Effect of different biofloc levels on microbial activity,water quality and performance of Litopenaeus vannamei in a tank system operated with no water exchange

In zero-exchange superintensive culture systems, flocculated particles (bioflocs) accumulate in the water column. Consequently, some control over the concentration of these particles must be performed. The objective of this study is to evaluate the effects of three concentrations of bioflocs on microbial activity, selected water quality indicators and performance of Litopenaeus vannamei in a tank system operated with no water exchange. A 44-day study was conducted with juvenile (6.8 g) shrimp stocked in twelve 850 L tanks at a stocking density of 459 shrimp m⁻³. Biofloc levels were expressed as three presets of total suspended solids (TSS) concentrations, as follows: 200 mg L⁻¹ (T200), 400–600 mg L⁻¹ (T400–600), and 800–1000 mg L⁻¹ (T800–1000). TSS levels were controlled by attaching a 40 L settling tank to each culture tank. Reduction of TSS to concentrations close to 200 mg L⁻¹ decreased the time of bacterial cell residence and significantly reduced the nitrification rates in the water (P < 0.05). The tanks in the T200 treatment had a greater variability of ammonia and nitrite (P < 0.05), which led to the need to increase the C:N ratio of the organic substrate to control ammonia through its assimilation into heterotrophic bacterial biomass. But the higher production of heterotrophic bacteria in T200 (P < 0.05) increased the dissolved oxygen demand. Nitrification rates were higher (P < 0.05) in tanks with TSS concentrations above 400 mg L⁻¹, and ammonia and nitrite were significantly lower than in the T200 tanks. We suggest that ammonia and nitrite in the T400–600 and T800–1000 tanks were controlled primarily by nitrifying bacteria, which provided higher stability of these parameters and of dissolved oxygen. Regarding shrimp performance, the reduction of TSS to levels close to 200 mg L⁻¹ was associated with better nutritional quality of bioflocs. Nevertheless, differences in biofloc levels and nutritional quality were not sufficient to affect the weight gain by shrimp. The rate of shrimp survival and the final shrimp biomass were lower (P < 0.05) when the TSS concentrations were higher than 800 mg L⁻¹. Analysis of the shrimps’ gills showed a higher degree of occlusion in the T800–1000 treatment (P < 0.05), which suggests that the shrimp have an intolerance to environments with a solids concentration above 800 mg L⁻¹. Our results show that intermediate levels of bioflocs (TSS between 400 and 600 mg L⁻¹) appear to be more suitable to superintensive culture of L. vannamei since they create factors propitious for maintaining the system’s productivity and stability     

Nutrient discharge,sludge quantity and characteristics in biofloc shrimp culture using two methods of carbohydrate fertilization

The aim of this study was to evaluate the effect of two methodologies of carbohydrate fertilization on the volume and characteristics of effluent from intensive biofloc shrimp cultivation. Six fiberglass circular tanks (50 m² each) were divided into two treatments. In the treatment called continuous (CONT), the tanks received daily molasses fertilization throughout the entire rearing period. In the treatment named initial (INI), molasses was used only in the early weeks of cultivation. Juvenile Litopenaeus vannamei (0.87 ± 0.10 g) were stocked at a density of 180 animals m⁻² and cultured during 12 weeks until they reached an average weight of 12 g. The tanks were operated with no water exchange and the total suspended solids concentration were kept between 300 and 400 mg L⁻¹ using settling chambers. The sludge produced and the wastewater at harvest were quantified and their characteristics were determined. The production of TSS in the CONT treatment was higher (0.25 kg of solids per kg of applied feed) than in the INI treatment (0.16 kg kg⁻¹) (P < 0.05). The analysis of the sludge revealed a high amount of volatile solids in both treatments, between 636 and 702 g kg⁻¹. However, due to the elevated sludge nitrogen content, the carbon to nitrogen (C:N) ratio was low, with values of 6.4 ± 1.4 and 7.5 ± 1.6 for INI and CONT respectively. The BOD:TSS ratio was also low in both treatments, but the INI showed lower values (10.3 ± 0.6%) than the CONT (14.9 ± 0.0%) (P < 0.05). Both fertilization strategies were able to modify the characteristics of sludge produced during cultivation. Moreover, the high nitrogen and sulfate content of the sludge in both treatments indicated that it may be difficult to use an anaerobic digestion process to treat sludge. In the INI treatment tanks, the sludge is partially stabilized, while in the CONT there was a greater need for stabilization.     

