive sample or by certifying that the GW treatment process always provides 4-log (99.99%) inactivation of viruses. 
• GWUDI and SW sources likely have pathogen sources in the water (e.g., beavers, point discharges). Pathogenic organisms are expected and GWUDI/SW systems must meet pathogen log removal/inactivation requirements in the Surface Water Treatment Rule (SWTR). The required treatment is based on three target pathogens:
• Cryptosporidium: 2-log (99%) removal. 
• Higher levels may be required based on LT2 source water monitoring.
• Giardia lamblia: 3-log (99.9%) removal/inactivation.
• Viruses: 4-log (99.99%) removal/inactivation. 

For SW/GWUDI systems: filtration and disinfection are two complementary critical barriers for protecting public health. Drinking Water Policy 4 outlines the removal credits for various filtration types (e.g., conventional filtration, direct filtration). The disinfection treatment must be designed and operated to provide any remaining log inactivation required in the SWTR. These processes work together to ensure that drinking water is safe for the public. 

Next time, we will discuss disinfection log inactivation and the critical parameters for design and compliance demonstration. 

Below are some resources that will give more specific information about the requirements outlined above. If you have questions, please contact Melanie Criswell at melanie.criswell@state.co.us. 

Resources

• Quick guide: Disinfectant residual 
• Quick guide: Minimum disinfection for groundwater 
• Drinking Water Policy 4 (DW004): Surface water treatment rule performance assessment, proper operation and fostering improved operations
• ​Drinking Water Policy 4 - Policy guidance
• Additional information about disinfection is available at the division's Revised Total Coliform Website

➽ Melanie Criswell - Lead Service Line, Corrosion, and Emerging Contaminants Engineer

Thank you to Colorado’s Certified Operators

To Colorado’s Certified Operators: Thank You for What You Do

From all of us on the staff of Colorado’s Water and Wastewater Facility Operators Certification program, we want to take a moment to speak directly to you – the certified operators who keep Colorado’s water systems running safely, reliably, and with professionalism every single day. 

Quite simply: what you do matters, and it matters a lot.

Every time a community turns on the tap expecting clean, safe drinking water or flushes a toilet without a second thought, it’s because of your expertise. Behind the scenes, often unnoticed and sometimes underappreciated, you protect public health and safeguard the environment across our beautiful state. Colorado’s quality of life depends on you.

A Profession Built on Skill, Commitment, and Trust

Water and wastewater operations are not “just jobs.” They are professions that demand technical knowledge, sound judgment, adaptability, and a deep sense of responsibility. From managing complex treatment processes and regulatory requirements to responding to emergencies at all hours, operators are asked to do more with less and to do it flawlessly. 

Obtaining certification isn’t easy, and it isn’t meant to be. It represents your commitment to learning, accountability, and excellence. When you earn and maintain your certification, you demonstrate to your community, your employer, and your peers that you are a trusted professional. We see the effort that goes into studying, testing, continuing education, and staying current in a field that is constantly evolving. 

Showing Up When It Counts

Whether you work for a large utility on the Front Range or a small system in a rural mountain or plains town, your role is critical. During wildfires, floods, droughts, freezes, power outages, and equipment failures, operators are among the first to respond and often the last to leave. You show up when conditions are difficult, when systems are stressed, and when communities need stability the most.

In recent years especially, operators have navigated staffing shortages, aging infrastructure, tightening regulations, and increasing public expectations. Through it all, you’ve kept systems compliant and water safe. That resilience does not go unnoticed. 

More than Compliance

While regulations and certifications are part of the job, the heart of this profession goes beyond compliance. It’s about stewardship – of water resources, public health, and future generations. It’s about pride in doing work that truly serves people.

We also recognize that many of you mentor new operators, share knowledge across systems, and contribute to a culture of professionalism in Colorado’s water sector. That willingness to teach, learn, and support one another strengthens the entire industry. 

Our Commitment to You

As a program, we see our role as partners in your success. Our goal is to support a fair and credible certification process that upholds high standards while acknowledging the realities of the work you do. We continually work to improve guidance, communication, and outreach, as a strong operator workforce is essential to the strength of our water systems. 

Your feedback matters to us. Your experience in the field helps shape how we do our work, and we value the dialogue we have with operators across the state. 

Operator Certification Program staff can be reached at (719) 225-7339, info@coloradocwp.com, cdphe.facilityoperator@state.co.us, or cdphe.wwfocb@state.co.us. 

You can also find everything you need to know about Colorado operator certification using the Operator Resources webpage. 

Thank You

So, to Colorado’s certified water and wastewater operators: thank you for your dedication, your professionalism, and your service. Thank you for the long hours, the problem-solving, the vigilance, and the pride you bring to your work. Thank you for protecting public health and the environment, often without recognition and always with integrity. 

On a personal note, I (Jessica Morgan) also want to share my gratitude as I prepare to step away from public service after nine years, including three and a half years with the Water and Wastewater Facility Operators Certification program. It has been an honor to serve the operator community and to support the important work you do every day. I am deeply grateful for the opportunity to have worked alongside such knowledgeable, committed professionals, and I will carry a lasting appreciation for this field and the people in it. 

We are proud of the operator community in Colorado, and we are grateful to work alongside you – supporting a profession that is as essential as it is honorable. Keep doing what you do. Colorado is better because of it. 💧

➽ Jessica Morgan, Liaison for the Water & Wastewater Operator Certification Board 

➽ Larisa Oringdulph, Program Director of Colorado Certified Water Professionals