Demonstration of a novel ethanol-packed membrane biofilm reactor for denitrification at the Tokyo Sea Life Park

• An ethanol-packed membrane biofilm reactor (MBfR) was developed for denitrification.
• The MBfR was operated under aerobic conditions in a recirculating aquaculture system.
• With two types of maintenance, the MBfR has been working for more than a year.
• The denitrification rate ranged 0.7–1.1 gN m−2 d−1, depending on nitrate load.
• Ethanol refilling (every 6 months) and air expelling (every month) were required.

A novel ethanol-packed membrane biofilm reactor (MBfR) was used to develop a simple denitrification technology applicable to aerobic tanks in recirculating aquarium or aquaculture systems. First, the basic properties of the module of the MBfR were assessed via a series of batch tests. The results of the batch tests indicated that (1) the module could obtain denitrifying capability after a 2-week submergence in aerated seawater without having direct inoculation onto the module, (2) the denitrification rate was proportional to the ethanol supply rate, and (3) the type of supporting medium had little effect on denitrifying capability. On the basis of these results, a module for the full-scale demonstration was made of 30 cm × 30 cm square of non-woven fabric coated with a 0.07-mm-thick PE film on the one side. An openable tap was incorporated to supply additional ethanol, and 100 mL of ethanol was packed into each module before use. Then, the appropriate number of modules was used for a full-scale demonstration in two types of recirculating aquarium systems for up to 400 days. In an aquarium rearing small fishes, the three MBfR modules submerged in the aerobic tank prevented nitrate accumulation for 77 days. However, the surface denitrification rate (0.68 gN m−2 d−1) was smaller than the expected value, probably because the nitrate concentration was very low. In another aquarium mainly rearing lobsters, the 17 submerged modules prevented nitrate accumulation at the rate of 1.0–1.1 gN m−2 d−1 for approximately 6 months without ethanol refilling. After that, it was also observed that the dilution of ethanol decreased the denitrification rate; however, ethanol refilling resulted in immediate recovery of denitrification capability. Taken together, our results indicate that the proposed ethanol-packed MBfR could be employed as a denitrification technology for recirculating aquariums or aquacultures, and offers the practical advantage of eliminating the need for a pump for ethanol supply and an additional loop or tank exclusively for denitrification.