A review of the properties of biosludge and its relevance to enhanced dewatering processes

io ri troleum , Iran and A method. td. All rights reserved. issue of these plants due to high required capital and oper- ating cost for sludge treatment from one side and stricter regulation on sludge disposal from the other side. Incinera- tion, composting and landfill are the most common sludge disposal methods used over the years. But these methods are no longer suitable due to both economical and environmental logies are needed to ows that the mass is usually more than source of energy by of biofuel or other chemical substances. Thus any improvement in reducing the volume of this biosludge can be beneficial from both economical and environmental points of view. Dewatering the sludge to 60% water is key for the cost-effective reduction in the volume [3,4]. Reducing sludge water content before * Corresponding author. Tel.: þ1 4169788517. Available online at www.sciencedirect.com .co b i om a s s a n d b i o e n e r g y 5 8 ( 2 0 1 3 ) 3 6 5e3 7 8 E-mail address: [email protected] (D.G. Allen). proper management of this quantity of sludge is the major organic materials. Further processing of the produced bio- sludge is difficult as it may contain large amount of toxic 1. Introduction The rapid growth of industrialization and population in recent decades has resulted in the production of a huge amount of activated sludge from wastewater treatment plants. The constraints and more sustainable techno solve this problem. Recent research sh fraction of water in produced biosludge 98% [1,2], can be used either directly as a combustion or processed for production Chemical pretreatment Enzymatic pretreatment recommendations have been made for optimal application of each ª 2013 Elsevier L Sludge pretreatment Physical pretreatment drawbacks of different methods are described and the dominance of one over the others is discussed mostly with respect to energy requirement and environmental impacts. Some b Department of Chemical Engineering Ontario, Canada M5S 3E5 a r t i c l e i n f o Article history: Received 27 July 2012 Received in revised form 3 September 2013 Accepted 3 September 2013 Available online 3 October 2013 Keywords: Sludge dewatering 0961-9534/$ e see front matter ª 2013 Elsev http://dx.doi.org/10.1016/j.biombioe.2013.09. pplied Chemistry, University of Toronto, 200 College Street, Toronto, a b s t r a c t Difficulties in dewatering of biosludge result in economical and environmental issues for wastewater treatment plants. Various attempts have been made to overcome this problem by achieving some pretreatment on biosludge. The main purpose of all pretreatment methods is to modify the biosludge characteristics in such a way to boost settling of cells and solid particles of sludge, and to ease the release of water molecules from extracellular polymeric substances and cells and to facilitate flow of water through forming filter cake. The present work presents an overview of different properties of sludge and their mea- surement, the main reasons of sludge dewatering difficulty, the fundamentals of sludge dewatering and various proposed methods for sludge pretreatment. The advantages and a Environmental Research Centre in Pe Engineering, Shiraz University, Shiraz and Petrochemical Industries, School of Chemical and Petroleum D. Mowla , H.N. Tran , D. Grant Allen * a b b, Review A review of the properties of b relevance to enhanced dewate http: / /www.elsevier ier Ltd. All rights reserved 002 sludge and its ng processes m/locate/biombioe . the flocs and the “free water” which behaves as the bulk water The bound water content which is the sum of interstitial, vicinal and hydration waters is one of the major limiting A cell ( ) ( ) A floc Free water ( ) Interstitial Water ( ) Hydrationwater Vicinal Water Fig. 1 e Schematic model of various forms of water in biosludge. 80 90 Water content % b i om a s s a n d b i o e n e r g y 5 8 ( 2 0 1 3 ) 3 6 5e3 7 8366 and can be easily separated [5]. However, Vesilind and Martel [6] defined four different forms of water in sludge as follows: - Free water: water which is not attached to solid particles and is separated easily by simple gravitational settling. It freezes at the normal freezing point of water. - Interstitial water: water that is trapped within the floc structure or within a cell. This can be separated only when it is released by breaking up the floc or disruption of the cell. Only small amounts of interstitial water might be removed by mechanical dewatering systems such as centrifugation or vacuum filtration. Due to high dissolved solid concen- trations, this type of water freezes at temperatures lower than normal freezing point. - Vicinal water: water molecules which are physically bound to solid particles surface due to their nature and cannot be separated by any mechanical means. It could be frozen only at very low temperature. - Water of hydration: water molecules which are chemically bound to the solid particles and do not enter the ice crystal lattice upon freezing. They can be released only by thermo chemical destruction of the particles at temperatures above further processing reduces capital, transportation and oper- ational costs. Unfortunately, biosludges are naturally difficult to dewater due to the existence of colloidal materials and extracellular polymeric substances (EPS) within them, which bind water molecules strongly to solid surfaces or capture them inside the cells or flocs. This high water content is the bottleneck in the biosludge treatment. Indeed, there are three limiting factors in dewatering of biosludge; low settleability of colloidal solid particles, high compressibility of the sludge solids and high affinity of EPS for trapping water molecules in them. Any attempts to improve these three factors can be beneficial in sludge dewatering. Coagulation or flocculation of solids particles to improve settleability, addition of proper skeleton builder or filter aids to reduce the compressibility of the sludge solids and using different techniques to release the trapped water molecules in the EPS mesh are among these attempts. In this paper, different properties of biosludge, the reasons of its low dewaterability, fundamentals of biosludge dewatering, and various conditioning methods for increasing of dewatering efficiency proposed by different investigators are discussed. Finally these methods are assessed for appli- cation at an industrial scale. 2. Fundamentals of sludge dewatering A better understanding of the fundamentals of sludge dew- atering mechanisms, can lead to better pretreatment ap- proaches for improving the sludge dewatering process. In any water-solid system such as biosludge, different aspects have been visualized for interaction between water molecules and solid materials. There could be an interaction energy binding or some structural binding between water molecules and the solid particles [1]. Often, thewater in sludge is categorized into “bound water”, which is held physically, chemically or both to 105 �C [7,8]. Fig. 1 represents a schematic model of various forms of water in biosludge. factors in dewatering efficiency because removing it requires much more energy. Based on the experimental data of Lee et al. [5] and Chu et al. [9], Wang et al. [3] illustrated a rela- tionship between dewatering energy demand and sludge water content. According to their findings, which is sche- matically shown in Fig. 2, in the high water content range, the binding strength is low and nearly 20% of the water is easily removed. Once the water content is decreased to about 80%, the binding strength and resulting dewatering energy demand is increased sharply. In another study, Chu et al. [10] employing TGA and DTA tests for dewatering of activated sludge from a wastewater treatment plant, found similar re- sults for variation of bond energy with residual water content for original and flocculated sludges. They reported that the bond energy was close to 1 kJ kg�1 at residual water contents of more than 30 kg kg�1 DS and it exceeded 1MJ kg�1 when the water content was less than 0.5 kg kg�1 DS. These imply that at lower residual water content, the sludge can no longer be dewatered, due to the inability of conventional dewatering equipment to provide sufficient force. There are different parameters which could affect the capture of water molecules in the flocs and cells or binding them to the surfaces of solid particles and therefore affect the dewaterability of sludge. Among these parameters, solids particle size and particle size distribution and chemical composition of solids could be very important. Particle size and particle size distribution play a major role in the deter- mination of settleability of solid particles and porosity and permeability of the solid cake. So flocculation of small gel-like solid particles in sludge into larger and stronger aggregates with less affinity for water, can improve sludge settle-ability and filterability. Physical [11e22] and chemical [23e36] Fig. 2 e Variation of dewatering energy demand with sludge water content. pretreatment of biosludge are mainly for favorable changes in particle size and particle size distribution in sludge to improve settleability of solid particles and porosity and permeability of forming filter cakes. Another important property of sludge is its composition. In biological wastewater treatment, usually bacteria tend to aggregate forming flocs, biofilms, or even granules [37]. It was postulated that some filamentous mi- croorganisms form a backbone to which the floc-forming bacteria are firmly attached [38,39]. The floc-forming bacte- ria convert organic substances to some extracellular mate- rials. These materials form an extracellular polymeric contents could be obtained at pressures of 300e400 kPa, however by increasing of pressure up to 6e10 Mpa, 60% water content are considered as the most important ones which affect the dewaterability of sludge. Zeta