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SRTmasterTM
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Control of Activated Sludge Waste Flow |
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1. Importance of food-to-mass ratio Food to mass ratio (F/M) is one of the most important parameters of an activated sludge system. The best performance of activated sludge is only achieved when F/M is maintained at optimum constant value. An increase of F/M ratio over an optimum value may cause low D.O. filamentous bulking, poor removal of pollutants in the aeration basin, dispersed growth of biomass, and overload of the thickening facility. A decrease of F/M ratio below an optimum value may cause low F/M filamentous bulking and foaming, an increased oxygen demand, and increased clarifier loading. The F/M ratio is controlled by the amount of wasted sludge: the increase of wasted sludge mass decreases F/M, and vice versa. Due to the difficulties in measuring the amount of incoming food in a timely manner (BOD5 results are known only in 5 days), wasting is usually done based on constant solids retention time (SRT) criterion, also known as sludge age. SRT is a ratio of the mass of solids in the activated sludge system to the mass of solids that have left the activated sludge system. SRT is related to F/M by the following formula: Y (F/M)-k=1/SRT Y - yield coefficient k - decay coefficient According to this formula, if SRT is kept constant it means that F/M will also be constant. SRT is much easier to calculate than F/M because instead of BOD data, it uses the total suspended solids concentrations (TSS). 
2. Comparison of SRT control criterion with other popular waste control criteria: 2.1 SRT vs. MLSS:Constant MLSS criterion will not provide constant F/M because when the amount of food changes, the mass stays the same. Because MLSS depends not only on food but also on ratio between influent and return sludge flows, wasting based on constant MLSS criterion may cause significant variability of the F/M, the flow, and the mass of wasted sludge.2.2 SRT vs. F/M:F/M criterion requires BOD data or surrogate of BOD measurements. These measurements are more expensive to obtain than TSS, and they are not as reliable.2.3 SRT vs. constant sludge depth in the clarifiers:A change in sludge depth may be caused by reasons other than a change in F/M, such as variations in return sludge flow to influent flow ratio, or a change in microbiological population.The advantages of wasting based on SRT control criterion are recognized by practically every wastewater treatment professional, and this method of wasting is advocated in most publications discussing the design and operation of activated sludge systems.
3. SRT control methods that do not utilize TSS data The following are the most popular waste control methods that do not require knowledge of suspended solids concentration and are aimed at maintaining a constant SRT: 3.1 Constant hydraulic wasting of mixed liquor.This is the simplest method of SRT waste control. The SRT is held equal to the ratio between aeration tanks volume and waste flow. This method, however, has two major problems:Increased volume. The volume of the mixed liquor waste is several times greater than the volume of return activated sludge waste. This increase necessitates larger waste sludge and recycled transmission facilities (pumps, pipes), as well as larger sludge thickening facilities. The operational costs of waste sludge and recycled stream pumping and sludge thickening are higher because of low concentration of waste sludge. Variable mass of waste sludge. It is difficult to maintain constant mass loading to the sludge thickening facility, and as a result the thickening process is not optimized. 3.2 Constant hydraulic wasting of return activated sludge.This method does not require measuring the solids concentration either. However, even a small unavoidable change in the ratio between return sludge flow and influent flow will drastically affect the SRT. It is also difficult to maintain mass loading to sludge thickening facility, and as a result the thickening process is not optimized.
4. Traditional SRT control Operators of majority activated sludge systems utilize the SRT control method that uses the regular SRT formula: SRT= (Mass of solids under aeration)/(Mass of wasted solids) Traditionally SRT control includes the following steps:
5. Automatic SRT control Automation of the SRT control routine using on-line TSS measurements can significantly simplify and improve the wasting process. Automation of the SRT control process has the following benefits compared to the traditional approach: Continuous monitoring and control. Automatic SRT control takes into in the account all changes in solids under aeration and in waste stream for the entire 24 hours. This is in contrast to the traditional approach of taking a snapshot of TSS concentrations once a day and making a change in wasting only once a day. Reduction of errors. Negative effects of lab errors are significantly minimized and SRT calculation errors are avoided. Reduction of TSS samples. TSS sampling is reduced by 70- 90%. An automatic SRT control system requires a controller and 2 TSS meters per treatment train (in some cases 1 TSS meter is enough). There are several options in the automation of the SRT control routine.
6. Methods of SRT control automation There are three methods of automation using constant SRT control criterion. 6.1 Calculation of the waste flow using instantaneous SRTThis method is easy to implement. However, it has the following problems:Significant variability of waste flow and mass. Due to high variability of MLSS and RASSS concentrations, waste flow will vary significantly and negatively affect waste sludge thickening process. Instantaneous SRT is not equal to MCRT. SRT calculated based on instantaneous MLSS and RASSS values will vary significantly and will not reflect true Mean Cell Residence Time (MCRT). 6.2 Calculation of waste flow using the dynamic sludge age (DSA) formulaRecognizing the shortcomings of instantaneous sludge age calculations, in 1985 dynamic sludge age calculations were introduced. DSA calculations more accurately compute the true MCRT for an unsteady-state system. The DSA calculations are more complicated than the traditional sludge age calculations, but with the ready availability of computing devices this is not a serious drawback. However, it was shown that waste flow calculated based on DSA formula is even more unstable than waste flow calculated based on instantaneous SRT formula. This volatility becomes especially dangerous if one of the meters provides a faulty signal. Drastic changes in waste flow will have a significant negative impact on the thickening facility, similar to the waste flow calculated using instantaneous SRT formula. As a result it is advised to use extreme caution when DSA formula is directly applied to automatic waste control calculations.Another problem with the previous two methods is the high probability of process oscillation. 6.3 Calculation of waste flow utilized in SRTmasterTMIn the mid-1990's Dr. Ekster pioneered the use of a computerized model of activated sludge process for tuning of SRT controller. Computer modeling consists of several steps. First, the filed data is collected and an activated sludge computer model (Fig.1) is calibrated using this data.
Fig.1 Picture of modeling software layout used for system modeling. Then using methodology developed by Dr. Ekster, time basis for SRT calculations is selected. Time basis is plant specific and depends on wastewater flow and characteristics, plant design, etc. A criterion for time basis selection is closeness of calculated SRT and dynamic SRT. The computer model is used to simulate an effect of waste flow change on both dynamic SRT and calculated SRT. An example of simulation results is shown on Figure 2. 
Fig.2 Results of computer simulation. After selection of time basis for SRT calculations, PID tuning coefficients are found for SRT controller with help of computer modeling. Finally, the behavior of controller is modeled for "what if" situations. Based on the modeling results SRTmasterTM final tuning coefficients are selected. The benefits of applying Dr. Ekster's approach to solving the "real world" problems are well documented. Software based on Dr. Ekster's approach is now successfully controlling activated sludge processes worldwide.
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