Optimization of chitosan flocculation for phytoplankton removal in shrimp culture ponds

The removal of phytoplankton cells from aquaculture systems generally results in the reduction of nitrogenous waste and improves water quality. With this study, the effects of chitosan concentration, environmental condition and pH adjustment on flocculation of phytoplankton in marine shrimp (Litopenaeus vannamei) culture tanks were investigated. The remaining phytoplankton and suspended solids in the system were indicators for evaluating the efficiency of chitosan on flocculation. The results indicate that the flocculation efficiency of chitosan was highest (>85%) and remained fairly constant at a chitosan concentration of 40–80 mg L⁻¹ and a pH range of 7–9 after chitosan addition. With this novel technique including 40 mg L⁻¹ chitosan addition, pH adjustment to 6.5 and then to 8.5, high efficiency and consistency of flocculation were achieved. This technique could also be applied with various water alkalinity up to 400 mg CaCO₃ L⁻¹. The experiment for phytoplankton removal by chitosan flocculation in the recirculating aquaculture system showed that flocculation efficiency remained constant even though flocculation was repeated several times.     

Population dynamics and fishery benefits of a large legal size of a pelagic sportfish,the Talang queenfish,Scomberoides commersonnianus,in northern Australia

The life history of an increasingly important pelagic commercial and sport fish, the Talang queenfish, Scomberoides commersonnianus, was studied in northern Australia to investigate the stock status and assess current management of the species using minimum legal lengths (MLL). Estimated von Bertalanffy growth parameters were L_∞ = 1404 mm FL, K = 0.10 year⁻¹ and t₀ = −1.21 year⁻¹. There was no significant difference in growth between sexes. Ages ranged from 1 to 11 years with age composition differing between the commercial (mainly 6–7 years) and sport fishery (mainly 2–4 years). Females matured (L₅₀) at 635 mm FL and 4–5 years. Spawning occurred between August and March when mature females were estimated to produce 259,488–2,859,935 eggs per spawning. Natural mortality (M) was estimated as 0.16–0.26 year⁻¹, while the combined fishing mortality (F_current) from commercial and sport fisheries was 0.38–0.48 year⁻¹. Yield-per-recruit analyses revealed that under current MLL limits (no MLL or 45 cm TL) and natural mortality (M = 0.16, 0.2 and 0.26 year⁻¹), F_current exceeded the reference points F_max (0.15–0.22 year⁻¹) and F_0.1 (0.10–0.15 year⁻¹), suggesting the stock may be growth overfished if the current situation remains unchanged. Although a stock–recruitment relationship is unknown, spawning stock biomass-per-recruit analysis indicates the stock may also be recruitment overfished since F_current exceeded the reference points F_25% (0.19–0.24 year⁻¹) and F_40% (0.11–0.15 year⁻¹). Increasing the MLL corresponding to L₅₀ of females (70 cm TL) will greatly improve the yield and long-term sustainability of the stock, and also enhance the sport fishery by increasing the number of larger trophy fish.     

Hydroponic plate/fabric/grass system for treatment of aquacultural wastewater

Hydroponic plants can efficiently absorb and uptake soluble compounds in wastewater but they have low abilities to remove suspended solids due to the lack of culture media to trap solids. This paper presented an improved hydroponic method for effective treatment of the wastewater from the backwash of recirculating aquacultural systems. The ryegrass (Lolium perenne Lam) was cultured with improved media consisting of perforated plastic plates and several layers of unwoven cotton fabric. The plate/fabric/grass cells with one, three, five, and seven layers of fabric were studied. After one vertical filtration pass through the cells, the removals were 48, 59, 60 and 63% for total solids (TS), 48, 58, 63 and 69% for volatile solids (VS), and 4, 7, 14 and 25% for suspended solids (SS), respectively, for different cells with one, three, five, and seven layers of fabric. It was found that increasing the number of vertical filtration passes through the cells improved the solids removal. The 1-day treatment in the recycling irrigation and treatment system with five cells ( = 0.8 m² grass) removed 66% TS, 71% VS, and 91% SS, and absorbed 72% total nitrogen (TN), 80% total phosphorus (TP), 63% chemical oxygen demand (COD), and 85% total ammonia nitrogen (TAN). This hydroponic plate/fabric/grass system is a simple and efficient technology for the effective eco-treatment of aquacultural wastewater with relatively high concentrations of suspended solids.     

