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

Coming in 2025: Renewing the General Permit for Water Treatment Plant Wastewater Discharges (COG641000)

The division is in the process of renewing the general permit for Water Treatment Plant Wastewater Discharges (COG641000). This permit was last renewed on November 1, 2005 and has been administratively continued and remains in effect beyond the expiration date of October 31, 2010. The general permit covers discharges of wastewater from drinking water treatment plants such as filter backwash water, to surface waters of the state and/or groundwater that is hydrologically connected to surface water. For this renewal, the division conducted a stakeholder process that included public meetings on March 14, 2023, April 5, 2023, June 6, 2023 and April 2, 2024. All current permittees were invited to participate in the stakeholder engagement process. Once a draft of the general permit is complete, it will be noticed for public comment in 2025. Following the public comment period, the division will respond to public comments in writing and make any applicable changes to the draft. Then the division will issue the final general permit and begin the process of renewing all the existing permit certifications.  

For more information, consult the division’s webpage about the COG641000 updates.

➽ Engineering Section and Clean Water Program Permits Section

Record of Approved Waterworks and Sanitary Survey Significant Deficiencies

The Field Services Section (FSS) just wrapped up the 2024 inspection year (IY24) in September 2024 and the new IY25 inspection year started in October 2024! We appreciate all the assistance from public water systems in completing 497 sanitary surveys in 2024. In this article we will discuss one of the most frequently cited significant deficiencies from IY24: T119 Proper treatment operation of surface water or groundwater under the direct influence (GWUDI) systems. 

Many suppliers of surface water or groundwater under the direct influence (GWUDI) have participated in the Disinfection Outreach and Verification Effort (DOVE) led by the Engineering Section. DOVE reviews typically include a detailed  assessment of the disinfection process and thorough documentation of all other unit processes at all active surface water treatment plants. The two end results of the DOVE review are 1) a facility-specific Monthly Operating Report (MOR)  and 2) a Record of Approved Waterworks (RAW). The RAW is issued by the Engineering Section and it covers all the supplier’s approved treatment and storage facilities and water sources. To find you facility's RAW please visit this website and type in your PWSID or the name of your facility. 

The RAW includes conditions of approval specific to the treatment plant that are evaluated during the sanitary survey. These conditions of approval are important operational requirements specific to each facility that are in place to ensure the production of safe drinking water and protection of public health. All suppliers are encouraged to be familiar with their RAW; especially the “conditions of approval” for each facility. 

Frequently seen RAW conditions of approval that are evaluated during the sanitary surveys include but are not limited to:

1. For suppliers reporting the log inactivation ratio achieved in their MOR:
• Clearwell levels and flow rate must be measured and recorded at least as often as each disinfectant residual monitoring event as defined in Section 11.8(3)(c) of Regulation 11. The records of clearwell volume and flow rate are subject to Division review during sanitary surveys.
• Water temperature and pH must be measured and recorded as often as necessary given source water quality conditions but at least as often as once every week. These records are subject to Division review during sanitary surveys.
2. For suppliers reporting the lowest daily chlorine residual value in their MOR:
• The minimum contact time volume must be measured and recorded at least as often as each disinfectant residual monitoring event as defined in Section 11.8(3)(c) of Regulation 11. The records of contact time volume is subject to Department review during sanitary surveys.
• Tank level sensors typically are required to be verified at least annually to ensure measurements are calibrated and accurate. Records of level sensor measurement confirmation must be kept and  are subject to review during sanitary surveys.
3. Any contact tank bypass valves (if present) must be confirmed as being operationally closed at least once per year by appropriately actuating the valve. If the bypass is operated at any time, the supplier must notify the Division prior to use. Records of bypass valve annual actuating and maintenance must be maintained and are subject to Division review during sanitary surveys. A common method for verifying valve closure and valve actuator integrity is to count, and keep a record of the number of actuator turns from open to closed and verify that the number of turns counted is correct for the valve type and size.
4. Suppliers using alternative filtration technology must continuously meet the design, performance, and operation and maintenance requirements established in Sections 4.3.9.6 – 4.3.9.8 of the Design Criteria and in the Department’s Acceptance letter for the specific filtration technology/vendor. 

