Effect of biocarrier material on nitrification performance during start-up in freshwater RAS

Abstract

Biofilters are key components in recirculating aquaculture systems (RAS). For ensuring cost-efficient function of biofilters, characteristics of biocarriers such as shape, material, surface area, density, longevity, and price needs to be evaluated. In this study, we compared otherwise identical biocarriers made of polyethylene (PE) against ones made of polypropylene (PP) and PP biocarriers with three different levels of pigment (carbon black) addition (without added pigment, semi-pigmented, fully-pigmented). The four types of biocarriers were tested in 12 parallel moving bed bioreactors under identical conditions receiving water from a semi-commercial freshwater RAS with rainbow trout. Measurements of total ammonium nitrogen (TAN) and nitrite-nitrogen (NO2--N) removal rates and nitrification kinetics were carried out by spiking events with NH4Cl or NaNO2 two, four, and six weeks after start-up. The results showed all biofilters removed TAN only after two weeks of operation, and that biofilters with PP biocarriers had a shorter maturation time, reaching their maximum TAN removal rate at week 4, while PE biofilters reached equal maximum at week 6. Biofilters with PE had generally lower NO2--N removal rates and first-order reaction constants than PP biofilters. Pigmentation level had a minor effect on TAN and NO2--N removal rates and kinetics, as fully pigmented biocarriers initially were found to have lower removal rates and first order constants for TAN and NO2--N than semi-pigmented or biocarriers without added pigment. Overall, the results suggest that PE is inferior biocarrier material than PP, due to the longer start-up period and nitrite accumulation, which might be related to the differences in the surface characteristics between these two materials. Furthermore, the results indicate that pigment addition does not provide any extra benefit for nitrification.