【Weekly Topic】-006-Relationships between Biochemical Operation Parameters

Generally, when the water temperature is low, for example below 15°C, a higher level of dissolved oxygen is required； When the water temperature is high and dissolved oxygen is still kept at a high level, excessive oxidation occurs, which affects the proper functioning of the activated sludge and leads to malfunctions.

Here, the water temperature in winter is only 14°C, and the dissolved oxygen level can be kept around 4； However, when the water temperature is 35°C in summer, the dissolved oxygen level hardly exceeds 2; it is usually around 1, and an increase in this level actually results in a decrease in the removal efficiency.

This post was last edited by zhaolijun on 2015-6-23 09:11. The impact of temperature changes on wastewater treatment efficiency: Microorganisms constitute the main component of activated sludge; since bacteria are difficult to monitor, changes in the population of protozoa within the activated sludge are generally used as an indicator of the quality of the sludge. The growth, reproduction, and metabolism of activated sludge are closely related to changes in water temperature. Under normal conditions, activated sludge reproduces once every 4 hours. However, when the water temperature is below 25°C, the metabolism of the activated sludge slows down, which reduces its efficiency in breaking down pollutants. When the water temperature is below 18°C, if the sludge concentration is not adjusted, the degradation efficiency declines further; the number of certain protozoa decreases or even disappears. When the water temperature exceeds 25°C, the metabolic activity of activated sludge increases significantly; the number of protozoa rises, their activity improves, and the efficiency of pollutant degradation also increases markedly. The sedimentation properties of the sludge improve as well. If the sludge load is not reduced in a timely manner at this stage, the supernatant fluid resulting from the sedimentation of the activated sludge will become cloudy with many suspended particles. The effect of water temperature changes on the degradation efficiency of pollutants is quite significant. When the dissolved oxygen concentration in the biological tank is maintained at 2–3 mg/L throughout the year, the sludge concentration averages 3,500 mg/L and the ratio of nutrients added remains constant, variations in water temperature from month to month still result in noticeable differences in the degradation efficiency of COD.

This post was last edited by Gorō Bagua Stick on 2015-6-23 09:19. The impact of water temperature changes on the properties of activated sludge: 1. Low water temperatures can lead to sludge bulking. At low temperatures, the activity of bacteria in flocs is low, and it is not easy to increase their activity and reproduction rate by adding nutrients. As a result, the growth rate of filamental bacteria is higher than that of bacteria in flocs. Moreover, due to their larger specific surface area, filamental bacteria have a significant advantage over bacteria in flocs when it comes to accessing BOD5 substances in wastewater as well as obtaining the oxygen needed to oxidize those substances. As a result, filaments become the dominant microorganisms in the aeration tank and multiply in large numbers, leading to sludge bulking and the formation of biological foam. Furthermore, most of these microorganisms are filamentous or branched in structure, allowing them to form networks that can capture particles and bubbles, etc., and bring them to the surface. Bubbles surrounded by a mesh increase the surface tension of their surfaces, making it harder for them to burst and thus enhancing the stability of the foam. The foam on the surface of the biological tank prevents oxygen from the air from entering the biological system. At the same time, it blocks the beneficial effect of sunlight on the bacteria in the biofilm, thereby exacerbating sludge bulking. 2 The effect of water temperature changes on the uptake and utilization of nutrients by bacterioplankton: When treating wastewater using the activated sludge method, the concentration of nutrients such as nitrogen and phosphorus in the wastewater is crucial for maintaining microbial activity. If these nutrients are insufficient, it is necessary to add them to ensure an adequate supply of nutrients for the microorganisms. The efficiency of microorganisms in absorbing nutrients cannot be described through specific data. Studies have found that, while maintaining dissolved oxygen in the biological treatment stage, the inhibitory effect of nutrient addition on filamental fungi is significantly influenced by changes in water temperature. In this experiment, when the water temperature is above 23°C and the carbon-nitrogen-phosphorus ratio in the wastewater is kept within 100:2.5:0.5, filamental bacteria can be effectively suppressed, with the sludge volume index SVI ranging from 80 to 120 mL/g ； When the water temperature is between 16 and 23°C, to effectively suppress filamentous bacteria while keeping the sludge volume index SVI below 120 mL/g, it is necessary to increase the carbon-nitrogen-phosphorus ratio in the wastewater to 100:(3–5):(0.5–1) ； When the water temperature is below 16°C, as the temperature drops, even if the ratio of nitrogen to phosphorus in the wastewater continues to increase, the inhibitory effect on filamental bacteria gradually diminishes until it becomes negligible, and the Sludge Volume Index SVI will reach over 150 mL/g.

The water temperature should ideally be kept around 30 degrees. When the temperature is low, a higher DO level is necessary; when the temperature is high, keeping the DO at 1-2 is sufficient, as higher levels result in unnecessary power consumption

This parameter contains a lot of information, and it would be best to present it using charts. However, this is a rather complex task that requires the collection of large amounts of data; moreover, the data tables provided by different sources cannot completely replace each other, as they each have their own characteristics. Firstly, the relationship among water temperature, DO, and organic load is quite complex. Water temperature and DO can be used to create a curve, but organic load also takes into account factors such as hydraulic retention time, sludge age, SV, SVI, B/C ratio, nitrogen and phosphorus concentrations, as well as the extent of nitrification and denitrification; therefore, it’s not possible to create a simple curve in this case.

This question is too general to be answered in just a few sentences. First, it is necessary to consider the nature of the wastewater being treated (domestic wastewater, industrial wastewater), the design specifications, and the operating mode of the process; for more details, refer to \"Theory and Technology of Activated Sludge Process\" by Li Yalin. This book can help you.

COD is easy to remove, but the removal rate of ammonia nitrogen is not high :(:(:(

When the water temperature is low, the dissolved oxygen level can be maintained around 4; when the temperature is high, it can be kept at 2. Excessively high levels of dissolved oxygen lead to excessive growth of sludge

Excessively high dissolved oxygen will accelerate sludge aging and cause biochemical operation failures