Streeter–Phelps equation / REF / 408 / 2014

The Streeter–Phelps equation is used in the study of water pollution as a water quality modelling tool. The model describes how dissolved oxygen (DO) decreases in a river or stream along a certain distance by degradation of biochemical oxygen demand (BOD). The equation was derived by H. W. Streeter, a sanitary engineer, and Earle B. Phelps, a consultant for the U.S. Public Health Service, in 1925, based on field data from the Ohio River. The equation is also known as the DO sag equation.

Streeter-Phelps equation

The Streeter–Phelps equation determines the relation between the dissolved oxygen concentration and the biological oxygen demand over time and is a solution to the linear first order differential equation
This differential equation states that the total change in oxygen deficit (D) is equal to the difference between the two rates of deoxygenation and reaeration at any time.
The Streeter–Phelps equation is also known as the DO sag equation. This is due to the shape of the graph of the DO over time.

Critical oxygen deficit

On the DO sag curve a minimum concentration occurs at some point, along a stream. If the Streeter–Phelps equation is differentiated with respect to time, and set equal to zero, the time at which the minimum DO occurs is expressed by
To find the value of the critical oxygen deficit, , the Streeter–Phelps equation is combined with the equation above, for the critical time, . Then the minimum dissolved oxygen concentration is
It should be noted that, mathematically it is possible to get a negative value of , even though it is not possible to have a negative amount of DO in reality.
The distance travelled in a river from a given point source pollution or waste discharge downstream to the (which is the minimum DO) is found by
where is the flow velocity of the stream. This formulae is a good approximation as long as the flow can be regared as a the a plug flow (turbulent).

Estimation of reaeration rate

Several estimations of the reaeration rate exists, which generally follow the equation

The constants depend on the system to which the equation is applied, i.e. the flow velocity and the size of the stream or river. Different values are available in the literature.

The software “International Hydrological Programme” applies the following equation derived on the basis of values used in published literature

Temperature correction

Both the deoxygenation rate, and reaeration rate, can be temperature corrected, following the general formula.

Mixing of rivers

When two streams or rivers merge or water is discharged to a stream it is possible to determine the BOD and DO after mixing assuming steady state conditions and instantaneous mixing. The two streams are considered as dilutions of each other thus the initial BOD and DO will be

Numerical approach

Nowadays it is possible to solve the classical Streeter–Phelps equation numerically by use of computers. The differential equations are solved by integration.

History

In 1925, a study on the phenomena of oxidation and reaeration in the Ohio River in the US was published by the sanitary engineer, Harold Warner Streeter and the consultant, Earle Bernard Phelps (1876–1953). The study was based on data obtained from May 1914 to April 1915 by the United States Public Health Service under supervision of Surg. W.H. Frost.
More complex versions of the Streeter-Phelps model were introduced during the 1960s, where computers made it possible to include further contributions to the oxygen development in streams. At the head of this development were O'Connor (1960) and Thomann (1963).O'Connor added the contributions from photosynthesis, respiration and sediment oxygen demand (SOD). Thomann expanded the Streeter-Phelps model to allow for multi segment systems.

Applications and limitations

The simple Streeter-Phelps model is based on the assumptions that a single BOD input is distributed evenly at the cross section of a stream or river and that it moves as plug flow with no mixing in the river. Furthermore only one DO sink (carbonaceous BOD) and one DO source (reaeration) is considered in the classical Streeter-Phelps model. These simplifications will give rise to errors in the model. For example the model does not include BOD removal by sedimentation, that suspended BOD is converted to a dissolved state, that sediment has an oxygen demand and that photosynthesis and respiration will impact the oxygen balance.

Expanded model

In addition to the oxidation of organic matter and the reaeration process, there are many other processes in a stream which affect the DO. In order to make a more accurate model it is possible to include these factors using an expanded model.
The expanded model is a modification of the traditional model and includes internal sources (reaeration and photosynthesis) and sinks (BOD, background BOD, SOD and respiration) of DO. It is not always necessary to include all of these parameters. Instead relevant sources and sinks can be summed to yield the overall solution for the particular model. Parameters in the expanded model can be either measured in the field or estimated theoretically.

Background BOD

Background BOD or benthic oxygen demand is the diffuse source of BOD represented by the decay of organic matter that has already settled on the bottom. This will give rise to a constant diffuse input thus the change in BOD over time will be

Sedimentation of BOD

Sedimented BOD does not directly consume oxygen and this should therefore be taken into account. This is done by introducing a rate of BOD removal combined with a rate of oxygen consumption by BOD. Giving a total rate for oxygen removal by BOD

Oxygen can be consumed by organisms in the sediment. This process is referred to as sediment oxygen demand (SOD). Measurement of SOD can be undertaken by measuring the change of oxygen in a box on the sediment.(benthic respirometer)

Nitrification

Ammonium is oxidized to nitrate under aerobic conditions

NH₄⁺ + 2O₂ → NO₃^- + H₂O + 2H⁺

Ammonium oxidation can be treated as part of BOD, so that BOD = CBOD + NBOD, where CBOD is the carbonaceous biochemical oxygen demand and NBOD is nitrogenous BOD. Usually CBOD is much higher than the ammonium concentration and thus NBOD often does not need to be considered.

Photosynthesis and respiration

Photosynthesis and respiration are performed by algae and by macrophytes. Respiration is also performed by bacteria and animals. Assuming steady state (net daily average) the change in deficit will be

Due to the variation of light over time, the variation of the photosynthetic oxygen can be described by a periodical function over time, where time is after sunrise and before sunset
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