Essence of disposing the excess sludge and optimizing the operation of wastewater treatment:...

Chemosphere 105 (2014) 1–13

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Chemosphere

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Review

Essence of disposing the excess sludge and optimizing the operationof wastewater treatment: Rheological behavior and microbial ecosystem

0045-6535/$ - see front matter � 2014 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.chemosphere.2013.12.067

⇑ Corresponding author. Permanent address: No. 34-5-703#, Wushan Campus, Guangdong University of Technology, Tianhe District, Guangzhou 510643, PR Ch+86 20 39322295; fax: +86 20 38457257.

E-mail address: [email protected] (B. Tang).

Bing Tang ⇑, Zi ZhangSchool of Environmental Science and Engineering, Guangdong University of Technology, 510006 Guangzhou, PR China

h i g h l i g h t s

� The key factors in determining therheological behavior of sludge weredemonstrated.� The essence of operation and

management of a WWTP wasdissected.� Future research direction in this area

was recommended.

g r a p h i c a l a b s t r a c t

a r t i c l e i n f o

Article history:Received 18 September 2013Received in revised form 25 December 2013Accepted 26 December 2013Available online 24 January 2014

ext>Anaerobic digestion

Keywords:Excess sludge disposalWastewater treatmentRheological behaviorMicrobial ecosystemMembrane bioreactorAnaerobic digestion

a b s t r a c t

Proper disposal of excess sludge and steady maintenance of the high bioactivity of activated sludge inbioreactors are essential for the successful operation of wastewater treatment plants (WWTPs). Sincesludge is a non-Newtonian fluid, the rheological behavior of sludge can therefore have a significantimpact on various processes in a WWTP, such as fluid transportation, mixing, oxygen diffusion, masstransfer, anaerobic digestion, chemical conditioning and mechanical dewatering. These are key factorsaffecting the operation efficiency and the energy consumption of the entire process. In the past dec-ade—due to the production of large quantities of excess sludge associated with the extensive constructionof WWTPs and the emergence of some newly-developed techniques for wastewater purification charac-terized by high biomass concentrations—investigations into the rheology of sludge are increasinglyimportant and this topic has aroused considerable interests. We reviewed a number of investigations intothe rheology of sludge, with the purpose of providing systematic and detailed analyses on the relatedaspects of the rheological behavior of sludge. It is clear that, even though considerable research hasfocused on the rheology of sludge over a long time period, there is still a need for further thorough inves-tigation into this field. Due to the complex process of bio-treatment in all WWTPs, biological factors havea major influence on the properties of sludge. These influences are however still poorly understood, par-ticularly with respect to the mechanisms involved and magnitude of such impacts. When taking note ofthe conspicuous biological characteristics of sludge, it becomes important that biological factors, such asthe species composition and relative abundance of various microorganisms, as well as the microbial com-munity characteristics that affect relevant operating processes, should be considered.

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2 B. Tang, Z. Zhang / Chemosphere 105 (2014) 1–13

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22. Rheological parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

2.1. c, s and rheogram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32.2. Yield stress (s0, Pa) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32.3. Viscosity (l, Pa s) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32.4. Hysteresis loop area (Ha) and reduced hysteresis area (rHa) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42.5. Shear sensitivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

3. Quantitative measurement and description of the rheological behavior of sludge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3.1. Flow measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43.2. Dynamic measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53.3. On-line (or in situ) measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
4. Factors influencing the rheological behavior of sludge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
4.1. Solids content . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64.2. Surface charge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64.3. Extracellular polymeric substances (EPS) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64.4. pH value. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
5. Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
5.1. Optimizing the activated sludge process of a WWTP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75.2. Optimizing the performance of MBRs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75.3. Evaluation of filamentous bulking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85.4. Excess sludge conditioning and dewatering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85.5. Anaerobic digestion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
6. Discussion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

1. Introduction

As a result of the massive increase in human population sizeduring the past few decades, the amount of domestic sewagewastewater has increased significantly. Due to the oxygen-con-suming organic pollutants present in such wastewater, it poses aserious threat to the aquatic environment. Purification of domesticsewage wastewater is therefore increasingly important in order toabate the pollution to the aquatic environment and to avoid a crit-ical global water-shortage crisis. The biodegradable characteristicsof organic substances contained in wastewater have led to thedevelopment of numerous biological processes for the treatmentof wastewater, particularly for the treatment of organic pollutantsin domestic sewage wastewater. Biological treatment of wastewa-ter containing organic pollutants utilizes the metabolism of micro-organisms to degrade organic pollutants and to improve effluentquality (Nielsen et al., 2010). During the process of bio-purification,organic pollutants are decomposed to smaller molecules, such asCO2, H2O, CH4, N2 and H2S, either by aerobic or anaerobic pro-cesses. Uptake of some organic pollutants into somatic cells in cer-tain microorganisms also takes place, leading to the growth ofmicroorganisms and the accumulation of biomass in bioreactors.When microorganisms proliferate in abundance and flock together,they form a bio-sludge aggregation in aerobic or anaerobic operat-ing bio-treatment tanks, referred to as mixed liquid suspended sol-ids (MLSS), or ‘‘activated sludge’’. Bioactivity and concentration ofactivated sludge are essential components of bio-treatment pro-cesses (Han et al., 2012), but excessive solids may have a negativeeffect on the operation and the quality of effluents. The surplussludge, generally with a synthesis yield in the range of 0.4–0.6 kgVSS (volatile suspended solids) kg�1 COD (Khursheed and Kazmi,2011), must be discharged from the bioreactor. The bio-sludge thusacts as a substantial component for the removal of organic pollu-tants on one hand. On the other hand, it may also be the end-result

of a bio-conversion of organic pollutants and is often dischargedfrom bioreactors as excess sludge.

In the past decade, efforts to achieve effective purification ofwastewater containing organic pollutants have aroused concernson two related aspects. One aspect relates to the development ofsome bio-treatment techniques that are characterized by high bio-mass concentrations, i.e. the membrane bioreactor (MBR) (Linet al., 2012; Skouteris et al., 2012; Smith et al., 2012). The other as-pect relates to is the extensive construction of WWTPs, which hasresulted in the creation of large volumes of excess sludge (Hazratiand Shayegan, 2011; Troiani et al., 2011; Imbierowicz and Chacuk,2012). Both aspects have a close relationship with sludge in termsof hydrodynamic behavior (or fluid type) (Braak et al., 2011). Forexample, the MBR combines a traditional activated sludge processwith membrane separation, leading to a high accumulation of bio-mass within the system, which results in improved effluent qualitywith a low demand for space. Due to the effective entrapment ofsludge by the membrane modules, an operating MBR may accumu-late very high concentrations of biomass (Krampe and Krauth,2003), to as high as 20 g L�1 (Hasar et al., 2004), which is aboutten times higher than that of a traditional activated sludge process(Mori et al., 2006). Such a concentration has a strong influence tothe hydrodynamic type of sludge within the bioreactor and thusdetermines the efficiency of the operation process in terms of masstransfer, oxygen diffusion (Guimet et al., 2007), fluid flow, andmembrane fouling (Meng et al., 2007; Al-Halbouni et al., 2008).Additionally, in terms of satisfying the increasing demand ofwastewater purification, however, numerous facilities for treatingdomestic and industrial wastewater containing organic matter,have been successfully constructed and operated using bio-pro-cesses. As a result, huge volumes of excess sludge are produced,leading to serious environmental problems in many countries (Ap-pels et al., 2008; Smith et al., 2009; Abe et al., 2011; Hait and Tare,2011). Maximum volume-reduction of excess sludge is a great

B. Tang, Z. Zhang / Chemosphere 105 (2014) 1–13 3

challenge to the successful operation of WWTPs. The drive to de-velop methods for the safe treatment and disposal of excess sludgewithout any secondary pollution is therefore a serious worldwideconcern (Abe et al., 2011; Chon et al., 2011; Abelleira et al.,2012; Cheng et al., 2012). The processes employed for the treat-ment and disposal of excess sludge include anaerobic digestion,pumping, fluid transportation, conditioning and dewatering. Theseare also greatly influenced by the fluid type of the excess sludge.

