Lignite aided dewatering of digested sewage sludge

water research 43 (2009) 623–634

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Lignite aided dewatering of digested sewage sludge K.B. Thapa, Y. Qi, S.A. Clayton, A.F.A. Hoadley* Department of Chemical Engineering, Building 35, Clayton Campus, Monash University, Victoria 3800, Australia

article info

abstract

Article history:

Mechanical dewatering is commonly used to increase the solids content of municipal

Received 22 July 2008

sludge prior to its disposal. However, if the rate of filtration is slow, mechanical dewatering

Received in revised form

can be expensive. In this study, the use of lignite to improve the sludge dewatering is

9 November 2008

investigated. The effectiveness of lignite conditioning of polyelectrolyte-flocculated sludge

Accepted 10 November 2008

is examined using mechanical compression tests. Results show that lignite conditioning in

Published online 21 November 2008

conjunction with polyelectrolyte flocculation gives much better dewatering than the polyelectrolyte flocculation alone. Using Darcy’s filtration theory, the specific cake resis-

Keywords:

tance and permeability of the compressed cakes are obtained. Both of these parameters are

Sludge conditioning

significantly improved after lignite conditioning. Mercury porosimetry tests on compressed

Flocculation

cakes show that the porosity of the lignite-conditioned sludge cake is much higher than

Filtration

that of the polyelectrolyte-flocculated sludge and it increases with increasing doses of

Filter aid

lignite. The mercury porosimetry results show that the lignite pore volume of pores greater

Cake resistance

than 0.5 mm are reduced with increasing sludge ratio indicating that sludge is trapped

Polyelectrolyte

within these pores, whereas smaller pores are unaffected. The yield stress curves for

Permeability

sludge, lignite and sludge-lignite mixtures show that the sludge filter cake is very

Lignite

compressible, but the lignite-conditioned cake has a range of compressibility which although more than lignite indicate that the cake is relatively incompressible at low pressures. Thus, lignite conditioning acts to maintain the permeability of the filter cake during compression dewatering by resisting cake compression. This leads to a trade-off between the rate of dewatering and the solids content of the compressed cake. With lignite conditioning, the dewatering rate can be increased by a factor of five for the same degree of water removal. Crown Copyright ª 2008 Published by Elsevier Ltd. All rights reserved.

1.

Introduction

Management of sewage sludge is a challenging issue for water industries. The direct disposal of the sludge to the environment has been found to impact on the environment and human health, due to the presence of heavy metals, pathogens and pesticides in the sludge (Parr et al., 1978; Singh et al., 2004). Methods like incineration of dried sludge to generate heat, electricity and reusable ash (Okufuji, 1990); and

composting the sludge to produce fertilizers and soil conditioners (Parr et al., 1978) have been considered as potential sludge management alternatives. However, when the moisture content in the sludge is high, these methods are not cost effective. Anaerobic sludge digestion is an effective method for reducing the volume of sludge and another benefit is the gas production during digestion. Anaerobic sludge digestion has been widely used for managing primary and activated sludge. However, sludge is still produced from digestion with

* Corresponding author. Tel.: þ61 3 99053421; fax: þ61 3 99055686. E-mail address: [email protected] (A.F.A. Hoadley). 0043-1354/$ – see front matter Crown Copyright ª 2008 Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.watres.2008.11.005

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Nomenclature A c H K ML MS DP P Py(4) R

Filter cake area (m2) Concentration of solids in the slurry (kg m3) Sample thickness (m) Compressed cake permeability (m2) Mass of lignite solids (g) Mass of sludge solids (g) Pressure drop across the filter cake (N m2) Compression pressure (N m2) Compressive yield stress (MPa) Filter medium resistance (m1)

