If you operate a surface water treatment plant, the SWTR is the rule you live with every shift. It is the reason you monitor turbidity, calculate CT, and maintain disinfectant residuals. I deal with these requirements daily at my plant in California, and studying for the T-5 forced me to understand not just the numbers, but why the rule is structured the way it is.
The key concept is the multibarrier approach: no single treatment step is trusted to do the entire job. Filtration handles part of the pathogen removal. Disinfection handles the rest. The two work together, and the rule defines exactly how much each one contributes. If you understand that framework, the numbers and tables in this guide will make sense instead of feeling like a list to memorize.
What Does the SWTR Require?
The Surface Water Treatment Rule, finalized in 1989, requires all public water systems using surface water or groundwater under the direct influence of surface water (GWUDI) to provide filtration and disinfection. It established minimum removal and inactivation requirements for Giardia lamblia and viruses, set turbidity performance standards for filtered systems, and introduced the CT concept for calculating disinfection credit.
That last point is worth pausing on. Before the SWTR, there was no standardized way to measure whether disinfection was actually achieving what it needed to achieve. CT gave operators and regulators a common framework: concentration times contact time equals inactivation credit. Every CT calculation you run on shift traces back to this rule.
The SWTR was later supplemented by three additional rules (IESWTR, LT1ESWTR, LT2ESWTR) that tightened turbidity standards and added Cryptosporidium requirements. Those are covered in a separate guide. This guide focuses on the original SWTR framework that the enhanced rules build on.
What Are the Log Removal Requirements?
The SWTR requires a total of 3-log (99.9%) removal or inactivation of Giardia lamblia and 4-log (99.99%) removal or inactivation of viruses. These are the numbers you need to know cold for the exam.
| Pathogen | Total Reduction Required | Log Removal |
|---|---|---|
| Giardia lamblia | 99.9% | 3-log |
| Viruses | 99.99% | 4-log |
| Legionella | No specific numeric standard | Treatment for Giardia and viruses is considered sufficient |
The math is straightforward. Filtration provides a fixed credit based on the type of filter you are using. Whatever the filtration credit does not cover, disinfection must make up through CT. For example, conventional filtration gets 2.5-log Giardia credit. You need 3-log total. That means your disinfection must achieve at least 0.5-log inactivation of Giardia through CT.
The SWTR does not set a specific numeric requirement for Legionella. EPA determined that treatment sufficient to remove Giardia and viruses would also adequately address Legionella. This is a common exam question.
How Much Credit Does Each Filtration Type Provide?
Different filtration technologies receive different log removal credits under the SWTR. This is not negotiable. The credit is assigned by rule, not measured at your plant. If you run conventional filtration, you get 2.5-log Giardia credit regardless of your actual performance.
| Filtration Type | Giardia Credit | Virus Credit |
|---|---|---|
| Conventional | 2.5-log (99.7%) | 2-log (99%) |
| Direct | 2-log (99%) | 1-log (90%) |
| Slow Sand | 2-log (99%) | 2-log (99%) |
| Diatomaceous Earth (DE) | 2-log (99%) | 1-log (90%) |
The remaining log removal must be achieved through disinfection CT. For a conventional plant, that means disinfection needs to provide at least 0.5-log Giardia inactivation and 2-log virus inactivation. For a direct filtration plant, the disinfection burden is higher because the filtration credit is lower.
The exam will give you a filtration type and ask how much additional disinfection credit is needed. The math is simple: total requirement minus filtration credit equals disinfection requirement. Conventional: 3-log minus 2.5-log equals 0.5-log Giardia via CT. Direct: 3-log minus 2-log equals 1-log Giardia via CT. Know these by heart.
What Are the Turbidity Standards?
Turbidity standards are how the SWTR verifies that your filters are actually performing. The original 1989 rule set the standards below. The IESWTR later tightened the conventional and direct filtration standards to 0.3 NTU (95th percentile) and 1 NTU (maximum), which is what you operate under today.
| Filter Type | 95th Percentile Standard | Maximum |
|---|---|---|
| Conventional/Direct (original SWTR) | 0.5 NTU in 95% or more of monthly measurements | Never exceed 5 NTU |
| Conventional/Direct (IESWTR, current) | 0.3 NTU in 95% or more of monthly measurements | Never exceed 1 NTU |
| Slow Sand | 1.0 NTU in 95% or more of measurements | Never exceed 5 NTU |
| Diatomaceous Earth | 1.0 NTU in 95% or more of measurements | Never exceed 5 NTU |
For the exam, you need to know both the original SWTR values and the IESWTR tightened values. The exam may specify which rule it is asking about, or it may ask you to identify which rule changed the standard from 0.5 to 0.3 NTU.
The 0.3 NTU standard applies to the combined filter effluent, measured at the 95th percentile of all measurements taken during the month. It does not mean every individual reading must be at or below 0.3 NTU. It means 95% of them must be.
What Are the Disinfection Residual Requirements?
The SWTR requires a minimum disinfectant residual at two points in the system: the entry point to distribution, and throughout the distribution system.
At the entry point to the distribution system: the residual must be 0.2 mg/L or higher. It may not drop below 0.2 mg/L for more than 4 hours in any 24-hour period. There are no exemptions from this requirement.
In the distribution system: a detectable residual must be present in at least 95% of monthly samples. If you find a sample with no detectable residual, an HPC (heterotrophic plate count) result of 500 CFU/mL or less at that same location is accepted as an equivalent demonstration.
That 0.2 mg/L entry point requirement is straightforward on paper. In practice, maintaining it consistently at a large plant means managing your chloramine or free chlorine dosing carefully, especially when flows change or source water quality shifts. The 4-hour window is tighter than it sounds when you are on a 12-hour shift and something unexpected happens.
