08/23/2022 | Drinking Water | 8 MINUTE READ

The Importance of Dissolved Oxygen in Drinking Water

drinking water

There are numerous components that can be found in drinking water, one of which is dissolved oxygen. The dissolved oxygen in water is the total amount of oxygen that has already been dissolved in a water sample. When dissolved oxygen is at the right concentration, aquatic ecosystems can thrive. On the other hand, too little dissolved oxygen means that aquatic species might not have enough dissolved oxygen to survive.

When it comes to drinking water, relatively high levels of dissolved oxygen can lead to a better taste. Bodies of water obtain oxygen directly from the atmosphere as well as from aquatic plants. In the event that DO levels have dropped precipitously in a body of water, the entire ecosystem could suffer. Keep in mind that running water results in more oxygen being dissolved when compared to still water that’s found in a lake or pond.

Measuring dissolved oxygen is a straightforward process that can be accomplished with a dissolved oxygen sensor. The right sensor can provide you with quick and precise readings that allow you to make the necessary changes to ensure dissolved oxygen concentration remains at an optimal level. This guide takes an in-depth look at dissolved oxygen and why it’s important for drinking water.

clean water

What is Dissolved Oxygen?

Dissolved oxygen is the amount of free and non-compound oxygen that’s present in a body of water or other type of liquid. Because of how dissolved oxygen influences any organisms that are present in the water, dissolved oxygen concentration is regularly measured to identify current water quality. If DO levels are too low or too high, it’s likely that the water quality would worsen and that aquatic life would be harmed.

Free oxygen involves any oxygen that isn’t directly bonded to another element. Dissolved oxygen occurs when free oxygen molecules are present in the water. Keep in mind that the bonded oxygen molecule that’s found in H2O is considered to be part of a compound, which is why it doesn’t count when measuring dissolved oxygen levels. It’s believed that the free oxygen molecules that get into water dissolved in the same way that sugar and salt do whenever stirred.

Why is it Important to Evaluate Dissolved Oxygen?

There are several ways that dissolved oxygen matters when it’s present in water. When it comes to bodies of water like lakes and rivers, dissolved oxygen is essential for aquatic animals. In fact, every aquatic animal requires dissolved oxygen to breathe. When DO levels drop too low, hypoxia occurs. In the event that there’s no dissolved oxygen in the water, this is referred to as anoxia.

It’s common for these conditions to take place when sizable algal blooms are present in the water. These blooms are effectively decomposed by the microorganisms in the water. While this process is ongoing, the dissolved oxygen is consumed. Low amounts of dissolved oxygen are usually found towards the lowermost portion of the water, which means that any organisms that live directly in the sediments are affected.

It’s common for DO levels to change based on the season or period of the year. If DO levels drop by a considerable amount, animals that are sensitive to DO fluctuations may choose to move away from the waters in question. If these animals don’t move, they may suffer from worse health or eventually die.

As mentioned previously, water that’s constantly being agitated usually contains high levels of dissolved oxygen. In comparison, stagnant water consists of much less dissolved oxygen. Water that moves quickly in a large river or mountain stream will come with the highest DO concentrations. If bacteria is present in a sample or body of water, the bacterial will consume surrounding oxygen while organic matter decays, which means that the presence of too much organic material in rivers and lakes could lead to eutrophic conditions.

These conditions occur when the body of water only contains small amounts of oxygen, which means that the water body is effectively dead. When water temperatures are high during the summer and early fall months, dissolved oxygen levels are low. In this situation, aquatic animals may find it difficult to survive when living in standard water with high amounts of organic material present.

warm ocean

In warmer weather, the water around the lake’s surface can be too warm for the aquatic animals. On the other hand, water around the bottom of the lake may have too little oxygen. During extended period of hot weather with very little wind to cause agitation, a considerable amount of fish could die.

Since temperature directly affects dissolved oxygen concentration along surface waters, these waters have a daily and seasonal cycle. Aquatic animal ecosystems are typically at their healthiest during the winter and spring months when water temperatures are low.

Keep in mind that dissolved oxygen is consumed by all aquatic life, which is why measuring dissolved oxygen levels can be helpful when determining the health of a stream or lake. While oxygen mainly gets to a body of water from the atmosphere, it can also do so via a groundwater discharge.

