Microbiology, BOD Kinetics & Monod Growth

Quick Answer: BOD is oxygen demand. First-order decay for carbonaceous BOD. Monod describes growth vs substrate. SRT controls biomass and nitrification.

BOD Kinetics

$$L_t = L_0 e^{-k_d t}$$

Ultimate BOD L₀; deoxygenation rate k_d (day⁻¹). NBOD from ammonia oxidized separately.

Monod Equation

$$\mu = \mu_{max} \frac{S}{K_s + S}$$

At low S, growth is first-order in substrate; at high S, μ → μmax.

Activated Sludge Parameters

Oxygen Requirements

1 mg/L NH₃-N oxidized ≈ 4.6 mg/L O₂ stoichiometrically.

Exam Traps

• BOD₅ is not ultimate BOD — only fraction in 5 days.
• Lower temperature → slower kinetics → longer SRT needed.

Multi-Step Workflow

List givens with units, select the governing relationship, convert to a consistent unit set, solve, and compare to a rough estimate.

BOD Kinetics and Monod Growth

Worked BOD

If $k=0.23\,d^{-1}$ (base e) and $L_0=200$ mg/L, BOD remaining at t=5 d: $L_5=200 e^{-0.23\times5}\approx 63$ mg/L. BOD exerted = $200-63=137$ mg/L.

On the Exam: Confirm whether $k$ is base-10 or base-e; NCEES problems state the convention. Mixing them flips the answer.

BOD Kinetics and Monod Growth

Worked BOD

If $k=0.23\,d^{-1}$ (base e) and $L_0=200$ mg/L, BOD remaining at t=5 d: $L_5=200 e^{-0.23\times5}\approx 63$ mg/L. BOD exerted = $200-63=137$ mg/L.

On the Exam: Confirm whether $k$ is base-10 or base-e; NCEES problems state the convention. Mixing them flips the answer.

BOD Kinetics and Monod Growth

Worked BOD

If $k=0.23\,d^{-1}$ (base e) and $L_0=200$ mg/L, BOD remaining at t=5 d: $L_5=200 e^{-0.23\times5}\approx 63$ mg/L. BOD exerted = $200-63=137$ mg/L.

On the Exam: Confirm whether $k$ is base-10 or base-e; NCEES problems state the convention. Mixing them flips the answer.

Additional review point: verify assumptions, boundary conditions, and whether the problem is steady-state or transient before selecting an answer.

Additional review point: verify assumptions, boundary conditions, and whether the problem is steady-state or transient before selecting an answer.

Additional review point: verify assumptions, boundary conditions, and whether the problem is steady-state or transient before selecting an answer.

Additional review point: verify assumptions, boundary conditions, and whether the problem is steady-state or transient before selecting an answer.

Additional review point: verify assumptions, boundary conditions, and whether the problem is steady-state or transient before selecting an answer.

Additional review point: verify assumptions, boundary conditions, and whether the problem is steady-state or transient before selecting an answer.

Additional review point: verify assumptions, boundary conditions, and whether the problem is steady-state or transient before selecting an answer.

Additional review point: verify assumptions, boundary conditions, and whether the problem is steady-state or transient before selecting an answer.

Additional review point: verify assumptions, boundary conditions, and whether the problem is steady-state or transient before selecting an answer.

Additional review point: verify assumptions, boundary conditions, and whether the problem is steady-state or transient before selecting an answer.

Additional review point: verify assumptions, boundary conditions, and whether the problem is steady-state or transient before selecting an answer.

Additional review point: verify assumptions, boundary conditions, and whether the problem is steady-state or transient before selecting an answer.

Additional review point: verify assumptions, boundary conditions, and whether the problem is steady-state or transient before selecting an answer.

Additional review point: verify assumptions, boundary conditions, and whether the problem is steady-state or transient before selecting an answer.

Additional review point: verify assumptions, boundary conditions, and whether the problem is steady-state or transient before selecting an answer.

Additional review point: verify assumptions, boundary conditions, and whether the problem is steady-state or transient before selecting an answer.

Additional review point: verify assumptions, boundary conditions, and whether the problem is steady-state or transient before selecting an answer.

Additional review point: verify assumptions, boundary conditions, and whether the problem is steady-state or transient before selecting an answer.

Additional review point: verify assumptions, boundary conditions, and whether the problem is steady-state or transient before selecting an answer.

Additional review point: verify assumptions, boundary conditions, and whether the problem is steady-state or transient before selecting an answer.

Additional review point: verify assumptions, boundary conditions, and whether the problem is steady-state or transient before selecting an answer.