HOW TO DESIGN RECIRCULATING AQUACULTURE SYSTEM COMPONENTS.
Uzukwu, P.U., Ph.D.
CEO/Managing Consultant, Armtec Nigeria Enterprises.
piusuzukwu@gmail.com; https://piusuzukwu.medium.com;
+2348055352214.
INTRODUCTION
Definition
Recirculating Aquaculture System (RAS) or water recirculation system (WRS) is a high-technology, hyper-intensive type of fish culture system in which water outflow from the fish rearing ponds/tanks is continuously treated and recycled back to the ponds/tanks (Figure1).
Figure 1. RAS utilizing 90% recycle per day.
Source: Uzukwu (2008).
RAS is best suited for locations where land and water are limiting and need to be conserved, that is, where: (i) land space is scarce or very expensive, (ii) water is scarce or very expensive, and (iii) power supply is cheap and constant. RAS can be used to grow catfish, tilapia, carp, crabs, oysters and other exotic fish species.
The publication of this article is aimed to show that RAS components design technology had been developed and domesticated in Nigeria and Africa in the past but lack of power supply to drive lift pumps has continued to constrain its general adoption. This is very frustrating and embarrassing, and the only way to solve this power supply problem in Nigeria and Africa is to declare a state of emergency on power. The effective diversification of any economy begins with provision of power to drive economic activities in that economy.
Commercial Implications
Commercial fish farming is land intensive, and adequate, cheap or low-cost land is available only in rural settings where big time entrepreneurs do not like to invest. RAS is the fish culture system that is most suited to low density, urban settings where wealthy businessmen live and invest in close proximity to busy urban markets. RAS allows for raising fish in indoor and outdoor ponds and tanks at high stocking densities (300 to 350 fish per m2) compared to 2 to 5 fish per m2 in static (stagnant) culture system or 50 to 100 fish per m2 in flushing culture system. This high stocking density in RAS is achieved by deploying water treatment unit operations and processes such as mechanical filtration, solid waste (sludge) removal, biological filtration (nitrification), water sterilization and aeration.
Nearly all the functional RAS installed in Nigeria were designed in Western countries. An attempt to develop, optimize and domesticate the technology of RAS using local materials for massive fish production is of great commercial and economic value for developing countries such as Nigeria. In this connection, Uzukwu et al. (2010b) showed that trickling biological periwinkle shell filter for closed recirculating catfish system was cheaper and more cost-effective than imported plastic filter block. This publication serves as an advocacy for adoption of RAS as one of the streams of domestic fish supply in Nigeria and beyond. Domestication of RAS technology begins with the knowledge of design of RAS components of which Deekae and George (2002), Uzukwu (2008) and other workers have done some pioneering effort in Nigeria and Africa. However, it is proper to first of all trace the origins of the wastes and contaminants in RAS which are continuously treated as effluent water cycles back to the fish ponds and tanks.
Origin of Water Quality Contaminants in RAS
All aquaculture production systems must provide a suitable environment to promote the growth of the aquatic crop. The critical environmental parameters include the concentrations of dissolved oxygen (DO); un-ioized ammonia-nitrogen (NH3-N); nitrite-nitrogen (NO2-N), and carbon dioxide (CO2) in the water (Losordo et al., 1999; Masser et al., 1999). Others are nitrate (NO3-N) concentration, pH, alkalinity and chloride levels within the system. In order to produce fish in a cost-effective manner, good water quality must be maintained in the production system (Losordo et al., 1999). Also, fish must be fed high-protein, pelleted feed (diets) at rates ranging from 1.5 to 15% of fish body weight per day (%FBW) depending on the size and species (15% for juveniles, 1.5% for market size) (Losordo et al., 1998).
Factors that adversely affect system water quality are: feeding rate, feed composition, fish metabolic rate, and quantity of unconsumed feed. The by-products of fish metabolism include carbon dioxide (CO2), ammonia-nitrogen (NH3-N) excreted across the gill membrane, urine and feacal solids. If uneaten feeds and metabolic by-products are left within the culture system, they will generate additional CO2, NH3-N, reduce the dissolved oxygen (DO) content of the water, and impact negatively on the health of the cultured products (Losordo et al., 1998). Ammonia is toxic to fish and can exert sub-lethal stress at concentrations less than 0.05mg/L of ammonia-nitrogen (NH3-N), resulting in poor growth and lower resistance to disease (McGee and Cichra, 2000). The foregoing traces the origin of contaminants in RAS and underscores the importance of effluent treatment in RAS. Before embarking on the task of designing RAS components, it is necessary to explore the principles of water recirculation and filtration in RAS upon which the designs of the system components are based.
