Air quality management in the uMhlathuze municipality using air dispersion modelling.

Abstract

Air pollution has increased over time due to human population growth, industrialisation and other economic activities which have led to global and localised deterioration in air quality. The uMhlathuze Municipality, located on the KwaZulu-Natal North Coast is one such local area that has a rapidly developing Industrial Development Zone, currently comprising many large and small scale industries. These large-scale operations are amongst South Africa’s largest process industries and operate continuous combustion processes which release significant quantities of air pollutants into the atmosphere. These pollutants include reduced sulphur gases, mercaptans, hydrogen sulphide, sulphur dioxide (SO2), sulphur trioxide, carbon dioxide, particulate fluoride and ammonia. In light of the promulgation of the National Environmental Management: Air Quality Act (AQA) of 2004 and the need to assess ambient air quality, the contribution of air dispersion modelling to ambient air quality management in the uMhlathuze Municipality was assessed using SO2 as an indicator pollutant. The Gaussian puff urban air dispersion model called Calpuff was used to model five scenarios including a control run with actual emissions data; a worst-case run using permitted emissions data; and three emissions reduction scenarios using 25%, 50% and 75% reductions of the permitted data.. The results of these modelling scenarios were compared with results of other modelling studies recently conducted in the uMhlathuze Municipality, as well as with the South African Ambient Air Quality Standards (SAAAQS) for SO2. The results revealed that the permitted emissions scenario led to exceedances of the SAAAQS 1-hour and 24-hour average concentrations over most of the uMhlathuze Municipal area. The use of the permitted emissions values produced higher SO2 concentrations over the study area than the control run that comprised current emissions values. The control scenario produced similar results to the scenario in which there was a 50% reduction in permitted emissions data and suggests that the industries are operating at half of their permitted levels of SO2 emissions. The reduction of the permitted emission by 75% shows a significant decrease in the area exceeding the SAAAQS 1-hour standard, and compliance with the SAAAQS 24-hour and annual average standards. The results of this study for the control scenario based on actual emissions were higher than previous studies conducted in uMhlathuze due to a larger quantity of SO2 emissions used in the modelling exercises, different meteorological data sets and different air dispersion models used. However, there is a close correspondence between the Airshed (2006) results and this study when similar quantities of SO2 emissions were modelled in the permitted emissions scenario. In view of the exceedances experienced in the control run and permitted emissions scenarios, it is likely that under the AQA, some reduction in emissions will be required. In line with the Department of Environmental Affairs and Tourism National Framework classification system, the City of uMhlathuze is likely to fall under a Class 4 area, in which ambient concentrations of SO2 can pose a threat to the health and well-being of people. Immediate air quality management action plans that have specific timeframes for compliance with the ambient standards are required. The National Framework notes that the air quality impact of an industry will be assessed before an Atmospheric Emission License is granted and implies that each industry is required to undertake an air quality specialist study to determine its individual impact on ambient air quality. The air quality specialist study should include air dispersion modelling to assess the ambient SO2 concentrations; a health risk assessment based on the results of the dispersion modelling; and mitigation measures that are required to ensure compliance with ambient standards through the use of the Best Practicable Environmental Option (BPEO).