Direct and continuous dissolved CO2 monitoring in shallow raceway systems: From laboratory to commercial-scale applications

Direct and continuous measurement of dissolved CO₂ (dCO₂) is crucial for intensive aquaculture, especially in shallow raceway systems (SRS). In this work the performance of a portable dissolved CO₂ probe analyzer was tested for the effects of different aqueous solutions, pure oxygen injection and agitation. Laboratory results showed significant (p < 0.05) solution effects on probe performance for low (10–20 mg L⁻¹) and high (30–50 mg L⁻¹) dCO₂ concentrations. Globally performance was better in deionized water, followed by marine fish farm water and artificial seawater. Accuracy and response time were the parameters most affected by the type of solution tested. Linearity was always observed (R² = 0.995–0.999). The probe was sensitive to 1 mg L⁻¹ dCO₂ increments for concentrations <6 mg L⁻¹ in artificial seawater. Pure oxygen injection did not affect probe readouts, and agitation was needed for better accuracy and response time. In real marine SRS with tanks in series dCO₂ dynamics was revealed using the probe coupled to a developed flow cell. A prototype SRS was built and used to study dCO₂ dynamics without endangering cultivated fish. Generally, results obtained indicate that the probe tested although precise, is better suited for discrete, single-point dCO₂ monitoring, being a limited resource for the special needs of shallow raceway systems. As SRS represent a paradigm change in aquaculture, new water quality monitoring strategies and instrumentation are needed, especially for dCO₂. Fiber optic sensors can be a solution for continuous, multipoint monitoring, thus contributing to the understanding of water quality dynamics in hyperintensive aquaculture systems.     

The effects of food and the sum of effective temperature on the embryonic development of the seahorse,Hippocampus kuda Bleeker

The effect of different food (live Acetes spp., live Mysis spp., frozen Mysis spp., and mixed food of 50% live Acetes spp. and 50% frozen Mysis spp.) on gonad development of seahorses, Hippocampus kuda Bleeker, was evaluated in this experiment. The developmental durations of testes and ovary of seahorses were significantly different among the four experimental treatments. The live Acetes spp. treatment presented the shortest developmental durations to stage V, which were 87.6 ± 3.84 days and 89.2 ± 3.71 days, respectively, for ovary and testes. The frozen Mysis spp. treatment had the longest developmental durations (F_3,15 = 13.284, P < 0.05). The relationship between developmental duration of the ovary and gonad developmental stages could be formulated: Y₁ = 12.04x + 24.36 (r² = 0.9722, n = 16, P < 0.05). The gonadosomatic index (GSI) of parent seahorses among the four treatments differed significantly (F_3,15 = 18.364, P < 0.05). The standard GSI of seahorses fed live Acetes spp. was 15.64 ± 1.65%, which was the highest. Feeding live food had a significant effect on the fecundity and spawning of seahorses. The fecundity and spawning number of the live Acetes spp. treatment were 598 ± 45.49 and 552 ± 49.19 individuals, respectively, which were dramatically higher than those of frozen treatment (F_3,19 = 34.152, P < 0.05). Live food also had a similar effect on the fertilization and hatching rate during the embryonic development of seahorses (F_3,19 = 11.386, P < 0.05). Food treatment could also induce an indirect effect on survival rate of juveniles through gonadal and embryonic development of the parents (F_3,15 = 14.519, P < 0.05). In this experiment, the mortality within parent seahorses in the frozen Mysis spp. treatment was the highest (15.1 ± 6.55%), and the survival of juveniles was the highest in the live Acetes spp. treatment (90.4 ± 2.26% at 10 days). In conclusion, feeding live Acetes spp. significantly benefited the gonadal and embryonic development of H. kuda.The effect of temperature (22 °C, 24 °C, 26 °C, 28 °C, 30 °C and 32 °C) on the hatching time of H. kuda was also studied. We demonstrated that the higher the temperature, the shorter the hatching time, as well as the higher the hatching speed. The relationship between hatching time and temperature could be expressed: T = − 39.337 t + 677.75 (r² = 0.9755, n = 30, P < 0.05). In this finding, we provided the sum of effective temperature (SET) and threshold temperature of embryonic development of H. kuda (14066.9 °C h^− 1 and 13.7 °C, respectively). This new information on the effect of feeding type and culture temperature is beneficial for the commercial rearing and breeding industry of this species.     

