The different units' performance and bed length effect of constructed wetland on the removal of organic substances, solids and nutrients from municipal wastewater: a case study, Iran

This study investigated the overall performance and bed length effect of the horizontal subsurface flow constructed wetland (CW) process for full-scale municipal wastewater treatment. 168 sam-ples were collected, including 84 samples in the hot and 84 in the cold season. Seven sampling points were selected as follows: (1) inlet raw wastewater, (2) effluent from an anaerobic pond, (3) from the first 5 m of the selected reed bed, (4) 5 m second, (5) 5 m third, (6) 5 m fourth, and (7) 5 m fifth from the reed bed. The evaluation parameters included biochemical oxygen demand (BOD5), chemical oxygen demand (COD), total suspended solids (TSS), total nitrogen (TN), and total phosphorus (TP). The results showed that the removal efficiency for the BOD5 , COD, TSS, TN, and TP was equal to 90.6%, 90.1%, 94%, 50.2%, and 35.4%, respectively. In the final treated effluent, the concentration of all studied parameters was less than the maximum allowable amount for discharge to the receiving water. The results showed that although the treatment plant's pol-lution load for BOD5 and COD parameters in winter was significantly higher than in summer, the treatment plant efficiency in winter was not significantly different from summer. The results also showed that a 15-m bed in the CW system is suitable for achieving the standard of the environ-mental protection organization of Iran for municipal wastewater treatment in the hot season; how-ever, it seems not to be enough in the cold season. So, 25 m of reed bed length will be required in practice. Finally, it can be concluded that the Qasr-e-Shirin wastewater treatment plant has the proper capability to remove municipal wastewater pollutants in different temperature conditions and the quality of the final effluent indicates the accuracy of the design criteria for this system.