Optimizing Biological Nitrogen Removal Processes

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Optimizing Biological Nitrogen Removal Processes
Jeanette A. Brown, P.E., DEE Executive Director
SWPCA CSWEA April 4 2006
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Presentation

• Review of Theory
• Nitrification
• Denitrification
• Characteristics of bioreactors
• Aerated
• Un-aerated
• Nitrification Optimization
• Denitrification Optimization
• Problems and Troubleshooting

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Nitrification

• Oxidation of ammonia nitrogen to nitrite nitrogen
  by nitrosamonas group
• NH4 O2 2H NO2-
• Oxidation of nitrite nitrogen by nitrobactor
  group
• NO2- O2 NO3-

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Denitrification

• Using methanol as carbon source
• 6 NO3- 5 CH3OH N2 5 CO2
• 7 H2O 6 OH-
• Using an endogenous carbon source
• C5H7NO2 4.6 NO3- 2.8 N2 5 CO2
  1.2 H2O 4.6 OH-

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Characteristics of Bioreactors
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Aerated Bioreactor
O2 Pollutants Microorganisms
RAS (microorganisms)
WAS
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Characteristics of an Aerated Bioreactor

• Aerobic
• Microorganisms
• Heterotrophic-use carbon for the formation of new
  biomass
• Autotrophic-derive cell carbon from carbon
  dioxide
• Requires net input of energy
• More energy for synthesis than heterotrophs
• Lower cell yields and growth rates

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Aerobic, Heterotrophic Metabolism
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Aerobic, Autotrophic Metabolism
Nutrients
C5H7 O2N (New Cells)
CO2
Synthesis
---Bacteria---
Energy
NO2- NO3 -
NH3-N
O2
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Un-aerated Bioreactor (Anoxic Zone)
Nitrate Recycle
Primary Effluent
Anoxic
Aerobic
RAS
WAS
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Characteristics of an Un-aerated Bioreactor

• Anoxic
• Microorganisms
• Facultative heterotrophic-use carbon for the
  formation of new biomass
• Use nitrate/nitrite instead of oxygen
• Oxygen is preferred

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Facultative Heterotrophic Metabolism (Anoxic)
Nutrients
C5H7 O2N (New Cells)
Synthesis
Organic Compounds
---Bacteria---
Energy
N2 CO2 H2 O
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Chemotrophs

• Organisms that derive their energy from chemical
  reactions
• May be heterotrophic or autotrophic
• Energy-production is through oxidation-reduction
  reactions
• Involves the transfer of electrons from an
  electron donor to an electron acceptor
• Electron donor is oxidized
• Electron acceptor is reduced
• Can be either organic or inorganic compounds

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Chemotrophs

• The metabolic process consists of the separate
  yet simultaneously occurring reactions-synthesis
  and respiration.
• Synthesis uses a portion of the waste matter
  (food) to produce new cells.

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Chemotrophs

• Respiration is the coupled release of energy
  through the conversion of food material to lower
  energy-containing compounds-CO2, H2O, and
  oxidized forms of nitrogen.
• The precise nature of the products formed depends
  on process design, reaction time, temperature,
  and process loading.

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Chemotrophs

• The synthesis of new cells is reversible because
  the cells can also use their own protoplasm as
  food to provide the energy needed to maintain
  life. This is known as endogenous respiration.
• Maintenance energy requirements exist independent
  of the presence of substrate outside the cell.

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Chemotrophs

• When endogenous respiration predominates, the
  growth of the microorganisms does not cease, but
  is exceeded by cellular degradation.
• This results in a net decrease in the mass of
  microbial cells. The extended aeration process is
  one example of a process variation that can
  successfully operate in endogenous respiration.