Because of how dissolved oxygen builds up in bodies of water, there are “dead zones” in the world where dissolved oxygen levels are too low to support the majority of life. Most dead zones are seasonal, which means that the condition typically occurs during the warm summer months. Dead zones take place in portions of the Gulf that are south of Louisiana. Another zone commonly develops along the Mississippi Delta.

When looking at oxygen depletion in the Gulf, this issue mainly occurs as a result of the nutrient-rich discharge from the Atchafalaya and Mississippi Rivers. The nutrients present in this water may lead to the stimulation of algal blooms. As mentioned previously, bacteria eat algal blooms when they die, which results in dissolved oxygen levels dropping substantially. If oxygen levels drop below 2.0 mg/L, water becomes hypoxic and a dead zone is formed.


What can Dissolved Oxygen Reveal About the Water Quality?

Every organism in a body of water has its own specific DO tolerance range. However, the majority of species will have difficulties surviving when DO concentrations dip below 3.0 mg/L. If Do levels drop to around 1-2 mg/L, the water is considered hypoxic and will likely be completely devoid of life.

When looking specifically at drinking water, healthy water should consist of dissolved oxygen levels that range from 6.5-8.0 mg/L. While high DO levels will make drinking water taste better, it’s important to understand that corrosion becomes more likely in this scenario, which is something that drinking water facilities must take into account. If DO levels drop below 6.5 mg/L, it’s likely that the water is contaminated and may be unsafe to drink.

water quality

Measuring Dissolved Oxygen

Measuring dissolved oxygen in drinking water is essential towards ensuring that the water is safe to drink. However, many facilities automatically use water with a low dissolved oxygen concentration to ensure that equipment doesn’t corrode. Modern dissolved oxygen sensors are relatively small and mostly electronic.

These sensors consist of a probe that’s positioned around the end of a cable. Since dissolved oxygen levels depend on the current temperature, the sensor will need to be calibrated to take this temperature into account before every use. Healthy water will need to have a DO concentration that’s at least around 6.5-8.0 mg/L, which equates to 80-110%.

Along with a dissolved oxygen sensor that uses electrochemical analysis to identify dissolved oxygen concentration, it’s also possible to use chemical analysis and photochemical analysis. Keep in mind, however, that dissolved oxygen sensors provide the most accurate results.

The two primary types of dissolved oxygen sensors at your disposal include galvanic dissolved oxygen sensors and optical dissolved oxygen sensors. Both types of sensors are known to provide accurate results. If you select a dissolved oxygen sensor that comes with digital communication technology, calibration data will be stored directly in the sensor’s cap, which means that you won’t need to calibrate the sensor before every use.

When you begin to measure for dissolved oxygen concentrations, there are numerous conditions that must be taken into account, which include:

  • In the event that air pressure is high, a large amount of oxygen will be dissolved, which means that DO levels will increase. As such, areas with high air pressure are considered to be better sources for drinking water.
  • Cold water will consist of higher concentrations of dissolved oxygen when compared to warm water.
  • The increase of water movement will lead to a higher dissolved oxygen concentration
  • Oxygen is produced directly from aquatic plants through the photosynthesis process, which leads to more dissolved oxygen getting into the water. This dissolved oxygen can then be consumed by aquatic plants and fish.

Since dissolved oxygen takes up space in water, these factors must be considered.

Measuring dissolved oxygen can give you a better idea of the quality of drinking water. If dissolved oxygen levels are relatively high and are at least above 6.5 mg/L, you should be able to consume the water without issue. On the other hand, water with DO readings below 6.5 mg/L should be avoided if possible. You can measure dissolved oxygen levels with a dissolved oxygen sensor of your choice.

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Posted by Dominic O'Donnell on August 23, 2022

Sensorex is a global leader in the design and manufacture of quality sensors for water quality and process applications. The company offers more than 2000 sensor packages for pH, ORP, conductivity, dissolved oxygen, free chlorine, chlorine dioxide, UV transmittance and other specialty measurements, as well as a full line of sensor accessories and transmitters. Its expert technical support engineers solve analytical sensor challenges with custom designs and off the shelf products.

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