By: Kate
Email:kate@aquasust.com
Date: 24th December 2024
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 content of dissolved oxygen 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 bodies.
The dissolved oxygen value is a basis for studying the self-purification ability of water. If the dissolved oxygen in the water is consumed and it takes a short time to return to the initial state, it means that the water body has a strong self-purification ability, or 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 loses its self-purification ability.
Most of the sewage treatment today is a combination of aerobic and anaerobic sewage treatment processes. Dissolved oxygen plays a vital role in the actual wastewater treatment operation. The deterioration or excessive fluctuation of this indicator will quickly lead to fluctuations in the activated sludge system, thereby affecting the treatment efficiency. Therefore, it is necessary to strictly control the dissolved oxygen content in the actual treatment process. Today, let's discuss in detail what dissolved oxygen is.
1. Definition and understanding of dissolved oxygen (DO)
It should be said that, in theory, when the DO value monitored at each point in the aeration tank is slightly greater than 0 (such as 0.01 mg/L), it can be understood that the oxygenation just meets the requirements of microorganisms in the activated sludge for dissolved oxygen. But in fact, we still do not simply control the dissolved oxygen at a level greater than 0, but apply the textbook method to control DO within the range of 1-3 mg/L. The reason is that, for the entire aeration tank, the distribution of dissolved oxygen and the dissolved oxygen demand in each aeration tank area are different. In order to conservatively stabilize the demand for dissolved oxygen in the decomposition of organic matter or its own metabolism by activated sludge, DO is controlled at 1-3 mg/L.
However, the actual operation is often different from the fixed and rigid DO theoretical value on paper. It cannot just follow the theoretical value on paper, but also fully combine it with the actual situation!
From the actual situation, it is found that in actual operation, it is unnecessary to control the dissolved oxygen at 1-3 mg/L in many cases, especially controlling it over 3 mg/L is meaningless, the only result is the waste of electric energy and the presence of fine suspended particles in the effluent. Therefore, the dissolved oxygen should be reasonably controlled according to the written theory and the actual situation.
2. What are the effects of too high dissolved oxygen (DO)?
Taking the commonly used activated sludge system as an example, the ratio of the total amount of COD supplied to the aeration tank every day to the total amount of activated sludge in the aeration tank is the food-microorganism ratio (where the supplied COD can be regarded as food provided to microorganisms). The food-microorganism ratio calculation formula is as follows:
F/M=Q*COD/(MLVSS*Va)
Where:
F: Food represents food, the amount of food entering the system (BOD)M: Microorganism represents the amount of active matter (sludge amount)Q: water volume, COD: the difference between inlet and outlet CODMLVSS: activated sludge concentrationVa: aeration tank volume
Usually, the appropriate range of food-microorganism ratio is between 0.1-0.25kgBOD5/kgMLSS.d. A high food-microorganism ratio indicates that there is an excess of microbial food and the aeration tank is in a high-load operation state. A low food-microorganism ratio indicates that the aeration tank is in a low-load operation state.
What will happen if the food-microorganism ratio is too high or too low?
When the aeration tank is operating in the appropriate feed-micro ratio range, the activated sludge floc structure is good, the sedimentation performance is excellent, and the effluent is clear and transparent.
When the aeration tank is operating in a high feed-micro ratio state, or even overloaded, the activated sludge sedimentation performance deteriorates due to excess food, the effluent is turbid, and the BOD in the wastewater is difficult to be completely degraded.
When the aeration tank is operating in a low feed-micro ratio state, the activated sludge is prone to aging due to insufficient food.
Long-term low feed-micro ratio operation may cause sludge deflocculation and even induce expansion of activated sludge filamentous bacteria. When the activated sludge ages and causes sludge deflocculation, the activated sludge floc structure will become looser, and the effluent will carry many fine sludge fragments, resulting in a decrease in the clarity of the effluent and deterioration of water quality.