Activated sludge and excess sludge are considered as a non-Newtonian fluid since the shear stress (s, Pa) is not linearly linkedto the shear rate (c, s�1). Theoretical concepts relating to suchfluids are part of the science rheology, a discipline that deals withthe deformation of bodies under the influence of external stress.The flow type determines the mass transfer, energy exchangeand momentum interactions between sludge and externalenvironmental factors. The rheological behavior of sludge is thusa crucial factor to consider when investigating processes such asfluid transportation, oxygen diffusion, mixing, settling, condition-ing, mechanical dewatering and anaerobic digestion (Sanin andVesilind, 2005; Ormeci, 2008). These processes are especiallyimportant to the development of new bio-techniques for wastewa-ter purification, as well as the highly-effective operation ofWWTPs, and they have resulted in a new focus on understandingthe basic properties of sludge, particularly the rheological behaviorand bioactivity of sludge, which are essential for the successfuloperation and management of bio-processing WWTPs. The presentpaper therefore first focuses on a critical review covering the fol-lowing topics: rheological parameters and models; factors influ-encing the rheological behavior of sludge; application ofrheological information with the purpose of optimizing wastewa-ter treatment; and the disposal of excess sludge; Second, consider-ing the importance of some microbial factors, involving thecomposition and the functioning of microbial communities (or eco-systems) of sludge, they may have various influences on the rheo-logical behavior of sludge, and are also discussed for the purpose ofextending the investigation of this field. We hope the presentedanalyses will be useful for developing and optimizing a new typeof bioreactor with high biomass concentrations as well as achiev-ing effective volume-reduction of excess sludge, which is still acrucial limiting factor that impedes the successful operation andmanagement of modern WWTPs.

2. Rheological parameters

Rheological parameters refer to those factors that are used toprovide a quantitative description of the rheological behavior ofa given sludge. They may be influenced by environmental factorsand the intrinsic properties of sludge. In the present section wehave limited the discussion to parameters that relate to the rheo-logical behavior of sludge.

2.1. c, s and rheogram

c is defined as the variation of velocity perpendicular to theforced direction, sometimes called the ‘‘velocity gradient’’, whichrelates to the deformation rate caused by the exerted force fromthe external environment. s is defined as the component of stresscoplanar with a material cross section, which arises from the forcevector component parallel to the cross section. In an engineeringsense, s may be considered as being generated in the form of aer-ation, mixing or pumping, which may be used to quantitativelycharacterize the required energy to sustain the proper operationof a unit of wastewater treatment. s can be manipulated as a con-trol parameter to enhance microbial processes in a bioreactor, dueto its significant influence on the structure, mass transfer, produc-

tion of exo-polysaccharides, and the metabolic/genetic behavior ofbiofilm, aerobic and anaerobic granules (Liu and Tay, 2002; Pevereet al., 2009).

c and s are closely interrelated parameters. Their quantitativerelationship constitutes a rheogram (Pevere et al., 2007) that iscurrently the most widely used tool to describe the rheologicalbehavior of sludge. In a rheogram, the rheological behaviors ofsludge are intuitively expressed with the rheological curves, whichis convenient to judge the type of a non-Newtonian fluid. Up todate, a rheogram can be accurately drawn by measuring the rela-tionship of c and s with a modern rheometer (Dentel et al.,2005); its application covers the fields such as optimizing thebio-purification of wastewater (Pollice et al., 2006; Khongnakornet al., 2010), chemical conditioning and mechanical dehydrationof the excess sludge (Chaari et al., 2003; Baudez, 2006).

2.2. Yield stress (s0, Pa)

Sludge with a certain concentration of solids can resist consid-erable stress, which indicates the occurrence of s0. It is commonlyaccepted that s0 is an indication of the strength of sludge with highsolids contents, and generally regarded as one of the basic proper-ties of a non-Newtonian fluid.

From a practical viewpoint, s0 is a measure of the resistance ofsludge to deformation, until the exerted stress exceeds a specifiedvalue that initiates flow (Møller et al., 2006), which—depending onthe measurement method—is generally classified as static or dy-namic s0. The s0 measured in an undisturbed way is called a statics0, which is a measure of the energy needed to overcome thestrength of the original structure, to initiate flow, while the dy-namic s0 is obtained by extrapolating the equilibrium flow curveto the zero point of the exerted c, which implies the structure ofsludge being totally devastated (Yang et al., 2009).

2.3. Viscosity (l, Pa s)

l is the most frequently used parameter in the field of rheology,which mainly expresses the resistance of a fluid to s. It is generallydefined by the following equation:

l ¼ s=ðdu=dyÞ ð1Þ

where du/dy is c or the velocity gradient (s�1).In Newtonian fluids, l is an intrinsic constant of the concerned

fluid, while in a non-Newtonian fluid, viscosity is a variable that isinfluenced by many factors. In such a case, only the apparent vis-cosity (la, Pa s) can be measured and then calculated. la of sludgedescribes the internal and external interactions and forces occur-ring between the sludge flocs and fluids. Accurate measurementof the la of sludge is a difficult task due to many complex influenc-ing factors. To obtain a reliable and repeatable measurement, it isnecessary to adopt a suitable protocol, based on the origin of thesludge. In the past decade some methods have been used to char-acterize the la of different sludge. Traditional methods all rely onan ex situ approach, which refers to sampling from an operatingapparatus and then measuring the viscosity outside of the appara-tus. Two commonly-used patterns are flow measurement and dy-namic measurement (Seyssiecq et al., 2003). The former patternis the simplest approach, which allows for characterization of thestructure of sludge under a laminar flow, while the latter mainlydetermines the viscoelastic properties of the sludge involved andthe limits between viscoelastic and plastic behaviors.

To accurately measure la, it is important to choose a suitablerheometer. A capillary rheometer and a rotational rheometer havebeen frequently used to measure la. In capillary rheometers, thekey component is a capillary tube that makes the sludge sampleflow in a laminar type through the tube at a controlled rate. The


rheological data can be obtained by coupling the differential pres-sure and the fluid flow rate. A capillary rheometer is a simple, low-cost apparatus and can attain high c, but needs large volumes ofsludge for measurement. A rotational rheometer is composed oftwo concentric cylinders, one of which is rotated at a controlledrate to provide a s. The rheological data are obtained by measuringthe torque on one of the two cylinders. The rotational rheometer iscommercially available and is thus frequently used.

