Nitrification in moving bed and fixed bed biofilters treating effluent water from a large commercial outdoor rainbow trout RAS

摘要

The nitrification performance of two fixed bed (FB) biofilters and two moving bed (MB) biofilters was evaluated. They received the same cold (8pC) influent water from a commercial outdoor RAS facility producing rainbow trout (average density 32kgmpd). The filters were constructed as four identical 5.5mpd tanks with different filter media inside and tested simultaneously in duplicate. FB filters were filled with 4.2mpd polyethylene netshaped cylinders (Bioblokp, 200mpo mpd), and MB filters with 2mpd polypropylene carriers (Biomedia, 850mpo mpd). Nitrification rates were measured 3p#/po months after start-up, and inlet water was supplemented with ammonium chloride in order to determine maximum nitrification rates (0-order kinetics). The filters were conditioned at an inlet TAN concentration of 2.89pl0.1mglp# and water in-flow ranging from 1lsp# to 4lsp#. Expressed as volumetric total ammonia-N (TAN) removal rate, the MB filters had significantly higher removal rate (231pl17gNp# mpd dp#) compared to the FB filters (92pl2gNp# mpd dp#). Expressed as surface specific TAN removal rate MB filters had significantly lower removal (0.27pl0.02gmpo dp#) than FB filters (0.46pl0.01gmpo dp#). When conditioned to a higher inlet TAN concentration (6.27pl0.39mglp#) for 2 weeks, the FB filters increased the removal rate (146pl3gmpd dp# or 0.73pl0.01gmpo dp#) while the MB filters had unaltered performance. The results indicate, that the more heterogeneous and stratified biofilm to be expected in FB, can react more flexibly when challenged with changes such as differences in TAN loading. The effect of dissolved oxygen level on FB filter nitrification rates was additionally tested at TAN 5.35pl0.06mglp#. Below approximately 60% saturation (7.1mgO lp#) measured at the filter outlet, nitrification rates started decreasing rapidly. An exponential expression (y =10.05+10.48(1e."#ix ), R po=0.96) was found to model the whole data range from 40% to 80% DO saturation well.