6. Treatment Plants for Nitrification
By Jes la Cour Jansen, Poul Harremoes and Mogens Henze
Ordinary, municipal wastewater contains 20-50 mg/1 of nitrogen compounds, especially in the form of ammonium (NH4 and NH.3) or organic compounds which are
easily converted into ammonium in a treatment plant. There are three reasons why
the presence of these compounds is not wanted in the treated wastewater, and why
attempts are made to remove or convert them before their discharge into the
recipient:
- On biological oxidation (nitrification), ammonium will consume a considerable amount of oxygen. In ordinary municipal wastewater, the oxygen
requirement for this oxidation will be approx. 40% of the total oxygen
requirement. Hence biological treatment without nitrification will not in
many cases, be sufficient to prevent oxygen depletion in the recipient.
- Ammonia (NH3) is a strong fish poison. Fortunately the content of ammonia
in the wastewater is low as the chemical equilibrium NH4 ~ NH.3 + H+ at
normal pH is shifted towards the left. Only at a pH of more than 8.5,
ammonia begins to form an essential part of the total content of ammonium
compounds. Especially in streams, the pH can vary considerably. In Denmark it is required that the ammonia content of the stream is less than 0.025
mg/1 if the stream is to be graded a fishing stream. This drastic requirement
of the ammonia content means that in most cases it is necessary to oxidize
the ammonium components before discharge.
- Along with phosphorus, nitrogen constitutes the most important nutrient
for plant growth in the recipients. Nitrification in itself does not mean that
the nutrient content will change as the plants absorb nitrogen in the form of
nitrate or ammonia. However, nitrification is necessary if nitrogen is to be
removed from the wastewater (in an economical way) since biological
denitrification, where nitrate is converted into free nitrogen, requires that
the ammonium compounds are oxidized into nitrate before they are reduced
to gaseous nitrogen. Chapter 7 gives a more explicit explanation of the
linking of such processes in treatment plants.
As a consequence of the above effects of ammonium discharge, nitrification (in
addition to the removal of organic matter) is also required to an increasing extent
when wastewater is discharged into streams, lakes and rivers.
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By Jes la Cour Jansen, Poul Harremoes and Mogens Henze
Ordinary, municipal wastewater contains 20-50 mg/1 of nitrogen compounds, especially in the form of ammonium (NH4 and NH.3) or organic compounds which are
easily converted into ammonium in a treatment plant. There are three reasons why
the presence of these compounds is not wanted in the treated wastewater, and why
attempts are made to remove or convert them before their discharge into the
recipient:
- On biological oxidation (nitrification), ammonium will consume a considerable amount of oxygen. In ordinary municipal wastewater, the oxygen
requirement for this oxidation will be approx. 40% of the total oxygen
requirement. Hence biological treatment without nitrification will not in
many cases, be sufficient to prevent oxygen depletion in the recipient.
- Ammonia (NH3) is a strong fish poison. Fortunately the content of ammonia
in the wastewater is low as the chemical equilibrium NH4 ~ NH.3 + H+ at
normal pH is shifted towards the left. Only at a pH of more than 8.5,
ammonia begins to form an essential part of the total content of ammonium
compounds. Especially in streams, the pH can vary considerably. In Denmark it is required that the ammonia content of the stream is less than 0.025
mg/1 if the stream is to be graded a fishing stream. This drastic requirement
of the ammonia content means that in most cases it is necessary to oxidize
the ammonium components before discharge.
- Along with phosphorus, nitrogen constitutes the most important nutrient
for plant growth in the recipients. Nitrification in itself does not mean that
the nutrient content will change as the plants absorb nitrogen in the form of
nitrate or ammonia. However, nitrification is necessary if nitrogen is to be
removed from the wastewater (in an economical way) since biological
denitrification, where nitrate is converted into free nitrogen, requires that
the ammonium compounds are oxidized into nitrate before they are reduced
to gaseous nitrogen. Chapter 7 gives a more explicit explanation of the
linking of such processes in treatment plants.
As a consequence of the above effects of ammonium discharge, nitrification (in
addition to the removal of organic matter) is also required to an increasing extent
when wastewater is discharged into streams, lakes and rivers.
195