**2. Characteristics of sewage sludge as a potential soil amendment/ fertilizer**

Sludge is rich in organic matter, nitrogen, phosphorus, and other macro and microelements, which makes it a useful raw material to be used in agriculture. Dry SS contains on average 50–70% organic matter and 30–50% mineral components [8]. Physicochemical and biological characteristics of agricultural soils, which are amended with the organics-rich SS, can be considerably improved. Particularly, a reduced bulk density leads to an increased soil porosity and soil-air recirculation, as well as improved soil structure and water holding capacity. Besides, the concentration of soil humus is increased. Organic matter of SS enhances soil nutrient storage, soil biota, and diversity, as well as reduces exposure to erosion. High organic matter content facilitates the formation of stable organic complexes with humic acids, thus reducing metal availability [11]. A slow release of mineral elements from SS to soil also changes the physical, chemical, and biological parameters of soil and benefits from increased gas exchange, better water infiltration, and its retention. The compounds of SS are available for a longer period [12].

Mbagwu and Piccolo [13] found that the decomposition of organic materials in sludge enhances the availability of nutrients such as nitrogen and phosphorus substantially. Application of SS at a rate of 200 t ha−1 increased the total nitrogen of soil aggregates by 57% and available phosphorus by 64.2%.

The formation of organic and inorganic acids throughout the decomposition process of SS components under aerobic conditions increases soil acidity. Soil salinity positively correlates with the increased application rate of SS. Amendment of loamy-clay soil with SS at dose 60 t ha−1 increased soil carbon content from 0.16% to 1.45% [14].

Comparison of physicochemical characteristics of SS of different origins showed that average concentrations of nitrogen, phosphorus, and potassium are similar and reach up to 3.20%, 1.75%, and 0.5% per kg of treated dry SS, respectively [15–17]. Often the potassium content in SS is considered insufficient for plant nutrition [8]. Some studies indicate that SS is an efficient replacement for chemical fertilizers, especially phosphorus. Indeed, Switzerland, Germany, and Austria are developing legislation to make P recovery mandatory from municipal SS [18, 19].

Nevertheless, there are some site-specific factors (e.g., applied technology, quantity, and the origin of raw wastewaters, which differ by the composition of macroand microelements and risk compounds), that make each SS unique, hence this specificity must be considered to predict the outcome of SS treatment. Determination of these factors remains challenging [20].