moisture contents ranging from 90 to 99% and this needs to be further managed. For these various reasons sludge dewatering has become an important part of the water industry. High pressure mechanical dewatering has been used to increase the solids content of sludge. The presence of organic components, mainly bacterial cells and EPS (Extracellular Polymeric Substances), in the sludge makes it very difficult to dewater even at high pressures. Chemical conditioning prior to pressure dewatering is usually employed to reduce the effect of these organic components. Various chemical conditioning methods have been tried. These include pH reduction and the use of surfactants (Chen et al., 2001), acidification or alkalization and consecutive oxidation by H2O2 (Vosteen and Weissenberg, 2000), coagulation with metal salts followed by flocculation with an amphoteric polymer (Watanabe and Tanaka, 1999) and coagulation with cationic polyelectrolytes followed by flocculation with non-ionic polyelectrolytes (Lee and Liu, 2000). This chemical conditioning changes the properties of the sludge solid, such as its surface charge and particle size, and as a result, a significant improvement in solid–liquid separation is obtained. Although chemical conditioning is effective, the rate of dewatering that can be achieved is controlled by the permeability of the compressed filter cake (Sorensen and Hansen, 1993). The solid–liquid separation theory developed by Buscall and White (1987) uses the yield stress curve and permeability as the fundamental physical properties that determine the dewaterability of the flocculated suspensions. The yield stress curve determines the maximum possible extent of dewatering for a given applied compressive stress and the permeability determines the rate at which this extent of dewatering can be achieved. The gel point is the solids concentration at which a network forms. At a solids concentration higher than the gel point, the inter-connected particle network has a physically measurable strength. For all the solids concentrations below the gel point, the compressive yield stress is zero. The rate of sludge dewatering in pressure filtration can be increased by reducing the filter cake compressibility and increasing the cake permeability. Physical conditioners often known as skeleton builders have been employed to decrease the compressibility of sludge. These physical conditioners form a more rigid lattice structure which can remain porous under high pressure during mechanical dewatering. Carbonbased materials such as char and coal have been used as physical conditioners to improve sludge dewatering. Hirota

t V X

Filtration time (sec) Filtrate volume (m3) Cake thickness (m)

Greek letters a Specific cake resistance (m kg1) m Filtrate viscosity (N s/m2) Solid density (kg m3) rs Lignite density (kg m3) rL 4 Solids volume fraction, dimensionless Solids volume fraction at the gel point, 4g dimensionless

et al. (1975) used a bed of pulverized coal as a filter medium to dewater organic sludge and similarly Albertson and Kopper (1983) used a slurry of fine-coal to condition sludge. They found an improvement in moisture removal and proposed that the improvement in moisture removal after the addition of coal slurry was due to the higher strength of the coal particles, which provided resistance to the compactive force and improved the drainage matrix. Sander et al. (1989) and Broeckel et al. (1996) used carbon powder or coal in conjunction with flocculation to improve the dewatering performance of sewage sludge. Smollen and Kafaar (1997) used pyrolysed domestic refuse (char) and a small amount of polyelectrolyte to improve sludge dewatering. They proposed that the large and strong sludge particles formed by the addition of char provided a porous, permeable and rigid lattice structure to the loose flocs formed after the addition of polyelectrolyte. Other inert materials like fly ash from municipal sludge incineration, cement kiln dust, bagasse, recycled wood chips, wheat dregs and gypsum have also been used as skeleton builders (Nelson and Brattlof, 1979; Benitez et al., 1994; Lin et al., 2001; Zhao, 2002). In addition to the improvement in dewatering properties, the addition of physical conditioners to sludge also reduces the problem of expansion or ‘spongy rebound’ that is commonly experienced when the pressure is released from the polymer-conditioned compressed sludge cakes during the pressure filtration (Zall et al., 1987). Lignite is a low rank coal and, like other carbon-based materials, could be expected to have a beneficial effect on sludge dewatering. A large deposition of lignite is present in Victoria, Australia, which is being used for power generation. Due to the low ash content (<5% db) and high calorific value approximately 25 MJ/kg db (Durie, 1991), there is a potential for the use of lignite to increase the calorific value of sewage sludge which is then possible to be managed by incineration. The high moisture holding capacity and the presence of humic substances in the lignite can also improve the moisture distribution and aggregate stability in soil (Piccolo et al., 1997). Therefore, lignite conditioning can also make the dewatered product suitable for use as a soil conditioner. Although there have been previous studies of carbonbased materials as conditioners, there has not been a quantitative analysis of the role of the conditioners in determining the filter cake properties, such as the cake porosity, permeability and resistance to compression as measured by the cake yield stress. Therefore, the aim of this work is to provide

