Integrated biofilters with Ulva and periphyton to improve nitrogen removal from mariculture effluent

Biofilters composed of Ulva or periphyton reveal great potential in removal of excess nitrogen in mariculture effluent while producing protein-rich edible biomass. However, TAN depletion is requisite to improve nitrate removal performance. In the current study we compared the contribution of Ulva fasciata and periphyton in removal of nitrogen, particularly in the form of nitrate, from fishpond effluent. For this purpose, we integrated U. fasciata or periphyton as a secondary biofilter downstream to a primary U. fasciata biofilter, and measured growth and nutrient removal performance in each of these two-step biofilters and in their constitutuent biofiltration units. The upstream U. fasciata biofilters removed nearly all TAN from fishpond effluent, leaving less than 5 μM of TAN but excess NO₃-N in the effluent leaving these biofilters for the downstream biofilters. The downstream U. fasciata biofilter removed 76% of NO₃-N from its inlet water. While this was about three times more efficient than its periphyton counterpart, the periphyton's specific removal rate of this nutrient (1.5 g NO₃-N g⁻¹ DW d⁻¹) was much faster than that of U. fasciata. Overall, integration of two sequenced U. fasciata biofilters (‘Ulva-Ulva’) yielded more biomass and removed more nitrogen than the Ulva-periphyton biofilter (removing 25 vs. 18 g N wk.⁻¹, respectively). Despite the differences in N removal rate and efficiency, the protein level in upstream U. fasciata and downstream periphyton was relatively similar (~25% DW), while somewhat lower in the downstream U. fasciata. Results of the current study favour the two-step U. fasciata biofilter, but periphyton's faster specific removal of NO₃-N and its contribution to oxygen generation should not exclude further improvement of this technology.