• For bag and cartridge filtration systems, this includes the requirements to not exceed the maximum specified pressure differential and keep a record of daily routine pressure differential readings and filter change outs and these records will be reviewed during the sanitary survey. Specific spare parts may also be required to be on hand and will be checked during the sanitary survey. 
• For membrane systems, suppliers must maintain an operation and maintenance manual for the filtration system. All integrity tests and Clean In Place (CIP) procedures must follow manufacturers prescribed procedures. The supplier must also keep records of the following operational parameters and the records will be reviewed during the sanitary survey:
• Integrity test date, results (pass or fail), and initials of person performing the test
• Clean in place dates with clean water permeability and integrity test results.
• Filter maintenance and fiber repair results
• Filter replacement date and reason for replacement.

Please review your RAW and make sure you are aware of the requirements established by any conditions of approval and keeping the required records. The RAW conditions of approval and site-specific records will be reviewed during the sanitary survey and if there is an issue, a T119 Significant Deficiency may be issued. According to Regulation 11, Section 11.3(72), a significant deficiency means: any situation, practice, or condition in a public water system with respect to design, operation, maintenance, or administration, that the state determines may result in or have the potential to result in production of finished drinking water that poses an unacceptable risk to health and welfare of the public served by the water system. Significant deficiencies must be addressed in writing within 45 days after the sanitary survey letter and any significant deficiencies that remain unresolved after 120 days or past an department approved schedule will be referred to compliance for a type 45-violation that requires Tier 2 public notice (30 day public notice with health effects language). 

Please check the conditions of approval in your RAW to protect public health and avoid significant deficiencies during your next sanitary survey. If you have any questions regarding your RAW, please contact the Engineering Section. If you would like additional assistance on technical issues or sanitary survey preparation, please sign up for individualized coaching here.  For any questions or concerns about sanitary surveys please email our Field Services team at cdphe_wqcd_fss_questions@state.co.us. 

Thank you for all your efforts to protect public health!

➽ Heather Young, PE, CWP, Field Services Section Manager

➽ Andrew Rice, PE, Engineering Section

PFAS Treatment and Best Available Technologies

As we discussed in the May 8th Aqua Talk article May 8th Aqua Talk article, EPA has released its final PFAS MCLs. The May 8th article summarizes the new rule and gives an overview of what water systems will need to do to come into compliance with the new rule. Today we will talk about typical PFAS removal technologies and  the upcoming update to the Design Criteria for Potable Water Systems, also known as Safe Drinking Water Program Implementation Policy #5, to incorporate PFAS treatment technologies. 

Policy #5 is used as a basis for reviewing design applications for drinking water treatment works in Colorado. The division routinely updates the document to maintain industry standards with input from stakeholders. The division is initiating a stakeholder process in 2024 and asking for feedback on portions of the drinking water design criteria to be updated. If you are interested in participating in this process please visit the Drinking Water Design Criteria stakeholder engagement website Drinking Water Design Criteria stakeholder engagement website. 

The division is aware that there are drinking water systems in Colorado that are looking to implement PFAS removal prior to the completion of the design criteria update. For those systems there are additional resources available. The EPA has released a guidance document for water systems titled Treatment Options for Removing PFAS in Drinking Water. The EPA has identified best available technologies (BAT) for PFAS removal using six criteria: 

• removal efficiency, 
• historical full scale operation, 
• geographic applicability, 
• compatibility with other treatment processes, 
• ability to bring the entire water system into compliance, 
• and a reasonable cost to large as well as medium sized systems

The technologies identified are granular activated carbon (GAC), ion exchange (IX), and reverse osmosis (RO). 

