Recirculating Aquaculture System Effluent Research Articles

Comparison of three unionid mussel species in removing green microalgae grown in recirculating aquaculture system effluent

Global increase in aquaculture production has created a need to reduce its environmental impacts. Nutrients could be recycled especially at land-based recirculating aquaculture systems (RAS) by cultivating green microalgae in aquaculture effluent. However, microalgae are difficult to harvest. As a multi-trophic solution, mussels could be used in harvesting microalgae. We tested three European freshwater mussels (duck mussel Anodonta anatina, swan mussel A. cygnea, and swollen river mussel Unio tumidus) for filtering two common green microalgae (Monoraphidium griffithii and Selenastrum sp.) grown in RAS effluent. Mussels decreased microalgal concentrations in the tanks 42–83% over three consecutive trials. Algal concentrations at the end of each trial were lowest for both microalgae in tanks containing Anodonta mussels. Clearance rates were higher for Anodonta mussels than for U. tumidus. Mussels biodeposited more microalgae to tank bottoms when M. griffithii was filtered. Ammonium concentration decreased or did not change in tanks with M. griffithii, but increased in tanks containing Selenastrum sp. These results suggest that of the tested species Anodonta mussels and M. griffithii show best potential for RAS effluent bioremediation application. We conclude that a co-culture of microalgae and unionid mussels could be used for recycling nutrients in aquaculture.

Nascent application of aerobic granular sludge for recirculating aquaculture system effluent treatment: Performance, granule formation, and microbial community

This study presents for the first time an evaluation of the feasibility of aerobic granular sludge (AGS) for treating recirculating aquaculture system (RAS) effluent in a sequential batch reactor configuration for nutrient removal. An AGS process was started using synthetic wastewater to grow the granules, and the feed was then switched to RAS effluent, and a systematically decreasing carbon supplementation was applied. Total nitrogen removal significantly decreased from around 75 % to as low as 13 %, but granules could restore their performance when allowed enough time (2 weeks) to acclimate to the change in feed. The dynamics of AGS microbial communities were followed by Illumina sequencing. A high abundance of microbial populations—indicating dense and stable granules—was observed after 97 days of operation with RAS wastewater. In particular, the genera Neomegalonema, Hydrogenophaga, Thauera, Bdellovibrio, Flavobacterium, and Pseudomonas represented most of the community, showing the heterotrophic, denitrifying, and phosphorus-accumulating potential of the studied operational design. The AGS showed promising results for a small-footprint solution for RAS treatment, but the energy consumption of aeration and carbon addition still requires further development.

Simultaneous utilization of CO2 and nitrate wastes from compact recirculating aquaculture system for improving algal biomass (Scenedesmus armatus) production

Tilapia cultivation in an air-tight culture tank of a recirculating aquaculture system (RAS) was conducted for 8 days to produce CO2 for growing Scenedesmus armatus in bubble column photobioreactors. The average CO2 concentrations in the headspace of the culture tank were determined at 925 ± 73.4 and 1619 ± 197.7 ppm for fish stocking densities of 3 and 10 kg/m3, respectively. These CO2 concentrations were higher than CO2 concentrations at the tank inlet (496 ± 13.9 ppm) exposed to the atmosphere. Using the captured CO2 and nitrate from the effluent of a commercial tilapia farm for cultivating S. armatus yielded higher algal biomass concentration (621 ± 41.2 mg/L) than using ambient air (410 ± 30.1 mg/L). Insufficient nitrate in RAS effluent is the limiting factor for CO2 utilization and algal biomass production. In conclusion, this study demonstrates the possibility of repurposing CO2 from RAS to enhance algal cultivation.

