 Fig. 1   Sewage contamination of a small stream.

DIFFERENTIAL EQUATION FOR DO SAG CURVE

Victor M. Ponce

Professor of Civil Engineering

San Diego State University



 ABSTRACT The differential equation for the dissolved oxygen sag curve (DO sag curve) is derived. The solution of this differential equation can be shown to be essentially the same as that of the well known Streeter-Phelps equation (Streeter and Phelps, 1925). Unlike the latter, the differential equation derived herein can be solved numerically and, therefore, does not require integration. Moreover, the differential equation is valid for all deoxygenation and oxygenation constants, unlike the Streeter-Phelps equation, which is undefined when these constants are equal.

INTRODUCTION

The discharge of a sewage effluent in a stream produces a biochemical oxygen demand (BOD) which decays exponentially in time and space. This oxygen demand causes an oxygen deficit, or oxygen shortage. The greater the oxygen deficit, the greater the rate of natural oxygen replenishment from the atmosphere into the stream. These two concurrent processes of oxygen consumption and oxygen replenishment produce an oxygen sag curve, i.e., a curve which sags initially due to the increased oxygen demand and recovers asymptotically downstream due to the increased rate of oxygen replenishment. The derivation of the DO sag equation is the objective of this article.

OXYGEN DEMAND EQUATION

The exponential decay of oxygen demand can be modeled as follows (Tchobanoglous and Schroeder, 1985):

D = Du e-kdt

in which:

D = biochemical oxygen demand (BOD) (mg/L);

Du = ultimate BOD immediately downstream of effluent discharge (mg/L);

kd = deoxygenation constant (d-1);

t = time (d).

The differential equation for oxygen demand is:

dD/dt = - kd Du e-kdt

Using the chain rule, the oxygen demand equation is converted to the spatial domain:

(dD/dx) (dx/dt) = - kd Du e-kdt

dD/dx = - (kd/v) Du e-(kd/v)x

in which:

x = distance along the stream, measured downstream of effluent discharge (m);

v = stream velocity (m/d).

ULTIMATE BOD

At the upstream boundary, a mass balance leads to:

Du = (QsDs + QeDe) / (Qs + Qe)

in which:

Du = ultimate BOD immediately downstream of effluent discharge (mg/L);

Qs = stream discharge (m3/s);

Ds = BOD in stream, immediately upstream of effluent discharge (mg/L);

Qe = effluent discharge (m3/s);

De = BOD of effluent discharge (mg/L);

Assuming the upstream flow is clean: Ds ≅ 0:

Du = [Qe/(Qs + Qe)] De

OXYGEN SUPPLY EQUATION

The differential equation for oxygen supply in a stream can be modeled as follows (Tchobanoglous and Schroeder, 1985):

dS/dt = ko (Ss - S)

in which:

ko = oxygenation constant (d-1);

Ss = dissolved oxygen concentration at saturation, a function of temperature, salinity, and atmospheric pressure (mg/L);

S = dissolved oxygen concentration (mg/L).

The quantity (Ss - S) is the oxygen deficit.

Using the chain rule, the oxygen supply equation is converted to the spatial domain:

dS/dx = (ko/v) (Ss - S)

DO SAG EQUATION

The change of oxygen in space is given by:

dO/dx = dS/dx + dD/dx

dO/dx = (ko/v) (Ss - S) - (kd/v) Du e-(kd/v)x

in which:

O = dissolved oxygen concentration (mg/L).

Since S = O:

dO/dx = (ko/v) (Ss - O) - (kd/v) Du e-(kd/v)x

In a control volume (computational reach) of length L (m), with index (j) upstream and index (j+1) downstream:

(Oj+1 - Oj)/L = (ko/v) (Ss - Oj) - (kd/v) Du e-(kd/v)xj+1

Oj+1 = Oj + L (ko/v) (Ss - Oj) - L (kd/v) Du e-(kd/v)xj+1

INITIAL DO

For j = 0: Oj = O0

At the upstream boundary, a mass balance leads to:

O0 = (QsOs + QeOe)/(Qs + Qe)

in which:

