Spiliotopoulou, Aikaterini

Nedgangen i den vilde fiskebestand og den stigende efterspørgsel efter mad og fiskeprotein har ført til fiskeopdræt i det marine miljø. Den intense fiskeproduktion i snævre rum (fx havbure) har medført forringelse af vandmiljøet (næringsopbygning fra ufordøjet foder og udskillelse forårsaget af eutrofiering, forurening, kemiske forbindelser, sygdomsudbrud, undslip osv.), som har givet videnskabelig og offentlig opmærksomhed. Miljøpåvirkningen fra akvakultur i kombination med konkurrencen om arealanvendelse (fritidsaktiviteter, æstetisk påvirkning, turisme, boligudvikling mv.) og ønsket om rentabel virksomhed har bidraget til et skifte tilbage mod landbaserede akvakultursystemer.Recirkulerende akvakultursystemer (RAS) giver en stabil fiskeproduktion. På grund af det kontrollerede produktionsmiljø, hvor vandkvalitetsstyringssystemer og affaldsbehandlingsteknologier er implementeret, kan fiskekvaliteten og væksten forøges, mens risikoen for fiskeundslip og diverse uheld minimeres. Men på grund af den høje recirkulationen (op til 99 %) og den øgede strømning akkumuleres affald fra fiskefoder og fiskesekreter. Dermed bliver kontrollen af organisk stof i RAS af stor betydning for at sikre en god forvaltning af anlægget. Høj organisk stofbelastning i vandet forringer vandkvaliteten, hvilket favoriserer den mikrobielle blomstring, der kan føre til sygdomsudbrud. Derudover vil øgede niveauer af organisk materiale reducere effektiviteten af de forskellige vandbehandlingsteknologier.Flere teknologier er blevet installeret for at fjerne partikler og organisk materiale fra vandet. Imidlertid er vandbehandlingsteknologierne ikke fuldt optimerede, og interaktionen mellem dem ikke forstået. Ozon er blevet implementeret i flere vandbehandlingssystemer så som spildevandsrensning, svømmebassiner mv., som et sekundært behandlingstrin for at forbedre vandkvaliteten. Derudover har ozon bakteriedræbende egenskaber, der sikrer desinfektion. Dog er ozon giftig for vandlevende organismer i ekstremt lave koncentrationer (0.01 mg O3/l). Frygt for at miste fisk fra overdosering af ozon hæmmer brugen af ozonbehandling i RAS.Formålet med denne ph.d.-afhandling er at designe et ozonsystem til RAS baseret på ozonbehovet i det specifikke recirkulerede akvakultur system, hvor vandkvaliteten forbedres uden at kompromittere fiskens velfærd. Herudover har målet været, at kunne overvåge ozonkoncentrationen i systemet med en ny, præcis, realtids målemetode baseret på fluorescens-følsomhed overfor ozon. Det er velkendt, at vandopløst organisk stof har fluorescens egenskaber og er meget reaktivt med ozon. Vandprøver indsamlet fra flere anlæg omkring Danmark blev analyseret med fluorescerende spektroskopi for at bestemme RAS-vandets fluorescerende karakter og for at identificer ligheder i fluorescensbestanddelene i vandprøver fra de forskellige RAS. Prøverne blev derefter udsat for ozonering og efterfølgende analyse med fluorescensmetoden for at validere ozon-effekten. Det viste sig, at bestanddele med ensartede fluorescensaktiviteter var til stede i vandprøver fra alle involverede RAS, men med forskellig ozon-reaktivitet. Dette tyder på, at en fluorescens baseret sensor kan anvendes som et indirekte ozondoserings-bestemmelsesværktøj i vand, da fluorescensintensitet og det opløste organiske materiales nedbrydning ved ozonering var godt korreleret. Potentielle anvendelsesområder af en sådan ozon-måle-metode er også blevet vurderet.For at designe et optimalt ozonsystem skal vandet fra det specifikke anlæg analyseres med hensyn til ozonforbrug og ozonlevetid. Hvert anlæg er unikt, da fiskeart, foderstrategi, tætheder/biomasse, vandbehandlingseffektivitet og andre driftsbetin¬gel¬ser varierer meget, og som resultat giver en vandmatrix der varierer – ikke blot fra anlæg til anlæg, men også over produktionscyklus på det aktuelle anlæg. Ozonreaktionerne i det specifikke vand bør undersøges inden installationen for at sikre, at mængden af ozon, der kræves for at forbedre vandkvaliteten, er tilstrækkelig og ozon vil blive nedbrudt længe før det når biofilterne eller kulturtankerne.Efter at ozonforbrug og kinetik blev eksperimentelt bestemt, blev de forudbestemte ozondoser anvendt i pilotskala-RAS for at verificere effekten af ozondoser på vandkvaliteten af kontinuert anvendte ferskvands-RAS. Flere