## 1 Introduction  
Wastewater treatment processes typically result in the production of a by-product material, known as sewage sludge. Sewage sludge is produced at various points along the treatment train of wastewater treatment plants (WWTPs), namely in the primary sedimentation stage, the biological stage (e.g., aeration tanks), and the secondary sedimentation stage. Hence, depending on the treatment stage, sludge can be characterized as primary, secondary, biological, mixed (a mixture of primary and secondary sludge) or tertiary (from tertiary or advanced treatment stages) [1]. As any wastewatersource, sludge can be originated from households, commercial and municipal areas, industrial facilities, agro-industries, surface runoff and stormwater. Sludge production is continuously increasing following a respective increase in the population numbers connected to centralized WWTPs and the adoption of stricter environmental legislation [2]. Typical sludge production in activated sludge plants, i.e., the most widely applied wastewater treatment method, is up to 2.5 kg per individual per day [1]. This means that, for example, for a population of 100,000 inhabitants, the generated sludge would reach 250 t/day. Municipal sludge was produced in the European Union (EU) at an annual rate of 8 million tons in 2016 [3,4], a figure that is estimated to further increase and reach 13 million tons of dry solids (ds) this year [5]. Although the sludge volume represents a very small fraction (typically less than 1%) of the total wastewater volume that is treated at a WWTP, the overall cost for sludge management and handling could account for up to 50% of the total operation costs of the WWTP [1]. In addition, considering the circular approach based on which all waste should be viewed as a valuable resource rather than as waste [5], sustainable sludge management options are now required. Moreover, the target of eliminating landfilling or energy-consuming incineration means that alternative eco-friendly solutions are in focus [5]. Various technologies have been applied for sludge dewatering, such as mechanical treatment (i.e., decanters, centrifuges, and screw-presses) and simple sand drying beds in warmer climates, as have environmentally friendly solutions, such as sludge treatment reed beds (STRBs), also known as sludge treatment wetlands [1]. Sludge treatment in reed beds/constructed wetlands was developed in the late 1980s and today represents a cost-effective and easy-to-run nature-based solution for the dewatering and stabilization of sludge that can be applied at small and also at large scales [6, 7, 8]. Comparing STRBs with mechanical dewatering systems and sludge disposal, STRB technology typically has slightly higher investment costs but significantly lower operation costs. A life cycle cost analysis and comparison of STRB and mechanical systems for the dewatering of activated sludge of 550 t ds/year revealed that the STRB system is a cost-effective alternative [9,10]. As an eco-friendly wetland technology, STRBs do not require the use of polymer coagulants for the dewatering process, and due to the small amount of energy input needed, the absence of complex electro-mechanical equipment, and the use of natural materials and processes, STRBs as all treatment wetland systems are viewed as a green treatment technology with a minimal carbon footprint [1, 10, 11, 12, 13]. STRB technology is widely applied mostly for municipal sludge dewatering in Europe. For example, this technology has been used in Denmark for almost 30 years. Sewage sludge contains organic matter and nutrients; hence, it is widely used as a soil amendment, as a fertilizer in agriculture, and in other environmental applications (e.g., forestry and land reclamation) [8, 14, 15]. However, sewage sludge may also contain contaminants such as micropollutants, emerging contaminants, and trace elements (e.g., heavy metals and pathogenic microorganisms) that may have an adverse environmental impact [14, 16, 17].