water research 43 (2009) 623–634

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intercept of the line, a and R can be determined. Equation (1) assumes an incompressible cake, but if the cake is highly compressible which is the case for sewage sludge there tends to be little or no cake formation followed by a long compression phase (Stickland et al., 2005). Therefore, the estimation of filtration parameters from Equation (1) for these samples is difficult and can be inaccurate. In this study, flocculated sludge is conditioned with lignite which has a much lower compressibility at low pressures. Hence, the estimation of filtration parameters from the equation is possible. For the cases where constant pressure cannot be attained instantaneously, Svarovsky (2000) has proposed to integrate Equation (1) starting from a point (ts,Vs) which correspond to the time and filtrate volume at the beginning of the truly constant pressure period.

P P

ðt  ts Þ amc Rm ¼ ðV þ Vs Þ þ ðV  Vs Þ 2A2 DP ADP

(2)

If ‘‘a’’ is the slope and ‘‘b’’ is the intercept of the line obtained by plotting (t  ts)/(V  Vs) vs V then a and R can be determined using the following equations.

a

b

a¼

2A2 DPa mc

(3)

R¼

ADPðb  aVs Þ m

(4)

Fig. 1 – Mechanisms of cake filtration (a) The cake formation stage. (b) The cake compression stage.

a quantitative explanation of the improvement in dewatering performance when using a porous filter aid, by measuring these properties for different ratios of sludge solids to lignite solids.

2.

Theory

2.1.

Cake filtration

Cake filtration is a process of concentrating a solid–liquid suspension based on a filter medium which is permeable to the liquid. It has two stages; the cake formation stage and the cake compression stage, as shown in Fig. 1. At the start of filtration, a layer of solid is formed on the filter medium and subsequent filtration takes place on top of this cake increasing the thickness of the filter cake. The total resistance to the filtration during the cake formation stage is the sum of the medium resistance (R) and the filter cake resistance. The filter cake resistance is described in terms of the specific cake resistance (a) and it is commonly used to describe the filtration behaviour during the cake formation stage. The specific cake resistance (a) and the filter medium resistance (R) can be determined by Equation (1), derived from Darcy’s Law as presented by Coulson and Richardson (1968), where t is the time from the commencement of filtration, V is the filtrate volume at time t, c is the concentration of solids in the slurry, m is the absolute viscosity of water (0.001 N s/m2) and P is applied pressure. t amc Rm Vþ ¼ V 2A2 DP ADP

(1)

Equation (1) indicates that if the pressure is held constant, a plot of t/V vs V will give a straight line. From the slope and

2.2.

Cake compression

As the filtration progresses, the solid–liquid interface increases in height and eventually comes in contact with the surface of the compression device as illustrated in Fig. 1b. Further application of pressure on the filter cake compresses the cake reducing its permeability until it reaches an equilibrium stage where the compression force is balanced by the reactive force of the filter cake. The equilibrium permeability of the compressed cake (K ) can be determined by Darcy’s law by passing liquid through the cake. K¼

QmX ADP

(5)

Q is the volumetric flow rate of the fluid through the compressed cake, m is the fluid viscosity of water (0.001 N s/m2), X is the cake thickness, A is the flow area, and DP is the fluid pressure drop across the cake.

3.

Materials and methods

3.1.

Materials

Sludge samples were collected from the anaerobic digester outflow of Melbourne Water’s Eastern Treatment Plant in Victoria, Australia. The sludge samples comprise the product of anaerobic digestion of primary and activated sludge. The solids content of the sludge samples was approximately 2% w/ w. To minimise the microbial activity after collection, the samples were stored in a refrigerator maintained at 4  C. The stability of the sludge sample was checked by monitoring

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Table 1 – Characteristics of sludge and lignite samples. Sample pH

Sludge Lignite

Particle size Mean particle Solids Total solids Dissolved solids Ash content Conductivity Zeta sizeb (mm) density content (% wb) content (% wb) (% db) (mS/cm) potentiala range (mm) (mV) (kg/m3)

7–8 3–4c

1.4–1.5 58–60

0.2 –

29.7 6.2

5–6 0.5–1

17–20 24–27

5–1000 5–3000

40 400

1475 1400

a Zeta potential of centrate of raw sludge or lignite slurry. b Volume-average means particle size. c Lignite slurry pH.

the total solids content, pH, conductivity and zeta potential of one batch of the sludge every two weeks for three months from the day of collection. It was found that the values of the solids content, pH, conductivity and zeta potential remain relatively constant throughout the storage period. Lignite sample was obtained from the Loy Yang mine in Victoria, Australia. The lignite samples were milled and sieved to less than 3 mm in particle size. The characteristics of the sludge and lignite are presented in Table 1.

