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Therefore, the DO supply could promote a suitable COD and BOD removal reaching 67.0 and 86.2% respectively. Regarding the stabilization stage (pretreatment) the organic matter in DP1 influent presented low concentrations values (about 100 mgCOD L−1 in average). The surface loading rates applied along the seasons are shown in Table 2. A low carbon concentration is expected for polish treatment stage and considering duckweed as autotrophic organism this fact does not detract nutrient uptake efficiency. By contrast, heterotrophic organism populations could decrease in low carbon environment. In similar conditions Sims et al. (2013) studied two DPs in series cites 83% of COD removal from average concentration of 111 mg·L−1. Considering higher inlet concentration, Ran et al. (2004) using Lemna gibba found a COD removal efficiency of 67.5% from 98.2 mg·L−1. The COD measured in this study and in the others mentioned comprises the total COD, in other words, particulate and soluble organic matter.

Table 2

Mean surface loadings rates applied in pilot system during each season

ParametersSLR springSLR summerSLR autumnSLR winter
COD 22.2 17.7 22.1 44.7 
TN 19.8 12.7 10.8 14.3 
NH3-N 15.8 10.6 10.7 10.1 
TP 2.0 1.2 1.6 1.8 
ParametersSLR springSLR summerSLR autumnSLR winter
COD 22.2 17.7 22.1 44.7 
TN 19.8 12.7 10.8 14.3 
NH3-N 15.8 10.6 10.7 10.1 
TP 2.0 1.2 1.6 1.8 

SLR: Surface loading rate (kg·ha−1·day−1).

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