ENV-24

First-order BOD remaining

Lt = L0 e^{−k t}. Carbonaceous BOD as a first-order decay.

Reading speed

Water qualityPhelps

Governing equation

L_t=L_0 e^{-kt}

where

L_0: Ultimate BOD (mg/L)
k: Rate constant (1/d)
t: Time (d)
L_t: BOD remaining (mg/L)
y_t: BOD exerted (mg/L)

Lecture brief

Historical brief

Streeter–Phelps (1925) oxygen sag, settling theory and Guldberg–Waage kinetics made water and air quality a rate problem. The lab computes sag, overflow and a snapshot of reactor mass balance. This sheet (ENV-24 — First-order BOD remaining) is the form associated with Phelps. Working symbols: $L_0$ , $k$ , $t$ $\rightarrow$ $L_t$ , $y_t$ . Phelps modelled the remaining oxygen demand as exponential. Ultimate L0 and k are fitted from bottle tests.

Purpose

Purpose: compute

L_t

y_t

from

L_0

k

t

in Environmental via

L_t=L_0 e^{-kt}

Lt = L0 e^{−k t}. Carbonaceous BOD as a first-order decay. Use it when a real environmental question must be answered in SI before a code check.

Live realistic example

In symbols

Live case. Given

L_0 = 250.000\,\mathrm{mg/L}

k = 0.230\,\mathrm{1/d}

t = 5.000\,\mathrm{d}

, the governing relation

L_t=L_0 e^{-kt}

yields

L_t = 79.16\,\mathrm{mg/L}

y_t = 170.84\,\mathrm{mg/L}

. A bottle, a decaying L bar, a 5-day tick. Move a slider: the numbers are this situation, not a canned story.

Calculator

Inputs

Ultimate BOD L_0250 mg/LRate constant k0.23 1/dTime t5 d

Outputs

BOD remaining L_t79.16 mg/L
BOD exerted y_t170.84 mg/L

Reading speed

Watch on YouTube

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Narration of this film

A bottle, a decaying L bar, a 5-day tick.

Phelps modelled the remaining oxygen demand as exponential. Ultimate L0 and k are fitted from bottle tests.

Reading speed

Watch on YouTube