6.2 The CT Disinfection Concept & Basin Hydraulics

6.2 The CT Disinfection Concept & Basin Hydraulics

Under the federal Safe Drinking Water Act (SDWA) Surface Water Treatment Rule (SWTR), Interim Enhanced Surface Water Treatment Rule (IESWTR), and Long Term 1 and 2 Enhanced Surface Water Treatment Rules (LT1/LT2ESWTR), all public water systems utilizing surface water or Groundwater Under the Direct Influence of Surface Water (GWUDI) must ensure continuous, verified destruction of microbial pathogens. In Arizona, where utilities treat surface water from the Colorado River via the Central Arizona Project (CAP), the Salt and Verde River watersheds, and mountain impoundments, regulatory compliance centers on the CT Disinfection Concept codified under Arizona Administrative Code (A.A.C.) R18-4-301.

---

Regulatory Basis & Pathogen Log-Inactivation Standards

The SWTR establishes legally enforceable health performance standards based on log-reduction benchmarks. Log-reduction measures the mathematical reduction of viable pathogens on a logarithmic scale:

$\text{Log Reduction} = \log_{10}\left(\frac{N_0}{N}\right)$ $\text{Percent Inactivation} = \left(1 - \frac{1}{10^{\text{Log Reduction}}}\right) \times 100\%$

• 1-log Inactivation: 90.0% reduction ($N/N_0 = 0.10$).
• 2-log Inactivation: 99.0% reduction ($N/N_0 = 0.01$).
• 3-log Inactivation: 99.9% reduction ($N/N_0 = 0.001$).
• 4-log Inactivation: 99.99% reduction ($N/N_0 = 0.0001$).

Minimum Multi-Barrier Compliance Targets

The EPA mandates that all surface water treatment systems deliver at least:

1. 3-log (99.9%) reduction of Giardia lamblia cysts: Giardia is a flagellated protozoan parasite whose thick outer cyst wall resists environmental stress and mild chemical exposure, causing severe human gastrointestinal illness (giardiasis).
2. 4-log (99.99%) reduction of enteric viruses: Enteric viruses (rotaviruses, noroviruses, enteroviruses, Hepatitis A) are minuscule infectious virions transmitted via fecal contamination.
3. Cryptosporidium oocyst protection: Highly chlorine-resistant protozoan oocysts regulated under LT2ESWTR through physical filtration removal credits (2.0 to 3.0-log) and, where source water Bin classification dictates, advanced disinfection (UV irradiation, ozone, or chlorine dioxide).

Treatment Credits: Physical Filtration vs. Chemical Inactivation

The regulatory framework recognizes that water treatment is a sequential, multi-barrier process. Facilities receive predetermined physical removal credits for properly operated coagulation, sedimentation, and granular media filtration systems that meet stringent filtered effluent turbidity limits ($\le 0.30$ NTU in 95% of monthly measurements):

For conventional surface water treatment plants in Arizona, the chemical disinfection process must achieve at least 0.5-log Giardia inactivation and 2.0-log virus inactivation every single operating day.

---

Kinetic Formulation of the CT Concept

Disinfection kinetics follow the classic Chick-Watson Law, which states that the rate of microbial inactivation is directly proportional to the disinfectant concentration ($C$) raised to a coefficient of dilution ($n \approx 1.0$) multiplied by the contact exposure time ($T$):

$\ln\left(\frac{N}{N_0}\right) = -k \cdot C^n \cdot T$

Under EPA drinking water regulations, this relationship is simplified into the CT Value:

$CT_{\text{calc}} = C \times T_{10}$

• $C$ = Disinfectant Residual Concentration (mg/L): The measured free or combined chlorine residual in milligrams per liter taken at or before the exit point of the contact basin or pipeline segment.
• $T_{10}$ = Effective Contact Time (minutes): The minimum contact time (in minutes) that 90% of the water volume remains in the basin under peak hourly flow conditions.

