Recirculating Aquaculture Systems

Recirculating aquaculture systems may require an internal disinfection process to control population growth of pathogens and heterotrophic bacteria. Ozonation and ultraviolet (UV) irradiation are two technologies that have been used to treat relatively large aquaculture flows, including flows within freshwater systems that recirculate water. The objective of the present study was to evaluate the effectiveness of ozone application alone or ozone application followed by UV irradiation to reduce abundance of heterotrophic and total coliform bacteria in a water reuse system. Results indicate that when only ozone was applied at dosagesdefined by the product of the ozone concentration times the mean hydraulic residence time (Ct) -that ranged from 0.10 to 3.65 min mg/L, the total heterotrophic bacteria counts and total coliform bacteria counts in the water exiting the contact basin were reduced to, respectively, 3-12 cfu/mL (1.1-1.6 LOG 10 reduction) and 2-18 cfu/100 mL (1.9-3.1 LOG 10 reduction). Bacteria inactivation appeared to be just as effective at the lowest ozone ct dosage (i.e., 0.1 mg/L ozone after a 1 min contact time) as at the highest ozone ct dosage (i.e., 0.2 mg/L ozone after a 16.6 min contact time). As with our previous research on UV inactivation of bacteria, we hypothesize that the recirculating system provided a selection process that favors bacteria that embed within particulate matter or that form bacterial aggregates that provides shielding from oxidation. However, when ozonation was followed by UV irradiation, the total heterotrophic bacteria counts and total coliform bacteria counts in the water exiting the UV irradiation unit were reduced to, respectively, 0-4 cfu/mL (1.6-2.7 LOG 10 reduction) and 0-3 cfu/100 mL (2.5-4.3 LOG 10 reduction). Thus, combining ozone dosages of only 0.1-0.2 min mg/L with a UV irradiation dosage of approximately 50 mJ/cm 2 would consistently reduce bacteria counts to near zero. These findings were orders of magnitude lower than the bacteria counts measured in the system when it was operated without disinfection or with UV irradiation alone. These findings indicate that combining ozonation and UV irradiation can effectively disinfect recirculating water before it returns to the fish culture tank(s). #

The purpose of this study was to develop a mathematical model to assist in the determination of design numbers for color destruction by ozone as a function of feed rate. A mass balance model was developed that takes into account methods of introduction and removal of color, including removal by ozone. Because direct measurement of color mass or concentration is difficult, a representative unit was developed during this study called the color mass equivalent (CME). The CME represents a direct measurement of absorbance at 436 nm, which is a unitless measurement, multiplied by the system volume. The CME is directly proportional to the true concentration of color causing compounds at that wavelength. Once the model was developed, two studies were run to test the model. A 1,500-L recirculating fish system was set up with a 57-L bead filter and six 1-L fluidized sand beds. Approximately 45 kg of channel catfish Ictulurus puncfatus were placed in the system and fed 0.25 kg of 32% protein feed per day (0.55% body wt.). Once the system reached stable conditions, the system was flushed with clean water and samples were taken to estimate the accumulation rate of color in the system. After 5 wk, an ozone unit was activated and samples were taken to determine the destruction rate for color. Once an accumulation rate had been determined, a second study was performed to corroborate this value. The same ozone unit was installed on a 5,000-L recirculating fish system being fed 0.49 kg per day with a 171-L bead filter. Samples were taken to determine the destruction rate for color. The accumulation rate for color was 12.6 CME/kg feed and the destruction rates were 1.7 CME/g 0, in the first study and 0.82 CMWg 0, in the second study. This calculates to a range of 7 to 15 g OJkg feed to remove the color produced by the feed.

Under laboratory-scale conditions, two types of bioflocs were used as a feed ingredient for Pacific white shrimp (Litopenaeus vannamei). This feeding trial was conducted indoors in recirculating aquaculture systems with seawater renewal. Bioflocs were produced in sequencing batch reactors (SBR) using carbon supplementation (sucrose) and in a membrane biological reactor (MBR) without carbon supplementation. Both types of bioflocs were produced using tilapia farm effluent as a feed for the SBR and MBR systems. The bioflocs were dried and were used in shrimp feed as a replacement for fishmeal and/or soybean protein. A control diet (without bioflocs) was compared against four diets with MBR bioflocs (at 10, 15, 21, and 30% inclusion levels) and SBR bioflocs (at 10, 15, and 21% inclusion levels). At these biofloc inclusion rates, soy protein was replaced completely and fishmeal by as much as 67%. The control and biofloc diets were formulated to be equivalent for levels of crude protein, total fat, crude fiber, calcium, magnesium, phosphorus, potassium, and sodium. Seven juvenile shrimp were stocked per tank and each dietary treatment was tested using 8 replicates over a 35 day feeding trial. Dietary treatments had some impact on shrimp performance. Even though no differences (P N 0.05) were observed between means for final survival (92.9 to 100%) or harvest biomass (536 to 574 g/m 2), some evidence (P b 0.05) of faster growth rates was observed for diets with bioflocs inclusion compared to the control diet. Growth rates ranged from 1.44 to 1.66 g/week. Therefore, data indicate that bioflocs produced using either SBR or MBR systems could replace fishmeal and soybean protein. Furthermore, the SBR and MBR systems can be used to treat effluent from a tilapia farm thus reducing effluent waste into the environment.

