Thermochemical treatment of biogas digestate solids to produce organic fertilisers: The effect of acidification and drying on N losses, soil N turnover and plant fertilizer value

Research output: Book/ReportPh.D. thesisResearch

  • Athanasios Pantelopoulos
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 (Paper I),
ii) determine their C and N dynamics after soil incorporation (Paper II) and iii) assess the plant N
and P uptake of ryegrass amended with different thermochemical treatments of the solids (Paper
III). For a more mechanistic understanding of the processes involved in the N cycle in the solidssoil-
plant system, 15N has been utilized at the second and third study.
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
Original languageEnglish
PublisherDepartment of Plant and Environmental Sciences, Faculty of Science, University of Copenhagen
Number of pages118
Publication statusPublished - 2015

ID: 160641829