Principles of Water Recirculation and Filtration in RAS
Recirculating aquaculture systems (RAS) are designed to minimize or reduce to the barest minimum the dependence on water exchange and flushing fish culture units (McGee and Cichra, 2000). Water is typically recirculated when there is a specific need to minimize water replacement to maintain water quality conditions which differ from the supply water, or to compensate for an insufficient water supply. There are innumerable designs for RAS and most will work effectively if they accomplish:
Removal of particulate matter.
Water loaded with particulates (faecal solids) leaves the fish tanks and are collected in the main line. From the main line water is passed through a micro-strainer or sedimentation tank where particles over a certain size are removed before water is pumped to a biological filter.
Biological filtration to remove waste ammonia and nitrite
The biofilter may consist of a submerged part and a tricking part (McGee and Cichra, 2000). In the biological filter a degradation of toxic, organic materials (for example, ammonia to nitrite, nitrite to nitrate) and organic matter takes place by activity of bacteria adhering to a biofilter matrix.
Aeration.
Aeration of the water is carried out in the tricking part of the biological filter. Aerators or oxygen cones may also be placed in the recycled water to ensure sufficient aeration. Levels of aeration should be sufficient to sustain dissolved oxygen levels above 60% saturation (5 mg/L) throughout the system (McGee and Cichra, 2000).
Buffering of pH.
The biological filtration processes will result in a build-up of nitrate and a drop in pH. Therefore, the system is equipped with facilities for de-nitrification and pH control and adjustment (BAS, 2006).
Disinfection.
After biological filtration, water is ready for fish tanks. Before it re-enters the tanks, water is exposed to UV radiation (BAS, 2006). Normally 10% of the total volume of water in system is changed daily. This amount can vary depending on the requirement of fish, water availability and local regulation. Process water exchanged from a fish farm should be treated further for example through land application in agricultural lands. Sludge generated might also be used as fertilizer in agricultural lands (BAS, 2006).
In pond fish culture, proper environmental conditions are maintained by balancing the inputs of feed with the assimilative capacity of the pond. The pond’s natural biological productivity (algae, higher plants, zooplankton, and bacteria) serves as a biological filter that processes the wastes (Losordo et al., 1998). The carrying capacity of tank systems must be higher than earthen ponds (2.27 to 3.18 kg of fish per 0.0045 m3 of pond water) to provide for cost–effective fish production because of the higher initial capital costs of tanks compared to earthen ponds (Losordo et al., 1998). Now that the necessary foundation has been laid it is ripe to embark on the design of RAS components.
DESIGN PROCEDURE OF RAS COMPONENTS
System recirculation Rate (R)
First, we select the system recirculation rate (R). This is the number of times a given water particle cycles round the system per unit time. The system recirculating rate (R) of 10 times per hour may be selected for a trickling system. This is based on LSE (2003) who reported that a very fast system might recirculate all the water in 5 minutes, that is, 12 times per hour. This is achieved by selecting a lift pump whose flow rate corresponds to the selected R.
System Recirculation Ratio(Z)
The recirculation ratio (Z) expresses the ratio of system water to that of makeup water. The recirculating ratio of 90% per day selected for any system means, 10% of makeup (new) water addition per day.
Fish Culture Tanks
Here the material of tank construction and dimensions of tank should be selected. The fish culture tanks of plastic drums of dimension 0.29m (radius), and 0.9m (height) selected for this exercise may be suitable for experimental purposes only. A freeboard of 0.10 m may be provided, giving a water depth of 0.80 m, and water volume of 0.21m3. The bottom of the plastic drums may be flat or conical. Ansa (2006) reported that plastic tank is one of the good fish production facilities.
Sedimentation Tank
(i) Detention Time (TD)
The detention time (TD) should now be selected for the sedimentation tank. According to The Open University (1985), the detention time for settling primary organic wastes in a 3m depth settling basin without settling plates is 1.98 hours. But if settling plates are provided the depth may reduce to 1m, with the detention time still maintained at about 2 hours.
(ii) Volume
The volume (VST), of the sedimentation tank should now be selected for the system. The volume is computed using the following expression:
Q (M3/hr) = Volume,VST(M3)/Detention time(TD)
Where:
Q = pump flow rate
TD = detention time
(iii) Settling plates
The sedimentation tank may be provided with plastic settling plates inclined at 60 degrees to enhance settling efficiency.
