A sulfur-based cyclic denitrification filter for marine recirculating aquaculture systems

Area of Science:

• Environmental Science
• Environmental Microbiology
• Aquaculture Engineering

Background:

• Saline wastewater poses challenges for biological nitrogen removal due to salinity's impact on microbial processes.
• Recirculating aquaculture systems (RAS) require effective nitrogen management to maintain water quality.
• Existing denitrification methods may be less effective or costly in high-salinity environments.

Purpose of the Study:

• To evaluate the efficacy of a novel sulfur-autotrophic cyclic denitrification filter (CDF) for nitrogen removal in marine recirculating aquaculture systems (RAS).
• To assess the performance of the CDF under varying operational conditions and fish loads.
• To determine the nitrogen mass balance and overall efficiency of the CDF in a saline RAS environment.

Main Methods:

• A sulfur-autotrophic cyclic denitrification filter (CDF) was implemented and tested in a marine RAS setting.
• Operational parameters such as residence time were varied to optimize performance.
• Water quality parameters including ammonia, nitrite, nitrate, and sulfide concentrations were monitored.
• A nitrogen mass balance was conducted, accounting for fish assimilation, passive denitrification, CDF removal, and discharge.

Main Results:

• The CDF successfully maintained low concentrations of ammonia, nitrite, and sulfide at residence times of 4–12 hours.
• After introducing fish (Poecilia sphenops), key nitrogen forms (NH₄⁺-N, NO₂^--N, NO₃^--N) were kept below critical levels (1, 1, and 60 mg/L, respectively).
• The CDF accounted for 60% of the total nitrogen removal, with fish assimilation and passive denitrification contributing smaller percentages.
• Daily freshwater addition was minimal (<2% of fish tank volumes), indicating efficient water use.

Conclusions:

• The sulfur-autotrophic CDF is a promising and potentially low-cost technology for effective nitrogen removal from saline wastewater in marine RAS.
• The system demonstrates efficient denitrification capabilities even with the presence of fish waste and varying operational conditions.
• The CDF offers a sustainable solution for managing nitrogenous waste in saline aquaculture, reducing the need for excessive freshwater exchange.