Determination of the fertilizer value of processed digestate products (VLACODIGEST)

From digestion plants of organic materials such as manure, organic waste, energy crops and the like, two products come out: biogas and - as a by-product - digestate. Digestate can be further processed and/or composted and become available on the market as an organic fertilizer. 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. The research questions are thus: 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?...

At the ILVO composting site, several composting experiments were started where the solid fraction of mechanical digestate separation was mixed with other biomass types. The goal was to find the optimal mix and optimal conditions to compost solid fraction. The end-products of these composting experiments and a range of other processed digestate products were characterized with several incubation and pot experiments. We first determine nitrogen mineralization in an incubation trial in a climate chamber, applying digestate products to a soil at a dose of 170 kg N-total/ha (MAP IV standard for "other fertilizers"). At fixed intervals, we record the mineral N content of the soil (NO3- and NH4+). We calculate the percentage of 'active nitrogen' (% of applied N that mineralizes). Some products are also analyzed for P and K activity via a pot trial in greenhouse conditions with Westerwold ryegrass. On the one hand, we measure the crop yield and, on the other hand, the P and K contents of the grass. Finally, we study the decomposition of organic matter from the digestate product in an incubation trial in a climate chamber, applying digestate products to a soil. Through monitoring of CO2 release, we calculate how much organic matter is degraded in the soil. This allows conclusions about the exact contribution of the digestate product to the effective organic matter content of the soil.

The composition (dry matter content, organic matter content, NPK, etc) of crude digestate products can be quite different. Because pure (unprocessed) digestates are less suitable as a nutrient for the field, people often still process the products. Mechanical separation, drying, pelletization, mixing with other products, composting, etc. is done to obtain a higher-value marketing product (soil conditioner and fertilizer). 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