In the near future, the discharge of nitrate from recirculating aquaculture systems (RAS) to receiving water bodies is expected to be constrained by environmental regulations. Following wastewater treatment terminology, nitrate removal that makes use of the organic solid wastes generated within the RAS as the energy source for denitrification, may be termed 'single-sludge denitrification'. In this approach, the costs associated with the addition of an external carbon source, the treatment of solid-wastes generated in the RAS, and the supplementation of alkalinity are reduced. The simple and economical operation of such a process can be realized by adopting the conventional activated sludge (AS) methodology. Organic solid-wastes taken from the solids-filter of a RAS growing gilthead seabream were characterized for their chemical and biodegradation properties. The results were used to generate a conceptual model to predict the performance of single-sludge denitrification in RAS. The model was run under typical operational conditions, employing the mean solids retention time (SRT) as the key operational parameter. Results indicated that in order to attain high denitrification rates, the ratio of the flux of organic solids (as COD) supplied to the denitrification reactor to the flux of NO 3 − reduced should be between 4.0 and 6.0 g COD (g NO 3 −N) − 1 and the SRT values should be lower than 10 d. At these conditions, sludge production was estimated to be between 40% and 60% of the solids feed mass (in terms of COD), and NH 3 −N production as a result of ammonification was estimated to be less than 10% of the NO 3 − -N removed. Empirical verification of the model is presented elsewhere. The model can be used as a design tool and for predicting the performance of the process at any operational conditions.

A lab-scale activated-sludge type reactor was used to induce denitrification by using the organic solid waste of a typical Recirculating Aquaculture System (RAS) as the electron donor ('Single-sludge denitrification'). The results were compared with the predictions of a stoichiometry-based model. As predicted by the model, reactor's performance was found to be strongly related to the mean solids retention time (SRT) employed. Measured denitrification rates conformed very well to model predictions. High nitrate removal rates of up to 590 mg N (L reactor d d) − 1 were recorded at a relatively low SRT of 4 d. Oxygen, that entered the reactor via both atmospheric diffusion and with the stream used to simulate the influent from a fish tank, reduced the amount of organic matter available for denitrification, resulting in lower denitrification rates. This interference was more significant when the system was operated at the longer SRTs. Most of the excess ammonia released to the aqueous phase through ammonification was oxidized (presumably by anammox bacteria) under the prevailing anoxic conditions, resulting in very low effluent TAN concentrations. Phosphate release to the aqueous phase was significantly lower than predicted, suggesting above-typical microbial P assimilation. Reaction kinetics was found to be zero order with respect to nitrate at concentrations of above 1.5 to 2.0 mg N L − 1 . Taken together the findings indicate that intensive single-sludge denitrification for treating RAS effluents is technically feasible, and that the process appears to be a cost-effective solution to reducing both the nutrient and the organic loads generated by intensive fish farms. The main advantages of the method include minimal formation of undesired by-products, small reactor volume and simple control and operation. Furthermore, the process is well described by a conceptual mathematical model, allowing its application as a part of any RAS design.

Maintenance of optimal water quality and removal of nitrogen compounds pose challenges to aquaculture worldwide. Presence and activity of different bacteria involved in nitrogen cycling in the biofilm of a moving bed biofilm reactor (MBBR) connected to a Koi carp tank were investigated experimentally. For this MBBR system, a nitrogen removing rate of 3.5 g nitrogen per day was found in activity assays, whereas the predicted nitrogen removal rate was calculated to be 58 g nitrogen per day. The presence of ammonium-oxidizing and nitrite-oxidizing bacteria in the biofilm was demonstrated by situ hybridization (FISH) analysis. A large number of planctomycetes were shown to be present in the biofilm of the reactor, but they did not hybridize to oligonucleotide probes of known genera of anammox bacteria. Molecular 16S rRNA gene analysis of the MBBR biofilm revealed a novel group of planctomycete cells was present in this biofilter, which may explain the difference in experimentally found and calculated nitrogen removal of the system. The novel 16S rRNA gene sequences showed a low similarity to described anammox bacteria. This and other studies may implicate that the anammox reaction can be incorporated in biofilter systems for aquacultural freshwater systems, which makes it possible to remove nitrogen compounds from these systems in a relatively inexpensive and sustainable manner.

Common practice of aquaculture in Vietnam and other countries in South East Asia involves frequent discharge of polluted water into rivers which results in eutrophication and degradation of receiving water bodies. There is therefore a need to develop improved aquaculture systems which have a more efficient use of water and less environmental impact. The aim of this study was to assess the suitability of using constructed wetlands (CWs) for the treatment of fishpond water in a recirculating aquaculture system in the Mekong Delta of Vietnam. Water from a fishpond stocked with Nile tilapia (Oreochromis niloticus) and common carp (Cyprinus carpio) was recirculated through horizontal and vertical flow CWs. The CWs were able to keep a good water quality with DO (N 1 mg/l − 1 ), BOD (b 30 mg l − 1 ), TAN (b 1 mg l − 1 ) and NO 2 − (b 0.07 mg l − 1 ) at acceptable concentrations for growth of the fish. There was a good removal of organic matter measured as oxygen demand with up to 50% removal of BOD and COD in both types of CWs despite the high loading rates and low concentration levels. However, the vertical flow CWs performed better than the horizontal flow CWs as they had higher nitrification rates and higher DO concentrations in the outlets. The ornamental Canna × generalis planted in the CWs grew faster and took up more N and P in the vertical flow CWs. The aquaculture fish had a feed conversion ratio of 1.53 based on feed dry weight, and 31% and 34% of N and P input, respectively, were incorporated into fish biomass. Only minor quantities of phytoplankton algae were removed in the CWs but abundance of toxic algae such as Microcystis was low. It is concluded that particularly vertical flow CWs have great potential for treatment of fishpond water in recirculating aquaculture systems in the tropics as the discharge of polluted water and the associated environmental impact can be significantly reduced.

In recirculating aquaculture systems (RAS), particulates (including feces, uneaten feed, bacteria, and algae) can cause several problems, in that they may harbor pathogens, can physically irritate the fish, and upon decomposition, release ammonia and consume oxygen. Mechanical filters, foam fractionators, and other engineered devices are used to remove particles quickly from aquaculture systems, in order to improve fish health and decrease the load on biofilters and oxygenators. Ozone is used in RAS as a disinfectant, to remove organic carbon, and also to remove turbidity, algae, color, odor and taste. Ozone can effectively inactivate a range of bacterial, viral, fungal and protozoan fish pathogens. But the effectiveness of ozone treatment depends on ozone concentration, length of ozone exposure (contact time), pathogen loads and levels of organic matter. In spite of ozone is a very effective oxidizing agent, higher ozone concentrations are a risk to cultured fish stocks causing gross tissue damage and stock mortalities, and also are a risk to bacterial films on the biofilter.