After understanding the feed-micro ratio, let's look at the effect of dissolved oxygen on the treatment effect. High dissolved oxygen accelerates the metabolism of microorganisms.
When the aeration tank is operating in a high feed-micro ratio state, it is beneficial to maintain a relatively high dissolved oxygen, which can accelerate the degradation rate of organic matter in the wastewater.
When the aeration tank is in a low food-micro ratio operation state, if the dissolved oxygen is still maintained at a high level, the lack of food will accelerate the endogenous metabolism of the activated sludge, and eventually lead to the deflocculation of the activated sludge, which is commonly known as over-aeration. Therefore, in the operation of the aerobic system, the control of the dissolved oxygen concentration should be closely related to the control of the food-micro ratio. A high food-micro ratio can control a higher dissolved oxygen concentration and promote the effective degradation of organic pollutants. On the contrary, when the food-micro ratio is insufficient, the relatively low dissolved oxygen concentration should be controlled to reduce the rate of endogenous metabolism to avoid sludge aging and sludge deflocculation, and at the same time reduce power consumption and save operating costs.
3. Control basis and optimization of dissolved oxygen (DO)
Main basis: raw water quality (organic matter, nitrogen, phosphorus), concentration of activated sludge, sludge settling ratio, pH, temperature, food-micro ratio (F/M), etc.
Of course, the theoretical values given in writing: the dissolved oxygen concentration under general aerobic conditions is ≥2.0 mg/L, the dissolved oxygen concentration under anaerobic conditions is ≤0.2 mg/L, and the dissolved oxygen concentration under anoxic conditions is 0.2-0.5 mg/L. The specific situation should be grasped according to the actual situation.
1. Raw water quality:
Generally, the more organic matter there is in the raw water, the more oxygen consumption of microbial decomposition and metabolism, and the demand for dissolved oxygen for nitrification reactions, so when controlling dissolved oxygen, attention should be paid to the changes in the influent water volume and the content of organic matter in the influent water.
2. Activated sludge concentration:
When the pollutants are removed and the discharge concentration is reached, the concentration of activated sludge should be reduced as much as possible, which is very beneficial for reducing aeration volume and reducing power consumption. At the same time, in the case of low activated sludge concentration, it is more important not to over-aerate, otherwise sludge expansion will occur, making the effluent turbid; of course, high activated sludge concentration requires higher dissolved oxygen, otherwise hypoxia will occur, which will inhibit the sewage treatment effect.
3. Sludge settling ratio:
Excessive aeration will cause fine bubbles to attach to the flocs of activated sludge, causing the activated sludge to float to the liquid surface, making the sludge settling performance worse. This problem should be paid attention to in actual operation, especially when sludge filamentous expansion occurs, it is more likely to cause aeration of fine bubbles to attach to the flocs, and then cause a large amount of scum to appear on the liquid surface.
4. pH:
Through the influence on the concentration of activated sludge and microorganisms, it indirectly affects the amount of dissolved oxygen. Therefore, in the control of sewage treatment, in addition to fully understanding the function of the regulating tank, it is also necessary to establish contact with the discharge unit to understand the sewage water quality so as to add appropriate reagents to neutralize abnormal pH.
5. Temperature:
Under different temperatures, the dissolved oxygen concentration in the sewage is different, which will affect the concentration of activated sludge and microorganisms. Low and high temperatures will affect the dissolved oxygen and microbial activity in the water, making sewage treatment inefficient. For low temperatures in the north, underground or semi-basement or indoor treatment is usually established; for high temperatures, the temperature in the pool is adjusted through a regulating pool to improve treatment efficiency.
6. Food-to-microbe ratio (F/M):
The higher the food-to-microbe ratio, the lower the oxygen demand. This shows that we use the food-to-microbe ratio to achieve energy saving in the water treatment process, that is, to maximize the food-to-microbe ratio while ensuring the treatment effect, so as to avoid unnecessary aeration consumption.