vandparametre blev undersøgt, herunder fluorescensorganisk stof, ikke-flygtigt organisk materiale, kemisk iltforbrug, biologisk iltbehov, ammoniak, nitrat og nitritniveauer, partikelantal og -størrelse og mikrobiel vandkvalitet. Den samlede vandkvalitet blev signifikant forbedret ved ozonering, proportional med den anvendte ozondosering. Under ozonprøverne blev der ikke observeret fiskedødelighed.Vandmatrixen har stor indflydelse på udformningen af et ozonsystem. I havvand, hvor bromid er naturligt til stede, dannes bromoxider, der er giftige for fisk, i tilstedeværelse af ozon. Bestemmelsen af ozon i havvandsprøver er mere kompliceret end i ferskvand. Forsøg på at måle hurtig ozon i bromeret vand blev lavet som grundlag for en modificeret analysemetode. Yderligere undersøgelser er nødvendige for at øge nøjagtigheden og for at verificere det kritiske punkt mellem ozon- doseringen, der er nødvendig for at forbedre vandkvaliteten og undgå dannelsen af bromerede biprodukter i havvands-RAS.Afslutningsvis konkluderer denne ph.d.-afhandling, at når ozon er korrekt implementeret i RAS, da vil vandkvaliteten blive bemærkelsesværdigt forbedret, når der tages hensyn til ozonforbruget og ozons levetid for det pågældende system. Til bestemmelse af ozonbrug og ozons levetid blev fluorescensspektroskopi anvendt og det viste sig at være en god indikator for akkumulering af organisk stof i RAS og meget følsom over for ozonbehandling. Den detaljerede analyse af det opløste organiske stof i fluorescens optaget i RAS-vand afslørede fire uafhængigt varierende fraktioner med forskellig reaktivitet og reaktion over for ozon, hvilket tyder på, at en fluorescensbaseret sensor rettet mod en specifik bølgelængdeovergang kunne anvendes til at bestemme indirekte ozonkoncentrationen i vand.Τhe decline of wild fish stock and the increasing demand for food and fish protein have accelerated the development of fish farming. The intense fish production in limited space (e.g. sea cages) has potentially deteriorated the aquatic environments (nutrient accumulation from uneaten feed and excreta cause eutrophication, pollution, chemical compounds, disease outbreaks, escapes etc.) gaining the scientific and societal attention. The environmental impact of aquaculture in combination with the competition for land use (leisure activities, aesthetic impact, tourism, residential development etc.) and the desire for a profitable business have contributed to a shift to land-based aquaculture systems. Recirculating aquaculture systems (RAS) are closed-containment systems where fish are farmed in reused water and provide a stable annual fish production. Due to the controlled farming environment, where water quality control systems and waste treatment technologies are installed, the fish quality and growth are improved while the risk of fish escapees and miscellaneous incidents are minimized. However, because of the recirculation (up to 99%) and the increased stocking density, waste derived from fish feed and fish excreta is accumulated. The control of organic matter in a RAS is of high importance for the good management of the facility. High organic matter loading in the water deteriorates the water quality, favouring the microbial blooming which might lead to disease outbreak directly affecting the fish. Additionally, increased levels of organic matter reduce the efficiency of the various water treatment processes.Several technologies have been installed to remove waste, and consequently organic matter, from the water stream of a closed system. However, waste removal processes are not fully optimized and the interactions between the waste treatment units are not well understood. Ozone has been implemented in several water treatment applications as a secondary treatment step to improve the water quality by oxidizing the organic matter and miscellaneous dissolved compounds in the water, having also bactericidal properties ensuring simultaneously disinfection. However, ozone is also toxic for aquatic organisms in extremely low concentrations (0.01 mg O3/L). Therefore, the risk of losing fish because of overdosing inhibits the full implementation of ozone in aquaculture industry.The aims of this PhD thesis were to design an ozonation system for RAS, based on the ozone demand of the specific system, where