Recirculating aquaculture systems (RAS)

The cul­ti­va­ti­on of fish on land has been great­ly opti­mi­zed over the past cen­tu­ries. In par­ti­cu­lar, the clo­sed recir­cu­la­ting aquacul­tu­re sys­tem (RAS) has now estab­lished its­elf along­side tra­di­tio­nal aquacul­tu­re methods such as pond sys­tems, net cages and flow-through sys­tems. In order to get a con­cre­te over­view of the dif­fe­ren­ces bet­ween the­se methods, the cha­rac­te­ristics of tech­no­lo­gi­cal recir­cu­la­ting aquacul­tu­re sys­tems are com­pared with tra­di­tio­nal bree­ding methods. In addi­ti­on, the SEAWATER Cube is used to explain the con­s­truc­tion of an inno­va­ti­ve clo­sed recir­cu­la­ti­on sys­tem in detail in the indi­vi­du­al pro­cess steps.

Traditional aquaculture vs. high-technological recirculation systems

If you compa­re exten­si­ve or semi-inten­si­ve methods such as pond sys­tems, net cages and flow-through sys­tems with clo­sed recir­cu­la­ti­on sys­tems, the dif­fe­ren­ces quick­ly beco­me clear with regard to the fol­lo­wing over­view. From an eco­lo­gi­cal point of view, the RAS have a clear advan­ta­ge and also with regard to the regio­na­li­ty of pro­duc­tion and the reduc­tion of emis­si­ons the­se sys­tems can score points.

Design of a closed recirculation system using the example of the SEAWATER Cube

The SEAWATER cir­cuit, as a clo­sed cir­cuit for rea­ring fish, is com­po­sed of 10 essen­ti­al com­pon­ents for water tre­at­ment. The fish bea­ring tank, the drum fil­ter, the cir­cu­la­ti­on pump and the bio­fil­ter with aer­a­ti­on by the com­pres­sor are loca­ted in the main cir­cuit. Also, the skim­mer with addi­ti­on of ozone by the ozone gene­ra­tor, the CO2 desorp­ti­on and the deni­tri­fi­ca­ti­on are in the main cir­cuit, but they are only char­ged with a part of the water flow. The sedi­men­ta­ti­on is loca­ted in a bypass of the drum fil­ter and the fish bea­ring tank.

  1. The fish bea­ring tank is com­ple­te­ly fil­led with water and ser­ves as habi­tat for the ani­mals. Fish feed and oxy­gen are put into it. In addi­ti­on, the excre­ments of the fish (feces, CO2, ammo­nia or ammo­ni­um) are trans­por­ted by the strong cur­rent of the water to the first com­po­nent of the water tre­at­ment. Rele­vant and mea­sura­ble para­me­ters are among others the sali­ni­ty (in salt­wa­ter aquacul­tu­re), tem­pe­ra­tu­re, pH-value and redox poten­ti­al as well as oxy­gen content.
  2. The drum fil­ter is used to dischar­ge solids and par­tic­les gene­ral­ly lar­ger than 40µm (depen­ding on the mesh size of the fil­ter gauze)
  3. The par­tic­le-con­ta­mi­na­ted back­wa­sh water is led into the sedi­men­ta­ti­on for water recla­ma­ti­on. The­re the par­tic­les sett­le on the bot­tom. This for­med sludge is spe­ci­fi­cal­ly remo­ved from the sys­tem, whe­re­as the cla­ri­fied water in the upper part of the sedi­men­ta­ti­on is retur­ned to the system.
  4. The cir­cu­la­ti­on pump trans­ports the water of the sys­tem from the pump well, with the drum fil­ter insi­de, into the water tre­at­ment com­pon­ents and thus gene­ra­tes the neces­sa­ry flow for the water treatment.
  5. The bio­fil­ter uses the bio­lo­gi­cal pro­cess of nitri­fi­ca­ti­on to con­vert the ammo­nia or ammo­ni­um excre­ted by the fish first to nitri­te and then to nitra­te. This task is per­for­med by auto­tro­phic, aero­bic bac­te­ria, which adhe­re to pla­s­tic car­ri­ers (car­ri­ers or fil­ter pel­lets) by forming a biofilm.
  6. This for­mer pro­cess requi­res oxy­gen, which is trans­por­ted by the com­pres­sor into the fil­ter component.
  7. The ozone gene­ra­tor sup­ports the pro­cess of foam for­ma­ti­on in the sub­se­quent step and pro­mo­tes the detach­ment of pro­te­ins from par­tic­le surfaces.
  8. The skim­mer is a phy­si­cal­ly working device, also cal­led „flo­ta­ti­on“ or „pro­te­in skim­mer“. In this pro­cess step, sur­face-acti­ve pro­te­in com­pounds are adhe­red to the air bubbles intro­du­ced in a coun­ter­cur­rent pro­cess. This pro­du­ces a foam for­ma­ti­on. The foam is then dischar­ged with the ampho­ly­tic sub­s­tances, viru­s­es and par­tic­les (e.g. bac­te­ria) adhe­ring to it. In the skim­mer all small par­tic­les (< 40 µm) are remo­ved from the water which could still pass the drum filter.
  9. CO2 desorp­ti­on is a degas­sing com­po­nent to remo­ve the car­bon dioxi­de excre­ted by fish and bac­te­ria into the water. In addi­ti­on, the out­side air intro­du­ced in the CO2 desorp­ti­on ser­ves to cool the water.
  10. The anae­ro­bic bio­fil­ter, the so-cal­led deni­tri­fi­ca­ti­on, also works with bac­te­ria and cor­re­spon­ding car­ri­ers. Deni­tri­fi­ca­ti­on essen­ti­al­ly repres­ents the con­ver­si­on of nitra­te to ele­men­tal nitro­gen (N2) and works only at low oxy­gen con­cen­tra­ti­ons (approx. < 0.1 mg/L; anae­ro­bic con­di­ti­ons = under oxy­gen exclu­si­on). The nitro­gen is released into the ambi­ent air.

