SLUDGE RETENTION TIME CONTROL SYSTEM …

1 SLUDGE RETENTION time CONTROL SYSTEM . FREQUENTLY ASKED QUESTION. Revised 1/18/06 SRT FAQ. Why automatic SRT (solids RETENTION time or SLUDGE age) CONTROL is better than current methods of SRT. CONTROL ? 1. Automatic SRT CONTROL takes into account all changes of solids under aeration and in the waste stream within an entire 24 hour period rather than taking a snap shot of it once a day. 2. The 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 thickening facilities.

2 Why is maintaining constant SRT important? Optimum activated SLUDGE performance can be achieved only by maintaining constant food to microorganisms (F/M) ratio. SRT and food to microorganisms ratio are directly connected by the following relationship: Y(F/M)-k = 1/SRT. Y and k coefficients do not change, unless wastewater characteristics significantly change. How is SLUDGE age calculated? SLUDGE age is calculated by dividing the mass of solids under aeration by wasted solids. Why are the solids in clarifiers not included into the SLUDGE age equation? Solids in clarifiers are not participating in the BOD removal process.

3 Normally, the average daily change of biomass in clarifiers should be close to zero. If average daily change of biomass in clarifiers is significantly different from zero, the SRT Controller detects the abnormality and alerts the operator. Why are solids in the effluent not counted as wasted? The mass of these solids is negligible compared with the mass of solids wasted through the waste line. How is mass of solids under aeration calculated? By multiplying the MLSS concentration by aeration volume. How does the controller know the volume under aeration? Aeration tank volume is preprogrammed at the factory.

4 An operator then enters the number of aeration tanks in service into the controller. Any physical change in the number of aeration tanks in service should cause the operator to update the menu settings. If an operator forgets to do so, the controller eventually detects the changes in solids under aeration and in the waste stream and alerts the operator. How is the SRT target selected? Usually the SRT target for a domestic BOD removal activated SLUDGE process is selected based on achieving good settleability of SLUDGE . Achieving the needed degree of nitrification is a criterion for SRT.

5 Target selection at nitrification plants. The ability of the activated SLUDGE process to sustain shock and toxic loading is often a consideration in selecting an SRT target for industrial plants. Sometimes plants maintain higher SRT than necessary because of the inability to maintain constant SRT. Automatic SRT. CONTROL may allow for the reduction in an SRT target, and as a result reduce aeration cost, reduce clarifier solids loading and increase gas production in the anaerobic digester. It may also stop undesired nitrification in BOD removal from the activated SLUDGE process and improve SLUDGE settleability as well as foam CONTROL .

6 When should a change in the SRT target be considered? A change of SRT target should be considered in cases of temperature change, in anticipation of shock and toxic loading, a significant change in wastewater characteristics or SLUDGE settling characteristics. How does one avoid unauthorized changes in controller settings? Controller settings are protected by two levels of security: one for settings affecting the alerts, another one for settings affecting the actual waste flow. Does the controller change waste flow in response to a diurnal flow fluctuation? No. But, any change in solids concentration caused by diurnal flow fluctuation is included in the calculations.

7 The SRT controller changes waste flow to maintain only long term F/M ratio since growth of biomass is a fairly slow process. Instantaneous F/M and SRT values still may change significantly during the day. Operators of activated SLUDGE processes that are subject to shock and toxic loading should consider this fact and choose an SRT target conservatively. What if waste flow or wasting time has to be temporarily changed (for instance in case of dangerously high SLUDGE blanket level)? An operator can override the controller. Mass of wasted solids in the manual mode still will be included into controller calculations when automatic waste mode is reinstated.

8 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 the weekend? The controller can handle any desired wasting pattern. How important is accuracy of the waste flow meter? Accuracy of a waste flow meter is extremely important. We strongly recommend calibration of the waste flow meter prior to installation of the SRT controller. How often should the suspended solids meters be recalibrated? We recommend daily comparison of MLSS and RAS suspended solids meters readings with readings of the portable TSS meter.

9 If difference in readings exceed 5%, the permanent meter should be recalibrated. Weekly calibration of portable meters against lab data is recommended. What happens if one of the TSS meters suddenly provides a faulty reading? The SRT controller detects abnormality and alerts the operator. Usually such occasions are extremely rare due to the high reliability of the TSS meters. What if the PID CONTROL of the actuator fails? The SRT controller detects this abnormality and alerts the operator. What if one of the solids meters is located very far from the controller and there are no electrical conduits connecting the two devices?

10 Radio communication between two devices could be provided. Sanitaire Royce Technologies 14125 South Bridge Circle Charlotte, NC 28273. 800 347 3505. Tel 704 409 9898 Fax 704 409 9899. European Office: London Asia-Pacific Office: Australia