Marine Recirculating Aquaculture System Research Articles

Denitrification performance evaluation and kinetics analysis with mariculture solid wastes (MSW) derived carbon source in marine recirculating aquaculture systems (RAS)

Biological denitrification using mariculture solid wastes (MSW) carbon source is a promising solution for removing nitrate (NO3−-N) and disposing MSW in marine recirculating aquaculture systems (RAS). To enhance denitrification performance, heating (HT), rhamnolipid (RL), alkali (AL), thermophilic bacteria (TB) pre-treated MSW acidogenic fermentation effluents were prepared as carbon sources. Profiles of soluble organics in four types of fermentation effluents were first evaluated. The highest volatile fatty acids (VFAs) yield (52.1%) was obtained from TB treated MSW after acidification. RL and TB treated MSW acidogenic fermentation effluents showed high NO3−-N removal efficiency (NRE) (around 97%). Acidogenic fermentation effluent from TB treated MSW presented a high biodegradability, with the minimum effluent chemical oxygen demand (COD) amount (35 mg/L). Denitrification kinetics parameters were also analyzed; high fraction (74.5%) of the most readily biodegradable organics (SS) demonstrated that TB treated MSW acidogenic fermentation effluent is a high-quality carbon source for enhancing denitrification.

Activated sludge denitrification in marine recirculating aquaculture system effluent using external and internal carbon sources

Stringent environmental legislation in Europe, especially in the Baltic Sea area, limits the discharge of nutrients to natural water bodies, limiting the aquaculture production in the region. Therefore, cost-efficient end-of-pipe treatment technologies to reduce nitrogen (N) discharge are required for the sustainable growth of marine land-based RAS. The following study examined the potential of fed batch reactors (FBR) in treating saline RAS effluents, aiming to define optimal operational conditions and evaluate the activated sludge denitrification capacity using external (acetate, propionate and ethanol) and internal carbon sources (RAS fish organic waste (FOW) and RAS fermented fish organic waste (FFOW)). The results show that between the evaluated operation cycle times (2, 4, and 6 h), the highest nitrate/nitrite removal rate was achieved at an operation cycle time of 2 h (corresponding to a hydraulic retention time of 2.5 h) when acetate was used as a carbon source. The specific denitrification rates were 98.7 ± 3.4 mg NO3−-N/(h g biomass) and 93.2 ± 13.6 mg NOx−-N/(h g biomass), with a resulting volumetric denitrification capacity of 1.20 kg NO3−-N/(m3 reactor d). The usage of external and internal carbon sources at an operation cycle time of 4 h demonstrated that acetate had the highest nitrate removal rate (57.6 ± 6.6 mg N/(h g biomass)), followed by propionate (37.5 ± 6.3 mg NO3−-N/(h g biomass)), ethanol (25.5 ± 6.0 mg NO3−-N/(h g biomass)) and internal carbon sources (7.7 ± 1.6–14.1 ± 2.2 mg NO3−-N/(h g biomass)). No TAN (Total Ammonia Nitrogen) or PO43- accumulation was observed in the effluent when using the external carbon sources, while 0.9 ± 0.5 mg TAN/L and 3.9 ± 1.5 mg PO43--P/L was found in the effluent when using the FOW, and 8.1±0.7 mg TAN/L and 7.3 ± 0.9 mg PO43--P/L when using FFOW. Average sulfide concentrations varied between 0.002 and 0.008 mg S2-/L when using the acetate, propionate and FOW, while using ethanol resulted in the accumulation of sulfide (0.26 ± 0.17 mg S2-/L). Altogether, it was demonstrated that FBR has a great potential for end-of-pipe denitrification in marine land-based RAS, with a reliable operation and a reduced reactor volume as compared to the other available technologies. Using acetate, the required reactor volume is less than half of what is needed for other evaluated carbon sources, due to the higher denitrification rate achieved. Additionally, combined use of both internal and external carbon sources would further reduce the operational carbon cost.

