ENV 2101 Principles of Environmental Engineering

Sag Curves

  1. DO content is one of the most widely used indicators of overall ecological health of a body of water
  2. If a river was healthy before we began discharging wastewater, a significant factor in its continued health or illness is the BOD added to it by wastewater
    1. At the outfall, BOD of the river/wastewater mixture (L0) is given by:

    2. This BOD is comparable to what we have in our stoppered bottle at the beginning of our BOD test
    3. Lt = L0e-kDt

  3. We are probably more interested in how much DO remains, which depends both on the rate of deoxygenation (as in our bottle) and on the rate of reoxygenation or reaeration (which doesnít occur in our bottle)
  4. rR = -kR*D with

    kR = reaeration time constant

    D = DO deficit = DOs-DO

    kR,20įC = 3.9u1/2/H3/2

    u = average stream velocity

    H = average stream depth

    Table 3-2 Reaeration constants


    Water body

    Ranges of kR at 20įC,
    base e

    Small ponds and backwaters


    Sluggish streams and large lakes


    Large streams of low velocity


    Large streams of normal velocity


    Swift streams


    Rapids and waterfalls

    Greater than 1.15

    Source: Peavy, Rowe and Tchobanoglous, 1985

  5. The DO at any point downstream depends on these competing processes:
  6. rate of deficit increase = rate of deoxygenation - rate of reaeration

  7. To start with, DO is being depleted faster than it can be replenished

Typo corrected 6/4/2010, otherwise this is unchanged since 2001. There's a more detailed treatment of oxygen depletion in rivers at rivers.pdf.

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ANTHONY G BENOIT abenoit@trcc.commnet.edu
(860) 885-2386

Revised 6/4/10