For the convenience of measurement and accurate characteriza-tion of the rheological behavior of sludge, some researchers haveextended the concept of viscosity by comprehensively consideringthe effects of shearing duration and the physico-chemical charac-teristics of supernatants. They have proposed the concepts of ‘‘limitviscosity’’ (l1) (Tixier et al., 2003b; Pevere et al., 2006) and ‘‘spe-cific viscosity’’ (lsp) (Guibaud et al., 2004). For a given sludge undera certain c, la is a function of time. When it is sheared for a periodof time, a constant value will be reached, which is defined as thelimit viscosity. This definition enables the characterization of theglobal properties of sludge. Water in sludge usually contains somedissolved substances, which significantly change the property ofsludge resulting in considerable variation in sludge properties.Such variation cannot usually be neglected, thus, the proposedlsp is a useful concept for characterizing the influences of superna-tants (including water and dissolved substances).

2.4. Hysteresis loop area (Ha) and reduced hysteresis area (rHa)

Increasing c and then decreasing it to the start point may forman enclosed curve in a rheogram. Such a curve is referred to as ahysteresis loop, with the enclosed area being called Ha. This defini-tion enables the calculation of Ha as follows:

Ha ¼Z t

0sdcðtÞ ð2Þ

A hysteresis loop and its area can be used to express the thixo-tropic property of sludge, which can then be quantitatively charac-terized using Eq. (2) in term of Ha. Higher Ha values indicate moreobvious thixotropic properties. rHa, calculated by dividing Ha bythe sample volume, is another parameter that can be used to char-acterize the thixotropic properties of sludge.

Filamentous bulking sludge has poor settling ability and is a keyfactor causing biomass loss from secondary settling tanks, whichmay restrict the success of separating solids from effluents inWWTPs (Nielsen et al., 2009). An accurate and fast method for esti-mating the bulking of sludge is thus important for the routineoperation of WWTPs. Non-filamentous activated sludge is weaklythixotropic, thus, the parameter rHa may provide an effective ap-proach for detecting filamentous bulking of sludge due to its exhi-bition of an obvious thixotropic property. Investigations by Tixierand Guibaud have indicated that the parameter rHa can be usedto evaluate the magnitude of thixotropy in a filamentous sludge(Tixier et al., 2003c; Guibaud et al., 2005), which suggests that sucha parameter could be an alternative to the traditional microscopicexamination and sludge volume index (SVI) measurement.

2.5. Shear sensitivity

The solids components of activated sludge are composed of pri-mary particles and large flocs, indicating a characteristic of the bi-modal size distribution. Although primary particles, includingsingle bacteria and some colloidal matter, are not the main constit-uents of sludge by volume and mass, they may have a negative im-pact on settling rate, quality of effluents and mechanicaldewatering. The formation of large flocs is determined by the com-prehensive effects of c and interactions between primary particles.

This is affected by surface chemistry and rheological parameters(Mikkelsen, 2001), which are essential for successful sludge set-tling and dewatering. Shear sensitivity is defined as a parameterfor quantifying the dispersion of solids suspensions in sludge (Mik-kelsen and Kelding, 2002). As an operational parameter, it is suit-able for describing the dispersion of sludge at low total solidscontent and intermediate turbulent c. It may therefore be usedas a tool for monitoring sludge properties over time and for testingthe role of various biological, chemical and physical factors on flocstability (Sponza, 2003).

3. Quantitative measurement and description of the rheologicalbehavior of sludge

Three types of assessment, involving flow measurement, dy-namic measurement and on-line measurement, have been usedfor providing complementary information on the rheologicalbehavior of sludge in related processes.

3.1. Flow measurement

Flow measurement is the most commonly-used method fordescribing the rheological behavior of sludge. This generally in-volves sampling sludge from relevant bioreactors and then usinga capillary or a rotational rheometer to measure viscous and visco-plastic properties of the sludge in a steady state of laminar flow.The obtained results are expressed with rheological models, whichgive a quantitative comparison to the rheological behavior ofsludge from different origins. These models mainly include the fol-lowing: Herschel–Buckley model (Eq. (3)), Bingham model (Eq.(4)), Ostwald de Vaele model (or Power law model) (Eq. (5)), Siskomodel (Eq. (6)) and Casson model (Eq. (7)) as expressed below.

s ¼ s0 þ Kcn ðs0 > 0Þ ð3Þs ¼ s0 þ lc ðs0 > 0Þ ð4Þs ¼ Kcn ð5Þs ¼ l1cþ Kcn ð6Þs1=2 ¼ s1=2

0 þ ðlcÞ1=2 ðs0 > 0Þ ð7Þ

s0 represents the critical resistance that must be overcome beforeflow occurs. In some boundary conditions, i. e. when s0 equals tozero, Eqs. (4) and (7) become the expression of Newtonian fluid.In the Bingham model, la is referred to as the Bingham viscosity(lB); l1 corresponds to the maximum dispersion of flocs underthe influence of c; K is the consistency index (Pa sn), and n is adimensionless parameter to represent the flow behavior index.When n is close to 1, the fluid property gradually tends towardsthe behavior of Newtonian fluid. The commonly used models fordescribing rheological behavior are summarized in Table 1.

In Table 1, it shows a considerable complexity in both the originand the operational process of the related sludge. Sludge from dif-ferent origins exhibits distinctive features (Eshtiaghi et al., 2013) interms of rheological behavior and need suitable rheological modelsto describe. The Herschel–Buckley model describes a generalrelationship between the behavior of a non-Newtonian fluidand s0 followed by a shear-thinning zone. When the flow behaviorindex n equals to 1 in the Herschel–Buckley model, the consistencyindex K then changes into the lB. The Bingham model describes abehavior similar to that of Newtonian fluids, when the appliedstress exceeds the initial resistance s0 and the fluid is sometimesreferred to as a pseudo-Newtonian fluid. The Ostwald de Vaelemodel describes a shear-thinning fluid without s0, which quanti-fies the overall viscosity range and degree of deviation fromNewtonian behavior, and is suitable for characterizing the behaviorin an intermediate range of c. There is no term for s0 in the Sisko

Table 1Comparison of various models.

Origins of sludge Solids content (g L�1) Shear rate (s�1) Most suitable model Ref.

Non-digested pasty sewage sludge �85 wt% 0–1000 Herschel–Buckley model Baudez and Coussot (2001)Activated sludge 4–43 0–1000 Mori et al. (2008)Slurry prepared with municipal wastewater sludge and coal 58–64 wt% 0–100 Liu et al. (2012)Activated sludge in a sMBR 3–12 1–4 Ostwald de Vaele model Hasar et al. (2004)Secondary sludge 10–40 0.08–46 Pham et al. (2010)Activated sludge 4 1–245 Civelekoglu and Kalkan (2010))Settled sludge 10Thickened sludge 20Digested sludge 75Activated sludge in a sMBR 4–23 3–1312 Bingham model Laera et al. (2006)Activated sludge 1–6 0–1100 Guibaud et al. (2004)Filamentous-bacteria-enriched sludge in SBR 3–5 0–500 Casellas et al. (2004)Anaerobic sulphidogenic sludge suspensions 0–23 0–800 Pevere et al. (2007)Anaerobic hydrogen-producing sludge 53 0–1000 Sisko model Mu et al. (2007)Thickened sludge 27–57 0–60 Mori et al. (2006)Raw secondary sludge 10 0.08–46 Casson model Pham et al. (2010)Fenton oxidized sludge 25 0.08–46Secondary sludge 3–8 0–50 Weiss et al. (2007)Activated sludge in an airlift MBR 3–10 25–1000 Yang et al. (2009)


