Hydrogen peroxide oxygenation and disinfection capacity in recirculating aquaculture systems

Hydrogen peroxide (H₂O₂) treatment is an alternative for disinfection in aquaculture, which may be advantageous as it dissociates and disinfects while increasing water oxygen concentration. Yet, accurate dosing remains undeveloped in Recirculating Aquaculture Systems (RAS). Dosage requirements can depend on organic burden, stocking density, feeding frequency, salinity, temperature and biofilter performance. The present case study investigated the dual effect of H₂O₂ application for oxygen enrichment and disinfection when continuously applied to a RAS rearing European seabass. H₂O₂ addition equivalent to 2.4 and 15.8 H₂O₂ mg L⁻¹ were applied for 4 h per day in three 5-days experiments. H₂O₂ was injected at the inlet of protein skimmer and/or the rearing tanks in or without combination with traditional disinfection methods. Water microbial load and oxygen saturation were determined, along with stress markers glucose and cortisol in blood plasma of fish. Doses of 15.8 mg L⁻¹ H₂O₂ steadily increased oxygen levels in holding tank water from ∼50 % to over 100 % saturation while reducing microbial load (from 604.4 CFU ml⁻¹ in the rearing tanks before dosing to 159.8 CFU ml⁻¹ after application), achieving suitable conditions for commercial fish densities in RAS. The doses used had negligible impact on biofilter performance and did not affect the fish in terms of stress. Overall results indicate H₂O₂ is effective for disinfection and oxygenation of RAS systems when applied at appropriate dosage and we recommend the protein skimmer as the safest position in order to protect the bacterial community of the biofilters and the reared fish.     

Microbial dynamics in RAS water: Effects of adding acetate as a biodegradable carbon-source

This study evaluated the effect of an abrupt increase in easily biodegradable carbon (acetate) on bacterial activity and abundance in the water of recirculating aquaculture systems (RAS). The study included a batch experiment with RAS water only, and an experiment at system level where twelve pilot scale RAS were used. The batch experiment was made to test how acetate concentration would influence the microbial state in RAS water. Further, we wanted to observe if the selected microbial analysis tools would be able to detect these changes. The second experiment was carried out in twelve identical and independent RAS that had been operated under constant loading conditions (1.6 kg/m³ make-up water) for five months prior to the trial. The twelve RAS were divided into four treatment groups in triplicates: i) control with submerged biofilter (Ctrl + bf); ii) control without submerged biofilter (Ctrl-bf); iii) acetate addition in RAS with submerged biofilter (Ac + bf); and iv) acetate addition in RAS without submerged biofilter (Ac-bf). The biofilter media from the groups without submerged biofilter (Ac-bf and Ctrl-bf) was removed just 5 h prior to the start of the trial. The two acetate treatment groups (Ac + bf and Ac-bf) were spiked with 40 mg/L of acetate three consecutive times (0, 24 and 48 h). Consumption of acetate, bacterial abundance and bacterial activity were followed for 72 h after the first acetate spike for both experiments. Bacterial activity was quantified by BactiQuant^® and hydrogen peroxide (HP) degradation assay. Bacterial abundance was assessed by quantifying micro-particles and free-living bacteria. In the batch experiment we observed a significant increase in bacterial activity proportional to the amount of acetate added, and a corresponding significant increase in microparticles (1–3 μm). In the pilot scale RAS experiment, the acetate addition in RAS with submerged biofilter did not cause an increase in bacterial activity, or in the number of microparticles in the water phase but a significant increase in bacterial activity and number of microparticles were observed in the RAS without submerged biofilter (Ac-bf). These changes were particularly pronounced shortly after each acetate spike.In RAS with submerged biofilters, the acetate was presumably consumed primarily by the bacterial community within the biofilm, and consequently, only minor changes were observed in densities of free-living bacteria in the water phase. The results of the study suggest that heterotrophic bacteria in the submerged biofilter have a high capacity to handle fluctuation of organic matter loading in RAS, thereby stabilizing the abundance and activity of bacteria in the water column.     