3.2.

sludge with the optimum polyelectrolyte dose using an overhead stirrer. The optimum mixing conditions were determined to be a stirrer speed of 300 rpm during the first 20 s after the addition of the flocculant and 60 rpm for a further 40 s.

3.2.2.

Methods

3.2.3.

Raw sludge is flocculated with polyelectrolyte and further conditioned with lignite. Water is separated from the conditioned sludge using a compression dewatering cell. The permeability of the compressed cake is measured by forcing water through the compressed cake. After the permeability test, the cake is dried in an oven and further analysed for its porosity distribution by mercury porosimetry and for its surface structure by scanning electron microscope.

3.2.1.

Flocculation test

P

Compression test

The lignite-conditioned flocculated sludge is compressed in a mechanical pressing cell as shown in Fig. 2a. The cell consists of a sintered-metal filter medium (40 mm pore size) attached at the base and a stainless steel piston which fits inside the cavity of the cell. The piston has an open channel at the centre which runs from the bottom to the top end of the piston. A connecting tube with a valve is fitted at the top end where a pressure sensor can be attached. The sludge sample is poured into the cavity of the cell and the piston is placed on the top of the sample. An Instron Mechanical Testing device is used to apply an accurate pressure to the sample via the piston. The piston position is simultaneously recorded to an accuracy of 2 mm. Before the actual compression test starts, the air inside the sample is removed by lowering the piston to bleed a few drops of water out from the top and bottom outlets of the compression cell. In order to measure the hydraulic pressure drop across the cell, a pressure sensor is attached to the top outlet. As the sample is compressed, water flows through the filter medium and the solids are retained on the filter medium. A single sample is

A linear cationic polyelectrolyte is used to flocculate the raw sludge. The polyelectrolyte is copolymers of polyacrylamide and quaternised N,N-dimethylaminoethyl acrylate methylene chloride. It has ultra high MW with intrinsic viscosity of 17 (dL g1) and low CD with 24 mole% of ionic functional groups. The optimum dose of the polyelectrolyte is determined by settling tests, where a set of identical sludge samples are mixed with different amounts of the polyelectrolyte and allowed to settle for 80 min. The dose which gives the lowest solid height is considered as the optimum dose. The sludge samples are flocculated by mixing a given amount of the Pressure sensor

Conditioning of flocculated sludge with lignite

The sludge is first flocculated with the optimum dose of polymer as described in the previous section. A slurry of lignite (60 g/L, dry basis) is added to the flocculated sludge 30 s before the completion of flocculation test and the mixture is then stirred at the same speed for 30 s.

P Piston

Sample

Water

Filter

a

b

Fig. 2 – (a) Compression test for separating water from the conditioned sewage sludge. (b) Permeability test for the determination of compressed cake permeability.

627

tested in a stepped pressure test for a range of pressures starting at 0.75 MPa and stepping in intervals of 0.25 MPa up to 1.5 MPa. Each compression is held until the rate of descent of the piston is less than 3 mm/min or the hydrostatic pressure drop across the sample is less than 1% of the initial applied pressure. At this point, the sample is considered to be in static equilibrium and the compression test is stopped. If a permeability test is to be conducted, a clamping device as shown in Fig. 2b is used to fix the position of the piston keeping the sample thickness constant. The applied pressure is then released from the piston allowing a second compression cell (with the same internal diameter as the original cell) containing water to be fitted to the mechanical testing machine. The procedure for the permeability tests is explained in the next section.

3.2.4.