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Chemotrophs

•  The electron acceptor may be available within
  the cell during metabolism (endogenous) or it may
  be obtained from outside the cell, for example
  dissolved oxygen, (exogenous)

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Summary Table
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Nitrification
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Effective Nitrification

• Achieved by
• Effective nitrification
• Adequate Aerobic SRT
• Temperature
• Sufficient Oxygen Transfer Capacity
• Maintain a DO of 2 mg/l at peak loadings
• pH gt 6.5, preferably gt7
• Accomplished by sufficient alkalinity (Effluent
  concentration at least 50 mg/l
• No inhibitory materials

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Nitrifier Minimum Aerobic SRT Varies with
Temperature.
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Maintain Adequate Dissolved Oxygen

• Oxygen Stoichiometry
• 4.6 mg of oxygen demand per mg of NO3-N produced
• Nitrification is inhibited by low dissolved
  oxygen
• Ensure 2 mg/l DO throughout aerobic zone

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Maintain pH 7.0 or Greater

• Alkalinity Stoichiometry
• Nitrification consumes 7.2 lbs alkalinity as
  CaCO3 per lb ammonia-N oxidized
• Denitrification produces 3.6 lbs alkalinity as
  CaCO3 per lb nitrate-N denitrified

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Importance of Alkalinity

• Insufficient alkalinity
• Causes drop in pH
• May cause inhibition of nitrification process
• May result in higher operating costs
• If effluent alkalinity is less than 50 mg/l
• Add supplemental alkalinity
• Or improve efficiency of dentirification process

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Alkalinity Consumption

• Alke Alko 7.14 (NO) 3.57 NO-(NO3-- N)e
• Where
• NO influent nitrogen converted to oxidized
  nitrogen
• (NO3-- N)e effluent nitrate
• Determined from reaction stoichiometry

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Simplified Calculation of Alkalinity Requirement
Given Plant Influent Flow 10 mgd Primary
Effluent NH3-N 28 mg/l
Alkalinity Consumed by Nitrification In
lbs/day (10) mgd x (28 mg/l) x (7.2)
x (8.34) 16,813 lbs alkalinity Flow
NH3-N conc. lbs. of alkalinity
as CaCO3 per day (mgd)
(mg/l) required per lb of
ammonia-N
nitrified
In mg/l (28 mg/l) x (7.2) 202 mg/l
alkalinity as CaCO3 consumed
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Alkalinity Sources

• Types of chemicals used
• Hydrated lime
• Quicklime
• Soda ash
• Caustic soda
• Sodium bicarbonate

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Nitrification OptimizationSummary

• Test nitrification rate occasionally
• Select appropriate SRT
• Keep DO at 2 mg/l
• Keep pH about neutral (optimal 7.5 to 8.5)
• Provide sufficient alkalinity

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Denitrification
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Denitrification Stoichiometry

• Oxygen Equivalent of NO3-N is
  2.86 mg O2/mg NO3-N. (2.86 lb oxygen demand
  satisfied / lb NO3 -N generated)
• Alkalinity Produced Due to Consumption of Acid
  (HNO3). (3.6 mg CaCO3 produced / mg NO3 -N
  generated)

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Effective Denitrification

• Effective Denitrification
• Sufficient Anoxic Volume (Anoxic SRT)
• Sufficient Carbon
• Sufficient mixed liquor recirculation

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Anoxic Zone (Selector)

• Size based on anoxic SRT
• Typically 1 to 2 days depending on temperature
• Must exclude dissolved oxygen
• ML Recycle and RAS below surface
• Reduce DO going to ML Recycle pumps

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Denitrification is Controlled by Mixed Liquor
Recirculation.
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Denitrification with Supplemental Carbon
Methanol or other carbon source
Primary Effluent
Nitrate Recycle
Anoxic
Aerobic
Aerobic
Anoxic
RAS
WAS
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Supplemental Carbon

• Methanol
• Stoichiometry
• 2.5 (NO3-N) 1.5 (NO2-N) 0.87 (DO)
• Or, approximately 3 mg CH3OH/mg NO3-N
• Requires 1 to 3 day SRT in secondary anoxic zone
  depending on temperature
• Other carbon sources technically feasible but
  generally more expensive.