The Inactivation Ratio (IR)

To demonstrate compliance, an operator compares the calculated operational CT ($CT_{\text{calc}}$) against the regulatory benchmark value ($CT_{\text{req}}$) published in EPA compliance tables for the specific pathogen, temperature, pH, and residual concentration:

$\text{Inactivation Ratio (IR)} = \frac{CT_{\text{calc}}}{CT_{\text{req}}}$

$\text{Daily Log Inactivation Achieved} = \text{Required Log Target} \times \text{IR}$

• If $\text{IR} \ge 1.0$, the disinfection segment meets or exceeds regulatory compliance.
• If $\text{IR} < 1.0$, the facility has under-disinfected and is in violation of the treatment technique requirement unless downstream contact basins provide additive credit.
• When water flows through multiple disinfection segments in series (e.g., rapid mix basin $\rightarrow$ sedimentation basin $\rightarrow$ clearwell $\rightarrow$ transmission pipeline), the system calculates total compliance by summing individual inactivation ratios:

$\text{Total Daily Inactivation Ratio} = \sum \text{IR}_i = \text{IR}_1 + \text{IR}_2 + \text{IR}_3 + \dots \ge 1.0$

---

Basin Hydraulics: Short-Circuiting & Baffling Classifications

In a hypothetical, ideal hydraulic system, water moves through a contact chamber in a uniform, flat velocity profile known as plug flow, where every parcel of water resides in the basin for the exact same duration.

Theoretical Hydraulic Detention Time ($T_d$)

Theoretical detention time reflects idealized hydraulic retention based purely on geometric basin volume and flow rate:

$T_d = \frac{\text{Basin Volume (gallons)}}{\text{Flow Rate (gallons per minute, gpm)}}$ $T_d = \frac{\text{Volume (gal)}}{\text{Flow (MGD)} \times 1,000,000 / 1,440}$

The Problem of Short-Circuiting

In actual clearwells and contact tanks, plug flow never exists naturally. Internal dead zones, stagnation pockets, density currents caused by temperature or salinity differentials, wind-induced surface circulation, and high-velocity inlet jetting cause a substantial portion of the incoming water to bypass the bulk volume and travel directly to the effluent weir in a fraction of $T_d$. This phenomenon is called short-circuiting.

If an operator relied on theoretical detention time ($T_d$) for CT calculations, short-circuiting water would be severely under-disinfected, allowing viable Giardia cysts and viruses to pass directly into the distribution system.

Defining $T_{10}$

To protect public health against short-circuiting, the EPA mandates that contact time be evaluated using $T_{10}$:

[!NOTE] $T_{10}$ Definition: The time required for 10% of a chemical tracer (or water volume) entering a basin to exit at the discharge point. In other words, 90% of the water molecules remain inside the basin for a time equal to or greater than $T_{10}$.

EPA Baffling Factors ($BF$)

Where empirical tracer studies have not yet been conducted, regulatory agencies permit estimating $T_{10}$ by applying an empirical Baffling Factor ($BF$) to the theoretical detention time:

$T_{10} = T_d \times \text{Baffling Factor (BF)}$

---

Empirical Determination of $T_{10}$ via Chemical Tracer Studies

Under ADEQ engineering review guidelines, large municipal utilities verify basin baffling factors by performing on-site chemical tracer testing rather than relying on assumed default factors.

Tracer Testing Methodology

1. Tracer Selection: A non-reactive, conservative chemical tracer is dosed into the basin inlet. Approved tracers include fluoride ion ($F^-$) dosed via hydrofluorosilicic acid (carefully monitored to avoid exceeding the 4.0 mg/L primary MCL), sodium chloride ($NaCl$) monitored via continuous electrical conductivity, or lithium chloride ($LiCl$).
2. Step-Dose Injection vs. Slug (Pulse) Injection:
   • Step-Dose Method: Tracer is injected continuously at a steady concentration ($C_0$). The effluent concentration ($C$) is recorded at regular intervals until it matches $C_0$. A breakthrough curve of $C/C_0$ versus time is plotted. The time at which $C/C_0 = 0.10$ (10% breakthrough) is the empirical $T_{10}$.
   • Pulse (Slug) Method: A single discrete mass of tracer is injected instantaneously at the inlet. Effluent concentration is monitored until the entire tracer cloud discharges. The concentration curve is integrated to generate a cumulative residence time distribution; the time where 10% of the total mass has discharged represents $T_{10}$.
3. The Peak Hourly Flow Mandate: Federal and ADEQ rules dictate that tracer studies must be evaluated under the peak hourly flow rate ($Q_{\text{peak}}$) of the operating day. Using average daily flow ($Q_{\text{avg}}$) is strictly prohibited for compliance determination because peak flows generate the highest velocities and shortest retention times, representing the facility's period of greatest microbial vulnerability.