Nursery of young Litopenaeus vannamei post-larvae reared in biofloc- and microalgae-based systems

A 13-day nursery trial was conducted to evaluate the performance of young Litopenaeus vannamei post-larvae (from PL₆ to PL₁₈) reared in both biofloc and microalgae-based systems at a stocking density of 67 PLs L⁻¹. The effects of different concentrations of total suspended solids (TSS) on PL performance were also evaluated. One experimental group was reared in a conventional microalgae-based system with daily water exchange and daily addition of microalgae (herein called microalgae treatment). The other two experimental groups were reared using biofloc technology (BFT) with daily dextrose addition and no water exchange, but in the “Biofloc-500” treatment, TSS were maintained at around 500 mg L⁻¹, while in the “Biofloc-700” treatment, TSS were maintained at around 700 mg L⁻¹. Water quality variables remained within the appropriate range for larval culture. In microalgae treatment, ammonia control was likely associated with its assimilation into microalgae biomass and daily water exchange. In biofloc tanks, however, the addition of dextrose stimulated the production of bacterial biomass from ammonia. This system required only 12.9% of the water used by the microalgae treatment since water was not exchanged during the culture. The nursery of young PLs resulted in similar (P > 0.05) performance in all treatments: survival >94%, PL length ∼ 11.5 mm, and PL dry weight ∼ 1.2 mg. In addition, the salinity stress test (>90.0%) was not significantly different among treatments. Our results indicate that BFT can be as effective as the microalgae-based system for the nursery of young L. vannamei post-larvae. We also found that post-larvae performance was similar (P > 0.05) between biofloc treatments, indicating that organisms can tolerate environments with large quantities of solids.     

Production of Akoya pearls from the Southwest coast of India

The Indian pearl oyster Pinctada fucata (Gould) is typically capable of producing pearls of 3–5 mm diameter. The feasibility of production of pearls similar to Akoya pearls of 6–8 mm diameter was studied from the southwest coast of India. Along with this, mortality and retention rates of implanted oysters, rate of nacre production, thickness of nacre deposited, quality and type of pearls produced and effect of hydrographic variations on the mortality of implanted oysters were also studied. A total of 706 oysters were implanted, 311 with 5 mm, 395 with 6 mm nuclei and stocked in 30 cages for a period of 317 days. The mortality rates were highest, 0.173 ± 0.22 for the 6 mm nucleus implanted oysters followed by 5 mm nucleus implanted oysters at 0.107 ± 0.025 during the first 30 days after implantation. These rates were significantly different (P < 0.05) from the mortality rate of the control oysters (0.042 ± 0.04). The retention rates based on the surviving oysters, ranged from 33 to 61% (average 45.9 for 5 mm) and 31 to 60% (average 48.9% for 6 mm). The nacre deposition rates on the nuclei were found to be 4.0 ± 1.0 μm day^− 1 and 3.0 ± 1.0 μm day^− 1 for 6 and 5 mm nuclei respectively. Of the total 131 pearls obtained, 27.6% were A-grade, 31.3% B-grade, 19.8% C-grade, 7.6% baroques and 13.7% rejects or trash. The total suspended solids (TSS) in the water were found to be positively correlated (P < 0.05) with the monthly mortality rate of the implanted oysters. The study showed that it was possible to obtain relatively thick nacre within a short period of 10 months, the deposition rate being about 9 times higher than that observed in Japanese waters and 2.2 to 2.3 times more than that along the Indian southeast coast.     

Effect of different total suspended solids concentrations on the growth performance of Litopenaeus vannamei in a BFT system

Decreased Litopenaeus vannamei performance resulting from excess total suspended solids (TSS) has been highlighted in previous studies. Therefore, the aim of this study was to evaluate the effect of different TSS concentrations on the L. vannamei growth performance in a BFT system for 42 days. Five TSS concentrations were used—250, 500, 1000, 2000, and 4000 mg L⁻¹—in three replicates identified as T250, T500, T1000, T2000, and T4000, respectively, in 200 L-tanks each. Dissolved oxygen concentration (DO) was maintained above 5 mg L⁻¹. Shrimp with an initial average weight of 4.57 ± 1.07 g were stocked at a density of 277 shrimp m⁻². The physical and chemical parameters were monitored. Water quality parameters and animal performance were subjected to analysis of variance (ANOVA − one way). The physical and chemical parameters were within the recommended range for L. vannamei. Weekly weight gain, feed conversion rate, survival, and productivity showed no significant differences (p > 0.05). The high TSS concentrations did not seem to affect the performance of this species when DO concentrations were maintained above 5 mg L⁻¹.     

