Salinity affects nitrate removal and microbial composition of denitrifying woodchip bioreactors treating recirculating aquaculture system effluents

Abstract

This study investigated the effect of salinity on microbial composition and denitrification capacity of woodchip bioreactors treating recirculating aquaculture system (RAS) effluents. Twelve laboratory-scale woodchip bioreactors were run in triplicates at 0, 15, 25, and 35 ppt salinities, and water chemistry was monitored every third day during the first 39 days of operation. Microbial communities of the woodchips bioreactors were analyzed at the start, after one week, and at the end of the trial.Woodchip bioreactors removed nitrate at all salinities tested. The highest NO3-N removal rate of 22.0 ± 6.9 g NO3-N/m 3 /d was obtained at 0 ppt, while 15.3 ± 4.9, 12.5 ± 5.4 and 11.8 ± 4.0 g NO3-N/m 3 /d were obtained at salinities of 15, 25 and 35 ppt, respectively. Nitrate removal rates thus decreased with salinity, being 54-69% lower than at 0 ppt. Leaching of total ammonia nitrogen (TAN) and orthophosphate (PO4-P) from woodchips was initially higher at saline treatments compared to 0 ppt, while initial leaching of BOD5 appeared to be similar across all treatments. Production of alkalinity per g NO3-N removed was higher at 0 (3.6 ± 0.5 gCaCO3/gNO3-N) and 15 ppt (3.5 ± 0.8) than at the more saline treatments (25 ppt: 2.0 ± 0.9, 35 ppt: 1.12 ± 0.5 gCaCO3/gNO3-N ), indicating that heterotrophic denitrification was the dominant nitrate removing process at 0 and 15ppt, while autotrophic denitrification processes probably interfered with the alkalinity balance at 25 and 35 ppt. In the woodchip reactors, Gammaproteobacteria was the most abundant taxa. However, salinity shaped the woodchip microbiome, resulting in an increase in the abundance of sulfide oxidizing autotrophic denitrifiers, but decrease in the overall abundance of denitrifying microbes at higher salinities, which presumably explained the reduced nitrate removal rates at elevated salinities.This study demonstrates that woodchip bioreactors can be applied to remove nitrate from saline RAS effluents albeit at lower nitrate removal rates compared to freshwater installations.

Nitrate removal microbiology in woodchip bioreactors: A case-study with full-scale bioreactors treating aquaculture effluents

“…In addition to microbial abundances, community composition of nitrate removing microbes can be reflected to NO 3 removal rates, as has been shown in salt marsh sediments (Angell et al 2018) or recently, in laboratory-scale woodchip bioreactors (von Ahnen et al 2019). Based on nirS sequencing, the denitrifying communities within the bioreactors were found to be different from the ones found in the inlet and outlets (Figure 2), suggesting that the bioreactor communities develop in the same direction under the prevailing conditions and are less dependent on the operating conditions.…”

“…Denitrification is a sequential process with four enzymatic steps, and the genes coding for enzymes reducing nitrite (NO 2 -) to nitric oxide (NO), nirS and nirK, are the most widely used molecular markers to study denitrifier populations. Previously, nir gene abundance has been found to increase with NO 3 -N removal rates in experimental denitrification beds including woodchips (Warneke et al 2011) and in laboratory-scale woodchip bioreactors treating RAS effluents (von Ahnen et al 2019), but knowledge on the abundance and communities of microbes carrying these genes under less-controlled full-scale woodchip bioreactors is currently missing. Denitrification can also be incomplete, producing nitrous oxide (N 2 O) instead of N 2 .…”

Comparison of microbial communities in replicated woodchip bioreactors

“…It is possible that the woodchips used for microbial characterization may have decayed more quickly based on their finer size, as others have observed over longer monitoring periods (Ghane et al., 2018). Potential solutions for this may be to divide larger samples for both analyses (von Ahnen et al., 2019). The role of influent composition on microbial community membership and succession mechanisms warrants further study because woodchip bioreactor substrate may be managed with respect to site nutrient characteristics and hydrology to promote consistent nitrate removal.…”

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

“…Woodchips contain a high C/N ratio, acting as a good carbon source in anoxic conditions (Warneke et al 2011). 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).…”