Membrane aerated biofilm reactor in recirculating aquaculture system for effluent treatment

ABSTRACT The implementation of fish farming has been increasing worldwide over the last decades, as well the search for alternative production systems and the treatment of their generated effluent. Recirculating Aquaculture System (RAS) is a compact solution for future intensive fish farming. However, few configurations of treatment technologies were tested in RAS, such as systems with a Membrane Aerated Biofilm Reactor (MABR). In this scene, this study aimed to evaluate the RAS effluent treatment efficiency device for intensive Nile tilapia (Oreochromis niloticus) production, the fish species most cultivated worldwide. The novel RAS configuration was composed of a cultivation tank (CT), a Column Settler, and a MABR. The RAS performance was evaluated by pH, temperature, turbidity, dissolved oxygen (DO), total nitrogen (TN), ammonia, nitrite, nitrate, total solids (TS), and chemical oxygen demand (COD). The obtained results in average values for temperature, pH, and DO inside the CT were 25.22 ± 1.88°C, 7.61 ± 0.33, and 3.80 ± 1.30 mg L−1, respectively, as ideal for tilapias survival. Average removal efficiencies found in the RAS for turbidity, COD, TN, nitrite, nitrate, ammonia, and TS were 50.0, 40.5, 11.7, 40.2, 13.1, 35.0, and 11.4%, respectively. Overall, we observed removals for all parameters studied, with good results, particularly, for COD, turbidity, nitrite, and ammonia. The evaluated system proved an effective alternative for water reuse in RAS capable of maintaining water quality characteristics within the recommended values for fish farming.

Formaldehyde degradation in denitrifying woodchip bioreactors: Effects of temperature, concentration and hydraulic retention time

Formalin is applied in certain aquaculture systems to control parasites infestations as well as bacterial and fungal diseases. This study investigated the capacity of end-of-pipe denitrifying woodchip bioreactors to remove potentially harmful amounts of residual formaldehyde (FA) from aquaculture effluents. Formaldehyde was readily removed by experimental- and field-scale denitrifying woodchip bioreactors and the removal of FA was found to be a combination of an initial adsorption of FA to woodchip surfaces (52 ± 2.8 g FA/m3 woodchips) and microbial degradation. Volumetric FA removal rates reaching 261 ± 27 g FA/m3/d were found at FA inlet concentrations of 90 mg FA/L and hydraulic retention times (HRT) of 5 h. High FA removal efficiencies ranged from 88.3 ± 4.6–99.8 ± 0.2% found for FA inlet concentrations –up to 105 mg FA/L and HRTs between 3.4 and 15 h. Microbial FA degradation rates in woodchip bioreactors were positively correlated to temperature with a Q10 value of 2.27 and a corresponding Arrhenius temperature coefficient of 1.086 for the investigated temperature range of 7–23 °C. At a commercial, outdoor recirculating aquaculture system (RAS) three full-scale woodchip compartments, achieved an average volumetric FA removal rate of 29.4 ± 0.2 g FA/m3/d and a removal efficiency of 82.5 ± 0.8% during the first 24 h following addition of FA. The results demonstrated that woodchip bioreactors are efficient in removing residual FA from RAS effluents and that nitrate removal was transiently enhanced during FA removal.

Effect of different C/N ratios and hydraulic retention times on denitrification in saline, recirculating aquaculture system effluents

