Treatment of flow-through trout aquaculture effluents in a constructed wetland

A study on effluent treatment with sub-surface flow (SF) constructed wetlands was conducted in a small commercial scale Bavarian (Germany) flow-through trout farm. Under limited spatial and financial conditions a most suitable wetland was constructed. The wetland treatment efficiency at high hydraulic loading rates during raceway runoff and cleaning situation in comparison to sedimentation as initial treatment method was examined.

The constructional solution involved the alteration of six existing sedimentation basins (SB) to SF horizontal flow constructed wetlands with a pre-sedimentation area. As constructional materials only local, cheaply available materials were used in order to reduce the costs. The SF wetland had high treatment efficiencies in the two operational modes examined. During cleaning situation at a hydraulic loading rate (HLR) of 13.6 m/day treatment efficiency for total suspended solids (TSS) was highest and reached 68%. While during raceway runoff situation total ammonia nitrogen (TAN) treatment efficiency of 88% overtopped the efficiency of the other nutrients examined at a HLR of 10.6 m/day. In both treatment situations the SF wetland efficiency was significantly higher than the effect of the SB. SF constructed wetlands treating high hydraulic loading rates accompanied with short retention times were effective on dissolved nutrient treatment only for TAN and nitrite nitrogen (NO₂–N), while other dissolved nutrients like nitrate nitrogen (NO₃–N) and phosphate phosphorous (PO₄–P) showed no or even negative treatment effects through the wetland passage. To reduce these nutrients, other treatment conditions or wetland configurations are needed.     

A partial-reuse system for coldwater aquaculture

A model partial-reuse system is described that provides an alternative to salmonid production in serial-reuse raceway systems and has potential application in other fish-culture situations. The partial-reuse system contained three 10 m³ circular ‘Cornell-type’ dual-drain culture tanks. The side-wall discharge from the culture tanks was treated across a microscreen drum filter, then the water was pumped to the head of the system where dissolved carbon dioxide (CO₂) stripping and pure oxygen (O₂) supplementation took place before the water returned to the culture tanks. Dilution with make-up water controlled accumulations of total ammonia nitrogen (TAN). An automatic pH control system that modulated the stripping column fan ‘on’ and ‘off’ was used to limit the fractions of CO₂ and unionized ammonia nitrogen (NH₃---N). The partial-reuse system was evaluated during the culture of eight separate cohorts of advanced fingerlings, i.e., Arctic char, rainbow trout, and an all female brook trout × Arctic char hybrid. The fish performed well, even under intensive conditions, which were indicated by dissolved O₂ consumption across the culture tank that went as high as 13 mg/L and fish-culture densities that were often between 100 and 148 kg/m³. Over all cohorts, feed conversion rates ranged from 1.0 to 1.3, specific growth rates (SGR) ranged from 1.32 to 2.45% body weight per day, and thermal growth coefficients ranged from 0.00132 to 0.00218. The partial-reuse system maintained safe water quality in all cases except for the first cohort—when the stripping column fan failed. The ‘Cornell-type’ dual-drain tank was found to rapidly (within only 1–2 min) and gently concentrate and flush approximately 68–88% (79% overall average) of the TSS produced daily within only 12–18% of the tank’s total water flow. Mean TSS concentrations discharged through the three culture tanks’ bottom-center drains (average of 17.1 mg/L) was 8.7 times greater than the TSS concentration discharged through the three culture tanks’ side-wall drains (average of 2.2 mg/L). Overall, approximately 82% of the TSS produced in the partial-reuse system was captured in an off-line settling tank, which is better TSS removal than others have estimated for serial-reuse systems (approximately 25–50%). For the two cohorts of rainbow trout, the partial-reuse system sustained a production level of 35–45 kg per year of fish for every 1 L/min of make-up water, which is approximately six to seven times greater than the typical 6 kg per year of trout produced for every 1 L/min of water in Idaho serial-reuse raceway systems.     

