DIFFUSOR BASED AERATION FOR CONTROL OF DISSOLVED OXYGEN IN AQUACULTURE PONDS AND CAGES Authors: A. Bergheim, IRIS*; M. Gausen, OV; N. Hovden, OV; H. Grundvig, TI; P. Makridis, UPAT; E. Mente, UTH; L. Ribeiro, IPMA; P. Pousão-Ferreira, IPMA and J. Colt, NAAO * (IRIS: International Research Institute of Stavanger, Stavanger, Norway.
[email protected])
Abstract
As a basic part of the AirX project, funded within the EU 7th Framework Programme, experimental tests have been carried out in order to maximize the efficiency of hose diffusers for aeration in fish stocked ponds and shallow cages. At Oxyvision’s laboratory, oxygen transfer efficiency of air injection in a seawater tank (36‰ sal.) without fish has been monitored combining a range of diffuser prototypes possessing different characteristics (involving hose material, coating, wall thickness, air pore density, etc.) with air supplied from a blower. The test procedure also involved combinations of air pressure and the resulting air flow. In each test, the transfer rate was frequently measured throughout the injection period from 10 – 30% to 80 – 100% of dissolved oxygen saturation. The oxygen transfer rate, KLaT, was calculated for each test and converted to the standard rate at 20ºC (KLa20). Based on the measured transfer rate and the electrical consumption, the standardized aeration efficiency, SAE, can be calculated in terms of kg oxygen transferred per KWh. In the lab, a SAE of c. 2.7 kg O2KWh-1 was calculated for the best in test diffuser hose at low air pressure. Compared to reported transfer rates in aerated ponds – raceways for freshwater fish production, the small-scale tests in lab indicate promising efficiency. The AirX diffuser system is presently being employed and tested in Portuguese raceways and in Greek cages stocked with sea bass, sea bream and meagre. Parameters, such as dissolved oxygen and total gas pressure in the water column, and pond sediment accumulation and characteristics, are routinely monitored with and without aeration. The performance of the fish stock will be balanced against the costs of the aeration attempt.
Test procedure
The lab-scale facility is presented in Image I with a diffuser hose prototype on the tank bed, a current creator for efficient mixing and a heater to stabilize the water temperature. Monitoring of oxygen transfer is based on average oxygen concentration (DO) increase from three probes. Additionally, water temperature and salinity are monitored.
Best in test diffuser hose (AirX) KLaT = (lnC1 – lnC2)/(18/60)h = (1.64 – 0.38)/0.30 = 4.2 h-1 KLa20 approx. KLaT = 4.2 h-1 SOTR = KLa20 x Csat x V(m3)/1000 = 0.082 kg O2 h-1 SAE* = SOTR/KWh = 0.082 kg O2 h-1/0.03 KWh = 2.7 kg O2KWh-1
*SAE: standard aeration efficiency Air flow 1.0 kg air h-1
Energy supply 0.03 kW
O2 concentration Tank Start: C1, 28% saturation End: C2, 80% saturation
Diffuser hose design
In brief, the diffuser consists of two perforated hoses where an inner hose is encompassed by an outer hose. The inner hose distributes air to the space between the hoses and thus ensures a constant pressure along the diffuser irrespective of whether the diffuser is operated horizontally or sloping/uphill. The outer hose is the actual diffuser creating the fine bubbles. More than 40 different diffusers combining different hose materials and
Saturation Csat (100%) 7.2 mg/L (20ºC)
Water temp 20.3ºC
perforation, such as hole size and density, have been subject to lab-scale efficiency tests.
Test outcome
Oxygen Transfer Rate, KLaT, was calculated for each test and converted to standard rate at 20 °C (KLa20). This coefficient was further used to calculate SOTR (Standard Oxygen Transfer Rate, unit: kg O2h-1). Based on the measured transfer rate and electrical consumption, SAE (Standard Aeration Efficiency, unit: kg O2KWh-1) can be calculated. The highest performance of all tested diffusers indicated a SAE level of c. 2.7 kg O2KWh-1 at low air pressure (calculations summarized beneath). Most reports of aerators commonly employed in freshwater ponds and raceways demonstrate lower efficiency under large scale conditions compared to the best in test AirX diffuser under lab conditions.
Figure I. Growth rate and feed utilization of Sea bream during 30 days at three dissolved oxygen levels (40-60%, 60-80%, 80-100%), July 2013. IPMA research station, Olhão, Portugal
In-field tests
Greece An AirX diffuser was introduced in a cage with low fish density in summer 2013. The diffuser at 4 m depth is run with lowpressure air from a petrol-powered blower. In order to control the DO concentration in the water column above 70% of saturation, the aeration is run 24 hrs. per day during the period August – October (> 26 – 28 ºC) and during parts of the day at early-morning and in stagnant periods with DO deficit. Long-term tests emphasizing DO concentration and fish performance in cages with – without aeration will be performed throughout summer and early autumn 2014.
