Biology of Decomposition

Decomposition is carried out by bacteria and fungi.

The first step in decomposition is to break the organism down into "bite sized pieces" so the bacteria can make use of the material. Large molecules of protein, cell membrane, cell wall, fats, sugars and oils have to be broken down into smaller molecules than can be transported into the tiny bacterial cell for processing. This first step (hydrolysis) is the slowest and the cause of many problems in ponds. Special bacterial and fungal enzymes are required to initiate the decomposition, and they are rarely present in large enough numbers at the right time. The result is the accumulation of sludge in the “dead zones” in the pond. The dead zones are those areas in the pond where water current cause sediment and sludge to accumulate.

In a perfect pond, with no dead zones, the organic material would form into flocs in the water: these would begin to decompose as bacteria worked within these flocs. As well, the flocs would become food for pond animals such as copepods and prawns.

In many ponds, the “dead” zones accumulate sludge and decomposition commences. Fermenting bacteria secrete enzymes that begin the process. These same fermenting bacteria grow and secrete products back into the sludge, often leading to putrefaction. The sulphate reducing bacteria use the fermentation products and produce sulphide and CO₂. If the C:N ratio is not adequate, ammonia and nitrite are produced from the sludge. As well, inadequate C:N will prevent the system from working effectively, leading to slower decomposition in the sludge and release of toxic intermediate byproducts into the water.

C:N ratio of organic waste

When there is adequate oxygen, bacteria grow quickly until all the food is used up. However, the food must be properly balanced in particular with respect to C and N.

If bacteria are using dead algae as food, the C:N molar ratio of the food must be at least 10:1 in order for the decomposition to proceed effectively. If it is less than this, ammonia will be produced. The molar C:N of algae is in fact only 7:1 and of bacteria (including cyanobacteria) only 5:1, so ammonia will definitely be released unless further C is added.

Decomposition can not proceed effectively without addition of extra organic C. Because decomposition is slowed down by the low C:N, sludge will accumulate and build up in mass.

Decomposition in sludge requires more C than aerobic decomposition, as more steps are involved with more different types of bacteria. If C is limiting, toxic by products will accumulate. As well, the quantity of sludge will build in the pond.

Addition of organic C can help to prevent accumulation of toxic sludge by improving the efficiency of microbial degradation processes. The type of organic C is important, and it must be readily and quickly available to the bacteria. Molasses is ideal as it is high in sugars.

When food runs out, bacteria undergo auto-oxidation and decompose themselves as follows:

4 (C₅H₇NO₂) + 20O₂ ------------------> 20CO₂ + 4NH₃ + 8H₂O

This shows that dying bacteria release ammonia as well as CO₂.