Permeability test

Permeability tests are conducted on the compressed cake by passing the water through the sample as described by Clayton et al. (2006). The pressure applied to the water via the piston forces the water out of the cell via a small channel at the centre of the piston and flows to the compression cell containing the compressed cake as shown in Fig. 2b. The flow rate of the water through the cake is measured by the piston descent rate into the water-containing cell. Since the mechanical tester is capable of measuring small changes in piston position, even a very low permeability can be determined using Equation (7). Low water pressures are applied to reduce the possibility that the hydraulic gradient through the sample could cause deformation of the pore structure in the filter cake. The applied water pressures used for this study ranged from 0.05 to 0.4 MPa. First, the test at the lowest pressure (0.05 MPa) is carried out. The pressure is gradually increased to 0.4 MPa and then decreased to 0.05 in the consecutive tests and the permeability is the average of these data. After the permeability test has been conducted and the equipment is disconnected, the sample may be compressed to a higher pressure.

Solids fraction (wt%, wet basis)

water research 43 (2009) 623–634

70 60

40 wt% (dry basis) sludge

50 40

100% sludge

30 20 10 0 0

10

20

30

40

50

Compression time (hr) Fig. 3 – Average sludge solid fractions of flocculated sludge and lignite-conditioned flocculated sludge at 1.0 MPa compression pressure. Initial solids content of flocculated sludge is 1.5 wt% (wet basis) and lignite-conditioned sludge is 3.25 wt% (wet basis).

effect of lignite conditioning on the dewatering performance of the flocculated sludge. A compression pressure is applied to the sample until the sample is considered to be in static equilibrium. The results of the compression test for 1.0 MPa are presented in Fig. 3. The rate of dewatering of ligniteconditioned sludge can be compared with the flocculated sludge (without lignite) by adjusting the lignite-conditioned sludge for the volume of lignite added to the slurry. The adjusted height of the lignite-conditioned sludge is given by Equation (6). Hadjusted ¼ Hactual  HLignite

(6)

4.

Results

HLignite is the height of the raw lignite (15 g) determined by conducting a separate compression test on the lignite alone under the same compression conditions. It can be clearly seen in Fig. 3 that the dewatering rate of the flocculated sludge is much slower than that of the ligniteconditioned flocculated sludge. The flocculated sludge takes 47 h to reach the equilibrium state whereas the ligniteconditioned flocculated sludge takes less than 10 h. It can also be seen that the equilibrium solids content is higher (approximately 65% w/w) for the flocculated sludge. This indicates that by conditioning the flocculated sludge with lignite, the rate of dewatering can be improved significantly, but the achievable equilibrium solids content is reduced. The slower rate of dewatering in the case of the flocculated sludge can also be observed in Fig. 4 by comparing the hydraulic pressure drop obtained from the pressure transducer attached to the piston. As the flow rate of water reduces, this pressure reduces and the applied pressure from the piston is transferred to the solid. It can be seen that the hydraulic pressure drop decreases very rapidly for the ligniteconditioned flocculated sludge, but remains high for the unconditioned flocculated sludge.

4.1.

Dewatering performance

4.2.

3.2.5.

Mercury porosimetry test

Mercury porosimetry tests are performed on the compressed cakes of lignite, flocculated sludge and lignite-conditioned flocculated sludge to determine the average cake porosity using a commercial mercury porosimeter. The compressed cakes are dried in an oven at 105  C for 3 h prior to the porosimetry tests. Shrinkage occurs during drying and this is discussed in Section 4.6. The dried cakes are crushed into millimetre sized pieces to fit into the penetrometers used for the tests. By performing the low pressure and high pressure mercury intrusion cycles, the porosity of the samples is determined. The maximum pressure applied in the high pressure cycle is 400 MPa.

Compression tests are carried out on the flocculated sludge and lignite-conditioned flocculated sludge to investigate the

Compressive yield stress

The effect of lignite conditioning on the cake compressibility is investigated by measuring the compressive yield stresses

water research 43 (2009) 623–634

equilibrium solid volume fraction with increasing compression pressure is much higher for the flocculated sludge compared to the lignite and the lignite-conditioned flocculated sludge. If sufficient time is given, a solid volume fraction of almost 90% can be achieved for the flocculated sludge. However, this requires more than 70 h which would not be practical for an industrial operation.

