Explore the "Influence Code" of Sludge Concentration on Nitrogen and Phosphorus Removal.

Sludge concentration refers to the content of suspended solids and organic matter in sewage during the wastewater treatment process. Nitrogen removal treatment means converting the nitrogen compounds in wastewater into nitrogen gas through specific processes, so as to achieve the purpose of reducing the discharge of nitrogen pollutants. Then, does a higher sludge concentration mean greater advantages for nitrogen removal treatment? This article will conduct an in-depth discussion on this issue.

(I) Impact on Denitrification Reaction

During the aerobic nitrification process, a higher sludge concentration implies a relatively higher concentration of nitrifying bacteria. Therefore, the rate of the aerobic nitrification reaction will be relatively higher under the condition of high sludge concentration. A certain sludge age is a necessary condition to ensure the existence of nitrifying bacteria in biological sludge. Meanwhile, creating favorable living conditions for nitrifying bacteria can further increase their proportion in the microbial flora and then enhance the concentration of nitrifying bacteria. Moreover, under high sludge concentration, more BOD (Biochemical Oxygen Demand) will be consumed in the anaerobic stage, and the BOD/TKN will be relatively lower when entering the aerobic stage. Studies have shown that the proportion of nitrifying bacteria in activated sludge is inversely proportional to BOD/TKN. Since nitrifying bacteria are autotrophic bacteria, the concentration of organic substrates is not a limiting factor for their growth. However, if the concentration of organic substrates is too high, heterotrophic bacteria with a higher growth rate will multiply rapidly and compete for dissolved oxygen. As a result, the growth of autotrophic bacteria will slow down, and aerobic nitrifying bacteria will not be able to gain an advantage, ultimately leading to a reduction in the nitrification rate.

(II) Impact on Denitrification Reaction

During the denitrification reaction process, the absence of molecular oxygen is required so that denitrifying bacteria can decompose organic matter by utilizing the ionic oxygen in nitrates and nitrites. In a biological system with high sludge concentration, the dissolved oxygen value can be appropriately reduced during the nitrification process while maintaining the nitrification effect. In this way, the dissolved oxygen at the end of the nitrification can be decreased, effectively reducing the content of dissolved oxygen carried in the nitrate return flow, minimizing the impact of molecular oxygen on the denitrification process in the anoxic zone, and enhancing the denitrification ability of denitrifying bacteria to utilize carbon sources. Moreover, a high sludge concentration has a relatively stronger endogenous metabolic oxygen demand of its own, which can further consume the dissolved oxygen in the return flow and the anoxic section. Furthermore, a very high sludge concentration will change the viscosity of the mixed liquor and increase the diffusion resistance, thereby reducing the dissolved oxygen carried by the return flow. In some treatment processes that use open channels as return flow channels, it can also reduce the oxygenation caused by the return flow falling. All in all, high sludge concentration plays a significant role in reducing the DO (Dissolved Oxygen) value in the denitrification stage during the actual operation of the process.

Since denitrifying bacteria are heterotrophic facultative bacteria and exist in large quantities in wastewater treatment systems, increasing the sludge concentration in the system can effectively raise the concentration of denitrifying bacteria. The denitrification reaction rate is basically independent of the concentrations of nitrates and nitrites, but has a first-order reaction relationship with the concentration of denitrifying bacteria. Therefore, in the actual operation of the process, high sludge concentration can shorten the denitrification time and reduce the effective volume of the anoxic section. Under the condition that the effective volume of the anoxic section is fixed, the denitrification reaction with high sludge concentration can make better use of the relatively refractory organic matter in the organic matrix as a carbon source for the denitrification reaction. This is particularly important for nitrogen and phosphorus removal processes, especially when there is a shortage of carbon sources. In addition, under high sludge concentration, the diameter of the microbial floc is relatively large. During the nitrification reaction, due to the low dissolved oxygen, the oxygen pressure gradient is small, and an anoxic environment is easily formed inside the floc, resulting in the occurrence of denitrification reaction. That is to say, high sludge concentration can promote simultaneous denitrification.

(III) Impact on Biological Phosphorus Removal

The key to biological phosphorus removal lies in increasing the proportion of phosphorus-accumulating organisms (PAOs) in the activated sludge system. In the anaerobic zone, high sludge concentration has a positive impact on PAOs. Because PAOs can release phosphorus in an anaerobic environment, a high sludge concentration leads to a relatively larger number of PAOs, which reserves sufficient "strength" for the subsequent phosphorus uptake process.

Under different sludge ages and sludge concentration conditions, the efficiency of biological phosphorus removal will change. When the sludge age is short and the sludge concentration is moderate, PAOs can maintain good activity and a dominant position within the system. They can absorb a large amount of phosphorus in an aerobic environment, thus achieving a relatively high phosphorus removal efficiency. However, if the sludge age is too long, it may lead to sludge aging, and the activity of PAOs will be affected. Even if the sludge concentration is high, the phosphorus removal efficiency may decline. On the contrary, if the sludge concentration is too low, the number of PAOs is limited. Even if other conditions such as sludge age are appropriate, it will be difficult to achieve the desired phosphorus removal effect. Therefore, reasonably controlling parameters such as sludge concentration and sludge age is crucial for improving the efficiency of biological phosphorus removal. It is necessary to continuously optimize and adjust according to the actual wastewater treatment situation to ensure that the phosphorus removal capacity of the entire wastewater treatment system reaches a good level.

All in all, sludge concentration is like the "behind-the-scenes conductor" in the wastewater treatment system, influencing key processes such as nitrification, denitrification, and biological phosphorus removal. From the regulation of bacterial concentration in the nitrification reaction, to the optimization of dissolved oxygen and carbon source utilization in the denitrification reaction, and then to the stimulation of the vitality of phosphorus-accumulating organisms in biological phosphorus removal, its role cannot be underestimated. In the actual operation of wastewater treatment plants, only by accurately grasping the sludge concentration and flexibly adjusting it according to factors such as the composition of wastewater and treatment processes can high-efficiency and stable nitrogen and phosphorus removal be achieved, allowing wastewater to be purified and reborn, laying a solid foundation for the sustainable development of the ecological environment, and opening a new chapter in the recycling and utilization of water resources.