potential which represents the surface characteristics of sludge and stability of colloids [25,67] and Yield stress which represents the rheological behavior of the sludge [68] are other properties of sludge. Sludge dewaterability can be assessed using different criteria for sludge as follows: - Capillary suction time (CST), which is a relatively easy to be measured property of sludge and is expressed by the time (in seconds) that water in sludge requires to travel a given distance when it is exposed to a standard capillary suction. The original setup for CST measurement consists of a cy- lindrical container in which a column of sludge is placed. The sludge column is centered in the middle of two concentric electrodes located at diameters d1 and d2 resting on a Whatman-17 filter paper. The time required for the waterfront to move from electrode 1 to electrode 2 is b i om a s s a n d b i o e n e r g y 5 8 ( 2 0 1 3 ) 3 6 5e3 7 8 367 substances (EPS) matrix, to which various bacteria and mi- croorganisms are enmeshed [37]. The EPS are highly charged polymers that interact withwatermolecules as a gel [39]. They absorb large amount of water and swell, but do not dissolve in water. EPS can exist in two different forms outside of cells: bound EPS and soluble EPS. Bound EPS are closely bound to cells, while soluble EPS are either weakly bound to cells or dissolved in the solution [40,41]. Nielson and Jahn [40] depic- ted a two layer model for the structure of bound EPS as shown in Fig. 3. The first layer consists of tightly bound EPS (TB-EPS), which is bound tightly and stably to the cell surface. The second layer consists of loosely bound EPS (LB-EPS), which is a loose slim layer without an obvious end. A gel-like floc structure model composed of EPS, different cells and boundwatermolecules is schematically shown in Fig. 4. The structural and functional integrity, and hence, the physico-chemical and biological properties of flocs are mainly controlled by EPS. Indeed, the presence of EPS in biosludge can cause difficulties in dewatering [42]. Rupturing of EPS by means of enzymatic [43e47], ultrasonic [1,48e57] or thermal [3,58,59] pretreatment, can improve dewaterability of the treated sludge. Due to the different nature of water molecules in bio- sludge, different approaches may be employed in the bio- sludge dewatering process depending upon the required consistency of the product sludge. These are [4,60]: a) Sludge thickening: which is the application of weak me- chanical strains or simple thickening apparatuses such as rotary drum or gravity belt by which the solid content of sludge is increased from 2% or less to at most 15%? This step is useful for separating free water molecules. b) Sludge dewatering: which is the application of pressure for forcing out the water molecules from sludge. Screw press, belt press, filter press, rotary press and centrifuge are LB EPS TB EPS Trapped water In EPS Fig. 3 e Schematic diagram of two-layer model for extracellular polymeric substances (EPS). solid content cakes can be produced. Although pressure is an important factor in dewatering, there is a threshold pressure above which the cake structure would be deteri- orated significantly [8,62]. c) Sludge drying: in which usually heat is used for destruc- tion of water molecular bonding to solid surfaces and re- leases them for evaporation. Rotary drum [63] and fluidized bed dryers [64e66] are common devices for this step which are able to reach a solids content of more than 90% [58]. 3. Sludge properties Among different properties of sludge, EPS content and bound conventional sludge dewatering apparatuses, in which the sludge solids content can reach up to 50%. Laheij et al. [61] studied the filtration and compression of a flocculated sludge and found that solid cakes with 35e40% solids Interstitial water Vicinal water Hydration water Trapped water in EPS Filamentous Bacteria EPS Fig. 4 e Gel-like floc structure model. measured by an automatic timing device and reported as CST [1,69], ender et al. [72], EPS which are present both outside of cells and in the interior of microbial aggregates, bind with cells to b i om a s s a n d b i o e n e r g y 5 8 ( 2 0 1 3 ) 3 6 5e3 7 8368 cells, also have a significant effect on microbial flocculation ability. Many investigators reported that although various components of EPS have different effects on the flocculation of microbial aggregates, the total EPS content in general has a negative effect on sludge flocculation [86e88]. Since EPS are negatively charged, they increase the surface charge of mi- crobes, resulting in an increase in the repulsive forces be- tween cells. These repulsive forces decrease the flocculability and thus the settleability ofmicrobial aggregates [82,84,85]. An form a vast net-like structure with plenty of water that protects cells against dewatering. There are many charged functional groups in EPS that in- fluence on the surface charge of microbial aggregates. Although different components of EPS have different effects on the surface charge of microbial aggregates, in general, the EPS content has a positive effect on the net negative surface charge of sludge [81e83]. The interaction between EPS and - Specific filtration resistance (SFR), which evaluates the sludge dewatering performance by measuring the filtration rate when exposed to a constant pressure difference. Application of Darcy’s law for flow through porous media to evaluate the filterability of sludges provides the following equation [70]. dt=dV ¼ m=ADPðacV=Aþ RmÞ (1) in which DP is the applied pressure difference, V is the volume of filtrate, t is the time, A is the filter area, c is the sludge total solids concentration, m is the filtrate viscosity and Rm is the resistance of the filter medium. According to this equation, a plot of dt/dV versus V, should give a straight line, whose slope gives a, the Specific Filtration Resistance (SFR). Generally, the SFR for activated sludges are in the range of 10e100 Tm kg�1 [71]. 3.1. Extracellular polymeric substances (EPS) In a biosludge most of the microorganisms are present in the form of flocs, biofilms and microbial granules. As shown by electron microscopy, all these microbial aggregates and even pure cultures contain a complex mixture of macromolecules, called extracellular polymeric substances, simply abbrevi- ated as EPS (by Wingender et al. [72]). EPS are the product of lysis and hydrolysis of macromolecules which may adsorb some organic matters from wastewater to its matrix [73]. Although polysaccharides and proteins are the major com- ponents of EPS, humic substances may also be present in as much as approximately 20% of the total amount [74]. In addition, EPS may contains nucleic acids, lipids, uronic acids and some other components depending upon the source of biosludge and EPS extraction methods [75e78]. EPS can have significant effects on different properties of microbial ag- gregates such as mass transfer [79,80], surface charge [81e83], settleability [82,84,85], flocculation [86e88], and dewaterability [37,89e92]. For example, according to Wing- increase in EPS content leads to poorer sludge dewaterability due to: - Creation of repulsive forces between cells which reduce flocculation ability and settleability of microbial aggregates. - Retention of much water and so increase in the amount of interstitial water in sludge flocs. - Formation of a stable gel-likematerial which preventswater seepage from the pores of flocs. - Formation of a thin layer on the surface of filtering media which acts as a barrier for water passage. - Increased sludge viscosity. It has been reported that after EPS removal from sludge, its dewaterability would be improved [93]. But Houghton et al. [91] proposed that the effect of EPS on dewaterability of sludge depends on the content of EPS in sludge. They found that, in spite of all the above mentioned effects of EPS, the dewater- ability of sludge is initially improved with the EPS content, but then decreased once the EPS content exceeded a certain threshold. This may be explained by the fact that low EPS content can act together with multivalent ions to aid the cells to bindmore tightly which improves flocculation but at higher EPS content the retained water by EPS is significantly increased which results in a lower sludge dewaterability [94]. 3.2. Bound water As it wasmentioned earlier, the boundwater content is one of the major limiting factors affecting sludge dewatering effi- ciency. The chemical potential and binding strength of bound water are naturally different from those of free water [95]. Thermodynamically, it does not behave as pure water. For example more energy is needed for releasing and separating bound water compared to free water [3,5,9] and it does not freeze at temperatures below the normal freezing point of water. Various methods have been proposed for measuring the bound water, all based on the principle that although bound water represents the aqueous part of sludge, it has different properties from pure water. So following the behavior of humidity in the sludge, a discontinuity in prop- erties such as freezing point, dewatering, binding energy or spectroscopic properties is observed which indicates the different nature of water [5,7,96,97]. Although different tech- niques such as: drying test [5,6,96], dilatometry [96,97], dif- ferential thermal analysis (DTA) [98], differential scanning calorimetry (DSC) [99,100], water vapor sorption isotherms [95], nuclear magnetic resonance (NMR) [101], suction pres- sure [102], centrifugal settling test [5,103] and constant pres- sure expression test [5,9] have been proposed for measuring the bound water content. However, due to the lack of a stan- dard reference sludge for assessment of these techniques, the obtained values tend to be operationally defined values which depend on the measurement technique and other variables [5,96]. Among these techniques, the following are most commonly used for this purpose: Drying