Removal of nitrogen by Algal Turf Scrubber Technology in recirculating aquaculture system

Ongoing research in recirculation aquaculture focuses on evaluating and improving the purification potential of different types of filters. Algal Turf Scrubber (ATS) are special as they combine sedimentation and biofiltration. An ATS was subjected to high nutrient loads of catfish effluent to examine the effect of total suspended solids (TSS), sludge accumulation and nutrient loading rate on total ammonia nitrogen (TAN), nitrite and nitrate removal. Nutrient removal rates were not affected at TSS concentration of up to 0.08 g L^?1 (P > 0.05). TAN removal rate was higher (0.656 ± 0.088 g m^?² day^?1 TAN) in young biofilm than (0.302 ± 0.098 g m^?² day^?1 TAN) in mature biofilm at loading rates of 3.81 and 3.76 g m^?² day^?1 TAN (P < 0.05), respectively, which were considered close to maximum loading. TAN removal increased with TAN loading, which increased with hydraulic loading rate. There was no significant difference in removal rate for both nitrite and nitrate between young and mature biofilms (P > 0.05). The ATS ably removed nitrogen at high rates from catfish effluent at high loading rates. ATS‐based nitrogen removal exhibits high potential for use with high feed loads in intensive aquaculture.     

Physiological changes in rainbow trout held under crowded conditions and fed diets with different levels of vitamins E and C and highly unsaturated fatty acids (HUFA)

Rainbow trout (Oncorhynchus mykiss) maintained in crowded (100 kg m^− 3) and uncrowded (20 kg m^− 3) conditions were fed 42 days with five experimental diets having different levels of vitamin E (25.6 and 275.6 mg kg diet^− 1), C (0 and 1000 mg kg diet^− 1) and HUFA (highly unsaturated fatty acids, 12.5 and 320.5 g kg diet^− 1): −E−HUFA, −E+HUFA, +E−HUFA, +E+HUFA, −C+E+HUFA. Cortisol, plasma metabolites, tissue glycogen, fish composition, and tissue fatty-acid profile were evaluated at the end of the experimental period. In general, no changes in cortisol levels were associated with crowding, although +E+HUFA and −C+E+HUFA fish showed higher levels (mean ± SE, 55.5 ± 11.1 and 78.0 ± 11.3 ng ml^− 1) as a consequence of a possible interaction between chronic crowding and diet composition. Protein and glucose con-centration in plasma displayed no effect of crowding, but liver glycogen showed a general tendency to decrease in −E−HUFA, −E+HUFA, +E−HUFA, +E+HUFA, −C+E+HUFA crowded groups (70.2 ± 2.1, 52.1 ± 2.5, 73.4 ± 7.4, 91.7 ± 3.3, 74.2 ± 8.4 mg g^− 1 tissue, respectively) compared to uncrowded groups (108.9 ± 14.2, 82.7 ± 8.8, 92.4 ± 10.7, 99.1 ± 10.0, 103.5 ± 15.6 mg g^− 1 tissue, respectively), thus proving significant in −E+HUFA fish. Variations in total lipids, triglycerides, total cholesterol and HDL as well as LDL cholesterol in plasma were manifested under crowding conditions, displaying a certain influence of vitamin E and HUFA dietary content. Final body composition, in general, showed no change attributable to fish density, but some differences associated with diet composition were found in lipid and moisture percentages of crowded fish. Liver and muscle fatty-acid profile revealed a clear effect of the dietary lipid source that was more evident in muscle than in liver at normal fish density, and in some cases this effect was modulated by dietary vitamin E and C content and fish-culture conditions.     

Performance of plastic biofilter media with different configuration in a water recirculation system for the culture of Nile tilapia (Oreochromis niloticus)