O0 = dissolved oxygen concentration immediately downstream of effluent discharge (mg/L);

Qs = stream discharge (m3/s);

Os = dissolved oxygen concentration in stream, immediately upstream of effluent discharge (mg/L);

Qe = effluent discharge (m3/s);

Oe = dissolved oxygen concentration of effluent discharge (mg/L);

Assuming the effluent is anoxic: Oe ≅ 0:

O0 = [Qs/(Qs + Qe)] Os

COMPARISON WITH STREETER-PHELPS EQUATION

The classical way of solving for the dissolved oxygen sag equation is the Streeter-Phelps equation, which dates back to 1925 (Streeter-Phelps, 1925; Tchobanoglous and Schroeder, 1984).

The Streeter-Phelps equation is:

O = Ss - [kd/(ko - kd)] Du [e-(kd/v)x - e-(ko/v)x] - (Ss - O0) e-(ko/v)x

in which:

O = dissolved oxygen concentration (mg/L) at distance x (m);

Ss = dissolved oxygen concentration at saturation, a function of temperature, salinity, and atmospheric pressure (mg/L);

O0 = dissolved oxygen concentration immediately upstream of effluent discharge (mg/L);

The Streeter-Phelps equation is an algebraic equation derived by integrating the differential equation governing the oxygen sag. The differential equation derived herein is based on the same principles as that of Streeter-Phelps. However, the differential equation is not integrated as in the case of Streeter-Phelps, but rather it is solved directly, using finite differences. In most cases, both methods will lead to the same answer. Significantly, the Streeter-Phelps equation suffers from being undefined for equal values of the oxygenation and deoxygenation constants, while the numerical method (this article) is not. Thus, the numerical method is an all-around better predictor than the Streeter-Phelps model, applicable for all oxygenation and deoxygenation constants, regardless of their values.

SUMMARY

The dissolved oxygen concentration at saturation Ss is a function of temperature, salinity, and atmospheric pressure.

The dissolved oxygen concentration at the upstream boundary, at j = 0, immediately downstream of the effluent discharge, is estimated as follows:

O0 = [Qs/(Qs + Qe)] Os

The ultimate BOD immediately downstream of the effluent discharge, is estimated as follows:

Du = [Qe/(Qs + Qe)] De

The dissolved oxygen concentration at node j+1 is:

Oj+1 = Oj + L (ko/v) (Ss - Oj) - L (kd/v) Du e-kd (xj+1/v)

Given Qs (m3/s), Qe (m3/s), Ss (mg/L), Os (mg/L), De (mg/L), v (m/d), L (m), kd (d-1), and ko (d-1), the dissolved oxygen concentration along the stream (DO sag curve) may be calculated.

REFERENCES

Streeter, H. W., and E. B. Phelps. 1925. A study of the pollution and natural purification of the Ohio River. III. Factors concerned in the phenomena of oxidation and reaeration. U.S. Public Health Service, Bulletin No. 146.

Tchobanoglous, G., and E. D. Schroeder. 1984. Water quality: Characteristics, modeling, modification. Addison-Wesley, Massachussets.

NOTATION

D = biochemical oxygen demand (BOD) (mg/L);

De = BOD of effluent discharge (mg/L);

Ds = BOD in stream, immediately upstream of effluent discharge (mg/L);

Du = ultimate BOD immediately downstream of effluent discharge (mg/L);

kd = deoxygenation constant (d-1);

ko = oxygenation constant (d-1);

L = length of control volume, or computational reach (m);

O = dissolved oxygen concentration (mg/L);

O0 = dissolved oxygen concentration immediately downstream of effluent discharge (mg/L);

Os = dissolved oxygen concentration in stream, immediately upstream of effluent discharge (mg/L);

Oe = dissolved oxygen concentration of effluent discharge (mg/L);

Qe = effluent discharge (m3/s);

Qs = stream discharge (m3/s);

S = dissolved oxygen concentration (mg/L);

Ss = dissolved oxygen concentration at saturation, a function of temperature, salinity, and atmospheric pressure (mg/L);

t = time (d);

x = distance along the stream, measured downstream of effluent discharge (m); and

v = stream velocity (m/d).