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CHOICE OF AERATION SYSTEM IN THE POND

 


The choice of an aeration device must take into account, as well
as cost and reliability,
- the efficiency of aeration in standard conditions which must be reconverted to the actual level of consumption (a value not always given by the manufacturer and difficult to control anyway).
- the size of the ponds to aerate for ponds or basins of more than afew thousand m2, aerators which generate good water circulation should be favoured (aerators with blades, hydro-ejectors)
- the ease of transport if the device is to be used at several sites.
- the ease of repair (standard motors)                    

It is important to bear in mind when using aerators under production conditions, that is to say in the pond, that the values of transfer rate of oxygen indicated by the manufacturers of different aerators have been obtained in standardised conditions, that is to say with 'tap’ water not containing oxygen at the beginning 0f the test.
In the pond, where the water is full of biological and chemjcal activity, the aerators Will not be so efficient.
The performance that can be expected principally depends, partly on the initial oxygen concentration and partly on temperature.
The following table indicates the factors to apply to correct the standard rate of transfer according to the true oxygen conditions and the temperature of the pond.





Aerator positioning


The water circulation generated by an aerator depends on its positioning.

• The Control of Oxygen and the Use of Aerators

Many manufacturers attempt to automate the control of dissolved oxygen in the pool with measuring devices which are able te switch on the aerators when the oxygen content measured falls below a certain threshold.
Unfortunately, these devices are very expensive and, in addition, are not reliable in commercial use.
The main problem is linked to the growth of algae on the probes (“fouling”) distorting the measurements, and to the heterogeneity of the oxygen distribution in the water mass when this is considerable.
The technological improvement of these materials should allow, before long, these problems to be resolved and the cost to be lowered.
At present, in the wait for the outcome of this work, two strategies can be proposed for the use of these pond aerators.
- Either the aerators are only used when the oxygen content of the water reaches dangerously low levels for the fish. This is what is known as ‘urgent aeration’, and assumes either more-or-less continuous monitoring or a prediction in the development of the oxygen content over the course of the night.
- Or the aerators are run every night for a certain duration during all the period of risk (summer). This is called systematic nocturnal aeration.
 






 


To contact AQUALOG


aqualog@aqualog.fr                Tél. : 0033 (0)4 94 10 26 26




 Prediction of deoxygenations and urgent aeration of the pond


Urgent aeration requires the monitoring of dissolved oxygen levels during periods of risk, mainly during the night, so that if necessary the aerators can be switched on.
In an attempt to avoid time-consuming night measurements, a prediction model of the oxygen content at dawn can be used.
The simplest model uses two measurements carried out the previous evening.
The drop in oxygen at night being practically linear, these two values, one taken at dusk, the other 2 or 3 hours later, allow the content at dawn te be estimated.
As a result of the general principle of this method, the precision of the prediction is weak but the fish farmer can improve
this by recording the true values that are observed at dawn.
He can therefore learn to assess the true functioning of his pond and therefore refine his/her predictions with the passing season. This method does not however allow a prediction, for example in the case of severe deoxygenation due to mass mortality of algae.
On the other hand, the elements which increase the risk of deoxygenation must also be taken into account and periods of risk can be specified beforehand.
It is not necessary to carry out oxygen measurements throughout the whole of the year, only in certain critical periods:
- in semi-intensive aquaculture, in our latitudes, it is between 15 July and the 15 September that risks of deoxygenation are the greatest, and the period of May to June can also cause certain risks when the temperature is above average.
- a water temperature greater than 25°C must be considered critical and should prompt an increase in surveillance.
- an increased oxygen content in the evening (greater than 18 mg/l) is the result of intense algal activity which increases the risk of deoxygenation at night.
- stormy periods in summer can be the origin of serious deoxygenation and it pays to take weather conditions into account.
These prediction methods therefore allow time consuming checks of the oxygen levels at night or the unnecessary use of aerators to be avoided.
Their use however carnes the inconvenience of a certain risk of error, which, is all too important in intensive conditions where the flnancial stakes are high, but remains genenally acceptable in semi-intensive or extensive conditions.
In the case of noticed or predicted deoxygenation, fish farmers need to resort to urgent aeration.
The aerators used are either electric aerators already in place
in the tank (ventilators with blades are the most common) or aerators operated by tractor power which are placed only when necessary.








To contact AQUALOG


aqualog@aqualog.fr                Tel. : 0033 (0)4 94 10 26 26




 Systematic (noctural) aeration of the ponds


To mitigate the numerous difficulties involved in by urgent aeration, many farmers pratice systematic aeration of their ponds.
Electric aerators in place in each pond are programmed to work for a certain duration in the night and/or the day.
Studies which have been carried out to determine the optimum duration of aeration have given controversial results and fisg farmers generally limit themselves to an aeration of 6-9 hours a night.
In French conditions only some case by case studies, taking into account the technical and economical aspects (valorisation of production), are capable of recommending night aeration or not, a practise which seems reserved, at the present time, only for exploitation systems which are already slightly intensified.


 


Dimensioning


In intensive aquaculture, the “norm” when it comes to the size of an aeration installation is to plan a power of 2.5-3 kW/ha for a high intensity of fish, leading to a feeding rate of 50 kg/hectare/day.
In practice, installed power varies between 1 and 5 kW/ha in ponds of more than 0.5 hectares having a production of 5 to 12 tonnes of fish per hectare.
These dimensions “guarantee” the survival of the fish in normal intensive aquaculture conditions.
However, in particularly unfavourable conditions (algal bloom) leading to severe nocturnal de-oxygenation, the application of this norm does not exclude the use of extra aerators.
In the economic plan of things, intensive aquaculture in ponds is too recent an activity in France to provide reliable data.
We can point out only that, in American conditions, hydro-ejectOrs are the most efficient for the aeration of ponds with a size less than 0.4 hectares.
For bigger ponds, it is the aerators with blades which have been shown to be the most effective.


To contact AQUALOG


aqualog@aqualog.fr                Tel. : 0033 (0)4 94 10 26 26




 


 







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Aqualog
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