Test: The drying test has been used to measure bound water content based on the premise that the resistance of bound water to evaporation should be larger than free water. Herwijn et al. [95] proposed that the water content with bond enthalpy >1 kJ kg�1 is referred to as the bound water. In this method the rate of drying of sludge is plotted versus its water content at constant temperature, as shown in Fig. 5, to - by reducing the sludge solids compressibility to improve Fig. 5 e Variation of rate of drying with water content. b i om a s s a n d b i o e n e r g y 5 8 ( 2 0 1 3 ) 3 6 5e3 7 8 369 locate the water content corresponding to the transition points between drying periods [5,6,9,96]. Dilatometry: In this technique the sludge freezable water (free water) content is measured by an apparatus called dila- tometer. The difference between the total water content and the free water content is reported as bound water. In dila- tometer, a weighted amount of sludge is added to some vol- ume of a suitable measuring fluid (xylene or hydraulic oil) and then the dilatometer is placed in a freezer at usually�20 �C for one night. Using the standard cooling and freezing curves of distilled water and measuring fluid, allows quantifying the amount of contraction and expansion of each fluid. In this way, the volume of freezable water can be calculated. By dry analysis of the sludge, total water content can be obtained. The difference between total water content and the freezable water gives the amount of bound water [5,7,96,97]. Expression test: In this test, the residual moisture content in sludge is measured when it is in equilibrium with a con- stant applied pressure such as 20.7 MPa [5,9]. Centrifugal settling test: In this test sludge is centrifuged at very high rotational speeds of 3500e4000 rpm and the water content of the sludge sediment is reported as bound water of Hydration Water Vicinal Water Interstitial Water Free Water Rate of drying Water Content the sludge [5,103]. 4. Sludge dewatering difficulties The difficulties in biosludge dewatering can be related to the highly compressible nature of sludge solids, high organic contentandcolloidalmaterials in it. Indeedsludge is a colloidal system inwhich small solid particles form a stable suspension in water which is very difficult to be separated from the water phase. Although it is not clearly understood which properties of these hydrous sludges caused them to bindwatermolecules more tightly than others, it is expected that dewaterability of sludge is related to the particle size and particle size distribu- tion of the solids within it, chemical composition and compressibility of solids andother factors suchas the lengthof fibers presented in the sludge. It is generally accepted that the presence of large fraction of long fibers in sludge makes it easier to be dewatered compared to the sludges containing sludge cake filterability (SFR). - by disintegration of sludge using techniques such as ozona- tion, enzyme treatment or sonication for rupturing flocs or cells to release the trapped (bound)watermolecules fromEPS. Various methods are used for this purpose ranging from a simple addition of flocculating agents such as lime and alum, to physical alteration in the structure of the cake by addition of some inert materials with high porosity such as wood chips or lignite. More sophisticated methods include thermal, chemical or biochemical alterations in the system such that some favorable changes occur in the form and properties of the pro- duced flocs. In addition to dewatering properties, conditioning processes can have significant impacts on other properties of sludge and so theymust be carefully integrated to provide both economic and environmental benefits. The known methods of conditioning can be categorized as physical or chemical methods as described in the next two sections: 5.1. Physical conditioning Among the methods employed for improving the dewater- ability of biosludges, physical conditioning methods are large quantities ofminute fibrous particles [4]. The presence of long fibers in the sludge modifies the structural rigidity or compressibility of thefilter cake in amanner to improve sludge dewaterability. The presence of fine particles causes cake filter or filters media blinding by migration of fines into the filter cake or filter media pores. From the chemical composition point of view, it was observed that the difficult-to-dewater sludges usually contain an excessive amount of water- swellable EPS, composing of polysaccharides, pectics, hemi- cellulose, etc. It was hypothesized that the presence of such substances could be partly responsible for the difficulties in dewatering. Comparison of the chemical composition of two fractions of solid particles presented in a ground wood pulp sludge, showed that the smaller size fractions, which aremore difficult to be dewatered, contained higher fractions of hemi- cellulose (lignin and pectic substances), while the larger size fraction contained higher quantities of cellulose relative to hemi-cellulose. So hemi-cellulose and pectic substances could be considered as the responsible for binding large quantities of water in the small size fractions of particles [4] or presence of fine particles, trapped in the filter cake or filtermedia, could be considered as a blinding agent for solids cake which makes difficult sludge dewatering. 5. Sludge conditioning methods In order to improve biosludge dewaterability, proper sludge conditioning is a prerequisite. Conditioning is intended to alter sludge properties to achieve effective dewatering. This is usually attained in three different ways: - by coagulation or flocculation of sludge solids particles to improve settleability of the sludge. generally more attractive from economical and environmental points of view. Physical conditioning is achieved through awide b i om a s s a n d b i o e n e r g y 5 8 ( 2 0 1 3 ) 3 6 5e3 7 8370 range of technologies. The most popular techniques are described below. 5.1.1. Non-chemical additives The most well-known classical physical conditioning method is the addition of some high porous inert minerals such as fly ash [13,14], lime [104] and gypsum [16,17] or carbonaceous materials such as coal [21], wood chips and wheat dregs [20], bagasse [17,19e21], rice shell and rice bran [22], lignite [12], etc. which act as skeleton builders or filter aids due to the role that they have in reducing the compressibility of sludge and improving the mechanical strength and permeability of solids existing in sludge during compression. These conditioners form a rigid lattice structure of solids which can remain porous while they are compressed in mechanical dewatering devices. Even mixing of sludge with high inorganic solid content and better properties of dewatering such as primary sludge in pulp and paper industries, can improve the dew- aterability of secondary sludge (i.e. biosludge) by rendering it less compressible and transformed the flocs into a more rigid structure capable of maintaining high porosity under pres- sure. Carbonaceous materials as physical conditioners are advantageous over mineral materials because of their low ash content, high heating value and generally high porosity. Their use not only improves the sludge dewatering, but also allows for the ultimate management of the solids mixtures by incineration which will improve the overall economics of the method. Physical conditioners could be used alone to improve sludge dewatering [27] or following a coagulation or floccula- tion with a chemical conditioner. In the latter case, optimal sludge dewatering can be achieved since without chemical conditioners, which are used to manage sludge colloids, physical conditioners alone usually cannot function as filter aids to some extent or at all, unless at very high doses. It has been found that simultaneous application of chemical and physical conditioners causes interaction between the condi- tioners and sludge colloids due to their charged nature. This interaction helps to form a homogenous and permeable solid structure, which leads to larger improvements in sludge dewaterability [11]. Thapa et al. [12] used lignite with a poly- electrolyte to improve sludge dewatering. They studied the SFR, permeability and porosity of the produced filter cakes using mechanical compression tests and showed that the use of lignite along with polyelectrolyte flocculation gave much better dewatering than polyelectrolyte alone. Indeed the presence of lignite particleswithmacropores on their surfaces in the compressed filter cake, create channels or pores in the filter cake increasing its porosity and decreasing its compressibility by making a rigid lattice in filter cake. The desirable properties of physical conditioners include rigid structure, high porosity, low compressibility and reasonably large particle size [11]. 