Three kinds of locally available plastic biofilter media with different configurations (plastic rolls, PVC pipes and scrub pads) were evaluated for their efficiency in organic waste removal from the effluents of an intensive recirculating tilapia culture system. A set of three types of solid-removing filters consisting of screened sedimentation; upflow sand as well as plastic bead filtration accomplished the mechanical filtration. Values of critical metabolic wastes like total ammonia nitrogen (TAN) (0.92 ppm) and nitrite-nitrogen (NO₂-N) (0.22 ppm) were found to be well within the acceptable limits, while other water quality parameters in the culture water were also maintained within the normal range by the filtration system. Removal rates of 3.46 g TAN/m³ per day and 0.77 g NO₂-N/m³ per day, as well as TAN and NO₂-N removal efficiencies of 29.37 and 27.3% respectively, were established to be the best for the plastic-roll biofilter medium as compared to PVC-pipe and scrub-pad media. Percent removal of TAN and NO₂-N per pass of the biofilter (25.49 and 26.3% respectively) and the specific TAN and NO₂-N removal rates (43 and 9.6 mg/m² per day) of plastic rolls were also found to be superior to the other two biofilter media. Pieces of PVC pipes as biofilter medium is recommended to be used in the biofilters in view of their cheaper cost.     

Reactive nitrogen and phosphorus removal from aquaculture wastewater effluents using polymer hydrogels

We have developed poly(allyl amine hydrochloride) (PAA · HCl) polymer hydrogels, that efficiently remove nitrate (NO₃⁻), nitrite (NO₂⁻), and orthophosphate (PO₄³⁻) nutrient anions from the aquaculture wastewater. The hydrogels were prepared by chemically crosslinking linear PAA · HCl chains with epichlorohydrin (EPI). The anion binding capacity of the pH sensitive polymer gels was measured in standard solutions and studied as a function of gel synthesis parameters. Equilibrium NO₃–N, NO₂–N, and PO₄–P loading of 15, 1.6, and 17 mg/g of dry gel, respectively, were calculated from the measurement of decrease in anion concentration in aqueous solutions using UV–vis spectrophotometry. Batch experiments showed that nutrient concentrations in aquaculture wastewater effluents decreased with regard to PO₄–P by 98+%, NO₃–N by 50+% and NO₂–N by 85+% within 3 h of reaction. The regeneration of the hydrogels was demonstrated by the release of bound nutrient anions upon washing the gels with a 1 N NaOH solution. These results have demonstrated that the hydrogels are appropriate materials for treating aquaculture wastewater effluents, and reducing the nutrient anion concentrations to levels, less than 10 mg/l NO₃–N, 0.08 mg/l NO₂–N, and 0.3 mg/l PO₄–P, suitable for discharge into natural surface waters.     

Effects of body weight, water temperature and ration size on ammonia excretion by the areolated grouper (Epinephelus areolatus) and mangrove snapper (Lutjanus argentimaculatus)

The effects of body weight, water temperature and ration size on ammonia excretion rates of the areolated grouper Epinephelus areolatus and the mangrove snapper Lutjanus argentimaculatus were investigated. Under given experimental conditions, L. argentimaculatus had a higher weight-specific ammonia excretion rate than E. areolatus. Weight-specific ammonia excretion rates of fasted individuals of both species showed an inverse relationship with body weight (W, g wet wt.), but a positive relationship with water temperature (t, °C). The relationships for total ammonia nitrogen (TAN) were: E. areolatus: TAN (mg N kg⁻¹ d⁻¹)=21.4·exp^0.11t·W^−0.43 (r²=0.919, n=60); L. argentimaculatus: TAN (mg N kg⁻¹ d⁻¹)=121.5·exp^0.12t·W^−0.55 (r²=0.931, n=60). Following feeding, the weight-specific ammonia excretion rate of E. areolatus increased, peaked at 2 to 12 h (depending on temperature), and returned to pre-feeding levels within 24 h. A similar pattern was observed for L. argentimaculatus, with a peak of TAN excretion being found 6 to 12 h after feeding. Stepwise multiple regression analysis indicated that weight-specific TAN excretion rates of both species increased with increasing temperature and ration (R, percent body wt. d⁻¹): E. areolatus: TAN (mg N kg⁻¹ d⁻¹)=22.8·t−28.8·R−378.2 (r²=0.832, n=24); L. argentimaculatus: TAN (mg N kg⁻¹ d⁻¹)=22.9·t−25.4·R−216.4 (r²=0.611, n=24). The effect of body weight on weight-specific postprandial TAN excretion was not significant in either species (p>0.05). This study provides empirical data for estimating ammonia excretion of these two species under varying conditions. This has application for culture management.     

