Automatic Sludge Age Control: Wasting Pounds Instead of Gallons

ProblemThe Town of Morrison owns and operates a 100,000 gallon per day conventional activated sludge plant to treat municipal wastewater. The operation of the plant is limited by a finite volume of aerobic digester capacity followed by sludge drying beds subjected to freezing weather. It was determined that increasing the concentration of sludge wasted to the digester, along with wasting sludge by calculating pounds of solids instead of gallons would reduce solids loading variability and volume of water sent to the digester; resulting in a reduction of sludge stored and hauled away when the drying beds are frozen. The staff at Morrison decided the best method to implement this process would be via an automatic wasting program to waste pounds of sludge instead of volume; comprised of an online suspended solids probe, magnetic flowmeter, the existing threeway valve, and logic via the programmable logic controller. Staff determined that with the addition of mixed liquor suspended solids measurement, complete control of sludge age could be implemented and tested, relieving the one full time and one part time operators from the daily task of wasting manually. With few operational examples of automatic wasting to learn from, the staff at Morrison partnered with Hach Company to create and implement a unique solution to meet their needs.ObjectivesThe objectives of this study are threefold: to determine if an automatic wasting process with an online TSS meter would reduce the water and increase the concentration of solids sent to the digester; to determine if such a system were reliable enough to operate without daily human intervention; and to determine if that system could be expanded to reliably control sludge age.MethodsIn order to measure the suspended solids concentration and calculate the pounds wasted, an online suspended solids probe was necessary to measure the RAS concentration continuously. The Hach SOLITAXsc™ Inline Suspended Solids and Turbidity probe was chosen for this application based on the range of measurement and willingness of Hach's staff to support the research effort. A magnetic flowmeter was already installed on the line to the digester, and integrated into the PLC. The automatic wasting system was implemented in three phases. The first phase of was to prove the measurement system is reliable by measuring the RAS concentration over time and observing the response time of the analytical system while determining the concentration curve during a typical cycle. Return activated sludge is wasted to the aerobic digester via an automatic three-way valve from the return activated sludge line; where the RAS pump operates via an on/off timer depending on the desired return rate. Phase two consisted of creating and proving a program in the PLC to control the amount of wasting via totalized pounds through wasting the thickest sludge during the RAS cycle. Lastly, phase three consisted of adding a suspended solids sensor to the mixed liquor and additional programming in the PLC to create and evaluate a Sludge Age controller based on real time values. The P&ID for the Sludge Age controller is shown in figure 1, below.ResultsThe project is currently in phase three; the Sludge Age controller is being evaluated for reliability. The system was proven to reliably trend a typical RAS cycle and it was determined that a simple 30 second offset between the time the pump began operating and the three way valve opened allowed for the low solids water in the pipe to be discharged to the aeration tank and not to the digester (figure 2).Comparing the results of the actual computer controlled daily sludge age versus the desired (entered) setpoint shows good correlation with no settleability or effluent problems (figure 3). Large deviations between the desired and actual sludge age setpoint are due to mechanical failures at the plant such as pump failures.Likewise, comparing the actual pounds wasted daily against the legacy calculated waste sludge units finds an excellent correlation; even though the sludge age controller calculates solids concentration by mass and waste sludge units are calculated by volume (figure 4).During the time of this study the sludge quality did not decrease, as measured by effluent water quality and the 30 minute settleability test, compared to operating sludge age via the legacy waste sludge units system.ConclusionThese results should be considered small-scale, wasting between 30 and 386 lbs/day. Slight inaccuracies in flow or concentration measurement can significantly influence the actual sludge age calculation, so it is important to not focus solely on the correlation between desired sludge age and actual sludge age, but instead focus on the correlation between wasted pounds and waste sludge units, the 30 minute settleability, and effluent water quality results. It is expected with a larger facility the ability to control the sludge age would be more accurate as more pounds of solids would be wasted in a day.The results show that an automatic wasting system and automatic sludge age controller are both feasible and reliable, resulting in a reduction in the necessary daily workload for the operations staff.