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In the transition from a fossil to a bio-based economy, it has become an important challenge to maximally recuperate valuable nutrients coming from waste streams. Nutrient resources are rapidly depleting, significant amounts of fossil energy are used for the production of chemical fertilizers, whereas costs for energy and fertilizers are increasing. In the meantime, biogas production through anaerobic digestion produces nutrient-rich digestates. In high-nutrient regions, these products cannot or only sparingly be returned to agricultural land in its crude unprocessed form. The consequent processing of this digestate requires a variety of technologies producing lots of different derivatives, which could potentially be re-used as green fertilizers in agriculture. As such, a sustainable alternative for fossil-based mineral fertilizers could be provided. This study aims to characterize the physico-chemical properties of digestates and derivatives, in order to identify the fertilizer value and potential bottlenecks for agricultural re-use of these products, in line with European legislative constraints. In addition, the economic and ecological benefits of substituting conventional fertilizers by bio-based alternatives are quantified and evaluated. Waste water from acidic air scrubbers for ammonia removal shows potential for application as N-S fertilizer. Analogously, concentrates resulting from membrane filtrated liquid fraction of digestate show promise as N-K fertilizer. Substituting conventional fertilizers by digestate derivatives in different cultivation scenarios can result in significant economic and ecological benefits for the agriculturist. Starting from theoretical scenarios outlined in the current study, field test validation will be required to confirm the potential substitution of fossil-based mineral fertilizers by bio-based alternatives.

In the transition from a fossil to a bio-based economy, it has become an important challenge to maximally recycle valuable nutrients that currently end up in waste streams. Nutrient resources are rapidly depleting. Significant amounts of fossil energy are required for the production of synthetic fertilizers, whereas costs for energy and fertilizers are increasing. Meanwhile, biogas production through anaerobic digestion produces nutrient-rich digestates, which could potentially be reused as green fertilizers in agriculture, thereby providing a sustainable substitute for synthetic fertilizers. The aim of this study was to evaluate the impact of using bio-digestion waste derivatives instead of synthetic fertilizers and/or animal manure on soil and crop production. In a field trial, nutrient balances were assessed and the physicochemical soil fertility and quality were evaluated. The biogas yield of the harvested energy crops was determined. An economic and ecological evaluation was conducted. Application of bio-digestion waste derivatives induced small, albeit statistically insignificant improvement in crop yield, soil fertility and quality compared to current common practices using animal manure and synthetic fertilizers. Moreover, the use of these products might stimulate nutrient mobilization from the soil, thereby increasing the use efficiency of soil minerals. For all reuse scenarios the calculated economic and ecological benefits were significantly higher than the reference. It is clear that the reuse of bio-based products as nutrient supply in agriculture should be stimulated in European legislation. Further field research is on-going in order to validate the results and evaluate the impact on soil quality in the longer term.

Nutrient recovery from digested biodegradable waste as marketable products has become an important task for anaerobic digestion plants to meet both regulatory drivers and market demands, while producing an internal revenue source. As such, the present waste problem could be turned into an economic opportunity. The aim of this study was to provide a comprehensive overview and critical comparison of the available/emerging technologies for nutrient recovery from digestate, and a classification of the resulting end-products according to their fertilizer characteristics. Based on the stage of implementation, the technical performance, as well as financial aspects, struvite precipitation/crystallization, ammonia stripping and (subsequent) absorption using an acidic air scrubber were selected as best available technologies to be applied at full-scale for nutrient recovery as marketable fertilizer commodities. The resulting end-products can and should be classified as renewable nitrogen–phosphorus (N/P) precipitates and nitrogen–sulfur (N/S) solutions, respectively, in fertilizer and environmental legislations. This would stimulate their use and foster nutrient recovery technology implementation.

In the framework of the Phos4You (P4Y) project funded by Interreg North West (NW) Europe 6 different Phosphorous(P)-recovery technologies will be demonstrated. The University of Liège is developing one of the processes, called PULSE (Phosphorus ULiège Sludge Extraction) process, to recover P from fully or partially dried sewage sludge. The PULSE process is a modification of the PASCH process developed at RWTH Aachen to extract P from sewage sludge ashes [1]. In the PULSE process P is recovered from partially or fully dried sludge using acidic leaching. Purification of the leach liquor will be carried out by reactive extraction to separate P and other nutrients from co-leached metals. Finally, depending on the leaching and extraction approach used above, the final product of the PULSE process can either be obtained as phosphate salt or phosphoric acid. Nevertheless, production of novel P products requires a novel standardized methodology for its quality assessment and valorisation on the market. In the first part of the research, the experiments for the unit operations of the PULSE process are conducted at lab-scale and metals, P and other macronutrient content in each step of the process sequence is monitored and benchmarked against the legislative limits. Comparison of the standardized sludge digestion method with nitric acid and/or aqua regia with modified sulphuric and hydrochloric acid will be conducted in order to establish a standard for sludge characterization especially for heavy metals determination. The data obtained for the different process options of each unit operation are evaluated using the methodology of 'Cascaded Option Trees' [2] to select the most feasible and optimum option. A solid-liquid equilibrium speciation model developed in MATLAB is further used for optimizing process parameters. In the second step, the PULSE process will be demonstrated on a pilot-plant scale at 4 different locations in NW Europe. The novel P product will be thoroughly analysed using quality methods selected by project partners responsible for quality assessment in the P4Y project for P availability and inorganic characterization, which provides feedback to the technology producer for improvement. In the presentation, the concept of the PULSE process will be explained along with the results from the lab experiments and evaluation of process options. The concept of solid-liquid equilibrium speciation model and its application to optimize the PULSE process operation will also be presented. The relation between the quality of the P source and P product will be showcased. Further comparison of the quality of PULSE product with the regional and EU regulations on P fertilizers will also be presented. ; Peer reviewed