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

Wildfire Planning and Recovery Playbook - 2025 Updates!

After a wet spring and variable monsoon season, wildfire season is again upon us in Colorado. As many of you are aware, our public water systems and local communities face a diverse and significant array of challenges when planning, responding, and recovering from wildfires. The best time to start planning for wildfires is right now, in advance of fires.

The Water Quality Control Division (WQCD), along with many state, federal, and local partners, have released a revised and updated version of our Wildfire Planning and Recovery Playbook, available on our Source Water Assessment and Protection website.  Several authors also hosted a webinar on July 25th, with the slide presentation and a recording available.

Pre-fire planning, response, and recovery is a team effort, and requires coordination across multiple jurisdictions, and administrative and physical boundaries. Each community wildfire event may present a unique set of circumstances that must be understood and conveyed to effectively navigate wildfire incidents. The centerpiece of the playbook is the comprehensive critical contacts list, outlining necessary points of contact along with each representative’s roles and responsibilities within the planning, response, and recovery process. Below is an example of the critical contacts list contained in the playbook.

The playbook also provides various actionable steps through each phase of the fire cycle, from planning through recovery.  Examples include identifying your values at risk, forming a recovery group and identifying partners, understanding prefire actions and resources, and roles and responsibilities of partners throughout the different phases of a wildfire incident. The playbook also includes 2 full pages of links to additional resources, including a list of funding programs and technical assistance partners.

The playbook is concise, usable, and accessible. The target audience for this playbook is public water systems, municipalities, counties, and tribes. The updated version reflects lessons learned from recent urban and suburban wildfires and the new Wildfire Ready Watersheds framework from the Colorado Water Conservation Board. Please contact the source water protection team at cdphe.wqswap@state.co.us with any questions or for more information.

➽ Robert Murphy, CPSS, Source Water Protection Program Coordinator

➽ Kristen Hughes, Source Water Protection Specialist

➽ John Duggan, Source Water & Emerging Contaminants Unit Manager

Aqua Answers: Bag and Cartridge Filters in Surface Water Treatment

Dear Aqua Answers,

I’m the operator for a surface water treatment system that uses bag and cartridge filters, and I have a few questions!

___________________________________________________________________________

Question 1: What’s the difference between compliance filters and other bag or cartridge filters at my plant?

For suppliers of surface water or groundwater under the direct influence of surface water (SW/GWUDI), the treatment system must be designed to meet the requirements of Section 11.8 of Regulation 11, also known as the Surface Water Treatment Rule (SWTR). This rule requires the treatment process to remove specific levels of Giardia and Cryptosporidium to ensure public health protection.

One way to meet these requirements is by using bag or cartridge filtration. These filters use a straining process where water passes through a disposable bag or cartridge housed in a permanently installed filter housing. Each filter and housing combination used for compliance filtration must be approved by the Colorado Department of Public Health and Environment (the Department) through the alternative technology approval process. Typically, this approval is obtained by the filter manufacturer rather than through a site-specific approval.

Every installation of bag or cartridge filters at a public water system (PWS) must also be reviewed by the Department as part of a design submittal.

Additional filters, sometimes called “roughing filters” may be installed upstream of the compliance filters. These do not require separate Department alternative technology approval but usually still require review as part of the design submittal.

For more details on design requirements, see the State of Colorado Design Criteria for Potable Water Systems (DCPWS), Section 4.3.9.

Question 2: How do I know which cartridges or bags I should use in my compliance filters?

Many SW/GWUDI suppliers have been issued a Record of Approved Waterworks (RAW) that lists all the supplier’s approved treatment and storage facilities and water sources. To find your facility’s RAW, visit the Department’s RAW webpage and enter your PWSID or facility name.

If you don’t have a RAW, you can find this information in the approval letter issued by the Department for your filtration system, or you can contact the Engineering Section for assistance.

Your RAW (or approval letter) will specify the approved filter manufacturer, model number, and the Department’s alternative technology acceptance letter. You can find the acceptance letter on our drinking water alternative technology website.

Important: Many bag and cartridge filters on the market have not been approved by the Department. Using unapproved filters or filter/housing combinations for compliance filtration can result in a treatment technique violation or a significant deficiency noted during a sanitary survey—both of which would require the supplier to issue a public notice.

Question 3: I have a sanitary survey coming up. Is there anything I should know about my bag or cartridge filters?

Yes! Suppliers using alternative filtration technology must continuously meet the design, performance, and operation and maintenance requirements in Sections 4.3.9.6 – 4.3.9.8 of the DCPWS and in the Department’s acceptance letter for the specific filtration technology.

For bag and cartridge filtration systems, this typically includes:

• Not exceeding the maximum specified pressure differential.
• Keeping daily records of pressure differentials and filter change-outs. These records will be reviewed during the sanitary survey.
• Maintaining specific spare parts on-site, which may also be checked during the survey.

Be sure to review your RAW and acceptance letter to understand all conditions of approval and ensure you’re keeping the required records. Both your RAW conditions and site-specific records will be evaluated during the sanitary survey.

Question 4: I’m a contract operator managing multiple public water systems. Do the requirements for bag and cartridge filters differ by system type?

Yes, the requirements can vary based on system size and type (e.g., community, non-community, or transient systems). These differences may include NSF 61 certification, the number of redundant filters required, and other system-specific considerations. The DCPWS outlines these requirements in detail, but if you have any questions, please reach out to the Department’s Engineering Section for assistance.

Sincerely,

Aqua Answers