Frequently Asked Questions About SRTmaster^TM

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SOLIDS RETENTION TIME (SRT) CONTROL METHOD

1. Why is maintaining constant SRT important?


2. How is SRT control accomplished?


3. How is sludge age (SRT) calculated?


4. How is mass of solids under aeration calculated?


5. How is mass of wasted solids calculated?


6. Why are solids in clarifiers not included in the equation?


7. Why are solids in the effluent not counted as wasted?


8. Can SRT control be accomplished by wasting flow from mixed liquor channel or from aerators?

SRT TARGET (SET POINT)

9. How is the SRT target selected?


10. When should a change of SRT target be considered?


11. How much should the SRT target be changed?

AUTOMATIC SRT CONTROL METHOD

12. Why is automatic SRT (solids retention time or sludge age) control is better than traditional SRT control?


13. What is so unique about the SRTmaster^TM control algorithm?


14. What is the advantage of using SRTmaster^TM vs. a generic SRT formula control?


15. After the automatic SRT system is installed, how long does it take to notice improvements?


16. Does SRTmaster^TM change waste flow in response to the diurnal flow and solids fluctuation?


17. How does the controller know volume under aeration?


18. How many SRT controllers does a plant need?

CONTROLLER FUNCTIONALITIES

19. Does the SRTmaster^TM select the SRT target?


20. What if the desired wasting pattern is a combination of continuous and intermittent modes such as wasting continuously during the weekdays and the termination of wasting on weekend?


21. What if waste flow or wasting time has to be temporarily changed, as for a dangerously high sludge blanket level)?


22. What happens if one of the meters suddenly provides faulty readings?


23. What if the PID controller of the actuator fails?


24. What happens if the SRT controller (PC or stand-alone embedded controller) fails?

SOLIDS RETENTION TIME (SRT) CONTROL METHOD

1. Q. Why is maintaining constant SRT important?
   A. Optimum activated sludge performance can only be achieved by maintaining a constant food to microorganisms (F/M) ratio. The following formula represents the relationship between SRT and food to microorganisms ratio: Y (F/M)-k=1/SRT. Y and k are coefficients that do not change, unless wastewater characteristics significantly change.



2. Q. How is SRT control accomplished?
   A. It is usually accomplished by wasting a fraction of the flow returning from clarifiers to the aerators.



3. Q. How is sludge age (SRT) calculated?
   A. Sludge age is calculated by dividing the mass of solids under aeration by the mass of wasted solids.



4. Q. How is the mass of solids under aeration calculated?
   A. By multiplying the mixed liquor TSS concentration by aeration volume.



5. Q. How is mass of wasted solids calculated?
   A. By multiplying the waste sludge TSS concentration by wasted sludge flow.



6. Q. Why are solids in clarifiers not included in the equation?
   A. Solids in clarifiers are not participating in the BOD removal process. Normally the average daily change of biomass in clarifiers should be close to zero. If it is significantly different from zero, an automatic SRT controller detects this abnormality and alerts operators.



7. Q. Why are solids in the effluent not counted as wasted?
   A. The mass of these solids is negligible compared to the mass of solids wasted through the waste line.



8. Q. Can SRT control be accomplished by wasting flow from mixed liquor channel or from aerators?
   A. Yes, it can be. The advantage of this method is that only flow measurement is required. However, compared to wasting of return activated sludge, this method has the following problems: a waste sludge transmission system (pipes and waste pumps) and a recycling (recycle pipes and pumps) have to be significantly larger; the sludge processing facility has to be oversized as well; and the operational cost of waste sludge transport and processing is increased. Stable sludge mass loading of the sludge processing facility is impossible using this method. Thus, despite its simplicity, it is a very expensive SRT control method.




SRT TARGET (SET POINT)

9. Q. How is the SRT target selected?
   A. Selection of the SRT target is a tedious process that usually requires considerable time, effort and activated sludge process expertise. Ekster and Associates have developed a unique methodology of selecting the best SRT and offers it as an optional service.



10. Q. When should a change of SRT target be considered?
    A. Changing an SRT target should be considered in case of temperature change, when anticipating shock and toxic loading, or if there is significant change in wastewater characteristics or sludge settling characteristics.



11. Q. How much should the SRT target be changed?
    A. To avoid a process upset, it is recommended to change the SRT target by no more than 10-15% a day.