Increased sulfate availability in saline water promotes hydrogen sulfide production in fish organic waste

The risk of hydrogen sulfide (H₂S) production can be a challenge in marine land-based recirculating aquaculture systems (RAS). Hydrogen sulfide is a toxic gas that can cause massive fish mortality even at low concentrations, and in addition, serious odour problems in the surroundings. It is a bacterial by-product originating from the degradation of organic matter in sulfur-rich waters such as marine waters. In order to hinder H₂S production in marine land-based RAS, more information on the H₂S production conditions and the associated microbiology is needed. In this study, the production of H₂S from rainbow trout (Oncorhynchus mykiss) organic waste was examined using a novel H₂S measurement method under a range of salinities (0, 5, 10, 15, 25 and 35 g/L) in anaerobic mixed reactors, and the microbial communities as well as abundance of sulfate reducing bacteria (SRB) were characterized. The maximum H₂S concentration increased from 23.1 ± 8.2 mg H₂S/L at 0 g/L salinity to 153.9 ± 34.1 mg H₂S/L at 35 g/L salinity. Similarly, the H₂S production rate increased from 5.6 ± 0.2 at 0 g/L salinity to 26.4 ± 12.7 mg of H₂S produced per day at 35 g/L salinity. The highest H₂S production was recorded after increased availability of volatile fatty acids, which were produced by fermentative bacteria from phyla Firmicutes and Bacteroidetes that dominated the microbial communities after day 5. The traditional sulfate reducing bacteria (SRB) were found only at 0 and 5 g/L salinity, while at higher salinities, H₂S production was carried out by novel unquantifiable SRB. The results demonstrate that H₂S can be a pronounced problem in marine RAS, although it can be controlled through preventing anaerobic conditions within the system.     

Changes in microbial water quality in RAS following altered feed loading

Intensive recirculating aquaculture systems (RAS) with its hyper-eutrophic water offer ideal conditions for bacterial growth, abundance and activity, potentially affecting fish and system performance. Feed composition and feed loading in particular will have significant impact on organic and inorganic nutrients available for microbial growth in RAS. How these nutrient inputs affect and regulate bacteria in RAS water is, however, unclear. To investigate this relationship and the associated water quality dynamics, the effects of altered feed loading on microbial water quality in RAS was studied.The study included six independent, identical pilot-scale RAS, each with a total volume of 1.7 m³ (make-up water: 80 L/day) stocked with juvenile rainbow trout (Oncorhynchus mykiss). All systems had been operating with constant and identical feed loading of 3.13 kg feed/m³ make-up water for a period of three months before the experiment was initiated. Three controlled levels of feed loading where established in duplicates: no feed (0 kg feed/m³), unchanged feeding (3.13 kg feed/m³), and doubled feeding (6.25 kg feed/m³). The experimental period was seven weeks, where microbial and chemical water quality was monitored weekly. Bacterial activity was measured using Bactiquant^®, and microbial hydrogen peroxide degradation. Bacterial abundance was quantified by flow cytometry, and water quality parameters by standardized methods The study showed that water quality as well as bacterial activity and abundance were affected by the changes in feed loading. The microbial water quality parameters, however, did not respond to feed loading changes as quickly and straightforward as the physicochemical parameters such as nitrate, chemical oxygen demand and biological oxygen demand. It was presumed that the fixed bed biofilter suppressed microbial response in the water phase. Hydrogen peroxide degradation assay proved to have considerable potential for assessing overall bacterial load in RAS water although further adjustments and standardization procedures are required.     

Synthetic fibre as biological support in freshwater recirculating aquaculture systems (RAS)

