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Design and evaluation of a commercial recirculating system for half-smooth tongue sole (Cynoglossus semilaevis) production
Aquacultural Engineering 54 (2013) 104–109
Contents lists available at SciVerse ScienceDirect
Aquacultural Engineering journal homepage: www.elsevier.com/locate/aqua-online
Design and evaluation of a commercial recirculating system for half-smooth tongue sole (Cynoglossus semilaevis) production Zhitao Huang a , Xiefa Song a,∗ , Yanxuan Zheng a , Lei Peng a , Rong Wan a , Timothy Lane b , Jieming Zhai c , Eric Hallerman b , Dengpan Dong a a
Department of Fisheries, Ocean University of China, Qingdao 266003, PR China Department of Fish and Wildlife Conservation, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061, USA c Laizhou Mingbo Aquatic Co., Ltd., Lai Zhou 261418, PR China b
a r t i c l e
i n f o
Article history: Received 3 October 2012 Accepted 28 December 2012 Keywords: Recirculating aquaculture system Half-smooth tongue sole Cynoglossus semilaevis
a b s t r a c t A recirculating aquaculture system was designed for producing half-smooth tongue sole (Cynoglossus semilaevis). The recirculating system included twenty-eight 40-m3 circular culture tanks. The bottom discharge from the culture tanks was treated by passage across a bowed screen, and then water was pumped through a foam fractionator and a series of three submerged biofilters which contained plastic media, BIO-BLOK® element, and porous plastic media, respectively. Water then was disinfected with UV irradiation and supplied with pure oxygen before flowing back to the culture tanks. The system was stocked with 33,073 fish (mean weight 305 ± 57 g) in March 2009 and operated with greater than 95% water reuse. At the maximum feeding rate, the cumulative feed burden and loop strength were 7.25 kg feed/m3 of water and 11.8 mg/l, respectively. Over 8 months, fish survival rate was more than 97%. Fish were harvested from the system in October 2009 at a mean weight of 1246 ± 166 g, with a specific growth rate (SGR) through this time of 1.2% body weight per day and feed conversion ratio of 1.1. The system maintained water quality appropriate for the fish, with mean temperature of 19.7 ◦ C, DO of 6.7 mg/l, pH of 7.37, un-ionized ammonia of 0.012 mg/l, and nitrite of 0.044 mg/l. Approximately 99% of heterotrophic bacteria and coliform bacteria were inactivated by UV irradiation. Power usages per cubic meter water and per kg of production were 0.11 kW and 6.6 kW, respectively. Our results demonstrated the utility of recirculating aquaculture systems for producing half-smooth tongue sole. We suggest that regular backwashing of submerged biofilters may raise production capacity and allow use of biofilters with fewer stages. © 2013 Published by Elsevier B.V.
1. Introduction Half-smooth tongue sole (Cynoglossus semilaevis) is an important fishery product in East Asia. However, with the decrease of wild populations, fishery harvest was less than 1 metric ton in 2005 (Jiang and Wan, 2005). To meet market demand, half-smooth tongue sole has become an important aquaculture product in China (Fang et al., 2010), with captive production in 2009 totaling 7120 metric tons (Ni et al., 2010). The traditional culture method for producing tongue sole is a flow-through system in which water is pumped from a well into circular tanks, from which effluent is discharged to the sea by gravity through the bottom drain (Liu et al., 2006; Ni et al., 2010). With more and more intensive
∗ Corresponding author at: Aquaculture Building, 5 Yushan Road, Qingdao 266003, PR China. Tel.: +86 532 82032522; fax: +86 532 82032833. E-mail address: [email protected] (X. Song). 0144-8609/$ – see front matter © 2013 Published by Elsevier B.V. http://dx.doi.org/10.1016/j.aquaeng.2012.12.004
flow-through culture systems placed into production, however, two serious problems became evident: (1) a growing shortage of saline groundwater, and (2) environmental pollution due to untreated effluent being discharged to the receiving ecosystem. Commercial companies must obey the aquaculture wastewater discharge regulations subsequently promulgated by the government. For example, for saline aquaculture wastewater discharges, SS < 100 mg/l, COD < 6.0 mg/l, DO > 3.0 mg/l, and un-ionized ammonia < 0.06 mg/l are required to meet first-grade criteria required by the regulations of Zhejiang Province. Under these conditions, recirculating systems have become an attractive production alternative. Against this background, we designed and evaluated a recirculating system for producing half-smooth tongue sole in Lai Zhou, China. We evaluated fish production and water quality during commercial on-growing of half-smooth tongue sole in order to provide information useful for designing and operating systems for producing these valuable fish.