Environmental Drivers: Water Temperature and pH

Pathogen inactivation kinetics are profoundly influenced by two environmental parameters: water temperature and water pH.

Water Temperature Impact

Chemical reaction rates decrease significantly as temperature drops, following the Arrhenius principle (reaction velocity decreases by roughly 50% for every 10°C drop in temperature). Cold water slows down the rate at which chlorine molecules cross pathogen cell membranes and disrupts metabolic enzyme inactivation. Consequently, colder water requires dramatically higher CT values to achieve an identical log-inactivation.

Water pH Impact

As detailed in Section 6.1, higher water pH shifts the chemical equilibrium away from hypochlorous acid ($HOCl$) toward the hypochlorite ion ($OCl^-$). Because $OCl^-$ is 80 to 100 times less germicidal than $HOCl$, higher water pH drastically increases the required CT value for free chlorine.

EPA CT Table Reference Values for Giardia Inactivation

The following data excerpted from EPA compliance tables illustrates the dramatic impact of temperature and pH on the required CT for 0.5-log Giardia lamblia inactivation using free chlorine at a residual concentration of 1.0 mg/L:

[!WARNING] Seasonal Operational Shift in Arizona: During northern Arizona winters (e.g., Flagstaff, Show Low, Verde Valley) or winter runoff events, surface water temperatures drop below 5°C to 10°C. If raw water pH remains elevated (~8.0), the required CT more than quadruples compared to summer operations. Operators must proactively boost chlorine dosage or lower filtration throughput to ensure $T_{10}$ contact time remains sufficient.

---

Step-by-Step Operator Compliance Math Example

Problem Statement: A conventional water treatment plant in central Arizona treats surface water at a peak hourly flow rate of 8.64 MGD. Finished water passes through an on-site rectangular clearwell with a total storage volume of 450,000 gallons. The clearwell is equipped with serpentine multi-pass baffle walls classified as superior baffling ($BF = 0.7$).

Water quality monitoring at the clearwell discharge records:

• Water Temperature: 10°C
• Water pH: 7.5
• Effluent Free Chlorine Residual ($C$): 1.2 mg/L
• EPA CT Table Requirement for 0.5-log Giardia (at 10°C, pH 7.5, $C = 1.2$ mg/L): 26 mg·min/L
• EPA CT Table Requirement for 2.0-log Enteric Viruses (at 10°C, pH 7.5): 6 mg·min/L

Calculation Steps:

1. Convert Peak Flow to Gallons Per Minute (gpm):

$Q = \frac{8.64 \text{ MGD} \times 1,000,000 \text{ gal/MG}}{1,440 \text{ min/day}} = 6,000 \text{ gpm}$

1. Calculate Theoretical Detention Time ($T_d$):

$T_d = \frac{\text{Volume}}{Q} = \frac{450,000 \text{ gal}}{6,000 \text{ gpm}} = 75.0 \text{ minutes}$

1. Calculate Effective Contact Time ($T_{10}$):

$T_{10} = T_d \times \text{Baffling Factor} = 75.0 \text{ min} \times 0.7 = 52.5 \text{ minutes}$

1. Calculate Operational $CT_{\text{calc}}$:

$CT_{\text{calc}} = C \times T_{10} = 1.2 \text{ mg/L} \times 52.5 \text{ min} = 63.0 \text{ mg}\cdot\text{min/L}$

1. Evaluate Inactivation Ratios (IR):

$\text{IR}_{\text{Giardia}} = \frac{CT_{\text{calc}}}{CT_{\text{req}}} = \frac{63.0 \text{ mg}\cdot\text{min/L}}{26.0 \text{ mg}\cdot\text{min/L}} = \mathbf{2.42}$

$\text{IR}_{\text{Virus}} = \frac{CT_{\text{calc}}}{CT_{\text{req}}} = \frac{63.0 \text{ mg}\cdot\text{min/L}}{6.0 \text{ mg}\cdot\text{min/L}} = \mathbf{10.50}$

Conclusion: Because both Inactivation Ratios exceed 1.0 ($\text{IR}{\text{Giardia}} = 2.42 \ge 1.0$ and $\text{IR}{\text{Virus}} = 10.50 \ge 1.0$), the disinfection process fully satisfies ADEQ and federal SWTR requirements.