Granular Activated Carbon (GAC) is a process where adsorptive media is contained in pressure vessels or atmospheric filter beds. The PFAS contaminated water passes through the vessel or bed and the dissolved contaminants are pulled from the water and onto the surface of the media. In addition to removing PFAS, GAC can remove other contaminants including taste and odor compounds, volatile organic compounds, disinfection byproduct precursors, and others. For this reason, GAC is great for water systems looking to address multiple concerns at once. GAC will generally not have a significant impact on the corrosivity of the water. After a period of time, the GAC media will not be able to adsorb any more contaminants and will need to be replaced by fresh media and properly disposed of. The EPA has published an Interim Guidance on the Destruction and Disposal of PFAS and Materials Containing PFAS. Disposal methods include landfills and thermal treatment for regeneration or destruction. 

Ion exchange (IX) is similar to GAC where contaminated water is passed through a media, but rather than adsorbing contaminants, the PFAS in the feed water are exchanged for an ion, such as chloride, on resin beads contained in a pressure vessel. The IX process continues until the resin is out of chloride exchange sites. Unlike other IX treatment applications like water softening, the spent resin can not be regenerated by backwashing. The resin will need to be periodically disposed of and replaced with fresh media. This treatment process generally targets PFAS only and will not remove additional contaminants. The IX process tends to require a smaller footprint than GAC. Due to the addition of chloride ions into the IX effluent, this treatment process may impact the corrosivity of the finished water. Spent IX resin must either be landfilled or incinerated. 

There are other PFAS selective media available on the market today. For any of these newer media, GAC or IX resins, pilot testing is recommended to determine what method works best for a specific water quality. Piloting allows the water system to look at factors like comparative costs, treatment efficacy, and secondary impacts to finished water quality whether beneficial or not. 

The third technology that EPA has identified is reverse osmosis (RO). RO is a membrane separation process that physically removes contaminants from drinking water. This process results in two streams of water: the clean effluent and a reject stream that contains concentrated PFAS and other contaminants. The disposal of this continuous waste stream can be a substantial challenge in Colorado. The most common disposal method for land-locked states is deep well injection or evaporation ponds. There are other disposal methods in the research phase. 

There are many factors that should be considered as a water system addresses compliance with the PFAS MCLs. For some systems there may be a non-treatment option that is a better fit. Non-treatment options include adjusting source water blending ratios to prioritize sources with lower PFAS levels, an interconnect into a system with existing PFAS treatment, or consolidation into an existing system. If you would like to talk to us more about what available treatment options and grant funding opportunities, please email us at cdphe_pfas@state.co.us.

Thank you,

➽ Chelsea Cotton, P.E. Lead Drinking Water Engineer

Positive impacts of enhancing disinfection requirements - 2000s, 2010s, and today

We have written numerous articles over the years about disinfection and maintaining a proper chlorine residual.  Just search the blog for chlorine or disinfection and you will find excellent information and references from simple fixes to profound policy positions on appropriate disinfection residuals. Did you ever wonder how effective chlorination is in preventing drinking water contamination and what happened to disinfection waivers in Colorado?

In Regulation 11 rulemaking in 2010, the Water Quality Control Commission (commission) decided to remove the authority of the department to grant disinfection waivers. So at that time all waivers were evaluated and held static. The rulemaking also established more rigorous regulatory oversight for waivered systems. The rulemaking also required that all disinfecting groundwater systems had to maintain 0.2 mg/L at their entry points (surface water already had that requirement). Then, in the 2015 rulemaking to update the total coliform rule, the commission chose to establish a storage tank inspection regulation and also require that all systems maintain at least 0.2 mg/L chlorine residual throughout the distribution system. All of these rule changes were meant to recognize best practices performed by most water systems and to compel the few systems that were not up-to-speed to better protect their distribution systems. While the yearly data is noisy, it is important to recognize that the overall number of E.coli positive samples reported to us per year went dramatically down around the mid-2010s. See the graph below and table below.

It is also worthwhile to look back further at the trends over a few decades:

It’s apparent that over time E. coli violations dramatically decreased. Since about 2014, the majority of E. coli violations occurred at public drinking water systems that were out of compliance with the state’s disinfection requirements. 