The aim of this study was to evaluate the use of synthetic fibre as a biological support for the adhesion of nitrifying bacteria in an aquaculture recirculation system (RAS). It was developed from three assays over 120 days. In the first assay, the synthetic fibres used as biological support were introduced in tanks of biological filtration of the system for posterior respirometry analysis and scanning electron microscopy (SEM). Respirometry and SEM were performed 10 days after inoculation with nitrifying microorganisms. Water quality parameters were monitored daily, and the respirometry showed that the bacteria in this assay were consuming the following amounts of ammonium: concentrations [C1]35.369 mg NH₃/L, R² = 0.9912; [C2]51.628 mg NH₃/L, R² = 0.9883; [C3]79.494 mg NH₃/L, R² = 0.986; and [C4]215.225 mg NH₃L, R² = 0.9934. In the second assay, a 1920-L tank was stocked with 120 Nile tilapia, Oreochromis niloticus, with an initial weight 32.11 ± 7.6 g and a biomass of 3.8 kg. After 60 days, the tank and its contents were assessed to evaluate zootechnical parameters and physical–chemical parameters of water quality. From these results, a third assay was developed in which the biomass of fish was increased to challenge the recirculation system. The tank was stocked with 480 jundiá Rhamdia quelen (initial weight 11.34 ± 2.4 g and biomass 5.4 kg) for 60 days. In both the tilapia and jundiá assays, the fish were fed four times per day with a commercial diet of 35% crude protein and 42% crude protein, respectively, at 5% of each individual fish’s body weight. At the end of the zootechnical assays, the synthetic fibres used showed efficient biological support for bacterial growth, as confirmed by scanning electron microscopy. The fibres also demonstrated maintenance of the water quality, which allowed good fish growth in the recirculating aquaculture system, and the maintenance of up to 11.19 kg/m³ of biomass of fish.     

Process requirements for achieving full-flow disinfection of recirculating water using ozonation and UV irradiation

A continuous water disinfection process can be used to prevent the introduction and accumulation of obligate and opportunistic fish pathogens in recirculating aquaculture systems (RAS), especially during a disease outbreak when the causative agent would otherwise proliferate within the system. To proactively prevent the accumulation of fish pathogens, ozonation and ultraviolet (UV) irradiation processes have been used separately or in combination to treat water in RAS before it returns to the fish culture tanks. The objective of the present study was to determine the process requirements necessary to disinfect the full RAS flow, using ozonation followed by UV irradiation, just before the flow was returned to the fish culture tank(s). We found that a proportional-integral (PI) feed-back control loop was able to automatically adjust the concentration of ozone (O₃) generated in the oxygen feed gas (and thus added in the low head oxygenator) in order to maintain the dissolved O₃ residual or ORP at a pre-selected set-point. We determined that it was easier and effective to continuously monitor and automatically control O₃ dose using an oxidative reduction potential (ORP) probe (in comparison to a dissolved ozone probe) that was located at the outlet of the O₃ contact chamber and immediately before water entered the UV irradiation unit. PI control at an ORP set-point of 450 and 525 mv and a dissolved O₃ set-point of 20 ppb provided almost complete full-flow inactivation of heterotrophic bacteria plate counts (i.e., producing <1 cfu/mL) and improved water quality (especially color and %UVT) in a full-scale recirculating system. Achieving this level of treatment required adding a mean dose of approximately 29 ± 3 g O₃ per kg feed. However, because water is treated and reused repeatedly in a water reuse system, the mean daily O₃ demand required to maintain an ORP of 375–525 mV (or at 20 ppb dissolved O₃) was 0.34–0.39 mg/L, which is nearly 10 times lower than what is typically required to disinfect surface water in a single pass treatment. These findings can be used to improve biosecurity and product quality planning by providing a means for continuous water disinfection in controlled intensive RAS.     

Effect of oxidation–reduction potential on performance of European sea bass (Dicentrarchus labrax) in recirculating aquaculture systems

