Urban Wastewater Discharge Impact Assessment on the Chattahoochee River

Case Study 1: Urban Wastewater Discharge Impact Assessment on the Chattahoochee River

Scenario Project Type: Municipal wastewater treatment plant (WWTP) compliance review and effluent upgrade feasibility study. Location Context: Downstream of Atlanta, Georgia, USA — a regulated stretch of the Chattahoochee River with sensitive trout habitat and downstream drinking water intakes. Flow is moderate (12–18 m³/s), summer water temperature averages 24°C, and regulatory DO minimum is 5.0 mg/L for coldwater biota. Constraints: Tight permitting window (<90 days); no field DO monitoring data available; must demonstrate compliance at 12 km downstream (critical mixing zone near Morgan Falls Dam); limited budget for physical sampling.

Given Data

• Initial dissolved oxygen (DO₀): 7.2 mg/L (measured upstream of discharge)
• Initial BOD (L₀): 28.5 mg/L (effluent BOD₅ from WWTP influent characterization)
• Deoxygenation rate constant (k_d): 0.25 day⁻¹ (calibrated for 24°C using Thomas’ method)
• Reaeration rate constant (k_a): 0.38 day⁻¹ (based on USGS stream velocity & depth data for this reach)
• Saturation DO (DOₛ): 8.4 mg/L (temperature-corrected for 24°C)
• Distance downstream (x): 12 km

Calculation Using the Streeter–Phelps dissolved oxygen sag curve model:

DO(x) = DOₛ − (DOₛ − DO₀)·e^(−k_a·t) + (k_d·L₀)/(k_a − k_d)·[e^(−k_d·t) − e^(−k_a·t)]

First, convert distance to travel time t using average flow velocity (v = 0.65 m/s ≈ 56.2 km/day): → t = x / v = 12 km / 56.2 km/day ≈ 0.2135 days

Now compute:

• e^(−k_a·t) = e^(−0.38 × 0.2135) ≈ e^(−0.0811) ≈ 0.922
• e^(−k_d·t) = e^(−0.25 × 0.2135) ≈ e^(−0.0534) ≈ 0.948
• (DOₛ − DO₀) = 8.4 − 7.2 = 1.2
• (k_d·L₀)/(k_a − k_d) = (0.25 × 28.5)/(0.38 − 0.25) = 7.125 / 0.13 ≈ 54.81
• Term1 = 1.2 × 0.922 ≈ 1.106
• Term2 = 54.81 × (0.948 − 0.922) = 54.81 × 0.026 ≈ 1.425
• DO(12 km) = 8.4 − 1.106 + 1.425 ≈ 8.72 mg/L

(Note: The calculator internally handles time conversion using empirical velocity–distance relationships; user inputs distance directly and tool applies site-specific velocity calibration.)

Result and Decision Predicted DO at 12 km = 8.72 mg/L, well above the 5.0 mg/L regulatory threshold and even above the critical DO minimum (4.2 mg/L at sag point). However, sensitivity analysis revealed that during low-flow drought conditions (velocity drops to 0.3 m/s → t ≈ 0.46 days), DO dips to 4.6 mg/L — near violation. As a result, the engineering team recommended installing an inline fine-bubble aerator at the outfall and tightening secondary treatment to reduce L₀ to ≤18 mg/L — achieving margin-of-safety under all hydrologic scenarios.

Lesson Always perform scenario-based sensitivity analysis (especially low-flow, high-temperature extremes) — a single-base-case DO prediction can mask critical vulnerabilities in regulated rivers.