1.2 1.0

100% sludge

0.8 0.6 40 wt% sludge

0.4

4.3.

0.2 0.0

0

10

20

30

40

50

Compression time(hr) Fig. 4 – Change in liquid pressure during the compression of flocculated sludge and lignite-conditioned flocculated sludge.

(Py(4)) of the lignite-conditioned sludge cakes at various compression pressures. The sample is compressed at a given pressure in the compression cell until static equilibrium is reached. The solid volume fraction (4) of the sample at equilibrium is determined from the sample thickness and the total mass of the solids in the cake using Equation (7). MS ML þ r rL 4¼ S Heq A

Compressive yield stress, Py (φ) (MPa)

3.5 3.0 2.5 2.0 Determined from settling tests

1.5

0% sludge 40% sludge 57% sludge

1.0 0.5

100% sludge

0.0 0

a

(7)

10

20

30

40

50

60

70

80

90

Solid volume fraction, φ (v/v%)) Fig. 5 – Compressive yield stresses of lignite, flocculated sludge and lignite-conditioned flocculated sludge. Solid volume fractions corresponding to zero compressive yield stress are determined from settling tests.

5000

4000

t/V (sec/m3)

where MS is the mass of sludge solids, rS is the density of sludge solids, ML is the mass of dry lignite solids, rL is the density of lignite solids, Heq is the equilibrium sample thickness and A is the cross-sectional area of the compression cell. The gel point (4g) or the maximum solids concentration that a sample can attain at zero compression pressure is determined by performing equilibrium batch settling tests in 300 mL measuring cylinders as described by Northcott et al. (2005). The results of the settling tests and the high pressure compression tests are shown in Fig. 5. For a fixed compression pressure, the equilibrium solid volume fraction increases with increasing mass fraction of the sludge in the cake. It can be seen that the change in

Specific cake resistance

The effect of lignite conditioning on the properties of the filter cake during the cake formation stage is investigated by estimating the specific cake resistance of the filter cake. A number of constant pressure filtration tests are performed on ligniteconditioned flocculated sludge samples containing different mass fractions of sludge. The specific cake resistance is estimated by plotting t/V vs V, as shown in Fig. 6a. The linear portion of the data represents the cake formation stage and the slope of this line is substituted in Equation (3) to obtain the specific cake resistance. Fig. 6b shows that the specific cake resistance of the flocculated sludge on its own is significantly higher than the lignite. The specific cake resistance decreases

3000

2000

1000

Regression line

0 0

0.05

0.1

0.15

0.2

V (m3)

b

Specific cake resistance (m/kg)

Hydraulic pressure drop (MPa)

628

1.E+09

1.E+08

1.E+07

1.E+06

1.E+05 0

10

20

30

40

50

60

70

80

90

100

Mass fraction of sludge (wt%, dry basis) Fig. 6 – (a) t/V vs V plot for 33% sludge solids (dry basis) showing cake formation and compression stages. (b) Effect of lignite conditioning on the average specific cake resistance of the filter cake at applied pressure of 0.75 MPa. The initial concentration (4o) of the flocculated sludge is 1.08 wt% (wet basis) and the solids concentration at gel point (4g) is 1.13 wt% (wet basis).

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when the flocculated sludge is conditioned with increasing amounts of lignite. For the sample containing 40% w/w (dry basis) sludge and 60% w/w (dry basis) lignite, the specific cake resistance is approximately twenty seven times lower than that of the flocculated sludge alone. Repeat tests performed on the lignite-conditioned flocculated sludge show that the variation in specific cake resistance between the tests is less than 10%.

4.4.

Compressed cake permeability

The rate of water removal during the cake compression stage depends on the permeability of the compressed cake. A number of compressions followed by flow-through permeability tests are conducted to investigate the permeability of the compressed cake. By keeping the mass of the raw lignite fixed (15 g) and changing the quantity of raw sludge, a range of sludge mass fractions in the cake are obtained. The average compressed cake permeability results for lignite, flocculated sludge and lignite-conditioned flocculated sludge at various compression pressures are presented in Fig. 7 as a function of compression pressure (Fig. 7a) and the average solid volume fraction (Fig. 7b). The average permeability is calculated from six flow-through tests at different water pressures. It can be seen in Fig. 7a that at any pressure the permeability of the

