Oxygen --- Identification of Water Pollution

"Do you know why do we need to measure the oxygen concentration in the water?"
"To test whether it is enough for the fish to breath?"

Ok that was a sample answer most probably will be given by normal people who are not into Chemistry or Water Science. The basic knowledge that we know about oxygen is, all living things need oxygen to breath and stay alive. From the perspective of Environmental Science, the oxygen concentration in the water means something more, and we call the oxygen 'dissolved oxygen' (DO). Oxygen is soluble in water, that's why aquatic life can breath underwater!

(image taken from Michael Prescotts's Blog)

Conditions Affecting DO Concentration in Water

At normal room temperature and pressure, oxygen, like any other gases, is the most stable in gaseous form. So they can easily escape from the water into the atmosphere if the water condition is not 'favourable' for it to remain in dissolved form. These are a few key conditions that affect the solubility of oxygen in water:

Temperature

When we boil the water, DO absorbs heat and expands, making it much lighter than the water and thus leading to the 'escape' as it rises. This is the reason we don't change the water in fish tank with boiled water, which is deprived of oxygen.

Atmospheric pressure

At a higher altitude such as on a mountain, we might not be able to breath as easy as when we are at sea level due to low atmospheric pressure up there. Because of the low oxygen concentration in the air under low atmospheric pressure, the DO concentration in water will be lowered as well.

Salinity (saltiness)

Salty water contains salt, which is ionic compound. These ionic compounds will attract and attach to water molecules, causing the water to hold less oxygen molecules, which are neutral. Thus, sea water can dissolve less oxygen compared to fresh water!
[if you are interested to know, the solubility of oxygen in fresh water (at 25°C & sea level) is 8.3 mg/L while in sea water, it is 6.7 mg/L]

(image taken from Stevens Water website)

Relating DO Concentration with Indication of Water Pollution

Sometimes, the fastest way to wipe out the entire aquatic life in a region is not through poisoning them with toxic pollutants, but through taking away the oxygen in the water which is vital for life. The only consequence of depleted oxygen in water towards the aquatic life is death. According to the National Water Quality Standards for Malaysia & Department of Environment Water Quality Index Classification, water belongs to Class I should have DO concentration more than 7 mg/L. What does it mean when the DO concentration is lower than that?

Accumulation of Organic Matter

Organic matter sounds like good stuff for the growth of plants, but when they accumulate in the water, it turns out to be not so good. Chemistry-cally speaking, organic matter is made of carbon chains which will be break down by microorganisms into simpler form so that it can be used as nutrients. The carbon will be converted into carbon dioxide which requires oxygen as shown in the equation below:

C  +  O₂  à  CO₂

If there is a lot of organic matter, there are a lot of carbons. So a lot of oxygen will be used, causing the oxygen in the water to run low. By doing some simple calculations, you will know how many grams of carbon will immediately consume 7 mg of oxygen in 1 L of water. These organic matter might come from the animal wastes of farms, discarded internal organs from slaughterhouses, dead animals carcases or plants etc.

Accumulation of  Nutrients

Accumulation of nutrients in water can be caused by the accumulation of organic matter, but it can be also resulted from the direct discharge of nutrients into the water, most probably due to the excessive chemical fertilizers used in farms that are washed into the water by rain. The nutrients can come from the detergent we use too (to be more specific, the nutrient mentioned is phosphate)! It is true that the aquatic plants will benefit from these excessive nutrients available for their growth, but there are some 'bad guys' who will feast on these nutrients as well and quickly multiply in number. These guys, again, are microorganisms. They are given more incentives to grow their population, thus raising the competition of grabbing any oxygen available with other aquatic life.

Infestation of Microorganisms 

This one is obvious, to put it in a simpler way, the water will be too crowded with microorganisms gasping for air to breath. It can be indirectly caused by the two points I mentioned above, but same as nutrients, the direct discharge of microorganisms especially bacteria into the water happens too, where untreated sewage effluent is channeled into the water. Where does the sewage come from and what kind of microorganisms it contain? Er...it comes from your toilet...?

How about Phytoplankton that Produces Oxygen?

Phytoplankton behaves just like plants living on land or in the water. They carry out photosynthesis to produce oxygen, so they constantly supply the water with the goodness all aquatic life will be happy with. Like other aquatic plants, they react positively towards the excessive nutrients available in the water and all of the sudden they bloom, covering the water surface with a tint of green (usually). There comes the disaster. The aquatic plants at the bottom of the water will not get enough sunlight as the phytoplankon covers up the water surface. Eventually, those plants die and the story begins with accumulation of organic matter, moves on to the accumulation of nutrients, and then the infestation of microorganisms. There goes all the oxygen. This phenomenon is called eutrophication.

(image taken from Slide Player)

Differences between DO and Biochemical Oxygen Demand (BOD)

If you have not heard of DO, you might not have heard of BOD as well. DO is the current dissolved oxygen concentration of the water, while BOD is the rate of the dissolved oxygen being consumed by the microorganisms in the water. So, if DO goes up, BOD will go down. To perform this analysis, the water sample will be collected using BOD bottle, a glass bottle with a glass stopper which can make sure that no air goes into the water sample collected.

The legendary BOD bottle. There is another version which is in amber colour to avoid penetration of sunlight so that the microorganisms in the sample will not carry out photosynthesis which will influence DO reading. We should always hold the bottle by its neck if we are going to perform analysis related to the microorganisms in the water because our body temperature might cause changes to the results (microorganisms like warm environment).

Then we will take the initial reading of the DO concentration which will be performed immediately after collecting the water sample. The analysis can be done either with the traditional Winkler method, or with the modern technology - DO meter. Winkler method involves using a chemical which captures dissolved oxygen in the water so that we can proceed to analyse how much oxygen-capturing chemical has been used to calculate the concentration of dissolved oxygen. Ok that sounds like too much work. So it's faster if we use the DO meter!

The DO meter, the most convenient invention ever compared to the traditional Winkler method! By just inserting the probe into the sample, the reading can be recorded when it becomes stable.

Thanks to the improvement on the instrument we are using now, this DO probe has a mini stirrer attached to it so that we don't have to stir while taking the reading from the DO bottle.

DO meter probe in action! It fits so well into the BOD bottle that we don't have to hold it and air in the surrounding won't get into the bottle and affect the actual DO content in the sample!

Up to this point, we have just measure the DO, not the BOD. The BOD bottle containing the sample will be sealed and placed inside an incubator at 20°C for 5 days, giving the microorganisms some time to use the oxygen in the bottle so that we can measure the DO again to see how much oxygen has been used up in the 5-day time.

So...

Now you know the reasons we shouldn't throw food waste into the drain or wash the detergent into the river. Whatever input we give to the water, there will be an outcome which might not be desired to everyone.




References:

1) Radojevic, M. & Bashkin, V. N. (1999). Practical Environmental Analysis. Cambridge, UK: The Royal Society of Chemistry.

2) Manahan, S. E. (2009). Environmental Chemistry (9^th Ed). Florida, United States: CRC Press.