**9. Future direction, challenges, and scope of sewage sludge as a soil fertilizer**

The efficient use of waste-derived fertilizers in agriculture needs more empirical knowledge on markets with further research focused on variability, interactivity, and uncertainty. The site-specific factors (e.g., applied technology, quantity, and the origin of raw wastewater differed by the composition of macro- and microelements and risk compounds, soil types, and crops) make each SS unique, hence this specificity must be considered to predict the outcome of SS treatment. New efficient technologies for onsite sludge disinfection are necessary and urgent. Interdisciplinary activities on the safe use of SS upon treatment and application need to be thoroughly analyzed and developed, for example, planning, servicing, diagnosing, storing, and others. Furthermore, macroeconomic factors can considerably influence technology stocks. Soaring gas prices directly affected the production of synthetic fertilizers costs. High prices as well as the disruption of transport and production logistics lead to a real threat of a dramatic reduction in supply on the mineral fertilizers market. Combined with higher prices, an increase in demand for less volatile organic fertilizers can be expected.

### **10. Conclusions**

Summarizing our experimental data on optimization of SS treatment and its application in agriculture, as well as recent findings of other authors in this field, the following conclusions were drawn:

The technology, which was newly developed by Earth Revival Ltd., offers an innovative and comprehensive solution to the problem of SS disposal and soil degradation, which includes aspects of agriculture, healthcare, epidemics, ecology, economics, and the social sphere. Costs can be recuperated through sludge treatment service fees and fertilizer sales.

The infrared heating system, used for SS disinfection, has shown consistently successful results. For the material with a humidity of 80%, it took 15 min, considering the heating time of the layer to 95°C, which is below the temperature of the organic matter decomposition. Research and experiments related to the neutralization of the spore-forming bacteria are planned to be realized in the next stage of the project.

The technology (SS transportation system, maturation process) can be used for fast, safe, and efficient SS processing into organic fertilizer. It also combines well with the anaerobic digestion process as it can complete the digestive sludge transformation to a huge quality fertilizer.

Sewage sludge can replace mineral fertilizers in crop production. Attention should be paid to the amount and ratio of mineral elements available to the plant during plant growth. Sewage sludge may not fully provide plants with potassium and phosphorus. Sewage sludge is recommended for plants with a longer vegetation period due to the slow release of nutrients. A phytotoxic effect may occur during seed germination.