Average permeability (m2)

a

1.E-14 0% sludge

1.E-15

14% sludge

1.E-16

40% sludge 57% sludge 75% sludge

1.E-17 1.E-18 1.E-19

100% sludge

lignite compressed cake is significantly higher than that of the flocculated sludge and the lignite-conditioned flocculated sludge. The flocculated sludge on its own shows the lowest permeability. However, a significant improvement in the permeability of the flocculated sludge is achieved after it is conditioned with lignite. With increasing mass fraction of the lignite, the permeability of the compressed cake increases. Fig. 7b shows the permeability as a function of the solid volume fraction. For the 14% lignite series, the permeability results appear to extend from 100% lignite results. For the higher sludge ratios series, these results appear to extend back from 100% sludge case and there appears to be a discontinuity between the two groups of data.

4.5. Distribution of sludge and lignite in the compressed cake The distribution of sludge and lignite solids in the compressed cake is investigated by determining the ash content at various layers of the compressed cake. Since both the sludge and lignite have very different ash contents (Table 1), the amount of ash present at various locations of the cake will depend on how the lignite and sludge are distributed in the cake. The cake is divided into three layers of approximately 3 mm thickness: top, middle and bottom; and the ash content of each layer is determined using Australian Standard (AS 2434.8-2002). The results for the three different sludge solids to lignite solids ratios are shown in Fig. 8. The analysis of variance performed on the ash content measurements shows that the variation in the ash content values of the top, middle and bottom layers of the compressed cake is not significant ( p-value > 0.05). Thus, there is no significant variation in lignite or sludge content between the top and bottom of the sample. Therefore, it can be assumed that the lignite solids are homogeneously mixed in the compressed cake.

1.E-20 0.6

0.8

1

1.2

1.4

1.6

14.0

Average permeability (m2)

b

1.E-14 0% sludge 14% sludge 40% sludge 57% sludge 75% sludge 100% sludge

1.E-15 1.E-16 1.E-17 1.E-18 1.E-19

Ash content (w/w%, dry basis)

Compression pressure (MPa)

Top

Middle

Bottom

12.0 10.0 8.0 6.0 4.0 2.0 0.0

1.E-20 25

30

35

40

45

50

55

60

65

70

Average solid volume fraction (v/v%) Fig. 7 – The average cake permeability as a function of (a) compression pressure (b) the average solid volume fraction. The error bars shown for the 14% sludge case represents the standard deviation of three identical tests.

30

40

50

Mass fraction of sludge (wt%, dry basis)

Fig. 8 – Distribution of ash content in various layers of the compressed cakes for different sludge solids to lignite solids ratios. The error bars shown are standard deviations of three repeat tests.

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4.6.

Compressed cake porosity

Mercury intruded (mL/g total solids)

The effect of lignite conditioning on the porosity of compressed cake is investigated by performing mercury intrusion porosimetry (MIP) tests on the compressed cakes. The porosity is measured by the volume of the mercury intruded at a specific pressure level which is inversely proportional to the diameter of the pore opening. The MIP tests are performed on the dried cakes and a significant amount of shrinkage can occur during the drying process. The method of Bergins et al. (2007) is used to determine the shrinkage due to drying, where the shrinkage is calculated from the difference in the volumes of the voids per unit mass of solids before and after drying. The volume of the voids per unit mass of solids before drying is equivalent to the moisture content of the solids. This is measured from the change in sample weight after oven drying at 105  C. The volume of the voids per unit mass of dried solids is obtained from the final intrusion volume obtained from the mercury porosimetry. The amount of shrinkage measured for the lignite and lignite-conditioned sludge cakes with different sludge to lignite solid ratios is approximately 75  5%. The amount of shrinkage found for the lignite in this investigation is similar to the shrinkage found by Bergins et al. (2007) and Clayton et al. (2007) for lignite. Clayton et al. (2007) has shown that all the pores for the lignite shrink roughly to the same extent. Therefore, in this investigation, it is assumed that all the pores shrink to the same extent and the shrinkage due to drying is the same for all the conditioned sludge cakes. High pressures up to 300–400 MPa are applied during MIP tests. At these high pressures, compression of the solids occurs and, as a result, the amount of mercury intruded increases. The true volume of mercury intruded is obtained by correcting the data for sample compression as explained by Bergins et al. (2007). The intrusion pore volumes corrected for compression effects are presented in Fig. 9. Since the solid samples used for MIP tests are crushed into small pieces to fit into the penetrometers, many large pores or the channels can be formed around the crushed pieces. At the early stage of the mercury porosimetry tests, these large pores or the channels are filled

