Influence of fixed and moving bed biofilters on micro particle dynamics in a recirculating aquaculture system

Abstract

Accumulation of fine particulate organic matter in recirculating aquaculture systems (RAS) is a balance between system input (from feed to waste), internal transformation, removal and dilution. The mechanisms leading to fine particle accumulation in RAS are not fully understood, and neither is the potential influence of biofilters in this aspect.This study describes the effect of fixed bed biofilters (FBB) and moving bed biofilters (MBB) on particle size distribution and organic matter. It was conducted in an 8.5m3 RAS with four equal biofilters − two FBB and two MBB. The RAS was stocked with rainbow trout (Oncorhynchus mykiss), and operated under constant feed loading conditions (1kg feed/m3 of make-up water) for more than three months. Production or removal of micro particles according to biofilter mode of operation (FBB vs. MBB) was assessed by operating all biofilters simultaneously as well as separately.In periods where FBB and MBB effects were assessed separately, particle concentration was reduced by approximately 195 particles/mL (from 1117 to 922 particles/mL) per passage through FBB, and increased by 252 particles (from 2409 to 2667 particles/mL) per passage through MBB. In FBB, a 10% reduction in particle concentration also represented a 10% reduction in total particle surface area and particle volume. In MBB, a 10% increase in particle concentration also represented a 10% increase in total particle surface area, but had no effect on total particle volume. A volumetric reduction of particles >100μm, and an equivalent volumetric increase of particles <40μm, showed that MBB produced fine particles by disintegration of larger particles. A constant removal of particulate volume through all size classes by FBB demonstrates their function as secondary particle removal units.Net removal of organic matter (ConcentrationIN − ConcentrationOUT), as biochemical oxygen demand after 5days (BOD5), occurred at the same rates in both modes of operation. While FBB removed a higher amount of filtered BOD5 (material filtered through a 1.6μm filter) than MBB, MBB removed more particulate BOD5 (Particulate=Raw − Filtered) than FBB, presumably due to disintegration of particles in MBB. In the RAS, ammonia and nitrite were observed at concentrations below 0.20mgN/L throughout the majority of the experiment. However, during the phase where only MBB were in operation, TAN (Total Ammonia Nitrogen) and nitrite levels increased significantly. Nitrate levels ranged between 40 and 44mgN/L, reflecting stable operating conditions and constant feed loading.The trends observed when FBB or MBB were operated separately were also observed when all filters were operated simultaneously. Differences in biofilm formation, development and maintenance, coupled to reactor flow characteristics are discussed in relation to the fate of micro particles and organic matter when operating fixed or moving bed biofilters in RAS.