Dissolved oxygen

Overview

Molecular oxygen in the air dissolved in water is called dissolved oxygen. The content of dissolved oxygen in water is closely related to the partial pressure of oxygen in the air and the temperature of the water. Under natural conditions, the oxygen content in the air does not change much, so the water temperature is the main factor. The lower the water temperature, the higher the dissolved oxygen content in the water. Molecular oxygen dissolved in water is called dissolved oxygen, usually recorded as DO, expressed in milligrams of oxygen per liter of water. The amount of dissolved oxygen in water is an indicator of the self-purification ability of water.

Dissolved oxygen is closely related to the partial pressure of oxygen in the air, atmospheric pressure, water temperature and water quality. At 20°C and 100kPa, the dissolved oxygen in pure water is approximately 9mg/L. Some organic compounds undergo biodegradation under the action of aerobic bacteria, consuming dissolved oxygen in the water. If organic matter is calculated in terms of carbon, according to C+O2=CO2, it can be seen that every 12g of carbon consumes 32g of oxygen. When the dissolved oxygen value in the water drops to 5mg/L, some fish will have difficulty breathing.

Dissolved oxygen usually comes from two sources: one source is oxygen from the atmosphere seeping into the water when the dissolved oxygen in the water is not saturated; the other source is the oxygen released by plants in the water through photosynthesis. Therefore, the dissolved oxygen in the water will be continuously replenished by the dissolution of oxygen in the air and the photosynthesis of green aquatic plants. However, when the water body is polluted by organic matter, oxygen consumption is serious, and the dissolved oxygen cannot be replenished in time, anaerobic bacteria in the water body will multiply quickly, and the organic matter will turn black and smelly due to the corruption of the organic matter.

Dissolved oxygen value is a basis for studying the self-purification ability of water. The dissolved oxygen in the water is consumed and it takes a short time to return to the original state, which shows that the water body has strong self-purification ability, or that the water body is not seriously polluted. Otherwise, it means that the water body is seriously polluted, the self-purification ability is weak, or even the self-purification ability is lost.

Due to violent aeration and other reasons, the amount of molecular oxygen in the air dissolved in the water to become dissolved oxygen increases significantly, making the dissolved oxygen in the water supersaturated. The content of dissolved oxygen in water is closely related to the partial pressure of oxygen in the air and the temperature of the water. Under natural conditions, the oxygen content in the air does not change much, so water temperature is the main factor. The lower the water temperature, the higher the dissolved oxygen content in the water.

However, water conservancy projects can cause supersaturation of dissolved oxygen. For example, when there is overflow in a water release building under a high dam or large reservoir, or when a dam releases water through a flood discharge hole, the falling process of the water flow is accompanied by severe water and gas exchange, which often causes the dissolved gas content in the downstream water to increase due to severe aeration. Significant increase, causing further downstream range, thus causing adverse effects and harm to aquatic life, especially fish.

Application

Surface water monitoring, lakes, aquaculture, medicine, rural sewage monitoring, industrial sewage discharge, swimming pool, etc.

Details