model, and it only expresses a shear-thinning behavior within therange of intermediate and high c, with la tending to a limiting va-lue (l1). In this sense, the Bingham model, Ostwald de Vaele mod-el and Sisko model may all be regarded as special cases of theHerschel–Buckley model. The Casson model is commonly used todescribe the rheological characteristics of a non-Newtonian fluid,which represents shear-thinning behavior over a relatively largerrange of solids content. It has been demonstrated that in anairlifted MBR, the Casson model is suitable for describing the rhe-ological behavior of activated sludge over the entire MLSS concen-tration range (Yang et al., 2009). Pevere et al. (2007) studied ananaerobic sulphidogenic sludge, which exhibited very complexrheological behavior, the description of which required two ormore rheological models at different time periods. This phenome-non implies that the rheological behavior in bioreactors (in aerobicor anaerobic conditions) is complex and time-dependent and maybe influenced by many factors. In most cases, it is advisable to com-pare different models for obtaining an accurate description of therheological behavior of sludge.

Slatter (1997) divided the characterization of the rheologicalbehavior of sludge into three steps: (1) viscometry, (2) rheologicalmodeling, and (3) correlation of parameters. The second step, rhe-ological modeling, has been identified by many research workersas the most essential in sludge rheology (Guibaud et al., 2004; Yanget al., 2009; Abu-Jdayil et al., 2010; Civelekoglu and Kalkan, 2010;Dong et al., 2011; Eshtiaghi et al., 2012; Liu et al., 2012). Com-monly-used criteria to evaluate the adaptability of a model arethe correlation coefficient and the standard error, enabling thecomparison of several models with the same sludge sample. Thecompared results showed that some fitting curves were overlappedin a certain range of solids content (Hasar et al., 2004; Yang et al.,2009; Pham et al., 2010), which indicated the rheological behaviorcould be described by different rheological models in some cases.This phenomenon has been observed by several researchers, whichdemonstrates that judging the adaptability of a model only by thecorrelation coefficient and standard error is not sufficient for a pre-cise evaluation of the effectiveness of a particular rheological mod-el. In a recent review, Ratkovich et al. (2013) indicated thedifficulty to carry out a reliable rheological measurement, whichseverely affected the reliability of a rheological model and the pre-dictive power. For total evaluation of the adaptability of a rheolog-ical model, it is required a comprehensive consideration of thecorrelation coefficient and standard error over a wide range of sol-ids content, and thus, a more reliable measurement protocol is veryimperative to get a repeatable result.

3.2. Dynamic measurement

As was the case for flow measurement, it is necessary to obtaina sludge sample from a bioreactor, as a dynamic measurement, soas to measure the viscoplastic and viscoelastic behavior of thesludge in terms of sinusoidal oscillation. The main objective of adynamic measurement is to obtain the parameter of complex mod-ulus G*, including its real part and imaginary part (Mori et al.,2006), which represents the energy-storage and energy-loss mod-ulus, respectively. The complex modulus G* may act as a useful toolfor assessing the energy consumption in the processes of disposingexcess sludge and purifying organic wastewater. Other importantinformation provided by a dynamic measurement is the s0 value,either as an oscillatory strain or as an oscillatory stress sweep ata constant frequency, which has a higher accuracy than the calcu-lated value of the s0 obtained by an extrapolation method from or-dinary flow measurement data. In a dynamic measurement, themethod of small-amplitude oscillatory shearing is often adoptedto determine the storage modulus and the loss modulus, whichmay provide useful information for calculating the optimal poly-mer dose for sludge conditioning (Chen et al., 2005).

3.3. On-line (or in situ) measurement

The above two methods are all off-line (or ex situ) measure-ments, which involve sampling the sludge from a process (or a bio-reactor) at a given time interval and then determining therheological parameter, using a rheometer which can only reflect lo-cal and instantaneous rheological properties in a process (or a bio-reactor). For most processes related to the disposal of excess sludgeand the wastewater treatment, it is however of primary impor-tance to understand the information relating to variation in therheological properties of sludge during a whole process (Manonet al., 2011). This cannot be detected by an ordinary off-line mea-surement and needs an on-line (or in situ) measurement, to obtainthe real-time data.

An on-line measurement is also called systematic rheology,which is based on sensing the variation of a certain physico-chem-ical property of sludge and transforming it to the rheological signalthrough a conversion device. Until recently, the physico-chemicalproperties of sludge used for such analysis included the torqueand electrical properties. The method based on the torque propertyhas now been developed into a systematic method, referred to as atorque rheological method. Its main principle is based on the Metz-ner–Otto’s principle, as expressed below:


la ¼2pC

KpNd3 ð8Þ

where C is the torque (N m), N is the rotation speed (s�1), d is theimpeller diameter (m), and Kp corresponds to the dimensionlessconstant of a laminar power curve, which is a systematic constantand can be determined by a multi-point method with a fluid havingknown viscosity.

The commonly-used apparatus to perform an on-line rheologi-cal measurement is a torque-meter with a torque sensor acting asthe key component. This converts torque signals into digital sig-nals, which can be handled and stored by a computer. Such a pro-cess greatly increases the efficiency of data analysis and obtainingresults, providing a convenient condition for the automatic controlof the related processes. Using this method, Seyssiecq et al. (2008)successfully carried out an in situ triphasic characterization to therheological behavior of activated sludge in an aerated bioreactor.Örmer also completed a full-scale experiment to optimize thedewatering operation of wastewater sludge, based on torque rheol-ogy (Ormeci, 2007).

Dieude-Fauvel et al. (2009) established an on-line method toobtain information of the rheological behavior of sludge by mea-suring electrical resistivity, based on the linear relationship be-tween resistivity and l, expressed by the following equations:

q ¼ q0

1� /ð9Þ

l ¼ l0ð1þ 2:5/Þ ð10Þ

where q (X m) is the sludge resistivity; / is the volume fraction ofthe electrically insulating phase. The subscript ‘‘0’’ represents thesupernatant.

The electrical measurement, based on the relationship betweenresistivity and viscosity, is a non-destructive in situ characteriza-tion method, capable of continuously evaluating the rheologicalbehavior of sludge, particularly for the assessment of microorgan-ism activity.

4. Factors influencing the rheological behavior of sludge

4.1. Solids content

The composition of sludge can be roughly divided into waterand solids substances, usually expressed as water content (mois-ture content) and solids content, respectively. Water content ac-counts for a major proportion of sludge and determines manyflowing characteristics of sludge. When describing the influenceof dissolved substances, the term ‘‘supernatant’’ refers to water,some unsettleable and dissolved substances. Solids content canbe calculated by subtracting the moisture content from the totalamount of sludge, and can sometimes be expressed by the param-eter total suspended solids (TSS). Considerable researches (Forster,2002; Pevere et al., 2007; Pham et al., 2010) have indicated thatTSS content is a main parameter affecting the evolution of rheolog-ical behavior in biological sludge. Accurate measurement hasshown that, as the solids content increases, the fluid type of sludgegradually changes from a Newton fluid to a non-Newton fluid.There are also some evidences to demonstrate that an incrementin TSS content results in a decrease in the distance between theparticles of sludge, thus increasing the interaction between them.This results in an increased resistance of sludge to flow, when sub-jected to shear. The corresponding solids content of the criticalpoint of transition to exhibit an obvious s0 was determined to beabout 9–11 g L�1 (Forster, 2002). The solids components in sludgeare complex, their compositions and properties are determined bythe bioconversion step adopted by WWTPs. The organic matter

(including microorganisms and their insoluble metabolites) is thusgenerally the major solids component of sludge and determinesthe flowing properties of the sludge.