Over the past decade, the environmental impact of wastes discharged from high-density flow-through fish-production systems has been viewed with increasing concern. Recirculating aquaculture systems are gaining wider acceptance because of their ability to reduce waste discharges, improve quality control,
and reduce costs. The crucial processes that must be addressed in treating recirculating water are solids capture, biofiltration, aeration, degasification, and ion balance. One of the greatest potentials for cost reduction rests with the development of designs that integrate two or more of these processes. Expandable granular biofilters (EGBs) integrate solids capture and biofiltration in a single unit process, potentially reducing the cost of treatment. Backwash frequency, a major operational parameter, influences the rate of nitrification and the volume of sludge produced. Computer and mass balance models allow the relationship between solids retention time, sludge production, and nitrification rates to be examined. While infrequent backflushing decreases water loss and sludge production, nitrification rates decline for extended solids retention times. Declining nitrification rates reflect ammonification of the accreting solids mass and a decrease in the availability of oxygen due to heterotrophic competition and increased mass-transfer resistance. Nitrification appears to be optimized with solids retention times in the range of 2-3 days, for filters utilized as the primary solids capture device. Studies indicate that biofiltration can be optimized in the presence of solids capture, by manipulating the backwash regime. This paper focuses on the primary in-filter stabilization and the post-discharge, predisposal digestion of aquacultural solids. However, because in-filter stabilization of solids is accomplished concomitantly with nitrification in EGBs, the interplay of these two processes are considered.

Development of environmentally sustainable farming of marine and freshwater species using recirculating aquaculture systems (RASs) requires a complete understanding of the biological component involved in wastewater treatment. This component integrates biofilters composed of microbial communities whose structure, dynamics, and activities are responsible for system success. Engineering highly efficient, environmentally sound, disease-free, and economically viable systems necessitates a thorough knowledge of microbial processes involved in all facets of RAS biofilters and has only recently been the focus of comprehensive studies. These studies have included the application of molecular tools to characterize community diversity and have identified key processes useful for improving system performance. In this paper we summarize the current understanding of the microbial diversity and physiology of RAS biofilters and discuss directions for future studies.

From an environmental point of view, hydrogen peroxide (HP) has beneficial attributes compared with other disinfectants in terms of its ready degradation and neutral by-products. The rapid degradation of HP can, however, cause difficulties with regard to safe and efficient water treatment when applied in different systems. In this study, we investigated the degradation kinetics of HP in biofilters from water recirculating aquaculture systems (RAS). The potential effect of HP on the nitrification process in the biofilters was also examined. Biofilter elements from two different pilot-scale RAS were exposed to various HP treatments in batch experiments, and the HP concentration was found to follow an exponential decay. The biofilter ammonia and nitrite oxidation processes showed quick recuperation after exposure to a single dose of HP up to 30 mg L−1. An average HP concentration of 10–13 mg L−1 maintained over 3 h had a moderate inhibitory effect on the biofilter elements from one of the RAS with relatively high organic loading, while the nitrification was severely inhibited in the pilot-scale biofilters from the other RAS with a relatively low organic loading. A pilot-scale RAS, equipped with two biofilter units, both a moving-bed (Biomedia) and a fixed-bed (BIO-BLOK®) biofilter, was subjected to an average HP concentration of ∼12 mg L−1 for 3 h. The ammonium- and nitrite-degrading efficiencies of both the Biomedia and the BIO-BLOK® filters were drastically reduced. The filters had not reverted to pre-HP exposure efficiency after 24 h, suggesting a possible long-term impact on the biofilters.

This paper describes the performance characteristics of an industrial-scale air-driven rotating biological contactor (RBC) installed in a recirculating aquaculture system (RAS) rearing tilapia at 28 8C. This three-staged RBC system was configured with stages 1 and 2 possessing approximately the same total surface area and stage 3 having approximately 25% smaller. The total surface area provided by the RBC equaled 13,380 m 2 . Ammonia removal efficiency averaged 31.5% per pass for all systems examined, which equated to an average (AE standard deviation) total ammonia nitrogen (TAN) areal removal rate of 0.43 AE 0.16 g/m 2 /day. First-order ammonia removal rate (K 1 ) constants for stages 1-3 were 2.4, 1.5, and 3.0 h À1 , respectively. The nitrite first-order rate constants (K 2 ) were higher, averaging 16.2 h À1 for stage 1, 7.7 h À1 for stage 2, and 9.0 h À1 stage 3. Dissolved organic carbon (DOC) levels decreased an averaged 6.6% per pass across the RBC. Concurrently, increasing influent DOC concentrations decreased ammonia removal efficiency. With respect to dissolved gas conditioning, the RBC system reduced carbon dioxide concentrations approximately 39% as the water flowed through the vessel. The cumulative feed burdendescribes the mass of food delivered to the system per unit volume of freshwater added to the system daily -ranged between 5.5 and 7.3 kg feed/m 3 of freshwater; however, there was no detectable relationship between the feed loading rate and ammonia oxidation performance. Published by Elsevier B.V.

