Explain what might have caused changes in oxygen concentration downstream from the point of sewage entry
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ANSWER :
One of the most important measures of the health of the stream is the level of dissolved oxygen (DO) in the water. Oxygen (O2) dissolves in water through the mixing of the water surface with the atmosphere. The oxygen is used by fish and other animals in the water to "breath" through their gills or other respiratory systems and by plants. If the levels fall too low, many species of fish, macroinvertebrates, and plants cannot survive. At very low levels of oxygen, the stream becomes "septic" and smells rotten because low oxygen sulfur bacteria begin to dominate.
The level of oxygen dissolved in the water is inversely related to the water temperature. The lower the temperature, the more oxygen can dissolve in the water. At zero degrees centigrade, the maximum or saturation level of DO is 14.6 parts per million (ppm) or for every million molecules of water, there are 14.6 molecules of oxygen. Because of the molecular weight of water, ppm of dissolved substances in water is also equivalent to milligrams of the substance per liter of water. As water becomes warmer, the saturation amount of DO will drop. For example, at 30 degrees centigrade, the DO saturation is 7.56 mg/l while at 40 degrees it goes down to 6.41 mg/l.
Even in very clean streams, the DO never quite gets to the saturation DO level as animals and plants consume oxygen for respiration and bacteria decompose natural wastes, also using oxygen for respiration. When people discharge sewage or animal wastes into streams, those wastes are decomposed by bacteria. Because the discharge is a very large amount coming out of a pipe at one location, there is an immediate depression in oxygen levels from the mixing of the sewage volume with low oxygen levels with the stream water with higher oxygen levels:
Average DO = (DO_effluent*Sewage_Flow_+DO_stream*Stream_Flow)/
(Sewage_Flow_+Stream_Flow)
Once discharge occurs, bacteria begin to decompose the wastes, using oxygen from the water in the process. The amount of oxygen that might be used is measured by a test called the Biochemical Oxygen Demand (BOD). For each increment of time, a portion of the BOD is decomposed based on a decay rate for decomposition and as a result, an increment of oxygen is consumed. At the same time, additional oxygen is dissolved in the water in exchange with the atmosphere. The rate at which this reaeration happens is governed by the characteristics of the stream and is often expressed as a function of the stream depth and velocity. Taken together this deoxygenation and reaeration produce an oxygen sag curve that looks like figure 1.
The oxygen level continues to drop as the amount of oxygen consumed exceeds the rate of reaeration. Once enough of the BOD load has been consumed, the trend reverses as the rate of reaeration exceeds the rate of deoxygenation and the oxygen level continues to rise until it approaches the saturation level.
This system was modeled originally by Streeter and Phelps in 1925 and has become the basis for modeling the impacts of sewage treatment and industrial plants on oxygen levels in streams. In reality, the system is much more complicated as plants add oxygen to the water during photosynthesis and other biochemical processes consume oxygen at different rates. However, it is still a realistic approximation of the trends that can be used to test the impacts of sewage treatment plants on the receiving stream.
Using Stella, we have created a version of the model which represents the dissolved oxygen of Sugar Creek in Stark County, Ohio. The model is based on a much more complete and complex model of the system developed for USEPA (Brown and Barnwell, 1987). The Stella model is divided into two major sectors. The BOD Load sector calculates the initial organic waste load and its decay with time. The DO Balance sector calculates the original DO level as a function of temperature and the mixing of the waste load and then tracks the impacts of BOD decay on oxygen levels over