Handling of food waste and agro-industrial waste digestates, and fly and bottom wood ashes represents an economic and environmental problem for society and industries where they are produced due to the requirement of large capacity storage facilities with special conditions. Since both materials contain important nutrients for crops, two different blending combinations were examined as potential slow-release fertilizers to enhance the circular economy. High share of biomass ashes were selected aiming the chemical alkaline stabilization of the digestates. The concentrations of nitrogen, carbon, and phosphorus in the water-soluble extract of the blends, which was obtained by adding ten parts of milli-Q® per each part of sample and subsequent solid–liquid separation, were measured during a 10-h incubation at 22 °C and 100 rpm. The volatilization of ammonia was the main route of losing nitrogen, with the greatest rate of volatilization at the time of blending and approximately 50 mg NH₃-N/kg blend/hour by the end of the incubation. The hydrolysis taking place at alkaline pH was found responsible of the conversion of organic nitrogen to ammoniacal nitrogen and of the increase of the amount of water-soluble carbon, which rose progressively during the incubation. The release of CO₂ was prevented by the high pH (> 10) of the blends. The drop in the concentration of water-soluble orthophosphate at the time of preparing the mixtures was driven by adsorption onto the ashes. The desorption of some of the orthophosphate was found to be inversely related to the amount of ashes added to the digestates. Graphic Abstract

The use of the commercial simulator Aspen Plus^® could bring an amelioration in the accuracy of the predictions of the chemical species composition in the output streams of the anaerobic digestion process. Compared to the traditionally employed lumped models, which are elaborated from scratch, the models implemented in Aspen Plus^® have access to a broad library of thermodynamic and phenomena transport properties. In the present investigation, a process simulation model for anaerobic digestion has been prepared by including a stoichiometric-equilibria reactor to calculate the extent of the ionization of the molecules present in the anaerobic digestate. The model characterizes the technical feasibility of anaerobic digestate stabilization, by means of biomass ash-based treatment, for the production of an organic fertilizer and potential biogas upgradation with the synthesis of ammonium carbonate. First of all, the titration of the manure digestate with the hydrochloric acid showed that a dose of 3.18 mEq/g would be required to attain the targeted pH of zero-point charge, upon addition of the sewage sludge ash in a ratio to the manure digestate of 0.6 ± 0.2%. Secondly, the profiles of ammonia, carbon dioxide, and methane found in the biogas agree with the pH of the treated digestate and enable the upgrading of the biogas with the production of NH₄HCO₃. The model needs to be further developed to ensure the standards are attained in all output streams of stabilized anaerobic digestate, biomethane, and isolated added-value chemical fertilizers.

The process and energy intensifications for the synthesis of glycerol carbonate (GC) from glycerol and dimethyl carbonate (DMC) using an eggshell-derived CaO heterogeneous catalyst were investigated. The transesterification reaction between glycerol and DMC was typically limited by mass transfer because of the immiscible nature of the reactants. By varying the stirring speed, it was observed that the mass transfer limitation could be neglected at 800 rpm. The presence of the CaO solid catalyst made the mass transport-limited reaction process more prominent. Mass transfer intensification using a simple kitchen countertop blender as an alternative to overcome the external mass transfer limitation of a typical magnetic stirrer was demonstrated. A lower amount of the catalyst and a shorter reaction time were required to achieve 93% glycerol conversion or 91% GC yield, and the turnover frequency (TOF) increased almost 5 times from 1.5 to 7.2 min⁻¹ when using a conventional magnetic stirrer and countertop blender, respectively. In addition, using a simple kitchen countertop blender with 7200 rpm, the reaction temperature of 60 °C could be reached within approximately 3 min without the need of a heating unit. This was the result of the self-frictional heat generated by the high-shear blender. This was considered to be heat transfer intensification, as heat was generated locally (in situ), offering a higher homogeneity distribution. Meanwhile, the trend toward energy intensification was promising as the yield efficiency increased from 0.064 to 2.391 g/kJ. A comparison among other process intensification techniques, e.g., microwave reactor, ultrasonic reactor, and reactive distillation was also rationalized.