Activated sludge can withstand a maximum temperature of 40°C, with an optimal survival temperature of 25-35°C. In all activated sludge processes, unless the activated sludge is specially cultivated to withstand high-temperature water, it cannot withstand scalding.
When the wastewater temperature exceeds 38°C, the activated sludge will experience adverse reactions, and when the wastewater temperature reaches above 40°C, the activated sludge will die.
At this point, a layer of floating black-gray foam will appear on the surface of the biochemical reaction tank (especially in the corners). The larger the area and thickness of the foam, the more activated sludge has died and the more severe the pollution factor exceeds the standard.
High water temperature has a significant impact on sludge activity. Once the sewage temperature is too high, sludge activity will be inhibited, resulting in excessive ammonia nitrogen and COD levels in the effluent of the biochemical reaction tank.
Key Points to Consider for Aeration Tanks in Summer
1) Sludge Concentration and Air Supply
According to the traditional aeration tank oxygen demand calculation formula, endogenous respiration oxygen consumption increases with rising temperature. However, oxygen supply is limited by the air blower equipment.
In summer, blowers are more prone to surge, resulting in air supply failing to meet the required standard. Therefore, to address this conflict between oxygen supply and demand, reducing sludge concentration can be considered.
2) Preventing Sludge Bulking
Sludge bulking is more likely to occur in summer due to lack of oxygen. Therefore, frequent microbial microscopic examinations should be conducted during operation. If an increase in filamentous bacteria is detected, the process should be adjusted promptly, or chemical agents should be added to suppress excessive growth.
Key Points to Note for Secondary Sedimentation Tanks in Summer
Due to high temperatures, the rate of stratified sedimentation decreases, making secondary sedimentation tanks prone to mud sludging, which in turn increases effluent SS.
High temperatures accelerate the metabolic rate of microorganisms in secondary sedimentation tanks, often producing gas and causing sludge to float.
Therefore, during summer operation, ensure adequate oxygen supply and, if necessary, add flocculants such as Al2(SO4)3 to improve sludge settling performance.
Countermeasures and Cooling Recommendations
1. Add a water sprinkler system. Utilizing the principle of evaporative cooling, water is atomized and sprayed into the air. As the water evaporates, it absorbs heat, reducing the pool surface temperature by 3-5°C. This method is suitable for small-scale wastewater cooling needs, but care should be taken to minimize water waste.
2. Install cooling fans for forced ventilation, accelerating air flow and removing heat. This method allows for adjustable wind speed and direction to achieve optimal cooling. It is suitable for applications requiring rapid cooling, but consideration should be given to fan energy consumption and noise.
3. Divert half of the wastewater into a firefighting wastewater recovery tank, where it is mixed with the wastewater remaining at the bottom of the tank and then sent to a homogenizing tank. This can provide some cooling, allowing for further treatment.
4. Install sunshades, sunshades, or other shading measures above the structure to reduce direct or reflected sunlight and lower the internal temperature.
5. Adjust the wastewater treatment process, such as increasing the aeration volume and extending the aeration time, to enhance microbial activity while reducing heat generation. This approach not only reduces temperatures but also improves wastewater treatment efficiency.
6. Introduce a wastewater cooling tower. After wastewater enters the cooling tower, it comes into contact with a cooling medium, which absorbs heat from the wastewater. This heat is then dissipated into the atmosphere through ventilation equipment above the cooling tower.
7. A wastewater heat exchanger is a device that uses heat transfer between two fluids to exchange heat. By exchanging heat between high-temperature wastewater and a cooling medium (such as water), high-temperature wastewater can be rapidly cooled. There are many types of heat exchangers, including shell-and-tube and plate heat exchangers. Choosing the right type of heat exchanger can improve cooling efficiency.