Denitrification
Although it is important to transform the
nitrogen in wastewater from ammonia to nitrate, excessive amounts of nitrate
are also harmful. At high concentrations they threaten the health
of babies, as their digestive systems are not capable of transforming the
nitrates into less harmful forms of nitrogen. Once nitrates enter
the infant's bloodstream they interfere with oxygen transfer; starving
the body of the oxygen it needs. In most places nitrate levels are
not high enough to significantly threaten infant health, but lower concentrations
still have effects on the ecosystem. Nitrate can be used directly
by plants and algae as their nitrogen source. Releasing large amounts
of nitrates into surface waters is like adding fertilizer to the system.
With the added nitrogen algal growth can increase to damaging levels.
The high algae concentration blocks sunlight needed by other plants uses
the oxygen required for all marine life.
Reactions and Yield Constants
Denitrification is essentially the conversion
of nitrate to nitrogen gas. Denitrification is a natural process
carried out by many microorganisms when sufficient oxygen is lacking.
Instead of using oxygen as the final electron acceptor in metabolism nitrate
is used. The process involves many steps, and many undesirable intermediates.
If the microbes are stressed, their metabolism is incomplete and these
intermediates may be produced in relatively high amounts.
The denitrification reactions are closely
tied to cellular metabolism, producing energy for growth or as heat.
Using the organic matter in typical wastewater, the reaction can be written
as:
When this energy is used for growth, maximum
yield constants are nearly as high as for aerobic metabolism. When
written for the production of biomass, the equation becomes:
This reaction has a maximum yield of 0.44
kg biomass/kg organic matter, compared to about 0.50 kg biomass/kg organic
matter for aerobic metabolism.
In most wastewater ammonium ions (NH4+)
will be present, and if present the microbes will nearly always use them
as a nitrogen source. This reaction can be written as:
This reaction has an even better yield,
0.47 kg biomass/kg organic matter.
Rates of Reaction
Rates of denitrification can be described
by the Monod equation, but in most cases extra substrate is provided to
prevent substrate limitations, and as a result the observed rate is nearly
equal to the maximum possible rate. The general substrate removal
rate can be expressed as:
where:
When substrate limitations are limited
the rate expression can be simplified to:
as
SNO3 >> KS,NO3
S >> Ks
Rates of reaction are strongly influenced
by a number of factor that including the kind of substrate, the system
pH, and amount of oxygen.
Substrate Dependence: Denitrifying
bacteria are highly sensitive to the substrate available for their use.
Easily degraded materials such as methanol and acetic acid give the highest
rates of denitrification. Raw wastewater does not yield such a high
degradation rate, but it is significantly higher than what is observed
with non organic carbon sources.
Temperature Dependence:
Rates of
denitrification reactions increase with temperature in a manor similar
to what rates in nitrification reactions. They are not as limited
in range though, as some bacteria can continue to carry out the processes
at temperatures up to 60°C. These bacteria are relatively
rare, and as a result reactions are not usually carried out at such high
temperatures despite the expected increases in reaction rate. Click Here to see the effect temperate has on reaction
rates for several different substrates.
Oxygen Inhibition: Oxygen inhibits
denitrification by providing a better electron acceptor for metabolism.
When oxygen is present, the microorganisms will use oxygen instead of nitrate.
The concentration of oxygen of importance is the concentration at the cell's
surface, which may be significantly different than the bulk concentration.
pH Dependence: The optimum pH for
denitrification is in the 7-9 range, with rates falling off sharply outside
this region (see figure). At lower
pH the generation of N2O and NO is increaced.
Additional Link(s)
Denitrification (in greater depth)
References
Created by: Renay Jacob and Emily Cordaro
, RPI, Fall 2000
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