Valorization of agrowaste digestate via addition of wood ash, acidification, and nitrification

Moure Abelenda, Alejandro and Semple, Kirk and Herbert, Ben M. J. and Aggidis, George and Aiouache, Farid (2022) Valorization of agrowaste digestate via addition of wood ash, acidification, and nitrification. Environmental Technology and Innovation, 28: 102632. ISSN 2352-1864

Abstract

The valorization of low pollutant bioenergy waste streams as fertilizers is key for a more circular economy. In addition to supplementing nutrients, the wood fly ash could be used as stabilizing agent of an agrowaste digestate, to minimize the pollution of the environment. A two-stage experimental design with series of mild and severe acidifications was used to assess the impact on the availability of nitrogen, carbon, and phosphorus in the 25.26 ± 2.78 g of treated digestate. The 144-hour incubations at 20 °C and 100 rpm were done in closed chambers (i.e. 250-mL Schott Duran® bottles) with a 0.11 M sulfuric acid trap of 4.43 ± 0.12 mL. The proposed model for the interpretation of the results of the closed chamber indicated that the greatest rate of formation of ammoniacal nitrogen in the sulfuric fraction occurred during the first 48 h of incubation of the untreated PVWD (zero order kinetics: 2.31 10−11 mol [H2SO4] NH-N/s). During this period, the mass transfer coefficient was 1.81 10, after which the equilibrium was stablished at 58.47 ± 20.50 mg [H2SO4] NH-N/kg digestate. Up to 14.08 ± 3.52 % of the water-soluble nitrogen was converted to nitric nitrogen in the remaining 96 h of incubation of the control. The acidification kept all the nitrogen in the form of ammonium but increased 5.97 ± 0.19 times the leachability of phosphorus. The combined treatment with wood ash and hydrochloric acid prevented both the nitrification and the increase of the share of water-soluble orthophosphate in the digestate. The high share of inert carbon in the wood fly ash was found promising to restore the soil as a natural carbon sink, upon application of the treated anaerobic digestate. Routes for the activation of ash as sorbent that would require lower doses of acid need to be investigated.