Leaching properties of three different micropaticulate diets and preference of the diets in cod (Gadus morhua L.) larvae

Leakage of water soluble nutrients from larval microparticulated feeds is probably extensive and needs to be further investigated. Leaching rates of ¹⁴C-labelled serine, pepsin hydrolysed, protein enriched ¹⁴C-algae extract and intact protein enriched ¹⁴C-algae extract were measured from three microparticulated feeds for marine fish larvae (heat coagulated, protein bond feed; agglomerated feed; protein encapsulated feed). The effects of particle size (< 0.3 mm, 0.3–0.6 mm; 0.6–1.0 mm) and immersion time (1–60 min) in salt water were also tested. Leaching increased by decreasing molecular weight of leaching component (P < 10^− 5), by the feeds in order encapsulated, heat coagulated and agglomerated feeds (P < 10^− 5), by longer immersion time (P < 10^− 5), and by decreasing feed particle size (P < 10^− 5). Due to low protein content of the algae extract, the leaching rates of intact and hydrolysed algae extract did not represent absolute estimates for protein and hydrolysed protein leaching. A new estimate for leakage of hydrolysed protein was calculated based on molecular weight distribution of the hydrolysed algae extract analysed by cutoff centrifugation of the extract. Assuming that molecules < 300–600 or < 9–18 kD would leak, leakage of hydrolysed protein from the smallest feed particles after 5 min immersion would be 80–98%, 43–54% and 4–6% of the agglomerated, heat coagulated and protein encapsulated feeds, respectively. The feeds were also tested for preference in cod larvae of two different sizes (5.6 ± 2.5 mg and 15.8 ± 7.2 mg). The preference was highest for the heat coagulated feed in the first experiment (feed intake 0.32 ± 0.06 mg dry feed larvae^− 1) and the agglomerated in the second (2.04 ± 0.32 mg dry feed larvae^− 1), while the protein encapsulated feed was preferred at lower rates in both experiments (0.11–0.14 mg dry feed larvae^− 1).     

Does orientation of raft helps in augmenting yield during lean period?: A case study of Gracilaria edulis cultivation in open sea by vertical raft alignment along the south-eastern coast of India

The food grade agar in India has been almost exclusively obtained from Gracilaria edulis, but the industrial production overwhelmingly relies on exploitation of natural resources. United Nations efforts through Food and Agriculture Organization under Bay of Bengal Program highlighted the necessity of undertaking commercial farming of this species along Indian coast for socio-economic benefits. The pilot-scale experiments established viability of large-scale cultivation by floating raft method. Nevertheless, drastic reduction in yield and quality during summer months due to enhanced sedimentation and severe epiphytism is found to be a major hindrance. Altering the positioning of rafts from horizontal to vertical alignment improved the growth and yield under open sea condition at two different locations along south east coast of India. The average yield in horizontal raft was found to be 3.08 ± 0.61 kg fr wt raft⁻¹ with corresponding DGR of 1.87 ± 0.63% day⁻¹ while same in case of vertical rafts was 13.76 ± 3.86 kg fr wt raft⁻¹ and 5.00 ± 0.5% day⁻¹ in Gulf of Mannar under 45 days growth cycle. The corresponding values along Palk Bay were 2.98 ± 0.52 g fr wt raft⁻¹ and 1.38 ± 0.42% day⁻¹ for horizontal raft and 13.02 ± 6.06 kg fr wt raft⁻¹ and 4.14 ± 1.18% day⁻¹ for vertical raft. ANOVA clearly indicated that raft position significantly influenced the biomass yield and DGR at Palk Bay (F = 75.77; F = 112.81) as well as Gulf of Mannar (F = 27.21; F = 59.16) at p = 0.001. The increment of 1.9–2.6% in fresh weight of individual frond was reported in vertically aligned rafts. The computational fluid dynamics (CFD) based unsteady numerical simulations have confirmed that vertical alignment of raft facilitates relatively free movement of water due to which sedimentation and epiphytism are either minimised or eliminated. Thus these studies can help us to deduce important conclusions pertaining to management of sustained commercial cultivation of this alga in Indian waters.     

Anaerobic digestion of solid waste in RAS: effect of reactor type on the biochemical acidogenic potential (BAP) and assessment of the biochemical methane potential (BMP) by a batch assay

