## 1 Introduction
There is limited study of the fate of various microppollutants and emerging contaminants in STRBs. STRBs have been found to be capable of removing some pharmaceuticals from domestic sludge (e.g., diclofenac [18] and antibiotics, i.e., ciprofloxacin and azithromycin [19]) and antibiotic-resistant genes [20, 21], but generally, there is not sufficient information yet regarding the behaviour and performance of STRBs in the removal of such micropollutants in STRBs treating domestic and/or industrial sludge. STRBs are also much more efficient in the degradation of hazardous organic compounds, such as linear alkylbenzene sulfonate (LAS), nonylphenol polyethoxylates (NPE), di-(2-ethylhexyl)phthalate (DEHP), and certain polycyclic aromatic hydrocarbons (PAHs) than conventional methods [9, 11, 12]. These are important aspects for sludge reuse; for example, the Danish Environmental Protection Agency (DEPA) and the EU have regulated the contents of nutrients, heavy metals, and hazardous organic compounds in sludge spread on agricultural land (Table 1; [9, 11]). Generally, experience has shown that STRBs can treat most types of sludge with ds contents between 0.1 and 5% [1]. Some sludge types, especially those of industrial, i.e., non-domestic, sources may, in general, be difficult to dewater and even unsuitable for an STRB system, e.g., sludge rich in oil and fat. Industrial sludge is considered a more difficult application and technically more challenging for effective dewatering, since it may contain heavy metals, nutrients, hazardous organic compounds, oil, and fats at much higher levels than typical sludge of domestic origin; thus, its treatment, dewatering, and/or disposal can be problematic considering the legal requirements. This is why the vast majority of STRB facilities worldwide are designed for the dewatering of domestic sludge. On the other hand, there is an intensively increasing worldwide interest in the application of constructed wetlands (or need beds) technology for industrial wastewater treatment [24], which also encompasses the need bed system for sludge dewatering. Thus, over the last few years, there has also been a respective increase in research and development interest in the application of this green technology for industrial sludge dewatering. This gradual shift of research interest to industrial waste comes as a result of the proven high treatment efficiency of this natural treatment technology and the realization of the actual treatment capacity it possesses [13; 24], as well as of the better understanding of its multiple environmental, technical, and economic benefits and the desire for sustainable options in the global water/wastewater industry. However, as this tendency is relatively new, most of the available studies and reports in the international literature on STRBs deal with sludge from domestic WWTPs. The published literature on the feasibility and efficiency of industrial sludge treatment in STRB is very limited, and only very few case studies (mostly pilot projects) have been presented at international conferences or reported in journal articles and/or book chapters. This means that there is gap in the respective literature not only due to the small number of studies but also due to the fact that this information is scattered and not easily accessible. Therefore, the main goal of this study is to cover this literature gap and summarize the current status and state-of-the-art knowledge and experiences regarding the design, operation, and efficiency of STRBs for industrial sludge dewatering. For this, the available published literature is reviewed and summarized while new case studies are presented in the first single and comprehensive overview paper on this specific application of STRB technology. Thus, this paper presents an overview of STRB design and operation and briefly describes the concepts and the performance of the few existing