Protein skimmers in recirculation systems

Effec­ti­ve water tre­at­ment in aquacul­tu­re can­not do without the work of bac­te­ria, becau­se in spe­cial­ly built bio­fil­ters they often con­vert toxic excre­ments of the fish into ani­mal and envi­ron­ment­al­ly friend­ly sub­s­tan­ces. Examp­les are the pro­ces­ses of nitri­fi­ca­ti­on and deni­tri­fi­ca­ti­on. Howe­ver, if bac­te­ria are pre­sent in lar­ger num­bers in the bea­ring tank, they can cau­se unde­s­i­ra­ble side effects such as dete­rio­ra­ti­on of water qua­li­ty or the occur­rence of dise­a­ses. The­re­fo­re, in clo­sed recir­cu­la­ti­on sys­tems, mecha­ni­cal puri­fi­ca­ti­on sta­ges such as the skim­mer are used in addi­ti­on to bio­lo­gi­cal fil­ters. The pro­te­in skim­mer uses mecha­ni­cal and bio­che­mi­cal pro­ces­ses to ren­der the bac­te­ria harm­less and remo­ve them from the water circuit.
How does the skimmer work?

The skim­mer uses the flota­ti­on pro­cess to remo­ve smal­lest par­ti­cles and bac­te­ria. During flota­ti­on, very fine gas bub­bles are intro­du­ced in the bot­tom area of the device. It app­lies that with hig­her salt con­tent of the water finer gas bub­bles are for­med. Only from a salt con­tent of about 1.2% the use of a skim­mer makes eco­no­mic sen­se. The advan­ta­ge of finer gas bub­bles is the com­pa­ra­tively lar­ger reac­ti­ve sur­face. Both the­se fine gas bub­bles and the bac­te­ria are electri­cal­ly char­ged on their outer sur­faces and the­re­fo­re stick tog­e­ther. At the same time, the gas bub­bles in the water of the reac­tion cham­ber rise upwards due to their buoyan­cy and ther­eby trans­port the atta­ched par­ti­cles. By tape­ring in the head area of the reac­tion cham­ber, the gas bub­bles are collec­ted at the water sur­face and a foam is crea­ted. This foam pas­ses over the so-cal­led foam pot, wher­eby the bac­te­ria are remo­ved from the water cir­cuit tog­e­ther with the gas bub­bles. The foam collects in a chan­nel and col­lap­ses the­re over time. What remains is a sludge which is rin­sed out as was­te­wa­ter and thus con­tri­bu­tes to the loss of water in recir­cu­la­ti­on systems.

Inac­ti­va­ti­on of bac­te­ria by ozone

The effect of flota­ti­on can be impro­ved by adding ozone (O3). Ozone is pro­du­ced as requi­red in the so-cal­led ozone gene­ra­tor. The demand is main­ly based on the dai­ly amount of feed. In the ozone gene­ra­tor ozone (O3) is pro­du­ced from the oxy­gen (O2) in the ambi­ent air by means of high-vol­ta­ge elec­tro­des and the resul­ting electri­cal dischar­ge. The use of ozone sup­ports the hygie­niz­a­ti­on of the water in recir­cu­la­ti­on sys­tems in two ways:

  1. Ozone is a very reac­ti­ve sub­s­tance and che­mi­cal­ly breaks the bac­te­ri­al shell, kil­ling the bacteria.
  2. The sub­s­tan­ces from the insi­de of the bac­te­ria lead to a bet­ter foam for­ma­ti­on, wher­eby more par­ti­cles can be remo­ved at the same time.

During its reac­tion, the ozone decom­po­ses back into oxy­gen so that no resi­du­al sub­s­tan­ces remain in the water.

Tas­te advan­ta­ges through skimming

The instal­la­ti­on of a skim­mer in aqua­far­ming sys­tems brings not only clear water and the asso­cia­ted health aspects for the fish. It has also tas­te advan­ta­ges for the end pro­duct. Fish farms – both in ponds and indoor sys­tems – often have an off-fla­vor. This ear­thy, mus­ty tas­te is cau­sed by geos­min. Geos­min is pro­du­ced by cer­tain micro-orga­nisms and is the­re­fo­re often asso­cia­ted with tur­bid, par­ti­cle- or bac­te­ria-con­ta­mi­na­ted water. Fish with off-fla­vor are not par­ti­cu­lar­ly appe­aling in tas­te. The easiest way to remo­ve the off-fla­vor is to keep the ani­mals in sepa­ra­te tanks for several days or weeks, usual­ly with a con­ti­nuous high sup­ply of fresh water and without the addi­ti­on of food. A pro­cess which is very time- and resour­ce-inten­si­ve and which hides the usual­ly rather unfa­vor­able living con­di­ti­ons. The use of a skim­mer is the­re­fo­re an attrac­ti­ve alternative.

Conclusion

In the SEAWATER Cube we use the ozone-sup­por­ted skim­mer tech­no­lo­gy, which remo­ves not only bac­te­ria but also geos­min. Fish always swim in clear, almost uncon­ta­mi­na­ted water and can the­re­fo­re be taken direct­ly from the pro­duc­tion tank and pro­ces­sed without any advan­ce plan­ning or hol­ding out. This redu­ces stress for the ani­mals, saves working time and pro­du­ces the tas­tiest pos­si­ble end pro­duct. In addi­ti­on, ill­nes­ses and the use of medi­ca­ti­on are com­ple­te­ly avoided, pro­vi­ding the con­su­mer with a hig­her qua­li­ty food pro­duct and saving valu­able raw mate­ri­als such as ener­gy and water. Through the com­bi­na­ti­on with our bio­lo­gi­cal fil­ter pro­ces­ses, we achie­ve a total water recy­cling rate of 99%, so that only 1% (= 500 l) of was­te­wa­ter is pro­du­ced per day.

Refe­ren­ces:

— Image source: SEAWATER Cubes
— Wil­helm, S.: Was­ser­auf­be­rei­tung – Che­mie und che­mi­sche Ver­fah­rens­tech­nik. 7., aktua­li­sier­te und ergänz­te Auf­la­ge; Sprin­ger-Ver­lag Ber­lin Hei­del­berg, 2008.
—  Schwis­ter, K.; Leven, V.: Ver­fah­rens­tech­nik für Inge­nieu­re. 2., aktua­li­sier­te Auf­la­ge; Carl Han­ser Ver­lag Mün­chen, 2014.
—  Tim­mons, M., Guer­dat T., Vin­ci B. (2018) “Recir­cu­la­ting Aquacu­lu­re”. Itha­ca, NY.