A salty start: Brackish water start-up as a microbial management strategy for nitrifying bioreactors with variable salinity

Sharada Navada,Marianna Sebastianpillai,Jelena Kolarevic,Ragnhild O Fossmark,Ann-Kristin Tveten,Frédéric Gaumet,Øyvind Mikkelsen,Olav Vadstein

doi:10.1016/j.scitotenv.2020.139934

Sharada Navada, Marianna Sebastianpillai + Show 6 more

Open Access

https://doi.org/10.1016/j.scitotenv.2020.139934

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Abstract

Nitrifying biofilms developed in brackish water are reported to be more robust to salinity changes than freshwater biofilms. This makes them a promising strategy for water treatment systems with variable salinity, such as recirculating aquaculture systems for Atlantic salmon. However, little is known about the time required for nitrification start-up in brackish water or the microbial community dynamics. To investigate the development of nitrifying biofilms at intermediate salinity, we compared the startup of moving bed biofilm reactors with virgin carriers in brackish- (12‰ salinity) and freshwater. After 60 days, the brackish water biofilm had half the nitrification capacity of the freshwater biofilm, with a less diverse microbial community, lower proportion of nitrifiers, and a significantly different nitrifying community composition. Nitrosomonas and Nitrosospira-like bacteria were the main ammonia oxidizers in the brackish water biofilms, whereas Nitrosomonas was dominant in freshwater biofilms. Nitrotoga was the dominant nitrite oxidizer in both treatments. Despite the lower nitrification capacity in the brackish water treatment, the low ammonia and nitrite concentration with rapidly increasing nitrate concentration indicated that complete nitrification was established in both reactors within 60 days. The results suggest that biofilms develop nitrification in brackish water in comparable time as in freshwater, and brackish start-up can be a strategy for bioreactors with varying salinity.

Highlights

Variable salinity influents are often encountered in municipal and industrial water treatment systems, such as food processing, cities with seawater flushing, and land-based aquaculture (Lefebvre and Moletta, 2006; Navada et al, 2020)
The results suggest that biofilms develop nitrification in brackish water in comparable time as in freshwater, and brackish startup can be a strategy for bioreactors with varying salinity
During the first 12 days, the concentration of ammonia and nitrite decreased on some days despite no dilution water flow

Summary

Introduction

Variable salinity influents are often encountered in municipal and industrial water treatment systems, such as food processing, cities with seawater flushing, and land-based aquaculture (Lefebvre and Moletta, 2006; Navada et al, 2020). Recirculating aquaculture systems (RAS) are a rearing technology for producing fish in land-based facilities with the treatment and reuse of water. While the fish are physiologically adapted to tackle an increase in salinity, the microbes in the nitrifying bioreactors in RAS may be negatively impacted by salinity changes (Navada et al, 2019). In RAS, the bioreactors perform the vital task of oxidizing the ammonia produced by the fish to nitrite, and subsequently, to nitrate. As even very low concentrations of ammonia (b2 mg L−1 total ammonia nitrogen) and nitrite (b0.5 mgN L−1) are toxic to Atlantic salmon, it is essential to maintain high and stable nitrification in RAS

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