Abstract: Nowadays, traditional residential and industrial wastewater treatment methods have been mainly developed as complex systems that consider costly infrastructure, which requires advanced control systems and highly qualified labour for their operation. The use of wetland-type infrastructure has been recognized both, by scientists and authorities, as an efficient and effective method to obtain good results in these processes.
The most relevant elements in the design of horizontal subsurface filters are filtering media, as biofilm supporting material and flow control methods. Usually, these treatment systems use gravel as filling material. Despite the functionality of the stone material, its weight presents serious difficulties for its handling at source and on site. The use of a plastic support would lower transportation costs, improve manageability and reduce the probability to damage the underlying impermeable layer. In addition, it might extent the useful life of the reactor by cleaning it when clogged, and the potential use of recycled plastic would improve the sustainability of the process.
To verify the possibility of using a lightweight plastic material to replace heavy gravel, an unplanted pilot scale treatment system, composed of four independent treatment units in parallel was implemented. The treatment units differed in the filtering media and the input flow regime. Two of the treatment units used gravel (Specific surface 305 m2/m3, 1475 kg/m3 apparent density) and two units used plastic material (Specific surface 750 m2/m3, 172 kg/m3 apparent density).
To check the incidence level of passive aeration procedures, on the effectiveness of each material, two of the treatment lines used a continuous flow system and two of them used an automatic filling and emptying flow method that allows passive aeration of the support media.
A mixture of landfill leachate and septic tank wastewater was treated, and the evolution of turbidity, chemical oxygen demand, nitrogen and phosphorus was monitored. Results showed that, in general, there are no significant differences regarding the performance of the materials tested, whereas passive aeration notably improves the abatement and solids retention performance of the pilot units.
It is concluded that the plastic material tested can be used as a replacement for the stone material, without having appreciable losses in the efficiency of the system. Further research is needed to quantify the benefits associated with the use of this support in constructed wetlands-type technologies.
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