Z. Huang et al. / Aquacultural Engineering 54 (2013) 104–109 Table 1 Design parameters for recirculating aquaculture systems. Parameter
Value
Number of tanks Tanks diameter (m) Tank water depth (m) Fish size before stocking (g) Number of fish in system Final rearing density (kg/m3 ) Feeding rate (%body weight/day) Feed protein content (%) Inlet DO (mg/l) Unionized ammonia of inlet water (mg/l) Nitrate of inlet water (mg/l) Temperature (◦ C) pH Culture tank unionized ammonia level (mg/l) Nitrate in the system (mg/l) Biofilter efficiency (percentage of metabolite removed by a single pass through the three filters, %)
28 8 0.8 300 33,000 45 1.2 48 7.5 0 0 21 7.5 <0.02 300 35%
Reuse flow required based on TAN (m3 /h) Reuse flow required based on DO (m3 /h)
1281 978
Table 2 Characteristics of culture tanks and water treatment components in recirculating aquaculture system. Parameter or criterion Culture tanks (28) Volume Exchange rate Critical features Bowed screen Screen pore size Effective filter area Critical features
Foam fractionator Size Hydraulic retention time Ozone dose Series of submerged biofilters Size Media
2. Materials and methods 2.1. Water reuse determination A mass balance approach was used to design the fish culture system. The approach taken was to set water quality parameters that must be achieved to ensure good culture conditions and then to solve for the required reuse flowrate. Under steady state conditions, we estimated the flow to the biofilters required to maintain acceptable total ammonia nitrogen (TAN) concentration in the tank as (Timmons and Losordo, 1994): Qf =
PTAN − Q × CTAN ATAN × E
(1)
where Qf = flow rate to the filter, PTAN = production rate of TAN, Q = flow rate through the system, CTAN = concentration of TAN in the culture tank, ATAN = maximum allowable TAN concentration, and E = efficiency of the series of biofilters. We estimated the flow to the biofilter required to maintain a biofilter effluent dissolved oxygen (DO) concentration of 2.0 mg/l (Timmons and Losordo, 1994) as: + RNOD R Qf = BODf COfi − COf
(2)
where Qf = flow rate through biofilter base on DO mass balance, RBODf = rate of biochemical oxygen demand (BOD) expression within the filter, RNOD = DO consumption due to nitrification, COfi = concentration of DO in the biofilter influent, and COf = concentration of DO in the biofilter. Using the full set of input parameters defined in Table 1, the water reuse flowrate (Qf ) required to control unionized ammonia
105
Hydraulic loading rate Critical features Degassing unit (trickling filter) Media Packing depth Hydraulic loading rate Critical features UV irradiation units UV lamp (20) Intensity Dissolved oxygen contact unit Oxygen supply rate Critical features
Value 40 m3 Every 72 min Concrete construction; bottom-drain design with stand-pipe 0.25 mm 0.9 m2 All stainless steel construction; manually cleaned three times daily with brush and back-flushed weekly using high-pressure water 3 m diameter, 3 m height 1 min 15 g ozone/kg feed day−1 3 m × 3 m × 5 m each Elastic polyamide and polyolefin media (100 m2 /m3 ) BIO-BLOK® element (200 m2 /m3 ) Porous polypropylene media (380 m2 /m3 ) 2340 l/m2 min All concrete construction BIO-BLOK® element (200 m2 /m3 ) 4m 480 l/m2 min All concrete construction; spray diffuser; forced ventilation 53 W each 150 mW s/cm2 7.36 kg/h Pure oxygen supply; micropore diffusers
levels was estimated as 1281 m3 /h. The reuse flowrate (Qf ) required to maintain adequate dissolved oxygen levels in the biofilter was estimated as 978 m3 /h. Hence, 1281 m3 /h was the flowrate used for system design. 2.2. System description The recirculating system (Fig. 1) was constructed of elements taken from an existing flow-through system retrofitted to make it a recirculating system. Characteristics of culture tanks and the water treatment components in the recirculating system are presented in Table 2. As described below, the system contained 28 circular culture tanks, bowed screen, sump, six pumps, foam fractionator, a series of three submerged biofilters, ozone contact chamber, degassing unit (trickling filter), UV light, and dissolved oxygen contact unit.
Fig. 1. Recirculating system for producing half-smooth tongue sole. The units are: (1) 28 fish culture tanks, (2) bowed screen for coarse solids removal, (3) sump, (4) foam fractionators, (5) three types of biofilters for nitrification and treatment of carbonaceous compounds, (6) degassing unit (trickling filter), (7) UV disinfection and dissolved oxygen contact unit, (8) ozone contact chamber, and (9) liquid oxygen chamber.
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Z. Huang et al. / Aquacultural Engineering 54 (2013) 104–109
The twenty-eight 8-m inner diameter × 1.0 m-deep circular culture tanks were custom-fabricated of concrete. The circular tanks were operated at depths of 0.8 m to produce a water volume in each tank of approximately 40 m3 . An outside stand-pipe connected to the bottom-drain controlled the level of water. Water was injected into the culture tanks through two 5-cm inside diameter nozzles located symmetrically on the top of each culture tank. The tank water volume was exchanged approximately 20 times a day (once every 72 min) by a total system flow of 21.3 m3 /min. All water discharged through the bottom-center drain of each culture tank to the bowed (arc) screen, as described by Lekang et al. (2000). The screen was made of stainless steel (0.90 m2 ) and the screen pore size was 0.25 mm. During fish production, workers brushed the solid particulates away from the bowed screen 1 h after feeding the fish and washed the bowed screen using high-pressure water once a week. Filtered water then was pumped through a foam fractionator (3 m diameter × 3 m height, 2.5 m water depth). Air was delivered by Venturi fittings. The foam that was generated was removed through the spigot on the top. The hydraulic retention time of the foam fractionator was approximately 1 min. Ozone was generated (HRCF-G-3-200/YQ, Haoer Co., Ltd., Yantai, China) with pure oxygen as a feed gas, and was injected through a Venturi pipe into the foam fractionator at an approximate rate of 15 g ozone/kg feed day−1 . The recirculating system used a series of submerged biofilters, a design that is widely used among production systems in China. Such systems usually consist of two or three submerged biofilters with different bio-media (Ni et al., 2010). In the three respective biofilter units in the present study, elastic polyamide and polyolefin media (100 m2 /m3 , Zhongkehai Co., Ltd., Qingdao, China), BIO-BLOK® elements (200 m2 /m3 EXPO-NET, Denmark), and porous polypropylene media (380 m2 /m3 , Mingbo Aquatic Co., Ltd., Laizhou, China) were suspended from the water surface to bottom in each 3 m L × 3 m W × 5 m H tank. The hydraulic loading rate was approximately 2340 l/m2 min. All of the recirculating water passed through a forcedventilation degassing unit (a trickling filter, 6.5 m × 6.5 m × 4 m) containing a 2.4 m-depth of packed BIO-BLOK® elements (200 m2 /m3 EXPO-NET, DK), hydraulically loaded at 480 l/m2 min. A UV light chamber unit used to inactivate heterotrophic and coliform bacteria contained twenty 53-W lamps that supplied a total UV dose of approximately 150 mW s/cm2 . Finally, the water flowed to the dissolved oxygen contact unit. Pure oxygen was injected into the contact unit by micropore diffusers (Tean Technology Co., Ltd., Nanjing, China). The overall oxygen consumption rate was the sum of the respiration rate of the half-smooth tongue sole, the biochemical oxygen demand (BOD) of uneaten food and fish wastes, and the nitrogenous oxygen demand (NOD) of the biofilters under steady-state conditions, assuming that the flow into the system was equal to that of the outflow. The required rate of DO input to the system was estimated as 7.36 kg/h. 2.3. Fish and feeding The systems were used to rear a total of 33,073 half-smooth tongue sole (more than 99% were female) for 8 months (March 1st to October 31st, 2009). Initial mean size and stocking density were 305 ± 57 g/fish and 10.8 kg/m3 , respectively. The fish were fed a commercial feed, EP4, with 48% crude protein and 13% fat (Marubeni Nisshin Feed Co., Ltd., Tokyo, Japan). Fish were fed at a rate of 1.2% body weight/day over the first 3 months. After June 1st, fish were fed at a rate of 0.8% body weight/day. Feeding occurred three times a day at 08:00, 15:00 and 22:30 h. Weekly feed ration changed with mean fish weight. Mean weights were estimated weekly by sampling approximately 200 fish from six randomly chosen tanks. The maximum feeding rate (kg/day) was attained at the
