Environmental impact assessment for wastewater treatment plan
SMW Daldowie wastewater treatment plant in Daldowie, Scotland received a number of odour complains from the local residents and was given an Enforcement Notice issued by the Scottish Environment Protection Agency (SEPA). Sinclair Knight Merz (SKM) was contracted by SMW to undertake a design and performance review of the existing installation and to assess the available options for improving the odour abatement technologies at the plant in preparation of a response to SEPA. The project included a study of the effects of wind conditions and plant layout on flue gas dispersion. This provided vital information on how the odour gas is spread outside of the site boundary and how changes in plant layout can reduce the odour problem at Daldowie.
CFD modelling
A Computational Fluid Dynamics (CFD) study was undertaken by SKM to investigate the impact of wind loading and plant layout on flue gas dispersion around the wastewater treatment plant. The model covered an area of 800mx800m and included the main building, three exhaust stacks and a number of storage tanks as shown in Fig. 2. In the CFD model, a boundary layer wind profile was applied as a boundary condition upstream of the plant in order to model different wind speeds and directions. The mass flow-rate, temperature and physical properties of the flue gas were used in the modelling of the discharge of flue gas from the plant’s three stacks into the surrounding air.
Outcomes of CFD modelling
The CFD model predicted the flow of air and flue gas around the buildings for various wind directions and speeds. Recirculation zones containing high flue gas concentration were identified for several wind directions. Fig. 2 indicates that the main building effectively acts as a ‘bluff body’ when the wind is in a direction normal to it (i.e. approaching from the South East). In this case, the resulting recirculation zone behind the building acts to draw the flue gas from the stacks downward and entrains it. The CFD study suggested that a significant improvement in flue gas dispersion could be achieved by relocating the stacks, increasing their length or reducing their diameter (equivalent to an increase of gas discharge speed).