Odour impact simulation of a large urban wastewater treatment plant through the numerical solution of a Eulerian model

This study aims to develop a solver to calculate the dispersion of emitted odour from the main sources located in a large urban wastewater treatment plant (WWTP). Its seasonal odour impact on surrounding areas, including nearby populations, was also evaluated. Different seasons of the year were studied using the prevailing meteorological conditions in each case, within the framework of a Eulerian model. Dynamic olfactometry was used to measure the odour concentration (OC) of the main emission sources of the WWTP, with such data being input parameters of the model. The calculations were carried out by a robust and precise fully-implicit-temporal-discretisation scheme and an exponential spatial scheme (in the control volume formulation), which was solved using the Modified Strongly Implicit (MSI) method. A code in the programming language Fortran90 was developed to calculate the odour immission concentration (OIC). Odour emissions from the WWTP were found to derive mainly from the wastewater line, which contributed with 98.86% of the total emission of the facility, with odour emission rates (OERs) as high as 62,100 ou_E/s, 55,800 ou_E/s, 88,400 ou_E/s and 11,300 ou_E/s in the pre-treatment header, sand and fat removal, primary settling and biological treatment, respectively. Such values corresponded to summer, which is the season that registered the most intense emissions. The first two odour sources and the units for gravity thickening, flotation thickening and sludge dehydration consisted of odour treatment systems based on adsorption by granular activated carbon (GAC). Gravity thickening achieved the highest OER value (1500 ou_E/s) in the sludge line, but this only contributed with 1.14% to the total emission of the WWTP. Similar OER values were observed in other seasons of the year, although somewhat lower. The highest odour impact (538 ou/m³) was predicted in the south direction in autumn, which corresponded to the lowest wind speed in the main direction (1.23 m/s).