Acoustic analysis of volume variation in a bag-net within a set-net

An experiment designed to measure the volume variation of a bag-net within a set-net was conducted in Jaran Bay, Kosung, Korea. Three radio-acoustic-linked positioning (RAP) buoys, a time controller with a personal computer and seven pingers were used to measure the volumes of the bags. During the April neap tide, the minimum and maximum volumes of the bag-net were 4173 m³ (at 17.00 h) and 4757 m³ (12.00 h), respectively. The average current directions and speeds were 99.9°, 12.9 cm/s and 104.0°, 2.4 cm/s, respectively. During the spring tide, the minimum and maximum volumes of the bag-net were 2016 m³ (18.30 h) and 4454 m³ (15.00 h), respectively. The average current directions and speeds were 315.6°, 16.1 cm/s and 289.0°, 5.7 cm/s, respectively. The minimum (2016 m³) and maximum (5568 m³) volumes of the bag-net were observed during the period when the spring tide changed to the neap tide.     

Waste and particle management in a commercial, partially recirculating trout farm

The present case study, deals with a recently built aquaculture facility using 80–120 L s⁻¹ spring water for trout production. The farm consists of six raceways, discharging in a common outflow channel, leading to a drum filter equipped with 80 μm gauze. About 120 L s⁻¹ of the microscreen effluent is pumped back in the inflow channel of the six raceways. The remaining effluent is oxygenated with pure oxygen in gravity oxygenation units and led to two U-shaped raceways. The farm effluent is finally filtered by a drum filter with 63 μm mesh size. The microscreen backwash sludge is treated in a cone settler, where the sediments are extracted for agricultural manure application. The sedimentation supernatant is further led in a sub surface flow (SSF) constructed wetland prior to discharge.Due to the advanced effluent treatment within the farm, the total farm effect on the receiving effluent is kept to a minimum. The nutrient increase produced by the farm is only 0.03 mg L⁻¹ total phosphorous (TP), 1.09 mg L⁻¹ biological oxygen demand (BOD₅) and, 0.57 mg L⁻¹ total suspended solids (TSS) in the brook. Especially the incorporation of an intermediate microscreen prior to water recirculation, prevents leaching of dissolved nutrients from particulate matter, as large particles are effectively and as fast as possible removed from the water flow.At the pumping station, needed for water recirculation, the particle size distribution (PSD) was monitored with the previous microscreen in use and by-passed. When the screen was by-passed a significant crushing effect on PSD through pumping action was found. Through the removal of large particles, the crushing effect of the pumping station on the particles is prevented, as revealed by particle size distribution (PSD) measurement. Thus, leaching of dissolved nutrients is prevented twice.In consequence, the farm configuration can be recommended as an effective possibility for intensive trout production at sites with a small freshwater source and stringent effluent thresholds, even with the unexpected low treatment efficiencies measured for the microscreens. Both drum filters showed relative low treatment efficiencies of 33–53% for total suspended solids, respectively, while an efficiency of 70% should be expected from the measured PSD. With this impact, the farm still emitted a low nutrient amount, especially due to the highly effective offline microscreen backwash sludge treatment, where the SSF wetland efficiently reduced dissolved and particulate nutrients as nitrite (NO₂-N), nitrate (NO₃-N) and TSS. Thus this SSF wetland application might be suitable as a denitrification step in a closed recirculating trout farm.     

Application of microbead biological filters

The application of floating microbead filters to aquaculture is reviewed and discussed. The microbead filter is distinctly different from the more commonly used floating bead filters that are used today. Conventional bead filters work in pressured vessels and use a media that is only slightly buoyant. The required mass of beads for the volume required make the media a relatively expensive component of a floating bead filter in contrast to sand or microbead media that is much less expensive on a per volume basis. Microbead filters use polystyrene beads (microbead) that are 1–3 mm in diameter (floating bead filters use media approximately 3 mm in diameter also). Microbead have an overall bulk density of 16 kg/m³ and a specific surface area of 3936 m²/m³ (for 1 mm beads). This material can be obtained commercially in bulk for roughly US$ 4 kg⁻¹ of material. Biological filters that use microbeads for their nitrifying substrate can be thought of as a trickling bio-filter in terms of how the flow distribution and collection mechanics are designed and operated. For design purposes, microbead filters can be assumed to nitrify approximately 1.2 kg of TAN/m³ of media per day for warm water systems with influent ammonia–nitrogen levels from 2 to 3 mg/l. For cool water applications, rates should be assumed to be 50% of warm water rates or use rates similar to those used for fluidized sand beds. Designs and results in several applications are presented. Microbead filters have been used successfully by several commercial growers after being first introduced in the mid 1990s. Effects of capitalization for equipment and buildings upon production costs is discussed and presented in graphical form.     

