Elsevier

Chemical Engineering Science

Volume 63, Issue 21, November 2008, Pages 5283-5290
Chemical Engineering Science

Fundamental dewatering properties of wastewater treatment sludges from filtration and sedimentation testing

https://doi.org/10.1016/j.ces.2008.07.016Get rights and content

Abstract

The prediction and optimisation of the performance of dewatering devices such as filters and thickeners requires the laboratory measurement of fundamental material characteristics. This work presents the application of recently developed methods for the extraction of meaningful phenomenological properties from filtration and sedimentation testing of wastewater sludges, which has previously been limited due to theoretical and experimental constraints. The results for a sample of digested wastewater treatment sludge described a material with weak network strength that was quite permeable at low solids concentrations and the gel point was about 1 v/v%. The sample compressed to high solids concentrations at moderate pressures (up to 40 v/v% at 400 kPa) but became extremely impermeable at these high concentrations. The solids concentration dependencies of the material characteristics had a form that was expected for highly compressible materials such as wastewater sludges, but had not been reported previously. The extracted material characteristics were then used to predict the test results to show that the characterisation was valid.

Introduction

Industrial-scale domestic wastewater treatment (WWT) processes produce sludges that consist of a matrix of fibrous matter, organic and inorganic particles (such as micro-organisms, contaminants and dirt), and extracellular polymer (ECP). The ECP, which binds the other components together, consists of various proteins, organic acids, lipids and polysaccharides. Dewatering aids such as flocculants and coagulants are also added, further diversifying the sludge matrix. Common disposal methods for WWT sludge include application to land as a fertiliser, removal to landfill and incineration. Plant operators reduce the cost of transportation and disposal of sludge by using solid–liquid separation equipment (such as thickeners, centrifuges and filter presses) to increase the solids content.

Research into the compressional behaviour of wastewater sludges in dewatering devices aims to give accurate prediction of the performance of these operations, thereby enabling optimisation of throughput and final solids content such that the economic, social and environmental costs of sludge handling and disposal are reduced. Laboratory studies involving dead-end filtration and batch sedimentation tests are two ways to investigate the compressional dewatering properties of sludges. Until recently, theories for suspension dewatering (and subsequent analysis of experimental data) developed separately based on filtration and sedimentation modelling. Filtration theory (Ruth, 1946, Terzaghi and Peck, 1967, Tiller and Shirato, 1964) developed from considerations of packed bed behaviour while sedimentation theory (Kynch, 1952) developed from considerations of particle settling velocities. However, these theories have some shortcomings (detailed later) when applied to extremely compressible materials such as WWT sludges.

Buscall and White (1987) developed a phenomenological description of suspension dewatering in which the sludge forms a continuous network that is capable of withstanding a load at concentrations above the gel point, φg, where φ is the local solids volume fraction. The network strength is given by the compressive yield stress, Py(φ). When Py(φ) is exceeded, the network structure fails and dewaters at a rate given by the solid–liquid interphase drag or hindered settling function, R(φ). The third critical parameter is the solids diffusivity, D(φ), which is related to Py(φ) and R(φ):D(φ)=dPy(φ)dφ(1-φ)2R(φ)

By modelling the suspension network as a continuous yield structure and using the local volume fraction to define the local properties, phenomenological theory has been shown to be an accurate depiction of the dewatering of compressible inorganic networks, such as mineral slurries (de Kretser et al., 2001) and water treatment sludges (Harbour et al., 2004, Stickland et al., 2006a). Py(φ), R(φ) and D(φ) are state properties that describe the dewatering behaviour under any compressional forces and have been used to describe and model sedimentation (Buscall and White, 1987, Howells et al., 1990), filtration (Landman et al., 1991, Landman and White, 1997, Martin, 2004, Stickland et al., 2006a) and centrifugation processes (Barr and White, 2006, Stickland et al., 2006b). Related phenomenological modelling has been produced by Bürger and Concha (1998). The aim of this work is to measure the compressional rheological properties of wastewater sludges across a large volume fraction range for use in such models.

Filtration tests are used to determine the high volume fraction dewatering characteristics of sludges. Constant pressure filtration tests give results of the filtration time, t, as a function of the specific filtrate volume, V. For many sludges such as mineral and water treatment sludges, filtration is typically dominated by linear t versus V2 behaviour (cake formation) followed by short asymptotic behaviour (cake compression). An example of this “traditional” behaviour is shown in Fig. 1(a). The extraction of dewatering parameters usually relies on determining the slope of t versus V2 during cake formation as a measure of the permeability and the equilibrium solids concentration, φ, as a measure of the compressibility (de Kretser et al., 2001). Extended linear t versus V2 behaviour is observed for both initially un-networked (φ0<φg) and networked materials (φ0φg), where φ0 is the initial solids volume fraction.

Very different filtration profiles are observed for WWT sludges—there is little cake formation followed by long cake compression (see Fig. 1(b)). In such cases, the short linear behaviour competes with membrane resistance and initially variable pressure effects, while compressibility data are obtained only after extremely long filtration times and errors due to evaporation and/or sludge ageing increase. It has been shown (Stickland et al., 2005a) that such a behaviour is predicted by phenomenological theory provided that D(φ) predominantly decreases over the volume fraction range in question (φ0 to φ). The difficulty under these conditions is the experimental determination of the relevant dewatering parameters. The extraction of permeability cannot involve a classical analysis of the slope of the cake formation stage of a t versus V2 plot. In addition, the instability of wastewater sludges precludes extended filtration analysis.

