Organic matter

All organic matter contains carbon in combination with one or more other elements. All substances of animal or vegetable origin contain carbon compounds and are, therefore, organic.

When plants and animals die, they begin to decay. The decay process is simply the various naturally occurring micro-organisms converting the organic matter—the plant and body tissue—to simpler compounds. Some of these simpler compounds may be other forms of organic matter or they may be nonorganic compounds, such as nitrate and ortho-phosphate, or gases, such as nitrogen gas (N2), ammonia (NH3), and hydrogen sulfide (H2S).

When manure or other organic matter is added to water, the decay process occurs just as it does on land. Micro-organisms attack these organic materials and begin to consume and convert them. If the water contains dissolved oxygen, the organisms involved in the decay process are aerobic or facultative. Aerobic organisms require free (dissolved) oxygen to survive, while facultative organisms function in both aerobic (oxygen present) or anaerobic (oxygen absent) environments.

As the organisms consume the organic matter, they also consume free oxygen. The principal by-products of this aerobic digestion process are carbon dioxide (CO2) and water (H2O). Figure 3-1 is a schematic representation of the aerobic digestion cycle as it relates to nitrogenous and carbonaceous matter.

In a natural environment the breakdown of organic matter is a function of complex, interrelated, and mixed biological populations. However, the organisms principally responsible for the decomposition process are bacteria. The size of the bacterial community depends on its food supply and other environmental factors including temperature and pH.

If a large amount of organic matter, such as manure, is added to a water body, the bacterial population begins to grow, with the rate of growth expanding rapidly. Theoretically, the bacterial population doubles with each simultaneous division of the individual bacteria; thus, one divides to become two, two becomes four, four becomes eight, and so forth. The generation time, or the time required for each division may vary from a few days to less than 30 minutes. One bacterium with a 30-minute generation time could yield 16,777,216 new bacteria in just 12 hours.

Because each bacterium extracts dissolved oxygen from the water to survive, the addition of waste and the subsequent rapid increase in the bacterial population could result in a drastic reduction in dissolved oxygen in a stream. The point in a stream where the maximum oxygen depletion occurs can be a considerable distance downstream from the point where pollutants enter the stream. The level of oxygen depletion depends primarily on the amount of waste added; the size, velocity, and turbulence of the stream; the initial dissolved oxygen levels in the waste and in the stream; and the temperature of the water.

A turbulent stream can assimilate more waste than a slow, placid stream because the turbulence brings air into the water (re-aeration) and helps replenish the dissolved oxygen. In addition, cold water can hold more dissolved oxygen than warm water. For example, pure water at 10 °C (50 °F) has 10.92 mg/L of dissolved oxygen when fully saturated, while water at 30 °C (86 °F) has 7.5 mg/L at the saturation level.

An adequate supply of dissolved oxygen is essential for good fish production. Adding wastes to a stream can lower oxygen levels to such an extent that fish and other aquatic life are forced to migrate from the polluted area or die for lack of oxygen. The decomposition of wastes can also create undesirable color as well as taste and odor problems in lakes used for public water supplies.

The amount of organic matter in water can be determined with laboratory tests, including those for 5-day biochemical oxygen demand (BOD5), chemical oxygen demand (COD), and volatile solids (VS). Table 3-1 illustrates BOD5 values for a sampling of lagoon influents and effluents for various livestock facilities. The table is used for illustration only and shows how "strong" agricultural wastes can be, even after treatment. Concentrations will vary considerably from these values, depending on such factors as the age and size of the lagoon, characteristics of the waste, geographical location, and the amount of dilution water added.

The BOD5 value for raw domestic sewage ranges from 200 to 300 mg/L, while that for municipal wastewater treated to the secondary level is about 20 mg/L. Because municipal waste is so much more dilute, the concentrations of BOD5 are much lower than those in treated animal waste. Nevertheless, animal wastewater released to a stream, though smaller in total volume relative to municipal discharges, can be more concentrated and cause severe damage to the aquatic environment.

Table 3-1 A sampling of influent BOD5 concentrations and range of effluent concentration for various types of anaerobic lagoons

Source Lagoon influent Lagoon effluent ------------mg/L-----------

Table 3-1 A sampling of influent BOD5 concentrations and range of effluent concentration for various types of anaerobic lagoons

Source Lagoon influent Lagoon effluent ------------mg/L-----------



200 - 1,200



200 - 2,500



300 - 3,600



600 - 3,800

Figure 3-1 Aerobic cycle of plant and animal growth and decomposition as related to nitrogen and carbon

Figure 3-1 Aerobic cycle of plant and animal growth and decomposition as related to nitrogen and carbon

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