Taxonomic and functional profiling of nitrifying biofilms in freshwater, brackish and marine RAS biofilters

In recirculating aquaculture systems (RAS), the crucial step of eliminating toxic N compounds like ammonia and nitrite is mediated via nitrifying microorganisms and takes place in biofilters. In this study, analyses of microorganisms colonizing biocarriers of nine moving-bed biofilters of three different RAS operated with freshwater, brackish or marine process water uncovered site specific communities. Illumina-based amplicon sequencing of the V4-region of the 16S rRNA gene revealed a high microbial diversity with 1000–2500 species-level operational taxonomic units (OTUs) in all biofilters with the highest diversity in the brackish RAS. Proteobacteria, Bacteriodetes, Plantomycetes, Chloroflexi and Nitrospirae represented the most abundant phyla. 76 out of 674 known genera occurred in all nine biofilters and were defined as core-taxa, including nitrifying bacteria (Nitrosomonas and Nitrospira) as well as members of the (heterotrophic) genera Planctomyces, Blastopirellula, Nannocystis and Lewinella. Nitrifying communities composed of different, closely related and so far uncultured members of Nitrosomonas and Nitrospira were identified, strongly indicating that several potentially novel ammonia and nitrite oxidizing species are present in RAS biofilters. Relatives of known comammox Nitrospira were detected in the brackish biofilters, revealing 94–99 % identity of the 16S rRNA gene sequence to Ns. inopinata. Salinity tolerance tests with biocarriers derived from biofilters of the three distinct RAS showed an unexpected broad physiological flexibility with regard to salinity. Nitrification performance of freshwater nitrifiers was drastically reduced with increasing salinity and nearly completely inhibited at 15 PSU, while the brackish and marine nitrifiers showed a high resistance and maintained nitrification activity in a broad range of salt concentrations. This data can help to improve the nitrification process in RAS with changing salinity of the process water.

A sulfur-based cyclic denitrification filter for marine recirculating aquaculture systems

Nitrogen removal from saline wastewater is challenging due to adverse effects of salinity on biological processes. A novel sulfur-autotrophic cyclic denitrification filter (CDF) was tested for marine recirculating aquaculture systems (RAS) under varying conditions. Low ammonia, nitrite and sulfide concentrations were maintained at residence times between 4 and 12 h. After introduction of Poecilia sphenops, concentrations of NH4+-N, NO2−-N, NO3−-N were maintained below 1, 1, and 60 mg/L, respectively. Fish waste inputs to the CDF contributed to mixotrophic denitrification and low sulfate production. A mass balance showed that 7% of the feed nitrogen was assimilated by fish, 6% was removed by passive denitrification (e.g., in anoxic zones in filters), 60% in the CDF and 27% was discharged during sampling and solids removal. Daily fresh water addition was <2% of fish tank volumes. The results are promising as a low cost alternative for saline wastewater denitrification.

Effect of Mesh Size on Microscreen Filtration Combined with Foam Fractionation for Solids Removal in Recirculating Aquacultural Seawater

AbstractMicroscreen filtration and foam fractionation have been considered as effective solids removal technologies in marine recirculating aquaculture systems (RAS). To figure out the appropriate mesh size for highly efficient solids removal and management, this study investigated the effects of different mesh sizes (120, 90, 60, and 40 μm) on a combination treatment of solids filtration and fractionation removal in RAS for grouper. The experimental results showed that filtration and total removal efficiency increased with the decrease of mesh size on the whole; however, the produced fine particles (&lt;20 μm) increased simultaneously due to the breakup of relatively large particles caused by filtration. Although the best total removal efficiency was 32.33%, obtained with mesh size 40 μm, it was accompanied with a serious particle breakup. We suggest that the large amounts of fine particles are a big risk. Therefore, 60‐μm mesh was better in terms of solids removal efficiency and fine particles management. The effect of mesh size on microscreen filtration was undoubted, while the indirect effect on foam fractionation was negligible. However, fractionation could be effective for removing partial fine particles and its efficiency was quite close to that of filtration. Foam fractionation is thus recognized as an essential treatment in marine RAS.

