An experimental comparison of sediment-based biological filtration designs for recirculating aquarium systems

Abstract

Common sediment filtration designs for recirculating systems designed for marine ornamentals fall largely into two major categories: sandbed and plenum-based systems. To date, there has been no experimental comparison of the relative performance of these methods for handling nitrogenous wastes in marine aquaria. We compared nutrient levels in a factorial design of aquaria: (1) with or without a plenum; (2) with deep (9.0 cm) or shallow (2.5 cm) sediments; and (3) with coarse (2 mm) or fine (0.2 mm) mean particle sizes. None of these experimental treatments have a significant advantage in the processing of nitrogenous wastes in recirculating aquaria; final ammonia and nitrite concentrations were below detectable levels, and nitrate concentrations did not differ significantly among the experimental treatments. After an initial stabilization period, most experimental treatments responded equivalently to continuous ammonium input of up to 0.5 mg/l/day. Results were qualitatively similar whether experiments were carried out in the absence of animals in a lab with nutrient input via measured dosing of ammonium chloride, or in aquaria with live animals and natural sediments. Sediment depth and particle size had significant effects on a variety of water parameter measures throughout the experiment. Overall, coarse sediments had lower buffering capacity (pH, calcium and alkalinity) and much higher final phosphate concentrations than fine sediments. Death rates in the live animal experiments containing shallow sediments were roughly twice (2.91 ± 0.46) those of the deep sediment trials (1.47 ± 0.46). Regardless, the presence or absence of a plenum had little effect on water parameters throughout the experiment. These results suggest that there is little benefit to be gained from the addition of a plenum plate beneath the sediments in recirculating aquarium designs.