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Dissolved oxygen (DO) is one of the most vital indicators of water quality. High DO levels improve the taste of drinking water, but this can cause corrosion in water pipes. Alternatively, low dissolved oxygen levels below 5.0 mg/L stress aquatic life and creates hypoxic conditions.
Oxygen (O2) makes up around 21% of the Earth’s atmospheric gases. Oxygen diffuses into water bodies until it reaches a saturation point. When oxygen dissolves in water, it forms dissolved oxygen (DO). Oxygen dissolves in the water as a byproduct of photosynthesis or diffusing from the surrounding air.
Dissolved oxygen is a direct indicator of water quality. Measuring DO is important because we rely on water for drinking, washing, and growing produce, and aquatic organisms rely on good water quality for biological processes and survival.
The concentration of dissolved oxygen, salinity, or turbidity (the amount of suspended material in the water) influences water quality. Water quality refers to the different characteristics of water (chemical, physical, and biological) based on the EPA standards of usage. Water quality is paramount for monitoring the health of aquatic ecosystems, human safety, and determining if the water is polluted and requires treatment.
Streams or rivers with rapidly moving water usually contain more dissolved oxygen than stagnant water due to aeration increases. As organic matter decays, bacteria present in the water reduce the amount of dissolved oxygen. Therefore, water that contains high levels of organic matter can quickly become oxygen-deficient, affecting the water quality and causing aquatic organisms to die.
On the other hand, if the amount of dissolved oxygen is high at the surface, the water becomes nutrient-rich because of the increase in photosynthesis by algal communities. However, in deep water, dissolved oxygen levels are depleted as photosynthesis is limited from the surface algae.
When algae smother the water’s surface, light cannot penetrate deeper water, which causes the algae to die and sink to the bottom, consuming oxygen as the algae decompose.
In nutrient-poor water, the level of DO doesn’t usually change with depth. However, in the summer months, thermal layering can prevent mixing. When the water cannot mix, the surface water becomes supersaturated with oxygen, and the bottom becomes anoxic. Mixing can be encouraged by increasing physical turbulence using a dam or waterwheel.
Aquatic life will not tolerate DO levels below 3 mg/L, so sessile animals will move to areas where DO levels are within range. Plants and animals unable to move to oxygen-rich areas will die if DO levels are lower than 1 mg/L.
When plants and animals die and decompose, they release nutrients such as carbon and nitrogen into the water. Therefore, low dissolved oxygen levels in water affect drinking water supplies.
Drinking water containing high amounts of organic carbon can lead to bladder cancer and reproductive problems for people consuming the polluted water. Reducing the total organic carbon in untreated drinking water supplies is a top priority in water treatment systems.
High levels of nitrogen are also an issue in drinking water. If consumed, the skin can turn a gray or blueish color. In severe cases, the person may experience an excess heart rate, weakness, fatigue, or dizziness. Drinking water should contain no more than 10 mg/L of nitrogen to be considered safe.
Drinking water operations aim for DO concentrations above 6.5-8 mg/L and between 80-120% to maintain good water quality. While higher DO levels can improve the taste of the water, high DO levels can cause rapid corrosion of water pipes.
If dissolved oxygen corrodes pipes, it destroys the protective hydrogen film and oxidizes dissolved ions into insoluble forms. Corrosive pipes also restrict water flow and deteriorate water quality.
In wastewater treatments, DO is used to treat and stabilize the wastewater. Dissolved oxygen is required to break down organic matter that contains carbon and ammonia.
Testing the DO levels in wastewater indicates the general water quality by understanding the biodegradable organic matter and biological oxygen demand content.
Many factors can influence dissolved oxygen levels in the water, which in turn affects the water quality. These include the following:
Measuring DO levels is necessary to assess water quality. While there are many ways to test DO, an electrochemical or electroanalytical DO sensor/probe is the most accurate.
Dissolved oxygen probes measure the amount of oxygen that diffuses across a permeable or semi-permeable membrane inside the DO probe. When the probe detects oxygen, a chemical reduction reaction produces an electrical signal. The electrical signal is then displayed on the meter, giving you an accurate DO reading.
As dissolved oxygen is dependent on water temperature, it is essential to calibrate the probe before each use and also measure the temperature using a temperature probe.
Dissolved oxygen plays a critical role in measuring water quality. When water contains high amounts of DO, overproduction of plants can cause anoxic conditions, killing aquatic organisms, releasing nutrients, and making it inadequate for human use.
For that reason, dissolved oxygen is monitored in a wide range of industries and applications to maintain water quality.
If you want to learn more about dissolved oxygen or what water quality measuring probes we have to offer, do not hesitate to contact our world-class team at Atlas Scientific.
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