**4. Conclusions**

Municipal wastewater is a product of human life. With the help of natural processes, man has created suitable conditions for its biological treatment. Generated sewage sludge is of biogenic origin, its production is dynamic, and that is reflected in seasonal fluctuations of its quality and quantity. The results of dried sludge granules analysis over a 10-year period of operation at the CWWTPL are presented, as well as the year 2020, which is the starting year for the upcoming new ten-year period. 2020 is also a control year for many forecasts of improving the environment and preventing climate change. The data provide a comprehensive insight into the properties and potentials of treated sludge and allow thoughtful studies for use of this smart material in WtE, critical raw material recovery, organic matter recovery with pyrolysis, and agricultural use of the solid pyrolytic product or as raw granules.

The sewage sludge composition during the period 2010–2020 is similar, only 2010 differs markedly, which was characterized by heavy floods. They did not spare the capital of Slovenia, when a lot of urban area were under the water. Normal changes of anthropogenic or meteorological behavior do not have any special effect on the average annual values of granules. Within a year, these values fluctuate more markedly due to the influence of wastewater temperature on the properties of activated sludge and consequently on the final quality of granules.

Knowledge of the sewage sludge properties enables quick adaptation to the requirements of the revised legislation and, above all, prudent use. The chapter brings together and comments on existing legislation and also legislation in preparation. The data make it possible to compare the quality of granules with other UWWTPs and confirm the feasibility of the drying technology of dehydrated digestate.

For energy recovery, sewage sludge must be used in technically advanced facilities to prevent harmful emissions of substances into the air and prevent the circulation of harmful substances in nature.

For the use of sewage sludge in agriculture, it is important to emphasize that this is a material that would make sense to add to the mixture of components for fertilizer and not just as a stand-alone substrate, while controlling the quality of the soil and that the application to the soil is in the form of a dynamic approach. Annual application amount, soil quality control, seasonal application period, and type of crop must be planned. With a dynamic approach, it is necessary to prevent the limit values for polluting substances per hectare of agricultural land from being exceeded.

Pyrolysis is a specific thermochemical decomposition of biomass into a range of useful products without oxygen. Pyrogenic carbonaceous materials are by mass the major pyrolytic product of treated sludge, but it has not yet been included in the EBC feedstock list and is therefore not subject to the EBC guidelines.

The study on the water solubility of generated residues after thermal treatment of granules shows that pyrolysis offers a much greater opportunity for material recovery from sewage sludge. It was found that the water solubility of phosphorus in biochar produced at 450°C is higher compared to the residues from the oxidizing atmosphere; in contrast, the water solubility of K and Mg is lower.

Without good sludge quality control, a sustainable strategy for the use of treated sludge in the CE cannot be achieved. To have confidence in the results of research into the properties of sewage sludge, it is necessary to have a good sampling procedure, targeted end-use, and a list of required parameters. Only high-quality analysis enables confidence in the results and the setting of optimal guidelines for a successful CE. Such an approach to sewage sludge quality control improves the public acceptance of this material in the CE.

The scope of analysis to determine the properties of sewage sludge can be reduced if the quality of sewage sludge is well known and the technology of municipal wastewater treatment and its quality does not change significantly.

### **Author details**

Barbara Novosel<sup>1</sup> , Vesna Mislej<sup>2</sup> \* and Viktor Grilc<sup>3</sup>

1 UL, Faculty of Chemistry and Chemical Technology, Ljubljana, Slovenia

2 JP VODOVOD KANALIZACIJA SNAGA d.o.o., Ljubljana, Slovenia

3 Environmental Protection College, Velenje, Slovenia

\*Address all correspondence to: vesna.mislej@vokasnaga.si

© 2022 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

*Basic Morphological,Thermal and Physicochemical Properties of Sewage Sludge for Its… DOI: http://dx.doi.org/10.5772/intechopen.101898*
