REC was commissioned to undertake a Dispersion Modelling Assessment of potential atmospheric emissions from a proposed anaerobic digestion plant and associated 2MW biogas engine in Surrey. The local planning authority had requested consideration of potential air quality impacts associated with the development on both human and ecological receptors in the vicinity of the site due to concerns over increases in pollutant levels. A stack height assessment was also undertaken in order to determine a suitable design to control impacts to an acceptable level.
The Dispersion Modelling Assessment was undertaken using ADMS-4, an advanced modelling software package suitable for the prediction of plume dispersion from industrial sources. The assessment considered a number of potentially harmful emissions, including nitrogen dioxide, sulphur dioxide, carbon monoxide and volatile organic compounds (VOCs), as well as nitrogen and acid deposition at nearby ecological sites. A sensitivity analysis of different meteorological conditions was included in order to ensure all potential situations were considered and the maximum potential impacts could be indentified.
The results of the dispersion modelling indicated that emissions from the proposed plant would not cause significant air quality impacts at any human receptor in the vicinity of the site. Due to the sensitivity of the eleven ecological designations within the study area, including nationally and internationally protected sites, additional consideration of potential effects was required by Natural England. This included further dispersion modelling and also identification of potential impacts associated with ammonia emissions due to specific concerns raised by the case officer. Due to a lack of available information in regards to emissions from the combustion of biogas, an innovative ‘back modelling’ solution was utilised in order to determine a suitable emission limit value for the plant and provide Natural England with a suitable degree of confidence that the relevant designations would not be compromised as a result of the development.
Following REC’s involvement in the project, Natural England accepted emissions from the site would not cause significant impacts and as such had no objections to the proposals.