Analysis of the current state of sewage sludge treatment from the perspective of current European directives

Due to its produced quantity, sewage sludge is a serious environmental problem not only in Slovakia, but also on a global scale. The submitted study deals with the current state of sewage sludge treatment with a focus on assessing the advantages and disadvantages of selected technologies. The main objective of the review study is to evaluate the advantages and limitations of individual sewage sludge processing technologies. Based on the evaluation of the advantages and limitations, another objective of the study was to determine the most suitable, less suitable, or unsuitable technologies for sewage sludge processing. Another objective of the study was to evaluate innovative technologies (not only commonly used ones). The purpose of this objective was to monitor the most current trends in sewage sludge processing technologies and, by evaluating them, to expand knowledge in this scientific field. The presented analysis of sewage sludge treatment was carried out in accordance with current European directives. The main novelty of the review study is a comprehensive assessment of sludge processing technologies with a focus on their main advantages and limitations. Another novelty is the focus of the study on innovative methods of sewage sludge processing. As part of the analysis, the evaluated sludge treatment technologies were divided into three categories: conventional, alternative, and innovative. As a conventional method, composting is currently the dominant method of sewage sludge management in the Slovak Republic. Mono-incineration, which is one of the conventional methods, allows for an effective reduction of the volume of sewage sludge while removing all unwanted hazardous components at the same time. However, this sludge treatment technology causes simultaneously the loss of most of the valuable nutrients, contained in sewage sludge. Pyrolysis and gasification as alternative technologies are effective in terms of removing hazardous components, but they are complex technologies, and their economic viability depends on their ability to maximise efficiency and produce high-value energy carriers. Hydrothermal liquefaction has a major advantage over all other thermal sewage sludge treatment processes because it does not require sludge drying. Innovative methods of sewage sludge treatment such as bio-electrochemical system, supercritical water oxidation, and biodiesel production require a more detailed examination by the scientific community to implement these methods in practice due to the lack of information from operational experiments. Given the extensiveness of the sludge treatment issue, we plan to address the economic aspect of the technologies in a separate study in the future.