Combinations of Ulva and periphyton as biofilters for both ammonia and nitrate in mariculture fishpond effluents

Periphyton-based biofilters for aquaculture effluent possess multiple advantages, in water oxygenation, CO₂ reduction and production of useful biomass. The performance of a marine periphyton biofilter, in terms of uptake rate and efficiency in removing different forms of nitrogen, was investigated. A periphyton biofilter was paired with an upstream macroalgae biofilter stocked with Ulva to expose the periphyton to ammonia-depleted but nitrate-rich effluent. Three trials compared the removal of total ammonia-N (TAN), nitrate (NO₃-N), total N and phosphorus. The biofiltration and growth performance of (1) the periphyton downstream to the Ulva tank, (2) periphyton alone and (3) Ulva alone were compared. Biofiltration performance was evaluated at different areal loads of TAN and NO₃-N. Periphyton growth performance did not depend on the effluent nutrient composition or on N loads, and yielded between 7.3 and 10.6 g dry weight m⁻² d⁻¹. While the Ulva preferred uptake of TAN over NO₃-N, the periphyton showed no preference between them demonstrating flexible shifts between TAN and NO₃-N uptake. TAN uptake rate by the periphyton was not influenced by the effluent composition. However, periphyton NO₃-N uptake rate and efficiency rose about fivefold, up to 1.4 g NO₃-N m⁻² d⁻¹ and 63%, respectively, upon depletion of TAN areal load below 0.18 g N m⁻² d⁻¹ (<0.3 mg L⁻¹) by the Ulva pre-treatment. Normalizing nutrient uptake rate to biomass revealed similar uptake rate of TAN and phosphorus by periphyton and Ulva, while the periphyton took up nitrate much faster. By removing up to 76% of the total nitrogen, with specific removal efficiency of 97% of the TAN and of 67% of the NO₃-N, the novel dual Ulva-periphyton biofilter revealed a synergistic potential in treatment of nutrient-rich mariculture effluents.