Data on operation and performance of cost-effective solutions for end-of-pipe removal of nitrate from land-based saltwater recirculating aquaculture systems (RAS) are scarce but increasingly requested by the aquaculture industry. This study investigated the performance of a (semi)commercial-scale fixed-bed denitrification unit using single sludge for treating effluent from a commercial, saltwater RAS used for production of Atlantic salmon (Salmo salar). A fixed-bed denitrification reactor was fed continuously with 3-days hydrolyzed sludge from the commercial RAS, and was operated at different hydraulic retention times (HRTs; 1.82, 3.64, 5.46, or 7.28 h) or influent C/N ratios (3, 5, 7, or 10). Twenty-four h pooled samples were collected from the inflowing RAS water and the hydrolyzed sludge as well as from the denitrification reactor outlet, and samples were analyzed for nutrients and organic matter content.Nitrate removal rates increased consistently with decreasing HRT (from 64.3 ± 5.2–162.7 ± 22.0 g NO3-N/m3/d within the HRTs tested) at non-limiting C/N ratios, while nitrate removal efficiencies decreased (from 99.6 ± 0.3–58.2 ± 8.9 %). With increasing influent C/N ratios at constant HRT (3.64 h), nitrate removal rates increased until the removal efficiency was close to 100 % and nitrate concentration in the denitrification reactor became rate-limiting. A maximum nitrate removal rate of 162.7 ± 2.0 g NO3-N/m3/d was achieved at a HRT of 1.82 h and an influent C/N of 6.6 ± 0.5, while the most efficient use of hydrolyzed sludge (0.19 ± 0.02 g NO3-N removed/g sCOD supplied) was obtained with a HRT of 3.64 h and a C/N ratio of 2.9. Removal rates of organic matter significantly and consistently increased with decreasing HRT and increasing C/N ratio. In addition, reducing HRT and increasing C/N ratios significantly improved removal of total phosphorus (TP) and PO4-P.In conclusion, optimal management of the operating parameters (HRT and C/N ratio) in a single-sludge denitrification process can significantly reduce the discharge of nitrogen, organic matter, and phosphorous from land-based saltwater RAS and thus contribute to increased sustainability.

Coagulation of phosphorous and organic matter from marine, land-based recirculating aquaculture system effluents

Saline effluents from marine land-based aquaculture production can neither be disposed in common municipal wastewater treatment plants, nor disposed as landfill. Furthermore, stricter environmental regulations require the reduction of phosphorous and organic matter levels from marine environment discharges to minimize eutrophication. Chemical coagulation with FeCl3 and AlSO4 is commonly used for removing phosphorous and suspended solids in wastewater treatment. The capacity of these coagulants for creating particle aggregations depends on the characteristics and chemistry of the treated wastewater, such as the ionic strength or mixing conditions. Marine water has a higher ionic strength than fresh or brackish water, which may be beneficial when using chemical coagulants to treat the effluents from farms operated at high salinities. The following study compared the application of FeCl3 and AlSO4, to treat the two effluents discharged from a marine land-based recirculating aquaculture system (RAS) producing salmon (Salmo salar). The aim of the study was to determine; 1) in what effluent (sludge flow vs. exchange water overflow) at the end-of-pipe treatment the coagulant application is more efficient for the removal of PO43−-P, total suspended solids (TSS), total phosphorous (TP) and total chemical oxygen demand (TCOD); and 2) the optimal coagulant dose to apply and its associated chemical sludge production. The results show that more than 89 % removal of TCOD, TSS and TP is achieved when treating the sludge flow, arguably because the sludge flow contained the largest fraction of the target masses (P and organic matter) discharged from the system. Up to 80 % of TSS removal was achieved by simple sedimentation, and with the highest coagulant dose tested, up to 95 % of TSS could be removed from the effluent. To remove 90 % of PO43−-P, FeCl3 and AlSO4 need to be dosed at a molar ratio of 2.6:1 Fe:PO43−-P and 5.7:1 Al: PO43−-P, respectively. Dosing above 90 % removal efficiency did not significantly affect removal of PO43-P and TSS, but substantially increased the volume of chemical sludge produced. Finally, FeCl3 is proposed as a better overall alternative for P removal at the end-of-pipe treatment in marine land-based RAS.

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.