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.

The Nitrogen-Removal Efficiency of a Novel High-Efficiency Salt-Tolerant Aerobic Denitrifier, Halomonas Alkaliphile HRL-9, Isolated from a Seawater Biofilter.

Aerobic denitrification microbes have great potential to solve the problem of NO3−-N accumulation in industrialized recirculating aquaculture systems (RASs). A novel salt-tolerant aerobic denitrifier was isolated from a marine recirculating aquaculture system (RAS) and identified as Halomonas alkaliphile HRL-9. Its aerobic denitrification performance in different dissolved oxygen concentrations, temperatures, and C/N ratios was studied. Investigations into nitrogen balance and nitrate reductase genes (napA and narG) were also carried out. The results showed that the optimal conditions for nitrate removal were temperature of 30 °C, a shaking speed of 150 rpm, and a C/N ratio of 10. For nitrate nitrogen (NO3−-N) (initial concentration 101.8 mg·L−1), the sole nitrogen source of the growth of HRL-9, the maximum NO3−-N removal efficiency reached 98.0% after 24 h and the maximum total nitrogen removal efficiency was 77.3% after 48 h. Nitrogen balance analysis showed that 21.7% of NO3−-N was converted into intracellular nitrogen, 3.3% of NO3−-N was converted into other nitrification products (i.e., nitrous nitrogen, ammonium nitrogen, and organic nitrogen), and 74.5% of NO3−-N might be converted to gaseous products. The identification of functional genes confirmed the existence of the napA gene in strain HRL-9, but no narG gene was found. These results confirm that the aerobic denitrification strain, Halomonas alkaliphile HRL-9, which has excellent aerobic denitrification abilities, can also help us understand the microbiological mechanism and transformation pathway of aerobic denitrification in RASs.

Comparison of active biofilm carriers and commercially available inoculum for activation of biofilters in marine recirculating aquaculture systems (RAS)

A recirculating aquaculture system (RAS) for rearing Atlantic salmon requires nitrifying microorganisms in the biofilter in order to be fully functional. These microorganisms play a crucial role in water quality improvement by removing ammonia, but the nitrifying population is time-consuming to establish in the biofilter due to their slow growth rates. An efficient biofilter activation strategy is therefore necessary for an improved and short maturation period of biofilters in new RAS. In this study, microbial growth cultures added two different inoculums for RAS were compared. The observed biofilm establishment on clean biofilm carriers and quantified nitrification in the cultures simulated activation of new RAS biofilters. The inoculums used in the experiments included commercial inoculum for marine RAS and transferred biofilm carriers from a marine RAS biofilter operated 1.5 years after activation. The growth experiments revealed that the enrichment cultures inoculated with biofilm carriers from RAS initiated nitrite and nitrate production from ammonia on day 4, which is much earlier than in enrichment cultures with commercial inoculum where nitrite and nitrate production were detectable on day 15 and 18, respectively. The concentration of nitrite and nitrate measured at the end of the experimental period was also substantially higher in enrichment cultures inoculated with biofilm carriers. These results were supported by microbial community analyses of the enrichment cultures, showing that the populations in cultures added biofilm carriers had the highest relative abundance of Nitrosococcus and Nitrospira. Overall, our study shows that biofilm carriers from an established RAS were applicable as inoculum for biofilter activation. However, more knowledge on optimized growth conditions for nitrifying bacteria in RAS could benefit the colonization efficiency and the enrichment of these microorganisms during biofilter activation. In addition, methodical screening for pathogenic bacteria or viruses is needed before biofilter material can be successfully transferred between RAS without risk for disease transfer.