4.2. Surface charge

Due to their large surface area, the primary particles (or flocs) ofsludge are likely to adsorb some charged ions, leading to surfacecharging. Surface charge can be quantitatively represented by theparameter zeta potential (n), an important parameter of colloidchemistry for characterizing the stability of a multi-phase system.As for other colloid particles, the n created by the surface charge in-creases the interactions between the flocs in sludge and is heavilyinfluenced by the pH value of the solution surrounding the parti-cles (or flocs) of sludge. After a series experiment at various pH val-ues, Pevere et al. (2006) found that the n of an anaerobic granularsludge decreased as the pH increased, and that l1 simultaneouslyincreased. Some researchers have determined other factors thathave a strong influence on the surface charge of sludge. Variationof viscosity has a direct relationship with interactions betweengranules (or particles) of sludge, which may be enhanced by thesurface charge. It should be noted however that the surface chargeis not an independent variable, and can be affected by many phys-ico-chemical parameters. Surface charge, as a direct factor that hasgreat influence on the viscosity of sludge, is thus actually an inter-medium that reflects the comprehensive effects of general environ-mental conditions on the rheological behavior of sludge.

4.3. Extracellular polymeric substances (EPS)

EPS influence the properties of sludge, particularly the mainte-nance of the sludge floc in a three-dimensional matrix. Liu andFang (2003) reviewed the influences of EPS on the flocculation, set-tling, and dewatering of activated sludge, and also discussed the ef-fects of EPS concentration on floc structure, which indicated that arelationship existed between the EPS concentration and the rheol-ogy of sludge. Mori et al. (2006) found that the concentration of ad-sorbed EPS during the starvation phase of microorganismsdecreased when the sludge age was prolonged, therefore inducinga decrease in viscoplastic property and exhibited a shear-thinningproperty. It has been shown that EPS are composed of several kindsof biopolymers, such as polysaccharide, protein, glycoprotein andglycolipid. These form a matrix that holds microorganisms to-gether and provides nutrients for the embedded microorganismsduring starvation. The relative levels of the above-mentioned bio-polymers in EPS and their surface functional groups facilitate cer-tain electrostatic and gel-like interactions between sludge flocsand increase the bound water content, which strongly influencesthe physicochemical properties of EPS and are vital for the cohe-sion of sludge flocs (Marinetti et al., 2010). A decrease in the con-centration of EPS reduces the strength of sludge flocs thusdecreasing the viscoplastic properties. Although the types and con-centrations of EPS have obvious influences on the rheologicalbehavior of sludge, they are actually the metabolic products ofmicroorganisms, and their production, or utilization as carbonsources by the embedded microorganisms, is dependent on thewastewater characteristics and the growth phase of the microbialcommunities in sludge. The microbial communities and theirgrowth phase thus directly determine the increases and decreasesin EPS concentrations.

4.4. pH value

The pH value of sludge is a parameter that can be varied due tothe acids and alkalis produced by the metabolism of microorgan-isms, or it can be artificially adjusted. pH level has a significant ef-


fect on the physico-chemical properties of sludge, which is still awidely-used controlling parameter in many WWTPs due to its hea-vy influence on many operations in wastewater processing and ex-cess sludge disposal. pH value has a direct effect on sludgeproperties, this relates to the levels of H+ or OH� in the superna-tant, which is a major factor affecting the concentrations of somecomplexes in sludge. Such an effect leads to other significant vari-ations, affecting a number of factors, such as the surface propertyof sludge particles, which can change the interactions betweensludge particles and rheological behavior. Raynaud et al. (2012)also confirmed that a variation in the pH value may change floccu-lation systems associated with excess sludge and release fine par-ticles, which can cause difficulties in the succeeding compressiondewatering step.

Tixier et al. (2003b) investigated the effect of pH on the inter-particle interaction of activated sludge flocs, in terms of a rheolog-ical approach. They found that the viscosity increased with in-creased pH for all tested samples. At neutral pH levels, theincrement in viscosity was relatively small, while at low and highpH levels the increment in viscosity was much higher. The viscos-ity of sludge is conventionally regarded as a parameter reflectingthe interactions between particles and flocs of sludge, which isstrongly influenced by the surface charge of sludge particles. A de-crease in pH facilitates a decrease in the thickness of electrostaticdouble layers, thus reducing the electrostatic repulsion betweenparticles. This results in a decrease in viscosity.

5. Applications

In their review article, Seyssiecq et al. (2003) gave a detaileddescription of the applications of the rheology of sludge, whichmainly involve wastewater treatment by an activated sludge pro-cess and volume-reduction of excess sewage sludge by anaerobicdigestion. In recent years, however, many new techniques havebeen used for the disposal of excess sludge and wastewater treat-ment from both municipal and industrial fields (Lin et al., 2012).This has stimulated an interest in a number of processes associatedwith sludge—including mass transfer (transfer of oxygen, nutrientsand soluble microbial products (SMPs)), microbial activity, mem-brane fouling, anaerobic digestion, fluid transportation, mixingand solids/liquid separation—all of which are heavily influencedby the rheological behavior of sludge. Related research advancesin optimizing the process of wastewater treatment and disposingthe excess sludge are discussed below.

5.1. Optimizing the activated sludge process of a WWTP

The activated sludge process is globally the most widely-usedmethod for treating domestic wastewater. The activity of biomassis largely responsible for the effluent quality of WWTPs, and theefficiency and quality of wastewater purification are dependenton mass transfer processes, including oxygen diffusion, and thetransportation of nutrients and metabolic products. To improvethe purification efficiency and optimize operations, sufficient bio-mass and fluidity must be maintained in a bio-treatment reactor.Such efficiency is strongly affected by the rheological propertiesof sludge due to its dominant influence on the total energy con-sumption in WWTPs.

The effective supply of oxygen to the aerobic microorganismsin an activated sludge process is essential for maintaining thebioactivity of sludge and promoting the biodegradation of or-ganic pollutants. Rapid transport of oxygen in wastewater isdetermined to a large extent by the flowing state of MLSS. Inthe state of laminar flow, oxygen can be only transported bymolecular diffusion, which is not fast enough for a sufficient

supply of oxygen. Higher transport efficiency is attained in moreturbulent conditions when oxygen is mainly transported by con-vection. This leads to higher internal friction and also consumesmore energy.

The flowing state of a fluid is generally characterized by Rey-nold number (Re), which is defined by the followed equation:

Re ¼ LuPl

ð11Þ

where L is the feature size of the flow channel, u is the velocity ofthe fluid, P is the density of the fluid.

l is an important rheological parameter that provides a macro-scopic description of the rheological behavior of sludge and has aclose relationship with biomass concentration. Higher concentra-tions of biomass mean larger values for l, which promotes a de-creased value of Re. The amount of biomass in a bioreactortherefore has two contradictory effects on the removal of organicpollutant: high biomass concentration is important for the effec-tive removal of organic pollutants, but it also decreases the effi-ciency of oxygen transfer by reducing the fluidity of MLSS, whichmay have a negative effect on pollutant removal. The mass transferof other substances, including nutrients and metabolic products, issimilar to that of oxygen transportation, which is also dependenton the rheological characteristics of sludge.

