Modelling of Amino Acid Fermentations and Stabilization of Anaerobic Digestates by Extracting Ammonium Bicarbonate

Abstract

With the current increase in demand for animal and agricultural products, management of agrowaste has become critical to avoid greenhouse gas emissions. The present article investigates the applicability of ammonium bicarbonate synthesis via flash distillation to valorize and stabilize several types of anaerobic digestates which are produced from individual fermentations of amino acids. The content of CO2 in the digestate was found to be responsible for the OH alkalinity (0.4 equivalents of acid/kg digestate), while the partial and total alkalinities (0.8 eq/kg digestate) were essentially derived from the content of NH3. The most suitable conditions for the flash distillation were 95 °C and 1 bar with the condensation occurring at 25 °C. However, in order to attain the precipitation of NH4HCO3 in the distillate, it was necessary to consider digestates with a moisture content of 50 wt.%, since saturation levels of inorganic nitrogen and inorganic carbon were not attained otherwise. Even under these conditions, few amino acids (i.e., arginine, glycine, and histidine) were able to provide an anaerobic digestate upon fermentation that would be suitable for NH4HCO3 stabilization. The process of stabilization with a capacity of a t of digestate per h was improved by adding hydrochloric acid or sodium hydroxide at a rate of 44 kg/h, leading to production of 34 kg NH4HCO3/h. Given the role of the volatile elements of the biogas as endogenous stripping agents, it is recommended to use a fresh and saturated digestate as feed for the flash distillation.