The bioenergy-fertilizer nexus: a challenge achievable from municipal wastewater

Several billion tons of liquid and solid waste are produced every year globally, contributing to soil and water pollution, greenhouse gas emissions and loss of important nutrients, like C, N and P in the environment. On the other hand, soils are facing a strong reduction of organic matter content while agricultural, food and productive processes mainly rely on external inputs, i.e. mineral fertilizers and chemical pesticides. Biological Wastewater Treatment (WWT) systems play an important role in improving water quality and human health. When properly treated, wastewater is a valuable resource to reduce pollution and combat water scarcity. This chapter aims to give a brief overview of the conventional biological WWT plant and its treatment steps, with a specific focus on the solid residues of WWT plant, i.e. sewage sludge (SS), which could be also valorised by anaerobic digestion (AD), converting it into valuable commercial products: biogas and fertilizers, applying hence the important concept of the Circular Economy. Moreover, we show the effects of application of AD residues (i.e. digestate) on soils with a high risk of desertification, such as with a prevalent sandy texture, with low soil organic matter and high depletion of macronutrients. Results demonstrate fertilizers' effect on poor soils, an issue that is nowadays a persistent environmental problem, with the advancing of desertification and the even more frequent presence of arid soils. The etiological agents of this phenomenon deal with deforestation, urban expansion, unsustainable soil management and climate change, issues that Vietnam and Europe try to solve with different approaches. As result, these soils, even distant, show common features in terms of structure (erosion, compaction), chemical composition (loss of organic carbon, nutrient imbalance) and biodiversity that need to be addressed.