end of the production (after October 1st). Under the maximum feed loading at the end of the production, the loop strength (i.e., the total mass of feed divided by the amount of recycled water returned to the rearing tank daily) was 11.8 mg/l. Feed conversion rate equaled the total mass of feed fed divided by the total mass of fish produced. Unhealthy and dead fish were removed each morning. 2.4. Water quality Temperature, pH and dissolved oxygen in the tanks were monitored daily. Ammonia and nitrite were measured five times a month. Total ammonia and nitrite were analyzed using the phenate method and the sulfanilamide-NED method (APHA et al., 1995). Un-ionized ammonia was computed using the seawater Ammonia Calculator of Fish Hatchery Management appendices. Temperature and dissolved oxygen (DO) were measured using a YSI 85 probe (YSI, Inc., Yellow Springs, OH, USA). A pH meter was used to determine pH. The total heterotrophic and coliform bacteria were counted using plate culture, with counting of colonies produced from water samples collected every 10 days during fish production. 3. Results and discussion 3.1. Fish growth and production Half-smooth tongue sole grew quickly in the system during the 8 months of production without any catastrophic loss of fish; only 700 dead or unhealthy fish were removed from the system, for a survival rate of more than 97%. At the conclusion of production, mean individual fish weight was 1246 ± 166 g (Fig. 2), specific growth rate (SGR) was 1.2% body weight per day, and feed conversion rate was 1.1. 3.2. Performance comparison among recirculating system and previous flow-through systems For conventional flow-through systems (Table 3), the maximum fish density was 9 kg/m3 , the survival rate was more than 85%, and the feed conversion rate was 1.2–1.33 (Fu et al., 2011; Weifang Trans-Ocean Farm, unpublished data). Use of the experimental recirculating system improved the survival rate and the feed conversion ratio. The final fish density was 44.3 kg/m3 in the recirculating system, approximately five times greater than the conventional flow-through system (Table 3). The greater density within the recirculating system did not compromise water quality in the tanks. Use of the flow-through system required continuous pumping of deep groundwater. Operating the recirculating system required pumping of water, ozonation, use of a trickling filter, pure oxygen aeration, and a UV filter for inactivating bacteria. 6.6 kW of energy was required to produce per kg fish. A total of 0.11 kW of energy was required per cubic meter of recirculated water, which was lower than that for the conventional flow-through system (0.3 kW, Table 3). Hence, additional production was achieved at lower energy cost. 3.3. Water quality Water quality parameters for culture water in the recirculating system are shown in Table 3. Over the course of production, dissolved oxygen was maintained at more than 6 mg/l, which is appropriate for rearing half-smooth tongue sole (Peng et al., 2009). Fang et al. (2010) suggested that commercial farmers could feed juvenile tongue sole to satiation in order to obtain higher growth rate, as the maximum specific growth rate occurred in fish fed to satiation at 22 ◦ C. In this commercial production cycle, mean temperature was 19.7 ± 0.8 ◦ C, which satisfied the
Z. Huang et al. / Aquacultural Engineering 54 (2013) 104–109
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Fig. 2. Mean weight of half-smooth tongue sole (with standard error) and temperature through the production period.
requirements for on-growing half-smooth tongue sole. Mean TAN, un-ionized ammonia and nitrite concentrations were 1.34, 0.012 and 0.044 mg/l, respectively. These good results were expected, since this series of biofilters were designed to remove TAN and nitrite efficiently. The combination of the series of biofilters and trickling filter maintained unionized ammonia at less than 0.02 mg/l; half-smooth tongue sole require an un-ionized ammonia concentration of <0.06 mg/l (Xu et al., 2006). In aquaculture systems, ammonia maintains equilibrium between unionized (NH3 ) and ionized (NH4 + ) forms. Unionized ammonia (NH3 ) is toxic to fish and need to be controlled within the production system. Biofilters are used to oxidize TAN (the sum of the ionized and unionized forms of ammonia) to nitrate with nitrite as an intermediate component in the recirculating system. When we designed the system, we assumed 35% as the efficiency of the series of biofilters, which was the percentage of metabolite removed by a single pass through the filter, with 0.02 mg/l of ammonia (2.0 mg/l of TAN) was designed as the safe level. Our results showed that the mean TAN was 1.34 ± 0.38 mg/l, which was much less than the designed safe value, indicating that the efficiency of the series of biofilters was much more than 35% in the RAS. The series of biofilters were acclimated to ammonia loading before culturing the fish. The data collected during the production trial (Fig. 3) indicated stable nitrification performance. Concentration of unionized ammonia, with a slight increase over time, was relatively stable in the recirculating system (at less than 0.02 mg/l), at a lower level than in the flow-through system (Table 3). The
increase over time presumably was due to more feed applied to the system as the fish grew, with an associated lag time for the filters to respond to the increased loading. Nitrite ranged between 0.019 and 0.061 mg/l, with dynamics following those of unionized ammonia. Lower nitrite (Table 3) was found in the flowthrough system, likely because of the adequate water exchange rate. The series of submerged biofilters used in this recirculating system is typical of commercial fish production in China, where most commercial production has adopted this filtration methodology. Although the filtration did improve water quality, several issues arose. Suspended solids captured in the biofilters may have clogged and caused channeling in the biofilters, and would likely have been hydrolyzed and released as dissolved organic matter, favoring the growth of heterotrophs and thereby decreasing TAN removal rate (Tseng and Wu, 2004). Ni et al. (2010) reported that the series of submerged biofilters in their study would clog after 3–6 months. However, water quality was good and stable in this production trial, which may be because: (1) 8 months was a rather short time period, and (2) the majority of suspended solids were removed by the bowed screen and the foam fractionator. Most commercial production companies in China practice backwashing of filters only after a commercial production cycle. However, we suggest that appropriate biomass management or a sludge management system for use within the production cycle should be designed and evaluated. These approaches may improve water quality and decrease the capital costs of the biofilters.