Development of a liquid chromatography atmospheric pressure chemical ionization mass spectrometry method for determining off-flavor compounds and its application toward marine recirculating aquaculture system monitoring and evaluation of aeration as a depuration approach

The off-flavor compounds geosmin and 2-methylisoborneol (2-MIB) are well-known to impact the quality of farmed freshwater fish species, but little is known about off-flavors in marine aquaculture. To begin addressing this knowledge gap, a method for determining geosmin and 2-MIB using LC with atmospheric pressure chemical ionization (APCI) MS detection was developed. While 2-MIB was readily detected using LC-APCI/MS, geosmin exhibited on-column degradation that was independent of column chemistry and could not be eliminated. Optimized conditions were identified that balanced the separation and ionization efficiency of 2-MIB and geosmin while minimizing geosmin degradation, but the overall method sensitivity for geosmin was reduced by the on-column losses. The method was used with direct aqueous injections to determine the volatilization rates of geosmin and 2-MIB at ppb levels during aeration under laboratory conditions in both salt water and pure water to simulate marine and fresh water aquaculture, respectively. The volatilization rates of both compounds were 30% faster in salt water than in fresh water with or without aeration, but aeration was found to enhance the rate by a factor of 2.5 in both water types. The LC-APCI/MS method was combined with stir bar sorptive extraction (SBSE) to achieve greater sensitivity for determining off-flavors in recirculating aquaculture system (RAS) water. Using SBSE-LC-APCI/MS, the LODs for geosmin and 2-MIB were 70 ng/kg (part per trillion) and 6 ng/kg, respectively. The on-column losses resulted in a relatively high LOD for geosmin that renders this method unsuitable for determining geosmin at the low ng/kg levels expected in RAS. SBSE using both grab water samples and an in-situ diving unit were used to evaluate 2-MIB levels in the culture water of two separate marine RAS that were supporting the growth of European sea bass but had differing levels of water treatment. 2-MIB was readily detected using both SBSE approaches in the RAS with less sophisticated treatment when the animal stocking density was at its highest (50 kg/m3) but was not detected in the more sophisticated RAS regardless of stocking density. Geosmin was not detected in either system, but the results were inconclusive given its higher LOD. These limited results suggest that the anaerobic water treatment components, present only in the more sophisticated RAS, maintained the level of 2-MIB below the LOD.

Development of denitrification in semi-automated moving bed biofilm reactors operated in a marine recirculating aquaculture system

This study examined the performance of three independently operated denitrifying moving bed biofilm reactors (MBBRs) in a zero-exchange marine recirculating aquaculture system (RAS) stocked with European seabass (Dicentrarchus labrax). A semi-automated control strategy was applied to foster spontaneous denitrification. Process automation consisted of a pulsed carbon supply and an inflow of nitrate-rich, aerated process water controlled by the oxidation-reduction potential (ORP) in the MBBR. Carbon dosing frequency was adjusted manually if the process produced unwanted products (i.e., nitrite or ammonia). OPR-controlled inflow stimulated bacterial activities in the MBBRs until inflow reached the pre-set maximum at a hydraulic retention time (HRT) of 0.75 h. This allowed for a quick start-up of the denitrification processes in spite of high initial variability of process water inflow and of nitrate removal efficiency (NRE). A start-up with glycerol did not induce a stable denitrification process; however, after the process had been established with acetate, glycerol promoted efficient denitrification with NRE close to one. The successive application of the two carbon sources resulted in a high nitrate removal rate (NRR) of 2 kg nitrate-N m−3 day−1 in the biofilters. This diminished the concentration of nitrate-nitrogen (nitrate-N) in the RAS (volume 9 m3) from 176 to 36 g m−3 in 42 days with biofilters comprising only 1% of the RAS volume. The implications for the development of an automated denitrification process are discussed.

