3. Results and Discussion
3.1. Process performance
The COD and BOD5 concentration of the treated effluent was
below 100 and 30 mg/L at HRT levels above 17 h, respectively
(Fig. 2), and exceeded Iran's national permissible limit at below 17 h
HRT. The mean reductions of COD and BOD5 were 78 ± 0.81% and
88 ± 1.12% at 17 h HRT. Regression analysis (under steady-state)
showed a reasonable linear correlation (r
2 0.97 and r
2 0.94,
respectively) between the inflow and outflow COD and BOD5
loadings (Fig. 3). The efficiency of the system can be attributed to
the efficient oxygen diffusion and the high accumulation of
enhanced bacterial populations responsible for organic degradation
in the treatment unit. The dissolved oxygen inside the reactor
was estimated to be an average of 3.0 ± 0.15 mg/L O2 at 17 h HRT
(Fig. 4). Atmospheric diffusion is a relatively inefficient process that
seems to be relatively insignificant because the dense plant cover
reduces the surface gas-exchange in the process of phytoremediation
with attached growth. On the other hand, vegetation
plays an important role in transferring oxygen via the aerenchyma
tissue (Li et al., 2011).