It's a pretty big subject, and full of interesting little side roads concerning whether or not to inhibit nitrification or not, and whether there is any correlation between Biochemical Oxygen Demand and Chemical Oxygen Demand. One can get lost in the complexities of seed concentration and so on and so forth, but most of those who come here won't be interested in any of that, so I haven't bothered.
Bacteria degrade things. We all know that. Compost composts, your bin gets smelly if you leave nasty things in there for too long, and there is much trumpeting of biodegradable substances these days. Now bacterial degradation oxidises organic molecules to stable inorganic compounds. Aerobic (oxygen requiring) bacteria make use of oxygen dissolved in the solution to do this.
C 6 H 12 O 6 + 6O 2 ––> 6H 2 O + 6CO 2
Now this reduces oxygen content in the solution. The BOD 5 test measures how much oxygen is used by bacteria undertaking this reaction over five days. Water with as much oxygen in it as it can hold is used to dilute the sample to a level where, hopefully, a measurable reading will result, the oxygen content is measured, and then the whole kit and kaboodle is sealed inside a bottle with no additional air or oxygen for five days in an incubator at 20 degrees Celsius. At the end of this period the oxygen content is again measured, so the Biochemical Oxygen Demand of that sample can be determined.
Say you dilute a sample by a factor of 800:1 and there is 3.5 mg/l less oxygen in the solution in the bottle when you take it out of the incubator than there was when you put it in. Your sample's BOD would be 2800mg/l (this example is oversimplified but gives a good idea of what is being discussed).
When a biodegradable polluting substance hits a stream or river or any other watercourse, the bacteria start using up the oxygen in the water in order to complete the reaction given above. This, of course, is pretty bad news for fish, who need the oxygen in the water to keep their little fins flapping. Now if water is fully saturated it contains enough oxygen to degrade about 8-8.5mg/l of BOD. The BOD of domestic sewage is around 500 mg/l and the BOD of the stuff I'm looking at, dirty water, can be anywhere between 1500 and 3000mg/l. Not marvellous for the piscine parties in the water courses.
Dilution has a fair effect on pollutants entering water courses, but the current consent is still only 20mg/l. So treatment is very much a necessity.