Odour Emissions and Dispersion from Digestate Spreading

Abstract

Odour emissions from digestate applied on 21 October 2020 in a 2.4 ha field in the Po Valley (Casalino, 28060, Novara, Italy) were measured using dynamic olfactometry and a six-specialist odour panel, and two application techniques were compared. The measured odour emissions were 3024 and 1286 ou m−2 h−1, corresponding to the digestate application with surface spreading and direct injection, respectively. The odour dispersion for the different emission values was modeled to a distance of approx. 500 m from the center of the field and 15 m from the ground using a Lagrangian puff model (SCICHEM) in different meteorological conditions. The meteorological variables were measured at the closest station during the whole month in which the digestate application took place, mimicking a “worst-case scenario” characterized by the frequent applications along the considered period. The maximum odour concentrations within one square km area from the center of the field occurred in calm wind and stable atmospheric conditions. This study also evaluated the effect of a barrier downwind from the source. In the worst-case scenario (spreading technique with maximum emissions, no barriers), the average and maximum estimated odour concentrations were 3.2 and 18.9 ou m−3, respectively. The calculated probabilities of exceeding the threshold value of 1 ou m−3 were 36% and 47% for the whole period and the episodes of calm winds, respectively, and 14% on average for the episode of maximum wind gust. In the best emission scenario (direct injection), the average and maximum odour concentrations were 1.5 and 8.6 ou m−3, respectively, while the probabilities of exceeding 1 ou m−3 were 26% and 36% for the whole period and the episodes of calm winds, respectively, and 0.016% for the maximum wind gust episode. In the presence of a solid barrier downwind from the source and for the wind gust episode, the peak values of the concentrations and exceedance probabilities at the sampling height were found to be reduced by a factor close to 2.5 and 5 × 105, respectively. The study also evaluated the concentration field’s vertical distribution, showing that the odour plume’s vertical and horizontal dispersion slightly increased with the barrier. This is not a cause of concern unless the emitted substances causing odour nuisance are also atmospheric pollutants with potential harm to far-field ecosystems and human settlements at low concentration levels.