Aeration and fluid transportation are two important steps thatinfluence the energy consumption in WWTPs. Aeration efficiencyis controlled by the oxygen transportation in MLSS, and the diffi-culty of transporting a fluid is determined by the viscosity of therelated fluid, which is also influenced by the rheological behaviorof sludge. Controlling the suitable concentration of biomass in abioreactor is therefore essential for optimizing the operation of aWWTP. To achieve this it is important to have a good understand-ing of the rheological behavior of the relevant activated sludgewithin the WWTP.

5.2. Optimizing the performance of MBRs

MBR is a newly-developed technique for wastewater treatmentin the past decades, it combines the advantages of cost-effectiveactivated sludge methodology with a high efficiency separationof biomass from water by a membrane module. The membranemodule of MBRs completely retains the sludge from the effluent,leading to high concentrations of biomass within the reactor,which usually results in a high quality effluent. Two types ofMBR are now available (Bouhabila et al., 1998; Ravanchi et al.,2009): a conventional MBR system and the submerged membranebioreactor (sMBR). In the conventional MBR system, which is suit-able for easily-biodegradable domestic or industrial wastewaters,the membrane module is installed outside the aeration tank, whichgenerally implies a high head loss and excessive energy consump-tion. The sMBR, in which a membrane module is immersed in theaeration tank, discharges the filtrate via an exterior suction pump.MBRs have obvious advantages over conventional activated sludge(CAS) systems, in terms of higher efficiency of wastewater purifica-tion and less space requirements as well as the ability of MBRs tocompletely retain sludge. This allows the system to operate atmuch higher sludge concentration. In an MBR, the concentrationsof MLSS typically range from 10 to 25 g L�1 (Rosenberger et al.,2000), compared with concentrations of about 2–5 g L�1 in tradi-tional CAS systems (Seyssiecq et al., 2008). Thus the sludge in anMBR exhibits the obvious behavior of a non-Newtonian fluid,which considerably influences oxygen transfer and pollutantconversion.

Fouling of membrane modules (Meng et al., 2009; Li et al.,2012) is the most important operating problem associated with


MBRs, which is affected by the rheological properties of sludge inan MBR. Van Kaam et al. (2008) have undertaken a rheological-based study to develop methods for minimizing fouling in MBRs.Hasar et al. (2004) operated a laboratory-scale sMBR for 100 d toevaluate the rheological properties of the sludge from a sMBR atdifferent times. They found that the rheological behavior couldbe described by the Ostwald de Vaele model, and that the s de-creased with increasing temperatures. An exponential relationshipwas found to exist between la and the solids content (Azami et al.,2011), i.e. a high solids content leading to high viscosity, whichconsumed more energy during the aeration operation period thanwould be the case for a CAS system. There were also indicationsthat temperature and solids content were the two most importantparameters determining flow behavior, which therefore influencedthe operation of sMBRs. Yang et al. (2009) proposed a mathemati-cal model involving these parameters—including l, MLSS, temper-ature and c—to describe the rheological behavior in an airlift MBR.Their results induced a concept of activation energy for viscosity,which could be used to evaluate the effect of temperature on vis-cosity. Sludge in an airlift MBR demonstrated a shear-thinning rhe-ological behavior without thixotropic property, and high MLSSconcentrations led to high viscosity, thus requiring higher energyconsumption during the aeration operation and fluid transporta-tion. This significantly affected mass transfer and oxygen diffusionin the bioreactor.

Maintaining the bioactivity of biomass in an MBR is a prerequi-site for ensuring high effluent quality, which requires effective andrapid mass-transfer of oxygen, nutrients and metabolites. Fullyunderstanding the rheological behavior of sludge in an MBR is thusof primary importance for establishing an energy-saving process,which provides useful information for optimizing the operationof MBRs.

5.3. Evaluation of filamentous bulking

Filamentous bulking of sludge, the most frequent failureencountered in WWTPs (Nielsen et al., 2009; Guo et al., 2012), re-fers to the over-growth of filamentous bacteria in an aeration tankthat leads to difficulties in separating solids and liquids in the sub-sequent secondary settling tank. The traditional method for detect-ing filamentous bulking combines the results of microscopicobservation and SVI measurements of sludge samples, which is of-ten time-consuming and not cost-effective.

Tixier et al. (2003a) investigated the possibility of evaluatingthe settling performance of activated sludge, using the parametersof l1 and reduced hysteresis area (rHa) as indicators. Their resultsindicated an exponential relationship between l1 and TSS, whichmeant the TSS had an obvious influence on l1, particularly in therange of high TSS concentrations. At constant TSS levels, however,the measured l1 results varied greatly according to the differentorigins of sludge samples, even when the bio-processes were thesame, indicating sensitivity to changes in the sludge’s originalcharacteristics. An exponential relationship was also found be-tween rHa and TSS, but the essential influence was dependent onthe proliferation of filaments in activated sludge. Low-filamentsludge showed very low rHa values, even when the TSS contentwas high. In filamentous sludge, the rHa had high values, evenwhen TSS content was low. The rheological behavior of filamen-tous sludge has a close relationship with the entanglement of fila-mentous bacteria, so the two parameters l1 and rHa, weresuggested as indicators to assess the quality of sludge, particularlyin the case of filamentous bacteria over-growth in activated sludge.Relevant methods have been tested for feasibility as on-line mea-surements, for the evaluation of filamentous bulking in aerationtanks (Tixier et al., 2004).

5.4. Excess sludge conditioning and dewatering

Water is the main component of excess sludge, with levels ofabove 99% being recorded in the sludge newly discharged fromsecondary settling tanks. Following gravity concentration, thewater content in excess sludge may decrease to below 98%, and fol-lowing mechanical dewatering, it still remains at levels of 60–70%.Volume-reduction of sludge is of primary importance for amelio-rating its negative environmental impacts, and dewatering of ex-cess sludge is a key step in this process.

A commonly accepted classification of sludge-water includesfour categories: free water, surface water, interstitial water, andintracellular water. The accurate content of the above four catego-ries is very difficult to measure because of the complex occur-rences in sludge from different origins. In sludge, free waterrefers to those water molecules that have very weak interactionswith sludge particles/flocs, it can be easily separated from solidsparticles using simple gravitational settling. Surface and interstitialwaters have some interactions with solids particles/flocs, whichmay be stripped by a mechanical method only after the interac-tions have been weakened by inorganic or organic polymers(chemical conditioner). Chemical conditioning and mechanicaldewatering are now regarded as standard procedures for volumereduction of excess sludge prior to final disposal from a WWTP.

In the processes of traditional chemical conditioning and dewa-tering, capillary suction time (CST) and specific resistance to filtra-tion (SRF) tests are commonly-used indexes for quantitativeevaluation of the dewatering effect. Some investigations have how-ever indicated that these tests lack reproducibility and their appli-cations are limited. A recent report indicated that a rheologicalmethod might be a feasible tool for evaluating the effect of condi-tioning excess sludge (Stickland and Buscall, 2009). Ormeci andAbu-Orf (2006) likewise made use of torque rheology to describethe effects of chemical conditioning on the physical characteristicsof sludge, in which they presented a simple protocol to determinethe optimum polymer dose and identified the best performingpolymer for excess sludge conditioning. In a full-scale dewateringoperation, Ormeci (2007) compared two different methods forpolymer optimization. Their results, which included rheologicalanalysis, provided some useful insights into sludge propertiesand dewatering behavior, which could not have been obtainedwith traditional CST and SRF tests. In these investigations, the rhe-ological property of sludge provided a reliable control parameterfor the optimization of conditioning and dewatering operations.

