5.2 CT Calculations & Disinfection Byproducts Compliance

3.3 CT Calculations & Disinfection Byproducts Compliance

Primary disinfection efficacy is validated using the Concentration-Time ($CT$) concept established under the EPA Surface Water Treatment Rule (SWTR) and enforced by SC DES Regulation 61-58. Concurrently, utilities must balance primary pathogen inactivation against the formation of carcinogenic Disinfection Byproducts (DBPs) regulated under the Stage 1 and Stage 2 Disinfection Byproducts Rules.

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The Concentration-Time ($CT$) Concept

The $CT$ factor measures the total chemical disinfection dosage applied to water, defined as the product of the free disinfectant residual concentration ($C$) and the effective contact time ($T_{10}$): $CT = C \times T_{10}$

Where:

• $C$ = Free disinfectant residual concentration measured at the outlet of the contact basin (mg/L).
• $T_{10}$ = The contact time (in minutes) required for 10% of the water volume to pass through the contact tank ($90%$ of the water remains in the basin for at least $T_{10}$).

Hydraulic Detention Time & Baffling Factors ($BF$)

Theoretical hydraulic detention time ($T_{\text{theoretical}}$) assumes perfect mixing, calculated as: $T_{\text{theoretical}} = \frac{\text{Basin Volume (gallons)}}{\text{Plant Peak Flow Rate (gpm)}} = \frac{V}{Q}$

However, real-world tanks suffer from hydraulic short-circuiting and dead zones. To determine actual effective contact time ($T_{10}$), $T_{\text{theoretical}}$ must be multiplied by an empirically determined Baffling Factor ($BF$): $T_{10} = T_{\text{theoretical}} \times BF$

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SWTR Pathogen Log-Inactivation Requirements

Under the SWTR, surface water treatment plants must demonstrate cumulative reduction (removal plus inactivation) of target pathogens:

• Giardia lamblia: 3-log (99.9%) reduction minimum.
• Viruses: 4-log (99.99%) reduction minimum.
• Cryptosporidium: 2-log to 3.5-log (99% to 99.97%) reduction under Long Term 2 Enhanced SWTR (LT2ESWTR) based on raw water source bin classification.

Credit Allocation between Filtration and Disinfection

Well-operated surface water treatment plants receive default log-reduction credits for physical filtration when meeting effluent turbidity standards:

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Step-by-Step Worked $CT$ Calculation Example

Problem: A South Carolina water treatment plant treats a peak flow of $3,500\text{ gpm}$ ($Q$). The clearwell contact basin holds $210,000\text{ gallons}$ ($V$) and features superior serpentine baffling ($BF = 0.7$). The free chlorine residual measured at the clearwell discharge is $1.6\text{ mg/L}$ at a water temperature of $15^\circ\text{C}$ and pH $7.5$. Determine whether the system satisfies the required 0.5-log Giardia inactivation credit, given that EPA tables state the required $CT_{\text{required}}$ for 0.5-log Giardia at these conditions is $34\text{ mg}\cdot\text{min/L}$.

Solution:

1. Calculate Theoretical Detention Time ($T_{\text{theoretical}}$): $T_{\text{theoretical}} = \frac{V}{Q} = \frac{210,000\text{ gal}}{3,500\text{ gpm}} = 60.0\text{ minutes}$

2. Calculate Effective Contact Time ($T_{10}$): $T_{10} = T_{\text{theoretical}} \times BF = 60.0\text{ min} \times 0.7 = 42.0\text{ minutes}$

3. Calculate Actual Achieved $CT$ ($CT_{\text{calculated}}$): $CT_{\text{calculated}} = C \times T_{10} = 1.6\text{ mg/L} \times 42.0\text{ min} = 67.2\text{ mg}\cdot\text{min/L}$

4. Calculate $CT$ Compliance Ratio: $\text{Ratio} = \frac{CT_{\text{calculated}}}{CT_{\text{required}}} = \frac{67.2}{34.0} = 1.98$

Conclusion: Because the $CT$ ratio is $1.98 \ge 1.0$, the plant comfortably satisfies DES and SWTR primary disinfection requirements for 0.5-log Giardia inactivation.

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Disinfection Byproducts (DBPs) & Precursor Chemistry

When free chlorine or strong oxidants react with naturally occurring organic matter (NOM)—measured as Total Organic Carbon (TOC)—and bromide ions ($\text{Br}^-$) in raw water, chemical disinfectants form hazardous carcinogenic Disinfection Byproducts (DBPs).

Regulated DBP Classes & Maximum Contaminant Levels (MCLs)

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Stage 1 vs. Stage 2 DBPR Compliance Rules

• Stage 1 DBPR: Allowed utilities to determine compliance by averaging DBP concentrations across the entire distribution system using a System-Wide Running Annual Average (RAA). This allowed low DBP levels in short-pipe areas to mask dangerous DBP spikes in dead-end mains.
• Stage 2 DBPR: Replaced system-wide averaging with the Locational Running Annual Average (LRAA). Compliance is calculated independently at each specific sampling location based on the average of the current quarter and the previous three quarters. Every individual site must maintain an $\text{LRAA} \le \text{MCL}$.

Operational Evaluation Levels (OEL)

To prevent Stage 2 LRAA violations, utilities monitor the Operational Evaluation Level (OEL) each quarter for TTHM and HAA5 at every sampling site: $\text{OEL} = \frac{Q_1 + Q_2 + (2 \times Q_{\text{current}})}{4}$

Where $Q_1$ and $Q_2$ are the sample results from two quarters prior, and $Q_{\text{current}}$ is the current quarter's result. If the calculated $\text{OEL}$ exceeds $0.080\text{ mg/L}$ for TTHM or $0.060\text{ mg/L}$ for HAA5, the utility must conduct an operational evaluation study and submit a written report to SC DES within 90 days.

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DBP Control & Mitigation Strategies

1. Enhanced Coagulation: Lowering raw water pH to 5.5–6.5 using acid or higher coagulant doses (alum/ferric) to precipitate TOC before chlorine application.
2. Alternative Disinfectants: Utilizing UV light or chlorine dioxide for primary disinfection, and switching from free chlorine to chloramines for secondary distribution protection.
3. Distribution System Management: Routine flushing of dead-end mains, installation of storage tank mixers, and tank aeration to strip volatile TTHMs.