Membrane biological reactors (MBRs) utilize ultrafiltration membrane s that create a clean filtrate, while microbial processes occurring within mixed liquor solids facilitate nitrification and denitrification. Onsite research has shown that MBRs effectively reduce waste concentrations in aquaculture effluents and have potential for integrat ion within RAS. For example, a recent Freshwater Institute study found that single-vessel MBRs receiving RAS backwash produced acceptable water quality for rainbow trout Oncorhynchus mykiss and led to significant water savings . However, a range of operational deficiencies were identified including rapid membrane fouling, incomplete denitrification , and challenges to maintain low oxygen levels and proper mixing of the activated sludge. Th ese systems were redesigned by decoupling the membranes and the aerobic and anoxic zone s for improved access and control of each treatment process.
Six replicated RAS (9.5 m3 volume) three of which were integrated with the redesigned systems along with Mempulse® MB R modules (Dupont, Indiana, USA) The other three RAS were operated without MBRs and enough flushing to maintain < 75 mg/L nitrate-nitrogen (NO3-N) in the fish culture water . Equal numbers of 1- kg rainbow trout were stocked in each RAS to begin. Granulated sugar was continuously added to the MBRs as a supplemental carbon source at a rate of ~ per kg feed using repurposed belt feeders. flow was returned to RAS . After 3.5- months of operation, low-dose ozone was used
Water use was dramatically reduced in RAS equipped with MBRs. The mean system hydraulic retention time in MBR RAS was 82 days vs. 5 days in typically operated RAS. Water quality concentrations were maintained within acceptable limits. Average NO3-N in RAS with and without MBRs was 78 ± 2 and 68 ± 1 mg/L, respectively. When operating with ozone, MBR RAS maintained comparable color and turbidity despite limited dilution. Moreover, rainbow trout growth was similar between treatments. After 4.7-months of production, trout in RAS with and without MBRs weighed 2.807 ± 0.033 and 2.800 ± 0.129 kg, respectively. Changes to MBR design and operation solved the challenges encountered during the initial study; however, new problems were identified including solids clogging behind restricted valves of small diameter (1.27 cm) pipes. Overall, however, MBRs appear to be a viable water treatment option that substantially reduces RAS water use, which could enable facility siting in water-limited areas and/or expansion of fish production volumes.