Characterizing the settleability of grit particles

Abstract

Grit chambers are installed at the headworks of a water resource recovery facility (WRRF) to reduce the impact of grit particles to the equipment and processes downstream. This settling process should thus be designed and operated in an efficient way. Despite the importance of knowing settling characteristics for design and operation of grit chambers, previous grit definitions have been based only on particle size characteristics, and not on settling velocities. Thus, this study presents an evaluation of the performance of two promising settling velocity characterization methods, ViCAs and elutriation, to characterize wastewater particles in view of the design and the optimization of the efficiency of the grit removal unit. © 2019 Water Environment Federation • Practitioner points• Settling characteristics are the governing parameter for grit chamber design. Since grit particles are vastly heterogeneous, it is preferred to measure these characteristics directly rather than to estimate them from particle size (and assumptions of density, form factor, …). • More detailed settling information about grit particles can improve grit chamber design and estimation of removal performance. • Adapted ViCAs and elutriation methods for faster settling particles allow studying the particle settling characteristics in a grit chamber. These methods are simple, fast, and cheap and only require small wastewater samples. • A relationship was found between the influent TSS concentration and the location of the PSVD curve, with higher TSS concentrations corresponding to higher settling velocities. • It was demonstrated that the dynamics of the wastewater characteristics under dry, wet, and snowmelt weather conditions influence grit settling characteristics.

Side-by-Side Testing of Grit Separation Units

“…When vortices are formed asymmetrically around the body (with respect to its mid-plane) due to shape or uneven grease coverage, different lift forces develop on each side of the particle that cause motion traverse perpendicular to the flow line [13], increasing buoyancy and hindering removal. Moreover, separated grit moves along the bottom of the FVS to reach the hopper in the center of the unit and, if vortex-induced vibrations lift a particle off the surface, the upward flow near the discharge structure of the FVS can carry it over with the treated effluent [14]. Similarly, grit traps are often installed in combined sewer systems to remove grit before the material reaches the treatment facility but grit re-entrapment into the bulk of the liquid can occur if not designed properly [15].…”

Characteristics of sand recovered from the municipal wastewater treatment plant

“…An ideal sand particle considered in a design of grit chambers is a spherical, homogeneous one, with a size of 200 microns and a specific gravity of 2.65 N•m -3 (Tchobanoglous et al 2014). typical sand specific gravity ranges from 2.64 to 2.72 N•m -3 , although the overall specific gravity of the particles in wastewater is lower due to the layers of organic material that normally cover the particle surface and range from 1.1 to 2.65 N•m -3 (Plana et al 2020). In real life, the ideal conditions for sand separation are rarely met, as the material can be of a different origin, depending on the characteristic of the drainage area and the composition of raw wastewater -both these factors have an impact on sand characteristics (Kolosovska and bauer 2022;Judd et al 2017).…”