Efficient water treatment achieved in recirculating aquaculture system using woodchip denitrification and slow sand filtration

Petra Camilla Lindholm-Lehto,Jani Tapio Pulkkinen,Jouni Vielma,Tapio Kiuru,Juha Koskela

doi:10.1007/s11356-021-15162-0

Abstract

For a recirculating aquaculture system (RAS), a passive water treatment system was designed for efficient discharge nutrient removal and water reuse in RAS production. Denitrification in a woodchip bioreactor filled with birch wood (Betula pendula) followed by sand filtration was introduced into a side-loop of an experimental RAS rearing rainbow trout (Oncorhynchus mykiss). Denitrification efficiency remained high (96%) throughout the experiment and reached a nitrogen removal rate of 15 g NO3-N m−3 per day. Sand filtration was used to remove dissolved and particulate matter and improve water quality before being returned to water circulation. To ensure the absence of harmful substances in the system, heavy metals were quantified. Additionally, off-flavor-inducing compounds were quantified in the circulating water and in fish flesh. Significantly higher concentrations of geosmin (GSM) (p<0.05) were observed in the controls compared to side-looped systems, but a similar effect was not observed in the case of 2-methylisoborneol (MIB). Among heavy metals, concentrations of Co (30 μg L−1), Ni (40 μg L−1), and Pb (140 μg L−1) decreased to below 10 μg L−1 in the side-loop water after the start-up of the system. Only low concentrations of Cu (5–30 μg L−1) were found in the rearing tank water, in both the side-loop and controls. The results indicated that this type of process design is suitable for safely producing fish of high quality.

Highlights

The intensive recirculating aquaculture system (RAS) is a steadily growing type of aquaculture (Blancheton et al 2007)
After a month of the experiment, the nitrate removal efficiency increased to 96% after the woodchip bioreactor and another 17% in the sand filter, yielding a total efficiency of 99%
This is of great importance because none of the trace elements must pose a risk to the raised species or to fish consumers. The aim of this experiment was to study the potential risks of reusing RAS outflow water treated with a woodchip bioreactor and sand filter

Summary

Introduction

The intensive recirculating aquaculture system (RAS) is a steadily growing type of aquaculture (Blancheton et al 2007). In denitrification, oxidized nitrogen compounds (nitrite, NO2−, and nitrate, NO3−) are transformed to elemental gaseous nitrogen (N2), typically via heterotrophic microorganisms. Other microbial processes, such as anaerobic ammonium oxidation (Anammox) and chemoautotrophic denitrification can produce N2 gas (Burgin and Hamilton 2007). A carbon source is required as an electron donor and for microbial growth (Seitzinger et al 2006; Tallec et al 2008). In RAS applications, denitrification utilizing woodchips as a carbon source has occasionally been reported in small experimental (Lepine et al 2020; Lindholm-Lehto et al 2020), pilot scale (von Ahnen et al 2019), and full-scale applications (von Ahnen et al 2018; Lepine et al 2018)

Methods

Results

Discussion

Conclusion