End-of-Pipe Horticultural Reuse of Recirculating Aquaculture System Effluent: Comparing the Hydro-Economics of Two Horticulture Systems

To assist waste management decision-making, there is a need to assess the economics of commercial-scale reuse of recirculating aquaculture system (RAS) effluent in horticulture. This study compared the feasibility/viability of using two representative horticulture systems, considering their distinct hydrological characteristics, in horticultural reuse schemes for RAS effluent. These representative systems included a soil-based system in field conditions (SOIL-FIELD) and a hydroponic system in greenhouse conditions (HYDRO-GH). A novel two-step hydro-economic modelling approach was used to quantify and compare the effluent storage volume, total land area, capital expenditure and crop price required for feasible/viable end-of-pipe reuse in the two systems. The modelling assessed several water management scenarios across four Australian climates. Results showed HYDRO-GH, reusing 100% of the annual effluent load and targeting an internal rate of return of 11.0%, required approximately 3 times more land, 14 times more capital expenditure and 5 times the crop price of SOIL-FIELD, targeting a 3.6% internal rate of return. As well as comparing two horticulture systems, this study presents a method to assess feasibility/viability of horticultural reuse schemes for other industrial wastewaters, using a water balance design approach.

Nitrogen Removal Performance and Metabolic Pathways Analysis of a Novel Aerobic Denitrifying Halotolerant Pseudomonas balearica strain RAD-17.

An aerobic denitrification strain, Pseudomonas balearica RAD-17, was identified and showed efficient inorganic nitrogen removal ability. The average NO3−-N, NO2−-N, and total ammonium nitrogen (TAN) removal rate (>95% removal efficiency) in a batch test was 6.22 mg/(L∙h), 6.30 mg/(L∙h), and 1.56 mg/(L∙h), respectively. Meanwhile, optimal incubate conditions were obtained through single factor experiments. For nitrogen removal pathways, the transcriptional results proved that respiratory nitrate reductases encoded by napA, which was primarily performed in aerobic denitrification and cell assimilation, were conducted by gluS and gluD genes for ammonium metabolism. In addition, adding the strain RAD-17 into actual wastewater showed obvious higher denitrification performance than in the no inoculum group (84.22% vs. 22.54%), and the maximum cell abundance achieved 28.5 ± 4.5% in a ratio of total cell numbers. Overall, the efficient nitrogen removal performance plus strong environmental fitness makes the strain RAD-17 a potential alternative for RAS (recirculating aquaculture system) effluent treatment.

Understanding nutrient throughput of operational RAS farm effluents to support semi-commercial aquaponics: Easy upgrade possible beyond controversies.

The present research attempted to address a key industry-level question amidst Recirculating Aquaculture System (RAS) waste throughput and aquaponics limitations controversies. Nutrient throughput of three operational RAS farms with progressive size proportions (16, 130, 1400 m3), aquaculture intensity (24, 62, 86 kg stock m-3) were studied. Results suggest - daily total efflux and potency of nutrients in effluents should not be generalized, extreme variability exists. Consistencies of nutrients in wastewater (except N, Ca and Na) are higher than in sludge. Asynchrony between patterns of nutrient loading and effluent nutrient concentrations exist for secondary macronutrients and micronutrients (S, Mg, Fe, Cu, Zn, B, Mo). Macronutrient output generally increases with increasing farm size and culture intensity but same cannot be said for micronutrients. Deficiency in wastewater can be completely masked using raw or mineralized sludge, usually containing 3-17 times higher nutrient concentrations. RAS effluents (wastewater and sludge combined) contain adequate N, P, Mg, Ca, S, Fe, Zn, Cu, Ni to meet most aquaponic crop needs. K is generally deficient requiring a full-fledged fertilization. Micronutrients B, Mo are partly sufficient and can be easily ameliorated by increasing sludge release. The presumption surrounding 'definite' phyto-toxic Na levels in RAS effluents should be reconsidered - practical solutions available too. No threat of heavy metal accumulation or discharge was observed. Most of the 'well-known' operational influences failed to show any significant predictable power in deciding nutrient throughput from RAS systems. Calibration of nutrient output from operational RAS farms may be primarily focused around six predictors we identified. Despite inherent complexity of effluents, the conversion of RAS farms to semi-commercial aquaponics should not be deterred by nutrient insufficiency or nutrient safety arguments. Incentivizing RAS farm wastes through semi-commercial aquaponics should be encouraged - sufficient and safe nutrients are available.

