## 2 Sludge Treatment Reed Bed System and Industrial Sludge  
### Description  
STRBs are an established nature-based technology for sludge treatment. The concept of an STRB is similar to that of a vertical flow constructed wetland [1; 25]. Most of these systems are found in Europe and the USA, e.g., in France, Denmark, Germany, Poland, and Sweden [26; 27; 28; 29]. Almost 20% of the produced sludge in Denmark is treated in STRB systems [8; 11], mostly in villages and small cities with populations of up to 10,000 inhabitants, though there are large-scale systems (serving over 100,000 inhabitants) as well. Lately, this sustainable technology has been introduced into various Mediterranean countries, such as in Greece, Italy, and Spain [30; 31; 32; 33; 34], as well as in China [35] and the Middle East [36], due to the favorable, i.e., warmer, climates. An STRB is a concrete or trapezoidal earthen basin [32; 37], the bottom of which is typically covered by a low-permeability sealing material, such as a high-density polyethylene (HDPE) geomembrane, to prevent leaching to the underground formations and the groundwater. The basin is filled with gravel and sand of different sizes and thicknesses [1], while on top of the substrate layer, local wetland plant species are planted, such as common needs or cattails (_Pltragmites australis_ or _Typha tafifolia_). A sludge distribution pipe network spreads the feed sludge across the surface of the bed, where sludge is dewatered through passive vertical drainage and evapotranspiration (Figure 1) [16; 37; 38]. A drainage pipe network is placed into a cobble (i.e., large stone) layer above the geomembrane and collects the drained water. The drainage network is connected to pipes that facilitate the passive aeration of the bed, which has been found to be beneficial for the dewatering performance of the system [1; 37; 38; 39]. Vegetation and passive ventilation create favourable conditions for the conversion of degradable organic matter to a more stable humic form [33; 40]. The dewatering process along with organic matter mineralization results in the reduction of the sludge volume, and the accumulated residual sludge is continuously incorporated into the filter media layer where the plants are estab  
Figure 1: Schematic cross-section of a sludge treatment reed bed (STRB) system indicating the sludge layer, the porous media layer, the feeding pipes, and the drainage network (courtesy of Orbicon). the depth of the mineralized residual sludge layer increases at an approximate accumulation rate of 10-15 cm/year, which depends on the quality of the feed sludge [1]. Typically, after 8 to 12 (even up to 20) years of continuous operation, the layer of the dewatered and also mineralized residual sludge is removed from the basins and then recycled as a fertilizer or as a soil conditioner [8; 16; 37; 41; 42]. There is a wide range of sludge types that can be dewatered in STRBs such as activated sludge, digested (anaerobic) sludge, and sludge from waterworks (WW). Experience has shown that STRBs can treat sludge of varying qualities, possessing a comparable or even higher dewatering capacity than conventional dewatering methods. The effective dewatering and operation of an STRB depends on the quality of the feed sludge, the climatic characteristics, and proper design and construction [1]. The sludge loading rate (SLR; kg ds/m\({}^{2}\)/year) represents a key parameter in STRB design, and mostly depends on the sludge quality and the climate [1; 37]. Usually, a first pilot testing phase is recommended before the implementation of a full-scale STRB to determine the dewatering ability and efficiency, especially when an industrial sludge is to be treated. A typical pilot study would consist of 3-12 beds, each with an area of up to 2 m\({}^{2}\) (or more). A typical testing period lasts 4-12 months, although there are pilot experiments operated for up to 3 years.