Table 3 Comparison of water quality parameters between the focal recirculating system and two previous flow-through systems. Recirculating system
Flow-through systema
Flow-through systemb
Volume (m3 ) Maximum fish density (kg/m3 ) Water exchange rate (times per day)
1120 44.3 20
240 8.6 –
800 7–8 4–6
Fish survival rate Feed conversion rate
>97% 1.1
– 1.33
>85% 1.2–1.3
TAN (mg/l) Un-ionized ammonia (mg/l) Nitrite (mg/l) DO (mg/l) pH Temperature (◦ C)
1.34 ± 0.38 0.012 ± 0.003 0.044 ± 0.014 6.7 ± 0.69 7.37 ± 0.20 19.7 ± 0.78
1.04 0.15 0.016 >5 7.5–8.0 22
– – – – – 17–23
Energy usage kW/m3 of water flows kWh/kg of production
0.11 6.6
– –
0.3 –
a b
Fu et al. (2011). Weifang Trans-Ocean Farm.
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Z. Huang et al. / Aquacultural Engineering 54 (2013) 104–109
Fig. 3. Un-ionized ammonia (left scale), nitrite and total ammonia nitrogen (right scale) concentrations (mg/l) in the recirculating systems from March 2009 to October 2009.
Table 4 Total bacterial counts entering and exiting the UV filter. Bacteria
No. of samples
Heterotrophic bacteria Coliform bacteria
24 24
Bacteria counts (cfu/ml)
Removal (%)
Before UV
After UV
30,549 ± 19,104 121 ± 67
78 ± 32 <1
99.74 100
TAN flushed from the system due to water replacement was approximately 67 g daily. Use of the recirculating aquaculture system could reduce the nutrient pollution discharged to the environment compared to the conventional flow-through systems. Cumulative feed burden (i.e., the mass of food delivered to the system per unit volume of water added to the system daily, Colt et al., 2006) reflected the intensity of recirculation, and was a critical parameter in the design and management of the systems. Under the maximum feed loading at the end of the production, cumulative feed burden was 7.25 kg feed/m3 . The sludge flushed from the bowed screen was flocculated and used to grow algae.
becoming more and more popular in China, as they are effective and economical.
3.4. Bacterial inactivation
References
20 UV lamps (0.053 kW each) were installed to inactivate the bacteria in this recirculating system and supplied a total UV dose of approximately 150 mW s/cm2 . UV irradiation denatures the DNA of microorganisms, causing death or inactivation. It has been used to treat relatively large aquaculture flows, including flows within recirculating systems (e.g., Liltved et al., 1995; Blancheton, 2000; Liltved, 2002; Summerfelt, 2003; Sharrer et al., 2005; Summerfelt et al., 2004, 2009). The heterotrophic and coliform bacterial counts entering and exiting the UV lamps are shown in Table 4. The removal rates for heterotrophic and coliform bacteria were 99.7 and 100%, respectively. Disinfection equipment has seldom been installed in conventional flow-through systems in China; our results suggest their broader adoption there.
APHA (American Public Health Association), American Water Works Association, Water Environment Federation, 1995. Standard Methods for the Examination of Water and Wastewater, 19th ed. APHA, Washington, DC. Blancheton, J.P., 2000. Developments in recirculation systems for Mediterranean fish species. Aquacultural Engineering 22, 17–31. Colt, J., Lamoureux, J., Patterson, R., Rogers, G., 2006. Reporting standards for biofilter performance studies. Aquacultural Engineering 34 (3), 377–388. Fang, J., Tian, X., Dong, S., 2010. The influence of water temperature and ration on the growth, body composition and energy budget of tongue sole (Cynoglossus semilaevis). Aquaculture 299, 106–114. Fu, X., Ma, S., Zhu, J., Zhang, H., Qu, K., Zhou, Y., 2011. Analysis of the efficiency of water treatment and the cultured effect of Cynoglossus semilaevis Günther in closed re-circulating aquaculture systems. Chinese Journal of Environmental Engineering 5 (4), 745–751 (in Chinese with English abstract). Jiang, Y.W., Wan, R.J., 2005. Tongue Sole (Cynoglossus semilaevis) culture techniques. In: Lei, J.L. (Ed.), Marine Fish Culture Theory and Techniques. China Agricultural Press, Beijing, pp. 663–665. Lekang, O., Bergheim, A., Dalen, H., 2000. An integrated wastewater treatment system for land-based fish-farming. Aquacultural Engineering 22, 199–211. Liltved, H., Hektoen, H., Efraimsen, H., 1995. Inactivation of bacterial and viral fish pathogens by ozonation or UV irradiation in water of different salinity. Aquacultural Engineering 14, 107–122. Liltved, H., 2002. Ozonation and UV-irradiation. In: Timmons, M.B., Ebeling, J.M., Wheaton, F.W., Summerfelt, S.T., Vinci, B.J. (Eds.), Recirculating Aquaculture Systems, NRAC Publication No. 01-002, 2nd ed. Cayuga Aqua Ventures, Ithaca, NY, pp. 393–425. Liu, X., Zhuang, Z., Ma, A., Cheng, S., Sun, Z., Liang, Y., Liu, S., Zhai, J., Qu, J., 2006. Operative technologies for seedling rearing of Cynoglossus semilaevis Günther. Marine Fisheries Research 2 (27), 17–24 (in Chinese with English abstract). Ni, Q., Lei, J., Zhang, H., Yang, Z., 2010. Developing progress and status analysis on flatfishes recirculating aquaculture system in China. Fisheries Modernization 4 (38), 1–9 (in Chinese with English abstract). Peng, X., Shi, J., Zhou, R., Wu, W., You, Z., 2009. Cultivation of half-smooth tongue sole Cynoglossus semilaevis in a pond. Fisheries Science 28 (3), 156–158 (in Chinese with English abstract). Sharrer, M.J., Summerfelt, S.T., Bullock, G.L., Gleason, L.E., Taeuber, J., 2005. Inactivation of bacteria using ultraviolet irradiation in a recirculating salmonid culture system. Aquacultural Engineering 33, 135–149.