Image I. Experimental tank for small-scale tests of aeration efficiency of various diffuser hoses. Oxyvision’s test lab, Norway, Jan. 2013
Time 18 mins
Portugal At IPMA’s research station, effects of increased fish density and dissolved oxygen (DO) deficit on the performance of sea bream have been studied in indoor tanks. As an overall conclusion, this species seems to tolerate a density level up to 20 kg m-3, but DO concentration as low as 40 – 60% of saturation lasting for 30 – 60 days obviously decreases growth and feed utilization (Figure I). No significantly increased stress levels were indicated. Early version prototypes of the AirX diffuser system are being tested in earth ponds stocked with meagre. The principle of aeration in raceways implemented by AirX diffusers is presented in Figure II. Longitudinal or circular hoses kept on the pond bed supply approx. 20 kg air h-1 per pond in order to control the running DO concentration above 70 – 80 % of saturation. The performance of the AirX diffuser prototypes vs. control ponds with traditional air-jet alkno aeration system will be assessed in 2014.
Figure II. Cross-sectional area of a raceway where AirX diffusers are kept 5 – 10 cm above the bottom level
Acknowledgements
This work was supported by the 7th Framework Programme of the European Union. Project title: Oxygenation by efficient air diffusion system for aquaculture farms (AirX) Grant agreement no. 315412
THE POTENTIAL NEED FOR AERATION IN MEDITERRANEAN AQUACULTURE P. Makridis 1*, L. Ribeiro2, P. Pousão2, E. Mente3, H. Grundvig4, M. Gausen5, N. Hovden5 and A. Bergheim6 1 2 3 4 5 6
University of Patras, Rio Patras, Greece, email:
[email protected] Instituto Português do Mar e da Atmosfera, Olhão, Portugal University of Thessaly, Volos, Greece Teknologisk Institutt, Oslo, Norway Oxyvision AS, Norway International Research Institute of Stavanger, Stavanger, Norway
Need for oxygen The amount of oxygen present in water is a main limiting factor for the biomass of cultured fish. Within the production cycle, this limitation becomes more obvious in ponds or cages with high biomass and at high temperature. In Mediterranean aquaculture this is a common case during the end of the summer and beginning of autumn, as at this period the fish has gone through a period with high growth and thereby the biomass is high and at the same time the water temperature may reach very high levels (over 30oC) for short periods. This situation may lead to increased stress, lower digestive capacity of the animals, lower appetite and growth, disease outbreaks and even increased mortalities. The normal strategy of farmers in these cases is to reduce or stop feeding the fish. A possible solution for these critical periods is investigated by the AirX project to inject air through diffuser
with small pores resulting in small air bubbles which provide increased efficiency of aeration at a low cost. This approach could provide better growth, survival, better fish welfare and increased disease resistance of the fish.
Positioning the AirX diffuser prototype in a shallow cage in Greece.
Results Oxygen saturation level (%) in a cage with seabass (Dicentrarchus labrax L.) in a marine fish farm in Greece reached a low level some hours after feeding in tests done in November 2012. The fish were fed only during the morning hours from 8 to 12 AM. In some cases the level of oxygen was lower than 40% oxygen saturation (Figure 1B). This critical level was linked with low water current in the area (Figure 1D). The water temperature in this tests was 19-20oC. The situation is therefore far more dangerous in other instances earlier in the year (Aug/Sep) when water temperature can be as high as 30oC.
Drawing showing the way the prototype is hanging in the fish cage from four points. The position of the diffuser is stabilized by four weights, one at each corner of the diffuser frame (not shown in the picture).
Prospective The AirX diffuser, which is patented by Oxyvision, has been tested successfully in lab scale in Norway and in large scale field trials at fish farms in Greece and Portugal. Further efficiency testing of the AirX diffuser prototypes vs. control ponds with traditional air-jet aeration system will be assessed in 2014.
Prototype tested in a cage in a farm in Greece.
Figure I: Dissolved oxygen concentration in a cage stocked with sea bream and outside (Reference), A & B, and Current velocity outside the cage (Reference), C & D. Monitoring at 3 m depth during two periods in November 2012, Zervas - Kyriazis fish farm, Greece.
Acknowledgements
This work was supported by the 7th Framework Programme of the European Union. Project title: Oxygenation by efficient air diffusion system for aquaculture farms (AirX) Grant agreement no. 315412
Initial aeration by AirX diffusers in a raceway in Portugal.
