Trickle Down Theories

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Written by:

Greg Farmer, Operations Supervisor,  Littleton/Englewood WWTP

Charles Caudill, Process Development Analyst, Littleton/Englewood WWTP

The Littleton/Englewood WWTP (LE WWTP) is a 24 MGD Activated Sludge/Trickle filter plant located southwest of Denver, Colorado.  Beginning in 1993, the LE WWTP was required to significantly reduce levels of ammonia in the plant effluent.  Several options to reduce ammonia were considered in a Long Range Master Plan.  The most cost effective option was determined to be the construction of 2 Nitrifying Trickle Filters (NTFs) using plastic cross flow media.  The NTFs are located downstream of the Secondary Clarifiers and before Final Effluent Disinfection

Ammonia removal in an NTF is achieved through the utilization of ammonia as a nutrient source by nitrifying bacteria that grow on the surface of the NTF media.  These nitrifying bacteria cannot store food; therefore, if ammonia is not supplied consistently throughout the entire day, the bacteria will not become established and the full potential of the NTF will not be realized. The objective is to continually supply food (ammonia) as evenly as possible to all portions of the NTF media.

The LE WWTP natural influent diurnal (24 hour) flows and ammonia loading vary over 300%.  During the low flow, low loading periods which last up to six hours, the nitrifying bacteria in the upper portion of the NTF consume all of the available ammonia.  This then prevents the establishment and growth of nitrifying organisms in the remainder of the filter. When the loadings increase during the day in excess of what the upper organisms can consume, the ammonia passes through the lower portion of the NTF that has not established a population of nitrifying organisms.  The ammonia then passes untreated out of the filter. T his natural variability of ammonia loading significantly impacts nitrifier growth and overall ammonia removal efficiency of the NTF.  This natural variation in ammonia loading was addressed in the design and operation plan by storing and returning ammonia rich anaerobic sludge dewatering centrate to the plant at the low loading periods.

Diurnal ammonia and centrate return modeling studies were conducted to determine baseline ammonia loadings at the NTFs.  This information was used in conjunction with design detention time analysis to determine centrate return requirements necessary to fill the low loading periods at the NTF’s distributing the ammonia load to the NTFs over 24 hrs.  Two low loading periods were found at the NTFs: 2:00 - 7:00 am and 3:00 - 5:00 p.m.  The earlier period could accept the greatest centrate return load.  Based on the design detention time analysis, centrate was pumped from the centrate storage tank back to headworks at approximately 3:00 p.m. in order to arrive at the NTF at 2:00 am. The duration of pumping was limited to the 5 hour window width.  The second smaller window required a later start time and a shorter pumping duration.

The NTFs went on line in September of 1992.  Within 3 weeks some ammonia reduction was being realized.  Removal efficiency continued to improve as more nitrifying bacteria became established in the NTFs.  Based on these initial results it was presumed performance would continue to improve and eventually meet the plant discharge permit.  Beginning in June 1993, removal efficiency began decreasing and it became apparent that permit compliance was in jeopardy.

Through meetings held weekly between plant managers, operations personnel, and the project design engineers two problem areas were identified.  Coordination in operation of interrelated process areas and further testing to determine if a more effective centrate return time frame existed.

Operation of an NTF utilizing dewatering centrate to achieve even constant loading 24 hours per day, 7 days per week requires a very close coordination with many other plant processes and programs. Beneficial Use programs, anaerobic digestion and dewatering schedules, activated sludge process operation, centrate tank management, and others all must be carefully coordinated to achieve constant even NTF loading.  To achieve this end, a comprehensive centrate management program was developed and implemented.  A few key features of this program include:

    * Forecasting the volume of sludge pumped daily to the digesters and establishing weekly sludge dewatering schedules based on the forecast.
    * Anaerobic digested sludge is dewatered and the biosolids are transported from the plant 5 days a week. Utilization of the full storage capacity of the Centrate Tank to return this 5 day centrate production evenly over 7 days is dependent on accurate sludge production forecasting.
    * Maintaining the Trickle filter/Activated sludge process to achieve final clarifier TSS under 20 mg/l. Higher loadings have the potential to promote growth of carbonaceous organisms on the NTF media.

This program was instrumental in coordinating all interrelated processes and resulted in a significant improvement in evenly loading the NTF’s, over the full seven days of the week.

To determine if a more appropriate centrate return time existed a series of return time adjustments and performance tests were initiated. Nitrifying organisms have a slow growth rate. Therefore each return time adjustment was followed by 1 to 2 weeks of observation and close monitoring of NTF performance to gauge the effectiveness. Tests were conducted initially based on the modeling analysis to fill the two low loading windows. Diurnal ammonia studies were conducted in conjunction with these tests to determine its effectiveness. The results indicated that the earlier and larger window could accept a greater amount of centrate return. In addition, the performance of the two window return, although improved, indicated the second window return may have encroached on the second high loading period. Based upon these results, an additional pump was installed to increase the centrate return volume during the first window and pumping during the second window was terminated.  A single window return study was then initiated coupled with diurnal ammonia analyses to determine its effectiveness. T he results from these investigations were applied to the centrate return pumping plan, and some improvement in ammonia removal performance was realized.

Final effluent ammonia concentrations were still above permit requirements so another series of centrate return timing and duration trials were conducted.  These were coupled with diurnal ammonia monitoring studies to evaluate the effectiveness of each.  Centrate return pump start times were adjusted up to one hour on each side of the previous start time. Again, 1 to 2 weeks of monitoring followed each adjustment.  The results were inconclusive - neither the later nor the earlier start times resulted in improved performance.  Changes in duration of return was also inconclusive. The determination of the actual centrate arrival time at the NTF’s was being clouded by the broadening and diluting of the centrate as it passed through upstream plant processes. The centrate was also being masked by background ammonia concentrations.

To eliminate the masking effects of background ammonia on the determination of centrate return detention times and actual NTF arrival, a Lithium Tracer study utilizing anhydrous lithium chloride was conducted in October 1993. A known concentration of the lithium solution was introduced at the inlet of the activated sludge basins. Grab samples were collected and analyzed to determine lithium concentrations at different points in the basin at varying time intervals. The results of the study clearly defined actual detention times through the secondary system and actual arrival at the NTFs. It was found that significant short circuiting in the secondary system reduced the actual detention time by more than two hours. As a result the centrate had been arriving at the NTF more than two hours too soon.  In addition, it was found that the secondary process significantly broadened the return load.

In November 1993, the results of this test were implemented into the centrate return plan.  The start time was set back to 7:00 p.m.  A new higher output pump was installed in the centrate tank so that more centrate could be returned in less time.  The effects of these changes were realized almost immediately. Within days the performance of the NTFs dramatically improved.  The most stringent permit limits were soon met and even surpassed. Since this changed more a year ago, the NTFs have consistently met the most stringent permit requirements.