Effluent treatmjent and COD / BOD

Wastewater constituents

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The composition of wastewater varies widely. This is a partial list of what it may contain:

• Water ( > 95%) which is often added during flushing to carry waste down a drain;
• Pathogens such as bacteria, viruses, prions and parasitic worms;
• Non-pathogenic bacteria (> 100,000 / ml for sewage);
• Organic particles such as faeces, hairs, food, vomit, paper fibers, plant material, humus, etc.;
• Soluble organic material such as urea, fruit sugars, soluble proteins, drugs, pharmaceuticals, etc.;
• Inorganic particles such as sand, grit, metal particles, ceramics, etc.;
• Soluble inorganic material such as ammonia, road-salt, sea-salt, cyanide, hydrogen sulfide, thiocyanates, thiosulfates, etc.;
• Animals such as protozoa, insects, arthropods, small fish, etc.;
• Macro-solids such as sanitary napkins, nappies/diapers, condoms, needles, children's toys, dead animals or plants, body parts, etc.;
• Gases such as hydrogen sulfide, carbon dioxide, methane, etc.;
• Emulsions such as paints, adhesives, mayonnaise, hair colorants, emulsified oils, etc.;
• Toxins such as pesticides, poisons, herbicides, etc.

[edit]Wastewater quality indicators

Main article: Wastewater quality indicators

Any oxidizable material present in a natural waterway or in an industrial wastewater will be oxidized both by biochemical (bacterial) or chemical processes. The result is that the oxygen content of the water will be decreased. Basically, the reaction for biochemical oxidation may be written as:

Oxidizable material + bacteria + nutrient + O₂ → CO₂ + H₂O + oxidized inorganics such as NO₃ or SO₄

Oxygen consumption by reducing chemicals such as sulfides and nitrites is typified as follows:

S^-- + 2 O₂ → SO₄^--

NO₂^- + ½ O₂ → NO₃^-

Since all natural waterways contain bacteria and nutrients, almost any waste compounds introduced into such waterways will initiate biochemical reactions (such as shown above). Those biochemical reactions create what is measured in the laboratory as the Biochemical oxygen demand (BOD). Such chemicals are also liable to be broken down using strong oxidising agents and these chemical reactions create what is measured in the laboratory as the Chemical oxygen demand (COD).

Both the BOD and COD tests are a measure of the relative oxygen-depletion effect of a waste contaminant. Both have been widely adopted as a measure of pollution effect. The BOD test measures the oxygen demand of biodegradable pollutants whereas the COD test measures the oxygen demand of oxidizable pollutants.

The so-called 5-day BOD measures the amount of oxygen consumed by biochemical oxidation of waste contaminants in a 5-day period. The total amount of oxygen consumed when the biochemical reaction is allowed to proceed to completion is called the Ultimate BOD. The Ultimate BOD is too time consuming, so the 5-day BOD has almost universally been adopted as a measure of relative pollution effect.

There are also many different COD tests of which the 4-hour COD is probably the most common.

There is no generalized correlation between the 5-day BOD and the ultimate BOD. Similarly there is no generalized correlation between BOD and COD. It is possible to develop such correlations for a specific waste contaminants in a specific waste water stream but such correlations cannot be generalized for use with any other waste contaminants or waste water streams. This is because the composition of any waste water stream is different. As an example and effluent consisting of a solution of simple sugars that might discharge from a confectioneryfactory is likely to have organic components that degrade very quickly. In such a case the 5 day BOD and the ultimate BOD would be very similar . I.e there would be very little organic material left after 5 days. . However a final effluent of a sewage treatment works serving a large industrialised area might have a discharge where the ultimate BOD was much greater than the 5 day BOD because much of the easily degraded material would have been removed in the sewage treatment process and many industrial processes discharge difficult to degrade organic molecules.

The laboratory test procedures for the determining the above oxygen demands are detailed in many standard texts. American versions include the "Standard Methods For the Examination Of Water and Wastewater" ^[1]

[edit]Sewage disposal

Industrial wastewater effluent with neutralized pH from tailing runoff. Taken in Peru.

In some urban areas, sewage is carried separately in sanitary sewers and runoff from streets is carried in storm drains. Access to either of these is typically through a manhole. During high precipitation periods a sanitary sewer overflow can occur, causing potential public health and ecological damage.

Sewage may drain directly into major watersheds with minimal or no treatment. When untreated, sewage can have serious impacts on the quality of an environment and on the health of people. Pathogens can cause a variety of illnesses. Some chemicals pose risks even at very low concentrations and can remain a threat for long periods of time because of bioaccumulation in animal or human tissue.

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