Analysis of hydraulic conditions and HRT on the basis of experiments and simulations on soil column

The hydraulic residence time (HRT) is a key parameter affecting the soil aquifer treatment process (SAT) efficiency as tertiary treatment of wastewater through infiltration in vadose zone. In order to maximize the HRT, a pulsed feeding technique of a salt solution (used as conservative tracer) was adopted to allow the increase of HRT and soil aeration between pulses. For this reason, several laboratory column experiments combined with the use of a numerical model was conducted. The optimal number of pulsations that meet with the maximum of the HRT and the temporal variation of the water saturation into the column was determined for different applied hydraulic heads. It varies between 10 and 12. The maximum corresponding water stock in the porous media column was evaluated to about 84%, 79% and 73%, respectively, for an applied hydraulic head of 96, 75 and 50 cm. The applied hydraulic head had an important effect on the HRT. In fact, this parameter varies from almost 6.5 to 10.8 h for an applied hydraulic head of 96 and 50 cm. The change in the permeability of the soil had also a significant effect on the HRT. Hence, HRT is decreased of about 9% when the soil permeability at saturation is multiplied by 2, and increases of about 11% when the permeability is two times lower.

This study is based on the use of an experimental porous media columns and a numerical model, it shows that the HRT is very dependent on the applied hydraulic head height added and the number of water sequences feeding. Taking into account the calculated HRT values and the water stock evolution versus time, 10 or 12 water sequences feeding and 50 cm applied hydraulic head seem to be the best operating cycle. The corresponding HRT, which is about 10.8 h, maximize the contact time between the biomass and the wastewater percolating in the subsurface and consequently the dissolved pollution oxidation. The initial permeability of the porous media has also a significant effect on this parameter and consequently on the efficiency of the treatment. In fact, when the soil permeability is important, a rapid wastewater infiltration and consequently the renewal of the air phase which is mainly responsible for the pollution degradation are favoured. The specific conditions cited above, corresponding to the optimal HRT, should be applied in lab and in a second step in real sites. The clogging phenomena evolution of the superficial layer must be taken into account in the optimization of the overall management of the SAT process, especially for the determination of the infiltration and drying periods.