Abstract
The paper shows the results of a long-term research comprising FLUENT-based numerical modeling, in situ measurements and laboratory tests to analyze suspended solids (SS) transport processes in primary settling tanks (PSTs). The investigated PST was one of the rectangular horizontal flow PSTs at a large municipal wastewater treatment plant (WWTP) of a capacity of 500,000 population equivalent. Many middle-sized and large WWTPs are equipped with such PSTs. The numerical PST model was calibrated and validated based on the results of comprehensive in situ flow and SS concentration measurements from low (5 m/h) up to quite high surface overflow rates of 9.5 and 13.0 m/h and on settling and other laboratory tests. The calibrated and validated PST model was also successfully used for evaluation of some slight modifications of the inlet geometry (removing lamellas, installing a flocculation 'box', shifting the inlet into a 'bottom-near' or into a 'high' position), which largely affect PST behavior and performance. The investigations provided detailed insight into the flow and SS transport processes within the investigated PST, which strongly contributes to hydrodynamically driven design and upgrading of PSTs.
Highlights
Primary settling tanks (PSTs) of large wastewater treatment plants (WWTPs) are essential both for wastewater treatment and renewable energy production using digester gas
The main function of the inlet facility is the dissipation of both the kinetic and potential energy of the inlet jet, and to ensure a nearly uniform flow field, which is necessary for an efficient PST performance
This clearly shows that the inlet jet's kinetic and potential energy has to be satisfactory dissipated within the inlet facility, which requires a design strongly based on hydrodynamic principles
Summary
Primary settling tanks (PSTs) of large wastewater treatment plants (WWTPs) are essential both for wastewater treatment and renewable energy production using digester gas. In former times, the function of PSTs was to remove as high a rate of total suspended solids (SS) and organic carbon as possible in order to decrease SS load, aeration tank volumes, and aeration energy demands. The function and operation of PSTs have become much more complex including the control of readily biodegradable carbon between denitrification and anaerobic digestion (biogas production) (Dulekgurgen et al ; Alanya et al ) This requires detailed investigations to substantially improve our knowledge on the hydraulic and removal processes within PSTs. While secondary settling tanks (SSTs) have received much attention over the last decades (e.g. Larsen ; Günthert ; Krebs ; Deininger et al ; Patziger et al , ), behavior of PSTs has been poorly addressed
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