How Do You Calculate CT?
CT is the product of disinfectant concentration (C, in mg/L) and contact time (T, in minutes). It is how you demonstrate that your disinfection system is achieving the required log inactivation.
The formula is simple. CT = C x T.
However, the application requires attention to detail. You must use the minimum residual at the measurement point, not the average. You must calculate contact time using T10, which accounts for short-circuiting in your contact basin. And you must use peak hourly flow, not average daily flow, because that represents the worst-case scenario for contact time.
T10 is the theoretical detention time multiplied by the baffling factor. The baffling factor accounts for the reality that water does not flow through a basin in perfect plug flow. Some water moves faster, some moves slower. The baffling factor adjusts for that.
| Baffling Condition | Factor |
|---|---|
| Unbaffled (mixed flow, inlet/outlet at same end) | 0.1 |
| Poor (single or no intrabasin baffles) | 0.3 |
| Average (several intrabasin baffles) | 0.5 |
| Superior (perforated baffles or serpentine flow) | 0.7 |
| Perfect (plug flow, very high length-to-width ratio) | 1.0 |
CT calculations on the exam will give you a residual, a flow rate, a basin volume, and a baffling factor, then ask for the CT achieved. The steps are: (1) calculate detention time from volume and flow, (2) multiply by baffling factor to get T10, (3) multiply T10 by the residual to get CT, (4) compare to the required CT from the tables. Practice this until it is automatic.
The CT tables themselves vary by disinfectant, temperature, pH, and target pathogen. EPA publishes these tables in the SWTR guidance manual. For the exam, you do not need to memorize every value, but you need to understand the relationships: lower temperatures require higher CT, higher pH requires higher CT (for free chlorine), and different disinfectants have very different CT requirements.
What Are the Filtration Avoidance Criteria?
A system using surface water or GWUDI can avoid installing filtration only if it meets all of the following criteria simultaneously. Failing any single criterion triggers a requirement to install filtration within 18 months.
The criteria are strict because unfiltered surface water carries significant microbial risk. Very few systems qualify, and maintaining the criteria is an ongoing obligation, not a one-time demonstration.
To avoid filtration, a system must demonstrate:
Fecal coliform in source water must be 20 per 100 mL or less (or total coliform 100 per 100 mL or less) in at least 90% of samples over any 6-month period. Source water turbidity must not exceed 5 NTU. The system must maintain a watershed control program. An annual on-site inspection must be completed. No waterborne disease outbreak can be linked to the system. The system must comply with all SWTR and coliform MCLs.
There is no partial credit. A system that meets five out of six criteria must still install filtration. The 18-month deadline for installation does not include extensions.
Filtration avoidance is a high-frequency exam topic. The key fact is that all criteria must be met simultaneously, and failure of any single criterion triggers mandatory filtration installation within 18 months. The exam often presents scenarios where most criteria are met but one is not, testing whether you recognize that partial compliance is not sufficient.
The SWTR is the backbone of surface water treatment in the United States. Every enhanced rule that followed, from the IESWTR to LT2, builds on the framework this rule established. If you understand the multibarrier concept, the log removal math, and the CT calculation process covered here, the enhanced rules will feel like refinements rather than entirely new material.
I will continue building T-5 study guides on H2oCareerPro.com as I work through the exam content. If this guide was useful, I would appreciate you sharing it with a colleague who is also preparing. We are all in this together.
This guide is for educational purposes and reflects federal regulations as of April 2026. Always verify current requirements with your state regulatory agency. Regulations and their implementation may have changed since publication.
Frequently Asked Questions
What are the log removal requirements under the SWTR?
The SWTR requires 3-log (99.9%) removal or inactivation of Giardia lamblia and 4-log (99.99%) removal or inactivation of viruses. These totals are achieved through a combination of filtration credit and disinfection credit (CT). Conventional filtration provides 2.5-log Giardia and 2-log virus credit. Direct filtration provides 2-log Giardia and 1-log virus credit. Disinfection CT must make up the remainder.
How is CT calculated for disinfection credit?
CT equals the disinfectant concentration in mg/L multiplied by the contact time in minutes. Contact time is calculated using T10, which is the theoretical detention time multiplied by the baffling factor. Baffling factors range from 0.1 for unbaffled basins to 1.0 for plug flow. CT calculations must use peak hourly flow and minimum residual to ensure conservative compliance.
What is the minimum disinfection residual required at the entry point?
The SWTR requires a minimum disinfectant residual of 0.2 mg/L entering the distribution system. The residual may not drop below 0.2 mg/L for more than 4 hours in any 24-hour period. In the distribution system, a detectable residual must be maintained in at least 95% of monthly samples. An HPC result of 500 CFU/mL or less is accepted as an equivalent at sample points with no detectable residual.
What happens if a system fails to meet filtration avoidance criteria?
If a system using surface water or GWUDI fails to meet any of the filtration avoidance criteria, it must install filtration within 18 months. The criteria include fecal coliform limits in source water, turbidity limits, watershed control, annual inspection, no waterborne disease outbreaks, and MCL compliance. All criteria must be met simultaneously. Failure of any single criterion triggers the filtration requirement.
What is the difference between filtration credit and disinfection credit?
Filtration credit is the log removal automatically assigned to a filtration process based on its type. Conventional filtration receives 2.5-log Giardia credit. Direct filtration receives 2-log. The credit is assigned by rule, not measured at your plant. Disinfection credit is the additional log inactivation achieved through chemical disinfection, calculated using CT values. Filtration credit plus disinfection credit must meet or exceed the total requirements.