The direct impact of oxidation–reduction potential (ORP) on fish welfare and water quality in marine recirculating aquaculture systems (RAS) is poorly documented. In this study, the effects of the fish size (S₁, S₂, S₃) and ORP level (normal, four successive levels) on the performance of European sea bass (Dicentrarchus labrax) were investigated. Three size fish were distributed into two RAS (RAS and RAS O₃). Ozone was injected into RAS O₃ to increase the ORP level. The ORP was stabilized to four successive levels: 260–300, 300–320, 320–350, and 300–320 mV in fish tanks during four periods (P_1–4). At the last day of each period, the hematological parameters, plasma protein and mortality of sea bass were analyzed. Two-way ANOVA revealed that several hematological parameters, including pH, hematocrit, concentrations of oxygen, carbon dioxide, glucose (Glu), ionized calcium, kalium, and hemoglobin, were significantly influenced by the increased ORP levels over the experimental period. The alteration in blood Glu and plasma protein concentration showed that ORP around 300–320 mV started to stress sea bass. Once the ORP exceeded 320 mV in the tanks during the P₃ period, mortality occurred even when total residual oxidants/ozone-produced oxidants was only 0.03–0.05 mg L^?1 in the fish tanks. At the same time, plasma protein decreased notably due to appetite depression. After the decrease in ORP during the P₄ period, mortality continued. In conclusion, the results strongly suggest that for European sea bass in RAS, the ORP should not exceed 320 mV in the tanks. Once ozonation damaged fish, the effect seemed to be irreversible. However, how ORP affected related hematological parameters still need the further investigations.     

A case study of biofilter activation and microbial nitrification in a marine recirculation aquaculture system for rearing Atlantic salmon (Salmo salar L.)

Marine recirculation aquaculture system (RAS) is a prominent technology within fish farming. However, the nitrifying bacteria in the biofilter have low growth rates, which can make the biofilter activation a long and delicate process with periods of low nitrification rates and variations in water quality. More knowledge on the microbial development in biofilters is therefore needed in order to understand the rearing conditions that favour optimal activation of the biofilters. In this case study, we investigated the activation of two biofilters in a marine RAS for Atlantic salmon post‐smolt associated with either high or low stocking densities of fish by monitoring the microbial communities and chemical composition. The results showed that the microbial communities in both biofilters were similar during the first rearing cycle, despite variations in the water quality. Nitrifying bacteria were established in both biofilters; however, the biofilter associated with low stocking density had the highest relative abundance of ammonia‐oxidizing Nitrosococcus (1.0%) and nitrite‐oxidizing Nitrospira (2.1%) at the end of the first rearing cycle, while the relative abundance of ammonia‐oxidizing Nitrosomonas (2.3%–2.9%) was similar in both biofilters. Our study showed that low fish stocking density during the first rearing cycle provided low and steady concentrations of ammonium, nitrite and organic load, which can stimulate rapid development of a nitrifying population in new marine RAS biofilters.     

Practical use of response surface methodology for optimization of veterinary antibiotic removal using UV/H2O2 process

Three of the most commonly used veterinary antibiotics—enrofloxacin (ENR), sulfamethoxazole (SMX), and oxytetracycline (OTC)—were chosen as representative antibiotics for UV/H₂O₂ treatments. The objective was to determine the optimization of UV/H₂O₂ to remove antibiotics from aquaculture discharge water using response surface methodology. The degradation of the antibiotics was investigated under varying UV/H₂O₂ conditions in environments with different levels of pH, water matrices, humic acid, and constituent ions. The degradation results demonstrated that increasing the H₂O₂ dosage facilitated ENR degradation at a neutral pH while facilitating degradation of SMX and OTC at a slightly acidic pH. The optimum removal conditions for ENR, which was used in all influential effect experiments and the contact tank experiments, was obtained at 10 mM H₂O₂, a pretreated COD of 87.51 mg L⁻¹, and an initial pH of 6.15. Among the tested anions, only the presence of Cl^- showed slight positive effects on ENR degradation, due to the generation of secondary active radicals. During the reaction, the hydroxyl radical (OH) was present at a higher pH while singlet oxygen (¹O₂) was slightly present at a lower pH. The experimental results from H₂O₂ sequential addition indicated that freshly added H₂O₂ could quench the recently generated OH and therefore a high H₂O₂ concentration with frequent adding was not necessary. Our contact system reduced the ENR concentration in both the effluent reservoir and in the UV irradiation zone. The overall results supported the use of the UV/H₂O₂ system to treat remnant antibiotics in the discharge water.     