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Denitrification Optimization Summary

• Minimize DO in anoxic zone (lt 0.2 mg/l)
• Have 2Q to 4 Q recycle capabilities
• Provide sufficient carbon (readily biodegradable
  COD)
• Maximize use of secondary anoxic zones

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Troubleshooting and Problem Solving
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Nitrification Inhibition

• Inhibition is defined as
• Decrease in rate
• Inability to convert NH3 to NO2, or NO2 to NO3
• Indicators of Potential Inhibitors
• Increase in effluent NH3-N concentration
• Increase in NO2 concentration
• Failure to nitrify at appropriate SRT
• Decreased OUR
• White foam
• Increased effluent turbidity

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Probable Nitrification Inhibitors

• Metals
• Cadmium
• Lead
• Zinc
• Organic Chemicals
• Benzene
• Cyanide
• Thiourea
• Surfactants
• Inhibition can be acute or chronic

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Potential Sources of Inhibition

• Industrial Discharges
• Haul-in with sludge or septic
• In-plant chemical spills
• Incinerator scrubber return

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Potential Solutions

• Confirm presence
• Use simple nitrification test procedure with
  control
• Identify source
• Can use test procedure for system-wide detective
  work
• Remove source or modify treatment strategy
• Storage
• Side stream options
• Main stream options

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Solutions to Problems

• Foam on tanks
• Gray - brown - orange foam, viscous in nature -
  Nocardia type foam
• Lon SRTs, trapped surface, fluctuating SRT,
  fluctuating temperature
• Remove trapped surface, chlorinate foam
  selectively, chlorinate RAS
• White foam - looks like soap
• May have too low MCRT, not enough biomass in
  tank, excessive detergents

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Solutions to Problems

• High effluent ammonium, fluctuating effluent
  ammonium-N
• MCRT or DO may not be adequate in the aerobic
  zone to maintain nitrification.
• Increase MCRT.
• Evaluate Step Feed to increase MCRT without
  increasing MLSS to clarifier.

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Solutions to Problems

• Fluctuating chlorine demand
• Partial nitrification of ammonium-N to nitrite-N
  without further conversion to nitrate-N.
• Inadequate aeration to handle high flows,
• inadequate biomass in system to handle diurnal
  peak nitrogen loads, or
• inadequate biomass to handle spikes in influent
  TKN (e.g. sudden septage discharges).

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Solutions to Problems

• Fluctuations in basin DO (with periods of low
  DO)
• Check if sufficient blowers are operating for
  peak loads
• Consider adding more blowers or upgrading to fine
  bubble diffusers.
• Excessive DO at certain times of the year or
  during low flow periods
• Look into ways of adjusting aeration based on
  time of day. e.g. install timers or an
  automated DO Control system

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Solutions to Problems

• Gradual increase in secondary clarifier sludge
  blanket
• Evaluate the trend in SVI. Is SVI too high for
  the clarifier solids loading? What is the
  blanket level?
• If SVI is high because of filaments, are they low
  DO filaments? Where are these filaments growing?
  Is the anoxic zone behaving as a low DO zone? Is
  the aerobic zone suffering from low DOs. Can it
  be corrected?
• Initiate RAS chlorination to reduce SVI.
• Increase sludge wasting if MCRT can be reduced.

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Solutions to Problems

• SVI increases during and immediately after
  periods of high flows
• Does infiltration increase anoxic zone DO?
  Infiltration may also bring in filamentous
  bacteria.
• Plan on a maintenance dose of RAS chlorination.

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Solutions to Problems

• Increase in effluent soluble organic-N - possible
  causes or Increase in effluent TKN with increase
  in ammonium-N
• Reduction in SRT below that required for
  nitrification.
• Sudden increase in influent TKN - septage dose,
  etc.
• Sudden addition of inhibitor - industrial
  chemical, pesticide, preservative, etc.
• .

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Solutions to Problems

• Large clumps of sludge floating to top of
  secondary clarifier
• Denitrification in clarifier - Increase RAS flow
  rate to reduce time sludge spends in clarifier,
  increase activated sludge tank effluent DO.
• Too high of a blanket
• Check sludge scraper for proper operation.

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Summary

• Good troubleshooting and problem solving comes
  only with experience.
• Every plant is different.
• We never know what is coming into the plant that
  can harm the process.
• Even under perfect process control, we can still
  have problems.
• The more we learn, the more we need to learn.