4.2.2 Chemical characteristics of liquid wastes

Organic matter

Wastewaters from many different sources contain organic matter, which is a frequent cause of pollution in surface waters. If organic matter is released into a river or lake, bacteria and other micro-organisms that are naturally present in fresh water will degrade the waste and in the process they use dissolved oxygen from the water. If there is a lot of organic matter, then most or all of the dissolved oxygen may be used up, thus depriving other life forms in the water of this essential element. The oxygen taken up in degrading the organic matter is referred to as its oxygen demand. This can be determined by a measure called the biochemical oxygen demand (BOD). BOD tests are carried out in a laboratory and involve measuring the amount of oxygen used, usually over a period of five days, as the organic matter in the wastewater breaks down. The result is the amount of oxygen used in degrading the organic matter in the wastewater, which is expressed in milligrams per litre (mg l^-1).

There is also a chemical method of determining the quantity of organic matter called the chemical oxygen demand (COD) test. This test is much quicker than the BOD test, taking only about two hours to carry out. It depends on chemical oxidation of the organic matter rather than biological degradation. It involves boiling a sample of wastewater with a mixture of concentrated acids and a measured quantity of oxidising agent to oxidise the organic matter. The amount of oxidising agent remaining at the end of the test is measured. The amount that has been used up is equivalent to the amount of organic matter in the sample. The result is again expressed in mg l^-1. COD tends to give higher results than BOD because the chemical process can oxidise more material than the biological process.

Inorganic material

Wastewater also contains inorganic chemicals. This means any substance that has not come from animals or plants, so it includes a wide range of different chemicals as well as inert solids like sand and silt. Many inorganic chemicals are dissolved in the water and although some are harmless, others are pollutants that can damage aquatic life such as fish and other organisms that live in water. One example is ammonia (NH₃) which is present in human and animal excreta. Like organic matter, ammonia is broken down in the environment by natural processes. If ammonia is released into a river it is converted by the action of bacteria to nitrate (NO₃), which is less harmful. This natural conversion of ammonia to nitrate requires oxygen and is limited if there are excessive quantities of ammonia. Other examples of inorganic chemicals in wastewaters are chloride (from salt), phosphates (from chemical fertilisers and from human and animal wastes), and metal compounds (from mining operations or metal-plating plants).