Denitrification

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Denitrification/Gasification:

The question often arises as to why bubbles occasionally form in a secondary (or primary) clarifier, eventually bringing up clumps of sludge to the surface or as to why H₂S forms in the sludge going to a dewatering unit, etc. The answer is that the gasses are caused by a phenomena known as gasification, i.e.; the transformation of a soluble or suspended organic material into gas during anoxic waste decomposition.

This situation is caused by a lack of sufficient dissolved oxygen in the system, resulting in facultative bacteria (heterotrophic bacteria) using alternative chemical sources for the oxygen demand required to sustain metabolism of their carbon based food sources.

First: Metabolism is the biochemical process (a series of biochemical oxidation-reduction reactions) performed by living organisms to yield energy for synthesis, motility and respiration. All non photosynthetic microorganisms (microorganisms that do not fix sunlight as their source of energy, i.e.; don’t contain chlorophyll and, therefore, can’t utilize photosynthesis) are dependent on their ability to obtain energy from the metabolism of a substrate. Heterotroph’s use carbon molecules as their substrate, autotroph’s use an inorganic molecule as their substrate.

In autotrophic bacteria (bacteria which use an inorganic compound as an energy source, such as nitrifiers and sulfur degraders), reduced inorganic compounds (such as NH₃-N) are oxidized to yield energy .

NH₄⁺+1½ O₂^{. nitrosomonas bacteria .®}NO₂^- + 2H⁺ + H₂O + energy

NO₂^- + ½ O₂ ^{. nitrobacter bacteria .®}NO₃^- + energy

Heterotrophic bacteria utilize carbon molecules as an energy source;

2CH₃OH + 3O₂ ¾ ¾ ® 2CO₂+ 4H₂O

^(methanol)

The heterotrophic reaction above is an Oxidation reaction, adding an oxygen (note: an oxidation reaction could also remove hydrogen or electrons) to the substrate in which reduced organic compounds serve as hydrogen donors and oxidized organic or inorganic compounds act as hydrogen acceptors. The oxygen in the example above is the hydrogen acceptor.

The higher the ultimate hydrogen acceptor is on the energy scale, the greater the preferential use of that acceptor over alternative hydrogen acceptors (and the higher the energy yield). Strict anaerobic reactions have the lowest energy yield, and therefore, have the lowest priority in this reaction and take longer to achieve.

A Reduction reaction involves the removal of oxygen from the substrate or the addition of hydrogen or electrons).

Therefore, gasification (or denitrification) occurs when a system becomes depleted in dissolved oxygen and heterotrophic (carbon degrading) bacteria utilize an alternative oxygen source (hydrogen acceptor) to support their oxidation reaction metabolism. This is called an anoxic reaction.

Confused? Just remember that in an anoxic reaction, the goal of the bacteria is to use the oxygen in the molecules in support of their oxidation reactions for metabolism of an organic compound - they are not sulfur reducers or some special microbe. The sequential order of oxygen utilization as a hydrogen acceptor is as follows;

In order by decreasing energy yield:

Aerobic AH₂^* + O_2·¾ ¾ ® CO₂ + H₂O + energy

½ AH₂ + NO₃^- ¾ ¾ ® N₂+ H₂O + energy

facultative AH₂ + SO₄^2- ¾ ¾ ® H₂S + H₂O + energy

½ AH₂ + CO₂ ¾ ¾ ® CH₄+ H₂O + energy

Anaerobic AH₂ + B^** ¾ ¾ ® BH₂^*** + A + energy

· dissolved oxygen

* energy stored in organic matter

** an oxidized organic compound

*** a reduced organic compound

So, what does this mean to my plant? Allowing gassification to occur in your plant can cause you to violate on TSS permits, increase your demand for polymers on a clarifier or even float your entire bed or sludge blanket over the weirs eventually.

In sludge dewatering areas, an increase in H2S gas can cause serious health problems.

Total Nitrogen Balance Newsletter

Oh No, all my nitrifiers are Dead

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