In a review article published in the journal Discover Soil, authors Caterina Lucia, Luigi Badalucco, Santo Fabio Corsino, Antonino Galati, Massimo Iovino, Sofia Maria Muscarella, Sara Paliaga, Michele Torregrossa, and Vito Armando Laudicina provide a comprehensive overview of the opportunities and challenges in managing and reusing sewage sludge in the agricultural sector. Sewage sludge (SS), the semi-solid material left over from wastewater treatment, is a significant by-product that has long posed environmental and socioeconomic challenges. However, the authors argue that when managed properly, sewage sludge can be a valuable resource for a circular economy, as it is rich in organic matter and nutrients like nitrogen and phosphorus. This approach not only minimizes environmental impact but also promotes resource efficiency by providing a sustainable alternative to non-renewable resources. The European Union produces about 50 million tons (Mt) of sludge annually, with Germany, the United Kingdom, France, Spain, and Italy generating about 75% of this total.

The review highlights that applying treated sewage sludge to agricultural land can improve soil health and crop yields in several ways. It can supply organic matter and nutrients, stabilize soil structure, and make soil less vulnerable to erosion. For example, one field experiment found that applying domestic SS at a rate of 40 t/ha significantly increased wheat yield. This application rate also led to a 29% increase in organic matter and a 433% increase in macronutrients like total N and available P. The application of SS at a rate of 120 t/ha in a sandy loam soil in Tunisia resulted in a 2.92% increase in total organic carbon and a 0.18% increase in total N. Beyond physical and chemical changes, sewage sludge can also positively impact the soil’s biochemical properties. Since most microbial populations are heterotrophic and respond quickly to new amendments, the organic matter in sewage sludge can increase the amount of microorganisms and their activity. One laboratory study showed that after just 15 days, soil amended with a high concentration of sewage sludge (50 Mg/ha) had more than triple the microbial biomassBiomass is a complex biological organic or non-organic solid product derived from living or recently living organism and available naturally. Various types of wastes such as animal manure, waste paper, sludge and many industrial wastes are also treated as biomass because like natural biomass these More carbon (MBC) values compared to unamended soil.

Despite its benefits, sewage sludge poses risks due to potential contaminants, including heavy metals, pathogens, and emerging organic micropollutants like pharmaceuticals and microplastics. The European Directive on the agricultural use of SS (86/278/EEC) sets limits on heavy metals but does not address organic micropollutants, which is a major limitation. To mitigate these risks, the SS must undergo stabilization treatments, such as composting or pyrolysisPyrolysis is a thermochemical process that converts waste biomass into bio-char, bio-oil, and pyro-gas. It offers significant advantages in waste valorization, turning low-value materials into economically valuable resources. Its versatility allows for tailored products based on operational conditions, presenting itself as a cost-effective and efficient More, before it can be safely used. The conversion of SS to biocharBiochar is a carbon-rich material created from biomass decomposition in low-oxygen conditions. It has important applications in environmental remediation, soil improvement, agriculture, carbon sequestration, energy storage, and sustainable materials, promoting efficiency and reducing waste in various contexts while addressing climate change challenges. More through pyrolysis is a particularly important method for safe valorization. The pyrolysis process, which occurs at temperatures between 350 and 1000∘C , can reduce sludge volume by 80%, remove pathogens, and immobilize heavy metals, thus lowering their risk of leachingLeaching is the process where nutrients are dissolved and carried away from the soil by water. This can lead to nutrient depletion and environmental pollution. Biochar can help reduce leaching by improving nutrient retention in the soil.   More into the soil. This is because the higher ashAsh is the non-combustible inorganic residue that remains after organic matter, like wood or biomass, is completely burned. It consists mainly of minerals and is different from biochar, which is produced through incomplete combustion.   Ash Ash is the residue that remains after the complete More content of sewage sludge biochar acts as a basic buffer, preventing the release of heavy metals. Biochar’s porous structure improves water and nutrient retention in soil, and it can enhance soil aggregation and aeration. A study found that applying SS biochar increased soil organic carbon by 67–85%, mineral N by 55–145%, and available P by 45–54%. Additionally, the study noted that maximum improvements in plant yield were found at a pyrolysis temperature of 300∘C.

Composting is another widely used and beneficial process for stabilizing sewage sludge. This three-phase process involves a thermophilic phase where temperatures can reach 80∘C, which effectively eliminates pathogens and inactivates weed seeds and toxic compounds. Composted sewage sludge has been shown to have positive effects on soil biochemical properties and can lead to a 42% increase in microbial biomass carbon. The economic feasibility of various sewage sludge treatments is a key factor in their adoption. A comparison of four common SS treatments found that composting had the lowest life cycle cost due to low capital investment. Pyrolysis, on the other hand, generated the highest income from selling by-products like bio-oil and biochar. However, economic research on this topic remains limited, and factors like fluctuating market prices for energy, gas, and raw materials can significantly impact a project’s viability.

The authors conclude that sewage sludge is a valuable resource with the potential to improve soil health and support a circular economy. Realizing this potential will require an integrated approach that combines improved treatment technologies with updated legislation and a better understanding of the economic factors at play.

Source: Lucia, C., Badalucco, L., Corsino, S. F., Galati, A., Iovino, M., Muscarella, S. M., Paliaga, S., Torregrossa, M., & Laudicina, V. A. (2025). Management and valorisation of sewage sludge to foster the circular economy in the agricultural sector. Discover Soil, 2(80).