The use of acoustic acceleration transmitter tags for monitoring of Atlantic salmon swimming activity in recirculating aquaculture systems (RAS)

Successful operation of recirculating aquaculture systems is dependent on frequent monitoring of the optimal function of water treatment processes in order to maintain environmental conditions for optimal growth and welfare of the fish. Real time monitoring of fish status is however usually not an integrated part of automatized systems within RAS. The aim of this study was to evaluate the use of implanted acoustic acceleration transmitters to monitor Atlantic salmon swimming activity. Twelve salmon post-smolts were individually tagged and distributed in three tanks containing salmon at start density of 50 kg m⁻³. The tagging did not cause any mortality and all individuals increased their body weight during this study. Following initial recovery, acceleration data were continuously logged for one month, including treatment periods with exposure to hyperoxic (170% O₂ saturation) and hypoxic (60% O₂ saturation) conditions, and different tank hydraulic retention times (HRT; 23 and 58 min). Changes in-tank dissolved oxygen levels to hyperoxic and hypoxic conditions reduced the total activity of Atlantic salmon in this study. On the contrary, increased and reduced tank HRT increased the total activity levels. Feeding periods induced a sharp increase in the Atlantic salmon swimming activity, while irregular feeding caused larger oscillations in activity and also lead to increased swimming activity of the tagged fish. Atlantic salmon responded with a maximum recorded total activity to stress caused by technical problems within the system and consequent changes in the RAS environment. The results of this study indicate that Atlantic salmon respond quickly with changed swimming activity to changes in the water quality and acute stress caused by normal management routines within RAS. The use of acoustic acceleration transmitters for real time monitoring of swimming activity within aquaculture production systems may allow for rapid detection of changes in species-specific behavioural welfare indicators and assist in the refinement of best management practices. In addition, acceleration tag could potentially serve as a valuable research tool for behavioural studies, studies on stress and welfare and could allow for better understanding of interaction between fish and RAS environment.     

Pyocyanin as a safe aquaculture drug for the control of vibriosis in shrimp recirculating aquaculture system (RAS)

Diseases are one of the most critical limiting factors in aquaculture. Recirculating aquaculture systems (RAS) are one of the most functionally viable sustainable aquaculture production systems adopted world over. In the event of any eventuality caused by bacterial pathogens, antibiotics or other antibacterial agents cannot be applied due to the vulnerability of biological filters which form an integral part of the RAS. Because of this, newer drugs are required for the effective control of diseases in RAS which would not interfere with the activity of nitrifying bacteria used in the biological filters. The antagonistic activity of pyocyanin, a bioactive compound produced by Pseudomonas aeruginosa, against vibrios is well established. The purpose of this study was to prove the effectiveness of pyocyanin as an aquaculture drug for application in RAS by employing a pilot-scale shrimp culture under RAS. It was noted that at the concentration of 5 mg L^?1, pyocyanin could significantly bring down the population of Vibrio spp. in RAS without affecting noticeably the other natural heterotrophic bacteria. Also, pyocyanin at this concentration did not inhibit the activity of nitrifying bacterial consortia used in the SBSBR (stringed bed suspended bioreactor) of RAS. The reared shrimp (Penaeus monodon) showed 100% survival after the application of pyocyanin, besides exhibiting normal health signs. Pyocyanin was non-toxic to the shrimp hybrid cell line (PmLyO-Sf9) at the concentration required for its application in disease management (IC₅₀?=?419.26 mg L^?1). The present study has demonstrated that pyocyanin is effective as an environment-friendly and safe aquaculture drug for the application in RAS-based shrimp culture to control Vibrio spp. without impeding nitrification established through the deployment of nitrifying bioreactors.

     

Optimization of blue mussel (Mytilus edulis) seed culture using recirculation aquaculture systems

By introducing recirculation aquaculture systems (RAS) in the nursery phase of the blue mussel (Mytilus edulis) (17–18 mm), we aimed at a similar growth and survival and a similar water quality compared to the commonly used flow‐through systems (FTS). To calculate water flow and size of the biofilter, a series of experiments were done to determine clearance rate (9.26 mL min^?1), pseudo faeces threshold (60 000 cells Pavlova lutheri mL^?1), nitrogen production (0.00065 mg TAN h^?1 ind^?1 and 1.6 × 10^?5 mg NO₂–N h^?1 ind^?1) and oxygen consumption (0.03 ± 0.01 mg O₂ h^?1 ind^?1). RAS showed no significant differences in water quality (0.06 mg TAN L^?1; 7.7 mg O₂ L^?1) and growth performance of mussel seed specific growth rate (SGR = 5% day^?1) after the experimental period of 4 weeks compared with FTS. The low water refreshment, 10% per day, as well as the constant chlorophyll concentrations (9.76 ± 1.06 μg L^?1), suggests the potential of RAS as culture system for mussel seed.     

