Using organic carbon to offset excess N in low water exchange ponds.


Before proceeding with this section readers are advised to have a look at the section on pond bacterial processes first, then come back here to understand how excess N can be “removed” in low water exchange ponds.


The Theory of removing N


In low water exchange ponds ammonia-N will tend to accumulate when feed introduces more than about 0.6 mg N/L per day. This 0.6 mg figure is explained here. The amount of N introduced into the pond as feed can be calculated provided we know the total feed applied per day and the protein content. The following is an example of how to calculate the amount of N introduced to the pond:


  1. the total amount of 40% protein feed applied per day is 100 kg, so the total amount of protein applied is 40 kg

  2. 16% of protein is N so the total N applied per day is 16% of 40 kg or about 6.5kg

  3. about 25 % of the N is incorporated into the cultured animals, so about 75% or 4.9 kg is returned to the pond.

  4. The pond is 5 million litres, so the total amount of N introduced per day is 4.9 kg per 5 million litres or about 0.98 mg per L

  5. Recall that an outdoor pond can in average circumstances manage about 0.5 mgN/L per day, so the difference of 0.48 mg per L must be removed or will build up to produce excess ammonia in the pond.


We should also calculate the amount on organic carbon in the feed. The reason this has to be accounted for is because this source of organic carbon will assist in removing excess N. It does this because organic carbon is the major food source for heterotrophic bacteria, which along with algae and water exchange, are the main pathways for removal of excess N. More details are provided here.


Here is is how the total organic N can be calculated, using the same example as we used above for working out N.


  1. the total amount of feed applied per day is 100 kg

  2. the C content of feed is about 40%, so the total C applied (as organic C) is about 40kg

  3. about 43% of the 40 kg or about 17 kg of this organic C is returned to the pond as organic C in the form of faeces. This figure is based on the following assumptions: 1. the farmed fish or shrimp retain about 13 % of the added organic C and 2. the rest is split evenly between faeces and conversion by the animals' metabolism to C02.

  4. The pond is 5 million litres, so the total amount of organic C introduced per day is 17 kg per 5 million litres or about 3.4 mg/L


We know that a good C:N ratio for growth of bacteria is about 12:1, so for 0.48 mg/L N we need about 5.8 mg/L organic C. Our calculations indicate that we are short on organic C by about 2.4 mgC/L and so there is a strong likelihood that N will accumulate unless steps are taken to remove it. The two things that can be done are water exchange or addition of extra organic C.


Addition of organic carbon such as molasses will provide nutrition for the growth of heterotrophic bacteria which will consume ammonia as shown here. Molasses is around 20% C by weight. In our example we need to add 2.4 mg C per L, and the pond is 5 million litres, so we need to add 2.4 x 5 kg or 12 kg C, or 60 kg molasses.


The Practice of removing N


So much for the theory. In practice each pond is different, each day is different, and the organisms in the pond go about their business oblivious to the ruminations of all of us. The theory is useful as it helps to provide us with insights into what is happening in the ponds. But as farmers we must take a practical approach and work with our ponds, rather than expecting them to behave in the way we think they should behave. We must also consider the most economical way to achieve the desired end result which is maximum survival and optimal growth of the fish or shrimp we are growing.


The most economical way may be to exchange water. In fact in purely dollar terms, this usually is the cheapest way to maintain good water quality. The problem is that often the water available for pumping is poor quality and may contain pathogens. The risk of water exchange as the tool of choice for maintaining pond water quality is in many cases simply too high. Appreciation of this risk has led to many pond managers adopting a low water exchange management strategy. The issues with low water exchange are well known: a build up of N as ammonia. This section has shown that algae can utilise N up to a set maximum of about 0.7 mg per L. Over this limit the N will accumulate. Another way to manage excess N is to apply organic C and stimulate the heterotrophic bacteria.


















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


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

bacteria


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


These equations also show that the oxygen demand is relatively high with 20 oxygen molecules required for every 20 carbon atoms in the dying bacteria, compared with 16 oxygen molecules for every 36 carbon atoms while the bacteria are growing. In other words dying bacteria require more oxygen in order to die than they do in order to grow!


Biochemistry 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 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.


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 most 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.


C:N ratio of organic waste


When there is adequate DO, 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 9:1 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.



The speed of the composting process is dependent on the C:N ratio of the organic material to be composted. The desired C:N and C:P ratio is 25–35:1 and 75–100:1, respectively (NRC, 1981a; Edwards, 1982; Biddlestone, Ball and Gray, 1981; De Bertoldi et. al., 1985). If there is an excess of C compared to N in the material to be composted (ie. C:N ratio > 35) the biological breakdown process is slow as the micro-organisms must go through many life cycles, oxidizing off the excess carbon, until a more convenient C:N ratio for their metabolism is reached (the C:N and N:P ratio of microorganisms on a dry weight basis being 10:1 and 5–20:1, respectively; Alexander, 1961). By contrast, a low C:N ratio in the starting material (ie. C:N ratio > 15) would result in the loss of nitrogen from the system through ammonification (particularly at high pH and temperatures; NRC, 1981a, De Bertoldi et. al., 1985).



Decomposition can not proceed effectively without addition of extra 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.


Phosphate


Phosphorus accounts for 32% of the weight of phosphate PO4. The molar ratio of N:P in algae and bacteria is 16:1 and 12:1 respectively. These equate to N:P weight ratios of about 7:1 for algae and about 6:1 for bacteria. The equivalent N:PO4 ratios are around 2-2.5:1. This means that for every 1 ppm of inorganic N, around 0.4 ppm of inorganic PO4 is required otherwise P may become limiting. In natural systems, these ratios are extremely difficult to follow due to the fact that most organisms undertake “luxury consumption” of P and keep the excess in storage products inside the cell. Practice suggests that pond PO4 levels should be kept around 0.05 - 0.2 ppm until the Secchi transparency reaches about 45 cm. Experience suggests that addition of phosphate when Secchi is less than about 45 cm can stimulate blue green algae.