Useful information can be extracted from cake compression (de Kretser et al., 2001, Kapur et al., 2002). The analytical solution to constant pressure filtration (Landman and White, 1997) shows that the asymptotic behaviour during cake compression is given by a Taylor's series expansion that is logarithmic to first order and can be expressed ast=E1-E2ln(V-V)where E1 and E2 are constants and V is the equilibrium filtrate volume. A useful alternative is to replace V with h (the filter piston height in dead-end filtration) such that (V-V) is replaced by (h-h). E2 is given byE2=4Dh0φ0πφ2where h0 is the initial height and D is D(φ). h is related to φ through the conservation of volume:h=h0φ0φHere, φ and D are given by fitting Eq. (2) to experimental filtration results. Since Eq. (2) is the first-order term of the solution, only the end of the cake compression results are fitted and there is no need to determine the transition from cake formation to cake compression. The two unknown parameters are the number of data points and φ.

Conventional filtration theory (Ruth, 1946) predicts linear t versus V2 behaviour but applies only for material initially below the gel point. The extraction of permeability information requires a priori knowledge of the functional form of the constitutive equations and equilibrium measurements are seldom made (Tiller and Li, 2000). For materials initially above the gel point, consolidation modelling (Terzaghi and Peck, 1967) predicts logarithmic behaviour but is derived using more rigid assumptions than Landman and White (1997), namely that the consolidation coefficient (related to D(φ)Stickland et al., 2005b) is constant and the pressure distribution in the cake is known (that is, a functional form for Py(φ) is required). The transition to cake consolidation must also be known, which can be difficult to determine (Christensen et al., 2006). Further work has included increased complexity to resolve some of the inherent problems in the conventional theory, by including extra driving forces such as creep (Shirato et al., 1986) and osmotic pressure (Keiding and Rasmussen, 2003). However, these theories require known constitutive equations to extract average parameters. A key conceptual difference between the conventional filtration approach and phenomenological modelling is seen in the definition of specific cake resistance and solids pressure from the operating conditions of the filtration process rather than as general material properties defined independently of their application.

Sedimentation tests are used to extract low volume fraction information about the material, nominally at concentrations below the gel point, whereas filtration cannot access data in this concentration regime. Suspensions are poured into measuring cylinders and allowed to settle. The height of the interface, h, is measured with time. Conventionally, the slope during the initial linear phase is used to give the permeability at the initial concentration (Kynch, 1952) and multiple tests at various concentrations are used to give the permeability as a function of φ0. Lester et al. (2005) recently described an analytical method for extracting phenomenological properties as a function of concentration from a single transient test by measuring h(t) until equilibrium is approached. The method requires high volume fraction data from filtration or centrifugation testing to allow data interpolation close to φg.

This work presents results for the filtration and sedimentation testing of a sample of digested WWT sludge. The compression results from dead-end constant pressure filtration were analysed using logarithmic curve fitting to produce high volume fraction Py(φ) and D(φ) results. The results from transient batch sedimentation were analysed to give low volume fraction R(φ) and Py(φ) (including φg), and combined with the filtration results to give continuous functions of the material properties over a large concentration range. Finally, these material properties were used in a numerical filtration model (Landman et al., 1991) to predict the full form of the original filtration results, illustrating that the characterisation was valid.

Section snippets

Materials and methods

A digested WWT sludge was sampled from Luggage Point WWT Plant in Brisbane, Australia. The solids volume fraction, φ0, was measured by oven drying as 0.0254 v/v (using a solids density, ρs, of 1100kg/m3, also measured gravimetrically). In order to minimise biological activity, the sample was stored at 4 °C and allowed to warm to room temperature when required. The sample (approximately 200 ml) was dosed with flocculant (Zetag 7587) and stirred slowly with a magnetic stirrer bar for 2 min prior to

Results and discussion

A total of seven constant pressure filtration runs were performed at pressures ranging from 2 to 400 kPa using a fixed initial solids and initial filtration height. A summary of the final cake solids, φf, and filtrate volume, Vf, is given in Table 1. Repeat tests were performed at low pressures, and although reproducibility of the extracted dewatering parameters was good, direct comparison between filtration data sets was difficult as it was found difficult to keep the initial height and

Conclusions

Batch settling and pressure filtration experiments were performed to measure the compressional rheological properties of digested WWT sludge. High volume fraction compressibility and diffusivity data were extracted from the compressional data of a series of filtration tests. Low volume fraction permeability and compressibility data (including φg) were extracted from batch sedimentation. The results were combined to give continuous functions of Py(φ), R(φ) and D(φ). The properties varied by many

Notation

D(φ)solids diffusivity, m2/s
E1,E2logarithmic fitting constants, s
hiInterfacial height, m
Py(φ)compressive yield stress, Pa
ΔPapplied pressure, Pa
R(φ)hindered settling function, Pas/m2
ttime, s
Vspecific filtrate volume, m
Greek letters
ρssolids density, kg/m3
φsolids volume fraction, v/v
φggel point solids volume fraction, v/v
Subscripts
0initial
ffinal
equilibrium

Acknowledgements

The authors wish to acknowledge the financial support of the Particulate Fluids Processing Centre (a Special Research Centre of the Australian Research Council) and industrial sponsorship from United Utilities PLC, UK and Yorkshire Water Services Ltd, UK.

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