Diurnal dynamics of water quality parameters in an aquaculture system based on recirculating green water technology

Abstract

Recirculating greenwater aquaculture technology is an appropriate method for producing commercial quantities of tilapia in areas where water is scarce. Greenwater systems achieve better fish production, more than 90% water recycling and nutrient utilization. This study was designed to address the diurnal dynamics of certain water quality parameters in greenwater system. Culture tanks were stocked at a density of 40 fish/m 3 with mixed-sex Nile tilapia, Oreochromis niloticus fingerlings and fed ad libitum with floating pellets containing 34% protein. The water temperature was kept 28±1ºC, aeration was provided and sludge was collected twice daily. Values of critical chemical parameters in the water were found to be well within the acceptable limits based on diurnal water sampling. @JASEM Deprived freshwater resources and harsh climatic conditions limit the development of freshwater aquaculture in Saudi Arabia. However, an increasing demand for fresh fish is putting pressure for the establishment of water recirculating aquaculture technologies that intensify fish production and maximize water recycling. Recirculating greenwater aquaculture technology has been found to be an appropriate method for producing commercial quantities of tilapia in locations that have environmental constraints (Cole et al. 1997; Rakocy et al. 2000). Greenwater aquaculture systems aim to increase fish production, proper nutrient utilization, maximum water recycling and least water wastage. Water is green in greenwater systems because of the development of photosynthetic algae in the water column. Uneaten feed, fecal matter of the fish and dead algae is removed from the system in the form of sludge. Airlift pumping combined with a numbers of air diffusers keep some minute particles, detritus, feces and plankton in constant circulation. The nitrifying bacteria colonize this floating substrate and oxidize toxic ammonia into relatively harmless nitrate (NO 3-N) by creating a suspended growth treatment process (Rakocy et al. 2000). The process maintains adequate water quality for tilapia while recycling waste nutrients into plankton and bacteria. Elimination of the need for a biofilter not only cuts capital costs but also the management and maintenance needs of greenwater systems (Martin 2000). Water losses in greenwater systems are from evaporation and sludge removal. Small-scale greenwater systems require usually less than 1% of the total water volume per day. Water recirculating greenwater technology for semi-intensive fish culture and maximum water recycling was developed in Saudi Arabian climate to produce Nile tilapia. Overall system performance and fish production is determined by water quality, thus, major water quality parameters were regularly tested in order to establish the success of this system. This paper presents diurnal variation of certain water quality parameters in greenwater system aiming at dynamics of these parameters through 24 hours period. The experiment is expected to contribute some insight on the effects of physiological activities of the fish on water chemistry in a diurnal cycle. Determination of the periods of peak concentrations of major water quality parameters would be helpful to improve the performance of greenwater system by altering the management practices and minimizing the stress to fish. MATERIALS AND METHODS Each of the three replicates of the greenwater system had one fish culture tank (water volume 10.0 m 3) and one cone bottomed cylindrical clarifier (water volume 0.47 m 3) for solid settling. An inline airlift pump was installed before the cone clarifier to recirculate the water between the fish culture tank and cone clarifier at a rate of 7-8 L/minute. Fish culture tanks had a 3% slope towards the center of the bottom drain to enhance the solid removal. Water from central bottom drain of fish culture tank pumped to the cone clarifier through airlift pump. Cone clarifier had two baffles to aid the settling of detritus, feces and dead cells. The sludge was drained twice daily from the clarifier and the water lost in the sludge collection and due to evaporation was added back to the system. The outflow of the cone clarifier flows by gravity into the fish rearing tank through a 7.62 cm diameter pipe. Fish tanks were stocked at a density of 40 fish/m 3 with mixed sex Nile tilapia, Oreochromis niloticus (average weight 29.26 (±6.75) g/fish), on 22 nd November 2001. Floating pellets containing 34% protein and 5% fat were used to feed the fish to satiation for 20 minutes twice daily (0800 and 1600 hours). The water temperature was maintained at 28±1ºC by installing heaters with thermostats. Water flow rate was approximately 7.0 L/minute so that entire volume of the culture tank circulates through the clarifier once in 24 hours. Fifteen small fish (approximately 30 g each) were stocked in each clarifier to enhance the sludge settling. Aeration was provided to the water in