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

Accumulation of fine particulate organic matter in recirculating aquaculture systems (RAS) is a balancebetween system input (from feed to waste), internal transformation, removal and dilution. The mecha-nisms leading to fine particle accumulation in RAS are not fully understood, and neither is the potentialinfluence of biofilters in this aspect.This study describes the effect of fixed bed biofilters (FBB) and moving bed biofilters (MBB) on particlesize distribution and organic matter.

It was conducted in an 8.5 m3RAS with four equal biofilters −two FBB and two MBB. The RAS was stocked with rainbow trout (Oncorhynchus mykiss), and operatedunder constant feed loading conditions (1 kg feed/m3of make-up water) for more than three months.Production or removal of micro particles according to biofilter mode of operation (FBB vs.

MBB) wasassessed by operating all biofilters simultaneously as well as separately.In periods where FBB and MBB effects were assessed separately, particle concentration was reduced byapproximately 195 particles/mL (from 1117 to 922 particles/mL) per passage through FBB, and increasedby 252 particles (from 2409 to 2667 particles/mL) per passage through MBB.

In FBB, a 10% reductionin particle concentration also represented a 10% reduction in total particle surface area and particlevolume. In MBB, a 10% increase in particle concentration also represented a 10% increase in total particlesurface area, but had no effect on total particle volume. A volumetric reduction of particles >100 m,and an equivalent volumetric increase of particles <40 m, showed that MBB produced fine particles bydisintegration of larger particles.

A constant removal of particulate volume through all size classes byFBB demonstrates their function as secondary particle removal units.Net removal of organic matter (ConcentrationIN− ConcentrationOUT), as biochemical oxygen demandafter 5 days (BOD5), occurred at the same rates in both modes of operation.

While FBB removed a higheramount of filtered BOD5(material filtered through a 1.6 m filter) than MBB, MBB removed more par-ticulate BOD5(Particulate = Raw − Filtered) than FBB, presumably due to disintegration of particles inMBB. In the RAS, ammonia and nitrite were observed at concentrations below 0.20 mg N/L throughoutthe 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 40and 44 mg N/L, reflecting stable operating conditions and constant feed loading.The trends observed when FBB or MBB were operated separately were also observed when all filterswere operated simultaneously.

Differences in biofilm formation, development and maintenance, coupledto reactor flow characteristics are discussed in relation to the fate of micro particles and organic matterwhen operating fixed or moving bed biofilters in RAS