The guiding role of the settlement ratio in the operational management of wastewater treatment using the activated sludge process

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The guiding role of the settlement ratio in the operation and management of wastewater treatment using the activated sludge process. Source: Abstract: There are many methods for wastewater treatment, and for large and medium-sized wastewater treatment plants, the activated sludge process remains the preferred method, as it features stable operation, low operating costs despite high load capacities, easy maintenance, and good treatment efficiency. In the actual operation of wastewater treatment using the activated sludge process, there are many factors that affect the efficiency of wastewater treatment. For example, the mixed liquor concentration in the aeration tank (MLSS), sludge settling ratio, sludge load, sludge return ratio, retention time, dissolved oxygen (DO), volatile mixed liquor concentration (MLVSS), air-to-water ratio, water temperature, and pH value are all important factors that influence treatment efficiency. However, in the operational management of wastewater treatment processes, the mixed liquor concentration in the aeration tank, settling ratio, sludge index, and the quality of inlet and outlet water are generally used as the main parameters for guiding operations. Over the past decade or so since its operation, our plant has relied primarily on the sludge settling ratio in the aeration tank, along with other parameters, to guide the operation of the wastewater treatment process. This article examines and discusses the important role of the settling ratio in operational management. Keywords: settlement ratio; activated sludge process; management; guiding role; 1 Theoretical basis The treatment of wastewater using the activated sludge process relies on activated sludge microorganisms, which, in an aerobic environment, convert organic matter into new cellular substances or break it down through metabolic processes. Subsequently, the organic matter formed by these cells is aggregated, settled, and separated, thereby removing organic pollutants from the wastewater and purifying it. The diagram of microbial metabolism is as follows: http://www.epday.com/upimg/081004/12230B450Z94H9.gif. An important step in wastewater purification is the process by which organic substances in the wastewater are converted into flocs under the action of microorganisms in the aeration tank; another key step is the flocculation, sedimentation, and separation of these microbial organic substances ； It can be inferred from this, as well as confirmed by research, that the main factor affecting the quality of wastewater treatment is first and foremost the concentration of activated sludge (MLSS) formed from microbial organic matter in the aeration tank ； Next is the quality of the flocculation and sedimentation properties of the activated sludge. The sludge settling ratio (SV%) refers to the volume percentage of settled sludge to the volume of the mixed liquid in a 100 ml measuring cylinder, after the mixture is allowed to stand and settle for 30 minutes. Thus, on the one hand, it is possible to directly assess the quality of sludge coagulation and sedimentation properties ； On the other hand, the sludge settling ratio is also, to a certain extent, a quantitative reflection of the sludge concentration ； Therefore, the sludge settling ratio is an important parameter used to guide process operation. 1.1 MLSS is key to the removal of organic matter in wastewater. The metabolic process by which activated sludge microorganisms remove organic matter from wastewater consists mainly of the synthesis of microbial cell material (growth of the activated sludge), the oxidative decomposition of organic matter (including some of the cell material), and the consumption of oxygen. When oxygen supply is sufficient, the growth of activated sludge goes through a logarithmic growth phase, a decelerating growth phase, and an endogenous respiration phase. During each growth phase, the rate of organic matter removal, the oxygen utilization rate, and the characteristics of the activated sludge all vary. Studies have found that the ratio of organic matter (F) to microorganisms (M) (sludge load ratio F:M) is an important factor affecting the active sludge at different stages, and thus influencing the efficiency of removing organic matter from wastewater. 　　F:M=Ns=QLa/XV (KgBOD5/KgMLSS·d) Where: Q – wastewater flow rate, m3/d; La – organic matter concentration in the influent water (BOD5), mg/l; V – volume of the aeration tank, m3; X – concentration of suspended solids in the mixed liquor (MLSS), mg/l. In typical urban wastewater treatment plants, the volume of the aeration tank remains constant, and both the amount of wastewater and its quality (BOD5 concentration) are relatively stable. From this formula, it is clear that the MLSS concentration is the determining factor for the sludge load rate, and it directly affects the efficiency of removing organic matter from the wastewater. 1.2 Under normal circumstances, the sludge settling ratio is an intuitive indication of MLSS values. This can be proven by the following formula: MLSS (g/L) = SV/SVI, where SVI (ml/g) is the sludge index, which serves as a metric for assessing the coagulation and sedimentation properties of activated sludge. In a stable wastewater treatment process, since the SVI value remains within a certain stable range over time, the sludge settling ratio can generally reflect the concentration of the mixture in the aeration tank; it is directly proportional to the sludge concentration. 