Performance and microbial diversity of bioreactors using polycaprolactone and polyhydroxyalkanoate as carbon source and biofilm carrier in a closed recirculating aquaculture system

Nitrogen removal is essential for a successful management approach in a marine recirculating aquaculture system (RAS) through microbial remediation. In this study, two biodegradable polymers, polycaprolactone (PCL) and polyhydroxyalkanoate (PHA), were used as a carbon source and biofilm carrier for marine RAS wastewater treatment. Results showed that the use of an artificial saltwater nitrate (ASN) medium with seeded polymers had a high nitrogen removal efficiency regardless of flow rate, with a nitrate–nitrite removal efficiency of 95.4–98.9%. The TN removal efficiency was greater with PHA (95.1–95.3%) than PCL (86.1–90.1%). TP removal efficiency was greater with the use of PCL (64.1–67.3%) than PHA (48.2–52.5%). The use of marine aquaculture effluent showed an increased TN nutrient removal efficiency with PCL having a removal efficiency of 89%, with an increased TP removal efficiency using PHA (40.4%). Microbial community analysis demonstrated the complexity of microbial consortia within marine RAS. Two major classes identified in our study were Alphaproteobacteria (2–68%) and Flavobacteriia (0.5–58%). We identified Crocinitomix, Oceanicola, Meridianimaribacter, and Ruegeria as potential PCL degraders, while Winogradskyella, Muricauda, Marinobacter, and Alteromonas were identified as potential PHA degraders. The microbial communities responded to varying water chemistry differences and developed unique consortia based on biodegradable polymer types. Biodegradable plastics can be utilized in marine RAS to treat wastewater through microbial processes.

Bioremediation of organic sludge from a marine recirculating aquaculture system using the polychaete Abarenicola pusilla (Quatrefages, 1866)

Organic sludge, the principal residual product from recirculating aquaculture systems (RAS), is one of the main environmental and logistical challenges associated with aquaculture production. Polychaetes have been described as potential remediating organisms of aquaculture sludge due to its ability to survive in organic enrichment conditions and its capability to assimilate particulate organic waste from intensive aquaculture. The aim of this study was to evaluate the removal performance of Abarenicola pusilla (Quatrefages, 1866) when fed with organic sludge from a marine RAS. Experiments consisted of the addition of fish sludge (0.5, 2.0, 4.0 and 10.0% respect total inert sediment) at different densities of A. pusilla (60, 75, 150 and 200 organisms m−2), with 3 replicates each, and during 45 days. The highest removal rate of total organic matter (85.37%; 35.77 ± 0.05 g m−2 day−1), total N (33.63 ± 0.003 mg day−1) and total C (236.78 ± 0.003 mg day−1) were obtained with 10% of sludge and 200 organisms m−2. Assimilation was corroborated by obtaining significant increase (p < .05) in total carbon (24.75%), organic carbon (24.68%) and lipid content (0.65%) in the biomass of organisms. Highest specific growth rate achieved by A. pusilla was 3.06% day−1 and the highest survival was 91.67% over the 45 days trial period. These results indicated that A. pusilla could be used as remediating species of organic compounds contained in marine aquaculture sludge. The results also suggest that A. pusilla is a potential candidate for nutrient recycling from marine aquaculture systems on land. Further studies should determine the possible changes in the fatty acid profile of A. pusilla in order to use it in aquaculture feeds.

Impact of carrier on ammonia and organics removal from zero-discharge marine recirculating aquaculture system with sequencing batch biofilm reactor (SBBR).

Marine recirculating aquaculture system (MRAS) is an effective technology that provides sustainable farming of food fish globally. However, dissolved organics material (chemical oxygen demand, COD) and especially ammonia are produced from uneaten feed and metabolic wastes of fish. To purify the MRAS water, this study adopted a sequencing biofilm batch reactor (SBBR) and comparatively investigated the performances of four different carriers on ammonia and COD removal. Results indicated that the NH4+-N removal rates were 0.045 ± 0.05, 0.065 ± 0.008, 0.089 ± 0.005, and 0.093 ± 0.003 kg/(m3·d), and the COD removal rates were 0.019 ± 0.010, 0.213 ± 0.010, 0.255 ± 0.015, and 0.322 ± 0.010 kg/(m3·d) in the SBBRs packed with porous plastic, bamboo ring, maifan stone, and ceramsite carriers, respectively. Among the four carriers, ceramsite exhibited the best performance for both NH4+-N (80%) and COD (33%) removal after the SBBR reached the steady-state operation conditions. For all carriers studied, the NH4+-N removal kinetics could be well simulated by the first-order model, and the NH4+-N and COD removal rates were logarithmically correlated with the carrier's specific surface area. Due to its high ammonia removal, stable performance and easy operation, the ceramsite-packed SBBR is feasible for MRAS water treatment.

