Effect of slurry separation and air-plasma treatment on NH3 and VOC emissions from field applied biogas digestate and pig slurry to grassland

Different technologies can be utilised to mitigate environmentally harmful ammonia (NH₃) emissions after field application of liquid animal manure (slurry). After a solid-liquid separation, air-plasma technology can acidify the liquid fraction and enrich its nutrient value by increasing the amount of inorganic nitrogen. The present work investigates the emissions of NH₃ and volatile organic compounds (VOC) after field application of the following fractions of pig slurry and slurry digestate: i) untreated slurry (UN), ii) liquid fraction of slurry (LF), iii) liquid fraction of slurry treated with air from the plasma treatment (LP). Emissions were measured with a system of wind tunnels and a cavity ring-down spectrometer for NH₃ concentration measurements and a proton-transfer-reaction mass-spectrometer for measurements of VOC. For both slurry types, the cumulative NH₃ emissions were in the following order UN > LF > LP. All the differences were significant (P < 0.05), except between pig slurry LF and LP. The reduction in cumulative NH₃ emission obtained by the treatments compared to UN were 55–74% and 70–89% for LF and LP, respectively. The slurry separation decreased dry matter by 46–54% and resulted in a rapid decrease in slurry exposed surface area after application, presumably due to high infiltration. Several VOCs were measured after application of the slurry, but continuous emission was undetectable for all VOCs. The very low VOC emission was presumably due to high infiltration of the low dry matter slurry treatments and low concentration of VOC in the digestate.

Science4Impact Statement

This work demonstrates how treating slurry with plasma treated air can mitigate ammonia emissions after field application. The presented findings can be used for additional technology development and verification. Future research efforts should e.g. clarify what level of solid-liquid separation is needed before treating the liquid fraction with plasma treated air, to assess whether the additional ammonia reductions are profitable. Furthermore, the findings can be used by decision makers and advisory bodies to assess the compliancy of this slurry application technology with applicable environmental regulations. The work also highlights important remaining knowledge gaps that need to be investigated before the technology can be deemed fit for wider practical application.