2 Practical Applications In secondary wastewater treatment plants that use the activated sludge process for wastewater treatment, there are many factors that affect the efficiency of this treatment process. In the absence of empirical data, operation managers rely on the sedimentation ratio as the main parameter to guide their operations, firstly because it is simple to use and requires little time ； Secondly, operation managers and process engineers can monitor the flocculation and sedimentation processes of activated sludge at any time by measuring the sludge settling ratio, understand the characteristics of the activated sludge, determine its quantity, assess the performance of the aeration tank process, and provide a scientific basis for process adjustments, thereby controlling the efficiency of wastewater treatment. 2.1 Relationship between sedimentation ratio and sludge volume index (SVI) The process of measuring the sludge sedimentation ratio allows one to directly assess the quality of the sludge’s flocculation and sedimentation properties. During operation in our plant, when the SVI value is between 80 and 120, the sludge is brown in color, flocculent in structure, and exhibits good sedimentation properties ； When the SVI value is less than 80, it indicates that the sludge age is too long or the organic matter content is too low; in such cases the sludge is fine-grained, dark in color, and has poor activity ； When the SVI value is greater than 120, the sludge is too loose, appears light brown in color, and has poor sedimentation properties ； Additionally, after measuring the sludge settling ratio, it is possible to determine the oxygen supply in the aeration tank by observing how long it takes for the sludge in the measuring cylinder to float to the surface. If the sludge does not float after being left to settle for 3–4 hours and remains brown, it indicates that the activated sludge has good quality and has received sufficient oxygen supply through aeration ； If the sludge floats to the surface after about 2 hours of settling and appears black, it indicates that the sludge is in an anaerobic state, meaning that the oxygen supply in the aeration tank is insufficient. 　　During operation, if operating conditions such as the water inflow rate and excess sludge discharge volume remain stable, the sludge settling ratio will not change abruptly, and the SVI value will also remain stable; at such times, the sludge settling ratio corresponds to a certain concentration of activated sludge. However, when the sludge settling ratio changes suddenly due to the influence of influent water quality, temperature, or other operating conditions, it indicates that the growth phase of the activated sludge will be at a different stage, and the SVI value will inevitably be affected; at this point, the relationship between the sludge settling ratio and MLSS will also change. Below are two examples of conditions under which the settlement ratio undergoes a sudden change. 　　(1) When aeration in the aeration tank was stopped for a period of time due to certain reasons, the relationships between the sludge settling ratio and SVI values as well as MLSS levels before and after this stoppage are as follows: Settling Ratio (%), SVI (ml/g), MLSS (mg/l). Before stopping aeration: 15, 107, 1400; After resuming aeration: 43, 253, 1700.

(2) The relationships between the sludge settling ratio and SVI values as well as MLSS levels before and after a heavy rain are as follows: Settling Ratio (%), SVI (ml/g), MLSS (mg/l). Before the heavy rain: 14, 140, 1000; After the heavy rain: 40, 308, 1300. Although the conditions affecting these situations differ, the underlying reason for the changes is actually the same—the microorganisms are affected by external conditions (in the first case, it’s oxygen deficiency) ； The latter is affected by heavy rainfall, which loosens the microorganisms and prevents the formation of proper sludge flocs; as a result, the SVI value increases, and the concentration of suspended solids (SS) in the effluent is high at this time. However, this situation is temporary; by understanding the sludge settling ratio and determining the amount of excess sludge to be discharged appropriately, the MLSS value can be kept within an appropriate range. Once the adsorption capacity of the activated sludge flocs increases and the SVI value returns to normal, the effluent will become significantly clearer, at which point the water quality will meet the standards. 　　The above situation indicates that, on the one hand, operation managers and staff can identify problems through the sludge sedimentation process; by observing changes in the sludge sedimentation ratio, the color of the activated sludge, and its behavior after being left to settle, they can gain insight into the properties of the sludge as well as the level of aeration and oxygen supply ； On the other hand, operation managers can determine the amount of excess sludge to be discharged by observing the sludge settling ratio, thereby controlling the sludge concentration in the aeration tank and keeping the sludge load within the settling zone to ensure the quality of the effluent water. 