Other operations relating to the disposal of excess sludge, suchas transportation of sludge through pipes and pumps, might createan additional shear to the transported sludge, which could possiblybreak the structure of sludge flocs. This could expose them morenegatively charged surfaces, which has a significant effect on sub-sequent conditioning of sludge with polymers. Ormeci and Ahmad(2009) thus developed a rheological method to measure the addi-tional shear that a sludge network was exposed to during condi-tioning and dewatering.

The above investigations clearly indicate that the rheologicalmethod can be used to judge an optimal polymer dose and mayhave a potential application for automatically controlling a dewa-tering and conditioning process (Wolny et al., 2008).

5.5. Anaerobic digestion

Anaerobic digestion is a cost-effective approach for achievingvolume reduction of excess sludge (Evans et al., 2011) and biogasenergy recovery (Baudez et al., 2011; Li and Yu, 2011; Athanasouliaet al., 2012), as well as an effective method to reduce the infectiousrisk to public health prior to dewatering and final disposal, duringwhich, about 40–50% organic compounds may be reduced.


Maximum dehydration of sludge prior to anaerobic digestion isessential for achieving effective volume reduction of excess sludge,and also to achieve high efficiency and minimal space require-ments. This means that an anaerobic digester is usually operatedat high-solids-content sludge (Abbassi-Guendouz et al., 2012).Anaerobic processes are generally categorized as three phases (Sol-yom et al., 2011)—hydrolysis, acidogenesis and methanogenesis—during which the microbial community in sludge may experiencea series of variations. Only after thorough anaerobic digestion(methanogenesis), parts of organic substances in excess sludgeare converted to biogas and separated from the solids component,resulting in significant changes in the organic matter content of di-gested sludge.

In an anaerobic digester, the composition of sludge, in terms ofthe microbial community and the organic matter, is continually ina changing state, which leads to unique rheological behavior in di-gested sludge. Baudez et al. (2011) demonstrated that digestedsludge behaved as a linear viscoelastic solids at low s, while at veryhigh s, it behaved as a Bingham fluid. A similar phenomenon wasalso observed by Monteiro (1997). The rheological behavior of di-gested sludge was however nearly the same in quality even at dif-ferent solids concentrations. Results in terms of variation inquantity, were however uncertain due to the variation of theparameters of s0 and lB. Such a phenomenon implied that theinteractions between the sludge particles were dominated by a ste-ric effect more than an electrostatic effect. Dentel and Dursun(2009) evaluated the rheological behavior of digested sludge interms of shear sensitivity, using the strain sweep method, by mea-suring the s0. In comparison with the traditional CST shear test,they concluded that the rheological method would be more appli-cable for characterizing the dewatering behavior of digestedsludge. They also found that the rheological method could be fur-ther improved, through an immobilized cell, during the processof sludge concentration by dewatering.

The routine operations of an anaerobic digester include mixing,mechanical stirring and pumping, which generally account formost of the operating costs. Accurate estimates of the rheologicalbehavior are needed in order to calculate head losses and pumpingpower. Such rheological properties are very important for the de-sign and efficient operation of an anaerobic digester.

6. Discussion

Effective reduction of excess sludge volume is important for thesustainable operation of WWTPs, which includes minimizing theproduction of sludge and reducing the volumes of the produced ex-cess sludge. Meanwhile, some newly developed bio-techniques forwastewater treatment are generally characterized by their highconcentrations of biomass with high bioactivity, and the need fora fast proliferation of biomass within the bioreactors. The abovetwo aspects seem contradictory in terms of most WWTPs. To re-solve this issue, it is therefore important to obtain a full under-standing of the basic properties of sludge itself. The basiccharacteristics of sludge in terms of rheology and microbial ecolog-ical composition are still not fully understood, particularly in termsof the relationship between the rheological properties and themicrobial ecological composition. Among the reported literature,Khursheed and Kazmi (2011) have analyzed certain ecological ap-proaches with the purpose of reducing excess sludge, placing anemphasis on the overall mass balance of inputs and outputs, toevaluate which may produce less biomass during wastewatertreatment. Most reported literature on investigations into sludgerheology has focused on the observation of rheological phenomenaand fitting the relationship between different rheological parame-ters. It has been concluded that only those bioreactors that containcomplex multispecies of microbial communities under a state of

high concentration of biomass, can achieve a rapid transformationof contaminants, and none of the individual species of microorgan-isms is capable of completely degrading complex pollutants. How-ever, consideration has not yet been given to microbial factors thatinfluence the rheological behavior of sludge. Actually, as a consor-tium of microorganisms, bio-sludge is a definite microbial ecosys-tem that exhibits considerable bio-activity during the operation ofa bio-treatment process, and these microbial factors have a consid-erable impact on the rheological properties of sludge.

There are also some experimental evidences implying thatmicrobial factors have an important influence on the rheologicalbehavior of sludge.

� Ecological factors of microbial communities in a bioreactor

In a WWTP bioreactor, numerous microorganisms, and theirinsoluble microbiological products, aggregate to form a function-ing activated sludge (aerobic or anaerobic), which is responsiblefor the biodegradation of organic pollutants. In a stably-operatedbioreactor, the composition of the microbial community is quitecertain. Xia et al. (2010) found that the microbial community struc-ture in five WWTP bioreactors, in China and the United States, wassurprisingly consistent. A biological WWTP can be considered as anartificial ecosystem (Wei et al., 2003) with a stable food web, inwhich the relationships among different microorganisms can becategorized as producers, consumers and decomposers. Concentra-tions and characteristics of EPS and SMPs are dependent on thecharacteristics of the microbial community, which also impact onthe rheological behavior of sludge and determine the appropriatedisposal procedure for treating excess sludge.

� The effect of microorganism with special morphology and activ-ity on the rheological behavior of sludge

Tixier et al. (2003c) carried out rheological experiments onsludge containing similar solids content, and found that the con-centration of filamentous bacteria was a direct reason for signifi-cant changes in rheological properties. Following chloridization,the bioactivity of filamentous bacteria decreased markedly, whichweakened the structure of sludge flocs and reduced s0 (Ramirezet al., 2000). Sludge containing large amounts of filamentous bac-terium was noted to have an irreversible thixotropy, due to irre-versible changes in the structure of sludge floc under the s,whereas this property was not noted in sludge with a low concen-tration of filamentous bacteria (Tixier et al., 2003a). Brar et al.(2007) in an investigation into pesticide production associatedwith Bacillus thuringiensis (Bt) fermentation of hydrolyzed sludge,found that s increased during the active growing phase of Bt cells,and decreased during the stationary phase, which may have re-sulted from cell lysis and the subsequent release of spores. Thisindicated a relationship between total cell count, viable sporecount and rheological behavior. Although the solids content insludge is an important factor for determining rheological behavior,various concentrations of bacteria with unique structural features(Fig. 1) exhibit quite obvious differences in rheological behavioreven under similar conditions in terms of solids content. Duringdifferent growth phases of the microorganisms involved, changedbioactivity of sludge also leads to an obvious variation in the rhe-ological behavior, which implies that microbial factors have animportant influence on the rheological behavior of sludge.

