Recirculation aquaculture systems; a big step towards sustainability

The principles
The principle of recirculation aquaculture systems (RAS) is treatment of the culture water by suspended solid removal, biological filtration and application of oxygen. For many species temperature control is also practiced to always have optimal conditions for the fish. Such systems are intensive and expensive by nature and have a high output. Over the years RAS have been further optimised allowing a very low water and energy consumption. By using heat exchangers the warmth of the out going water and even the air can be regained. Since such systems are closed RAS offer unparalleled opportunities for health management and disease control. Another benefit is the total prevention of escapees from the farm so that no interbreeding with wild stocks can occur. Since the waste produced during the farming process is broken down and removed the impact on the environment is minimised and under control.

Water treatment
Fish produce solid faeces and dissolved un-ionised ammonia (NH3) as waste products. The water treatment system will breakdown and remove the waste to maintain a high water quality.
The first step of water treatment is solid particle removal with a mechanical filter e.g. a drumfilter or a sedimentation filter. Sedimentation filters remove, depending on efficiency, particles of 100 µm and more. Drumfilters remove particles depending on mesh size, usually of 60 µm and more. The primary objective of the biofilters is breaking down NH3 which in water takes the form of ionised ammonia (NH4). In moving bed filters or trickling filters nitrifying bacteria oxidise NH4 via nitrite (NO2) to nitrate (NO3) according to below equation:

Nitrification (aerobic process);
NH4+ + 1.5 O2 => NO2- + H+ + H2O (by Nitrosomonas bacteria)

NO2- + 0.5 O2 => NO3- (by Nitrobacter bacteria)

Total equation: NH4+ + 2 O2 => NO3- + H+ + H2O

In a submerged biofilter the remaining small particles are trapped and used to fuel the breakdown of the during nitrification formed NO3 to nitrogen gas (N2). This second biological step, denitrification, requires anoxic conditions and is performed by a variety of heterotrophic bacteria.

Denitrification (anoxic process);
5CH2O2 + 4 NO3- + 4 H+ => 2N2 + 5CO2 + 7 H2O

With the forming of atmospheric nitrogen (N2) the nitrogen cycle is completed and the dissolved waste eliminated.

Feed for RAS
Feeds for RAS have to meet high standards. Even more so since many farms use feeding systems that transport the pellets from silos to the feeders of individual tanks. This inevitably leads to friction and may result in the forming of dust. Dust can not be eaten by the fish and puts unnecessary pressure on the filters. At Coppens we therefore put extra emphasize on pellet durability and not only on pellet hardness. Next to that the pellets are made very water stable so that leaking of nutrients and pollution is prevented. By choosing only highly digestible ingredients the fish can utilise the feed to the maximum. As a consequence low feed conversion ratio's (FCR) are reached. The FCR is crucial not only for the farms performance but it also determines directly the amount of faeces or waste the fish creates. Furthermore, at the Coppens Research centre the protein/energy ratio's for RAS feeds have been fine tuned. By balancing this ratio the dietary protein is used for growth and not to supply energy. This protein sparing effect minimises the NH3 output of the fish and improves performance. In this way the RAS can handle more feed with a better performance of the fish and less waste.

Faeces binding
One of the latest developments to adapt fish feed for use in RAS is the application of components that bind the faeces of fish. Some of these components have the added benefit that they make faeces firmer and prevent disintegration and leaking of e.g. nitrogen and phosphorous. This can be of great importance since organic matter is transported through the RAS until the filters break it down. When waste disintegrates and dissolves it becomes increasingly difficult to eliminate.
During an evaluation at the Coppens Research Centre the best faeces binding option was selected for a long term field test. In such a trial the effect of faeces binding can be evaluated under intensive practical conditions over time. In a large commercial RAS catfish farm daily water analysis where made during the trial in order to follow the water quality. Also daily water samples where collected to check the visual water quality like transparency and the amount of sedimentation.
One of the first signs of improvement was less foam building in the RAS. Also the fish where showing a more eager eating behaviour apparently enjoying better conditions. The water analyses showed an improved nitrification reflected in lower values for NH4 and NO2. At the same time the water samples showed a higher transparency and, step by step, less sediment. Clearly the improved particle removal by the mechanical filter had a beneficial effect on the biofilters. Over the course of weeks the water quality kept on improving till it became stable. The feed intake of the fish increased due to the better conditions in the farm while growth and feed conversion where unaffected. The use of the faeces binder clearly improved the water quality and reduced pollution considerably. With the current line of feeds for recirculation Coppens can make an important contribution towards sustainable fish farming.