2.2 Relationship between sedimentation ratio and sludge concentration (MLSS) 　　(1) When the SVI value remains relatively stable, there is a certain linear or logarithmic relationship between the sludge sedimentation ratio and the sludge concentration. Through analysis of relevant data over the years, the relationship between the sludge settling ratio and sludge concentration at different SVI values was determined, as shown in Figures 1, 2, and 3: http://www.epday.com/upimg/081004/12230B451320109429.gif http://www.epday.com/upimg/081004/12230B4515P114432.gif. These three figures along with the corresponding equations indicate that when SVI < 120, there is a linear relationship between the sludge settling ratio and MLSS; specifically, when SVI < 80, the slope of the MLSS value versus the sludge settling ratio is greater than that when 80 < SVI < 120. When SVI > 120, there is a logarithmic relationship between the sludge settling ratio and MLSS. This indicates that when the SVI value remains relatively stable, there is a stable correlation between sludge concentration and sludge settling ratio. As the SVI value increases gradually, the variation in sludge concentration with the sludge settling ratio becomes smaller and smaller. 　　(2) Temperature affects, to a certain extent, the relationship between the sludge settling ratio and sludge concentration, that is, the value of the sludge index. 　　The relationship between the sludge settling ratio and sludge concentration changes primarily due to variations in the SVI value. Apart from being influenced by the biological growth phase and some random factors, temperature is the main factor that affects the SVI value. The figure below shows the SVI values corresponding to different months throughout the year. http://www.epday.com/upimg/081004/12230B451R0123456.gif This graph shows that throughout the four seasons, the SVI value changes significantly depending on the season. Generally, during the transition periods between seasons, the SVI value increases suddenly; afterward, as the system adapts to the seasonal temperatures, the SVI value gradually decreases, until another peak is reached at the start of the next season. As can be seen from Figure 4, SVI reaches higher values in January, May, and September, while it is lower in February, August, and December. Generally speaking, SVI is relatively high in spring and low in winter. Of course, since the seasonal temperature changes vary from year to year, and other factors also play a role, the curve showing how the SVI value changes throughout the seasons differs from one year to another. However, the impact of seasonal temperature differences on the SVI value remains unchanged, and its trend is essentially the same. 　　(3) Influence of sludge settling ratio on wastewater treatment efficiency Different sludge settling ratios result in varying wastewater treatment efficiencies. Figures 5, 6, and 7 show the relationships between the BOD removal rate, COD removal rate, and SS removal rate, respectively, and the sludge settling ratio. 　　As can be seen from the figure above, the BOD removal rate remains stable at over 80% when the settling ratio is greater than 5% and less than 50%; however, when the settling ratio exceeds 50%, the BOD removal rate becomes more variable. The COD removal rate is less stable when the sedimentation ratio is less than 15%; when the sedimentation ratio lies between 15% and 50%, the removal rate remains stable at over 80%. When the sedimentation ratio exceeds 50%, the COD removal rate shows a clear tendency toward instability. The SS removal rate is very unstable when the settling ratio is less than 15%; it can generally be maintained above 85% when the settling ratio is between 25% and 50%. When the settling ratio exceeds 50%, the SS removal rate also tends to be variable. Explanation of the three diagrams: When the settlement ratio is less than 15%, the concentration of the mixture in the aeration tank is low; the activated sludge does not develop properly and remains in an immature stage. As a result, the flocculation and sedimentation of the sludge are poor, the microbial flocs are loose, and the microorganisms in the activated sludge are inactive. This leads to unstable water quality in the effluent, which may even fail to meet the required standards ； When the settling ratio is between 15% and 50%, the activated sludge is mature, and the concentration of the mixed liquor is relatively high, usually around 2000–3000 mg/l. The sludge load falls within the settling range; the sludge exhibits good flocculation and sedimentation properties, the microorganisms are active, and the quality of the effluent remains stable. To reduce the aeration volume in the aeration tank and save energy, we generally keep the sludge settling ratio between 15% and 30%. 　　In summary, in wastewater treatment plants that use the activated sludge process, determining the sludge settling ratio is an important method for operation and management personnel, both theoretically and in practice, to guide the operation of the process. It is not only simple and easy to operate, but it also allows operation and management personnel to monitor at any time the concentration of activated sludge as well as the quality of the sludge in the aeration tank. This enables them to control the operating parameters of the entire process. By establishing a stable sludge settling ratio, it is possible to control the efficiency of wastewater treatment and ensure the quality of the treated water. http://www.epday.com/upimg/081004/12230B452040131S4.gifhttp://www.epday.com/upimg/081004/12230B4522P141W1.gif