the water quality is improved without compromising the fish welfare, and then, to be able to monitor the ozone within the system with a novel, accurate and real-time measuring method relying on the fluorescence sensitivity towards ozone.Aquatic dissolved organic matter has numerous fluorescent properties and it is highly reactive with ozone. Water samples were collected from several aquatic facilities around Denmark and then were analysed with fluorescence spectroscopy to determine the fluorescent character. RAS samples were subjected to ozonation to investigate the responsiveness towards ozone. Similar fluorescence components were present in all studied RASs, with different reactivity suggesting that a florescence based sensor could be used as an indirect ozone dosage determination tool in water, since the fluorescence intensities and dissolved organic matter degradation by ozone were well correlated. Furthermore, potential applications of such a sensor were proposed.To design an optimal ozonation system, the water from the specific facility should be analysed in terms of ozone demand and ozone lifetime. Each facility is unique since the process water content of solutes, the operational conditions, and the water treatment units, the fish species and stocking density vary greatly, resulting in different water matrix. Therefore, the ozone reactions in this particular water should be investigated prior to installation, to ensure that the amount of ozone required to improve water quality is sufficient and it will be degraded long before it reaches the biofilters or the culture tanks.Having experimentally determined the ozone demand and kinetics, the predicted ozone dosages were applied in pilot-scale RASs to verify the effect of ozone on water quality of continuous operated freshwater RASs. Several water parameters were investigated including non-volatile organic matter, chemical oxygen demand, biological oxygen demand, ammonia, nitrate and nitrite levels, particles number and size, and microbial activity. Fluorescence organic matter was analysed by fluorescence Excitation Emission Matrix (EEM) spectroscopy coupled with Parallel Factor analysis (PARAFAC) for a more accurate identification of the organic matter. The overall water quality was significantly improved upon ozonation, proportionally to ozone dosage applied, suggesting that the predicted ozone dosages matched with the needs of the water. During ozonation no fish mortality was observed.The water matrix has huge influence in the design of an ozonation system. The determination of ozone in seawater samples is more complicated than in freshwater. In seawater, where bromide is naturally present, brome-oxides are formed in presence of ozone, which are toxic to fish. Thus, it is vital to be able to determine the critical point between the ozone dosage required to improve water quality and avoiding the formation of brominated by-products. Attempts to measure fast ozone in brominated water were made setting the basis for a modified analytical method. Further investigations are needed to increase the accuracy and to verify the breakpoint between optimal ozonation and brominated by-product formation inhibition in seawater RAS.In conclusion, this PhD study elucidated that when ozone is properly implemented in a RAS, having taken into consideration the ozone demand and the lifetime of ozone for the system of interest, the water quality of a RAS will be remarkably improved. To determine the ozone demand and lifetime, fluorescence spectroscopy was used, since it was found to be a good indicator of organic matter accumulation in RAS and highly sensitive towards ozone treatment. The detailed analysis of the fluorescence dissolved organic matter contained in RAS water, revealed four independently varying fractions with different reactivity and responsiveness to ozone, suggesting that a fluorescence based sensor targeting a specific wavelength transition could be used to determine indirectly the ozone concentration in water.<br/

Online Research Database In Technology

Ozonation of semi-closed aquatic systems - Online control

http://orbit.dtu.dk/ws/files/167227610/Thesis_online_version_Aikaterini_Spiliotopoulou.pdf

Ozonation of semi-closed aquatic systems - Online control

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