순환여과양식시스템 결합 광생물반응기의 배양액 체류시간에 따른 성능 평가

Abstract

This study combined a laboratory‑scale recirculating aquaculture system (RAS) for Nile tilapia (Oreochromis niloticus) with a cylindrical photobioreactor (PBR) inoculated with Chlorella vulgaris in order to elucidate the effect of hydraulic retention time (HRT) of the culture medium on fish growth, microalgal productivity, and the removal of nitrogenous compounds and carbon dioxide. The PBR was operated continuously for three weeks at HRTs of 1, 3, and 5 days, while fish survival, growth indices, feed utilisation efficiency, and water‑quality parameters were monitored. Survival was 86 ± 1 % across all treatments, indicating that integration of the PBR exerted no adverse effect on fish viability. At an HRT of 3 days, the specific growth rate (SGR) reached a maximum of 1.23 ± 0.09 % day⁻¹, and the feed conversion ratio (FCR) decreased by 31 – 36 % (1.35 – 1.47) relative to the RAS‑only control (2.10 ± 0.24), demonstrating a significant improvement in feed efficiency. With respect to water quality, the PBR removed 100 % of dissolved CO2 under all HRT regimes, whereas dissolved inorganic nitrogen removal was strongly HRT‑dependent. Total ammonia nitrogen (TAN) removal efficiency increased with longer HRT, yet daily volumetric removal declined owing to reduced flow. Significant nitrate (NO3⁻‑N) abatement was observed only at HRT ≥ 3 days; prolonged high‑HRT operation elevated pH (≈ 10), suppressing nitrite reductase activity and leading to transient accumulation of nitrite (NO2⁻‑N). Maintaining pH at 7.5 – 8.0 through supplemental CO2 or buffering effectively mitigates this accumulation. Regression analysis of net specific growth rate (G) yielded a wash‑out threshold of 2.59 days; applying a 20 % safety factor produced an optimal design HRT of 3.1 days. Under this condition, microalgal productivity attained 0.63 ± 0.06 g DCW m⁻² day⁻¹ and 0.089 ± 0.009 g DCW L⁻¹ day⁻¹. Consequently, operating the PBR at an HRT of 3.1 days simultaneously enhances fish growth and feed efficiency, achieves concurrent nitrogen and carbon removal, and supports robust microalgal biomass production, providing a practical design reference for sustainable aquaculture.