Anaerobic digestion is a way to utilize the potential energy contained in solid waste produced in recirculating aquaculture systems (RASs), either by providing acidogenic products for driving heterotrophic denitrification on site or by directly producing combustive methane. In this study the biochemical acidogenic potential of solid waste from juvenile rainbow trout was evaluated by measuring the yield of volatile fatty acids (VFA) during anaerobic digestion by batch or fed-batch reactor operation at hydrolysis time (HT)/hydraulic retention time (HRT) of 1, 5, or 10 days (and for batch additional 14 and 20 days) in continuously stirred tank reactors. Generally, the VFA yield increased with time and no effect of the reactor type used was found within the time frame of the experiment. At 10 days HT or 10 days HRT the VFA yield reached 222.3 ± 30.5 and 203.4 ± 11.2 mg VFA g⁻¹ TVS₀ (total volatile solids at day 0) in batch and fed-batch reactor, respectively. For the fed-batch reactor, increasing HRT from 5 to 10 days gained no significant additional VFA yield. Prolonging the batch reactor experiment to 20 days increased VFA production further (273.9 ± 1.6 mg VFA g⁻¹ TVS₀, n = 2). After 10 days HT/HRT, 16.8–23.5% of total Kjeldahl N was found as TAN and 44.3–53.0% of total P was found as ortho-phosphate. A significant difference between reactor types was detected for the phosphorous dissolution at 5 days HT/HRT as a relatively steep increase (of a factor 2–3) in ortho-P content occurred in fed-batch reactors but similar steep increase was only notable after 10 days HT for batch reactors. No differences between reactor types at the other HT/HRT were recorded for P as well as (for all HT/HRT for) N. Based on this study a HRT of approximately 5 days would be recommended for the design of an acidogenic continuously stirred reactor tank in a RAS single-sludge denitrification set-up. The biochemical methane potential of the sludge was estimated to 318 ± 29 g CH₄ g⁻¹ TVS₀ by a batch assay and represented a higher utility of the solid waste when comparing the methane yield with the VFA yield (in COD units). This points toward a technological challenge of ultimately increase the acidogenic output to match the methane yield as both products are formed from the same source.     

End-of-pipe denitrification using RAS effluent waste streams: Effect of C/N-ratio and hydraulic retention time

Environmentally sustainable aquaculture development requires increased nitrogen removal from recirculating aquaculture systems (RAS). In this study, removed solids from a large commercial outdoor recirculated trout farm (1000 MT year⁻¹) were explored as an endogenous carbon source for denitrification. This was done by (1) a controlled laboratory experiment on anaerobic hydrolysis of the organic matter (from sludge cones, drumfilter, and biofilter back-wash) and (2) an on-site denitrification factorial experiment varying the soluble COD (COD_S)/NO₃-N ratio from 4 to 12 at hydraulic retention times (HRT) from 50 to 170 min in simple 5.5 m³ denitrification reactors installed at the trout farm.The lab-experiments showed that the major part of the readily biodegradable organic matter was hydrolyzed within 14 days, and the hydrolysis rate was fastest the first 24 h. Organic matter from the sludge cones generated 0.21 ± 0.01 g volatile fatty acids (VFA) g⁻¹ total volatile solids (TVS), and the VFAs constituted 75% of COD_S. Analogously, 1 g TVS from the drum filter generated 0.15 ± 0.01 g VFA, constituting 68% of the COD_S. Comparison of the laboratory hydrolysis experiments and results from the on-farm study revealed as a rough estimate that potentially 17–24% of the generated VFA was lost due to the current sludge management.Inlet water to the denitrification reactors ranged in NO₃-N concentration from 8.3 to 11.7 g m⁻³ and COD_S from 52.9 to 113.4 g m⁻³ (10.0 ± 1.2 °C). The highest NO₃-N removal rate obtained was at the intermediate treatments; 91.5–124.8 g N m⁻³_reactor d⁻¹. The effect of the C/N ratio depended on the HRT. At low HRT, the variation in C/N ratio had no significant effect on NO₃-N removal rate, contrary to the effect at the high HRT. The stoichiometric ratio of COD_S/NO₃-N was 6.0 ± 2.4, ranging from 4.4 (at the high HRT) to 9.3 (at the low HRT). A simple model of the denitrification reactor developed in AQUASIM showed congruence between modeled and measured data with minor exceptions. Furthermore, this study pointed to the versatility of the NO₃-N removal pathways expressed by the bacterial population in response to changes in the environmental conditions; from autotrophic anammox activity presumably present at low C/N to dissimilatory nitrate reduction to ammonia (DNRA) at high C/N, besides the predominate “normal” heterotrophic dissimilatory nitrate reduction (denitrification).     