4. Conclusion A recirculating system for producing half-smooth tongue sole was designed and evaluated in Lai Zhou, China. The system was operated at approximately 95% reuse. Water quality in the fish culture systems was found to be acceptable for producing halfsmooth tongue sole. Compared to two conventional flow-through systems, the recirculating system improved survival, growth, and feed conversion ratio, and also maintained five-time greater fish density. During the 8-month commercial production cycle, no catastrophic losses occurred, and 41.5 metric tons of fish were harvested with only 700 fish lost. The system still can be improved, for example by using dual-drain tanks and adopting better backwash practices for the biofilters. Other valuable fishes can be cultured in this recirculating system. Recirculating aquaculture systems are
Acknowledgements We thank Mingbo Aquatic Co., Ltd. and Weifang Trans-Ocean Farm for providing useful data. John Colt made useful suggestions that improved the manuscript. This study was supported by the National Key Technology Research and Development Program of China (Grant No. 2011BAD13B04).
Z. Huang et al. / Aquacultural Engineering 54 (2013) 104–109 Summerfelt, S.T., 2003. Ozonation and UV irradiation – an introduction and examples of current applications. Aquacultural Engineering 28, 21–36. Summerfelt, S.T., Sharrer, M.J., Hollis, J., Gleason, L.E., Summerfelt, S.R., 2004. Dissolved ozone destruction using ultraviolet irradiation in a recirculating salmonid culture system. Aquacultural Engineering 32, 209–224. Summerfelt, S.T., Sharrer, M.J., Tsukuda, S.M., Gearheart, M., 2009. Process requirements for achieving full-flow disinfection of recirculating water using ozonation and UV irradiation. Aquacultural Engineering 40, 17–27.
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Timmons, M.B., Losordo, T.M., 1994. Aquaculture Water Reuse Systems: Engineering Design and Management. Elsevier, Amsterdam. Tseng, K.F., Wu, K.L., 2004. The ammonia removal cycle for a submerged biofilter used in a recirculating eel culture system. Aquacultural Engineering 31 (1–2), 17–30. Xu, Y., Zhang, X., Qu, K., Ma, S., Li, J., Song, D., 2006. Acute toxic effects of nitrite and ammonia on Cynoglossus semilaevis at different dissolved oxygen levels. Marine Fisheries Research 27 (5), 28–33 (in Chinese with English abstract).
Contents lists available at SciVerse ScienceDirect
Aquacultural Engineering journal homepage: www.elsevier.com/locate/aqua-online
Design and evaluation of a commercial recirculating system for half-smooth tongue sole (Cynoglossus semilaevis) production Zhitao Huang a , Xiefa Song a,∗ , Yanxuan Zheng a , Lei Peng a , Rong Wan a , Timothy Lane b , Jieming Zhai c , Eric Hallerman b , Dengpan Dong a a
Department of Fisheries, Ocean University of China, Qingdao 266003, PR China Department of Fish and Wildlife Conservation, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061, USA c Laizhou Mingbo Aquatic Co., Ltd., Lai Zhou 261418, PR China b
a r t i c l e
i n f o
Article history: Received 3 October 2012 Accepted 28 December 2012 Keywords: Recirculating aquaculture system Half-smooth tongue sole Cynoglossus semilaevis
a b s t r a c t A recirculating aquaculture system was designed for producing half-smooth tongue sole (Cynoglossus semilaevis). The recirculating system included twenty-eight 40-m3 circular culture tanks. The bottom discharge from the culture tanks was treated by passage across a bowed screen, and then water was pumped through a foam fractionator and a series of three submerged biofilters which contained plastic media, BIO-BLOK® element, and porous plastic media, respectively. Water then was disinfected with UV irradiation and supplied with pure oxygen before flowing back to the culture tanks. The system was stocked with 33,073 fish (mean weight 305 ± 57 g) in March 2009 and operated with greater than 95% water reuse. At the maximum feeding rate, the cumulative feed burden and loop strength were 7.25 kg feed/m3 of water and 11.8 mg/l, respectively. Over 8 months, fish survival rate was more than 97%. Fish were harvested from the system in October 2009 at a mean weight of 1246 ± 166 g, with a specific growth rate (SGR) through this time of 1.2% body weight per day and feed conversion ratio of 1.1. The system maintained water quality appropriate for the fish, with mean temperature of 19.7 ◦ C, DO of 6.7 mg/l, pH of 7.37, un-ionized ammonia of 0.012 mg/l, and nitrite of 0.044 mg/l. Approximately 99% of heterotrophic bacteria and coliform bacteria were inactivated by UV irradiation. Power usages per cubic meter water and per kg of production were 0.11 kW and 6.6 kW, respectively. Our results demonstrated the utility of recirculating aquaculture systems for producing half-smooth tongue sole. We suggest that regular backwashing of submerged biofilters may raise production capacity and allow use of biofilters with fewer stages. © 2013 Published by Elsevier B.V.
1. Introduction Half-smooth tongue sole (Cynoglossus semilaevis) is an important fishery product in East Asia. However, with the decrease of wild populations, fishery harvest was less than 1 metric ton in 2005 (Jiang and Wan, 2005). To meet market demand, half-smooth tongue sole has become an important aquaculture product in China (Fang et al., 2010), with captive production in 2009 totaling 7120 metric tons (Ni et al., 2010). The traditional culture method for producing tongue sole is a flow-through system in which water is pumped from a well into circular tanks, from which effluent is discharged to the sea by gravity through the bottom drain (Liu et al., 2006; Ni et al., 2010). With more and more intensive
∗ Corresponding author at: Aquaculture Building, 5 Yushan Road, Qingdao 266003, PR China. Tel.: +86 532 82032522; fax: +86 532 82032833. E-mail address: [email protected] (X. Song). 0144-8609/$ – see front matter © 2013 Published by Elsevier B.V. http://dx.doi.org/10.1016/j.aquaeng.2012.12.004
flow-through culture systems placed into production, however, two serious problems became evident: (1) a growing shortage of saline groundwater, and (2) environmental pollution due to untreated effluent being discharged to the receiving ecosystem. Commercial companies must obey the aquaculture wastewater discharge regulations subsequently promulgated by the government. For example, for saline aquaculture wastewater discharges, SS < 100 mg/l, COD < 6.0 mg/l, DO > 3.0 mg/l, and un-ionized ammonia < 0.06 mg/l are required to meet first-grade criteria required by the regulations of Zhejiang Province. Under these conditions, recirculating systems have become an attractive production alternative. Against this background, we designed and evaluated a recirculating system for producing half-smooth tongue sole in Lai Zhou, China. We evaluated fish production and water quality during commercial on-growing of half-smooth tongue sole in order to provide information useful for designing and operating systems for producing these valuable fish.
