Simultaneous Nitrification and Denitrification at Fresno

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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

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

• High organic load (gt40 lb BOD/103 ft3)
• High temperature (80ºF)
• Easy nitrification
• Easy secondary clarifier denitrification
• Blanket on top!
• Poor sludge settleability
• High effluent TSS

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The Fresno Treatment System
Plant 2
B Side
To Percolation Ponds
Headworks
Primary Clarifiers
A Side
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A Side Activated Sludge
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Wastewater Characteristics
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Industrial Contribution
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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 _at_ 238 BOD

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Effluent Limitations
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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

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Historical Results

• Worked well with lower organic loading
• High-rate activated sludge
• Effluent BOD/TSS usually lt 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

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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

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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

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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

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Results

• 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

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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

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A Side Flow Rate
Flow, MGD
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Organic Load
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Aeration Tank Dissolved Oxygen
Very Low MCRT
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Air Flow Rate and Space Loading
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Oxygen Transfer Efficiency
AOTE SOTE(a)(ßCsw CL)/Cs?(T-20)
As CL get smaller, the transfer rate increases.
(ßCsw CL)/Cs For instance in a situation such
as Fresnos 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 ß 0.95, Csw 8.0 mg/L and Cs
9.17 mg/L. Nitrification would improve that
further by increasing a.
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MCRT
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SVI
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Organic Acids
Consistently high SVI from Thiothrix and type 021N
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Effluent Quality
High MCRT
High Organic Load
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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

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Results

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

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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

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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)

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Questions

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• ?
• ?

Ron Schuyler BugDr_at_rtweng.com