Performance of packed column aerator at low hydraulic loading

A laboratory-scale packed column aerator filled with ceramic Raschig rings was tested for its performance with hydraulic loadings in the range of 16–86·3 m³ m⁻² h⁻¹. Two columns of 0·19 m and 0·24 m inside diameter and packing sizes of 15, 25 and 36 mm were used. The system equation developed by previous workers for trickling filters in waste-water treatment was not generally applicable across the lower hydraulic loadings. The system coefficient incorporating the oxygen absorption coefficient (k_La) was found to vary within the range of hydraulic loadings studied. The oxygen transfer rate equation developed for surface and submerged aerators was used for estimating standard oxygen transfer efficiency of the packed column aerator, which, in this case, ranged from 6·2 to 22·6 kg O₂ kWh⁻¹.     

Waste excretion characteristics of Manila clams (Tapes philippinarum) under different temperature conditions

Water recirculating systems have been used in the shellfish industry for depuration and wet-storage. Knowledge of shellfish excretion characteristics is critical to recirculating system design. In this study, the excretion rate of total ammonia nitrogen (TAN), total Kjeldahl nitrogen (TKN), and 5-day biochemical oxygen demand (BOD₅) from Manila clams (Tapes philippinarum) were investigated under both laboratory and commercial conditions. The laboratory tests were conducted under temperatures ranging from 3 to 30°C. The experimental results showed that temperature was a key factor in determining the excretion rate of all the above parameters. The relationship between TAN excretion rate (R_TAN) and temperature (T) can be represented by an exponential function (R_TAN=0.57×1.25^T). For the temperature range between 3 and 20°C, the daily mean excretion rates of TAN, TKN and BOD₅ ranged between 1.5–46.1, 4.8–131.0 and 57.4–219.4 mg per kilogram of the clams (wet weight with shell on), respectively. There were linear correlations between TAN, TKN and BOD₅ production rates. The data presented in this paper can be used to estimate waste generation from a given shellfish processing operation and to size the waste treatment components for a recirculating depuration (or wet-storage) system.     

Assessing sediment removal capacity of vegetated and non-vegetated settling ponds in prawn farms

Sediment removal capacity is assessed for a constructed mangrove wetland, and a non-vegetated settling pond that are both used for filtering water in tropical aquaculture. The assessment is performed through sediment budget analysis using data of suspended sediment concentration collected from optical backscatter sensors. The sensors were deployed at the pond's inlet and outlet. These data sets provide a measure of trapping efficiency of each pond with different flow regimes and settling areas. The tides influenced flow in the wetland but none was felt in the settling pond. The average trapping efficiency obtained for the vegetated and the non-vegetated ponds was (40±33) and (70±36)%, respectively. The deposition rate calculated for the vegetated and non-vegetated pond ranges between 13–174 g/m² per h (average=63 g/m² per h) and 10–19 g/m² per h (average=14 g/m² per h), respectively. The efficiency of vegetated and non-vegetated ponds is likely to be improved by decreasing the aspect ratio (length/width) from the current value of 6 to 1 and of 5 to 1, respectively.     