Z. Huang et al. / Aquacultural Engineering 54 (2013) 104–109 Table 1 Design parameters for recirculating aquaculture systems. Parameter
Value
Number of tanks Tanks diameter (m) Tank water depth (m) Fish size before stocking (g) Number of fish in system Final rearing density (kg/m3 ) Feeding rate (%body weight/day) Feed protein content (%) Inlet DO (mg/l) Unionized ammonia of inlet water (mg/l) Nitrate of inlet water (mg/l) Temperature (◦ C) pH Culture tank unionized ammonia level (mg/l) Nitrate in the system (mg/l) Biofilter efficiency (percentage of metabolite removed by a single pass through the three filters, %)
28 8 0.8 300 33,000 45 1.2 48 7.5 0 0 21 7.5 <0.02 300 35%
Reuse flow required based on TAN (m3 /h) Reuse flow required based on DO (m3 /h)
1281 978
Table 2 Characteristics of culture tanks and water treatment components in recirculating aquaculture system. Parameter or criterion Culture tanks (28) Volume Exchange rate Critical features Bowed screen Screen pore size Effective filter area Critical features
Foam fractionator Size Hydraulic retention time Ozone dose Series of submerged biofilters Size Media
2. Materials and methods 2.1. Water reuse determination A mass balance approach was used to design the fish culture system. The approach taken was to set water quality parameters that must be achieved to ensure good culture conditions and then to solve for the required reuse flowrate. Under steady state conditions, we estimated the flow to the biofilters required to maintain acceptable total ammonia nitrogen (TAN) concentration in the tank as (Timmons and Losordo, 1994): Qf =
PTAN − Q × CTAN ATAN × E
(1)
where Qf = flow rate to the filter, PTAN = production rate of TAN, Q = flow rate through the system, CTAN = concentration of TAN in the culture tank, ATAN = maximum allowable TAN concentration, and E = efficiency of the series of biofilters. We estimated the flow to the biofilter required to maintain a biofilter effluent dissolved oxygen (DO) concentration of 2.0 mg/l (Timmons and Losordo, 1994) as: + RNOD R Qf = BODf COfi − COf
(2)
where Qf = flow rate through biofilter base on DO mass balance, RBODf = rate of biochemical oxygen demand (BOD) expression within the filter, RNOD = DO consumption due to nitrification, COfi = concentration of DO in the biofilter influent, and COf = concentration of DO in the biofilter. Using the full set of input parameters defined in Table 1, the water reuse flowrate (Qf ) required to control unionized ammonia
105
Hydraulic loading rate Critical features Degassing unit (trickling filter) Media Packing depth Hydraulic loading rate Critical features UV irradiation units UV lamp (20) Intensity Dissolved oxygen contact unit Oxygen supply rate Critical features
Value 40 m3 Every 72 min Concrete construction; bottom-drain design with stand-pipe 0.25 mm 0.9 m2 All stainless steel construction; manually cleaned three times daily with brush and back-flushed weekly using high-pressure water 3 m diameter, 3 m height 1 min 15 g ozone/kg feed day−1 3 m × 3 m × 5 m each Elastic polyamide and polyolefin media (100 m2 /m3 ) BIO-BLOK® element (200 m2 /m3 ) Porous polypropylene media (380 m2 /m3 ) 2340 l/m2 min All concrete construction BIO-BLOK® element (200 m2 /m3 ) 4m 480 l/m2 min All concrete construction; spray diffuser; forced ventilation 53 W each 150 mW s/cm2 7.36 kg/h Pure oxygen supply; micropore diffusers
levels was estimated as 1281 m3 /h. The reuse flowrate (Qf ) required to maintain adequate dissolved oxygen levels in the biofilter was estimated as 978 m3 /h. Hence, 1281 m3 /h was the flowrate used for system design. 2.2. System description The recirculating system (Fig. 1) was constructed of elements taken from an existing flow-through system retrofitted to make it a recirculating system. Characteristics of culture tanks and the water treatment components in the recirculating system are presented in Table 2. As described below, the system contained 28 circular culture tanks, bowed screen, sump, six pumps, foam fractionator, a series of three submerged biofilters, ozone contact chamber, degassing unit (trickling filter), UV light, and dissolved oxygen contact unit.
Fig. 1. Recirculating system for producing half-smooth tongue sole. The units are: (1) 28 fish culture tanks, (2) bowed screen for coarse solids removal, (3) sump, (4) foam fractionators, (5) three types of biofilters for nitrification and treatment of carbonaceous compounds, (6) degassing unit (trickling filter), (7) UV disinfection and dissolved oxygen contact unit, (8) ozone contact chamber, and (9) liquid oxygen chamber.