5.1.2. Cavitation pretreatment Recently, other physical conditioning methods in which the properties of sludge and forming flocs in the sludge are improved without the addition of external materials have been suggested. For this purpose, cavitation seems to offer immense potential and received special attention during recent years. Cavitation is a process during which the local pressure in the aqueous phase is reduced below the equilib- rium vapor pressure and some microcavities or microbubbles are formed, grown gradually and collapse violently after reaching to an unstable diameter. This process produces a shockwave causing high temperature (500e15,000 K) and high pressure (10e500 MPa) locally in the media at a lifetime of a few microseconds [51,105]. Among different methods of cavitation generation, only acoustic and hydrodynamic cavi- tations have been shown to be efficient in producing the desired changes in biological processes. Acoustic cavitation is generated by the use of low frequency ultrasound. The ultrasound is cyclic sound compression and expansion with a frequency greater than 20 kHz. Compression cycles exert a positive pressure and push the molecules together while expansion cycles exert a negative pressure and pull molecules from each other. Due to this cyclic negative pres- sure, acoustic cavitation or sonication occurs through me- chanical shear forces which are produced in the biosludge. So exposure of biosludge to ultrasonic energy can create physical (particle size and particle size distribution and turbidity [51,53]), chemical (soluble chemical oxygen demand, SCOD [51,54]) and biological (heterotrophic plate count and specific oxygen uptake rate, SOUR [51,55]) changes in its properties [1,47,48,50,105]. Ultrasonic stresses rupture the microbial cell wall and release the intracellular organics into the aqueous phase. Also sonication helps to deagglomerate the biological flocs and change large organic particles into smaller ones. It has been reported that ultrasonic wave propagation gives some sponge-like properties to the forming sludge and facil- itates the movement of water molecules in the channels and pores in the sludge. In spite of these changes, controversial conclusions were taken by different investigators regarding the effect of sonification on dewatering properties of sludge. For example, Bien and Wolny [106] and Hogan et al. [107], re- ported that ultrasonic pretreatment could enhance the dew- aterability of sludge, while Wang et al. [49] showed that the sludge dewaterability deteriorates with increases in ultra- sonication intensity due to cell lysis and release of bacteria and biopolymers from EPS into the aqueous phase. Xin Feng et al. [108] reported that the effect of ultrasonication on sludge dewatering was dependent on applied energy doses Ed, which is defined as, Ed ¼ P t=V TS0 (2) in which P is the ultrasonic power, t is the exposure time, V is the volume of sludge sample and TS0 is the initial total solid concentration of the sludge. According to their findings, low-energy dosage improve slightly sludge dewatering, while high-energy dosage deteri- orate significantly sludge dewatering. They obtained an opti- mum energy dosage of 800 kJ kg�1 TS considering CST and SFR for sludge dewatering evaluation. This energy dosage gener- ated sludge with EPS content of 400e500 g m�3 and particle size range of 80e90 mm. The major reasons for this behavior were considered to be increasing of EPS and decreasing of particle size in sludgewhen subjected to ultrasonication. Also, Wang et al. [50] studied the effect of sonication energy dosage on EPS release and found that the release of EPS increaseswith energy dosage. The presence of EPS increases the viscosity of b i om a s s a n d b i o e n e r g y 5 8 ( 2 0 1 3 ) 3 6 5e3 7 8 371 the sludge and also forms a thin layer on the surface of filteringmedia which acts as a barrier for water passage. Huan et al. [51] reported that if less than 2% of sludge mass is dis- integrated by sonication, the change in sludge floc structure is very limited and when it is more than 5%, more fine particles are produced and this increases the bound water content. So the sludge dewaterability will be improved if the degree of disintegration of sludge is between 2% and 5%. Although the dramatic effects of sonication leads to its more common application in bioprocessing industries, similar results can be achieved by hydrodynamic cavitation which is generated by hydraulic systems with much lower energy requirements [109]. Hydrodynamic cavitation can simply be generated by using constrictions such as orifice plate or venturi in liquid flow path in which velocity of liquid is increased. This in- crease in velocity causes a local pressure decrease which, if sufficient (less than vapor pressure of the media at operating temperature), can generate cavities which collapse as the liquid jet expands and pressure is recovered similar to what happens for sonication. This process is usually done in a high pressure homogenizer apparatus which consists of a feed tank, a high pressure positive displacement pump and an orifice plate. Caufield [110] referred to the use of this type of high-pressure homogenizer or, as he called it, cell disrupter for breaking down the bacteria’s tough cell walls and releasing their liquid contents in the case of waste activated sludge produced in pulp and paper industries. He noted that the treated sludge produced more biogas by anaerobic digestion, released more valuable nitrogen and phosphorus and con- tained fewer solids which reduce the required amount of polymer for sludge dewatering. 5.1.3. Thermal pretreatment Another well-known method for sludge conditioning is ther- mal pretreatment. In this method, liquid sludge is heated up in the temperature range of 60e180 �C. Although the lipids and carbohydrates of sludge are easily degradable, the proteins are protected from enzymatic hydrolysis by the cell wall. Thermal pretreatment in this range can destroy the cells wall and release the proteins for biological degradation [3,57,58]. In this process, the sludge gel network is broken and the water af- finity of the sludge solid is decreased [3]. The degree of disintegration depends on the applied temperature and holding time. A temperature of 175 �Chas been reported as the optimum for digested and undigested sludges with 10e30 min of holding time. The viscosity of heat-treated sludge decreases significantly and this can also improve the filterability of the treated sludge. 5.1.4. Freeze/thaw pretreatment Freeze/thaw conditioning is another physical efficientmethod for changing floc structure and reducing the bound water content in sludge [111e118]. In this method, the sludge is first frozen at temperatures below the normal freezing point of water, around �15 �C and kept at this state for some time and then it is thawed at room temperature. In this technique, separation of water and solid particles occurs during ice crystal formation. An ice crystal grows by taking water mol- ecules into its structure. Impurities such as solid particles are not accepted in the ice crystal lattice and are rejected to the boundaries of the crystals where they are consolidated. The ice crystals melt away during thawing, leaving the consoli- dated and dewatered solid particles. Martel [113] employed the technique of natural freezing and thawing for dewatering of three different kinds of sludge: aerobically digested, anaerobically digested, and alum sludge and obtained good results in laboratory, pilot, and full scales. They reached a solid content of 82% for alum sludge by this method. France- schini [117] studied the effect of freezing temperature and the number of freezing cycles on dewaterability of different sludges and reported that although freezing temperature was not very effective, increasing the number of freezing cycle improved the dewaterability of treated sludge. 5.2. Chemical conditioning Sludge particles are known to be positively or most often negatively charged. Chemical conditioners are chemical spe- cies often with the opposite charges, used to flocculate or coagulate the sludge colloids by charge neutralization and adsorption. The chemical conditioners are classified into two general groups of organic additives and inorganic additives as described below. 5.2.1. Organic additives Organic polymers in the form of polyelectrolytes have long been used in sludge conditioning to improve its mechanical dewaterability. Sludge conditioning by polyelectrolytes could be explained by two different mechanisms: charge neutrali- zation and interparticle bridging. The result of both is for- mation of flocs which reinforce the structure of solids to ease separation of solids from liquid. Bache et al. [26] used three types of cationic and anionic polymers for conditioning of an alum sludge derived from the coagulation of colored waters and proposed the factors which control the optimum dosage of polymers on SFR and CST improvement. Dieude-Fauvel and Dentel [8] studied the impact of different polymers dosages on the structure of flocs by both rheological and microscopical techniques. Rheology and microscopy give information on sludge structure at macro- scale and micro-scale respectively. Using the results of both techniques, they found that for a given groups of polymers (Acrylamide-based Zetag polymers), increasing the polymer dose up to an optimal dosage, bigger and denser flocs are produced after which the flocs structure tends to be stabilized. Sludge capillary suction time measurement showed a minimum CST at this optimum dose. So the dose of polymers should be precisely controlled, since overdosing will increase the operating cost and, if applied at too high a level, it can both reduce sludge dewaterability and cause adverse effects on humans and the ecosystem [36,119]. It is to be noted that the presence of these polymers in the treated water or even the sludge could have adverse effects on human beings and ecosystem. A critical review was given by Bolto and Gregory [28] on different types of poly- mers which are used in the water industry and the mech- anisms of flocculation and coagulation for them. According to this study, the polymers which are used in water treat- ment systems are classified as anionic, cationic or nonionic. Although these substances are sometimes biodegradable, Dursun et al. [131] also used the same enzyme with a dose of b i om a s s a n d b i o e n e r g y 5 8 ( 2 0 1 3 ) 3 6 5e3 7 8372 they are generally toxic to human and aquatic species at high concentrations [28]. Recently, some research was carried out to examine chemicals which may boost the performance of conventional polyelectrolytes used for sludge dewatering. Banerjee et al. [29,30] and Bradley et al. [31] found that the addition of very low dosage of cyclodextrines (CDs) to other polyelectrolytes, increases the capture rate of solids in belt pressing and de- creases the SFR of the sludge. In another study, Wang and Banerjee [120] showed that the hydrodynamic diameter of cationic polyacrylamides (c-PAM) in water can change dramatically in the presence of small amounts of b-CD. Turbidity measurements demonstrated that b-CD enhances the c-PAM induced flocculation of wood pulp fibers. A dual cationic and nonionic polyelectrolytes conditioning was pro- posed by Lee and Liu [25]. They compared sludge dewatering behaviors from single and dual polyelectrolytes conditioning by measuring SFR, CST, settling rate and zeta potential pa- rameters and reported a better dewaterability due to the for- mation of larger flocs and fine particle capture. The effect of adding a nano-colloidal silica to a conventional cationic polymer for conditioning of biosludge from a paper mill was studied by Yuan-Shing Perng et al. [121]. They found better results for CST and SRF if the cationic polymer was added first, followed by the anionic nano-silica. 