Analysis of the methods for determining the settling characteristics of sewage and stormwater solids

Abstract

In order to optimise the design and the management of sewer networks and settling tanks, there is a need to determine the settling velocity characteristics of sewage and stormwater solids. Devices and test methods for measuring settling velocities of these solids have been developed (Chebbo, 1992), (Michelbach and Wohrle, 1993) and (Tyack et al., 1993). In order to quantify the influence of the experimental procedure on the settling velocity grading curves, we have compared three selected methods (Cergrene, Aston, Camp) when tested with the same sample with the method developed by UFT (Michelbach and Wohrle, 1993). The results of the comparative tests indicate that the settling velocities are significantly higher with the UFT test procedure than the other methods. This is mainly explained by the fact that the UFT method takes only settleable solids into account. When the calculations of the four methods are made with the total mass of SS, the study has shown that the settling velocity curves of the same samples are different. The differences vary between the nature of the samples and the test procedures which were compared. The influence of the experimental procedure for separating a sample by settling velocities fractions is demonstrated. In conclusion, we explain the need to develop an international test procedure in order to be able to compare the results.

Characterizing the settleability of grit particles

“…Over the years, several static settling columns have been developed, for example, Aston column (Tyack, Hedges, & Smisson, ), Umwelt‐ and Fluid‐Technik (UFT) column (Michelback & Wöhrle, ), CERGRENE protocol (Chebbo, ), U.S. EPA column (O’Connor et al, ), and the newer ViCAs protocol (Chebbo & Gromaire, ). The height of these settling columns varies between 0.2 and 1.8 m, with sample volumes ranging between 1 and 40 L. Also, some protocols include a sample preparation step applying a pretreatment such as sieving or settleable solids preselection (Aiguier et al, ; Lucas‐Aiguier, Chebbo, Bertrand‐Krajewski, Gagné, & Hedges, ). In addition, on several occasions, these methods have been compared and the results show different PSVD curves (Aiguier et al, ; Krishnappan et al, ; Lucas‐Aiguier et al, ).…”

“…The height of these settling columns varies between 0.2 and 1.8 m, with sample volumes ranging between 1 and 40 L. Also, some protocols include a sample preparation step applying a pretreatment such as sieving or settleable solids preselection (Aiguier et al, ; Lucas‐Aiguier, Chebbo, Bertrand‐Krajewski, Gagné, & Hedges, ). In addition, on several occasions, these methods have been compared and the results show different PSVD curves (Aiguier et al, ; Krishnappan et al, ; Lucas‐Aiguier et al, ). Lucas‐Aiguier et al () pointed out that these differences are probably due to the different sizing of the columns.…”

A Water Elutriation Apparatus for Measuring Settling Velocity Distribution of Suspended Solids in Combined Sewer Overflows

“…Examples of columns belonging to the first group include the U.S. EPA Column (Dalrymple et al 1975;O'Connor et al 2002), Aston Column of the Aston University in the U.K. , Umwelt-und Fluid-Technik Apparatus, Germany (also known as the Brombach column; Michelbach and Wöhrle 1993), and the Cergrene apparatus (Chebbo 1992;Aiguier et al 1996). While the individual columns differ in their design and operational procedures (Aiguier et al 1996(Aiguier et al , 1998, all suffer from a limitation of quiescent settling, which is not typical for actual CSO settling tanks. The need to account for settling under flow turbulence led to the development of a more complex apparatus incorporating an oscillating grid (Rasmussen and Larsen 1996).…”

“…This proliferation of testing methods raises the question of how similar the settling velocity characterizations produced by these methods are, and consequently, a number of researchers conducted comparative studies (Aiguier et al 1996(Aiguier et al , 1998Tyack et al 1996;O'Connor et al 2002). Aiguier et al (1996) compared four columns (i.e., Aston Column, UFT column, Andreasen pipette and the Cergrene settling column) and observed that for the same sample, the different columns indicated different settling velocity distributions.…”

“…A similar conclusion was reported by O'Connor et al (2002), who compared the U.S. EPA column and the Aston column. In both of the above studies, the authors also have highlighted some of the practical difficulties associated with the settling column measurements, including the difficulty in establishing a uniform concentration of the particles in the settling column at the start of a test and the inability to measure the fast settling fraction of the sediment in the initial phase of the test (Aiguier et al 1996(Aiguier et al , 1998O'Connor et al 2002). Furthermore, a major drawback of the settling column measurement is particle settling under quiescent conditions; in full-scale, on-line settling tanks, particles undergo dynamic settling in the presence of flow.…”