An aquaponics production output comparison between drain-fill versus flooded bed cultures and between solids removal versus no solid removal, using African catfish and Swiss chard

Abstract

Aquaponics is one of the solutions for food self-sufficiency which constitutes a challenge for many countries. This technology is based on the integration of aquaculture and hydroponic production, through water recycling. The result is the availability of fresh fish and vegetables for the benefit of communities and the reduction of environmental pressure associated with conventional production systems. With this technology, the problem of eliminating solid waste from uneaten food and fish excretions arises. These wastes can become the ultimate source of water pollution in aquaponics. These pollutants are rich in nitrogen compounds, which are toxic in certain forms. Nitrogen exists in various forms, and among these forms are ammonium (NH4+), ammonia (NH3), nitrite (NO2-), nitrate (NO3-), and nitrogen gas (N2). In its NH3 form, it is toxic to aquatic life, but also in its NH4+ form, but at higher concentrations. However, nitrogen is important in its nitrate form. In this form, it assists in plant growth integrated with soilless growing technology, although plants can also absorb nitrogen in other forms. In addition, most vegetables are terrestrial, and their root system requires access to air. Drain-fill technology is used in hydroponics and has been shown to increase the biological filtration capacity of systems, and increases plant production, but this technology has not been widely studied in aquaponics. The aim of this study was to evaluate the performance of primary facultative pond (PFP) used to maintain solid waste in the system, and drain-fill technology in aquaponics. This was done by comparing water quality parameters, growth and health of African catfish (Clarias gariepinus) and Swiss chard (as a crop) between conventional systems where solids were removed (which acted as the control) and those with a PFP that maintain and treat solid waste as a loop in the system. The same parameters were compared between aquaponics systems with flooded grow beds (which acted as the control), to grow beds using drain-fill technology. Fish were significantly larger and about 37.6% heavier in the drain fill compared to the flooded aquaponics system after five months, and specific growth rate was significantly higher in the drain-fill system at 1.41±0.03% bw/day. The wet leaf biomass of Swiss chard was significantly different between the flooded (689.3±90.1 g) and drain-fill treatments (17529.6±2731.9 g). In the no-PFP treatment (where solid waste was removed from the system), fish weight and length were not significantly different to the PFP (where solid waste was maintained in the system). The wet leaf biomass of Swiss chard was also similar between the two treatments (18191.00±4765.39 g, in no-PFP and 21618.67±7805.50 g in PFP). This study contributed to the knowledge and understanding of the operation of different systems in aquaponics. It showed that the drain-fill technology is a better alternative to the flooded as it produced better plant and fish biomass, probably due to the aeration (and additional oxygen) associated with drain-fill technology. The addition of a PFP however did not appear to have a significant impact on fish and plant production, but this requires further investigation because the benefit of maintaining nutrients in the system (with the addition of the PFP) might have been masked by excessive nutrient availability and/or the inclusion of insufficient vegetable biomass in the system to realise the benefit of the additional nutrients that were made available by the PFP.