5.2.2. Inorganics additives Zhao [122] proposed a non-organic polymer approach for sludge conditioning, in order to reduce the unknown potential risks related to prolonged use of polymers in wastewater treatment. For this purpose, they referred to sludge ozonation [32] and the advanced oxidation process (AOP) of sludge by Fenton (Fe2þ/H2O2) and its related reagents [33e35] as poten- tial alternatives for alum sludge conditioners. In their exper- iments, Zhao et al. [122] found a 47% reduction in CST when Fenton and Fenton-like reagents were used for alum sludge conditioning. It is to be noted that this amount of reduction is not the same level when using polymers and also due to dual- reagent addition for Fenton reagents, the equipment and control costs may be increased. Cousin et al. [23] studied the effect of salinity on dewaterability of an industrial activated sludge. They found that increasing NaCl concentration in the sludge has adverse effects on its dewaterabiliy. They put for- ward two different explanations for their findings: - Sodium ions could have physiological effects on bacterial metabolism which result in denser flocs and so lower dewaterability. - Sodium ions could have physicochemical effects which change only the size of flocs and produce bigger flocs con- taining more hard-to- be removed water molecules. 5.3. Biological or enzyme conditioning Biological or enzyme pretreatment of sludge can improve it’s dewaterability by weakening the gel structure of the flocs through the hydrolysis of EPS present in the sludge. Since enzymes are proteins and therefore considered environmen- tally friendly, this method can be very attractive, especially if it replaces acrylamide based synthetic polymers which are 15 gm�3 in both laboratory- and pilot-scale tests. In laboratory scale, they obtained 33% solid content cake, instead of 20% without enzyme pretreatment. But the dewatering efficiency reached in laboratory scale could not be reached at the pilot- scale. Their results indicated that the higher shear applied in pilot-scaled centrifugation can deteriorate flocs and so be responsible for the lack of improved cake solids. Bymeasuring the network strength of sludge, they concluded that enzyme treatment disrupts the gel network of sludge and thus it is beneficial for low shear dewatering processes such as belt press, but not effective for high shear centrifugation. Wawr- zynczyk et al. [128] applied enzymes and enzymes combined with sodium tripolyphosphate (STPP) for extraction of EPS from sewage sludge. They concluded that enzymes combined with STTP released higher amounts of total EPS compared to enzymes alone. Sarker et al. [127] used cellulolytic and peroxidase enzymeswith andwithout flocculants (acrylamide based cationic polymers) for conditioning of anaerobically digested sewage sludge samples and found improvement in dewaterability with simultaneous reduction in polymer dose. They reported the effective doses of 40e60 g for cellulolytic and 25e50 g for peroxidase respectively both per dry tonne solids. Lu et al. [43,132] treated the primary sludge of a pulp and paper plant withmixed cellulose enzyme and found 50 �C as the optimum temperature for enzymatic hydrolysis of cellulose. This treatment caused rupture of fibers which leads to higher cake solids by dewatering due to better cake consolidation with the shorter fibers. 6. Assessment of conditioning methods A successful sludge dewatering process starts out by settling down the solid particles suspended in the sludge and then to produce a permeable and rigid lattice structured cake which can remain porous and incompressible under high positive pressure during the compression step, thereby maintaining the size of micropassages through which water is expressed. For biosludge, rupture of cells and flocs to release the captured water molecules within them can also be effective in dew- atering. Mechanical dewatering which has found extensive industrial applications in sludge dewatering mainly due to its low energy consumption in comparison with classical ther- neurotoxic. This method has been investigated by several re- searchers in recent years [123e130]. Parmar et al. [130] inves- tigated the effect of industrial microbial enzymes of cellulase, protease and lipase, individually or in combination, on reducing the disposal solid content of anaerobically digested sewage sludge. They found that a mixture of these enzymes, in equal proportion by weight, reduced total suspended solids by 30e50% and improved settling of solids. Ayol [129] and Ayol and Dental [124] showed that addition of low doses (10e15 g m�3) of a commercially available enzyme formula- tion (Enviro-Zyme 216) to polymer conditioned sludge sam- ples increased significantly the dewaterability in terms of CST and final solid content of cakes produced by Crown press for two anaerobically digested sludges. Abu-Orf et al. [125]. and mal methods, is not able to separate all the water content of sludge due to strong binding between the water molecules and solid surfaces. So sludge conditioning, before the me- chanical pressure is applied to force out thewater, is essential. Different methods of conditioning are applied for increasing the dewatering efficiency of sludge by improving one of the three items which are mentioned previously for achieving successful dewatering. For example the main role of all the chemical conditioners including inorganic coagulants such as lime, alum and ferric chloride and organic flocculants such as polyelectrolytes is to flocculate or coagulate the small solid particles and settle them out, but their role in creating a b i om a s s a n d b i o e n e r g y 5 8 ( 2 0 1 3 ) 3 6 5e3 7 8 373 porous and incompressible cake is minor. Although different optimum dosages have been reported for different chemical conditioners, but in general for polymers it is between 1.4 and 2.8 kg tonne�1, for lime it is 45.4e272.4 kg tonne�1 and for ferric chloride it is 22.7e90.8 kg tonne�1 [59]. Numerous re- searchers have conducted studies to evaluate the effective- ness of chemical conditioners on sludge dewatering properties [8,9,25,26,28,59,123,133,134]. In these studies, for each conditioner an optimum dosage at which dewatering efficiency is maximized has been obtained. For example Buyukkamaci et al. [133] found the optimum dosage of 0.9% mass fraction of initial total sludge for a cationic poly- electrolyte at which SFR and CST are both improved by 95%. They also reported 80% and 60% improvement of SFR by using alum (8%) and lime (8e12%) respectively. Hou et al. [134] found the optimum dosage of 15 mg kg�1 for another cationic poly- mer of polyacrylamide at which the SFR is decreased from 1.33 to 0.13 Tm kg�1 and CST is decreased from 20.95 s to 15 s. Due to the high volume of sludge which is to be conditioned and high cost of commercial chemical conditioners and also the adverse effects that they can have on environment and ecosystem, attempts have been done to reduce the dose of chemical conditioners, while keeping high the dewatering efficiency. The use of physical additives, other inorganic chemicals, sonication, thermal treatment or freezing are among these attempts. Physical additives such as fly ash, saw dust, wood chips, bagasse, etc. usually serve as filter aids and their role ismainly improving the porosity and permeability of filter cake by preventing of filter cake and filter media from blinding. A conceptual description of the role of chemical (alum, ferric chloride etc.) and physical (fly ash, wood chips etc.) condi- tioners is shown in Fig. 6 [16]. Benitez et al. [20] treated the primary sludge of a waste- water treatment unit which processed a mixture of industrial and municipal effluents with 1.3% (based on original dry solid content of the sludge) of a cationic copolymer coagulant (CALGONWT-2640) as chemical conditioner with and without P Filter media Physical conditioner P Filter Media Chemical conditioner under pressure Filter Media Chemical conditioner without pressure Fig. 6 e Conceptual description of the role of chemical and physical conditioners. bagasse as physical conditioner. They found that addition of 37% (based on original dry solid content of the sludge) bagasse to the system increased the net sludge solid yield (kg m�2 h�1) by 1260% when compared to conditioning only with polymer. Qi et al. [11] reviewed in detail the effects of mineral and carbon-based physical conditioners on sludge dewatering. They concluded that although both thesematerials are able to improve sludge solids properties and reduce the required dose of chemical conditioners, but carbonaceous materials give more significant improvement in sludge dewatering probably due to their porosity. They also pointed out two advantages of lower ash content and higher calorific value of produced solid cake when carbonaceous materials, are used, especially if the dewatered solid sludge is going to be used for combustion. In gathering and comparing the experimental data of various authors, they showed that for example addition of fly ash (0.5:1 solid mass to sludge ratio) to iron and lime chemical conditioners decreased the SRF of sludge from 45 Tm kg�1 to 19 Tm kg�1 or