End-of-pipe single-sludge denitrification in pilot-scale recirculating aquaculture systems

A step toward environmental sustainability of recirculat aquaculture systems (RAS) is implementation of single-sludge denitrification, a process eliminating nitrate from the aqueous environment while reducing the organic matter discharge simultaneously. Two 1700 L pilot-scale RAS systems each with a 85 L denitrification (DN) reactor treating discharged water and hydrolyzed solid waste were setup to test the kinetics of nitrate and COD removal. Nitrate removal and COD reduction efficiency was measured at two different DN-reactor sludge ages (high θ_X: 33–42 days and low θ_X: 17–23 days). Nitrate and total N (NO₃⁻ + NO₂⁻ + NH₄⁺) removal of the treated effluent water ranged from 73–99% and 60–95% during the periods, respectively, corresponding to an overall maximum RAS nitrate removal of approximately 75%. The specific nitrate removal rate increased from 17 to 23 mg NO₃⁻-N (g TVS d)⁻¹ and the maximal potential DN rate (measured at laboratory ideal conditions) increased correspondingly from 64–68 mg NO₃⁻-N (g TVS d)⁻¹ to 247–294 mg NO₃⁻-N (g TVS d)⁻¹ at high and low θ_X, respectively. Quantification of denitrifiers in the DN-reactors by qPCR showed only minor differences upon the altered sludge removal practice. The hydrolysis unit improved the biodegradability of the solid waste by increasing volatile fatty acid COD content 74–76%. COD reductions in the DN-reactors were 64–70%. In conclusion, this study showed that single-sludge denitrification was a feasible way to reduce nitrate discharge from RAS, and higher DN rates were induced at lower sludge age/increased sludge removal regime. Improved control and optimization of reactor DN-activity may be achieved by further modifying reactor design and management scheme as indicated by the variation in and between the two DN-reactors.     

Comparing a chemoautotrophic-based biofloc system and three heterotrophic-based systems receiving different carbohydrate sources

A 56-day study was conducted in which shrimp (Litopenaeus vannamei) were stocked at 300 m⁻³ into 16, 500-L tanks. Four treatments were created: chemoautotrophic (CA), heterotrophic sucrose (HS), heterotrophic molasses (HM), and heterotrophic glycerol (HG). The heterotrophic treatments were managed such that the C:N ratio of inputs (feed and carbohydrate source) was 22:1. The chemoautotrophic treatment received no added carbohydrate, only shrimp feed. Each treatment was assigned randomly to four replicate tanks. Nitrate-N was significantly greater in the CA treatment, accumulating to a peak mean concentration of 162 mg NO₃-N L⁻¹ and nitrate was typically below detection (<0.01 mg NO₃-N L⁻¹) in the heterotrophic treatments. 5-Day biochemical oxygen demand (BOD₅) was significantly greater in the heterotrophic treatments compared to the chemoautotrophic treatment. Total suspended solids concentration was significantly lower in the CA treatment compared to any other. Shrimp growth rate was significantly greater in the CA and HS treatments versus the HM treatment and there was no significant difference in growth rate between the HG treatment and any other treatment. These results indicate that differences in management and carbohydrate source can lead to substantial disparity in system function and shrimp production.     

Optimization of chitosan flocculation for phytoplankton removal in shrimp culture ponds

The removal of phytoplankton cells from aquaculture systems generally results in the reduction of nitrogenous waste and improves water quality. With this study, the effects of chitosan concentration, environmental condition and pH adjustment on flocculation of phytoplankton in marine shrimp (Litopenaeus vannamei) culture tanks were investigated. The remaining phytoplankton and suspended solids in the system were indicators for evaluating the efficiency of chitosan on flocculation. The results indicate that the flocculation efficiency of chitosan was highest (>85%) and remained fairly constant at a chitosan concentration of 40–80 mg L^?1 and a pH range of 7–9 after chitosan addition. With this novel technique including 40 mg L^?1 chitosan addition, pH adjustment to 6.5 and then to 8.5, high efficiency and consistency of flocculation were achieved. This technique could also be applied with various water alkalinity up to 400 mg CaCO₃ L^?1. The experiment for phytoplankton removal by chitosan flocculation in the recirculating aquaculture system showed that flocculation efficiency remained constant even though flocculation was repeated several times.     

Water delivery capacity of a vacuum airlift – Application to water recycling in aquaculture systems

