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INTRODUCTION
Many technologies for sewage sludge dewatering are available. They range from sophisticated technologies such as centrifugation and filtration using filter presses to simpler ones such as evaporation using drying beds (Metcalf & Eddy 2003) as well as reed bed sludge drying systems (Nielsen & Larsen 2016). Each technology has its advantages and disadvantages that may limit its use. The weather conditions and the simplicity of operation make drying bed technology the main technology used for sludge dewatering in Egypt. However, drying beds require a huge land area, which may pose an obstacle in expanding the existing treatment plants for future needs. This is considered a problem in areas where the available land is limited. Therefore, there is a need to find a solution to increase the ability of the existing drying beds, to accommodate the capacity needed for sludge dewatering in future expansions of wastewater treatment plants without using additional land area. The use of plant-assisted drying beds (Nielsen & Larsen 2016) seems to be an ideal solution for increasing the capacity of the existing drying beds.
Sewage sludge also carries different types of pollutants, since many wastewater treatment plants receive discharges from residential areas as well as industrial areas (Bright & Healey 2003; Dai et al. 2007). Some of these pollutants, such as heavy metals, are mobile, non-biodegradable, and become toxic at certain levels. The presence of heavy metals in sewage sludge may inhibit its application as a soil amendment, since metals tend to accumulate along the food chain (Dudka & Miller 1999; Amir et al. 2005). Based on investigations of pilot and field-scale systems, the technology of constructed wetlands employing aquatic plants has proven to be a promising alternative for wastewater treatment (US EPA 1988). This technology has been characterized by low investment, operation and maintenance costs (Cooper et al. 1996; Kadlec & Knight 1996). For several years, a number of constructed wetland systems have been employed to treat various kinds of wastewaters, including sewage sludge, from conventional treatment plants (Heinss & Koottatep 1998). They have the ability to efficiently remove suspended materials and nutrients from wastewater (El Zawahry & Kamel 2004). In addition, aquatic plants can reportedly be used as natural catalysts to absorb and accumulate heavy metals in their tissues...