Strategies for the production of a stable blended fertilizer of anaerobic digestates and wood ashes

Abstract

The wood ash-based treatment of anaerobic digestates is a novel technology to achieve the stabilization and improve the properties of organic amendments as slow-release fertilizers. The present work evaluated 3 strategies to optimize the blending of these materials under minimum acidification. The first strategy consisted in the titration of 2 samples of digestate and 2 samples of wood ash with 4 acids (H2SO4, HCl, HNO3, and CH3CHOHCOOH). The stability of the samples was inferred from the pH measurements and the simulations with Visual MINTEQ allowed to confirm some of the on-going phenomena. The bottom ash and the agro-waste digestate required less acid to reach the targeted pH of 5.5, which prevents ammonia volatilization, due to low reactivity of the coarse particles and the low ammoniacal nitrogen content, respectively. The calculation of the pH of zero point of charge further minimized the use of acid reagents, since the optimum pH for the treatment of the digestates could be 11.90 ± 0.50 and 12.43 ± 0.06, when employing the finer and coarser fractions of the ashes, respectively. In the second strategy tested, commercial doses of acids were applied to the digestates before the addition of the ashes. The sorption of ammonia onto the fly ash at pH 10.49 ± 0.76 was found to be key to preserve the nitrogen of the food waste digestate, while saving HCl or HNO3. The third strategy involved washing the ashes repeatedly with ultrapure milli-Q® water to remove the impurities and reach a pH closer to the zero point of charge without using acids. The removal of impurities and most reactive alkali elements (e.g. sodium and potassium) from the ashes led to approximately 5 % lower pH and higher carbon, calcium, and magnesium contents, which could enhance sorption processes upon combination of these materials with the digestates. The integration of washing the wood ashes and commercial acidification of anaerobic digestates would provide the best conditions for the preparation of stable blends. This procedure also minimizes the use of acids, which disrupts the closed-loop valorization of these materials.