The growing need for food and the increase in agricultural production, and consequently the increasing demand for water on the one hand, and the harmful environmental effects of agricultural drainage water on natural ecosystems and receptive water resources on the other, have led the world to look for methods and techniques that, along with reducing the harmful and adverse consequences of drainage water, make it possible to reuse them to meet part of human needs. In many areas facing water shortages for irrigation, drainage water is used to meet the crop's water requirement. Due to high volume and special quality, agricultural drainage water have a high potential for pollution in the environment, especially water resources. According to water shortage in Iran and population growth and increasing water needs in various uses, planning for the protection and quality management of water resources and optimal use of unconventional water, especially agricultural drainage water as alternative water resources has become more necessary.

For this purpose, the present study investigated the efficiency of a natural reed bed on a real scale in qualitative treatment of incoming drainage water of Khuzestan sugarcane during a period of one year (2010-2011). According to the trend of qualitative changes and wastewater treatment stages at the reed bed level, the length of the reed bed was divided into three parts, namely three stations ST1, ST2, and ST3 with a length and width of 3.5*1.2 hectares, respectively. The efficiency of the natural reed bed was evaluated by measuring parameters such as BOD, COD, PO4, and TP.

The results of the T-Test showed that there are significant differences in terms of BOD values between ST0-ST1, ST0-ST2, ST0-ST3, ST1-ST2, ST1-ST3, and ST2-ST3 study stations. The highest difference in BOD was observed between ST0 and ST3 stations equal to 6.96 mg/l. In addition, the lowest difference in BOD was detected between ST0 and ST1 equal to 1.73 mg/l. At the ST3 station, which is the farthest from the entry point, the highest percentage of BOD removal was obtained. There was a significant difference of 1% for COD between study stations. The largest difference in COD was observed between ST0 and ST3 stations equal to 9.62 mg/L followed by ST1-ST3 stations equal to 7.12 mg/L. There was also a significant difference of 1% levels in PO4 and TP of study stations and the highest difference in PO4 and TP with 0.2724 and 0.3790 mg/l, respectively, between stations. ST0-ST3 was observed. The results showed that the performance of the reed in eliminating the studied parameters was very good and acceptable. The removal efficiency of BOD, COD, PO4, and TP to the last station was noticeable and acceptable in all four seasons. Under different retention times (1.26, 1.10, 1.30, and 1.60 days), the removal percentage of BOD, COD, PO4, and TP was significant in ST1, ST2, and ST3 stations and with increasing the distance from the entry point, of the drain, the reed efficiency increased. Considering the different performances of the three stations in improving the quality of drainage and considering the efficiency target above 50% of all organic matter and phosphorus compounds, the ST2 station can be assumed as the optimal limit in terms of efficiency and cost.

In general, this system can be used as an efficient system in reducing common drainage water pollutants to the level of drainage water treatment standards and acceptable reduction of BOD, COD, PO4, and TP parameters of agricultural drainage water and improving the quality of this drainage water for discharge to surface and groundwater and also reuse for irrigation in agricultural fields.