Microbial decomposition process of organic matter in sinking particles,resuspendable particles,and bottom sediments at a coastal fish farming area

The possible impacts of resuspension of low-density bottom sediments on the microbial decomposition process of organic matter were investigated at a coastal fish farming area. Hydrolysis and mineralization rates were much higher in sinking particles, resuspendable particles, and bottom sediments than in seawater. The cell-specific mineralization rate of free-living bacteria in seawater was several times higher than that of particle-associated bacteria at the other three sites. Conversely, no significant difference was observed in the cell-specific ecto-enzymatic hydrolysis rate. These results indicated different strategies in the utilization of organic matter: free-living bacteria actively respire low-molecular-weight compounds produced from the high-molecular-weight compounds resulting from extracellular enzyme activity of particle-associated bacteria. Both hydrolysis and mineralization were higher in sinking particles than in the other three sites. Hydrolysis rates were higher in resuspendable particles than in bottom sediments. Furthermore, leucine aminopeptidase and mineralization rates tended to be highest during winter in all four sites within the water column. These results suggest that the microbial decomposition of organic matter is stimulated by the resuspension of bottom sediments, especially during winter, when vertical mixing is relatively strong.     

Prevention of fungal infestation of rainbow trout (Oncorhynchus mykiss) eggs using UV irradiation of the hatching water

Fungal infestation by water mold Saprolegnia spp. causes great losses in aquaculture and fish egg hatching. To find a safe and effective alternative for the fungal prevention, we studied continuous disinfection of the inlet water by UV irradiation and ozonation combined with low concentration hydrogen peroxide (H₂O₂) treatments in a rainbow trout (Oncorhynchus mykiss) egg hatching system. High dose of UV irradiation (400 mWs/cm²) of the inlet water decreased the mortality of rainbow trout eggs from the 77.3% to 14.3% in a 28 day trial. UV irradiation did not modify water quality parameters, while combination of UV irradiation and H₂O₂ caused up to fivefold increase in the formate levels, and combination of O₃ and H₂O₂ caused even ten-fold increase in the acetate and formate levels. UV suppressed the gradual increase of the heterotrophic bacterial counts on the fish eggs. Based on the molecular profiling high dose of UV reduced the growth of some of the dominating bacterial groups and combination of UV and H₂O₂ had a distinctive effect on the overall bacterial community structure on the fish eggs.     

Productive performance of juvenile freshwater prawns Macrobrachium rosenbergii in biofloc system

The aim of this study was to compare two rearing systems for freshwater prawn Macrobrachium rosenbergii: one with use of a recirculating aquaculture system with biofilters (RAS) and another with use of microbial flocs (F). Thirty postlarvae of freshwater prawn with an initial average weight of 0.13 ± 0.05 g were randomly stocked in six experimental units with 0.20 m² and volume of 50 L. The experiment lasted thirty days. Dissolved oxygen, temperature and pH were monitored daily; ammonia concentration was determined three times per week; nitrite concentration, alkalinity and hardness were measured weekly. For the formation of microbial floc, molasses was used to keep the ammonia concentrations within safe levels for prawn farming. The variables of water quality remained within the suitable range for the production of the species, except for ammonia concentrations at the F treatment, which exceeded the safe levels. At the end of the experiment, the following parameters were evaluated: survival, specific growth rate, weight gain and feed conversion rate. Differences were found only in feed conversion rate with better values on RAS treatment. The microorganisms present in the RAS and F treatment were also evaluated. The densities of rotifers, amoebas and total bacteria were higher at the F treatment although the same organisms were found at the RAS treatment. The results of this study showed the possibility of rearing M. rosenbergii in biofloc system technology.     

