Process control To maintain high levels of treatment performance with the activated-sludge process under a wide range of operating conditions, special attention must be given to process control. The principal approaches to process control are(1)maintaining dissolved oxygen levels in the aeration tanks, (2) regulating the amount of return activated sludge(RAS), and(3)controlling the waste-activated sludge(WAS). The parameter used most commonly for controlling the activated-sludge process is SrT. The mixed-liquor upended solids(MLSS) concentration may also be used as a control parameter. Return activated sludge is important in maintaining the MLSS concentration and controlling the sludge blanket level in the secondary clarifier. The waste activated-sludge flow from the recycle line is selected usually to maintain the desired SRT. Oxygen uptake rates(OURs) are also measured as a means of monitoring and controlling the activated-sludge process. Routine microscopic observations are important for monitoring the microbial haracteristics and for early detection of changes that might negatively impact sludge settling and process rformance Dissolved Oxygen Control. Theoretically, the amount of oxygen that must be transferred in the aeration tanks equals the amount of oxygen required by the microorganisms in the activated-sludge sy stem to oxidize the organic material. In practice, the transfer efficiency of oxygen for gas to liquid is relatively low so that only a small amount of oxygen supplied is used by the microorganisms. When oxygen limits the growth of microorganisms, filamentous organisms may predominate and the settleability and qual ity of the activated sludge may be poor. In general, the dissolved oxygen concentration in the aeration tank should be maintained at about 1.5 to 2 mg/L in all areas of the aeration tank. Higher DO concentrations (2.0 mg/L)may improve nitrification rates in reactors with high BOD loads. Values above 4 mg/L do not improve operations significantly, but increase the aeration costs considerably Return Activated-Sludge Control. The purpose of the return of activated sludge is to maintain a sufficient concentration of activated sludge in the aeration tank so that the required degree of treatment can be obtained in the time interval desired. the return of activated sludge from the final clarifier to the inlet of the aeration tank is the essential feature of the process. Ample return sludge pump capacity should be provided and is important to prevent the loss of sludge solids in the effluent The solids form a sludge blanket in the bottom of the clarifier, which can vary in depth with flow and solids loadings variations to the clarifier. At transient peak flows, less time for sludge thickening is available so that the sludge blanket depth increases. Sufficient return sludge pumping capacity is needed, along with sufficient clarifier deptl (3.7 to 5.5 m), to maintain the blanket below the effluent weirs. Return sludge pumping rates of 50 to 75 percent of the average design wastewater flowrate are typical, and the design average capacity is typically of 100 to 150 percent of the average design flowrate. Return sludge concentrations from secondary clarifiers range typically from 4000 to 12,000 mg/L (WEF, 1998) Several techniques are used to calculate the desirable return sludge flowrate. Common control strategies for determining the return sludge flowrate are based on maintaining either a target MLSS level in the aeration tanks or a given sludge blanket depth in the final clarifiers. The most commonly used techniques to determine return sludge flowrate are(1) settleability, (2) sludge blanket level control, (3) secondary clarifier mass balance, and(4)aeration tank mass balance Settleability. Using the settleability test, the return sludge ate is set so that the flowrate approximately equal to the percentage ratio of the volume by the settleable solids from the aeration tank effluent to the volume of the clarified liquid(supernatant) after sealing for 30 min in a 1000-mL graduated cylinder. This ratio should not be less than 15 percent at any time. For example, if the settleable solids occupied a volume of 275 mL after 30 min of settling, the percentage volume would be equal to 38 percent [(275 mL/725 mL)*100). If the plant flow were 2 m/s, the return sludge rate should be038×2m/s=0.76m3/s Sludge Blanket Level. With the sludge blanket level control method, an optimum sludge blanket level is maintained in the clarifiers. The optimum level is determined by experience and is a balance between settling depth and sludge storage. The optimum depth of the sludge blanket usually ranges between 0.3 and 0.9 m. The sludge blanket method of control requires considerable operator attention because of the diurnal flow and sludge production variations and changes in the settling characteristics of the sludge Several methods are available to detect the sludge blanket levels, including withdrawing samples using air-lift pumps, gravity-flow tubes, portable sampling pumps, and core samplers, or using sludge-supernatant interface detector7-9 Process Control To maintain