What is dissolved oxygen and how to monitor it
The current biological treatment in wastewater treatment mostly adopts a combination of anaerobic and aerobic treatment processes. Dissolved oxygen plays a crucial role in actual wastewater biological treatment operations. Inappropriate or excessive fluctuations in this indicator can quickly cause the activated sludge system to be impacted, thereby affecting treatment efficiency. Therefore, in actual biochemical treatment processes, it is necessary to strictly control the content of dissolved oxygen.
1. Overview of dissolved oxygen
Dissolved oxygen (DO) is a parameter that characterizes the concentration of oxygen in aqueous solutions, and is the free oxygen dissolved in water.
The unit of dissolved oxygen is mg/L, 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. High dissolved oxygen is beneficial for the degradation of various pollutants in water, thereby enabling faster purification of the water; On the contrary, low dissolved oxygen results in slower degradation of pollutants in the water.
2. Factors affecting dissolved oxygen
The dissolved oxygen content in water is affected by two factors: one is the oxygen consumption effect that reduces DO, including the oxygen consumption of aerobic organic matter degradation and advanced metabolic oxygen consumption; Another type is the reoxygenation effect that increases DO, mainly including the dissolution of oxygen in the air, aeration methods, etc. The mutual growth and decline of these two effects result in a spatiotemporal variation in the dissolved oxygen content in water.
The environmental factors that affect the content of dissolved oxygen in water include water temperature, oxygen partial pressure, salinity, and other factors.
1. Water temperature
When the oxygen partial pressure and salt content are constant, the saturation content of dissolved oxygen decreases with the increase of water temperature. The saturation content of dissolved oxygen at low temperatures varies more significantly with temperature.
2. Salt content
When the water temperature and oxygen partial pressure are constant, the higher the salt content of water, the smaller the saturation content of dissolved oxygen in water. The salt content of seawater is much higher than that of freshwater. Under the same conditions, the saturation content of dissolved oxygen in seawater is much lower than that in freshwater. The variation of salt content in natural freshwater bodies is very small, so the influence of salt content on the saturation content of dissolved oxygen is not significant, and can be approximately calculated based on the saturation content in pure water.
3. Partial pressure of oxygen
When the water temperature and salt content are constant, the saturated salt content of dissolved oxygen in water increases with the increase of oxygen partial pressure on the liquid surface.
Monitoring of dissolved oxygen (DO)
Due to the susceptibility of dissolved oxygen to factors such as oxygen, temperature, and humidity in the air, online detection instruments or portable dissolved oxygen detectors are often used for on-site monitoring. When testing, the entire aeration tank should be divided into several areas, and the dissolved oxygen monitoring values within the entire area should be statistically analyzed to understand the distribution of dissolved oxygen at different stages and time points of the system. This is very beneficial for the overall grasp of the subsequent system and the analysis of activated sludge faults. If such detection conditions are not available, the dissolved oxygen at the outlet of the aeration tank can be monitored as the final result of the organic matter degradation process in the activated sludge system. Under normal circumstances, the effect of oxygenation in winter is significantly better than that in summer. The main reason is that the water temperature is lower in winter and the saturation of dissolved oxygen is higher. On the contrary, the saturation of dissolved oxygen is lower in summer.