To mitigate the serious water pollution caused by the rapid expansion of the aquaculture industry in recent years, the development of improved aquaculture systems with more efficient water usage and less environmental impact has become essential. In this study, a land-based recirculating aquaculture system (RAS) was established that consisted of purification units (i.e., a primary biological pond, two parallel horizontal subsurface flow constructed wetlands [CWs], and a long ecological ditch) and 4-5 series-connected recirculating ponds. This system was mainly designed to stock channel catfish (Ictalurus punctatus), fifteen spine stickleback (Spinibarbus sinensis) and yellow catfish (Pelteobagrus fulvidraco), and the culture efficacy was evaluated based on a 2-year field experiment covering two growing seasons. According to the results, the primary biological pond played a role in sedimentation or nutrient retention, although this was not as evident when the CWs were functioning. The water flowing through the wetland system at a hydraulic loading rate (HLR) of 600 mm/day displayed lower values for the temperature, pH, dissolved oxygen (DO), suspended solids, organic matter and nutrients, whereas the electrical conductivity (EC) was higher, suggesting the accumulation of dissolved solids in the system. Due to the recirculation treatment, the trophic status of the recirculating ponds increased gradually along the direction of the flow and was notably lower in comparison to the control. As a result, the fish production responded to the variation of the water quality, which was reflected in the measurements of culture efficacy (final weight, survival rate, SGR and yield). The three main rearing species showed a decreasing trend along the direction of the flow, which was higher compared to the control, whereas an opposite trend was observed for filter-feeding fish. A Pearson correlation analysis revealed that the main culture species were inclined to live in meso-or oligotrophic conditions, and the silver carp adapted to more eutrophic conditions. Because RAS can provide better environmental conditions year-round, the present culture method could be more suitable for species that are sensitive to water quality in typical subtropical areas.

To date, farming fish in recirculating aquaculture systems (RAS) is one of the most environmentally friendly ways of producing fish. However, with the trend towards intensification, and consequently decrease in water exchange rates, these systems may accumulate substances, such as heavy metals, in the water and fish. Inductively-coupled plasma-optical emission spectrometry (ICP-OES) and inductively coupled plasma-mass spectroscope (ICP-MS) were used to determine Al, As, Cd, Cr, Cu, Fe, Mn, Ni, Pb and Zn, in the water and fish (Nile tilapia Oreochromis niloticus). Three RAS were used, differing in daily water exchange rates (30, 70 and 1500 l/kg feed/d). The concentrations of As, Fe, Mn, Ni and Zn in the water increased with decreasing water exchange rates, suggesting an accumulation of heavy metals as more water was re-used. Such accumulation in the water was, however, not translated into accumulation in the liver and muscle. Accumulation of heavy metals was always higher in the liver than in the muscle; however, As reached 1.61 mg/kg wet weight in the muscle of fish farmed in RAS-70 l/kg feed/d. However, these levels are considerably lower than permissible safety levels for human consumption.

The aim of the present work was to simulate water treatment practices with hydrogen peroxide (HP) in recirculating aquaculture systems (RAS). Six identical 1,700-L pilot-scale RAS were divided into two experimental groups based on daily feed allocation and operated under constant conditions for a period of 3 months. The organic and nitrogenous loadings of the systems differed fourfold between the two groups and were achieved by predefined constant daily feed loads and constant additions of water. The fixed cumulative feed burden was 1.6 × 103 mg feed/L in the low-intensity RAS and 6.3 × 103 mg/L in the high-intensity RAS. The decay of HP in rearing tanks and disconnected biofilter units was investigated by means of HP spiking experiments. The decay in high-intensity RAS rearing units and biofilters was orders of magnitude faster than that in low-intensity units. The application of HP impaired biofilter nitrite oxidation in low-intensity RAS but not in high-intensity RAS. The impact of HP exposure time on biofilter nitrification capacity was then assessed in biofilter bench-scale experiments with nitrite spiking. Exposure time was found to significantly affect nitrite oxidation. Compared with unexposed biofilter elements, nitrite oxidation was reduced more than 90% following 3 h of exposure to 15 mg HP/L, whereas 30 min of exposure had only minor negative effects on nitrite oxidation. The findings of this study demonstrate the potential for developing HP water treatment practices for RAS and contradict prevailing notions that HP cannot be used safely in RAS that employ biofiltration. The development of effective new HP treatment protocols for recirculating aquaculture could reduce the current dependence on formalin to improve water quality and control parasitic loads.Received January 12, 2011; accepted June 22, 2011

Factors such as limitations in water quality and quantity, cost of land, limitations on water discharges, environmental impacts and diseases, are driving the aquaculture industry toward more intensive practices. This will force producers to adopt environmentally friendlier technologies. Recirculating systems, with a biofilter as the most prominent characteristic, treat internally the water contaminated with dissolved organics and ammonia and reduce the amount of water use and discharge from aquaculture operations. This paper reviews the implications of the changing use of recirculating aquaculture systems (RAS) on biofiltration research for freshwater and marine operations. Demand for cost effective biofilters will increase with the expansion of recirculating systems, both as a complement and replacement of traditional ponds. For freshwater aquaculture, emphasis should be placed in cost competitiveness, low head operations, intensification of ponds with RAS biofiltration and the evaluation of suspended growth systems. In the marine systems, an increase in demand of oligotrophic and ultraoligotrophic systems is expected, particularly in the nursery systems. Sizing and cost efficiency of biofilters for nursery operations should be addressed. Problems in marine biofilter acclimation appear to justify the development of new acclimation procedures. Biosecurity concerns, land cost and storm threats will drive nursery systems inland, where saltwater supply and disposal will force an increased water reuse. Denitrification strategies will need to be redefined and optimized for the marine nursery environment. #

This paper describes a design of an industrial-scale rotating biological contactor (RBC) and its performance installed in recirculating aquaculture system culturing tilapia at El-Nenaeia Fish Farm which was described by Ali et al., 2006. The total surface area provided by the RBC equaled 2352 m2. Ammonia removal efficiency averaged 33.2%, equated to an average (± standard deviation) total ammonia nitrogen (TAN) volumetric loading and removal rate of 368.9126.2 and 133.368.8 g TAN.m-3.day-1, respectively.