Salinity affects nitrate removal and microbial composition of denitrifying woodchip bioreactors treating recirculating aquaculture system effluents

This study investigated the effect of salinity on microbial composition and denitrification capacity of woodchip bioreactors treating recirculating aquaculture system (RAS) effluents. Twelve laboratory-scale woodchip bioreactors were run in triplicates at 0, 15, 25, and 35 ppt salinities, and water chemistry was monitored every third day during the first 39 days of operation. Microbial communities of the woodchips bioreactors were analyzed at the start, after one week, and at the end of the trial.Woodchip bioreactors removed nitrate at all salinities tested. The highest NO3-N removal rate of 22.0 ± 6.9 g NO3-N/m3/d was obtained at 0 ppt, while 15.3 ± 4.9, 12.5 ± 5.4 and 11.8 ± 4.0 g NO3-N/m3/d were obtained at salinities of 15, 25 and 35 ppt, respectively. Nitrate removal rates thus decreased with salinity, being 54–69% lower than at 0 ppt. Leaching of total ammonia nitrogen (TAN) and orthophosphate (PO4-P) from woodchips was initially higher at saline treatments compared to 0 ppt, while initial leaching of BOD5 appeared to be similar across all treatments. Production of alkalinity per g NO3-N removed was higher at 0 (3.6 ± 0.5 gCaCO3/gNO3-N) and 15 ppt (3.5 ± 0.8) than at the more saline treatments (25 ppt: 2.0 ± 0.9, 35 ppt: 1.12 ± 0.5 gCaCO3/gNO3-N), indicating that heterotrophic denitrification was the dominant nitrate removing process at 0 and 15 ppt, while autotrophic denitrification processes probably interfered with the alkalinity balance at 25 and 35 ppt. In the woodchip reactors, Gammaproteobacteria was the most abundant taxa. However, salinity shaped the woodchip microbiome, resulting in an increase in the abundance of sulfide oxidizing autotrophic denitrifiers, but decrease in the overall abundance of denitrifying microbes at higher salinities, which presumably explained the reduced nitrate removal rates at elevated salinities.This study demonstrates that woodchip bioreactors can be applied to remove nitrate from saline RAS effluents albeit at lower nitrate removal rates compared to freshwater installations.

Optimization of nitrogen use efficiency by means of fertigation management in an integrated aquaculture-agriculture system

Recirculating aquaculture system (RAS) effluent has a high concentration of nitrogen, which can be a valuable asset as fertilizer for agricultural use in the vicinity of the RAS. A novel fully-automated experimental set-up of 24 lysimeter-plots was used to determine the optimal irrigation rate for cucumber plants fertigated with RAS effluents of three different nitrogen concentrations. Three RASs were stocked with 7, 14 and 21 kg of barramundi (Lates Calcarifer), which were fed 1.75% of their body mass daily. Then, 10% of the water in each RAS was exchanged daily, resulting in three nitrate–N concentrations of approximately 30, 60 and 90 mg/L. A synthetically fertilized control treatment was kept at 60 mg/L nitrate–N, and the other essential plant nutrients were kept at the same concentration for all treatments. The water from these four sources was then used to irrigate six plots per nitrate–N concentration treatment, at a rate of 1–6 times the amount transpired, which was measured in an automated fashion by the experimental set-up.Two yield response to nitrogen fertigation models fit well to the measured data, and although they led to different conclusions in terms of optimal fertigation can be a useful tool for decision support. The nitrogen use efficiency dropped with increased fertigation, from about 80% to 55%, and more fertigation led to an exponential increase in drainage and gaseous emissions of nitrogen. The observed data can be used to optimize irrigation for each nitrogen concentration, but the values are highly dependent on climate, plant type, and root zone characteristics, demonstrating the need for more inclusive modelling. Determination of how to optimally make use of RAS effluent water and the comparison with synthetic fertilizer can lead to the development of simplified protocols concerning integrated aquaculture agriculture systems, making it accessible to a wider range of growers.

