A New Grit Removal System for Applications in Municipal Wastewater Treatment Plants

  • Author / Creator
    Nandi, Biswajit
  • There is a fundamental need to improve grit removal techniques for municipal wastewater treatment plants (WWTP) to meet newer effluent standards and to deal with the rapid growth of urbanization and industrialization. In response to this need, secondary and tertiary treatment processes are being developed, and these will continue to be limited without significant improvement of grit removal systems. Modern grit removal units separate fat, oil, grease and organic materials (hereafter named FOG) coated on grit particles by energetic turbulent flows followed by settling of the cleaned grit particles in quiescent flows contained in the same tank. Generating these two flows in a single tank is a problem due to complexities of tank design and operation. This research proposed a simplified grit removal system that comprised of a mixing tank equipped with multiple transverse jets for the separation of FOG and grit, and a grit tank for settling grit particles. The preliminary experiments were carried out in a laboratory-scale hydraulic model of a mixing tank followed by a grit tank. The grit tank was a 1/15th scale model of a prototype aerated grit tank located at the Gold Bar WWTP in Edmonton, AB. The mixing tank, which was 42.7 cm square and 43.6 cm high, was set upstream of the grit tank. Laboratory experiments included measurements of residence time distribution (RTD) and planar laser-induced fluorescence (PLIF). A total of 23 RTD measurements were conducted with 18 repeats in the mixing tank, and six tests with three repeats were conducted in the PLIF measurements. The experimental variables were ten jet layouts (Λ), six jet-Reynolds numbers (Re) and two jet diameters (d) in the RTD measurements to determine the values of Λ, Re and d to achieve the maximum mixing performance of the mixing tank. Reynolds number of jets were varied in the PLIF measurements to test the effects of Re on the mixing performance and to visualize mixing flow in a plane. Based on the laboratory experiments, a layout, diameter and a jet-Reynolds number were determined for field experiments at Gold Bar WWTP to test the effectiveness of the mixing tank in removing grit and FOG. Three repeated tests were conducted with the specified conditions of jets and other three repeated tests were conducted without jets (control test). In analyzing the RTD measurements, a dispersive compartmental model (DCM) was developed with consideration of dispersive nature of the plug flow compartment. The DCM showed better performance in evaluating the effects of Λ, Re and d on the mixing performance of the mixing tank than the conventional models of reactors. The best mixing performance of the tank was achieved at a layout with 8 jets, 5.3 mm diameter, and Reynolds number larger than 20600. In PLIF experiments, the DCM was used to estimate the mixing performance, which was reasonably matched with the mixing performance estimated in the RTD measurements. Mixing performance was increased with increases of Reynolds number larger than 16700, where dead flow zones were observed at a corner of the tank. In the field experiments, the conditions of the tests with jets were the layout with eight jets, 5.3 mm jet-diameter, and Reynolds number of 41600. Reynolds number was increased to this value to ensure energetic turbulence that was required to clean grit from FOG. The results of the field experiments showed that concentration of total suspended solid (TSS) was reduced at the effluent of the grit removal system by 28.9% of influent to the system in the tests with transverse jets, whereas the tests without jet showed no significant difference in TSS between influent and effluent flows. The new system was able to remove larger amount of grit than the existing capacity of grit removal tank of Gold Bar WWTP. In addition, 95% of 44µm particles were removed in the new system. This indicated that the mixing tank was effective in cleaning grit from FOG that led to the settling of very fine particles in the grit tank. This inclusion of a mixing tank for grit cleaning can be used in new WWTPs and, most importantly, can be easily added to the thousands of existing WWTPs in Canada to synergistically work with the other advanced technologies for meeting effluent standards.

  • Subjects / Keywords
  • Graduation date
    Spring 2016
  • Type of Item
  • Degree
    Doctor of Philosophy
  • DOI
  • License
    This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.