The aquatic fern Azolla as a natural plant-factory for ammonia removal from fish-breeding fresh wastewater

This study has investigated the potential of an Azolla-Anabaena symbiosis, a marriage between the cyanobacterium Anabaena azollae and the aquatic fern (Azolla), to remove ammonia from freshwater fish breeding areas. Experiments were carried out under artificial light of 20, 70, and 140 μmol mâ^'2 sâ^'1. We investigated three different water temperatures for the growing Azolla, ranging from sub-optimal to optimal temperatures (15, 22, and 28 °C). The capability of Azolla to remove ammonia from wastewater was demonstrated, and the highest ammonia concentration tolerated by the symbiosis between Azolla-anabaena without any toxic effect on the aquatic ferns was ascertained. The shortest time taken to remove ammonia from wastes, 2.5 cm deep and at 28 °C, was 40 min. The ammonia removal rate (ARR) was both light and temperature dependent and the highest rate (6.394 hâ^'1) was attained at light intensity of 140 μmol mâ^'2 sâ^'1 and at a temperature of 28 °C; the lowest (0.947 hâ^'1) was achieved at 20 μmol mâ^'2 sâ^'1 and 15 °C. The depth of the fish-wastewater pool also affected the ARR with the relation between ARR and the depth being a hyperbolic function.