Comparison of different substrate media on the performance of Vertical Flow Constructed Wetlands

Vertical Flow Constructed Wetlands represent the state of the art in the field of ecological engineering and technology. These systems have been proved to be an effective and economical solution for wastewater treatment. However, further research on optimization of their performance is necessary, mainly because of the lack of studies in pilot- or laboratory-scale systems, where various parameters could be better examined through controlled experiments. With this in mind, five small-scale, cylindrical, vertical flow constructed wetlands have been designed, constructed and are currently operating in the laboratory, as part of a larger experiment containing ten units. Each unit has a drainage layer, 15 cm thick, made of cobbles, placed at the bottom. Different porous media were used in the five pilot-scale units to fill a layer of medium gravel, placed above the cobble layer: one unit contained carbonate rock obtained from a quarry, another contained igneous material obtained from a river bed, another contained carbonate material 50% mixed with natural zeolite, another unit contained 50% bauxite and the 5th unit contained zeolite. All units were planted with common reeds (Phragmites australis) and were fed with simulated municipal wastewater. Experiments started in early autumn 2008. Water samples were collected from the influent and from the effluents, and were analyzed in the laboratory for the determination of BOD5, COD, TKN, ammonia nitrogen (ΝΗ4+-N), nitrite and nitrate, total phosphorus (TP) and ortho-phosphate (PO4-3-P). Overall mean performance in all pilot-scale units was 73.1% for BOD5, 68.7% for COD, 50.7% for TKN, 47.4% for ammonia nitrogen, 37.2% for TP and 39.3% for PO4-3-P. Results showed that the use of substrate materials with specific characteristics, such as bauxite and zeolite, can enhance the removal of particular constituents (nitrogen or phosphorus, respectively). However, this improvement is not spectacular in vertical flow systems, because the contact time between the wastewater and the substrate is relatively short, which does not favor the adsorption onto substrate surface.