Simultaneous Nitrification and Denitrification at Fresno

1. Simultaneous Nitrification and Denitrification at Fresno Presented by Ronald G. Schuyler, PE, DEE, and Joe R. Tamburini Rothberg, Tamburini, and Winsor, Inc. and Steven Hogg and Kim Toepfer City of Fresno

2. Thanks to all of the Fresno-Clovis operations staff for making this approach work!

3. Operational Problems • High organic load (>40 lb BOD/103 ft3) • High temperature (80ºF+) • Easy nitrification • Easy secondary clarifier denitrification • Blanket on top! • Poor sludge settleability • High effluent TSS

4. The Fresno Treatment System Plant 2 B Side To Percolation Ponds Headworks Primary Clarifiers A Side

5. Plant 2 Primary Effluent Side-A Effluent to Percolation A Side Activated Sludge

6. Wastewater Characteristics

7. Industrial Contribution

8. Design Parameters • Original A-Side Design • Q = 50 MGD • Influent BOD/TSS – 240 mg/L • Based on CPE performed in May 2003 • Aeration basin limited • Primary effluent BOD of 238 mg/L • Two-year average value • 33.3 MGD @ 238 BOD

9. Effluent Limitations

10. Historical Control Approaches • Nitrification/denitrification issues exist even though nitrification was not required • Normal AS plants nitrify at normal MCRT at high T • Minimize denitrification in secondary clarifiers • Minimize nitrification in aeration tank • Very low MCRT • 1.5-2.0 Days • RSF high to minimize solids detention time in the clarifier • Lower DO – approximately 1.0 mg/L

11. Historical Results • Worked well with lower organic loading • High-rate activated sludge • Effluent BOD/TSS usually < 40/40 • Clarifier denitrification during low organic loading • High SVI, but controllable • Tanks off-line saved money • Increased organic loading mid 2001 • Food processing industries come on line • Zoogloeal slime production from sugars, acids and alcohols • Higher SVI

12. Project Objectives • Treat higher organic load • Increase the stability of mixed liquor • Reduce zoogloeal slime • Reduce SVI (historically very high) • Control nitrification/denitrification • Nitrify some for EC control • Minimize clarifier denitrification • Reduce effluent TSS • Minimize oxygen requirements

13. Possible Approaches • Anoxic section - MLE • High $ for barriers and recycle piping/pumps • Excess design and construction time • May not mesh with next plant upgrade • On/Off aeration • Old, unstable fine-bubble aeration grid that could fall apart • Square tanks with only air mixing and one influent point

14. Modified Approach • Increase MCRT to provide more stable MLSS • Reduce zoogloeal slime production • Reduce F/M to reasonable value • Use low DO environment • Minimize nitrification • Maximize denitrification in aeration tank • Minimize denitrification in clarifiers • Control low-DO filaments • 0.3-0.4 mg/L possibly too low for low DO filaments

15. Load Flow Organic Air DO Flow rate MCRT SVI Effluent quality BOD TSS Ammonia Nitrate – no data but less than 8-10 if limited clarifier DN Results

16. Operating Realities • System prone to nitrify even in less than ideal conditions • Operators shifted flows around the system as needed depending on the system conditions • DO controlled at the end of the aeration tanks only • DO probe calibration infrequent • Problems with instrument calibration accuracy • Problems convincing operators of requirement to maintain low DO

17. Project Initiated A Side Flow Rate Flow, MGD

18. Project Initiated Organic Load

19. Project Initiated Higher MCRT Lower MCRT Lower MCRT Aeration Tank Dissolved Oxygen Very Low MCRT

20. Project Initiated Air Flow Rate and Space Loading

21. Oxygen Transfer Efficiency AOTE = SOTE(α)[(βCsw – CL)/Cs]θ(T-20) As CL get smaller, the transfer rate increases. (βCsw – CL)/Cs For instance in a situation such as Fresno’s with little initial nitrification, a transfer efficiency increase of about 10% would be expected with a drop in DO from the 0.9 mg/L to 0.2 mg/L with β = 0.95, Csw = 8.0 mg/L and Cs = 9.17 mg/L. Nitrification would improve that further by increasing α.

22. Project Initiated MCRT

23. Project Initiated SVI

24. Organic Acids Consistently high SVI from Thiothrix and type 021N

25. Project Initiated Effluent Quality High MCRT High Organic Load

26. Results • Allowed significantly higher flow rate – 22% • Allowed 12% increase in BOD mass loading • Reduced effluent parameter concentrations • BOD - 7.5% • TSS - 14% • NH3 - 28% • Air requirements per MGD reduced 22% • Reduced zoogloeal slime

27. Results • Provided acceptable effluent quality under significantly overloaded conditions • Clarifier denitrification controlled • DO < 0.35 mg/L • Low numbers of low-DO filaments • DO < 0.4 mg/L • There are always process instabilities that make it difficult to identify a specific causative agent

28. Lessons Learned • Quality DO meter capable of controlling at ultra-low level • Accurate and frequent DO meter calibration required • Low-DO filament population would “tell us” proper DO level • Relying on “hard and fast” DO target a mistake

29. More Lessons Learned • Nose and eyes tell when DO was too low • Biology of the system “told us” the correct MCRT – now 5-6 days typically • Low DO, simultaneous N/DN proven successful • Stable process • Operational reliability • Trial and error testing required • Approach saves energy (and chemicals)

30. Questions • ? • ? • ? Ron Schuyler BugDr@rtweng.com