addition of Oyster shell (0.68:1 solid mass to sludge ratio) to alum, decreased the SFR of the sludge from 0.512 Tm kg�1 to 0.121 Tm kg�1. These experimental data show the large effect of physical conditioners on improving the structure of forming cake. Considering low cost and environmental friendly nature of these materials, their use is recommended in full-scale utilization. Especially if the pro- duced sludge is to be used for combustion purposes, addition of wastes such as wood chips or rice hull can increase also the heating value of the dewatered sludge [18]. Another technique for boosting the performance of chemical conditioners and reduce their required dosage is addition of other chemicals. For example, Wang et al. [120] and Bradley et al. [31] showed that addition of low doses (0.1e0.3 kg tonne�1) of Cyclodextrines (CDs), non-toxic chemicals used in pharmaceuticals, foods and cosmetics in- dustries, can modify the properties of cationic poly- acrylamides which are usually used as chemical conditioner in sludge dewatering. They proposed that since CDs increase the surface tension of cationic polyacrylamides polymers in water and reduce the turbidity, they are able to aggregate more the charged polymers and as a result the sludge particles treated with the polymer producing tighter flocs. Banerjee [29,30] used CDs along with chemical conditioners in several paper mills and industrial and municipal wastewater treat- ment plants. They noticed lower SFR, higher cake solids and drainage rates for primary or secondary sludge and as a result a 30% reduction in polymer consumption. From economical points of view, although CDs are approximately twice more expensive than polymer, but due to low dosage requirement, the cost benefit is attractive. In addition CDs are biologically derived products, prepared from starch, while polymers are known as toxic substances. So displacement of polymers by CDs carries both economic and environmental benefits [29]. Fenton’s reagent (Feþ2/H2O2) conditioning is another tech- nique proposed as potential alternative for polymer condi- tioning to decrease the probable long term risk associated with polymer residual in the environment [33e35,122,135]. Zhao et al. [122] reported a mean CST reduction of 47% for an alum sludge, using Fenton with a dose of Fe 2þ at 20 g kg�1 DS and H2O2 at 125 g kg �1 DS (dry solids). Park et al. [32] reported that sludge ozonation prior to chemical conditioning any secondary toxic compounds are generated. In addition content and higher heating value which are very important if the produced dewatered sludge is to be used for energy Thermal pretreatment in spite of its effectiveness in dis- b i om a s s a n d b i o e n e r g y 5 8 ( 2 0 1 3 ) 3 6 5e3 7 8374 many toxic and recalcitrant organic pollutants are broken into simpler chemical compounds. In spite of controversy on the effect of sonication time or sonication power, it is now generally accepted that ultrasonic energy density or dose (kJ kg�1) is more important for efficient sludge disintegration [43]. An energy dose of 800 kJ kg�1 TS has been recognized as optimum sonication energy. Lower energy dosages show very limited effects on sludge floc structure, while higher energy doses produce more fine particles and increase the bound water content [51]. The freeze/thaw technique is another efficient physical conditioning method for sludge dewatering. Although this methodwas investigated for different sludges andwas proved as an efficient technique, but it is too expensive to freeze so large amount of sludge. So it is not fit to be used in industrial scale when taking the cost into account except for plants located in cool regions of the world such as Canada and North America where natural freezing is feasible. The technique of thermal conditioning has been employed in several full-scale installations and an increase of 60e80% in sludge dewatering reported for it. But problems such as odor, corrosion and high strength COD liquor caused difficulties for this technique. In order to solve these difficulties, combined thermo-chemical pretreatment in which low temperature hydrolysis followed by acid or alkaline hydrolysis was pro- posed [136,137]. 7. Summary About 80% of water content of biosludge is recognized as bound water which cannot be separated by conventional dewatering devices such as centrifuge or filter press due to high strength binding between water molecules and sludge solids surface. The main limiting factors in biosludge dew- atering are: low sedimentation rate of sludge colloidal solid particles (settleability), high compressibility of sludge solids (filterability) and high affinity of EPS for water molecules capture (bound water). In order to improve the dewatering efficiently, sludge pretreatment is necessary. A sludge pre- treatment method is known as successful if it can improve at least one of the above limiting factors. The role of improved both settleability and dewaterability of an activated sludge. According to their results, the SFR of the sludge first deteriorated up to an ozone dose of 200 g kg�1 of DS and then improved considerably at higher ozone doses. A sludge mass reduction of 70% was achieved through ozonation at a dose of 0.5 g O3 g �1 DS. They claimed that this was a more economi- cally attractive method compared to other alternatives for small-sized wastewater treatment plants. The ultrasonication or acoustic cavitation technique is a promising one that disrupts sludge flocs and lyses biological cells. This technique acts by solubilization of organic matters, reduction of particle size and inactivation of microorganisms, can change settleability and filterability of treated sludge. Ultrasonic disintegration is essentially a physical process during which neither any chemical compounds are added nor chemical conditioning is mainly coagulation or flocculation of sludge solids particles by some polyelectrolytes to form integrating of EPS and reduction of bound water in sludge is limited because of high corrosion rate, unpleasant odor and high COD of obtained liquor. Ultrasonication, ozonation and enzymatic pretreatment of biosludge are also effective methods in floc and cell rupture and as a result on dewater- ability of sludge, but they are limited to narrow ranges of energy dose, ozone dose or enzyme concentration and required special control to provide favorable effects on sludge dewatering. Acknowledgment This work was supported by the Natural Sciences and Engi- neering Research Council and the Pulp and Paper Centre at the University of Toronto. We also thank Amir Mowla for his assistance in drawing the figures in this article. r e f e r e n c e s [1] Yin X, Han P, Lu X, Wang Y. A review on the dewaterability of bio-sludge and ultrasound pretreatment. Ultrason Sonochem 2004;11:337e48. [2] Elliott A, Mahmood T. Pretreatment technologies for advancing anaerobic digestion of pulp and paper biotreatment residues. Water Res 2007;41(19):4273e86. [3] Wang W, Lud Y, Qiao W. Possible solutions for sludge dewatering in China. Front Environ Sci Eng China 2010;4(1):102e7. [4] CANMET Energy Technology Centre. Pulp and paper sludge to energy-preliminary assessment of technologies. Canada: ADI Limited; March 2005. p. 152. Report No. 34, 0173e479.1. production by combustion. This technique due to low cost and environmental friendly nature of used materials is rec- ommended if it can provide the desired dewatering effi- ciency. Freeze/thaw pretreatment methods are able to improve dewatering properties, but due to high energy requirement, it is not recommended except perhaps for very cold regions of the world where natural freezing is possible. stable aggregates and improve settleability and also filter- ability of treated sludge to some extent. The potential adverse effects of using polymers, limit the use of this method. Attempts have been made for finding other chem- icals to boost the effects of these polyelectrolytes and reduce their required amount. Addition of very small dose of cyclodextrines or nano-silica particles showed considerable reduction in required polyelectrolytes dosage. Use of phys- ical additives such as fly ash or wood wastes which act as filter aids or skeleton builder, improve the porosity and permeability of forming solid cake. Carbon based materials are physical additives that are generally more effective in sludge dewatering than inorganic materials due to their generally higher porosity. In addition they have lower ash [5] Lee DJ, Hsu YH. Measurement of bound water in sludges: a comparative study. Water Environ Res 1995;67(3):310e7. b i om a s s a n d b i o e n e r g y 5 8 ( 2 0 1 3 ) 3 6 5e3 7 8 375 [6] Vesilind PA, Hsu CC. Limits of sludge dewatering. Water Sci Technol 1997;36(11):87e91. [7] Colin F, Gazbar S. Distribution of water in sludges in relation to their mechanical dewatering. Water Res 1995;29(8):2000e5. [8] Dieude-Fauvel E, Dental SK. Sludge conditioning: impact of polymers on floc structure. J Residual Sci Technol 2011;8(3):101e8. [9] Chu PC, Lee DJ. Moisture distribution in sludge: effects of polymer conditioning. Environ Eng 1999;125(14):340e5. [10] Chu CP, Lee DJ, Chang CY. Energy demand in sludge dewatering. Water Res 2005;39:1858e68. [11] Qi Y, Thapa KB, Hoadley AFA. Application of filteration aids for improving sludge dewatering propertiesea review. Chem Eng J 2011;171:373e84. [12] Thapa KB, Qi Y, Clayton SA, Hoadley AFA. Lignite aided dewatering of digested sewage sludge. Water Res 2009;43:623e34. [13] Chen Ch, Zhang P, Zeng G, Deng J, Zhou Y, Lu H. Sewage sludge conditioning with coal fly ash modified by sulfuric acid. Chem Eng Journal 2010;158:616e22. [14] Wang S, Viraraghavan T. Wastewater sludge conditioning by fly ash. Waste Manag 1997;17(7):443e50. [15] Zall J, Galil N, Rehbun M. Skeleton builders for conditioning oily sludge. Water Pollut Control Fed 1987;59(7):600e706. [16] Zhao YQ. Involvement of gypsum (Ca SO4.2H2O) in water treatment sludge dewatering: a potential benefit in disposal and