The process principle of the aerobic treatment system is to utilize the metabolism of aerobic microorganisms to convert organic pollutants in wastewater into harmless carbon dioxide, water, and the energy needed for their own survival. Oxygen is essential for maintaining the normal life activities of these microorganisms. Does that mean the higher the dissolved oxygen, the better the treatment effect of the aerobic system?
Before addressing this question, it is essential to grasp the concept of the food to microbe ratio in aerobic systems. Taking the commonly used activated sludge system as an example, the ratio of the total amount of Biological Oxygen Demand (BOD) supplied to the aeration tank each day to the total amount of active sludge in the aeration tank is known as the food to microbe ratio (where the supplied BOD can be considered as food provided to the microorganisms).
The calculation formula for the Food Micro Ratio is as follows:
F/M=Q*BOD5/(MLVSS*Va)
F: Food stands for food; the amount of food entering the system (BOD)
M: Microorganism signifies the active substance content (sludge volume)
Q: Water volume, BOD5: Value of incoming BOD5
MLVSS: Mixed Liquor VSS Concentration
Vessel: Aeration Tank Volume
The appropriate range for the food to microbe ratio is typically between 0.1-0.25 kgBOD5/kgMLSS.d. A high food to microbe ratio indicates an excess of microbial food, suggesting the aeration tank is operating under high load conditions. Conversely, a low food to microbe ratio indicates the aeration tank is operating under low load conditions.
What are the consequences of having too much or too little food microelements?
When the aeration tank operates within the appropriate food-to-microbe ratio, the activated sludge floc structure is robust, with excellent settling performance and clear, transparent effluent.
2. When the aeration tank operates at a high food to microbe ratio, even under overloaded conditions, the settling performance of the activated sludge deteriorates due to excess food, resulting in turbid effluent and the difficulty in complete degradation of BOD in the wastewater.
3. When the aeration tank operates at a low food to microbe ratio, aging of the active sludge can occur due to insufficient food.
Extended low-shear operation may lead to sludge deflocculation, and even trigger the expansion of filamentous bacteria in activated sludge.
When activated sludge shows signs of aging and絮凝 degradation occurs, the activated sludge floc structure becomes looser, resulting in many tiny sludge fragments being carried in the effluent, which reduces the clarity of the effluent and degrades water quality.
After understanding Food Microbe Ratio, let's examine the impact of dissolved oxygen on treatment effectiveness.
Maintaining a relatively high dissolved oxygen level is beneficial when the aeration tank operates at a high food to microorganism ratio, as it accelerates the degradation rate of organic matter in the wastewater.
When the aeration tank operates at a low food to microbe ratio, maintaining high dissolved oxygen levels can accelerate the endogenous metabolism within the active sludge due to insufficient food, leading to sludge disintegration, commonly known as over-aeration. High dissolved oxygen speeds up microbial metabolism. To put it in a more vivid example, it's like forcing a person to work tirelessly without enough food, which only accelerates their physical decline until they perish.
Therefore, during the operation of the aerobic system, the control of dissolved oxygen concentration should be closely related to the control of the food-to-microorganism ratio. A higher food-to-microorganism ratio can maintain a higher dissolved oxygen concentration, promoting the effective degradation of organic pollutants. Conversely, when the food-to-microorganism ratio is insufficient, it is advisable to control a relatively lower dissolved oxygen concentration to reduce the rate of endogenous metabolism, thereby preventing sludge aging and the occurrence of sludge deflocculation. This can also help lower electricity consumption and save operational costs. In practice, we can control the dissolved oxygen in the aerobic tank by adjusting the frequency, runtime of the fan, or the size of the vent valve.
