Valorization of Anaerobic Digestion Liquid Residue by Production of Organic Fertilizer with Fluidized-Bed Granulation

The use of fertilizers plays a major role within the world agriculture, with particular interest in the development of alternative production methods, less energy intensive, and compatible with organic agriculture. Anaerobic conversion of organic matter such as animal manure and crop residues into biogas provides not only a clean and renewable source of energy but also a nutrient-rich digestate for land applications. The recovery of nutrients from waste biodigestion and its reuse as fertilizer has become a major challenge for the development of sustainable agriculture. In countries where anaerobic digestion plants constitute a considerable proportion of the energy matrix, the application of liquid digestate as fertilizer is common. However, there are problems associated with the direct application of this liquid such as the leaching of nutrients and the costs associated with its transport and storage.

In the fertilizers industry, the granulation process is very important since it provides products with high resistance to breakage and low tendency to caking and lump formation. In particular, it can be considered as an alternative to overcome the drawbacks associated with direct application of the liquid digestate [1]. According to this, and taking into account the advantages of fluidized-bed granulation over other granulation techniques [2], it is the aim of this work to explore the alternative of granulating the liquid digestate with different seed particles in a lab-scale top-spray fluidized-bed unit.

Granulation was performed combining the liquid digestate with biochar, zeolite, dolomite or agricultural gypsum of different size fractions. The process conditions for appropriate operation were determined (i.e., bed temperature, fluidization air flowrate and temperature, digestate flowrate and temperature). Special attention was given to the ammonium content and a strategy to retain it through the granulation process was assessed. Posteriorly, friability and hardness of the final particles were measured and compared with the values recommended for solid fertilizers. Nutrient content of the final product was also analysed. Ammonium was evaluated through the Berthelot method [3]. After acid digestion of the product, the phosphorus content was measured by the Scheel [4] colorimetric method while the potassium content was determined by atomic absorption.

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[3] W.T. Bolleter, C.J. Bushman, P.W. Tidwell, Spectrophotometric Determination of Ammonia as Indophenol, Anal. Chem. 33 (1961) 592–594. doi:10.1021/ac60172a034.

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