The dual objective of sustainable aquaculture, i.e., to produce food while sustaining natural resources is achieved only when production systems with a minimum ecological impact are used. Recirculating aquaculture systems (RASs) provide opportunities to reduce water usage and to improve waste management and nutrient recycling. RAS makes intensive fish production compatible with environmental sustainability. This review aims to summarize the most recent developments within RAS that have contributed to the environmental sustainability of the European aquaculture sector. The review first shows the ongoing expansion of RAS production by species and country in Europe. Life cycle analysis showed that feed, fish production and waste and energy are the principal components explaining the ecological impact of RAS. Ongoing developments in RAS show two trends focusing on: (1) technical improvements within the recirculation loop and (2) recycling of nutrients through integrated farming. Both trends contributed to improvements in the environmental sustainability of RAS. Developments within the recirculation loop that are reviewed are the introduction of denitrification reactors, sludge thickening technologies and the use of ozone. New approached towards integrated systems include the incorporation of wetlands and algal controlled systems in RAS. Finally, the review identifies the key research priorities that will contribute to the future reduction of the ecological impact of RAS. Possible future breakthroughs in the fields of waste production and removal might further enhance the sustainabilty of fish production in RAS.

Waste discharge from recirculating aquacultural systems is typically in the form of sludge composed of partially stabilized excreta, uneaten food particles, and bacterial growth. The amount of solids produced can be estimated using an equation presented in this paper. In a typical recirculating system, total suspended solids (TSS) is the major pollutant of the sludge produced. The production of TSS ranges from 10 to 30% of the feeding rate on a dry weight basis. The ratio of 5-d biochemical oxygen demand to total suspended solids (BODflSS) of the sludge ranges from 0.10 to 0.2, the total Kjeldahl nitrogen (TKN) content of the TSS ranges from 4 to 6 % , and total phosphorus ranges from 0.2 to 2%.

The dual objective of sustainable aquaculture, i.e., to produce food while sustaining natural resources is achieved only when production systems with a minimum ecological impact are used. Recirculating aquaculture systems (RASs) provide opportunities to reduce water usage and to improve waste management and nutrient recycling. RAS makes intensive fish production compatible with environmental sustainability. This review aims to summarize the most recent developments within RAS that have contributed to the environmental sustainability of the European aquaculture sector. The review first shows the ongoing expansion of RAS production by species and country in Europe. Life cycle analysis showed that feed, fish production and waste and energy are the principal components explaining the ecological impact of RAS. Ongoing developments in RAS show two trends focusing on: (1) technical improvements within the recirculation loop and (2) recycling of nutrients through integrated farming. Both trends contributed to improvements in the environmental sustainability of RAS. Developments within the recirculation loop that are reviewed are the introduction of denitrification reactors, sludge thickening technologies and the use of ozone. New approached towards integrated systems include the incorporation of wetlands and algal controlled systems in RAS. Finally, the review identifies the key research priorities that will contribute to the future reduction of the ecological impact of RAS. Possible future breakthroughs in the fields of waste production and removal might further enhance the sustainabilty of fish production in RAS.

Reduced fishery harvests and increased consumer demand for seafood have precipitated an increase in intensive fish farming, predominantly in coastal and open ocean net-pens. However, as currently practiced, aquaculture is widely viewed as detrimental to the environment and typical operations are vulnerable to environmental influences, including pollution and endemic diseases. Here we report the development of a land-based, marine recirculating aquaculture system that is fully contained, with virtually no environmental impact as a result of highly efficient biological waste treatment and water recycling. Over 99% of the water volume was recycled daily by integrating aerobic nitrification to eliminate toxic ammonia and, for the first time, simultaneous, anaerobic denitrification and anaerobic ammonium oxidation, to convert ammonia and nitrate to nitrogen gas. Hydrogen sulfide generated by the separated endogenous organic solids was used as an electron source for nitrate reduction via autotrophic denitrification and the remaining organic solids were converted to methane and carbon dioxide. System viability was validated by growing gilthead seabream (Sparus aurata) from 61 g to 412 g for a total of 1.7 tons in a record 131 days with 99% fish survival. Ammonia nitrite and nitrate did not exceed an average daily concentration of 0.8 mg/l, 0.2 mg/l and 150 mg/l, respectively. Food conversion values were 16% lower than recorded levels for net-pen aquaculture and saltwater usage of less than 16 l/every kg of fish produced. The system is site-independent, biosecure, devoid of environmental contaminants and is not restricted to a single species.

Sea bass (Dicentrarchus labrax) (135 AE 4 g) were reared under tank-based recirculating aquaculture system for a 63-day period at four densities: 10, 40, 70, 100 kg m À3 . Fish performance, stress indicators (plasma cortisol, proteonemia plus other blood parameters-Na + , K + , glucose, pH, total CO 2 À ) and water quality were monitored. At the end of the 63-day period, resistance to infection was also studied by a nodavirus challenge. A 25-day test was performed on fish from two extreme densities (10 and 100 kg m 3 ) and one intermediate density (40 kg m 3 ).

Peracetic acid (PAA) is a powerful disinfectant with a wide spectrum of antimicrobial activity. PAA and hydrogen peroxide (HP) degrade easily to oxygen and water and have potential to replace formalin in aquaculture applications to control fish pathogens, for example the ectoparasite, Ichthyophthirius multifiliis. We studied water phase PAA and HP decay in three aquaculture situations, i) batch experiments with two types of system waters, ii) PAA decay at different fish densities, and iii) degradation of PAA in submerged biofilters of recirculating aquaculture systems (RAS). Furthermore, effect of PAA on the nitrification activity and the composition of the nitrifying population were investigated. PAA and HP decay showed first order kinetics. High dosage PAA/HP in water with low COD inhibited HP removal, which was not observed in water having a higher COD content. PAA decay was significantly related to fish stocking density, with half life constants for PAA of 4.6 and 1.7 h at 12 and 63 kg m − 3 , respectively. PAA application to RAS biofilter showed rapid exponentially decay with half life constants of less than 1 h, three to five times faster than the water phase decay rates. Biofilter surface specific PAA removal rates ranged from 4.6 to 13.9 mg PAA m − 2 h − 1 and was positively correlated to the nominal dosage. Low PAA additions (1.0 mg L − 1 ) caused only minor impaired nitrification, in contrast to PAA application of 2.0 and 3.0 mg L − 1 , where nitrite levels were significantly increased over a prolonged period, albeit without fish mortality. The dominant ammonium oxidizer was Nitrosomonas oligotropha and the dominant nitrite oxidizer was Nitrospira. Based on the present findings and other recent results from field and in vitro studies, application perspectives of PAA are discussed.