Effects of foam fractionation and chemical disinfection on the removal of different microalgae cultures

The recent development of recirculation aquaculture systems (RAS) potentially allows more sustainable and controlled rearing conditions. However, control of suspended microparticles and microalgae in RAS is challenging, as uncontrolled blooms of toxic algae or heterotrophic dinoflagellates can have catastrophic impacts on the production of fish. In this study, we tested the potential of protein skimmers to remove microalgae. In 100 L batch tests, protein skimmers were tested separately and in combination with ozone (O₃), ultraviolet (UV) or hydrogen peroxide (H₂O₂). Three different and distinct microalgae cultures were tested with densities from 30,000 to 120,000 algae cells/ml in a triplicated experimental setup. Trial 1 included twelve 24‐hr replicated tests where protein skimmers with air or with two levels of ozone (low and high O₃ exposure) were compared. The protein skimmer with air alone had a limited effect on the removal of microalgae compared to the untreated control algae tanks. When ozone and protein skimmers were combined, a clear additive effect was found, and all added microalgae were removed. Low O₃ dosage and protein skimmers completely removed the algae cultures within 12 hr, while more than 95% of the algae were removed within 7 hr when a higher ozone dose was used. The second trial compared the removal capacity of protein skimmers in combination with UV, H₂O₂ and O₃. These experiments showed no to limited additive effect of UV combined with protein skimming, but significantly increased removal efficiency (270% and 1,300%, respectively) was found when H₂O₂ and ozone were combined with a protein skimmer. The study showed an algae species‐specific response to a protein skimmer with ozonation and provided information on the transition from reactivity and consumption to accumulation of ozone‐produced oxidants.     

Effects of abrupt salinity increase on nitrification processes in a freshwater moving bed biofilter

The nitrification process is a widely used biological approach responsible for ammonia and nitrite removal in recirculating aquaculture system (RAS) biofilters. Given this pivotal role, the influence of different water quality parameter on nitrification efficiency is important information for RAS operations. One influencing parameter is salinity, and salinity fluctuations in freshwater RAS biofilters are reported to affect the nitrifying bacteria. This study investigated the effects of abrupt increase in salinity in freshwater RAS on substrate-dependent (1’-order) as well as substrate independent (0’-order) nitrification rates. A 100% inhibition was found for surface specific removal (STR) of total ammonia nitrogen (TAN) and surface specific nitrite removal (SNR) when salinity was abruptly increased to 25‰ and above. A fast turnover (i.e. steep decline in [NH₄-N⁺] and [NO₂-N⁻]) were observed at lower salinities (≤10‰), while limited/no degradation of either ammonia or nitrite was seen at salinities above 25‰. At low substrate loading (1’-order process), removal rate constants (k_1a) of 0.22 and 0.23 m d^-1 were observed for ammonia and nitrite degradation, respectively, declining to 0.01 m d^-1when adding marine RAS water increasing the salinity to 15‰. Similar observations followed at high nutrient loadings (0’-order process) with STR and SNR of 0.10 and 0.12 g N m^-2 d^-1, respectively, declining to 0.01 g N m^-2 d^-1 at 15‰. When salinities of 25‰ and 35‰ were applied, neither TAN nor nitrite degradation was seen. The results thus demonstrate a pronounced effect of salinity changes when freshwater RAS biofilters are subjected to fast/abrupt changes in salinity. RAS facility operators should be aware of such potential effects and take relevant precautions.     

Discharge management in fresh and brackish water RAS: Combined phosphorus removal by organic flocculants and nitrogen removal in woodchip reactors