reuse. Separation Sci Technol 2006;41(12):2785e94. [17] Zhao YQ. Enhancement of alum sludge dewatering capacity by using gypsum as skeleton builder. Colloids Surf A 2002;211:205e12. [18] Lin YF, Jing SR, Lee DY. Recycling of wood chips and wheat dregs for sludge processing. Bioresour Technol 2001;76:161e3. [19] Srivastava VC, Mall ID, Mishra IM. Treatment of pulp and paper mill wastewaters with polyaluminium chloride and bagasse fly ash. Colloids and Sur A 2005;260:17e28. [20] Benitez J, Rodriguez A, Suarez A. Optimization technique for sewage sludge conditioning with polymer and skeleton builders. Water Res 1994;28(10):2067e73. [21] Hirota M, Okada H, Misaka Y, Kato K. Dewatering of organic sludge by pulverized coal. Water Pollut Control Fed 1975;47(12):2774e82. [22] Lee DY, Lin YF, Jing Z, Xu J. Effects of agricultural waste on the sludge conditioning. J China Inst Environ Eng 2001;11:209e14. [23] Cousin CP, Ganczarczyk JJ. The effect of cationic salt addition on the settling and dewatering properties of an industrial activated sludge. Water Environ Res 1999;71(2):251e4. [24] Ward RF, Huston B. Wastewater sludge dewatering for Champion-Hamilton treatment facility. Water Sci Technol 1999;40(11e12):155e62. [25] Lee CH, Liu JC. Enhancement sludge dewatering by dual polyelectrolytes conditioning. Water Res 2000;34(18):4430e6. [26] Bache DH, Papavasilopoulos EN, Rasool E, Zhao YQ. Polymers in alum sludge dewatering: development and control. Water Environ J 2003;17(2):106e10. [27] Jing SR, Lin YF, Lin YM, Hsu CS, Huang CS, Lee DY. Evaluation of effective conditioners for enhancing sludge dewatering and subsequent detachment from filter cloth. J Environ Sci Health 1999;34(7):1517e31. [28] Bolto B, Gregory J. Organic polyelectrolytes in water treatment. Water Res 2007;41:2301e24. [29] Banerjee S. Sludge dewatering with cyclodextrins: a new cost effective approach. In: Thirteenth international water technology conference IWTC 13 March 2009. p. 395e401. Hurghada, Egypt. [30] Banerjee S., inventor; Georgia Tech Research Corporation, assignee. Method for improving the consolidation and dewatering of suspended particulate matter. United States patent US2008/0135194. 2008 Jun 12. [31] Hartong BH, Abu-Daabes M, Le T, Saidan M, Banerjee S. Sludge dewatering with cyclodextrins. Water Res 2007;41:1201e6. [32] Park KY, Ahn KH, Maeng SK, Hwang JH, Kwon JH. Feasibility of sludge ozonation for stabilization and conditioning. Ozone Sci Eng 2003;25(1):73e80. [33] Neyens E, Baeyens J, Weemaes M, Deheyder B. Pilot-scale peroxidation (H2O2) of sewage sludge. J Hazard Mater 2003;B98:91e6. [34] Buyukkamaci N. Biological sludge conditioning by Fenton’s reagent. Process Biochem 2004;39:1503e6. [35] Dewil R, Baeyens J, Neyens E. Fenton peroxidation improves the drying performance of waste active sludge. J Hazard Mater 2005;B117:161e70. [36] Bohm N, Kulicke WM. Optimization of the use of polyelectrolytes for dewatering industrial sludges of various origins. Colloid Polym Sci 1997;275:73e81. [37] Raszka A, Chorvatova M, Wanner J. The role and significance of extracellular polymers in activated sludge. Part I: literature review. Acta Hydrochim Hydrobiol 2006;34:411e24. [38] Wanner J. Activated sludge bukling and foaming control. PA, USA: Technomic Publishing Co Inc.; 1994. [39] Keiding K, Wybrandt L, Nielson PH. Remember the water: comment on EPS colligative properties. Water Sci Technol 2001;43:17e23. [40] Nielson PH, Jahn A. Extraction of EPS. In: Wingender J, Neu TR, Flemmin HC, editors. Microbial extracellular substances: characterization, structure and function. Berlin Heidelberg: Springer-Verlag; 1999. p. 49e72. Chapter 3. [41] Laspidou CS, Rittman BE. A unified theory for extracellular polymeric substances, soluble microbial products, and active and inert biomass. Water Res 2002;36:2711e20. [42] Nelson TC, Huang JYC, Ramaswami D. Decomposition of exopolysaccharide slim by a bacterio phage enzyme. Water Res 1998;22(9):1185e8. [43] Lu J, Rao S, Le T, Mora S, Banerjee S. Increasing cake solids of cellulosic sludge through enzyme assisted dewatering. Process Biochem 2011;46:353e4. [44] Pei H, Hu W, Lia Q. Effect of protease and cellulase on the characteristic of activated sludge. J Hazard Mater 2010;178:397e403. [45] Frolund B, Griebe TT, Nielson PH. Enzymatic activity in activated-sludge floc matrix. Appl Microbiol Biotechnol 1995;43(4):755e61. [46] Thomas N, Rolf K. Change of particle structure of sewage sludge during mechanical and biological processes with regard to the dewatering results. Water Sci Technol 1997;35(4):293e307. [47] Chen Y, Yang GG. Effect of acid and surfactant treatment on activated sludge dewatering and settling. Water Res 2001;35(11):2615e20. [48] Kumar Khanal S, Grewell D, Sung S, Leeuwen JV. Ultrasonic applications in wastewater sludge pretreatment: a review. Crit Rev Environ Sci Technol 2007;37:277e313. [49] Rokhina EV, Lens P, Virkutyte J. Low-frequency ultrasound in biotechnology: state of the art. Trends Biotechnol 2009;27(5):298e306. [50] Wang F, Ji M, Lu S. Influence of ultrasonic disintegration on the dewaterability of waste activated sludge. Environ Prog 2006;25(3):257e60. b i om a s s a n d b i o e n e r g y 5 8 ( 2 0 1 3 ) 3 6 5e3 7 8376 [51] Huan L, Yiying J, Mahar RB, Zhiyu W, Yongfeng N. Effects of ultrasonic disintegration on sludge microbial activity and dewaterability. J Hazard Mater 2009;161:1421e6. [52] Pilli S, Bhunia P, Yan S, LeBlac RJ, Tyagi RD, Surampalli RY. Ultrasonic pretreatment of sludge: a review. Ultrason Sonochem 2011;18:1e18. [53] Khanal SK, Isik H, Sung S, van Leeuwen J. Ultrasonic conditioning of waste activated sludge: evaluation of sludge disintegration and aerobic digestibility. In: Proceeding of IWA world water congress and exhibition 10e14 September 2006. Beijing, China. [54] Khanal SK, Isik H, Sung S, van Leeuwen J. Ultrasonic conditioning of waste activated sludge for enhanced aerobic digestion. In: Proceedding of IWA specialized conference- sustainable sludge management: state of the art, challenges and perspectives 29e31 May 2006. Moscow, Russia. [55] Khanal SK, Isik H, Sung S, van Leeuwen J. Effect of ultrasonic pretreatment on aerobic digestibility of thickened waste activated sludge. In: Proceedding of 7th specialized conference on small water and wastewater systems 7e10 March 2006. Mexico City, Mexico. [56] Wang F, Shan L, Ji M. Components of released liquid from ultrasonic waste activated sludge disintegration. Ultrason Sonochem 2006;13(4):334e8. [57] Rai CL, Struenkmann G, Muller J, Rao PG. Influence of ultrasonic disintegration on sludge growth and its estimation by respirometry. Environ Sci Technol 2001;38:5779e85. [58] Tuncal T. Improving thermal dewatering characteristics of mechanically dewatered sludge: response surface analysis of combined lime-heat treatment. Water Environ Res 2011;83(5):405e10. [59] Vlyssidea AG, Karlis PK. Thermal-alkaline solubilization of waste activated sludge as a pre-treatment stage for anaerobic digestion. Bioresour Technol 2004;91:201e6. [60] Dental SK. Conditioning, thickening, and dewatering: research update/research needs. Water Sci Technol 2001;44(10):9e18. [61] Laheij EJ, Kerkhof PJAM, Herwijn AJM, Cumans WJ. Fundamental aspects of sludge filteration and expression. Water Res 1996;30(3):697e703. [62] Zhao YQ, Bache DH. Integrated effects of applied pressure, time and polymer doses on alum sludge dewatering behavior. Waste Manag 2002;22:813e9. [63] Chun YN, Lim MS, Yoshikawa K. Development of a high efficiency rotary dryer for sewage sludge. J Mater Cycles Waste Manag 2012;14:65e73. [64] Mowla D. Simulation of fluidized bed driers. In: Proceeding of CHISA96 conference 1996. Prague, Czech Republic. [65] Adamiec J. Drying of waste sludges in a fluidized bed dryer with a mixer. Drying Technol 2002;20(4e4):839e53. [66] Shin YS, Kim HC, Chun HS. Drying of water treatment process sludge in a fluidized bed dryer. Korean J Chem Eng 2000;17(1):22e6. [67] Velegol D, Feick JD, Collins RL. Electrophoresis of spherical particles with a random distribution of zeta potential or surface charge. J Colloid Interface Sci 2000;230:114e21. [68] Lotito V, Spinosa L, Mininni G. The rheology of sewage sludge at different steps of treatment. Water Sci Technol 1997;36(11):79e85. [69] Huisman M, van Kesteren WGM. Consolidation theory applied to the capillary suction time (CST) apparatus. Water Sci Technol 1998;37(6e7):117e24. [70] Ruth BF. Correlating filteration theory with industrial practice. J Ind Eng Chem 1946;38:564e71. [71] Arhan Y, Zoturk I, Ciftci T. Settling and dewatering characteristics of sludge from baker’s yeast production wastewater treatment. Water Sci Technol 1996;34(3e4):459e67. [72] Wingender J, Neu T, Flemming HC. What are bacterial extracellular polymeric substances? In: Wingender J, Neu TR, Flemmin HC, editors. Microbial extracellular substances: characterization, structure and function. Berlin Heidelberg: Springer-Verlag; 1999. p. 1e18 [Chapter 1]. [73] Liu Y, Fang HP. Influence of extracellular polymeric substances (EPS) on flocculation, settling, and dewatering of activated sludge. Crit Rev Environ Sci Technol 2003;33:237e73. [74] Frolund B, Palmgren R, Keiding K, Nielson PH. Extraction of extracellular polymers from activated sludge using a cation exchange resin. Water Res 1996;30:1749e58. [75] Sheng GP, Yu HQ, Li XY. Extracellular polymeric substances (EPS) of microbial aggregates in biological wastewater treatment systems: a review. Biotechnol Advances 2010;28:882e94. [76] Dignac MF, Urbain V, Rybacki D, Bruchet A, Snidaro D, Scribe P. Chemical description of extracellular polymeric substances: implication on activated sludge floc structure. Water Sci Technol 1998;38:45e53. [77] D’Abzac P, Bordas F, Joussein E, van Hullebusch E, Lens PNL, Guibaud G. Characterization of the mineral fraction associated to extracellular polymeric substances (EPS) in anaerobic granular sludge. Environ Sci Technol 2010;44:412e8. [78] D’Abzac P, Bordas F, van Hullebusch E, Lens PNL, Guibaud G. Extraction of extracellular