The aim of this work was to develop a better understanding of a floating medium in a mechanical filtration mode. The experiments were carried out using a commonly available polystyrene floating medium filter with the grain size of 1 mm. A sand medium filter with the similar grain size was also tested for the comparison. A short-term trial of 2 h and a long-term of 20 days filtration times were conducted with three custom manufactured pressurized filters of 16 l. The filters were operated under three different configurations: (i) upflow with floating media (UFMF), (ii) downflow with floating media (DFMF) and (iii) downflow with a sand medium (DSF). The results of the long-term trial indicated that at a flow rate of 22 m/h, the UFMF and DSF had similar solid removal capacity with an average total suspended solids (TSS) removal efficiency of 60%. The DFMF could only remove 33% of TSS. However, during the short-term trial, TSS removal efficiency of the UFMF was better compared to the DSF (e.g., 71%, 56% and 57% of TSS removal in UFMF compared to 66%, 49% and 41% in the DSFF at the flow rates of 20, 25 and 31 m/h, respectively). The energy requirements of each filter were compared by measuring the pressure differential across each filter. The long-term trial indicated that the UFMF had a significantly less pressure differential (44 kPa) compared to the DSF (80 kPa) (p < 0.001). This was further confirmed that at different flow rates whereby the DSF displayed higher pressure differentials for filtration rates at 350, 450, 550 and 800 l/h.

Refreshment (make-up) water is used in recirculating aquaculture systems (RAS) mainly to purge off-flavors, to add alkalinity and sometimes for temperature control. Alternatively, alkalinity may be added by means of a chemical base and heat may be supplied by a heating system. The objective of this study is to show how the optimal (minimizing cost) mix of the three controls: water, base and heat, can be found for given temperatures and water prices.

The high levels of water-reuse in intensive recirculating aquaculture systems (RAS) require an effective water treatment in order to maintain good water quality. In order to reveal the potential and limitations of ozonation for water quality improvement in marine RAS, we tested ozone's ability to remove nitrite, ammonia, yellow substances and total bacterial biomass in seawater, considering aspects such as efficiency, pH-dependency as well as the formation of toxic ozone-produced oxidants (OPO). Our results demonstrate that ozone can be efficiently utilized to simultaneously remove nitrite and yellow substances from process water in RAS without risking the formation of toxic OPO concentrations. Contemporaneously, an effective reduction of bacterial biomass was achieved by ozonation in combination with foam fractionation. In contrast, ammonia is not oxidized by ozone so long as nitrite and yellow substances are present in the water, as the dominant reaction of the ozone-based ammonia-oxidation in seawater requires the previous formation of OPO as intermediates. The oxidation of ammonia in seawater by ozone is basically a bromide-catalyzed reaction with nitrogen gas as end product, enabling an almost complete removal of ammonia-nitrogen from the aquaculture system. Results further show that pH has no effect on the ozone-based ammonia oxidation in seawater. Unlike in freshwater, an effective removal of ammonia even at pH-values as low as 6.5 has been shown to be feasible in seawater. However, as the predominant reaction pathway involves an initial accumulation of OPO to toxic amounts, we consider the ozone-based removal of ammonia in marine RAS as risky for animal health and economically unviable.

The drumfilter effluent from a recirculation aquaculture system (RAS) can be used as substrate for heterotrophic bacteria production. These bacteria can be reused as aquatic feed. In RAS drumfilter effluents are organic carbon deficient for bacteria production. This is due to nitrogen accumulation in the system water. In the present experiment, a bacteria growth reactor (3.5l) was connected to the drumfilter (filter mesh size 60 μm) outlet of a recirculation system. To counteract carbon deficiency, different supplementation levels of molasses (organic carbon) were tested (carbon fluxes of 0.0, 3.2, 5.8, 7.8, 9.7 gC molasses/l/d; C:N ratios: 3.4, 6.4, 9.4, 13.0 and 16.5). The hydraulic retention time in the bacteria reactor was 6 h. For the maximum flux, the VSS and crude protein productions were about 168 gVSS and 95 g crude protein per kg feed. The maximum conversion of nitrate and ortho-phosphate was 24 g NO 3− N and 4 gP/kg feed. This equals a conversion of 90% of the inorganic nitrogenous waste and 98% of the ortho-phosphate-P. Furthermore the maximum substrate removal rate and the K s for molasses were determined (1.62 gC/l/h and 0.097 gC/l respectively). The maximum specific removal rate was 0.31 gC/gVSS/h and the related half saturation constant was 0.008 gC/l. The observed growth rate reached a maximum for C fluxes higher than 8 g/l/d. The present integration of heterotrophic bacteria production in RAS represents, therefore, an innovative option to reduce waste discharge by conversion. If the bacteria are reused as feed, system's ecological sustainability increases.

Photosynthetic bacteria were isolated from photosynthetic sludge for use in a recirculating aquaculture system. A strong denitrifying photosynthetic bacterium was identified from four isolates. The useful strain was taxonomically identified to be Rhodopseudomonas palustris. The parameters of growth were characterized in anaerobic flask culture. The optimum pH, temperature, and illumination intensity were 5.5, 31°C, and 5000 lux, respectively. The maximum specific growth rate and the rate of gas production by denitrification were 0.095 h − 1 and 0.2 ml N 2 h − 1 , respectively. Dissimilatory nitrate reduction to nitrogen gas by the isolate started after the late-log phase, and a small amount of nitrite was accumulated at the end of the culture. The maximum number of viable cells was 14 × 10 8 cells ml − 1 with 1.07 g l − 1 of dry-cell weight, and the maximum concentration of bacteriochlorophyll a was 0.17 OD 775 g − 1 of dry-cell weight.