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The twenty-eight 8-m inner diameter × 1.0 m-deep circular culture tanks were custom-fabricated of concrete. The circular tanks were operated at depths of 0.8 m to produce a water volume in each tank of approximately 40 m3 . An outside stand-pipe connected to the bottom-drain controlled the level of water. Water was injected into the culture tanks through two 5-cm inside diameter nozzles located symmetrically on the top of each culture tank. The tank water volume was exchanged approximately 20 times a day (once every 72 min) by a total system flow of 21.3 m3 /min. All water discharged through the bottom-center drain of each culture tank to the bowed (arc) screen, as described by Lekang et al. (2000). The screen was made of stainless steel (0.90 m2 ) and the screen pore size was 0.25 mm. During fish production, workers brushed the solid particulates away from the bowed screen 1 h after feeding the fish and washed the bowed screen using high-pressure water once a week. Filtered water then was pumped through a foam fractionator (3 m diameter × 3 m height, 2.5 m water depth). Air was delivered by Venturi fittings. The foam that was generated was removed through the spigot on the top. The hydraulic retention time of the foam fractionator was approximately 1 min. Ozone was generated (HRCF-G-3-200/YQ, Haoer Co., Ltd., Yantai, China) with pure oxygen as a feed gas, and was injected through a Venturi pipe into the foam fractionator at an approximate rate of 15 g ozone/kg feed day−1 . The recirculating system used a series of submerged biofilters, a design that is widely used among production systems in China. Such systems usually consist of two or three submerged biofilters with different bio-media (Ni et al., 2010). In the three respective biofilter units in the present study, elastic polyamide and polyolefin media (100 m2 /m3 , Zhongkehai Co., Ltd., Qingdao, China), BIO-BLOK® elements (200 m2 /m3 EXPO-NET, Denmark), and porous polypropylene media (380 m2 /m3 , Mingbo Aquatic Co., Ltd., Laizhou, China) were suspended from the water surface to bottom in each 3 m L × 3 m W × 5 m H tank. The hydraulic loading rate was approximately 2340 l/m2 min. All of the recirculating water passed through a forcedventilation degassing unit (a trickling filter, 6.5 m × 6.5 m × 4 m) containing a 2.4 m-depth of packed BIO-BLOK® elements (200 m2 /m3 EXPO-NET, DK), hydraulically loaded at 480 l/m2 min. A UV light chamber unit used to inactivate heterotrophic and coliform bacteria contained twenty 53-W lamps that supplied a total UV dose of approximately 150 mW s/cm2 . Finally, the water flowed to the dissolved oxygen contact unit. Pure oxygen was injected into the contact unit by micropore diffusers (Tean Technology Co., Ltd., Nanjing, China). The overall oxygen consumption rate was the sum of the respiration rate of the half-smooth tongue sole, the biochemical oxygen demand (BOD) of uneaten food and fish wastes, and the nitrogenous oxygen demand (NOD) of the biofilters under steady-state conditions, assuming that the flow into the system was equal to that of the outflow. The required rate of DO input to the system was estimated as 7.36 kg/h. 2.3. Fish and feeding The systems were used to rear a total of 33,073 half-smooth tongue sole (more than 99% were female) for 8 months (March 1st to October 31st, 2009). Initial mean size and stocking density were 305 ± 57 g/fish and 10.8 kg/m3 , respectively. The fish were fed a commercial feed, EP4, with 48% crude protein and 13% fat (Marubeni Nisshin Feed Co., Ltd., Tokyo, Japan). Fish were fed at a rate of 1.2% body weight/day over the first 3 months. After June 1st, fish were fed at a rate of 0.8% body weight/day. Feeding occurred three times a day at 08:00, 15:00 and 22:30 h. Weekly feed ration changed with mean fish weight. Mean weights were estimated weekly by sampling approximately 200 fish from six randomly chosen tanks. The maximum feeding rate (kg/day) was attained at the
end of the production (after October 1st). Under the maximum feed loading at the end of the production, the loop strength (i.e., the total mass of feed divided by the amount of recycled water returned to the rearing tank daily) was 11.8 mg/l. Feed conversion rate equaled the total mass of feed fed divided by the total mass of fish produced. Unhealthy and dead fish were removed each morning. 2.4. Water quality Temperature, pH and dissolved oxygen in the tanks were monitored daily. Ammonia and nitrite were measured five times a month. Total ammonia and nitrite were analyzed using the phenate method and the sulfanilamide-NED method (APHA et al., 1995). Un-ionized ammonia was computed using the seawater Ammonia Calculator of Fish Hatchery Management appendices. Temperature and dissolved oxygen (DO) were measured using a YSI 85 probe (YSI, Inc., Yellow Springs, OH, USA). A pH meter was used to determine pH. The total heterotrophic and coliform bacteria were counted using plate culture, with counting of colonies produced from water samples collected every 10 days during fish production. 3. Results and discussion 3.1. Fish growth and production Half-smooth tongue sole grew quickly in the system during the 8 months of production without any catastrophic loss of fish; only 700 dead or unhealthy fish were removed from the system, for a survival rate of more than 97%. At the conclusion of production, mean individual fish weight was 1246 ± 166 g (Fig. 2), specific growth rate (SGR) was 1.2% body weight per day, and feed conversion rate was 1.1. 3.2. Performance comparison among recirculating system and previous flow-through systems For conventional flow-through systems (Table 3), the maximum fish density was 9 kg/m3 , the survival rate was more than 85%, and the feed conversion rate was 1.2–1.33 (Fu et al., 2011; Weifang Trans-Ocean Farm, unpublished data). Use of the experimental recirculating system improved the survival rate and the feed conversion ratio. The final fish density was 44.3 kg/m3 in the recirculating system, approximately five times greater than the conventional flow-through system (Table 3). The greater density within the recirculating system did not compromise water quality in the tanks. Use of the flow-through system required continuous pumping of deep groundwater. Operating the recirculating system required pumping of water, ozonation, use of a trickling filter, pure oxygen aeration, and a UV filter for inactivating bacteria. 6.6 kW of energy was required to produce per kg fish. A total of 0.11 kW of energy was required per cubic meter of recirculated water, which was lower than that for the conventional flow-through system (0.3 kW, Table 3). Hence, additional production was achieved at lower energy cost. 3.3. Water quality Water quality parameters for culture water in the recirculating system are shown in Table 3. Over the course of production, dissolved oxygen was maintained at more than 6 mg/l, which is appropriate for rearing half-smooth tongue sole (Peng et al., 2009). Fang et al. (2010) suggested that commercial farmers could feed juvenile tongue sole to satiation in order to obtain higher growth rate, as the maximum specific growth rate occurred in fish fed to satiation at 22 ◦ C. In this commercial production cycle, mean temperature was 19.7 ± 0.8 ◦ C, which satisfied the
Z. Huang et al. / Aquacultural Engineering 54 (2013) 104–109
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Fig. 2. Mean weight of half-smooth tongue sole (with standard error) and temperature through the production period.
