How does the loading rate affect the performance of an overland flow system in treating domestic sewage?

Considering the low efficiency of traditional overland flow system denitrification, a 25-cm thick artificial substrate was used instead of clay to fill the surface. In this way, the wastewater can flow through the artificial substrate rather than moving by sheet flow over the surface in expectation to improve the pollutant removal efficiency. In order to evaluate the impact of loading rates, four plots (7.5 m x 10 m for each one) were constructed with a slope of 6%. The hydraulic loading rates (HLRs) of the four plots were 0.007, 0.01, 0.03, and 0.05 m(3)/m(2).d, respectively. In the second experimental phase, the pollutant loading rates (PLRs) of the four plots were 0.05, 0.15, 0.45, and 0.75 g biological oxygen demand (BOD)/m(2).d, respectively. The results indicated that the artificial substrate configuration improved the removal efficiency of pollutants and the ability to resist load shock. Both HLR and PLR were negatively correlated with the removal rate of contaminants. The optimal HLR and PLR were suggested as ranges of 0.01-0.03 m(3)/m(2).d and 0.15-0.45 g BOD/m(2).d, respectively. Under the optimal operation conditions, removal rates were 81.5% +/- 1.6% for chemical oxygen demand, 82.9% +/- 2.7% for NH4+-N, 60.1% +/- 2.0% for total nitrogen, and 88.8% +/- 1.1% for total phosphorus. Nitrifier quantity was reduced by an order of magnitude with an increase in depth from 25 cm to 40, 55, and 70 cm. The activity of nitrate reductase decreased with increasing depth, consistent with the trend of nitrifiers.