Typical composition of Sewage
Component Concentration
Solids Total 700 mg/L
Dissolved Total 500
Volatile 200
Suspended Total 200
Volatile 150
Settleable 10 ml/L
B.O.D. 200 ppm
T.O.C. 200
C.O.D. 500
Nitrogen Total as N 40
Organic 15
Ammonia (free) 25
Nitrate or nitrite 0
Phosphorus Total as P 10
Organic 3
Inorganic 7
Chloride 50
Alkalinity as CaCO3 100
Grease 100
There is much information here that is interesting, but only to someone who understands the vocabulary. First of all, the units were chosen to give convenient numbers. Sewage is almost entirely water, so the other ingredients are expressed as ppm, parts per million, or another unit that magnifies the numbers to avoid tiny decimals. Parts per million must have some basis such as grams per million grams, pounds per million pounds, or the like. This means that you must know the total weight and the weight of the ingredient to be tabulated.
For air pollution, we switch from weight to volume, and the ppm is volume per million volume units because gases are more easily measured by volume. Water has a density of about one gram per milliliter, and a thousand milliliters equals one liter. It is quick and easy to measure water by volume instead of finding and using a large scale, but water can contain dissolved solids that make its specific gravity greater than one. In that case, one liter will weigh more than one thousand grams. The water in most samples of interest to the environmental engineer will be so dilute in dissolved solids that there is very little error in assuming that one liter weighs one thousand grams, but it is more accurate and rigorous to express units as weight per liter. One milligram per liter and one ppm are almost the same most of the time, but the former is the modern way of expressing a concentration.
One of the analytical mainstays of environmental engineering is the BOD test for Biochemical Oxygen Demand. This is a slow, laborious, and inaccurate method, but its result is a good index of how pollution would affect a stream or other body of water. When fish are killed because pollution is added to a stream, they are not suddenly poisoned. In fact there may be no harm to the fish for a day or two. A simple, harmless nutrient such as sugar may be a deadly pollutant because it feeds the microorganisms in the water. If there are few microorganisms initially, their respiration may hardly dent the dissolved oxygen concentration. However, the microorganisms multiply, and soon their numbers and the stimulation to their metabolism by the pollution (their food) depletes the dissolved oxygen. When oxygen becomes scarce, the fish die from suffocation just as surely as they would die from a deadly poison.
There are many possible organic compounds that can be present in wastes. Furthermore, several different organic compounds are usually present, and it is difficult to select an appropriate analytical procedure. An analyst could perform many tests for a variety of pollutants, add up the results, and report the level of pollution. This is costly, and failure to measure some compound that was not expected to be present may render the report worthless. Often it is essential to know exactly what is present and at what concentrations in order to track down and to do something about the pollution. However, there are many, many situations where we want to know the approximate level of all pollutants that can be metabolized by microorganisms and cause reduction in dissolved oxygen. Using up the dissolved oxygen can change the appearance, odor, and taste of water, and desired aquatic species such as trout may leave a stream to be replaced by other fish or life forms that can tolerate reduced oxygen concentration.
The concept of BOD is the stoichiometry of carbon and oxygen to form carbon dioxide. Carbon compounds, however, may already be partially oxidized; a gram of alcohol will not exert the same BOD as a gram of fat. Nevertheless, measuring how much oxygen is consumed by a pollutant in a laboratory test is a good index of how the dissolved oxygen of a stream will be affected. The BOD test mixes dilution water that is rich in dissolved oxygen with sufficient sample to consume much but not all of the total dissolved oxygen (there may be a little in the sample). Oxygen is measured at the start and again after a time interval, and the amount consumed is corrected to the sample size and is called the BOD. The chemistry of the test expresses all concentrations in terms of oxygen. Thus the BOD is the oxygen consumed per unit volume of sample, e.g., pounds of BOD per million gallons, or more often parts per million, ppm.
A very serious drawback to the BOD test is the change with time. Reaching a final, stable concentration of dissolved oxygen takes roughly 25 days for a sample of a typical wastewater, although some industrial discharges that are composed of very easily metabolized compounds may take less time. Fortunately, the BOD often levels off fairly well or is close enough to a plateau in 5 days, and this has been accepted as the commonly reported BOD.
A peculiar feature of the BOD test is an increase at extended times to a second plateau. When the microorganisms exhaust the organic compounds that were in the sample, they seek an alternative source of energy. They can oxidize nitrogenous compounds, and this consumes some more dissolved oxygen.