Wood Ash Based Treatment of Anaerobic Digestate: State-of-the-Art and Possibilities

Abstract

The problem of current agricultural practices is not limited to land management but also to the unsustainable consumption of essential nutrients for plants, such as phosphorus. This article focuses on the valorization of wood ash and anaerobic digestate for the preparation of a slow-release fertilizer. The underlying chemistry of the blend of these two materials is elucidated by analyzing the applications of the mixture. First, the feasibility of employing low doses (≤1 g total solids (TS) ash/g TS digestate) of wood ash is explained as a way to improve the composition of the feedstock of anaerobic digestion and enhance biogas production. Secondly, a detailed description concerning high doses of wood ash and their uses in the downstream processing of the anaerobic digestate to further enhance its stability is offered. Among all the physico-chemical phenomena involved, sorption processes are meticulously depicted, since they are responsible for nutrient recovery, dewatering, and self-hardening in preparing a granular fertilizer. Simple activation procedures (e.g., carbonization, carbonation, calcination, acidification, wash, milling, and sieving) are proposed to promote immobilization of the nutrients. Due to the limited information on the combined processing of wood ash and the anaerobic digestate, transformations of similar residues are additionally considered. Considering all the possible synergies in the anaerobic digestion and the downstream stages, a dose of ash of 5 g TS ash/g TS digestate is proposed for future experiments.

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

“…Figure 5 represents all the possible combinations of the samples (i.e., discontinuous line between the Blend 1 and the Blend 2, represented in a quaternary diagram) that could be prepared to maintain a nutrient ratio C/N/P ≤ 10/1/1. This nutrient ratio is necessary to promote high CUE upon land application [49,63]. There was a difference between the Blend 1 (67.49 ± 3.14% PVWD, 16.03 ± 1.97% WFA, and 16.48 ± 2.87% WBA) and Blend 2 (65.03 ± 3.02% FWD, 17.13 ± 3.23% PVWD, and 17.8 ± 1.89% WFA) in terms of chemical stability because the FWD had a greater content of NH 4 + -N than the PVWD, which had a greater content of undigested fiber.…”

“…The preparation of blends of wood ashes and anaerobic digestates aims to produce organic soil amendments with a balanced nutrient ratio that enhances the use efficiency of these elements by plants and assimilation by soil microbes [27,[44][45][46][47][48]. Achieving the controlled release effect by reducing the availability of nutrients further prevents the air pollution and contamination of groundwaters [49]. A better management of the nutrients also makes the organic manures more competitive with chemical fertilizers [29,50] and directly implies a reduction in the consumption of mineral resources.…”

Aspen Plus(r) Process Simulation Model of the Biomass Ash-Based Treatment of Anaerobic Digestate for Production of Fertilizer and Upgradation of Biogas

“…The appraisal of novel processing conditions, like the biomass ash-based treatment of the anaerobic digestate [14], is much more justified by the use of the Aspen Plus® simulation package, which allows to specify a wide range of parameters. The biomass ashbased treatment of the anaerobic digestate aims to improve the properties of the organic material as slow-release fertilizer, due to the sorption processes taking place.…”

“…Advances on this technology should continue until reaching a high efficiency in the solid-liquid separation, given the high moisture content of the anaerobic digestate (95 wt.%) [15], the large quantities produced of this material (30,000 tonnes per year and per AD plant), and the cost of storage, transportation and land application (£10/tonne for a 10-mile delivery) [16]. The present article informs about the development of the process simulation model (PSM) of Rajendran et al [11] in Aspen Plus® to monitor the biogas production and the valorization of anaerobic digestate by means of biomass ash-based treatment [14]. An assessment of the synergistic approaches that could reduce the cost of processing and handling the anaerobic digestate, aims to increase the viability of the treatment of anaerobic digestate and promote an overall enhancement of the circular economy [1].…”

Dynamic Control of Liquid Biomass Digestate Distillation Combined with an Integrated Solar Concentrator Cycle for Sustainable Nitrogen Fertilizer Production

“…An apparent need still exists to integrate such medium-temperature solar heat generators with the conventional steam-powered reboilers in the existing distillation columns to overcome the intermittent nature of the solar energy and to achieve sufficiently robust and dynamically controllable operation (utility steam is available immediately at a constant temperature while solar heat cycles over several hours). One such example of an application that can benefit from solar heat harvesting and utilization is the distillation of biomass digestion liquid byproducts. − Recent focus has shifted from conventional air stripping to the efficient nitrogen capture with the concurrent carbon removal and recovery as a solid ammonium bicarbonate NH 4 HCO 3 fertilizer material . To obtain a solid NH 4 HCO 3 green fertilizer, it utilizes low-temperature heat in the reboiler and can be situated in remote agricultural areas that can provide plenty of land for solar trough installations.…”