high levels of treatment performance with the activated-sludge process under a wide range of operating conditions, special attention must be given to process control. The principal approaches to process control are (1) maintaining dissolved oxygen levels in the aeration tanks, (2) regulating the amount of return activated sludge (RAS), and (3) controlling the waste-activated sludge (WAS). The parameter used most commonly for controlling the activated-sludge process is SRT. The mixed-liquor suspended solids (MLSS) concentration may also be used as a control parameter. Return activated sludge is important in maintaining the MLSS concentration and controlling the sludge blanket level in the secondary clarifier. The waste activated-sludge flow from the recycle line is selected usually to maintain the desired SRT. Oxygen uptake rates (OURs) are also measured as a means of monitoring and controlling the activated-sludge process. Routine microscopic observations are important for monitoring the microbial characteristics and for early detection of changes that might negatively impact sludge settling and process performance. Dissolved Oxygen Control. Theoretically, the amount of oxygen that must be transferred in the aeration tanks equals the amount of oxygen required by the microorganisms in the activated-sludge system to oxidize the organic material. In practice, the transfer efficiency of oxygen for gas to liquid is relatively low so that only a small amount of oxygen supplied is used by the microorganisms. When oxygen limits the growth of microorganisms, filamentous organisms may predominate and the settleability and quality of the activated sludge may be poor. In general, the dissolved oxygen concentration in the aeration tank should be maintained at about 1.5 to 2 mg/L in all areas of the aeration tank. Higher DO concentrations (>2.0 mg/L) may improve nitrification rates in reactors with high BOD loads. Values above 4 mg/L do not improve operations significantly, but increase the aeration costs considerably. Return Activated-Sludge Control. The purpose of the return of activated sludge is to maintain a sufficient concentration of activated sludge in the aeration tank so that the required degree of treatment can be obtained in the time interval desired. The return of activated sludge from the final clarifier to the inlet of the aeration tank is the essential feature of the process. Ample return sludge pump capacity should be provided and is important to prevent the loss of sludge solids in the effluent. The solids form a sludge blanket in the bottom of the clarifier, which can vary in depth with flow and solids loadings variations to the clarifier. At transient peak flows, less time for sludge thickening is available so that the sludge blanket depth increases. Sufficient return sludge pumping capacity is needed, along with sufficient clarifier depth (3.7 to 5.5 m), to maintain the blanket below the effluent weirs. Return sludge pumping rates of 50 to 75 percent of the average design wastewater flowrate are typical, and the design average capacity is typically of 100 to 150 percent of the average design flowrate. Return sludge concentrations from secondary clarifiers range typically from 4000 to 12,000 mg/L (WEF, 1998). Several techniques are used to calculate the desirable return sludge flowrate. Common control strategies for determining the return sludge flowrate are based on maintaining either a target MLSS level in the aeration tanks or a given sludge blanket depth in the final clarifiers. The most commonly used techniques to determine return sludge flowrate are (1) settleability, (2) sludge blanket level control, (3) secondary clarifier mass balance, and (4) aeration tank mass balance. Settleability. Using the settleability test, the return sludge pumping rate is set so that the flowrate is approximately equal to the percentage ratio of the volume occupied by the settleable solids from the aeration tank effluent to the volume of the clarified liquid (supernatant) after sealing for 30 min in a 1000-mL graduated cylinder. This ratio should not be less than 15 percent at any time. For example, if the settleable solids occupied a volume of 275 mL after 30 min of settling, the percentage volume would be equal to 38 percent [(275 mL / 725 mL) * 100]. If the plant flow were 2 m3 /s, the return sludge rate should be 0.38 × 2 m3 /s = 0.76 m3 /s. Sludge Blanket Level. With the sludge blanket level control method, an optimum sludge blanket level is maintained in the clarifiers. The optimum level is determined by experience and is a balance between settling depth and sludge storage. The optimum depth of the sludge blanket usually ranges between 0.3 and 0.9 m. The sludge blanket method of control requires considerable operator attention because of the diurnal flow and sludge production variations and changes in the settling characteristics of the sludge. Several methods are available to detect the sludge blanket levels, including withdrawing samples using air-lift pumps, gravity-flow tubes, portable sampling pumps, and core samplers, or using sludge-supernatant interface detectors