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.

Nitrogen removal from water of recirculating aquaculture system by a microbial fuel cell

Maintaining low concentrations of nitrogen compounds in aquaculture water is a key requirement for a recirculating aquaculture system (RAS), due to the potential detrimental effects of ammonia or nitrate on fish growth and metabolic activities. Herein, a microbial fuel cell (MFC) was investigated to accomplish the removal of either nitrate or ammonia from real RAS water (with simulated daily nitrate/ammonium accumulation) while generating electricity, via aerobic nitrification in the cathode, electricity/concentration driven transport across anion exchange membrane, and subsequent heterotrophic denitrification in the anode chamber. The experiment went through two stages, nitrate removal (Stage I) and ammonia removal (Stage II). In Stage I when daily nitrate addition was performed to mimic nitrate accumulation (0.050 kg NO3−-N m−3 NCC d−1, NCC: net cathodic chamber volume) in the MFC cathode, a stable current density of 12.48 A m−3 could be achieved with a 73.3% nitrate removal and 91.3% COD removal at the end of day 15. To better mimic ammonium accumulation in the RAS effluent without a biofilter, daily ammonium addition (0.050 kg NH4+-N m−3 NCC d−1) was performed in the cathode in Stage II. The MFC system achieved a total inorganic nitrogen removal rate of 0.051 kg N m−3 NCC d−1, and a COD removal efficiency of 91.8% with a current density of 74.00 A m−3. A preliminary analysis of energy balance indicated that the proposed MFC could potentially achieve energy-positive RAS water treatment with a net energy production of 7.50 × 10−3 kWh m−3 treated RAS water or 0.145 ± 0.031 kWh kg−1 removed nitrogen. The results of this study indicate that MFCs have a potential to treat RAS water with simultaneous energy recovery.

Effects of sludge retention time on water quality and bioflocs yield, nutritional composition, apparent digestibility coefficients treating recirculating aquaculture system effluent in sequencing batch reactor

Bioflocs was produced in sequencing batch reactors (SBRs) that were treating recirculating aquaculture system (RAS) effluent using biofloc technology (BFT). The effects of sludge retention time (SRT=1 d, 2 d, 3 d, 4 d, and 6 d) on the water quality, bioflocs yield, and nutritional composition were investigated. A type of jellylike bioflocs pellet was designed, and the apparent digestibility coefficients (ADCs) in tilapia (GIFT Oreochromis niloticus) were studied. The results showed that SRTs longer than 2 d were good for water re-utilization of the SBRs effluent. The bioflocs yield could be as high as 2.30–2.54g VSS/g C (SRTs of 1–3 d). The protein values of solid waste (19.23±0.87%) in RAS effluent treated by BFT could be significantly improved, which were 22.58±1.14%, 23.38±1.76%, 24.08±0.94%, 23.92±0.72%, 22.56±1.64% in SRTs of 1–6 d, and that of the bioflocs was the highest in 3 d SRT. The results indicated that the frequently used protein/nitrogen conversion factor of 6.25 causes the protein content to be overestimated in bioflocs. SRT had no significant effects on the essential fatty acid and the polyunsaturated fatty acid contents in bioflocs. Jellylike bioflocs pellets could be successfully fed to tilapia, and the advisable ADCs of dry matter, protein, crude lipid, crude ash, and nitrogen were 57.28–58.64%, 67.04–69.77%, 76.90–80.80%, 53.50–59.82%, and 73.92–77.56%, respectively. In conclusion, a SRT of 3 d was the most advisable one.

