Dissolved oxygen (DO) refers to the total amount of oxygen currently present in the water. The amount of DO plays an important role in water quality and the livelihood of aquatic plants and animals. DO originates from the atmosphere or as a byproduct of photosynthesis. Once dissolved in water, it is available for use by living organisms and can play a significant role in many chemical processes in the aquatic environment and water/wastewater treatment.
Aeration: Aeration of water occurs when water and air mix, resulting in increased levels of DO in water. Natural aeration can occur through a movement of water caused by wind (creating waves), rapids, waterfalls, groundwater discharge or other forms of running water. Examples of artificial aeration include a paddle wheel or a fountain in the middle of a pond, the use of an air stone in an aquarium, and pumping air into aeration tank at wastewater treatment plants to sustain microbes that break down contaminants.
Photosynthesis: Another major source of DO is photosynthesis. While most photosynthesis takes place at the surface (by shallow water plants and algae), a large portion of the process takes place underwater (by seaweed, sub-surface algae and phytoplankton).
There are many reasons why it’s important to evaluate the amount of DO in water:
- Aquaculture: The volume of oxygen contained in water is the most critical parameter in fish farming and other forms of aquaculture. Every aquatic organism need DO to survive. When oxygen levels become too low, hypoxia will occur, where the concentration of dissolved oxygen decreases to a level that can no longer support living aquatic organisms.
- Wastewater Treatment Plants: Aeration tanks are used to suspend microorganisms in wastewater. After leaving the primary treatment stage, sewage is pumped into aeration tanks. The sludge is loaded with microorganisms and mixed with air or pure oxygen. As air is forced into the aeration tanks, it increases the activity of these microorganisms and helps keep the organic waste thoroughly mixed. DO is added to the aeration basin to enhance the oxidation process by providing oxygen to aerobic microorganisms so they can successfully turn organic wastes into inorganic byproducts.
- Industrial Settings: Boilers in industrial settings require low DO levels to avoid the buildup of scale and corrosion. When the amount of scale increases inside a boiler, the heat transfer process within the system will become increasingly inefficient. Unchecked corrosion will results in expensive repairs or equipment failures and subsequent replacement.
DO is expressed in many different units, but most often in mg/L or % saturation (DO%). The unit mg/L is straightforward, as it is the milligrams of gaseous oxygen dissolved in a liter of water.
Dissolved oxygen concentrations in water are affected by:
- Temperature: The most significant variable is temperature, so it is essential to measure it in conjunction with dissolved oxygen. The solubility of oxygen in water is inversely related to temperature – as temperature increases, DO decreases. Therefore, as a water body cools overnight, more oxygen can be dissolved. The same applies for cooler seasons – a water body in winter will have a higher DO concentration than in summer, assuming other variables are held constant. However, it is important to keep in mind the impact photosynthesis and respiration have on DO concentrations during the day and night.
- Salinity: The solubility of oxygen in water is inversely related to salinity – as salinity increases, DO decreases. Seawater can hold about 20% less oxygen under the same temperature and atmospheric pressure as freshwater. Therefore, it is critical to also measure salinity (using a conductivity sensor) when collecting DO data in estuaries, wetlands, coastal areas, aquaculture, or any other application where salinity can vary.
- Barometric Pressure: There is a direct relationship between barometric pressure and DO levels in water – as pressure decreases, DO decreases. At lower elevations, the barometric pressure is high, so there is more pressure to push gaseous oxygen from the atmosphere into water. But at higher elevations, the barometric pressure is much lower.
