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Circularity of Bioenergy Residues: Acidification of Anaerobic Digestate Prior to Addition of Wood Ash

The present study investigated the acidification treatment of an agrowaste digestate and a food waste digestate, which is necessary before the addition of the wood ashes to attain the pH of zero point of charge in the blend intended to behave as a slow-release fertilizer. The 336-h acidification treatments of the 2.39 ± 0.35 g of digestates were performed with high and low doses of four commercial acids (sulfuric, hydrochloric, nitric, and lactic acids) in 50-mL capped Corning® tubes. For analytical purposes, after the incubation, ultrapure milli-Q® water was added at a rate of 10 mL for each gram of digestate to create a water-soluble phase that allowed the measurement of the pH and the electric conductivity. The results showed that the optimum dose and type of acid were very dependent on the nature of the anaerobic digestate. The maximum buffer capacity of the agrowaste digestate was 0.07 mmol H+-H2SO4/g, but this increased by adding the food waste digestate with a greater content of ammoniacal nitrogen. The agrowaste digestate with a greater content of undigested fiber was more easily oxidized by nitric acid. On the other hand, sulfuric acid oxidized the food waste digestate to a greater extent than the other acids did. Since a high dose of acid was required to achieve a greater efficiency in the solid–liquid separation, which would ease any subsequent handling of the digestates, hydrochloric acid was considered to be the most suitable acid. Lactic acid promoted the growth of filamentous microbes in the agrowaste digestate and microbial colonies in the food waste digestate, which is an indication of the poor preservation of the organic matter under these conditions.