As the 2010s progressed, we developed rigorous protocols to evaluate systems that maintain disinfection waivers to make sure public health was being protected without disinfection present. Therefore, over time, the department revoked waivers at systems that could not maintain compliance with the rigorous regulatory requirements for waivered systems. Other waivered systems voluntarily began chlorinating their water systems based on concerns over liability and realization of the relatively low level of effort that chlorination requires. 

At present, only two public water systems still maintain disinfection waivers. These two systems are the last disinfection waivers that will be used in Colorado. These communities have shown through regulatory compliance and their track record of water quality that the public in those communities has a measure of protection from waterborne disease even though they do not chlorinate. However, these communities ultimately may choose to chlorinate their water in the long term. The department’s position would be that it is always best to chlorinate when possible. Also, through natural disasters, aging infrastructure, or unforeseen events, the two systems may ultimately end up having their waivers revoked due to failure to maintain compliance.  

As discussed in the previous blog postings about the importance of chlorination, Colorado has a proud tradition of protecting our public drinking water supplies to the greatest degree possible.

➽ Tyson Ingels, P.E Lead Drinking Water Engineer

➽ Ron Falco, P.E. Safe Drinking Water Program Manager

Cover Story

Filtration and Disinfection Monitoring Requirements

Public water systems that use surface water—or groundwater that is directly influenced by surface water—must remove or sufficiently inactivate microbiological contaminants to ensure that drinking water is safe. This requires at least two pathogen barriers, one to filter and one to disinfect. Water systems confirm proper filtration and disinfection by monitoring for turbidity, chlorine, pH, and temperature. We may cite systems during sanitary surveys if they are not properly monitoring for each parameter. 

For filtration, the state’s drinking water regulations establish performance criteria based on surrogate monitoring of turbidity, which is the industry standard. For chemical disinfection, performance criteria are based on pH, temperature, and chlorine residual, which correspond to an estimated treatment using log inactivation equations. Appropriate monitoring consists of several key steps.

Key steps

1. Monitoring in the correct locations
2. Selecting appropriate equipment/instruments to monitor the parameter in question
3. Using proper techniques to monitor
4. Calibrating and verifying instruments in accordance with state and federal requirements

Most of the requirements for turbidity and chlorine residual calibration originate in the federal Safe Drinking Water Act. Therefore, the state cannot set less stringent requirements. Below are summaries of the requirements for each parameter.

Turbidity 

• Online turbidity analyzers must be calibrated quarterly
• Continuous monitoring means that measurements must be recorded at least every 15 minutes
• Signals should be averaged over about 30 seconds to 1 minute but must not be averaged over 3 minutes
• Results should be verified once per week with an alternative method, but this is not required

Chlorine analyzers 

• Online chlorine analyzers must be verified with a handheld method once per week
• Continuous monitoring means that measurements must be recorded at least every 15 minutes
• Signals should be averaged over about 30 seconds to 1 minute but must not be averaged over 3 minutes

pH/Temperature 

• Must be analyzed immediately after collection and never sent to the lab
• pH must be calibrated daily 

A few systems in Colorado disinfect with ultraviolet light, chlorine dioxide, or chloramines, which require additional monitoring that will not be covered in this article. We work with systems that use these technologies on a case-by-case basis. Additional information can be found in our Policy 4 Guidance on Proper Operations of Water Treatment Processes.

Below are some resources that will give more specific information about the requirements outlined above. If you are concerned about your instrument monitoring system and calibration protocols, please contact Tyson Ingels at tyson.ingels@state.co.us.

Additional Resources

Colorado Primary Drinking Water Regulations

• 11.46 - Analytical requirements and laboratory certification rule

Safe Drinking Water Act

• Section 141.74 - Analytical and Monitoring Requirements
• Section 141.560 - Individual Filter Turbidity Requirements

➽ Tyson Ingels, lead drinking water engineer