Wastewater treatment by reed beds. An experimental approach

Abstract The present study consists of assessing the efficiency of a macrophyte (Phragmites australis) system in wastewater purification in an arid climate. The experimental system is made up of three beds differing in length (30, 40 and 50 m) and implanted with Phragmites australis. Wastewater inflow through these beds is horizontal with a flow of 10 l.s−1. Retention time varies between 1 and 4 h. The frequency of irrigation is weekly. The studied parameters are: the organic load (COD and TSS), nutrients (total Kjeldhal nitrogen: TKN and total phosphorus: TP) and the parasitical load (helminth eggs). Water, soil and plants are the three components of the experimental system we analyzed. Results reveal that in the output, the decrease in organic load, nutrients and parasitical load is very sharp in the large bed (COD: 62%, TKN: 43%, TP: 14%, helminth eggs: 93%). However, in the small bed (30 m), the decrease is slight (COD: 48%, TKN: 23%, TP: 5%, helminth eggs: 88%). Concerning parasitical load, the study shows that the three beds are effective in eliminating helminth eggs, even with a very short retention time (1–4 h). The quality of the effluent of the reed bed system should be classified within category B according to WHO guidelines (1989) for crops irrigation. Soil sample analysis from entrance to exit shows a decreasing gradient of nitrogen (N), phosphorus (P), organic carbon (Org.C), organic matter (OM) and parasitical helmith eggs. The gradient becomes more significant in the largest bed with decreasing rate, reaching respectively 49%, 44%, 37%, 40% and 100% for TKN, TP, Org.C, OM and helminth eggs. As for the plant material, the composition of major elements (nitrogen and phosphorus) of Phragmites aerial parts shows the same evolution. While the decreasing rate of nitrogen in the leaves reaches 50% in the largest bed, it does not exceed 8% in the smallest bed. Mean aerial productivity of the studied reed beds was estimated at 134 tons dry weight.ha−1. The aerial biomass exported contains only 5–6% of nitrogen and 10–12% of phosphorus with regard to their load at the influent.