Effects of Salinity and Immobilization Period on the Nitrification and Denitrification Co-processes during Biofilter Acclimation in a Marine Recirculating Aquaculture System

Effects of Salinity and Immobilization Period on the Nitrification and Denitrification Co-processes during Biofilter Acclimation in a Marine Recirculating Aquaculture System

Bacterial community analysis of marine recirculating aquaculture system bioreactors for complete nitrogen removal established from a commercial inoculum

An experimental recirculating aquaculture system was constructed under ambient seawater conditions to compare microbial community diversity of nitrifying and denitrifying biofilters that were derived from a commercial inoculum used for aquarium applications. Next generation sequencing revealed distinct and diverse microbial communities in samples analyzed from the commercial inoculant and the denitrification and nitrification biofilters. In all samples, communities were represented by a few dominant operational taxonomic units (OTUs). Bacteria having the capacity to carry out ammonia and nitrite oxidation were more abundant in the nitrification biofilter. Similarly, the proportion of the bacterial taxa known to carry out heterotrophic and autotrophic denitrification and participate in sulfur cycling were found in the denitrification bioreactor, and likely originated from the ambient environmental water source. Our results indicated that environmental seawater can be a favorable enhancement to the bacterial consortium of recirculating aquaculture systems biofilters.

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 (k1a) 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.

Denitrifying granules in a marine Upflow Anoxic Sludge Bed (UASB) reactor

Marine land-based Recirculating Aquaculture Systems (RAS) are generally perceived as environmentally friendly aquatic production systems. To promote their sustainability even further and reduce the discharge of nutrients, there is a need for cost-effective end-of-pipe treatment technologies for removing nutrients. This includes nitrate-nitrogen (NO3−-N) for which well-proven technologies for freshwater systems exists, while similar technologies for saltwater systems are less advanced. Granular technology has been developed since the 1970s in wastewater treatment under the upflow anaerobic sludge bed (UASB) concept. This concept is based on the enrichment of different bacterial aggregations into a compact granule, optimizing synergistic and syntrophic bacterial processes by reducing the diffusion distance of substrates between the different bacterial consortia forming the granule. The following study examined the: 1) granular formation; and 2) nitrate removal capacity of a marine Upflow Anoxic Sludge Bed (UASB) reactor operating at different up-flow velocities (0.40–2.11 m/h). The results showed that marine denitrifying granules developed within 27 days using preconditioned rainbow trout (Oncorhynchus mykiss) organic matter waste, and that the highest specific denitrification rate (321.9 ± 13.1 mg NO3−-N/g Total Volatile Suspended Solids (TVSS)/d) was found at an upflow velocity of 0.97 m/h. The marine UASB denitrifying granule reactor had a total capacity of removing 14.9 kg NO3−-N/m3 reactor volume per day at a hydraulic retention time of 1.9 h, making it a strong candidate for end-of-pipe denitrification of marine RAS effluent as well as for in-line treatment in marine systems.

