An integrated, two-step biofiltration system with Ulva fasciata for sequenced removal of ammonia and nitrate in mariculture effluents

Secondary treatment of fishpond effluent is imperative for the removal of excess nitrate and phosphorus. Biofilters comprised of the green macroalgae, Ulva, exhibit several advantages for this process, including usability of the produced edible biomass. However, even low levels of ammonia in fishpond effluent inhibit the uptake of nitrate by Ulva. In order to overcome this drawback we examined Ulva faciata in a two-step biofiltration system. The biofiltration system comprised of two sequenced tanks where the upstream Ulva biofilter was fed directly with mariculture effluent containing both ammonia (as total ammonium nitrogen - TAN) and nitrate (as NO₃-N) and the TAN-depleted but NO₃-N-rich effluent from this biofilter were transferred to the downstream Ulva biofilter. The performance of each separate Ulva biofilter as well as that of the whole system was measured on a weekly regime over a culture period of three weeks. Between 81 and 95% of the dissolved inorganic nitrogen (TAN + NO₃-N) in mariculture effluent was removed by the two-step biofilter, with specific removal of 89–100% of TAN and 57–78% of NO₃-N in mariculture effluent by the upstream and downstream Ulva, respectively. Exposure of Ulva in upstream biofilters to higher N input with both TAN and NO₃-N did not affect algal production rate, which was relatively similar to that in downstream biofilters fed with TAN-depleted, NO₃-N-rich effluent, with mean specific growth rate of 17.85% d⁻¹ (±3.1) in the different biofilters. Removal rates of TAN or NO₃-N by upstream and downstream Ulva, respectively, were relatively similar, between 90 and 135 mmol N m⁻² d⁻¹, while PO₄-P removal was much faster in downstream Ulva biofilters having only NO₃-N as a N source. While TAN removal induced production of Ulva biomass with higher N, P and protein content, culture in NO₃-N-rich effluent resulted in Ulva biomass with higher lipid content.