**Abstract**

Disposal of sewage sludge, which is a by-product of wastewater treatment, has become one of the greatest challenges of the twenty-first century. Conversion of sewage sludge to a soil amendment can be performed by a broad spectrum of methods, which greatly differ by substrate/amendment composition, treatment time, and physicochemical conditions. The book chapter is focused on (i) environmental and legislative aspects of sewage sludge application in agriculture; (ii) risk factors related to the abundance of pathogens in sewage sludge and methods of SS hygienization; (iii) optimization of the use of SS-derived fertilizers. Application of sewage sludge in combination with mineral fertilizers positively influenced crop growth and soil microbiological activity. An environmental impact of sewage sludge related to its disposal to agricultural areas has been analyzed in terms of global warming, ecotoxicity, and other internationally recognized issues. Narrowly targeted measures may aggravate the situation. Some site-specific factors make sewage sludge unique, hence this specificity must be considered to predict the outcome of its treatment. Determination of these factors remains challenging. Therefore, the complexity of sewage sludge can be reduced by employing integrated biorefinery approaches that will result in circular bioeconomy and industrial ecology solutions.

**Keywords:** circular economy, fertilizer, life-cycle assessment, plant growth, sewage sludge hygienization