Figure 2: Design of Sedimentation Tank
Source: Uzukwu (2008)
(iv) Dimension
The dimensions should be selected for the sedimentation tank. With settling plates, the depth can reduce to 1.00 m. This is based on the report of Losordo et al, (2000) who stated that using settling plates reduces the size requirement (settling distance) of a settling basin, thus saving space within the facility. This agrees with the Open University (1985) which stated that with settling plates a 3.00 m depth sedimentation tank can be reduced to 1.00 m.
(v) Settling velocity
The settling velocity should be selected for the sedimentation tank. This is computed using the expression:
Vs = d/TD
Where:
Vs = settling velocity of particles
d = depth of sedimentation tank without settling plates
TD = detention time
Vs = depth / detention time
The Open University (1985) stated that the settling velocity of primary organic particles is 0.42 mm/s (1.50 m/hr). The actual flow velocity in the sedimentation tank should be measured using flow meter or float method and result in this case is multiplied by a factor of 0.85 to obtain mean velocity. The sizing of the sedimentation tank is about 2.5 m3 per 25 tons of fish. The inlet and outlet of the sedimentation tank should be provided with baffle and weir respectively to reduce turbulence to the barest minimum. Sludge generated from the sedimentation tank should be physically characterized, treated and disposed probably by land application.
Water Pump Flow Rate
The pump flow rate, Q m3/hr, for the system should be selected to correspond to the system recirculation rate. This is because the pump is to accomplish both moving water through the tank and aeration of the system (McGee and Cichra, 2000). The pump should be provided with a suitable pump tank as shown below.
Figure 3: Design of Pump Tank
Source: Uzukwu (2008)
Biological Filter Media and Biotower
LSE (2003) and Uzukwu (2008) outlined the procedure for designing a trickling filter for Recirculating Aquaculture System (RAS) as follows:
1. Estimate the maximum amount of feed needed to feed the fish at any time in the crop cycle. This determines the maximum load the filter will need to handle.
2. Determine the total ammonia nitrogen (TAN) level that the fish will be able to tolerate with no ill effects, the ammonia removal rate, and the hydraulic loading rate.
3. Determine the volume of fish tank, the water pump flow rate, and the system recirculation rate.
4. Determine the total surface area for the biofilter packing based on the amount of feed to be used and the ammonia removal rate.
5. Select the appropriate specific surface area, SSA of the biological filter (390, 226, and 157 m2/m3).
6. Determine the volume of the biofilter by dividing the total surface area required by the SSA of the filter material.
7. Decide on the shape of the biofilter - circular, square, or rectangular.
8. Cross check the hydraulic loading rate, and adjust as appropriate.
The procedure outlined above should be followed in designing the biofilter media. The type of biofilter, trickling or submerged biofilter should be selected for the project. If trickling, the type of tricking biofilter selected should be stated say, vertical flow media. The biofilter media material should be selected, for example, periwinkle, Tympanotonus fuscatus, shells sandwiched between corrugated plastic sheets (Uzukwu et al.(2010a). The spines of this particular periwinkle species provide substrate for the attachment of ammonia-oxidizing bacteria as well as ventilation and drainage of the biofilter. The surface area, packing density, volume, cross-sectional area, and depth of the biofilter media may be computed using the following as an illustration:
Fish Tank volume = 0.21 m3
Crop Size = 84 kg of fish
Feeding rate = 3 % of fish body weight /day
Amount of feed = 3 kg/day at 42% crude protein (CP)
Allowable TAN = 1.5 ppm
Amount of ammonia produced = 0.03 x 3 kg/day = 90 g/day
Ammonia removal rate = 0.6 g/ (m2.d) at 6 gpm/ft2 (Fig. 4).
Surface area of media = 90 g/d/ ((0.6g/m2.d))
Packing density, SSA selected = 226 m2/m3
Volume of media = 150 m2/ (226 m2/m3) = 0.7 m3
Cross-sectional area of filter media = 0.59 m x 0.59 m = 0.35 m2
Depth of filter media = 0.7 m3/0.35 m2 = 2 m
This sizing of trickling biofilter is 1 ton of fish per 1 m3 of biofilter volume. A pilot RAS fish production using the trickling filter designed above and powered by a 2.0 KVA petrol engine electric generator was very efficient (Uzukwu et al.,2010c), it provided good water quality (Uzukwu et al.,2011) and no fish mortality was recorded.
