Production of solid fertilizers from anaerobic digestion liquid residue

Abstract

Anaerobic digestion is a process in which microorganisms convert organic matter into useful products in the absence of oxygen. From this process two basic products are obtained: biogas, which is a renewable energy resource, and digestate. The digestate is in the form of liquid (liquid digestate) and solid (solid or muddy digestate) and despite its inability to be further digested into gaseous products, has extraordinary agronomic qualities beneficial to crops, since it is highly rich in nutrients such as nitrogen, phosphorus, potassium, plant growth hormones, and other micronutrients. However, there are problems associated with the direct application of this liquid, such as nutrient leaching, which can lead to the eutrophication of natural water bodies, altering the habitat of animal and plant populations and the consequent degradation of the water quality. Also, the transport and storage of liquid fertilizer is difficult to perform, forcing this high-quality nutritional product to be discarded in wastewater treatment plants. For instance, the alternative of converting liquid digestate into a solid fertilizer would improve storage and transportation capabilities solving the need for sustainable engineering developments towards digestate utilization. In this context, it is the aim of this chapter to review all the explored alternatives for the conversion of liquid digestate into solid products (e.g. by pelletization, granulation, and other emerging technologies). In addition, information regarding their nutrient content and agronomical value will be summarized.