The susceptibility of salmonid fish to an atypical strain of Aeromonas salmonicida that infects goldfish, Carassius auratus (L.), in Australia

Abstract. An enzootic, Australian, atypical strain of Aeromonas salmonicida isolated from diseased goldfish, Carassius auratus (L.), was inoculated into Atlantic salmon, Salmo salar L., brown trout, S. trutta L., rainbow trout, S. gairdneri Richardson, and brook trout, Salvelinus fontinalis (Mitchill), fingerlings by intraperitoneal injection (i.p.) and by bath challenge, the latter with and without prior abrasion of skin. The 10-day LD₅₀ (i.p.) was estimated to be 7·4 × 10^-3 colony forming units (cfu) for Atlantic salmon, 3·0 × 10^-2 cfu for brown trout, 3·7 × 10² cfu for brook trout and 6·4 × 10³ cfu for rainbow trout. Brown, rainbow and brook trout succumbed to bath challenges with between 10⁵–10⁶ cfu/ml, developing ulcers of the skin and septicaemia. The organism was trasmitted from inoculated fish to five of 195 within-tank control fish via water and established a carrier state in one of 14 Atlantic salmon. It was concluded that the organism poses a significant threat to the salmonid farming industry and wild salmonid fisheries in Australia.     

A bath challenge model for furunculosis in rainbow trout, Salmo gairdneri Richardson, and Atlantic salmon, Salmo salar L.

Abstract. A natural bath challenge method has been developed for furunculosis using Atlantic salmon, Salmo salar L., and rainbow trout, Salmo gairdneri Richardson. Fish were placed in an enclosed, continuously circulating tank system, supplemented with additional oxygen, the temperature raised gradually (overnight) to 15–16°C, a low dose of Aeromonas salmonicida (strain 184/186) introduced into the tank and the challenge maintained for 14 days. The challenge strain was characterized with respect to possible virulence factors and possessed an A-layer, ability to auto-agglutinate, haemagglutinate, adhere to Atlantic salmon cells and resist destruction by serum. No caseinase activity was detected. LD₅₀ for salmon using this method was 1.8×10³ cells per millilitre while trout had an LD₅₀ of 9.5×10⁴ cells per millilitre. Onset of the disease appeared to depend on fish size with larger trout (50 g) taking up to 10 days to show signs of the disease while mortalities in smaller trout (8.5g) commenced on day 1 post-challenge.     

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.     

Wood chips and wheat straw as alternative biofilter media for denitrification reactors treating aquaculture and other wastewaters with high nitrate concentrations

This study evaluated wood chips and wheat straw as inexpensive and readily available alternatives to more expensive plastic media for denitrification processes in treating aquaculture wastewaters or other high nitrate waters. Nine 3.8-L laboratory scale reactors (40 cm packed height × 10 cm diameter) were used to compare the performance of wood chips, wheat straw, and Kaldnes plastic media in the removal of nitrate from synthetic aquaculture wastewater. These upflow bioreactors were loaded at a constant flow rate and three influent NO₃–N concentrations of 50, 120, and 200 mg/L each for at least 4 weeks, in sequence. These experiments showed that both wood chips and wheat straw produced comparable denitrification rates to the Kaldnes plastic media. As much as 99% of nitrate was removed from the wastewater of 200 mg NO₃–N/L influent concentration. Pseudo-steady state denitrification rates for 200 mg NO₃–N/L influent concentrations averaged (1360 ± 40) g N/(m³ d) for wood chips, (1360 ± 80) g N/(m³ d) for wheat straw, and (1330 ± 70) g N/(m³ d) for Kaldnes media. These values were not the maximum potential of the reactors as nitrate profiles up through the reactors indicated that nitrate reductions in the lower half of the reactors were more than double the averages for the whole reactor. COD consumption per unit of NO₃–N removed was highest with the Kaldnes media (3.41–3.95) compared to wood chips (3.34–3.64) and wheat straw (3.26–3.46). Effluent ammonia concentrations were near zero while nitrites were around 2.0 mg NO₂–N/L for all reactor types and loading rates. During the denitrification process, alkalinity and pH increased while the oxidation–reduction potential decreased with nitrate removal.

Wood chips and wheat straw lost 16.2% and 37.7% of their masses, respectively, during the 140-day experiment. There were signs of physical degradation that included discoloration and structural transformation. The carbon to nitrogen ratio of the media also decreased. Both wood chips and wheat straw can be used as filter media for biological denitrification, but time limitations for the life of both materials must be considered.     

