Phytoremediation of PAHs-contaminated Sludge in a Constructed Wetland

Wastewater sludge from metropolitan areas has become a source of polycyclic aromatic hydrocarbons (PAHs), and may result in pollution of soils and plants if applied to agricultural areas. It is important to stabilize wastewater sludge before its application to agricultural land. Constructed wetlands (CWs) have been widely applied for stabilization of sludge. In CWs, plants, especially reeds, promote degradation of pollutants in the rhizosphere and are the principal mechanisms for PAH-contaminated sludge phytoremediation. We studied PAH removal in sludge planted with Phragmites australis (common reeds) in onsite experiments over a 3-year period comprising 2 years of sludge application and 1 year of resting. The average PAH content in the feeder sludge was 5.7 mg kg–1 (DW), consisting of approximately 48% low-molecular-weight (2–3 ring) PAHs, 27% middle-molecular-weight (4 ring) PAHs and 25% high-molecular-weight (5–6 ring) PAHs. After 3 years, the average PAH content in stabilized sludge was 2.1 mg kg–1 (DW), which was lower than that of the feeder sludge. Overall, about 66% of low-molecular-weight PAHs (dominant type), 57% of middle-molecular-weight PAHs and 32% of high-molecular-weight PAHs were removed from the stabilized sludge. Additionally, there were spatial variations in PAHs in stabilized sludge, with PAHs being highest in the surface layer (12–14 cm) sludge, lower in the middle layer (6–8 cm) and lowest in the bottom layer (0–2 cm).