polymeric substances (EPS) from anaerobic granular sludges: comparison of chemical and physical extraction protocols. Appl Microbiol Biotechnol 2010;46:197e201. [79] Mu Y, Yu HQ, Wang G. Permeabilities of anaerobic CH4 producing granules. Water Res 2006;40:1811e5. [80] Zheng YM, Yu HQ. Determination of the pore size distribution and porosity of anaerobic granules using size exclusion chromatograph. Water Res 2007;41:39e46. [81] Wilen BM, Jin B, Lant P. The influence of the key chemical constituents in activated sludge on surface and flocculation properties. Water Res 2003;37:2127e30. [82] Liao BQ, Allen DG, Leppard GG, Droppo IG, Liss SN. Interparticle interactions affecting the stability of sludge flocs. J Colloid Interface Sci 2002;249:372e80. [83] Wang ZW, Liu Y, Tay JY. Distribution of EPS and cell surface hydrophobicity in aerobic granules. Appl Microbiol Biotechnol 2005;69:469e73. [84] Jin B, Wilen BM, Lant PA. Comprehensive insight into floc characteristics and their impact on compressibility and settleability of activated sludge. Chem Eng J 2003;95:221e34. [85] Yang SF, Li XY. Influence of extracellular polymeric substances (EPS) on the characteristics of activated sludge under non-steady state conditions. Process Biochem 2009;44:91e6. [86] Mogan JW, Forster CF, Evison L. A comparative study of the nature of biopolymers extracted from anaerobic and activated sludges. Water Res 1990;24:743e53. [87] Liu XM, Sheng GP, Yu HQ. DLVO approach to the floccubility of a photosynthetic H2-producing bacterium. Rhodopseudomonas acidophila. Environ Sci Technol 2007;41:4620e5. [88] Liu XM, Sheng GP, Yu HQ, Luo HW, Zhang F, Yuan SJ, et al. Contribution of extracellular polymeric substances (EPS) to the sludge aggregation. Environ Sci Technol 2010;44:4355e60. [89] Comte S, Guibaud G, Bandu M. Effect of extraction method on EPS from activated sludge: an HPSEC investigation. J Hazard Mater 2007;140:129e37. b i om a s s a n d b i o e n e r g y 5 8 ( 2 0 1 3 ) 3 6 5e3 7 8 377 [90] Ahn KH, Kim KS, Kang SW, Um CY, Lee WT, Ko KB. Settling and dewatering characteristics of granulated methane- oxidizing bacteria. J Environ Sci 2012. http://dx.doi.org/ 10.1016/S1001-0742(12)60021-7. [91] Houghton J, Quarmby J, Stephenson T. Municipal wastewater sludge dewatering and the presence of microbial extracellular polymer. Water Sci Technol 2001;44(2):373e9. [92] Neyens E, Baeyens J, Dewil R, de heyder B. Advanced sludge treatment affects extracellular polymeric substances to improve activated sludge dewatering. J Hazard Mater 2004;106:83e92. [93] Chen Y, Yang H, Gu G. Effect of acid and surfactant treatment on activated sludge dewatering and settling. Water Res 2001;35:2615. [94] Houghton J, Stephenson T. Effect of influent organic content on digested sludge extracellular polymer content and dewaterability. Water Res 2002;36:3620e8. [95] Coumans WJ, Kerkhof PJAM. The solidewater bound strength in sewage sludge. In: AIChE annual meeting, Miami Beach, Fla 1992. p. 1073e8. [96] Robinson J, Knocke WR. Use of dilatometric and drying techniques for assessing sludge dewatering characteristics. Water Environ Res 1992;64(1):60e8. [97] Smith KJ, Vesilind PA. Dilatometric measurement of bound water in wastewater sludge. Water Res 1995;29(12):2621e6. [98] Katsiris N, Kouzeli-Katsiri A. Bound water content of biological sludges in relation to filtration and dewatering. Water Res 1987;21:1319e27. [99] Lee HB, Jhon MS, Anderson JD. Nature of water in synthetic hydrogels. J Colloid Interface Sci 1975;51:225. [100] Lee DJ, Lee SF. Measurement of bound water content in sludge: use of differential scanning calorimetry (DSC). J Chem Tech Biotechnol 1995;62:359e65. [101] Haschemeyer AEV, Guschlbauer W, Devries AL. Water binding by antifreeze glycoproteins from Antarctic fish. Nature 1977;269:87e8. [102] Lewicki PP, Busk GC, Labuza TP. Measurment of gel-water binding capacity of gelatin, potato starch and carrageenan gels by suction pressure. J Colloid Interface Sci 1978;64:501e9. [103] Lee DJ. Measurment of bound water in waste activated sludge: use of the centrifugal settling method. J Chem Tech Biotechnol 1994;63:139e44. [104] Tay JH, Jeyaseelan S. Comparison of lime and alum as oily sludge conditioners. Water Sci Technol 1997;35(12):117e24. [105] Gogate PR, Kabadi AM. A review of application of cavitation in biochemical engineering/biotechnology. Biochem Eng 2009;44:60e72. [106] Bien J, Wolny L. Changes of some sewage sludge parameters prepared with an ultrasonic field. Water Sci Technol 1997;36(11):101e6. [107] Hogan F, Mormede S, Clark P, Crane M. Ultrasonic sludge treatment for enhanced aerobic digestion. Water Sci Technol 2004;50(9):25e32. [108] Feng X, Deng J, Lei H, Bai T, Fan Q, Li Z. Dewatering of waste activated sludge with ultrasonic conditioning. Bioresour Technol 2009;100:1074e81. [109] Gogate PR, Pandit AB. Hydrodynamic cavitation reactors: a state of the art review. Rev Chem Eng 2001;17(1):1e85. [110] Caufield P. Unlocking the value in sludge. Pulp Paper Can May/June 2012:18e9. [111] Vesilind PA, Wallinmaa S, Martel CJ. Freeze-thaw sludge conditioning and double layer compression. Can J Civ Eng 1991;18:1078e83. [112] Jean DS, Chu CP, Lee DJ. Freeze/thaw treatment of oily sludge from petroleum refinery plant. Separation Sci Technol 2001;36(12):2733e46. [113] Martel CJ. Fundamentals of sludge dewatering in freezing beds. Water Sci Technol 1993;28(1):29e35. [114] Vesilind PA, Martel CJ. Freezing of water and wastewater sludges. J Environ Eng Am Soc Civil Eng 1990;116(5):854e62. [115] Hellstrom D, Kvarnstrom E. Natural sludge dewatering. I: combination of freezing, thawing, and drying as dewatering methods. J Cold Regions Eng 1997; March:1e14. [116] Ormeci B. Freeze-thaw conditioning of activated sludge: effect of monovalent, divalent and trivalent cations. J Residuals Sci Technol 2004;1(3):143e50. [117] Franceschini O. Dewatering of sludge by freezing. Master thesis. Civil and Mining Engineering, Lulea University of Technology; 2010. [118] Martel CJ, Diener C. Pilot-scale studies of sludge dewatering in a freezing bed. Can J Civ Eng 1991;18:681e9. [119] Abu-Orf MM, Dentel SK. Automatic control of polymer dose using the measuring current detector. Water Environ Res 1998;70:1005e18. [120] Wang Y, Banerjee S. Cyclodextrins modify the properties of cationic polyacrylamides. J Colloid Interface Sci 2009;339:325e9. [121] Perng YS, Wang C, Yu ST, Gong HU, Dineh L, Kuo LS. Application of nano-silica to paper mill sludge dewatering. Taiwan J Sc 2006;21(3):353e62. [122] Zhao YQ, Koegh C, Maha AT. On the necessity of sludge conditioning with non-organic polymer: AOP approach. J Residuals Sci Technol 2009;6(3):151e5. [123] Shihab MS. Assessment of using chemical coagulants and effective microorganisms in sludge dewatering process improvement. J Environ Sci Technol 2010;3(1):35e46. [124] Ayol A, Dental SK. Enzymatic treatment effects on dewaterability of anaerobically digested biosolids- II: laboratory characteristics of drainability and filterability. Proc Biochem 2005;40:2435e42. [125] Abu- Orf M, Dental S, Dursun D, Turkmen M. Pilot-scale evaluation of enzyme pre-treatment to enhance dewatering: effects of shear and polymer. In: WEF/AWWA joint residuals and biosolids management conference, April 15e18, 2007. Denver, CO, USA: Water Environment Federation; 2007. p. 1180e95. [126] Thomas L, Jungschaffer G, Sprossler B. Improved sludge dewatering by enzymatic treatment. Water Sci Technol 1993;28(1):189e92. [127] Sarkar J, Shah J, Ramesh M. Method of dewatering sludge using enzymes, United State Patent, US 6733673B2. 2004. [128] Wawrzynczk J, Szewczyk E, Norrlow O, Szwajcer Dey E. Application of enzymes, sodium tripolyphosphate and cation exchange resin for the release of extracellular polymeric substances from sewage sludge. Characterization of the extracted polysaccharides/ glycoconjugates by a panel of lectins. J Biotechnol 2007;130:274e81. [129] Ayol A. Enzymatic treatment effects on dewaterability of anaerobically digested biosolids- I: performance evaluations. Proc Biochem 2005;40:2427e34. [130] Parmer N, Singh A, Ward OP. Enzyme treatment to reduce solids and improve settling of sewage sludge. J Ind Microbiol Biotechnol 2001;26:383e6. [131] Dursun D, Turkmen M, Abu-Orf M, Dental SK. Enhanced sludge conditioning by enzyme pre-treatment: comparison of laboratory and pilot scale dewatering results. Water Sci Technol 2006;54(5):33e41. [132] Lu J, Reye J, Banerjee S. Temperature dependence of cellulose hydrolysis of paper fibber. Biomass Bioenergy 2010;34:1973e7. [133] Buyukkamaci N, Kucukselek E. Improvement of dewatering capacity of a petrochemical sludge. J Hazard Mater 2007;144:323e7. [134] Hou CH, Li KC. Assessment of sludge dewatering using rheological properties. J Chinese Inst Eng 2003;26(2):221e6. [135] Tony MA, Zhao YQ, Fu JF, Tayeb AM. Conditioning of aluminium-based water treatment sludge with Fenton’s reagents: effectiveness and optimizing study to improve dewaterability. Chemosphere 2008;72:673e7. [136] Neyens E, Baeyens J. A review of thermal sludge pre- treatment processes to improve dewaterability. J Hazard Mater 2003;B98:51e67. [137] Saha M, Eskicioglu C, Marin J. Microwave, ultrasonic and chemo-mechanical pretreatment for enhancing methane potential of pulp mill wastewater treatment sludge. Bioresour Technol 2011;102:7815e26. b i om a s s a n d b i o e n e r g y 5 8 ( 2 0 1 3 ) 3 6 5e3 7 8378 A review of the properties of biosludge and its relevance to enhanced dewatering processes 1 Introduction 2 Fundamentals of sludge dewatering 3 Sludge properties 3.1 Extracellular polymeric substances (EPS) 3.2 Bound water 4 Sludge dewatering difficulties 5 Sludge conditioning methods 5.1 Physical conditioning 5.1.1 Non-chemical additives 5.1.2 Cavitation pretreatment 5.1.3 Thermal pretreatment 5.1.4 Freeze/thaw pretreatment 5.2 Chemical conditioning 5.2.1 Organic additives 5.2.2 Inorganics additives 5.3 Biological or enzyme conditioning 6 Assessment of conditioning methods 7 Summary Acknowledgment References