requirements for on-growing half-smooth tongue sole. Mean TAN, un-ionized ammonia and nitrite concentrations were 1.34, 0.012 and 0.044 mg/l, respectively. These good results were expected, since this series of biofilters were designed to remove TAN and nitrite efficiently. The combination of the series of biofilters and trickling filter maintained unionized ammonia at less than 0.02 mg/l; half-smooth tongue sole require an un-ionized ammonia concentration of <0.06 mg/l (Xu et al., 2006). In aquaculture systems, ammonia maintains equilibrium between unionized (NH3 ) and ionized (NH4 + ) forms. Unionized ammonia (NH3 ) is toxic to fish and need to be controlled within the production system. Biofilters are used to oxidize TAN (the sum of the ionized and unionized forms of ammonia) to nitrate with nitrite as an intermediate component in the recirculating system. When we designed the system, we assumed 35% as the efficiency of the series of biofilters, which was the percentage of metabolite removed by a single pass through the filter, with 0.02 mg/l of ammonia (2.0 mg/l of TAN) was designed as the safe level. Our results showed that the mean TAN was 1.34 ± 0.38 mg/l, which was much less than the designed safe value, indicating that the efficiency of the series of biofilters was much more than 35% in the RAS. The series of biofilters were acclimated to ammonia loading before culturing the fish. The data collected during the production trial (Fig. 3) indicated stable nitrification performance. Concentration of unionized ammonia, with a slight increase over time, was relatively stable in the recirculating system (at less than 0.02 mg/l), at a lower level than in the flow-through system (Table 3). The
increase over time presumably was due to more feed applied to the system as the fish grew, with an associated lag time for the filters to respond to the increased loading. Nitrite ranged between 0.019 and 0.061 mg/l, with dynamics following those of unionized ammonia. Lower nitrite (Table 3) was found in the flowthrough system, likely because of the adequate water exchange rate. The series of submerged biofilters used in this recirculating system is typical of commercial fish production in China, where most commercial production has adopted this filtration methodology. Although the filtration did improve water quality, several issues arose. Suspended solids captured in the biofilters may have clogged and caused channeling in the biofilters, and would likely have been hydrolyzed and released as dissolved organic matter, favoring the growth of heterotrophs and thereby decreasing TAN removal rate (Tseng and Wu, 2004). Ni et al. (2010) reported that the series of submerged biofilters in their study would clog after 3–6 months. However, water quality was good and stable in this production trial, which may be because: (1) 8 months was a rather short time period, and (2) the majority of suspended solids were removed by the bowed screen and the foam fractionator. Most commercial production companies in China practice backwashing of filters only after a commercial production cycle. However, we suggest that appropriate biomass management or a sludge management system for use within the production cycle should be designed and evaluated. These approaches may improve water quality and decrease the capital costs of the biofilters.
Table 3 Comparison of water quality parameters between the focal recirculating system and two previous flow-through systems. Recirculating system
Flow-through systema
Flow-through systemb
Volume (m3 ) Maximum fish density (kg/m3 ) Water exchange rate (times per day)
1120 44.3 20
240 8.6 –
800 7–8 4–6
Fish survival rate Feed conversion rate
>97% 1.1
– 1.33
>85% 1.2–1.3
TAN (mg/l) Un-ionized ammonia (mg/l) Nitrite (mg/l) DO (mg/l) pH Temperature (◦ C)
1.34 ± 0.38 0.012 ± 0.003 0.044 ± 0.014 6.7 ± 0.69 7.37 ± 0.20 19.7 ± 0.78
1.04 0.15 0.016 >5 7.5–8.0 22
– – – – – 17–23
Energy usage kW/m3 of water flows kWh/kg of production
0.11 6.6
– –
0.3 –
a b
Fu et al. (2011). Weifang Trans-Ocean Farm.
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Fig. 3. Un-ionized ammonia (left scale), nitrite and total ammonia nitrogen (right scale) concentrations (mg/l) in the recirculating systems from March 2009 to October 2009.
Table 4 Total bacterial counts entering and exiting the UV filter. Bacteria
No. of samples
Heterotrophic bacteria Coliform bacteria
24 24
Bacteria counts (cfu/ml)
Removal (%)
Before UV
After UV
30,549 ± 19,104 121 ± 67
78 ± 32 <1
99.74 100
TAN flushed from the system due to water replacement was approximately 67 g daily. Use of the recirculating aquaculture system could reduce the nutrient pollution discharged to the environment compared to the conventional flow-through systems. Cumulative feed burden (i.e., the mass of food delivered to the system per unit volume of water added to the system daily, Colt et al., 2006) reflected the intensity of recirculation, and was a critical parameter in the design and management of the systems. Under the maximum feed loading at the end of the production, cumulative feed burden was 7.25 kg feed/m3 . The sludge flushed from the bowed screen was flocculated and used to grow algae.
becoming more and more popular in China, as they are effective and economical.
3.4. Bacterial inactivation
References
20 UV lamps (0.053 kW each) were installed to inactivate the bacteria in this recirculating system and supplied a total UV dose of approximately 150 mW s/cm2 . UV irradiation denatures the DNA of microorganisms, causing death or inactivation. It has been used to treat relatively large aquaculture flows, including flows within recirculating systems (e.g., Liltved et al., 1995; Blancheton, 2000; Liltved, 2002; Summerfelt, 2003; Sharrer et al., 2005; Summerfelt et al., 2004, 2009). The heterotrophic and coliform bacterial counts entering and exiting the UV lamps are shown in Table 4. The removal rates for heterotrophic and coliform bacteria were 99.7 and 100%, respectively. Disinfection equipment has seldom been installed in conventional flow-through systems in China; our results suggest their broader adoption there.
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4. Conclusion A recirculating system for producing half-smooth tongue sole was designed and evaluated in Lai Zhou, China. The system was operated at approximately 95% reuse. Water quality in the fish culture systems was found to be acceptable for producing halfsmooth tongue sole. Compared to two conventional flow-through systems, the recirculating system improved survival, growth, and feed conversion ratio, and also maintained five-time greater fish density. During the 8-month commercial production cycle, no catastrophic losses occurred, and 41.5 metric tons of fish were harvested with only 700 fish lost. The system still can be improved, for example by using dual-drain tanks and adopting better backwash practices for the biofilters. Other valuable fishes can be cultured in this recirculating system. Recirculating aquaculture systems are
Acknowledgements We thank Mingbo Aquatic Co., Ltd. and Weifang Trans-Ocean Farm for providing useful data. John Colt made useful suggestions that improved the manuscript. This study was supported by the National Key Technology Research and Development Program of China (Grant No. 2011BAD13B04).
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