Denitrification-Potential Evaluation and Nitrate-Removal-Pathway Analysis of Aerobic Denitrifier Strain Marinobacter hydrocarbonoclasticus RAD-2

An aerobic denitrifier was isolated from a long-term poly (3-hydroxybutyrate-co-3-hydroxyvalerate) PHBV-supported denitrification reactor that operated under alternate aerobic/anoxic conditions. The strain was identified as Marinobacter hydrocarbonoclasticus RAD-2 based on 16S rRNA-sequence phylogenetic analysis. Morphology was observed by scanning electron microscopy (SEM), and phylogenetic characteristics were analyzed with the API 20NE test. Strain RAD-2 showed efficient aerobic denitrification ability when using NO3−-N or NO2−-N as its only nitrogen source, while heterotrophic nitrification was not detected. The average NO3−-N and NO2−-N removal rates were 6.47 mg/(L·h)and 6.32 mg/(L·h), respectively. Single-factor experiments indicated that a 5:10 C/N ratio, 25–40 °C temperature, and 100–150 rpm rotation speed were the optimal conditions for aerobic denitrification. Furthermore, the denitrifying gene napA had the highest expression on a transcriptional level, followed by the denitrifying genes nirS and nosZ. The norB gene was found to have significantly low expression during the experiment. Overall, great aerobic denitrification ability makes the RAD-2 strain a potential alternative in enhancing nitrate management for marine recirculating aquaculture system (RAS) practices.

Adding maize cobs to vertical subsurface flow constructed wetlands treating marine recirculating aquaculture system effluents: Carbon releasing kinetics and intensified nitrogen removal

This study aims to investigate the releasing behaviors of maize cobs with or without alkali (i.e. NaOH) pretreatment in seawater and distilled water, and to evaluate the effects of maize cobs addition (solid biomass and lixivium) on nitrogen removal in saline constructed wetland (CW) treating marine recirculating aquaculture system (RAS) effluents. Results revealed NaOH-treated maize cobs released carbon more efficiently, whether in seawater or in distilled water. Compared to distilled water, seawater conditions promoted carbon releasing. CW with maize cobs biomass and lixivium addition had high NO3-N removal efficiencies without significant difference, i.e. 94.9 ± 6.0% and 87.1 ± 13.2%, respectively. While CW with maize cobs biomass addition had higher effluent COD concentrations (16.3 ± 3.6 mg L−1) compared with those adding lixivium (2.1 ± 0.4 mg L−1). The study suggested adding maize cobs lixivium to be feasible and effective way to enhance nitrogen removal in saline CW.

Impact of hydraulic retention time, backflushing intervals, and C/N ratio on the SID-reactor denitrification performance in marine RAS

In recirculating aquaculture systems (RAS) the high water re-use in combination with insufficient treatment of the process water can lead to the accumulation of nitrate, among other metabolic end products. For the efficient removal of nitrate in a marine RAS, a Self-cleaning Inherent gas Denitrification Reactor (SID-Reactor) was investigated in this study. Within three consecutive experimental trials the effect of varying hydraulic retention time (HRT), backflushing intervals (BFI), and carbon to nitrogen (C/N) ratios on water quality parameters and denitrification performance (rate and efficiency) were monitored. Different HRTs of 2, 4, and 6 h and additionally BFIs of 10, 30, 60, and 90 min were evaluated. The tested C/N ratios, realized using methanol (MeOH) as a carbon source, were 1.1, 1.5, 1.9, 2.1, 2.3, 2.7, 3.1, and 3.5 (mg MeOH per mg NO3−-N). The experiments revealed that a HRT of 2 h resulted in the highest denitrification rate (497 g d−1 m3 biocarriers) but a lower denitrification efficiency of 64%. A HRT of 6 h had highest denitrification efficiency of 81% but a lower denitrification rate (253 g d−1 m3 biocarriers). Furthermore, it was evident that backflushing intervals every 10 min resulted in a decreased denitrification efficiency of 29%, while intervals every 90 min increased the maintenance effort. Overall, backflushing intervals every 30 and 60 min showed the best results. A C/N ratio of 2.3 seemed to be sufficient to ensure an optimal denitrification performance, incorporating all single tested water quality parameters. The results of this study allow an easy, efficient and safe application of a SID-Reactor with the purpose of nitrate removal in marine RAS.