Issues regarding the operation of activated sludge

There was a previous discussion on this topic; I’ll quote here the general solutions for sludge bulking. First category: Emergency measures. These are suitable for temporary emergency situations, with the main approaches being the addition of chemicals to improve the sedimentation properties of the sludge or directly killing the filamental bacteria. Adding coagulants such as iron salts and aluminum salts can directly improve the compressibility of sludge, ensuring the quality of the settled effluent. Additionally, adding some chemical agents, such as chlorine, to the return sludge can also help eliminate sludge bulking. Adding hydrogen peroxide and ozone can also serve to destroy filaments. 　　Using this method generally allows for a rapid reduction in the SVI value, but these approaches do not effectively control the growth of filamental bacteria; once the addition of chemicals is stopped, sludge bulking can reoccur. Moreover, the addition of chemicals may disrupt the microbial growth environment in the biochemical system, resulting in reduced treatment efficiency; therefore, this method can only be used as a temporary emergency solution. 　　Category 2: Improving the biochemical environment. When sludge bulking occurs in wastewater treatment plants, it is generally not possible to resolve the issue by altering the process flow, tank design, or aeration methods; instead, it is necessary to modify the microbial growth environment within the biochemical tanks, based on the existing operating procedures, in order to suppress or eliminate the excessive proliferation of filamental microorganisms. Under different processes and water quality conditions, it is difficult to find a one-size-fits-all solution. However, several issues that are common in biochemical processes must be taken into consideration. 　　1) Control of wastewater properties First, the pH value should be checked and adjusted. When the pH is below 5, it not only facilitates sludge bulking but also poses a certain risk to normal biochemical reactions; therefore, the pH should be adjusted promptly when it is too low. Additionally, in cold regions in the north, attention must be paid to the water temperature during winter; if the temperature is too low, it should be heated, as low temperatures can also lead to sludge bulking. Using forced aeration can effectively maintain higher water temperatures in winter. 　　When the nutrients in the wastewater are insufficient or unbalanced, they should be added supplementarily. The N and P contents should be controlled at around a BOD:N:P ratio of 100:5:1. 　　If digestion occurs prior to the biochemical system for wastewater treatment, the resulting low-molecular-weight organic acids will facilitate the growth of filamental bacteria; in such cases, pre-aeration of the wastewater in a regulating tank can be used to improve the situation. Generally, air diffusers are used to aerate the regulation tank with an effective water depth of 3–5 meters, and the air supply rate can be controlled at 0.5–1.0 m3 per cubic meter of wastewater per hour. It keeps the wastewater in the regulating tank fresh and effectively prevents odors caused by anaerobic conditions. 　　2) Maintaining sufficient dissolved oxygen in the tank is particularly important for high-load biochemical systems. 3) Generally, DO should be kept at least above 2 mg/L. 　　4) The sludge in the sedimentation tank should be discharged or recycled in a timely manner, 5) to prevent anaerobic conditions from occurring. If anaerobic conditions occur, 6) the various gases produced get adsorbed onto the sludge, 7) which also causes the sludge to float, 8) resulting in a reduced settling capacity. And 9) the return of anaerobic sludge also leads to massive proliferation of filaments. In such situations, in addition to sludge removal and cleaning of the dead zones in the sedimentation tank, 10) reducing the retention time of sludge in the tank, 11) it is also necessary to increase the DO level in the aeration tank, 12) to ensure that the water entering and leaving the sedimentation tank has a sufficient level of dissolved oxygen, 13) or to aerate and regenerate the sludge before it is returned to the biological treatment tank. As shown in the figure on the left. 　　After resolving the above issues, if sludge bulking still cannot be controlled, it is necessary to analyze the situation based on actual conditions. Below are some guiding methods for several common treatment processes, provided for reference by wastewater treatment professionals. 　　A. High-load activated sludge process 　At present, the design of activated sludge processes in China typically employs a moderate load (0.3 KgBOD5/(kgMLSS•d)), but in practice, economic considerations lead to the use of higher loads; therefore, sludge bulking under high loads is quite common in China. Under high load conditions, insufficient DO is the most common issue; therefore, measures such as increasing the gas-water ratio and enhancing aeration are taken, along with the use of jet aeration at the inlet of plug-flow aeration tanks. After observing for a period of time, the root cause of the problem is identified. 　　If the situation does not improve after implementing the above measures for a while, considering the addition of soft packing at the head of the aeration tank may be advisable. This part has a high removal efficiency for organic acids, thereby eliminating the factors that promote the growth of filamental bacteria and facilitating the growth of floc-forming bacteria. This method is relatively effective, but it is costly and causes difficulties in future maintenance and management. Alternatively, a selector with a hydraulic retention time of about 15 minutes can be installed before the aeration tank, which generally proves very effective in suppressing the growth of filamental bacteria. 　　 For the SBR process with intermittent water feeding, the reactor itself is fully mixed, and a concentration gradient of pollutants exists over time in its matrix; therefore, there is no need for an additional selector. Typically, the cause of sludge bulking in the intermittent SBR process is an excessively high sludge concentration, coupled with a low sludge load resulting from a low concentration of organic matter in the influent water or a small water volume. In such cases, reducing the discharge ratio, increasing the initial concentration of the substrate, and forcing sludge discharge from the SBR generally enable effective control of sludge bulking. For continuous-feed SBR processes such as ICEAS and CASS, if sludge bulking occurs, it is necessary to install a pre-reactor or bioreactor at the water inlet. 　　B. Low-load activated sludge process In the low-load activated sludge process, the substrate concentration in the aeration tank is low, allowing filamental bacteria to achieve high growth rates; therefore, this process is most prone to sludge bulking. Apart from finding solutions regarding water quality and aeration, the most fundamental and effective approach is to divide the aeration tank into multiple compartments and operate it in a plug-flow manner, or to add a small pre-aeration tank with compartments as a biological selector. In this selector, a high sludge load is used to absorb some of the organic matter and eliminate organic acids. This method not only helps to suppress sludge bulking but also effectively improves the efficiency of biochemical treatment. The method of adding packing in the aeration tank is also applicable to the low-load complete mixing process. 　　In the A/O and A2/O processes, by installing an anoxic and anaerobic stage before the aerobic stage along with a sludge return system, the mixed microbial community can be alternately placed in an anoxic and aerobic state, resulting in periodic changes in organic matter concentration. This not only controls sludge bulking but also improves the settling properties of the sludge. Systems with continuous water inflow, such as the alternating-operations oxidation ditch and UNITANK processes, have an effective control capability over sludge bulking because they inherently possess a practical \"selector\" in terms of time and space. If sludge bulking occurs in these two processes, the sludge load and DO in the tank can be adjusted by controlling aeration to regulate oxygen levels and the amount of return sludge; after some time of such adjustments, sludge bulking can generally be brought under control.