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ACE Recirculation systems for Eel farming

View of Eel ongrow system
Drumfilter

1 Introduction

ACE has build and designed 5 Eel farms over the last 4 years, ranging from a production capacity of 20 tons per year to 200 tons per year. These farms included 17 separate recirculation systems, either for raising glasseel to a size of 2-5 grams or for ongrowing of the 2-5 gram Elvers to market weights of 135-200 gram.

The system design is based on experience with Anguilla Anguilla, Anguilla mossambica, Anguilla bicolor and Anguilla marmorata and has been proven and perfected in practice. The systems are technically simple, very efficient with energy and easy to work with. More important, the Eel is very happy with the design, as proven by record growth rates, low food conversions and low incidence of disease and mortality.

 

2 System description

2.1 General

An eel recirculation system consists of round culture tanks and a watertreatment system. Centrifugal pumps move the water every 15-25 minutes from the fish tanks through the watertreatment and back.  This recirculation set-up allows for complete control of the culture environment, especially temperature, making sure that the culture environment is optimal throughout the year. It also allows farming to be done in high densities. As a result growth, food conversion and survival are better than in for example pond culture.

ACE designs and builds these systems for customers all over the world, not only for Eel, but also for, for example Tilapia, Seabream, Catfish and Pikeperch. Design and construction may also include the building and additional facilities like the well water supply; power generation or production of pure oxygen.


2.2 Culture tanks
The culture tanks vary in size from 2 meter diameter to 6 meter diameter, with water depths ranging from 70 to 100 cm. Each tank is equipped with an automatic feeder; either a rotofeeder or a pendel feeder. Rotofeeders are used to automatically dispense food and are used for small Eels. Pendel feeders are operated by the fish themselves and are used for the larger Eels.

The tanks are equipped with screens to prevent the Eels from swimming into the water inlet, as well as the water outlet. Brushing machines are used to prevent the fine outlet screens of glasseel and Elver tanks from clogging. The top of the tank wall is equipped with a baffle, to make sure Eels can not climb out of the tanks. Each tank is equipped with a diffuser hose to dispense pure oxygen, with an emergency overflow, with a water level sensor and with a standpipe that is used to move water and Eels to the grader.

2.3 Watertreatment

From the fish tanks the waste water flows to a drumfilter. The drumfilter sieves feces and other large particles out of the water, using a mesh size of 40 micron. The waste water then enters a pump sump, from where the water is pumped into the trickling biofilter by means of 2-3 centrifugal pumps. The trickling filter utilizes plastic media with a a surface area of 220 m2 per m3 that serves as attachment surface for the bacteria that remove ammonia and dissolved organic matter from the culture water.

Ventilation through the trickling filter takes care of carbondioxide stripping and at the same time adds oxygen. Ventilation also allows for increased evaporation of water, which in summer helps to cool the water.

After the water has passed the biofilter, pure oxygen is added in the patented Oxygen reactor. This reactor is characterized by the fact that it requires minimum pressure to operate, avoiding the need for a second pump step.

UV disinfection is sometimes incorporated in the watertreatment process, to keep bacterial counts low. Our experience with Eel is that UV disinfection is not needed.

The systems incorporate and automatic oxygen control system that measures oxygen concentrations in the water and decides how much pure oxygen to supply to the oxygen reactors. Another important component is the alarm system. Critical parameters are automatically monitored. If a threshold level is reached this triggers the alarm system to give a visible and audible alarm in the farm and to call out to a cell phone to warn the manager.

2.4 System performance
In the latest 100 tons production unit of Anguilla anguilla we received 475 kg glasseel in February 2010. Now, by the end of August the biomass has grown to a total weight of roughly 25 tons with a survival from glasseel of 94%. The average weight is now approximately 18 gram, but a large fraction of the fish already passed 60 gram. The average food conversion is still below 1.1 kg food per kg production, which is extremely good for this species. Daily food gift in the system already grew to 450 kg per day, which translates into a production that will easily exceed 100 tons.

The 100 tons unit has a total power requirement of 47 kW and a consumption of 750 gram oxygen per kg food and 250 liter water per kg food. Total production cost of the Eel is expected to be around 6 Euro per kg.
 

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