Novel Filamentous Algae Nutrient Scrubber (FANS) for Agricultural Drainage Nutrient Removal and Recovery

Filamentous algae nutrient scrubbers (FANS) have the potential to recover nutrients from agricultural drainage. This study was conducted on two dairy farms in North Waikato, New Zealand, with the aim of evaluating different FANS configurations for nutrient removal, algae productivity and nutrient recovery from agricultural drainage across seasons. We compared the performance of three different configurations of FANS: standard FANS with filamentous algae attached to a bottom surface (textured liner) (water depth: 19 mm; hydraulic retention time (HRT): 9 min; horizontal water velocity (HWV): 0.0006 m s⁻¹) and novel suspended FANS with algae attached to a floating mesh with a water depth of either 38 mm (shallow) or 214 mm (deep) (18 and 102 min HRT; 0.011, and 0.022 m s⁻¹ HWV respectively). Algae productivity and nutrient removal rates varied seasonally in all FANS configurations with summer values of up to five-fold (productivity), four-fold (nitrate removal) and three-fold (phosphate removal) higher than in winter. Shallow suspended FANS were the best configuration in warmer months with the highest biomass productivity on both farms (4.3–5.1 g VS m⁻².d in summer) and (3.7–3.8 g VS m⁻².d in spring). While during colder months, algae productivity was relatively similar for all FANS configurations. Moreover, shallow suspended FANS had the highest nutrient removal rates throughout the year of monitoring. This study highlights the potential of shallow suspended FANS as a better operation mode than standard FANS. Algae species relative abundance also varied across FANS configuration, with greater dominance of green filamentous algae with increasing FANS depth, providing a more consistent biomass quality year-round which may be important for beneficial use options.