**1. Introduction**

Sewage sludge (SS) is formed as a by-product at a wastewater treatment plant (WWTP) and represents a heterogeneous mixture. This complex suspension consists of solid organic and inorganic substances and colloids, which have been separated from the wastewater during the treatment process [1]. The global production of SS is estimated at 45 million t of dry matter per year [2, 3]. During the last decade the SS production in EU countries increased by 1.5 million t of dry matter (DM), that is,

from 11.5 million t in 2010 to 13 million t in 2020 [3], therefore, its management is a problem of great concern. The SS disposal reaches up to 60% of the total operating costs of WWTP, and, hence, makes this process problematic and expensive [4].

Sludge from WWTP is recovered by compost production, the application directly to agricultural and forest land, production of growing substrates, and energy recovery [5]. For practical and legal reasons, SS is increasingly reused rather than landfilled. This approach aims to minimize generated waste and promote the development of the bioeconomy that provides intelligent waste management, and, hence, is consistent with zero-waste strategy [3, 6]. Different countries have chosen different strategies for the use of urban SS. Analysis of the Eurostat data in the period from 2014 to 2018 showed that the use of SS in agriculture, in combination with compost, had been the main route for sludge disposal in the EU with 44.58%, followed by incineration (32.70%) and other methods of disposal (9.16%). Landfill disposal was at the level of 7.81%. Comparing the costs of different sludge disposal methods, the application on land and agriculture involves the lowest cost compared to composting, drying, incineration, and landfill.

At WWTPs, with more than 10,000 inhabitants, the sludge is divided into primary and secondary sludge. The primary sludge contains settling substances (from primary settling tanks), usually, it has a granular structure. Secondary sludge, also called excess sludge, consists of a mixture of microorganisms and settable substances from the biological stage of the WWTP. Primary sludge and secondary sludge are referred to as so-called raw sludge. The raw sludge is still microbially active, it can contain pathogenic microorganisms, with the total content of organic substances in the dry matter at about 70%. However, dewatered sludge (20–45% DM) is considered harmless and suitable for agriculture, because of high content of organic matter and biogenic elements (C, N, P), which increases soil fertility and is essential for plant growth and development as well as for soil microbiota [1]. Therefore, the use of SS on agricultural land is the best way to recycle the nutrients it contains, thus making the SS an important biological resource for sustainable agriculture [7–9]. On the other hand, the application rate is of great importance. Excessive concentrations of plant nutrients, mainly nitrogen and phosphorus, can also harm the environment, especially inland waters.

Another important issue is related to the abundance of hazardous and very persistent substances, such as heavy metals, polycyclic aromatic hydrocarbons, polychlorinated biphenyls, halogenated hydrocarbons, polychlorinated dibenzop-dioxins and dibenzofurans, pesticides, personal care products, hormonal substances, drugs and their metabolites, microplastics, and nanoparticles [8].

Therefore, the incorporation of sludge and its compost in the soil is regulated by various legislative acts [9]. The annual emission limit values for dry matter, heavy metals, total nitrogen, and total phosphorus are the maximum mass of these substances that can be applied per hectare of sludge or compost on average per year. Emission limit values for sludge dry matter vary considerably between the EU Member States, ranging from 1 to 10 t ha−1 per year. According to Mercl et al. [10], a high rate of SS composts applied once (60 t ha−1 compost in seedbed) is not recommendable since high nitrate concentration is not taken up by maize and increases the leaching risk. Furthermore, SS commonly contains high amounts of human pathogenic bacteria excreted in feces and urine, so the SS should be appropriately hygienized before application in agriculture.

The aim of this chapter was to summarize the main aspects of SS treatment for its application in agriculture, with emphasis on process efficiency, safety, and feasibility. *Application of the Sewage Sludge in Agriculture: Soil Fertility, Technoeconomic, and Life-Cycle… DOI: http://dx.doi.org/10.5772/intechopen.104264*

The dual role of SS as a fertilizer and amendment in the soil is widely described in the literature, referring to the supply of nutrients to plants and improving the soil's physical conditions, respectively. Our own results on SS treatment have been incorporated into the review of recent scientific literature and legislative documents.
