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Main research question/goal
Biogas is not the only by-product of biodigestion of animal manure, organic waste and energy crops. Crude digestate, another by-product, can be further processed and/or composted and become available on the market as an organic fertiliser. The goal in this project was (1) to investigate if the solid fraction of mechanical digestate separation can be composted, (2) to estimate the potential to build-up humus in the soil and (3) to estimate the NPK working coefficients of these processed digestates, which is different from the intrinsic organic matter and total NPK content. What are good practices for solid fraction composting? Are N, P and K working coefficients very differing among these products? Do the working coefficients increase or decrease over time? How much humus can we build-up in the soil per metric ton of applied product?...

Research approach
Several composting experiments where the solid fraction of mechanical digestate separation was mixed with other biomass types, were started on the ILVO composting site in a search to the optimal mix and optimal conditions to compost solid fraction. The products of these composting experiments and a range of other processed digestate products were characterized with several incubation and pot experiments. The N mineralization was determined in an incubation experiment where 170 kg N total/ha (MAP IV maximum allowed N application) was incorporated  to a soil. By monitoring the mineral N content (nitrate and ammonium) of the soil, the percentage of N mineralization per added unit of total N was calculated. In a pot experiment with annual ryegrass in the greenhouse, P and K working coefficients were determined, based on the dry matter crop yield and the P and K uptake by the crop. For this pot experiment we used a P and K deficient soil, and the N fertilization was sufficiently increased to avoid N from being the growth limiting nutrient. The decomposition of the organic material was tested in an incubation experiment with addition of the digestate products to a soil. By monitoring the CO2 flux, the amount of C mineralization and potential build-up of humus in the soil can be calculated.

Because crude digestate products are often further processed by mechanical separation, drying, pelletising, composting and blending with other organic materials, their composition (dry matter content, organic matter content, NPK, etc) can be very different. Both suppliers and customers (farmers) benefit from a good estimation of the NPK working coefficients and the potential of these products to build up the humus in the soil.

External partner(s)
Vlaams Coördinatiecentrum Mestverwerking
VLACO vzw - Vlaamse compostorganisatie
Effective start/end date1/01/1431/12/17

ID: 4151165