Modelling surface pH and emissions of hydrogen sulphide, ammonia, acetic acid and carbon dioxide from a pig waste lagoon

Pig slurry is a source of atmospheric pollutants. Emissions of basic and acidic compounds, such as hydrogen sulphide (H2S) from slurry are largely dependent on surface pH. In a storage system, the pH at the surface changes over time due to volatilisation of ammonia (NH3), carbon dioxide (CO2) and acetic acid (HAc). In this article, a gas emission–pH (GE–pH) coupled model is proposed to describe the release of H2S, NH3, CO2 and HAc from pig slurry. The GE–pH model accounts for changes in pH and concentration of buffer components in the surface layer due to volatilisation. The model was applied to calculate gaseous emissions from an undisturbed pig waste lagoon. The predicted pH at the surface layer in the undisturbed slurry lagoon increased over time from 8.0 (initial pH in the homogenised slurry) up to 8.64. H2S emissions predicted by the GE–pH coupled model (7.61 ± 0.56 μg m−2 min−1) showed a good agreement with measured data (5.92 ± 3.90 μg m−2 min−1) compared to previous gaseous models in which changes in surface pH caused by emission of NH3, CO2 and HAc were not considered (126.01 ± 1.53 μg m−2 min−1). This improvement supports the idea that emissions of important acidic and basic gases (such as CO2 and NH3), and changes in buffer component concentrations and pH at the surface layers due to volatilisation of these gases, should be taken into account when modelling the emissions of a specific (acidic or basic) target gas, such as H2S.