Final­ly, if you take a look at the dif­fe­rent sys­tems, it can be sta­ted that open aquacul­tures such as pond sys­tems are pro­ba­b­ly the „ide­al“ in the minds of con­su­mers due to their bio­di­ver­si­ty. Due to the enorm­ous amount of space requi­red, the­se bree­ding methods can­not be used to sus­tain­ab­ly increase the popu­la­ti­on in Ger­ma­ny. The high los­ses cau­sed by dise­a­ses, cli­ma­tic extre­mes or fish-eating ani­mals are also problematic.

The decisi­ve advan­ta­ge of clo­sed recir­cu­la­ti­on sys­tems is their inde­pen­dence from exter­nal influen­ces. Whe­re up to now main­ly lar­ge-sca­le plants were sta­te of the art, alter­na­ti­ve con­cepts are slow­ly deve­lo­ping. The SEAWATER Cube is an exam­p­le of a modern, com­pact and stan­dar­di­zed plant sys­tem. It enables the pro­duc­tion of ups­ca­le edi­ble fish, which small to medi­um-sized com­pa­nies can offer for sale regio­nal­ly. In addi­ti­on to the small space requi­re­ment, no major con­s­truc­tion and instal­la­ti­on work is requi­red for the com­mis­sio­ning of the plant. With inno­va­ti­ve com­pon­ents, we always ensu­re a very good water qua­li­ty in the plant and can thus great­ly redu­ce the occur­rence of dise­a­ses. In the SEAWATER Cube, the use of anti­bio­tics is com­ple­te­ly avo­ided and the important bac­te­ri­al cul­tures in the bio­lo­gi­cal fil­ters are spared. Fur­ther­mo­re, with this sys­tem we crea­te the pos­si­bi­li­ty of a trans­pa­rent pro­duc­tion, which the­r­e­fo­re offers the con­su­mer more trans­pa­ren­cy and crea­tes a pro­xi­mi­ty to the product.

Aquacul­tu­re is the fas­test-gro­wing area of food pro­duc­tion. Inves­t­ing in sus­tainable fish far­ming methods is wort­hwhile, as this is the only way to con­ser­ve the world’s oce­ans and make a decisi­ve con­tri­bu­ti­on to pro­tec­ting natu­ral stocks.


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— Lekang, O.-I.: Aquacul­tu­re Engi­nee­ring. Second Edi­ti­on, John Wiley & Sons, Ltd., 2013.

— Mar­tins, C. I. M.; Eding, E. H.; Ver­de­gem, M. C. J.; Heins­br­oek. T. N. et al: New deve­lo­p­ments in recir­cu­la­ting aquacul­tu­re sys­tems in Euro­pe: A per­spec­ti­ve on envi­ron­men­tal sus­taina­bi­li­ty. Aquacul­tu­ral Engi­nee­ring, Else­vier B.V., 2010.

— Orel­la­na, J.; Wal­ler, U.; Wecker, B.: Cul­tu­re of yel­low­tail king­fi­sh (Serio­la lalan­di) in a mari­ne recir­cu­la­ting aquacul­tu­re sys­tem (RAS) with arti­fi­ci­al sea­wa­ter. Else­vier B.V., 2013.

— Stein­bach, C.: Ent­wick­lung eines Sub­stra­tes für Fließ­bett-Deni­tri­fi­ka­ti­ons­stu­fen in Fluid­kreis­läu­fen der Mari­kul­tur. Mas­ter­the­sis, Hoch­schu­le für Tech­nik und Wirt­schaft des Saar­lan­des, 2014.

— Schaar, S.: Aus­le­gung eines deni­tri­fi­zie­ren­den Bio­fil­ters in einer geschlos­se­nen Kreis­lauf­an­la­ge für Wolfs­bar­sche. Bache­lor­the­sis, 2019.

—–15OE026-naturland-bergleiter-2017-kreislaufanlagen-aquakultur.pdf, auf­ge­ru­fen am 02.10.2019

—  Stav­ra­ki­dis-Zachou, O.; Ernst, A.; Stein­bach, C.; Wag­ner, K.; Wal­ler, U.: Deve­lo­p­ment of deni­tri­fi­ca­ti­on in semi-auto­ma­ted moving bed bio­film reac­tors ope­ra­ted in a mari­ne recir­cu­la­ting aquacul­tu­re sys­tem. Sprin­ger Natu­re Switz­er­land, 2019.

— van Rijn, J.: Was­te tre­at­ment in recir­cu­la­ting aquacul­tu­re sys­tems. Aquacul­tu­ral Engi­nee­ring, Else­vier B.V., 2012.