Performance characteristics of fluidised bed biofilters in a novel laboratory-scale recirculation system for rainbow trout: nitrification rates, oxygen consumption and sludge collection

A laboratory-scale recirculating aquaculture system for fluidised bed biofilter evaluation was engineered. The design included all components found in typical full-scale commercial production systems. The system included two identical units each with oxygenation, UV treatment, cooling, biofiltration and a particulates separation device. Water from the two systems was mixed in a degassing unit. A 1 month test period after biofilter maturation revealed stable concentrations of total ammonia nitrogen (TAN), nitrite and nitrate within the system. Mean nitrification rate was 0.27 and 0.21 g TAN m⁻² day⁻¹. Oxygen consumption in the biofilters ranged between 56 and 64% due to nitrifying activity. Mass balances on nitrogen indicated that 48%, added via the feed, was converted to nitrate within the system, with 6% of the added nitrogen being found in the sludge. The remaining 43% was either used during fish growth, left the system, as organic nitrogenous compounds (or unidentified nitrogenous compounds), via the outlet, or was lost to the atmosphere. At least 61% of the nitrate produced was generated by the biofilters. The system proved to be an exceptional set-up for evaluation of the performance of fluidised bed biofilters, allowing both pre- and post-filter measurements of various water quality criteria.     

Using a macroalgae Ulva pertusa biofilter in a recirculating system for production of juvenile sea cucumber Apostichopus japonicus

A simple indoor recirculating system for production of juvenile sea cucumber (Apostichopus japonicus) was operated on a commercial scale for 90 days during winter. The system consists of three 70 m³ sea cucumber rearing tanks and one biofilter tank where macroalgae (Ulva pertusa) was used as a biofilter in order to reduce water requirements. Effluent from the sea cucumber tanks drained into the macroalgae biofilter tank and were then returned to the sea cucumber tanks by a discontinuous-flow recirculation system. Survival and growth rates in the sea cucumber culture tanks were similar to those in the control tank (with one water exchange per day). The survival rate averaged about 87%. The average body weight increased from 3.5 ± 0.3 g to 8.1 ± 0.8 g and total sea cucumber biomass production over the experimental period was 745 g m⁻² after initial stocking densities of 375 g m⁻². The growth rate of U. pertusa was 3.3% day⁻¹. U. pertusa was efficient in removing toxic ammonia and in maintaining the water quality within acceptable levels for sea cucumber culture; there were only small daily variations of temperature, pH and DO. The U. pertusa tank removed 68% of the TAN (total ammonia-nitrogen) and 26% of the orthophosphate from the sea cucumber culture effluent; the macroalgae biofilter removed ammonia at an average rate of 0.459 g N m⁻² day⁻¹. It would be efficient to use the U. pertusa biofilter in a recirculating system for production of A. japonicus juveniles in winter.     

Modelling of nitrogen loads from the farming of yellowtail kingfish Seriola lalandi (Valenciennes, 1833)

Farming of yellowtail kingfish ( Seriola lalandi , Valenciennes, 1833) in the coastal waters of Australia is a relatively new aquaculture industry, and little is known about the magnitude of nutrient discharges from individual pens. In this work, we modelled the flow of nitrogen for each of two commercial pens in Fitzgerald Bay, upper Spencer Gulf, South Australia. The fish were fed commercial pellets with feed conversion ratios (FCRs) between 3.0 and 3.2 (dry weight feed/wet weight growth). These high values of FCR were reflected in the high nitrogen loads to the environment (176–195 kg N tonne⁻¹ growth) and a small retention of nitrogen in fish growth (14–16% of feed inputs). Considering an annual production of 2000 tonnes, total loads to the environment can reach 391 tonnes N year⁻¹. Eighty-two per cent of these loads are expected to be lost to the water column as dissolved wastes. The high nutrient loads and the importance of dissolved wastes compared with other aquaculture species, such as salmon and trout, reflect the distinctly higher metabolic rates of this pelagic predatory species. The nature of the wastes suggests low localized impacts at current production levels, but regional effects remain unknown.