Marine hatcheries represent high value, low waste systems for larvae that are sensitive to general infections by opportunistic bacteria. In intensive cultivation several procedures destabilise the microbial community of the rearing water and favour growth of potentially harmful microbes. Recirculation aquaculture systems (RASs) have properties that may contribute to microbial stabilisation, including long water retention time and a large surface area of biofilters for bacterial growth. Moreover, the microbial community in RAS biofilters has the possibility to stabilise at a similar carrying capacity as the larvae tanks, which could potentially limit the chances of proliferation of opportunistic microbes in the rearing water. The development of the microbial community in a RAS with moderate ozonation (to 350 mV) was compared to that of a conventional flow-through system (FTS) for the same group of Atlantic cod, Gadus morhua. The feed and intake water were the same for the two groups. The RAS developed and maintained a more diverse and stable microbial community composition compared to the FTS. Water treatment regime explained most of the variation in microbial composition during the live feed period, and in addition it accounted for ten times higher variation in the composition of bacteria in the FTS than in the RAS. Less variability in bacterial composition was found between replicate fish tanks of the RAS than between tanks of the FTS. The RAS had a more even microbial community structure with higher species diversity and periodically a lower fraction of opportunists. The fish in the RAS performed better than their siblings in the FTS, despite being exposed to an apparent inferior physiochemical water quality.

In a recirculation aquaculture system the drumfilter effluent can be used as substrate for heterotrophic bacterial production, which can be recycled as feed. Because the bacteria might contain pathogens, which could reduce its suitability as feed, it is important to characterize these communities. Bacteria were produced in growth reactors under different conditions: 7 h hydraulic retention time (HRT) vs. 2 h, sodium acetate vs. molasses, and ammonia vs. nitrate. The community of the drumfilter effluent was different from those found in the reactors. However, all major community components were present in the effluent and reactor broths. HRT influenced the bacteria community, resulting in a DGGE profile dominated by a band corresponding to an Acinetobacter sp.-related population at 2 h HRT compared to 7 h HRT, where bands indicative of a-proteobacterial populations most closely related to Rhizobium and Shinella spp. were most abundant. Molasses influenced the bacterial community. It was dominated by an Aquaspirillum serpens-related population. Providing total ammonia nitrogen (TAN) in addition to nitrate led to the occurrence of bacteria close to Sphaerotilus spp., Flavobacterium mizutaii and Jonesia spp. It was concluded from these results that a 6-7 h HRT is recommended, and that the type of substrate is less important, and results in communities with a comparably low pathogenic risk.

The accumulation of substances in Recirculating Aquaculture Systems (RAS) may impair the growth and welfare of fish. To test the severity of contaminants accumulated in RAS, early-life stages of fish were used. Ultrafiltered water from two Recirculating Aquaculture Systems (RAS), one RAS with a high accumulation of substances (water exchange rate 30 L/kg feed/day) and one RAS with a low accumulation of substances (water exchange rate 1500 L/kg feed/day), was used to incubate eggs and rear larvae of common carp Cyprinus carpio. A broad range of read-out parameters was used to determine the effect of accumulation level on the development of the early-life stages; from hatching dynamics to larvae length and dry weight. The water quality (temperature, pH, dissolved O 2 , conductivity, total bicarbonate, ortho-phosphate-P, TAN, NO 2 − -N, NO 3 − -N and minerals) was compared between the 2 treatments. Carp eggs developing in the highaccumulation water had higher mortality percentages (both for eggs and larvae), reduced hatching percentages, delayed hatching dynamics and reduced larvae length and body weight. However, these larvae exhibited fewer deformities than larvae incubated in the low-accumulation water. Furthermore, an accelerated development both of the embryo (appearance of heart beat, pectoral fin bud and tail movement) and yolk-sac larvae (depletion of the yolk sac) was observed in the high-accumulation water. The highaccumulation water had significantly lower pH and higher conductivity, NO 2 − -N, NO 3 − -N and orthophosphate-P. Most of the minerals (As, Cu, Mn, Ni, Zn, K, Mg, Na, P and S) including heavy metals, were present at a higher concentration in the high-accumulation water. The influence of these parameters on the embryonic and larval development of fish is discussed. It is suggested that in the high-accumulation water, the concentration of ortho-phosphate-P, nitrate and of the heavy metals arsenic and copper is likely to have impaired the embryonic and larval development and therefore deserves further research as potential growth inhibiting factors in RAS.

This paper analyses nutrient conversions, which are taking place in integrated intensive aquaculture systems. In these systems fish is cultured next to other organisms, which are converting otherwise discharged nutrients into valuable products. These conversions are analyzed based on nitrogen and phosphorous balances using a mass balance approach. The analytical concept of this review comprises a hypothetical system design with five modules: (1) the conversion of feed nutrients into fish biomass, the ''Fish-Biomass-Converter''; (2) the separation of solid and dissolved fish waste/ nutrients; the ''Fish-Waste-Processor''; (3) the conversion of dissolved fish waste/nutrients, the ''Phototrophic-herbivore-Converter''; (4 and 5) the conversion of solid fish waste, the ''Bacterial-Waste-Converter'', or the ''Detrivorous-Converter''. In the reviewed examples, fish culture alone retains 20-50% feed nitrogen (N) and 15-65% feed phosphorous (P). The combination of fish culture with phototrophic conversion increases nutrient retention of feed N by 15-50% and feed P by up to 53%. If in addition herbivore consumption is included, nutrient retention decreases by 60-85% feed N and 50-90% feed P. This is according to the general observation of nutrient losses from one trophic level to the next. The conversion of nutrients into bacteria and detrivorous worm biomass contributes only in smaller margins (e.g. 7% feed N and 6% feed P and 0.06% feed N 0.03 Â 10 À3 % feed P, respectively). All integrated modules have their specific limitations, which are related to uptake kinetics, nutrient preference, unwanted conversion processes and abiotic factors. #

Aquaculture saline water reuse BDD anode Electro-oxidation Nitrogen compounds COD a b s t r a c t The viability of the electro-oxidation technology provided with boron doped diamond (BDD) electrodes for the treatment and reuse of the seawater used in a Recirculating Aquaculture System (RAS) was evaluated in this work. The influence of the applied current density (5e50 A m À2 ) in the removal of Total Ammonia Nitrogen (TAN), nitrite and chemical oxygen demand (COD) was analyzed observing that complete TAN removal together with important reductions of the other considered contaminants could be achieved, thus meeting the requirements for reuse of seawater in RAS systems.

