Tailor made digestate products: blending with animal manure products, compost and mineral fertilizers

Vanden Nest, T. (Spreker op uitnodiging)

    Activiteit: Gesprek of presentatieLezing en mondelinge bijdrage


    At the end of the processing cascade, biomass processing in the bio-economy results in new products which can be used as alternative nutrient source and/or soil improver. 90% of the fresh weight biomass fed to biogas plants for energy production leaves the biogas reactor as digestate. Digestate contains large amounts of nutrients and organic matter, and has therefore a high potential as renewable nutrient source or soil improver. However the composition, nutrient ratios and nutrient availability are not always ideal for use in agricultural conditions. Digestate is often separated in a liquid fraction and a solid fraction, the latter often being dried to reduce transport costs. The liquid fraction can be further processed through a biological treatment to remove N, resulting in an effluent rich in K. The dried solid fraction of digestate (DD) is rich in P and N/P ratios are close to 1. These products contain too much P for a balanced crop fertilization. In Flanders and the Netherlands, i.e., regions with highly elevated soil P levels due to historical over-fertilization, their use is strongly legally restricted by the P content. DD products often have a less interesting N working coefficient. The high N mineralization reduces the value as soil improver, but is too low to replace (part of) mineral fertilizers. In the DIMA research project, funded by the Flemish government, actors from biogas sector, (organic) fertilizer industries, agricultural sector and researchers collaborate to produce tailor-made digestate products. By blending DD products with animal manure, compost and/or mineral fertilizers, it is possible to make (i) organic fertilizers with higher N/P ratios, K content and/or N working coefficients, or (ii) soil improvers with an elevated effective organic matter content and limited N mineralization. Products with an improved value for costumers, compared to DD. Because of the high nutrient content of the DD and DD based blends the application rate is often restricted to less than 10 Mg-1 ha-1, which is too low for common agricultural equipment such as a muck spreader. Therefore, pelletizing the blend is an interesting processing technique to produce organic fertilizers that are spreadable with common or pneumatic fertilizer spreaders or can be used in pot crops. Removing N from liquid fraction by biological treatment is currently under pressure, because N is lost to the air while simultaneously the very energy-intensive Haber-Bosch process is used to produce mineral N fertilizers from air-N. Alternative techniques such as reverse osmosis can be used to produce liquid concentrates. Fast working N(K) fertilizers can also be produced based on liquid fraction, by adding N(K) mineral fertilizers or products such as ammonium sulfate from scrubbers. Mineral K fertilizers can also be added to effluent to further improve the K content, achieving a replacement for common K fertilizers. In the DIMA research project, 11 types of pelletized organic fertilizers were produced based on DD, mineral fertilizers, byproducts such as vinasse and blood meal and /or animal manure with N/P ratios between 1.5 and 4.5. N incubation experiments in a sandy loam soil (pH-KCL 6.2, TOC 0.9%; 15°C, 116 days) revealed N working coefficients ranging from 32 to 88%. The effective organic matter content ranged between 208 to 296 kg Mg-1, calculated based on a C mineralization experiments with the same soil (21,5°C, 80-120 days). The plant availability of P from these products was also determined in pot trials with Westerwoldic ryegrass. The plant availability of P ranged from 57 to 73% relative compared to triple superphosphate. 4 types of mixtures with DD and composts were made to become soil improvers with low N working coefficients. N mineralization and effective organic matter content of these products ranged from 12 to 36% and 117 to 242 kg Mg-1. One solid fraction of digestate –without drying- was enriched with K, to result in a K rich soil conditioner especially for fruit orchards. In this sector there is increasing awareness of improving soil organic carbon, and additionally a need for K fertilization for higher fruit quality. The tested product had a DM% of 21%, and a K content of 4.1%, which was 100% plant available, compared to potassium chloride. Liquid fraction of digestate was mixed with mineral N and K and effluent was mixed with mineral K, to produce liquid N, K or NK rich fertilizers with a working coefficient close to that of mineral fertilizers. For all products the K working was 100% compared to mineral fertilizers. The N working coefficient was >85%. The mineral (N)K additions were soluble in both effluent and liquid fractions. However fibers in the liquid fraction caused precipitation and a less homogeneous product, reducing the use this product as such in greenhouses or with spraying equipment. Based on this research, we conclude that blending digestate products –especially dried solid fraction- and eventually pelletizing are promising steps to provide qualitative organic fertilizers. Although the potential market in Flanders and the Netherlands is probably small, these added value products can be exported to other regions. The biggest challenge is undoubtedly to deliver products with a constant composition, and constant nutrient availability over time. Results from the experiments and pot trials will be presented. *This Research was funded by the Flemish government (IWT-VIS 140995)
    Gehouden opVlaams Coördinatiecentrum Mestverwerking, België
    Mate van erkenningInternationaal