Design and operations of fine media fluidized bed biofilters for meeting oligotrophic water requirements

Fine media fluidized bed biofilters (FBB) have some unique characteristics, which become very important when extremely high water quality is required. They provide greater surface area per unit volume than other fixed film biofilters and are capable of operating as a plug flow on the liquid phase and mixed flow on the biological phase type reactor. As the concentration of pollutants decreases in an aquaculture system, the removal rate per unit surface area in a biofilter decreases, hence being able to obtain very high surface areas per unit cost becomes critical. As the concentration further decreases, conventional bioreactors that are either, mixed flow biological phase and mixed flow liquid phase (i.e. moving bed type reactor), or plug flow liquid and fixed biological phase (trickling filter or submerged filter) reach the minimum substrate concentration (S_Min), below which the bacteria cannot grow under steady state conditions. However, in a fine media FBB the discharge concentration can be below S_Min. This allows filters to be designed and operated in commercial aquaculture settings with over 90% removal of NH₃, and related biochemical oxygen demand (BOD) per pass. Fine media FBBs can be designed and operated for biological removal of 99.95% of slow biodegrading refractory organic pollutants like methyl tertiary butyl ether (MTBE) in a single pass with discharge concentrations <1 ppb (inlet 2000 ppb, 20 min contact time, S_Min = 20 ppb). The details of how and why these high performances at low concentrations are possible and why this oligotrophic water quality is desirable for maturation and larva rearing will be discussed.