We all know that water is two parts Hydrogen, one part Oxygen, this gives us the well known formula of H₂O, and we also known that almost all living organisms of superior order need oxygen to survive, like humans or animals that need it to breath, but there's also a deep relation between oxygen and water bodies, because the later are full with living beings that need oxygen as an energy source.

The organisms that need oxygen to survive are called aerobic, a word that comes from 'aēr' that means 'air' and 'bios' which means 'life', in direct contrast with these are the called anaerobic organisms, that can grow and exist without the need of oxygen in their environment. The balance of these organisms in a water body depends on the levels of oxygen present in itself.

When a water body lacks Oxygen, it enters in a state called "hypoxia", in this state most aerobic life dies, like fish. This condition is very likely due to a pollution process where some nutrients (Phosphorus and Nitrogen are the most commons) are added to the water, those nutrients allow the grow of plants in the water body, and this leads to a deceased transparency in the water column, resulting in a lack of solar light for photosynthesis, and thus a deficit in oxygen. The organic matter that comes from the dead bodies of fish also benefit the proliferation of bacteria that furthers the hypoxic state.

Since the presence of Oxygen is vital in a water body, it's imperative to know how to quantify this prescience, this can be done assigning it a measure, or several measures that can complement each other. In this article we will talk about some of the most common ones, along with video examples of the techniques associated with each one.

---

Dissolved oxygen

DO or dissolved oxygen is a measure expressed in solution units, like g/l (grams per litre) that reflects the quantity of oxygen present in a volume of water. Normally when the DO levels drop significantly the aerobic life begins to die. This is one of the reason why DO is one of the most important parameters studied to asses the quality of water, However, the levels of DO that can be present in a water body (maximum and minimum for life to subsist) can vary due to it's nature, temperature, salinity and pressure.

Both, increases in temperature and salinity lower the quantity of oxygen needed to reach the state of saturation (In which the water is holding as much oxygen as it can, also referred as 100% air saturation), while an increase in pressure also increases the demand of oxygen for saturation. To put it simple, warm saltwater in a shallow water body need less oxygen for saturation than, cool freshwater in a deeper (with higher pressure) environment.

We should also know that others factors affect the levels of DO, deep waters lack contact with the atmosphere, resulting in an absence of photosynthesis and a saturation regularly below 100 %, while shallow water can reach more than 100% air saturation thanks to aeration, atmospheric contact and photosynthesis. Whenever the air levels are over 100% in a water body, the air begins to dissipate until it evens out in 100%.

The levels of DO required for the aerobic life vary with the species of interest, almost every fish can survive a while in conditions of DO that doesn't met it's needs, although they eventually die if this time is prolonged for much. In freshwater, a level DO of at least 6 mg/l is needed to suit the development and sustaining of the majority of fishes, but levels below 11 mg/l delay the hatching of salmon eggs.

---

Biochemical oxygen demand

BOD or Biochemical oxygen demand refers to the amount of DO used in the processing of organic matter in water by microorganisms. Normally the organic matter associated with BOD comes from pollution like sewages. High levels of BOD result in less dissolved oxygen available for living organisms within the water body. This way BOD becomes a good indicator of pollution in water.

The complete decomposition of the organic matter by microorganisms takes at least 20 days, so theoretically, to get a precise reading of BOD, the test should be made on the 20th day after the discharge of wastes, in real life this period of time has been shortened by saying that after 5 days, the value of BOD reaches almost every time, around 70% of the BOD at day 20. Since temperature also affects the test, 20° C has been chosen as the standard temperature for a BOD test.

So, the standard time and temperature used to determine BOD are 5 days and 20° C, but it's common to see some countries doing another readings, at the 7th day. By doing that, and doing some measures at another days, a BOD graph can be constructed, this would show how the rate of decomposing can vary in time. The organic matter thats more easily consumed, it's processed first, this gives a kind of constant breakdown rate, but then as conditions change, the rate also does so, because it has to go through a little adjustment period. However, during the first 5 days this variation on the rate isn't present.

In short, the value of BOD is used as a representation of the level of pollution a water body, like a stream, might have, a high value indicates an elevated presence of organic matter in the sample, that also can be decomposed by aerobic organisms. While a low BOD can show a low volume of organic matter, or just that it's too complex to be processed by aerobic life.

---

Chemical oxygen demand

Also known as COD or, chemical oxygen demand, is also a means to measure the quantity of organic matter present in water sample, but in this case, to obtain results, a digestion reagent is used to decompose the organic material. This test is based in that almost every organic compound can be oxidized by the action of strong oxidizing agents.

Given that a chemical decomposition is more violent than a natural, or biological one, the COD test give a value of almost all degradable organic matter. This is why the value of COD always exceeds that of BOD. This test is especially important in the treatment of waste water that comes from the industry, as it's the most likely to contain organic matter that can only be reduced by using chemical reagents.

While measuring COD there are various chemical compounds that affect the results, such as chromium, sodium nitrate, sodium phosphate, and chloride. But the most common are those that come from chloride, because of this most test require adding mercury sulfate before hand to precipitate the chloride, but this only work when the amount added of mercury sulfate isn't so significant that changes the values of the test.

COD gives some advantages over BOD, in terms of measurement process and monitoring, like the time needed to run the test, while BOD takes at least 5 days, COD gives results in only 2 hours. As they are so related, is common to be interested in the COD/BOD ratio (Usually BOD at 5 days, 20° C), this ratio reflects the biodegradability of the water sample, when the rate approaches 1, it means that the compounds present are highly biodegradable.

---

Sources

• Fondriest Environmental, Inc. “Dissolved Oxygen.” Fundamentals of Environmental Measurements. 19 Nov. 2013. Web
• Encyclopaedia Britannica - Biochemical oxygen demand
• EnviroTech - The Importance of Biochemical Oxygen Demand (BOD) in the Water Analysis Sector. 01 May 2015. Web
• Hanna Instruments Blog. Guide to Chemical Oxygen Demand (COD) Testing. 10 May. 2017. Web
• YSI. Chemical Oxygen Demand in Influent Wastewater Monitoring. 25 Jan 2017. Web
• Sigma Aldrich. Measuring Chemical Oxygen Demand in Water Treatment Facilities. 14 Sep 2016. Web
• RMA Environmental Services. Chemical Oxygen Demand (COD) vs Biological Oxygen Demand (BOD). Web