End-of-pipe denitrification using RAS effluent waste streams: Effect of C/N-ratio and hydraulic retention time

Environmentally sustainable aquaculture development requires increased nitrogen removal from recirculating aquaculture systems (RAS). In this study, removed solids from a large commercial outdoor recirculated trout farm (1000 MT year−1) were explored as an endogenous carbon source for denitrification.

This was done by (1) a controlled laboratory experiment on anaerobic hydrolysis of the organic matter (from sludge cones, drumfilter, and biofilter back-wash) and (2) an on-site denitrification factorial experiment varying the soluble COD (CODS)/NO3-N ratio from 4 to 12 at hydraulic retention times (HRT) from 50 to 170 min in simple 5.5 m3 denitrification reactors installed at the trout farm.

The lab-experiments showed that the major part of the readily biodegradable organic matter was hydrolyzed within 14 days, and the hydrolysis rate was fastest the first 24 h. Organic matter from the sludge cones generated 0.21 ± 0.01 g volatile fatty acids (VFA) g−1 total volatile solids (TVS), and the VFAs constituted 75% of CODS.

Analogously, 1 g TVS from the drum filter generated 0.15 ± 0.01 g VFA, constituting 68% of the CODS. Comparison of the laboratory hydrolysis experiments and results from the on-farm study revealed as a rough estimate that potentially 17–24% of the generated VFA was lost due to the current sludge management.

Inlet water to the denitrification reactors ranged in NO3-N concentration from 8.3 to 11.7 g m−3 and CODS from 52.9 to 113.4 g m−3 (10.0 ± 1.2 ◦C). The highest NO3-N removal rate obtained was at the intermediate treatments; 91.5–124.8 g N m−3 reactor d−1. The effect of the C/N ratio depended on the HRT.

At low HRT, the variation in C/N ratio had no significant effect on NO3-N removal rate, contrary to the effect at the high HRT. The stoichiometric ratio of CODS/NO3-N was 6.0 ± 2.4, ranging from 4.4 (at the high HRT) to 9.3 (at the low HRT). A simple model of the denitrification reactor developed in AQUASIM showed congruence between modeled and measured data with minor exceptions.

Furthermore, this study pointed to the versatility of the NO3-N removal pathways expressed by the bacterial population in response to changes in the environmental conditions; from autotrophic anammox activity presumably present at low C/N to dissimilatory nitrate reduction to ammonia (DNRA) at high C/N, besides the predominate “normal” heterotrophic dissimilatory nitrate reduction (denitrification)