AUTOMATIC SRT CONTROL METHOD

12. Q. Why is automatic SRT (solids retention time or sludge age) control better than traditional SRT control?
    A.
    1. Automatic SRT control takes into account all changes of solids, both under aeration and in waste stream, during the entire day, rather than taking a snapshot of it once a day.
    2. Negative effects of lab errors are significantly minimized, and SRT calculation errors are avoided.
    3. TSS sampling is reduced by 70-90%.
    4. Significant improvements in sludge thickening can be achieved by maintaining the desired feed mode of the thickening facilities.


13. Q. What is so unique about the SRTmaster^TM control algorithm?
    A. SRTmaster^TM has the following unique features:
    1. Each control algorithm is tuned before delivery using a customized computerized model of the customer's activated sludge process.
    2. The unique sensor fault detection algorithm detects problems with the sensors and prevents use of faulty data.
    3. The unique output data filtration algorithm guarantees that maintaining of SRT target will never have a negative effect on the performance of sludge clarification and sludge processing.
    4. A special algorithm provides stable solids loading on sludge processing facility.
    5. The controller is an expert system and automatically notifies operators about potential problems and suggests possible solutions.


14. Q. What is the advantage of using SRTmaster^TM vs. a generic SRT formula control?
    A. Use of a regular SRT formula or Dynamic SRT formula for calculating waste flow causes these following well-documented problems:
    • Process oscillation
    • Significant variations in waste flow and waste mass. Such variations have a detrimental effect on sludge thickening.
    • If a sensor provides a faulty signal, this signal causes generation of a faulty waste flow signal. This fault is further amplified when a Dynamic SRT formula is used.
    SRTmaster^TM does not have these disadvantages because it uses a PI control algorithm and low-pass filters.



15. Q. After the automatic SRT system is installed, how long does it take to notice improvements?
    A. If the SRT target is selected properly improvements in sludge processing are observed fairly quickly. It takes considerably longer to see improvements in the activated sludge process due to the specifics of the biological process.



16. Q. Does SRTmaster^TM change waste flow in response to the diurnal flow and solids fluctuation?
    A. No. Although any change in solids concentration caused by diurnal flow fluctuation is included in the calculations, the SRT controller changes waste flow to maintain a constant long-term F/M ratio. A change in food mass over 24 hours cannot be compensated for by similar changes in biomass because growth of biomass is fairly slow. Therefore, instantaneous F/M and SRT values may still change significantly over 24 hours. Operators of activated sludge processes that are subject to shock and toxic loading should consider this fact and choose the SRT target conservatively. A proper control of return sludge flow can help to reduce diurnal variation of instantaneous SRT.



17. Q. How does the controller know volume under aeration?
    A. Aeration tank volume is preprogrammed using customer's information. An operator enters the number of aeration tanks in service into the menu. Upon physical change of the number of aeration tanks in service, operators should update menu settings. If the operators forget to do so, the SRTmaster^TM eventually detects the change in solids under aeration and in waste stream and alerts the operators.



18. Q. How many SRT controllers does a plant need?
    A. One controller per sludge wasting point (one per train).




CONTROLLER FUNCTIONALITIES

19. Q. Does the SRTmaster^TM select the SRT target?
    A. No. An operator enters the SRT target (set point) into the menu.



20. Q. What if the desired wasting pattern is a combination of continuous and intermittent modes such as wasting continuously during the weekdays and the termination of wasting on weekend?
    A. SRTmaster^TM can handle any desired wasting pattern.



21. Q. What if waste flow or wasting time has to be temporarily changed, as for a dangerously high sludge blanket level?
    A. An operator can override set point, waste flow, or wasting time calculated by the SRTmaster^TM. The mass of wasted solids in a manual mode will still will be included into the controller calculations when automatic waste mode is reinstated.



22. Q. What happens if one of the meters suddenly provides faulty readings?
    A. SRTmaster^TM detects the abnormality and alerts operators. It also automatically changes the control algorithm.



23. Q. What if the PID controller of the actuator fails?
    A. The SRT controller detects the abnormality and alerts operators.



24. Q. What happens if the SRT controller (PC or stand-alone embedded controller) fails?
    A. Waste flow set point will not change and there will be no short-term negative effect of such failure. Restarting of PC or embedded controller is required.