Effect of seeding biofloc on the nitrification establishment in moving bed biofilm reactor (MBBR)

Abstract

In recirculating aquaculture systems, nitrification is usually accelerated by inoculating nitrifier or mature biocarriers. In this study, the performance of the establishment of nitrification in the MBBR according to three different strategies: conventional method (Control group A), inoculation with biofloc recovered from a tilapia biofloc culture system (Group B), and addition with extra nitrite (Group C) in the Moving bed biofilm reactor (MBBR) was compared. Among them, the biofloc-inoculated group considerably accelerated the nitrification process in the MBBR (38 d), which is roughly 18 d faster than the control group (A) (56 d) and 21 d faster than group C (59 d). Less ammonia (8 mg/L NH4+-N, 10 mg/L in other groups) and external nitrite (2 mg/L NO2−-N) in the influent caused effluent ammonia to drop more slowly (5 d slower than the control group, 8 d slower than the B group), which is detrimental to the nitrification process' development. Notably, the influent's hydraulic retention time (HRT) was reduced from 12 h to 6 h following the successful establishment of nitrification. During the adaptation to reduced HRT, the MBBR inoculated with biofloc experienced short-term changes in the water quality index of the effluent water, whereas the other groups did not. The biofilm seeded with biofloc had the highest mean gray value ratio (1.42) of live/dead cell fluorescence, which grew better and could cover the entire groove under multiple microscope observations. However, the other groups did not demonstrate a similar trend. In summary, the research found that seeding biofloc use as nitrification bioaugmentation into the MBBR of the recirculating aquaculture system (RAS) to greatly speed up the nitrification process.