A study was undertaken to measure the water flow (Q_w) delivered by a vacuum airlift designed for recirculating aquaculture systems (RAS) in fresh (<1‰ of salinity) and sea water (35‰ of salinity). The vacuum airlift consists of two concentric tubes connected at their top to a depression chamber. The water rises in the inner tube as a result of air being injected in its lower section and flows back through the external downcomer tube. The vacuum airlift was adjusted at three different lengths: 2, 4 or 6 m and water discharge could be lifted from 0 to 30 cm. Air flow rate (Q_g) varied from 0 to 80 L min⁻¹. Different types of air injectors were tested, delivering different bubble sizes (0.1–5 mm) depending on porosity and functioning at low or high injection pressure. Results show an increase in water flow when pipe length and air flow were increased and lift height reduced. Water flow also depended on the type of water and ranged from 0 to 35 m³ h⁻¹ (0–580 L min⁻¹) for fresh water and only from 0 to 20 m³ h⁻¹ (0–330 L min⁻¹) for sea water (for a 6 m high vacuum airlift). This difference was attributed to the smaller bubble diameter and higher gas holdup (ɛ_g) observed in sea water (0–20%) compared to fresh water (0–10%). When bubbles were present in the downcomer tube, they created a resistance to flow (counter-current airlift) that slowed down liquid velocity and thus water flow. Increasing the vacuum made it possible to use low air injection pressures and high injection depths. Vacuum also increased bubble size and airflow (20 L min⁻¹ at atmospheric pressure to 60 L min⁻¹ at 0.3 barA) and thus water flow rates. With RAS, the presence of fish feed in water rapidly increased water flow delivered by the airlift because of changes of water quality and gas holdup. When working with low head RAS (under 0.3 m), vacuum airlift could save up to 50% of the energy required for centrifugal pumps. An empirical predictive model was developed and calibrated. Simulation shows a good correlation between predicted values and measurements (R² = 0.96).     

The start-up and saline adaptation of mesophilic anaerobic sequencing batch reactor treating sludge from recirculating aquaculture systems

Treatment of sludge from aquaculture is a matter of special importance and there is a need for salt-tolerant biological wastewater treatment to coincide with the development of brackish/marine aquaculture. The aims of the current study were to determine the ability of anaerobic sequencing batch reactor (ASBR) to anaerobically digest sludge from fresh-water recirculating aquaculture systems and the ability of adaptation to low saline conditions. The mesophilic ASBR were evaluated with loading rates between 0.12 and 0.41 g chemical oxygen demand (COD)/day at a 20-day hydraulic retention time (HRT) for start-up and with organic loading rates (OLR) of 0.39–0.41 g COD/L day at a 20-day HRT for saline adaptation. The average removal rate of total chemical oxygen demand (TCOD), total suspended solids (TSS) and volatile suspended solids (VSS) of the ASBR were above 97%, 96% and 91% during the stabilization period of the experimental reactors. The average daily gas production of ASBR was between 0.013 and 0.022 L/g TCOD from day 118. A sludge-mass reduction of up to 94 ± 2.3%, TCOD reduction of 44 ± 13% and VSS/SS of 39–70% were demonstrated for the reactor performance during the gas production period. However, the process of gas production was obviously inhibited, presumably by salt, and unstable due to the dissolved COD (DCOD), total ammonium nitrogen (TAN) and alkalinity of the effluents of the experimental reactors and TSS and sludge volume index (SVI) observed within the reactors. The daily gas production was observed to decrease during the saline adaptation period and stopped when the salinity of the effluents was higher than 8.7 ppt until the end of the experiment.     

Performance of single-drain and dual-drain tanks in terms of water velocity profile and solids flushing for in vivo digestibility studies in juvenile shrimp

In vivo digestibility determination in shrimp is a challenge because these animals are coprophagous, benthic and slow feeders and the small amount of feces that they produce is difficult to collect. The objective of this study was to evaluate an efficient tank design for the purpose of studying shrimp digestibility. Different tank designs were evaluated considering drain system (dual-drain and single-drain), water inlet flow rate (8, 12, and 16 L min⁻¹) and bottom drain diameter (6, 13, 19, 25 and 50 mm) and their effects on tank hydraulics, water velocity and solids flushing. A circular and slightly conical 500 L tank was adapted with a clarifier for the two dual-drain designs (Cornell-type and central-type) and settling columns for the two single-drain designs (Guelph-F and Guelph-L). Results showed that: (1) water rotational velocity profile was more homogeneous in tanks with larger bottom drain outlets, and water velocity increased with water inlet flow rate from almost zero up to 14.5 ± 0.7 cm s⁻¹; (2) solids flushing, measured as the percentage of feed pellets retained at both the bottom drain and in the settling devices, was positively correlated with the surface loading rate (L min⁻¹ flow per m²) and was more effective at the Guelph-L design fitted with a 150 mm diameter settling column. In this system 100% of the solids were removed at the inflow rate of 16 L min⁻¹. It can be concluded that among the systems evaluated, the Guelph-L at an inflow-rate of 12 L min⁻¹ was most efficient for both solids removal and water velocity profile and thus seemed more suitable for shrimp digestibility studies in high performance conditions. Technologies involving hydrodynamic must be intensively applied to solids removal for aquatic species production as well as research purposes like digestibility, which is highlighted in this study.