Evaluation of the giant reed (Arundo donax) in horizontal subsurface flow wetlands for the treatment of recirculating aquaculture system effluent

Two emergent macrophytes, Arundo donax and Phragmites australis, were established in experimental subsurface flow, gravel-based constructed wetlands (CWs) receiving untreated recirculating aquaculture system wastewater. The hydraulic loading rate was 3.75 cm day(-1). Many of the monitored water quality parameters (biological oxygen demand [BOD], total suspended solids [TSS], total phosphorus [TP], total nitrogen [TN], total ammoniacal nitrogen [TAN], nitrate nitrogen [NO(3)], and Escherichia coli) were removed efficiently by the CWs, to the extent that the CW effluent was suitable for use on human food crops grown for raw produce consumption under Victorian state regulations and also suitable for reuse within aquaculture systems. The BOD, TSS, TP, TN, TAN, and E. coli removal in the A. donax and P. australis beds was 94%, 67%, 96%, 97%, 99.6%, and effectively 100% and 95%, 87%, 95%, 98%, 99.7%, and effectively 100%, respectively, with no significant difference (p > 0.007) in performance between the A. donax and P. australis CWs. In this study, as expected, the aboveground yield of A. donax top growth (stems + leaves) (15.0 ± 3.4 kg wet weight) was considerably more than the P. australis beds (7.4 ± 2.8 kg wet weight). The standing crop produced in this short (14-week) trial equates to an estimated 125 and 77 t ha(-1) year(-1) biomass (dry weight) for A. donax and P. australis, respectively (assuming that plant growth is similar across a 250-day (September-April) growing season and a single-cut, annual harvest). The similarity of the performance of the A. donax- and P. australis-planted beds indicates that either may be used in horizontal subsurface flow wetlands treating aquaculture wastewater, although the planting of A. donax provides additional opportunities for secondary income streams through utilization of the energy-rich biomass produced.

Membrane biological reactor treatment of a saline backwash flow from a recirculating aquaculture system

A recirculating aquaculture system (RAS) can minimize water use, allowing fish production in regions where water is scarce and also placing the waterborne wastes into a concentrated and relatively small volume of effluent. The RAS effluent generated during clarifier backwash is usually small in volume (possibly 0.2–0.5% of the total recirculating flow when microscreen filters are used) but contains high levels of concentrated organic solids and nutrients. When a RAS is operated at high salinities for culture of marine species, recovering the saltwater contained in the backwash effluent could allow for its reuse within the RAS and also reduce salt discharge to the environment. Membrane biological reactors (MBRs) combine activated sludge type treatment with membrane filtration. Therefore, in addition to removing biodegradable organics, suspended solids, and nutrients such as nitrogen and phosphorus, MBRs retain high concentrations of microorganisms and, when operated with membrane pore sizes <1μm, exclude microorganisms from their discharge. In this research, an Enviroquip (Austin, TX) MBR pilot-plant was installed and evaluated over a range of salinities to determine its effectiveness at removing bacteria, turbidity, suspended solids, nitrogen, phosphorus and cBOD5 content from the approximately 22m3/day concentrated biosolids backwash flow discharged from the RASs at The Conservation Fund Freshwater Institute. The MBR system was managed at a hydraulic retention time of 40.8h, a solids retention time of 64±8 days, resulting in a Food: Microorganism ratio of 0.029day−1. Results indicated excellent removal efficiency (%) of TSS (99.65±0.1 to 99.98±0.01) and TVS (99.96±0.01 to 99.99±0.0) at all salinity levels. Similarly, a 3–4log10 removal of total heterotrophic microbes and total coliform was seen at all treatment conditions. Total nitrogen removal efficiency (%) ranged from 91.8±2.9 to 95.5±0.6 at the treatment levels and was consistent, provided a sufficient acclimation period to each new condition was given. Conversely, total phosphorus removal efficiencies (%) at 0ppt, 8ppt, 16ppt and 32ppt salinity were 96.1±1.0, 72.7±3.5, 70.4±2.3, and 65.2±5.4, respectively, indicating reduced phosphorus removal at higher salinities.

Development of a single-sludge denitrification method for nitrate removal from RAS effluents: Lab-scale results vs. model prediction

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 reactord) − 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.