Microbiology and technology of denitrification

In clo­sed aquacul­tu­re – e.g. in sys­tems with a water exchan­ge of less than 10% per day – the use of deni­tri­fi­ca­ti­on is essen­ti­al (read more in our blog post Nitri­fi­ca­ti­on and Deni­tri­fi­ca­ti­on). Deni­tri­fi­ca­ti­on con­verts the nitra­te (NO3) con­ver­ted from fish excre­ments (ammonium/ammonia) during nitri­fi­ca­ti­on with the help of spe­cial bac­te­ria, the so-cal­led deni­tri­fiers. Mole­cu­lar nitro­gen (N2) is for­med from the nitra­te in the absence of oxy­gen, which then escapes from the plant in gas­eous form. This is com­ple­te­ly harm­less to the envi­ron­ment, sin­ce the quan­ti­ties pro­du­ced are very small and the atmo­s­phe­re alrea­dy con­sists of almost 80% nitrogen.

Deni­tri­fi­ca­ti­on is thus an important sub-pro­cess in main­tai­ning water qua­li­ty and thus con­tri­bu­tes to less stress and mor­ta­li­ty among fish. Fur­ther­mo­re, it enab­les par­ti­cu­lar­ly low water exchan­ge rates, which are bene­fi­cial to the environment.

The deni­tri­fi­ca­ti­on pro­cess is rela­tively deman­ding and dif­fi­cult to con­trol. This means that every detail is important in the con­struc­tion of such a filter:

Biological aspects

Deni­tri­fi­ca­ti­on works only under oxy­gen exclu­si­on. This is becau­se deni­tri­fiers take oxy­gen from the nitra­te (NO3) (reduc­tion) and, tog­e­ther with a car­bon source (C‑source), con­vert it to more avail­ab­le ener­gy. A dis­tinc­tion can be made bet­ween deni­tri­fi­ca­ti­on with auto­tro­phic or hete­ro­tro­phic bac­te­ria. Auto­tro­phic deni­tri­fiers use the car­bon com­pounds natu­ral­ly pre­sent in water as a C‑source, whe­re­as hete­ro­tro­phic deni­tri­fiers use a car­bon source such as ace­tic acid (CH3COOH). This makes deni­tri­fi­ca­ti­on fil­ters more effi­ci­ent, sin­ce rea­di­ly avail­ab­le C‑sources can be used and added very pre­cise­ly matched to the pro­cess. Howe­ver, mea­su­ring tech­no­lo­gy must always be used to pre­vent over­do­sing of the C‑source, as this can have nega­ti­ve con­se­quen­ces for the ent­i­re recir­cu­la­ti­on sys­tem. In this case, auto­tro­phic deni­tri­fi­ca­ti­on has an advan­ta­ge des­pi­te signi­fi­cant­ly poo­rer efficiency.

Technical aspects

Deni­tri­fi­ca­ti­on can be car­ri­ed out as a fixed, floa­ting bed or floc reac­tor. Fixed-bed deni­tri­fi­ca­ti­on is most­ly auto­tro­phic, as the bac­te­ria both adhe­re to the car­ri­er mate­ri­al used for this pur­po­se and also break it down to use it as an ener­gy source. With floa­ting bed deni­tri­fi­ca­ti­ons, fil­lers are usual­ly made out of plastic, which can move free­ly in the reac­tor by means of a pump or agi­ta­tor. The­se fil­lers are only used to adhe­re on by the bac­te­ria and are not decom­po­sed. A car­bon source is added to a floa­ting bed reac­tor to gene­ra­te ener­gy. In a floc deni­tri­fi­ca­ti­on, the bac­te­ria accu­mu­la­ti­ons float free­ly in the water. The­re are no fil­lers or simi­lar to which they can adhe­re. The reten­ti­on of the bac­te­ria in the reac­tor is done by a fil­ter at the out­let. Most­ly this vari­ant is also ope­ra­ted hete­ro­tro­phi­cal­ly, e.g. with dosing of a car­bon source.

Constructive aspects

The deni­tri­fiers use part of the ener­gy to build up their own bio­mass. The­re­fo­re, it is a gre­at con­struc­ti­ve chal­len­ge that both the con­stant­ly gro­wing amount of bio­mass can be remo­ved in the mean­ti­me (e.g. drai­ning the sedi­ment at the bot­tom of the tank) and the most homo­ge­ne­ous mixing and thus nut­ri­ent sup­ply of the reac­tor is pos­si­ble. Espe­cial­ly thick and old bio­films block the reac­tor and its tubes. In addi­ti­on, the bac­te­ria in such a bio­film can hard­ly reach nitra­te or car­bon, so that the decom­po­si­ti­on capa­ci­ty of the reac­tor col­lap­ses for lack of nutrients.

No mat­ter which type of deni­tri­fi­ca­ti­on is used, each has its own advan­ta­ges and dis­ad­van­ta­ges. It is par­ti­cu­lar­ly important that the pro­cess of deni­tri­fi­ca­ti­on is care­ful­ly con­trol­led. Poor design or incor­rect­ly set para­me­ters can affect ani­mal wel­fa­re. The­se inclu­de unde­s­i­ra­ble by-pro­ducts such as nitri­te or hydro­gen sul­fi­de, which can be pro­du­ced in the event of car­bon under- or overdosage.

In the SEAWATER Cube, we rely on a floa­ting bed deni­tri­fi­ca­ti­on pro­cess with fil­lers spe­cial­ly adap­ted and shaped for this pro­cess. Due to the hig­her effi­ci­en­cy, an exter­nal car­bon source is also used. Auto­ma­ti­on con­stant­ly moni­tors and con­trols the deni­tri­fi­ca­ti­on pro­cess in order to best avoid risks to water qua­li­ty and ani­mal welfare.

You need a deni­tri­fi­ca­ti­on tailo­red to your plant or you have any questions?

Further informationen about Denitrifaction

Check out more facts about the tech­no­lo­gy. If you have any fur­ther ques­ti­ons, plea­se con­ta­ct us!

Refe­ren­ces

— van Rijn, J.; Tal, Y.; Schrei­er H. J.: Deni­tri­fi­ca­ti­on in recir­cu­la­ting sys­tems: Theo­ry and app­li­ca­ti­ons. Else­vier B.V., 2005
— Lom­pe, D.; Wies­mann, U.: Bio­lo­gi­sche Deni­tri­fi­ka­ti­on nitrat­hal­ti­ger Abwäs­ser und Grund­wäs­ser. VCH Ver­lags­ge­sell­schaft mbH, Wein­heim, 1991
— Lee, P. G.; Lea, R. N. et al.: Deni­tri­fi­ca­ti­on in aquacul­tu­re sys­tems: an examp­le of a fuz­zy logic con­trol pro­blem. Else­vier Sci­ence B.V., 2000
— Soares, M. I. M.: Bio­lo­gi­cal deni­tri­fi­ca­ti­on of ground­wa­ter. Klu­wer Aca­de­mic Publis­hers, 2000.

Image source
SEAWATER Cubes