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During the whole experimental period, the DPs pilot system performed a satisfactory efficiency with a high nutrient removal rates and fitting the final effluent in more restrictive standards legislation (<4 mgTP·L−1 in State law 14.675/14/CONSEMA and <20 mg NH3-N in CONAMA 430/11). The summary of the mean values could be seen in Table 1. The pH values remained near to neutrality (6.5 and 7.5), featuring a stable biological process. Due to their influence in equilibrium, a neutral pH is an important factor for nitrogen absorption and also for duckweed health (Caicedo et al. 2000). DO concentration in DPs strongly depends on the organic load applied, thus DO concentrations below 1.0 mg·L−1 were detected during some periods (during spring) with high loads. However, DO mean values between 1.2 and 2.8 mg·L−1 were found during summer, autumn and winter, showing an aerobic environment. Moreover, the strong presence of microcurstaceans and others organisms mentioned bellow corroborate to define the DPs evaluated as a constant aerobic environment, considering the surface layer (10 cm).

Table 1

Mean values of variables concentrations and duckweed pond (DP1 and DP2) efficiencies

Variables (mg·L−1)naInfluentDP1 Effl.(%)bDP2 Effl.(%)cFinal efficiency (%)
COD 41 99.7 ± 52.6 37.4 ± 11.6 62.4 32.9 ± 13.7 12.2 67.0 
BOD 41 76.1 ± 61.7 26.7 ± 22.6 64.9 10.5 ± 8.5 60.6 86.2 
TN 41 59.7 ± 19.1 20.6 ± 10.9 65.4 5.3 ± 5.0 74.2 91.1 
NH3-N 41 49.4 ± 16.2 15.5 ± 6.3 68.5 2.2 ± 2.8 86.2 95.6 
NO2-N 41 0.07 ± 0.3 0.1 ± 0.2 – 0.1 ± 0.1 – – 
NO3-N 41 0.02 ± 0.1 0.3 ± 0.5 – 0.4 ± 0.7 – – 
TP 41 6.7 ± 1.9 2.1 ± 1.2 68.9 0.5 ± 0.7 76.6 92.7 
PO4-P 41 4.1 ± 1.9 1.4 ± 0.8 68.8 0.2 ± 0.5 80.0 95.1 
DO 30 0.0 ± 0.0 1.8 ± 1.5 – 2.0 ± 1.2 – – 
Turbidity (NTU) 41 45.9 ± 37.4 3.9 ± 2.3 91.4 2.3 ± 1.5 42.1 95.0 
Variables (mg·L−1)naInfluentDP1 Effl.(%)bDP2 Effl.(%)cFinal efficiency (%)
COD 41 99.7 ± 52.6 37.4 ± 11.6 62.4 32.9 ± 13.7 12.2 67.0 
BOD 41 76.1 ± 61.7 26.7 ± 22.6 64.9 10.5 ± 8.5 60.6 86.2 
TN 41 59.7 ± 19.1 20.6 ± 10.9 65.4 5.3 ± 5.0 74.2 91.1 
NH3-N 41 49.4 ± 16.2 15.5 ± 6.3 68.5 2.2 ± 2.8 86.2 95.6 
NO2-N 41 0.07 ± 0.3 0.1 ± 0.2 – 0.1 ± 0.1 – – 
NO3-N 41 0.02 ± 0.1 0.3 ± 0.5 – 0.4 ± 0.7 – – 
TP 41 6.7 ± 1.9 2.1 ± 1.2 68.9 0.5 ± 0.7 76.6 92.7 
PO4-P 41 4.1 ± 1.9 1.4 ± 0.8 68.8 0.2 ± 0.5 80.0 95.1 
DO 30 0.0 ± 0.0 1.8 ± 1.5 – 2.0 ± 1.2 – – 
Turbidity (NTU) 41 45.9 ± 37.4 3.9 ± 2.3 91.4 2.3 ± 1.5 42.1 95.0 

aNumber of campaigns.

bDP1 efficiency.

cDP2 efficiency.

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