6.3 Thanos Pantelopoulos – University of Copenhagen

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WP 6.3: Assessment of soil quality effects and nutrient availability of manure ash and biochar based biofertilisers, Athanasios Pantelopoulos, UCPH 

Anaerobic digestion of animal manures has been proposed as a process with twofold advantage. The production of biogas, a renewable source of energy, and the treatment of animal manures to increase their agronomic value and reduce their environmental impact. However, the residual of anaerobic digestion, called digestate, may need a combination of treatments to improve its manageability, reduce its environmental impact and facilitate the exportation of excessive nutrients to other areas.

Often, mechanical separation, is implemented as a first treatment of the digestate. The resulting solids from digestate, are acknowledged for their potential to serve as organic amendments and fertilizers however, their characteristics constitutes them prone to N losses, and their management (handling, storage, transportation) costly. Thermal drying of manures is known to facilitate transportation by volume reduction, nutrient concentration and hygienization of the final product. However, thermal treatment of ammonium rich organic wastes such as digestate solids has been linked with relative high volatilization of NH3 and therefore decrease in N fertilizing value of the final product. Temperature and air velocity during thermal treatment influence the evaporation rate of water from the manure solids. At the same time, they also influence the ammonia emission rates, Lowering manure pH (controlling the NH4+ - NH3 equilibrium) can potentially reduce the loss rate. Furthermore, the changes occurring in the solid digestate after acidification and/or drying should impact its behavior in the soil specifically, their N and P availability.

The objective of this study was i) to assess the effects of different acidification levels (by addition of concentrated sulphuric acid) and drying conditions on solids digestate nitrogen content, ii) determine their C and N dynamics after soil incorporation and iii) assess the plant N and P uptake of ryegrass amended with different thermochemical treatments of the solids.

In conclusion, drying of digestate solids resulted in an end-product with increased stability and reduced mass/volume which can facilitate its storage and transportation. Nevertheless, dried digestate solids had low N fertilizing value due to the excessive loses of inorganic N during the drying process. On the contrary, acidification minimized ammonia volatilization from solids during the thermal treatment with direct impact on the N fertilizing value of acid treated solids. In addition, acidification promoted the stability of the organic matter in the acidified solids indicating a higher potential for C sequestration from this treatment.

Phosphorus solubility increased by drying and acidification however, no significant differences in plant P uptake were detected mainly as a result of soil properties on plant P availability. A combined acidification and drying treatment of digestate solids may be proved an interesting option for increasing the fertilizing value of the final product and reduce the mineral fertilizing dependency of the agricultural sector. Despite the added cost of acidification, in the digestate solids management chain, the fertilizing value and stability of the end product may compensate for the acid-related expenditures.

Nevertheless, alternatives to sulphuric acid should be exploited, to avoid excessive application of sulphur via acidified treatments, in the agricultural system. For a full utilization of acidification and drying as digestate solids treatment, a more systematic assessment of the effect of the thermochemical treatment on P availability is required. Moreover, the ameliorating properties of thermo-chemically treated solids should be assessed in comparison with known manure stabilizing treatments i.e. composting. Finally, the energy cost associated with thermal drying of bio-wastes is expected to largely determine the degree implementation in different areas. 

  • Net and gross Nitrogen turnover in soil amended with acidified and differently dried solids from biogas digestate. Article submitted.

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