Nutrient recovery from digestates : techniques and end-products

In nitrate vulnerable zones application of animal manure to land is limited. Digestate from anaerobic digestion plants competes with manure for nutrient disposal on arable land, which forms a serious hinder for the biogas sector to develop in these regions. Hence, one of its biggest challenges is to find cost-effective and sustainable ways for digestate processing or disposal. Furthermore, primary phosphorus resources are becoming scarce and expensive and will be depleted within a certain time. This urges the need to recycle P from secondary sources, like digestate or manure. From a sustainability point of view, it seems therefore no more than logical that digestate processing techniques switched their focus to nutrient recovery rather than nutrient removal. This paper gives an overview of digestate processing techniques, with a special focus on nutrient recovery techniques. In this paper nutrient recovery techniques are delineated as techniques that (1) create an end-product with higher nutrient concentrations than the raw digestate or (2) separate the envisaged nutrients from organic compounds that are undesirable in the end-product, with the aim to produce an end-product that is fit for use in chemical or fertiliser industry or as a mineral fertiliser replacement. Various nutrient recovery techniques are described, with attention for some technical bottlenecks and the current state of development. Where possible, physicochemical characteristics of the endproducts are given

Nutrient recovery from biodigestion waste (water) streams and reuse as renewable fertilizers: a two-year field experiment

The aim of this study was to evaluate the impact of using bio-digestion waste derivatives as substitute for synthetic fertilizers and/or as P-poor equivalent for 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 also determined. An economical and ecological evaluation was conducted. The highest biomass yields were obtained when the liquid fraction of digestate was used as P-poor fertilizer in addition to animal manure. Furthermore, the complete substitution of synthetic fertilizer N by air scrubber waste water resulted in the highest N and P use efficiencies. Finally, for all reuse scenarios the calculated economical and ecological benefits were significantly higher as compared to the common practice using animal manure and synthetic fertilizers

Techniques for nutrient recovery from digestate: inventory

Because of the historic presence of intensive livestock production and the limited amount of arable land for manure disposal, nitrate pollution in certain European areas is considerable. The European Nitrate Directive, implemented in 1991, is intended to improve water quality in Europe by preventing pollution of ground- and surface water by leaching of nitrates from agriculture. Through the Directive member states were obliged to set up an action programme and define vulnerable zones. The Flemish action plan, for example, consists of an obligation to process manure. Manure processing is defined as treating manure in such a way that the nitrogen present is not brought back on Flemish soil after treatment, so that it is either exported or converted to nitrogen gas or a mineral fertiliser. The restrictions on nutrient application on the fields combined with the presence of intensive livestock, implies that anaerobic digestion plants in Flanders and other nutrient rich areas, who are often obliged to co-digest manure, have no other choice than to invest in expensive digestate processing techniques. More recently, the focus in the development of manure and digestate processing techniques has switched from mere processing towards techniques that recover a maximal amount of nutrients (N, P, K) and produce dischargeable or re-useable water. This development is i.a. triggered by the increasing worldwide awareness of the depletion of phosphorus and potassium, which are nowadays extracted through mining. Another incentive is the volatile price of fossil-based mineral fertilizers. Estimates of the current phosphorus and potassium reserves are highly uncertain, but based on population growth and future nutrient demand, it is predicted that depletion will occur within 93 to 291 years for P and 235 to 510 years for K (Fixen and Johnston, 2012; Van Vuuren et al., 2010; Villalba et al., 2008; Smit et al.,2009). Geopolitical moves can however shift this date forward, making nutrient scarcity an imminent threat. These findings have generated awareness. The challenge for anaerobic digestion plants now is to achieve optimal recovery and recycling of nutrients from the digestate in a sustainable way. ARBOR is an Interreg IVb-project that focusses on the acceleration of bio-energy development in North-West Europe. The goal of action 9 in work package 2 of the project is to make an inventory of existing nutrient recovery techniques from digestate in North-West Europe and to stimulate marketing of the end-products by means of i.a. physicochemical characterisations and field trials. This report gives an overview of the inventory made in the framework of the ARBOR project. It shows how digestate can be used as a sustainable source of nutrients. A distinction has been made between currently used digestate processing techniques and nutrient recovery techniques, the latter being in an earlier phase of development, but very promising with respect to the demand for more sustainability in agriculture

Nutrient recovery from digestate: techniques & end-products

In nitrate vulnerable zones application of animal manure to land is limited. Digestate from anaerobic digestion plants competes with manure for nutrient disposal on arable land, which forms a serious hinder for the biogas sector to develop in these regions. Hence, one of its biggest challenges is to find cost-effective and sustainable ways for digestate processing or disposal. Furthermore, primary phosphorus resources are becoming scarce and expensive and will be depleted within a certain time. This urges the need to recycle P from secondary sources, like digestate or manure. From a sustainability point of view, it seems therefore no more than logical that digestate processing techniques switched their focus to nutrient recovery rather than nutrient removal. This paper gives an overview of digestate processing techniques, with a special focus on nutrient recovery techniques. In this paper nutrient recovery techniques are delineated as techniques that (1) create an end-product with higher nutrient concentrations than the raw digestate or (2) separate the envisaged nutrients from organic compounds that are undesirable in the end-product, with the aim to produce an end-product that is fit for use in chemical or fertiliser industry or as a mineral fertiliser replacement. Various nutrient recovery techniques are described, with attention for some technical bottlenecks and the current state of development. Where possible, physicochemical characteristics of the endproducts are given

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