Potential of Horizontal Subsurface-Flow Constructed Wetlands for Polishing of Treated Sewages

Abstract

Constructed wetlands (CWs) are increasingly being applied for wastewater treatment and as pretreatment before artificial recharge of the effluents. Three horizontal subsurface-flow CWs were operated in parallel for almost 7 months and fed with three types of treated effluents to analyze enhancement of water quality at an average hydraulic retention time (HRT) of 1.3 days. The wastewater effluents analyzed in this study were an on-site anaerobically pretreated sewage at an Al-Mazra’a house, tertiary effluent at Al-Bireh City, and secondary effluent at the Birzeit University campus in Palestine. The average dissolved organic carbon (DOC) removal of 48, 50, and 47%, chemical oxygen demand (COD) removal of 55, 45, and 50%, biochemical oxygen demand (BOD) removal of 57, 38, and 60%, ammonia-nitrogen (NH4+-N) removal of 96, 90, and 97%, nitrate-nitrogen (NO3−-N) removal of 88, 94, and 93%, total nitrogen (TN) removal of 72, 70, and 71%, phosphate (PO43−) removal of 63, 61, and 57%, total suspended solids (TSS) removal of 37, 41, and 42%, and fecal coliform (FC) removal of 90, 85, and 95% were achieved with the CWs of the Al-Mazra’a, Al-Bireh and Birzeit effluents, respectively. The dissolved solids in the effluent of the three investigated CWs were increased. The total dissolved solids (TDS) and electrical conductivity (EC) of the (influent; effluent) of Al-Mazra’a, Albireh, and Birzeit were (337, 366 mg/L; 680, 737 μs/cm), (327, 351 mg/L; 658, 695 μs/cm), and (299, 326 mg/L; 603, 653 μs/cm), respectively. The CW was efficient in terms of NH4+, NO3−, and BOD removal, and achieved the Palestinian standards for treated effluent reuse and discharge to wadis for recharge, despite the high evapotranspiration of approximately 24%. The maximum achieved specific removal rates of phosphorous (P), nitrogen (N), and BOD were 2,211, 15,329, and 5,980 kg/ha/year, respectively. The study clearly showed that CWs have a high potential for further polishing treated effluents in both urban and rural arid areas of Palestine, such as Al-Bireh and Ramallah Cities and the adjacent villages, while achieving the double goal of environmental protection and environmental restoration.