Figure 4: Ammonia removal rate at different hydraulic loading rate.
Source: LSE (2003).
Dissolved Oxygen Supply
The recirculating systems may be designed to be self-aerating. In that case no aerators will be deployed for dissolved oxygen supply, else adequate oxygen supply should be provided for the system.
Distribution plate
The distribution plate may be constructed using hard wood or fiber-glass. It is usually placed on top of the biofilter. Its dimensions, and depth should be specified. The diameter of the perforations on the bottom of the distribution plate is 6.00 mm, and a clearance of 0.3 m between biofilter media and distribution place are okay. This is based on the specification of GLUMRB (1996) cited by Lee and Lin (1999).
Figure 5: Design of Perforated Plate
Source: Uzukwu (2008)
Media Support System
The bottom layer of filter media module should be placed on wooden supports which should be 4.0 inches high to provide proper ventilation and drainage for the ammonia oxidizing bacteria in the biofilter. The wooden supports rest on the reception plate.
Reception Plate
The reception plate may be constructed with fiber-glass or hard wood lined with tarpaulin. Its dimensions: length, width, and depth should be specified for a given duty.
Figure 6: A Reception Place
Source: Uzukwu (2008)
SUMMARY
The underlying principles and design of RAS components using local materials which are available in Nigeria have been presented. It shows that the technology for design, fabrication, assembly and operation of hyper-intensive (RAS) aquaculture system is available in Nigeria and Africa. The major disincentive constraining RAS fish production in Nigeria and Africa is absolute lack of constant electric power supply to drive lift pumps; initial cost outlay of solar power supply is very high. If the political leaders can supply affordable and constant power in Nigeria and Africa, scientists and entrepreneurs are ready to play their roles. For more information on aquaculture systems, extensive, semi-intensive, intensive and hyper-intensive, etc, follow me at my Aquaculture in the Tropics blog using the link: https://piusuzukwu.medium.com
REFERENCES
Ansa, E.J. (2006). Fish production facilities. In: Proceedings of train the trainers workshop on intensive aquaculture enterprise, ARAC, Port Harcourt, Nigeria. 27th-29th July, 2006. p23–36.
BAS (Billand Aquaculture Services) (2006). Billand Aquaculture Services, APS Denmark. Retrieved 9/20/2006. http:/www.billand-aqua.dk/eng/4-
Deekae, S.N. and George, A.D.I. (2002). Use of recirculating systems in intensive fish production. Investment Fisheries Production Series №2. Munack Cont. Press, Port Harcourt. 33pp.
Lee, C.C. and Lin, S.D. (1999). Hand book of environmental engineering calculations. McGraw Hill book Company. p1.595- 1.608.
Losordo, M.T., Masser, M.P., Rakocy, J.E. (1999). Recirculating aquaculture tank production system: A review of component options, SRAC publication №453. p1–12.
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McGree, M. and Cichra, C. (2000). Principles of water recirculation and filtration in aquaculture. Retrieved 21/9/2006 from Univ. of Florida Inst. of Food and Agric. Science (UF/IFAS) web site. http://edis.ifas.ufl.edu/BODY-FAO50.
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Uzukwu, P.U., Leton, T.G., Ogbonna, D.N., and Obinna, F.C. (2010a). The design of trickling biological periwinkle shell filter for closed recirculating catfish system. Inter. Jour. of Natural and Applied Sciences,6(3):272–280.
Uzukwu, P.U., Ogbonna, D.N., Leton, T.G., and Obinna, F.C. (2010b). The economics of trickling biological periwinkle shell filter for closed recirculating catfish system. Proceedings of the 25th Annual Conference and fair of the Fisheries Society of Nigeria (FISON). p331–336.
Uzukwu, P.U., Ogbonna, D.N., Leton, T.G. and Obinna, F.C. (2010c). The efficiency of periwinkle shells filter as biofilter medium in closed recirculating catfish systems. Inter. Jour. of Trop. Agric. and Food systems. 4(2):139–145.
Uzukwu, P.U., Ogbonna, D.N., Leton, T.G., and Obinna F.C. (2011). Water Quality of Trickling Biological Periwinkle shell Filter for closed Recirculating Catfish System. Inter. Jour. of Trop. Agric. and Food Systems 4(4):302–309.