The current study combined P and N removal using organic flocculant chemicals and woodchip bioreactors in both freshwater and brackish water (7 ppm) recirculating aquaculture systems (RAS). The use of carbon (C) containing flocculant chemicals in the process was hypothesized to further stimulate C-demanding N removal (denitrification) in bioreactors. The trial of combined P and N removal consisted of four treatments: freshwater and brackish water RAS with and without the addition of supernatant from flocculation process to the woodchip reactor. Duplicate woodchip reactors were used per treatment and the trial was run for six weeks. 56% and 49% of P was removed from fresh and brackish sludge water, respectively. The nitrate-N (NO₃-N) removal rate was improved in the treatment when supernatant from flocculation process was used together with RAS discharge water when compared against the control. In brackish water RAS, the improvement was more pronounced (from 6.6–16.5 g NO₃-N m⁻³ d^-1) than in freshwater RAS (from 5.1–6.5 NO₃-N m⁻³ d^-1). In the freshwater bioreactors using supernatant, N was largely discharged as a nitrite-N (NO₂-N). High NO₂-N concentrations in freshwater reactors allude to incomplete denitrification reactions taking place. The results suggest that the organic flocculants did provide an additional C source for denitrification, which improved the N-removal process. However, in freshwater RAS this might have been partly due to untargeted processes such as DNRA (dissimilatory nitrate reduction to ammonium), and/or insufficient denitrification reactions taking place (excessive NO₂-N production).     

Micro particles and microbial activity in Danish recirculating rainbow trout (Oncorhynchus mykiss) farms

Increasing intensities of water reuse in recirculating aquaculture systems (RAS) lead to a build-up of micro particles (< 20 μm) in the water. This build-up may have consequences for other water quality parameters and for the fish. This baseline study was carried out to determine the variation in micro particle levels (numbers, volume and surface area) and accompanying bacterial activity in commercially operated outdoor RAS, as well as the effects of different components in the recirculation loop on micro particle dynamics. Water samples were obtained during spring 2017 from 7 Danish Model Trout Farms (MTFs) producing rainbow trout (Oncorhynchus mykiss) in a total of 20 separate RAS units. Micro particle numbers and size distribution, bacterial activity, and inorganic and organic nutrient concentrations were analysed. Micro particle numbers ranged between 6.0·10⁴ – 7.4·10⁵ ml⁻¹ and large variations were found between seemingly similarly operated RAS units within the same farm. There was a strong, positive correlation (p < 0.001) between micro particle levels and bacterial activity in the systems. Although not significant, biofilters generally seemed to trap particles whereas drum filters seemed to reduce particle volume while increasing particle numbers and surface area. The study sustains that bacterial activity in RAS is strongly associated with fine particle loading, and demonstrates for the first time the overall magnitude and level of variation in particle levels and bacterial activity that exists in commercially operated MTFs.     

Foam fractionation and ozonation in freshwater recirculation aquaculture systems

Foam fractionation is often considered an ineffective way of removing organic matter from freshwater due to the low surface tension of the water. There is, however, a lack of studies testing foam fractionation efficiency in replicated freshwater recirculating aquaculture systems (RAS). Foam fractionation can be applied with or without ozone. Ozone is a strong oxidiser previously shown to improve water quality and protein skimmer efficiency. To test the efficiency of foam fractionation and ozonation (20 g O₃ kg^-1 feed) separately and in combination in freshwater RAS, a two-by-two factorial trial was conducted with each main factor at two levels (applied or not applied). Each treatment combination was carried out in triplicates using 12 replicated pilot scale RAS stocked with juvenile rainbow trout (Oncorhynchus mykiss) and operated at a feed loading of 1.66 kg feed m^-3 make-up water. The trial lasted 8 weeks and samples were obtained once a week. Ozone applied by itself significantly reduced the number of particles (83%), bacterial activity (48%) and particulate BOD₅ (5-days biochemical oxygen demand; 54%), and increased ultra violet transmittance (UVT; 43%) compared to the untreated control group. Foam fractionation by itself lead to significant reductions in particle numbers and volume (58% and 62%, respectively), turbidity (62%), bacterial activity (54%) and total BOD₅ (51%). A combination of both treatments resulted in a significant additional improvement of important water quality variables, including a 75% reduction in total BOD₅, 79% reduction in turbidity, 89% reduction in particle numbers and 90% reduction in bacterial activity compared to the control. The removal efficiencies were within the same range as those observed in previous studies conducted with foam fractionators in saltwater systems (with or without ozone), corroborating that foam fractionation may become a useful tool for controlling organic matter build-up and bacterial loads in freshwater RAS.