Nutrient recycling from bio-digestion waste as synthetic fertilizer substitutes: a field experiment

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 used for the production of synthetic fertilizers, whereas costs for energy and fertilizers are increasing. In the meantime, biogas production through anaerobic digestion produces nutrient-rich digestates as a waste stream. In high-nutrient regions this product cannot or only sparingly be returned to agricultural land in its crude unprocessed form. The consequential processing of digestate requires a variety of technologies producing several 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. The aim of this study was to evaluate the impact of using bio-digestion waste instead of synthetic fertilizers and/or animal manure on soil and crop production. In a field trial, nutrient balances were assessed and the physico-chemical soil quality, including the nitrate residue, leaching, the salt content, pH, sodium adsorption ratio, as well as phosphorus and heavy metal accumulation, were evaluated. The biogas yield of the harvested energy crops was determined by means of an anaerobic digestion batch test. As such, the nutrients coming from the digestate are again recycled to the anaerobic digestion plant and nutrient cycles are maximally closed. Finally, an economic and ecological evaluation was conducted. In the current field-trial, application of waste water from acidic air scrubbers for ammonia removal, digestates and their liquid fraction caused small, yet insignificant, improvement in crop yield, physico-chemical soil fertility and soil quality compared to current common practices involving the use of animal manure and synthetic fertilizers. Moreover, it is observed that the use of bio-digestion waste can stimulate phosphate and potassium mobilization from the soil, thereby increasing the use efficiency of soil nutrients. For all re-use scenarios the energetic potential per hectare of harvested energy maize was slightly higher than in the reference scenario. Finally, the substitution of synthetic fertilizers by these bio-based alternatives resulted in significant economic and ecological benefits. Re-use of bio-digestion waste can so improve the economic viability of anaerobic digestion plants, especially in high-nutrient regions, which in turn can serve as a catalyst to meet the 2020 directives. It is clear that the re-use of bio-based products as nutrient supply in agriculture should be stimulated in European legislation. The results obtained in this research can help to better classify these products and serve as a support to stimulate their use. Further field research is on-going in order to validate the results and evaluate the impact on soil quality in the longer term. The presentation will first give a brief description of the nutrient problem, followed by a detailed description of the aims and set-up of the field experiment, as well as the results of the first fertilization year.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 used for the production of synthetic fertilizers, whereas costs for energy and fertilizers are increasing. In the meantime, biogas production through anaerobic digestion produces nutrient-rich digestates as a waste stream. In high-nutrient regions this product cannot or only sparingly be returned to agricultural land in its crude unprocessed form. The consequential processing of digestate requires a variety of technologies producing several 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. The aim of this study was to evaluate the impact of using bio-digestion waste instead of synthetic fertilizers and/or animal manure on soil and crop production. In a field trial, nutrient balances were assessed and the physico-chemical soil quality, including the nitrate residue, leaching, the salt content, pH, sodium adsorption ratio, as well as phosphorus and heavy metal accumulation, were evaluated. The biogas yield of the harvested energy crops was determined by means of an anaerobic digestion batch test. As such, the nutrients coming from the digestate are again recycled to the anaerobic digestion plant and nutrient cycles are maximally closed. Finally, an economic and ecological evaluation was conducted. In the current field-trial, application of waste water from acidic air scrubbers for ammonia removal, digestates and their liquid fraction caused small, yet insignificant, improvement in crop yield, physico-chemical soil fertility and soil quality compared to current common practices involving the use of animal manure and synthetic fertilizers. Moreover, it is observed that the use of bio-digestion waste can stimulate phosphate and potassium mobilization from the soil, thereby increasing the use efficiency of soil nutrients. For all re-use scenarios the energetic potential per hectare of harvested energy maize was slightly higher than in the reference scenario. Finally, the substitution of synthetic fertilizers by these bio-based alternatives resulted in significant economic and ecological benefits. Re-use of bio-digestion waste can so improve the economic viability of anaerobic digestion plants, especially in high-nutrient regions, which in turn can serve as a catalyst to meet the 2020 directives. It is clear that the re-use of bio-based products as nutrient supply in agriculture should be stimulated in European legislation. The results obtained in this research can help to better classify these products and serve as a support to stimulate their use. Further field research is on-going in order to validate the results and evaluate the impact on soil quality in the longer term. The presentation will first give a brief description of the nutrient problem, followed by a detailed description of the aims and set-up of the field experiment, as well as the results of the first fertilization year.C