ULVA, PERIPHYTON, AND THEIR COMBINATIONS FOR IMPROVING TOTAL NITROGEN REMOVAL IN MARICULTURE EFFLUENT

The environmental footprint of mariculture is a major obstacle towards the expansion of this agro-industry. Biofilters made of Ulva and periphyton differ in their effectiveness in removing ammonia and nitrate from mariculture effluents. In order to overcome the relatively minor uptake of nitrate by Ulva, a periphyton- based biofilter was integrated as a polishing unit in a combined Ulva-periphyton biofilter. The practicality of a combination of these two biofilters in improving the overall removal of dissolved N was evaluated by exposing the paired Ulva-periphyton biofilter to various areal loads of ammonia (as total ammonium nitrogen, TAN) and nitrate (as NO₃-N), the primary nitrogen forms in fishpond effluents. The upstream macroalgae biofilter was fed with fishpond effluents at different areal loads of ammonia and nitrate, while the downstream periphyton biofilter was fed with the remained nutrients in effluent of the upstream Ulva.

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.

Ulva removed ammonia at a rate of 0.7 - 5.4 g TAN m^-2 d^-1, in correlation with the TAN areal load, with Vmax of 5.1 and Km of 4.4 g TAN m^-2 d^-1. Downstream periphyton was exposed to a lower TAN, but nitrate-rich effluent, and revealed similar capacities for the removal of both N forms, at removal rates of up to 1.7 and 1.8 g N m^-2 d^-1, respectively. Compared to nitrate, areal load of TAN had a greater impact on the removal dynamics of both N forms by periphyton.

Overall, the paired biofilter resulted in a nearly total depletion of ammonia (97%) and efficient nitrate removal (67%), when areal loads in fishpond effluents were below 2 and 4 g N m^-2 d^-1 of TAN and NO₃-N, respectively.