Analysis of the biomass nitrogen content demonstrated a percentage
of 6.6% ± 0.8 of total nitrogen (dw), in average. Therefore,
knowing total biomass produced was possible to calculate that
28% of the nitrogen removal in DP1, that is 81 kg of TKN or 1.2 g
TKN/m2day was due to biomass absorption by the duckweeds.
Additionally, (72%) was removed by nitrification and denitrification
processes (Fig. 3). Strong denitrification efficiency can be justified
by several factors including aerobic and anoxic zones (2.1 mg
DO/L on the surface and 0.5 mg DO/L on the bottom) always being
present, a large area for a biofilm to attach, optimal pH and temperature
ranges, and availability of food (BOD) for heterotrophic
microorganisms (denitrificants). Also, nitrate (NO3) was detected
sometimes, usually during the summer, reaching 32 mg/L. However,
because nitrate can be used by both duckweeds and denitrificant
bacteria, the nitrate concentration ranged widely (Table 3).