BIOFILTRATION CAPACITY OF A MOVING BED BIOREACTOR IN SEQUENCE WITH A SUBMERGED FLOATING BEAD BED BIOCLARIFIER - APPLICATION TO RECIRCULATING AQUACULTURE SYSTEMS

Timothy J. Pfeiffer, PhDa, Rhine Perrina, Ronald F. Malonea, PhD, P.E
aAquaculture Systems Technologies, 108 Industrial Avenue, New Orleans, LA
tim.pfeiffer@astfilters.com

Biofiltration is a key process in recirculating aquaculture systems (RAS). The microbial oxidation of ammonia to nitrate is universal and vital to ensure that ammonia and nitrite do not accumulate in the fish culture system.  In moving bed bioreactors (MBBR) and submerged floating bead bed bioclarifiers, nitrifying bacteria colonization occurs as a fixed film on the media surfaces of each respective unit. This aerobic nitrification process is well described and numerous biofilter designs have been developed, tested, and reported. The numerous biofilter solutions are often site specific, based on operator preference, or country traditions. An ideal biofilter for all purposes does not exist and all types of biofilters have their pros and cons.  Much effort and research has been expended in developing guidelines for reporting biofilter performance and optimization. This presentation describes the performance limitations and application of a common RAS unit operation for solids removal and biofiltration, namely a bead filter in sequence with a moving bed bioreactor.

It has been observed that moving bed bioreactors placed behind bead filters with floating bead media (polygeyser, propeller-wash, or bubble-bead) are often flow limited in their ability to convert ammonia. That is, the MBBR ability to convert total ammonia nitrogen (TAN) to nitrate is limited by the transport of TAN into the unit. The bead filter must be sized to provide the necessary flow. Consideration of the need for flow and recognizing the TAN conversion capabilities of the bead filter leads to the sizing approach described under the assumption nitrogen is limiting. Commonly assumed bead filter and MBBR ammonia conversion data is used to model several scenarios of varying TAN concentrations and system volumes to describe the need to consider water flow in the treatment sequence. The STELLA (v 9.1) from ISEE Systems was used to develop and evaluated the varying sequence scenarios.