It has been suggested that fish cultured in recirculating aquaculture systems (RAS) grow less as compared with fish cultured in flow-through systems due to the accumulation of substances. In the Netherlands, the commercial culture of Nile tilapia Oreochromis niloticus in 300 and 600 MT's systems is done exclusively in RAS operated at water exchange rates as low as 30 L/kg feed/day due to nitrate control by single-sludge denitrification reactors. The use of such nearly closed RAS raises the question whether growth retardation (GR) is present in Nile tilapia. This study is the first to investigate the existence of growth retardation in Nile tilapia by comparing the growth, feeding behaviour and stress response of Nile tilapia cultured in RAS with different levels of substances accumulated. Three RAS, operated at 30 L/kg feed/day (HIGH accumulation), 70 L/kg feed/day (MIDDLE accumulation) and 1500 L/kg feed/day (LOW accumulation) were used. Each RAS contained 24 glass aquaria with individually housed fish. To determine whether GR is size-dependent, per RAS 3 fish size categories were tested in the 57 day experimental period: large (288.7 ± 34.2 g; N = 8), medium (162.4± 23.4 g; N = 8) and small (81.4 ± 21.0 g; N = 8). Experimental fish were fed ad libitum, twice per day. Feeding behaviour was determined once per week and was measured as the time taken by each fish to eat the first pellet (latency, LAT) and the total time spent feeding (total feeding time, TFT). Temperature, pH, conductivity, alkalinity, dissolved oxygen, dissolved CO 2 , nitrogen compounds (TAN, NO 2 -N and NO 3 -N), chemical oxygen demand and orthophosphate-P were measured over time. At day 57 fish were weighed, blood sampled and returned to their tanks for an extra experimental period of 15 days, and subjected at day 72 to an acute stress followed by blood sampling. Blood was analysed for glucose and cortisol. Results showed that the water quality parameters measured in the 3 RAS (with the exception of alkalinity) were still within the optimum range for growth of Nile tilapia. Large individuals showed a tendency to grow more in the LOW treatment (2.66 ± 1.35 g/kg 0.8 /day) as compared with the MIDDLE treatment (0.93 ± 1.63 g/kg 0.8 /day). On the contrary, small individuals grew significantly less in the LOW treatment (3.60 ± 1.74 g/kg 0.8 /day) as compared with the HIGH (7.22 ± 1.58 g/kg 0.8 /day) and MIDDLE (6.82 ± 4.54 g/kg 0.8 /day) treatments. Small fish were more motivated to eat (lower latency) in the MIDDLE (4.63 ± 5.24 min) as compared with the LOW treatment (8.94 ± 6.41 min). In the HIGH accumulation treatment higher glucose levels were observed in the small fish, before and after acute stress, as compared with the LOW accumulation treatment. In conclusion, this study showed that the extent to which the accumulation of substances in RAS affects growth depends on fish size: large individuals show a trend towards growth retardation in the highest accumulation RAS while small individuals, on the contrary, seem to grow better in such systems.

A recirculating aquaculture system (RAS) treating tilapia production wastewaters used a two-step process combining a simple clarifier and a submerged surface flow (SSF) constructed wetlands for suspended solids removal and removal of nitrogenous compounds. This system successfully supported a commercial scale level of production (&gt;35 kg/m3) for over 36 months of operation. The innovative SSF wetland design incorporated a high hydraulic loading rate (3.03 m/day), larger effective diameter media (380 mm), and a deeper bed depth (0.90 m) than previously suggested design guidelines. The SSF wetland flow pattern was characterized as plug flow with dispersion, but this analysis based on bed volume data indicated that media porosity was reduced from an assumed design value of 54–27% under operating conditions. The TSS, TAN, NO2-N, and NO3-N, percent mass removal for the SSF wetlands was 67.2, 46.0, 87.0, and 40.6, respectively. The TSS, TAN, NO2-N, and NO3-N mass removal for the SSF wet...

Geosmin and 2-methylisoborneol (MIB) are semi-volatile terpenoid compounds produced as secondary metabolites by benthic and planktonic cyanobacteria, several genera of fungi, and various actinomycetes. These off-flavor compounds pose a heavy economic burden in the aquaculture industry rendering fish unmarketable unless purified by purging with large quantities of clean water. In the present study, the presence of off-flavor compounds was examined in a recirculating aquaculture system (RAS) for tilapia culture. In this zero-discharge system, where water from the fish basins is recirculated through parallel aerobic (drum filter and a trickling filter) and anaerobic treatment loops (sedimentation/digestion basin), concentrations of geosmin and, in particular, MIB were highest in the aerobic treatment loop. Lowest concentrations were detected in the anaerobic treatment loop. This latter finding pointed toward a possible reduction of these compounds in this basin. Two bacterial strains of the streptomycetes family were isolated from the aerobic, organic-rich, drum filter and the nitrifying trickling filter. In vitro tests with these isolates, closely related to Streptomyces roseoflavus and Streptomyces thermocarboxydus, revealed that MIB production exceeded geosmin production under all conditions tested and was significantly higher under aerobic than under anoxic conditions. Under the latter conditions, with nitrate as an electron donor, the S. roseoflavus-like isolate was capable of denitrification. Based on the results obtained in this study, it was concluded that aerobic, organic-rich conditions stimulate the growth of actinomycetes and subsequent production of geosmin and MIB in the system. The observed reduction of these compounds in the anaerobic water treatment component may serve in designing treatment steps aimed at alleviating the problem of geosmin and MIB accumulation in recirculating systems.