� Influences of microbial secretion on the rheological behavior

As a result of metabolic activity, microorganisms secrete a con-siderable quantity of organic matter, which may change the phys-ico-chemical properties of sludge. Organic macromolecular

Thiothrix Epistylis urceolata

Spirulina Filamentous fungus(d)(c)

(a) (b)

Fig. 1. Common microorganism with special structure in sludge (40 � 10).


polymers, such as polysaccharides and lipopolysaccharides, may besecreted during the growth of microorganisms. These compoundsoften adhere to surrounding cells and form part of the EPS. Thespecies and amount of EPS are influenced by the growing phaseof microorganisms and are also closely linked to the bioactivityof sludge. The existence of EPS around the cells of microorganismsprovides an opportunity for the sludge particles to aggregate to-gether, even under a high zeta-potential (Velegol et al., 2000;Sponza, 2003), while ordinary slurries or other inorganic solidsparticle suspensions are usually stable under the same conditions.Other soluble secretions also have some influence on the rheolog-ical behavior of sludge, which can be quantitatively characterizedby the parameter supernatant viscosity.

� Influences of the structure of excess sludge and the cell mem-brane of microorganisms on the efficiency of physical pretreat-ment of excess sludge

The stable three-dimensional structure of excess sludge and thecell membrane of microorganisms have enough mechanicalstrength to resist the direct anaerobic digestion. For improvingthe efficiency of an anaerobic digestion, some physical methodssuch as ultrasonic pretreatment (Pham et al., 2009; Pilli et al.,2011) and microwave irradiation (Tang et al., 2010) were recentlyused to pretreat the excess sludge. After these pretreatments, theproperties of excess sludge, such as fluidity, biodegradability andrheological behavior, were obviously improved. The special struc-ture of excess sludge and the cell membrane of microorganismsare key factors to restrict the efficiency of physical pretreatment,which are also determined by the microbial compositions of sludge.

The above phenomena indicate a close relationship betweenmicrobial factors and the rheological behavior of sludge and alsoexhibit considerable complexity and diversity. Moreover, the engi-neering aspects of wastewater treatment also demonstrate that,

following an ecological succession, the microorganism speciescomposition and quantity in sludge, including bacteria, fungus,protozoa and metazoan, eventually form a stable well-structuredmicrobial ecosystem, with integrated functions under certain envi-ronmental conditions, in which there exists an explicit food webthrough which different microorganisms transfer mass and energy.This is an interesting topic for further investigations into the rela-tionships between the microbial factors and the rheological behav-ior of sludge.

7. Summary

Optimizing the operation of wastewater treatment and disposalof excess sludge are two essential aspects in successful running aWWTP. From a viewpoint of microbial ecology, the process of bio-purification of wastewater and the subsequent disposal of excesssludge can be considered as two related phases during the succes-sion in microbial ecosystems. Existing knowledge demonstratesthat the ecological composition of microorganisms in a stablewastewater treatment apparatus has a certain stability, and some-times undergoes a series of changes and variations when environ-mental factors change abruptly (such as in an anaerobic digester).During the succession, special rheological behavior may be mani-fested. The rheological behavior of sludge is a comprehensive reflec-tion of the interior structure of the microbial ecosystem, which isdetermined by the strength and nature of interior interactions.Therefore, the information obtained from the rheological investiga-tion of the related sludge is essential for disposing the excess sludgeand optimizing the operation of wastewater treatment. To our bestknowledge, many factors, including the physical, chemical and bio-logical properties of the targeted sludge, can influence rheologicalbehavior and have been shown to have complex effects. As a productof the WWTP bio-treatment processes, sludge is not just a consor-


tium of water and solid substances. It should be considered as amicrobial ecosystem that is affected by the nature of bio-processestaking place in a WWTP. The basic composition of a microbial eco-system influences the characteristics of sludge and, as a macroscop-ically kinetic property, the rheological behavior of sludge heavilyinfluences the transfer of momentum, mass and heat, which in turnaffects the composition of the ecosystem and its succession.

Various microorganisms in sludge comprise unique microbialecosystems and may undergo succession when subjected to varia-tion in external environmental conditions (Kim et al., 2012). Thiscan lead to changes in the structure of the microbial communityand influence the rheological behavior of sludge. An importantconclusion of the present review is that the composition of amicrobial ecosystem within sludge, and ecological successionwithin such an ecosystem, are the essential issues in determiningthe rheological behaviors of different sludge, which further influ-ence the efficiency of wastewater treatment, conditioning anddewatering of excess sludge. When conducting research on therheological behavior of sludge, they should be a primary consider-ation. On the basis of the above analyses, it is recommended thatfuture research attention should be focused on the followingconsiderations.

1. Because of the complexity and obvious microbial characteristicsof sludge, systematic research on sludge rheology should becarried out with regard to the succession in the microbial eco-system in a bioreactor.

2. From the start-up stage to the steady operation in WWTPs,microbial ecosystems within bioreactors undergo continuoussuccession. The microbial ecosystem that eventually becomesestablished in a bioreactor determines the successful operationof the WWTP and the subsequent disposal of excess sludge. Insitu studies should therefore be conducted to obtain real-timerheological data so as to increase understanding of the processof succession. Such research is fundamental to understandingthe microbial ecosystems in bioreactors.

3. Prior to the mechanical dewatering step, a chemical conditioningprocess is necessary for weakening the interactions between theparticles and liberating the bound water from the sludge flocs. Arheological approach has traditionally provided a powerful toolfor optimizing and controlling the dosage of chemical condi-tioner during chemical conditioning. Due to the influence ofmicrobial factors in sludge, the physico-chemical properties ofsludge are varied with the succession of the microbial communi-ties in sludge, they also influence the rheological behavior duringthe chemical conditioning. Analyzing the variations of the micro-bial ecosystem in excess sludge may provide useful informationfor choosing a more suitable strategy of chemical conditioning.

4. In excess sludge, microorganisms, organic and inorganic matterare agglomerated into a polymeric network formed by EPS ordivalent cations. This special structure increases the difficultyto achieve effective volume reduction with anaerobic digestionand mechanical dehydration. In recent years, however, somephysical pretreatment techniques, such as ultrasound andmicrowave irradiation, have also been used to disrupt the net-work. These physical pretreatment methods have the advantageof not producing chemical residues and potential toxicity, theinfluence of which on the rheological behavior of sludge is stilluncertain. Research into this aspect may provide useful infor-mation relating to the final disposal of excess sludge.

5. High concentrations of biomass in a bioreactor are beneficial forimproving the quality of effluent, but the energy consumptionduring the process is also high, due to restrictions relating tooxygen diffusion and mass transfer under the influence of therelated rheological behavior. These effects, together withenergy efficiency, which are the two most critical factors to

consider when operating a WWTP, are sometimes contradictoryso need comprehensive consideration. The rheological method-ology provides a useful tool for reconciling this problem. Agreater emphasis on this aspect is therefore recommended.

Acknowledgement

We are very grateful to the financial support of the NationalNatural Science Foundation of China (NSFC) under the Grant num-ber No. 51178120.

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