Biological filter? Like an aerobic digester or fluidized sand? The bead-filter plays this role to a certain extent, but the plants and planters & bubblers and in the pond are also doing this. The algae is an issue due to excessive phosphates in the water I reckon, not nitrogen. These nutrients would be coming from the planter soil and dusts (and cleaning products used in the villa?) which are (partially) removed via backwashing, particularly once I modify the filter intakes to remove tadpoles (toadpoles) which are apparently very high in phosphates. These mods will happen once the imported fish are established.
If you're talking about UV, no. Electricity is very costly around here and the client specifically wanted hyper-low maintenance. The water is crystal, so plankton (green water) isn't an issue.