0.3

with mercury. The scattering of the intrusion pore volume curves (Fig. 9) for the pores larger than 5 mm is suspected to be due to porosity introduced into the sample when it is crushed. Therefore, only the pores less than 5 mm are considered for the investigation. The compressed cake of lignite has the highest porosity whereas the flocculated sludge cake with no lignite has the lowest porosity. The porosity of the flocculated sludge increases after lignite conditioning. With increasing amounts of sludge in the compressed cake, the total porosity of the lignite-conditioned flocculated sludge cakes decreases.

5.

Discussion

5.1.

Improvement on rate of filtration

Conditioning of flocculated sludge with lignite has a beneficial effect on dewatering in both the cake formation stage and the cake compression stage. In the cake formation stage, the rate of filtration is significantly increased after lignite conditioning. The improvement in filtration rate is due to the lower specific cake resistance of the lignite-conditioned flocculated sludge as shown in Fig. 6. The reduction in specific cake resistance after lignite conditioning can be attributed to the greater permeability of the lignite fraction in the filter cake. The effect of lignite conditioning is also shown by the hydraulic pressure drop measurements, where a rapid decrease in pressure drop is found for the lignite-conditioned flocculated sludge compared to the flocculated sludge (Fig. 4). The slower decrease in pressure drop, in the case of the flocculated sludge, is due to the formation of a layer of low permeability on the filter medium. It is well known that sludge flocs are easily broken in a shearing flow (Langer et al., 1994). When water is pushed through the filter cake at the start of filtration there is both compression and shear and this can cause the flocs to be compressed against the filter medium forming an impermeable layer. The result of the formation of this impermeable layer is that the pressure drop across the bed stays high and the rate of filtration is significantly reduced. In contrast, when the flocculated sludge is conditioned with lignite, the lignite particles create a bridging effect between the sludge flocs which maintains the permeability of the filter cake facilitating the drainage of water.

0% sludge 0.25

5.2.

25% sludge 0.2 40% sludge 57% sludge

0.15 0.1

100% sludge 0.05 0 100

10

1

0.1

0.01

Pore diameter (mm) Fig. 9 – Mercury intrusion porosity profile plotted as a function of void volume per mass of total solids.

0.001

Improvement on compressed cake permeability

The conditioning of flocculated sludge with lignite also has a beneficial effect in the compression stage where a significant improvement in permeability of the compressed cake is achieved. The improvement in permeability is due to the reduction in the compressibility of the sample after lignite conditioning. The yield stress curves shown in Fig. 5 shows that the flocculated sludge has a much flatter curve than the lignite-conditioned flocculated sludge indicating that a much smaller change in pressure brings a much larger change in solids volume fraction. Since the permeability of the cake is inversely proportional to the solids volume fraction, the lignite-conditioned cakes which have lower solids volume

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fractions for an applied compression pressure will have higher permeability. This is shown most clearly in Fig. 7b, which shows that the permeability of the mixture of lignite and sludge falls into two bands. The upper band relates to the permeability of the lignite as the matrix material and the permeability falls steeply with increasing solid volume fraction. The lower band relates to the permeability of the sludge and this permeability which is already much lower than the lignite changes less with increasing solid volume fraction. The improvement in permeability is also due to the formation of channels or voids in the cake by the lignite particles which are formed when the cake is formed. During the conditioning process, the lignite particles are homogeneously mixed and trapped within the network structure of the flocculated sludge. This is demonstrated by the uniform ash content throughout the cake in the lignite-sludge sample shown in Fig. 8. The Environmental Scanning Electron Microscopic (ESEM) analysis on the wet sample (80% relative humidity) and Scanning Electron Microscope (SEM) on the dried samples of the flocculated sludge and lignite-conditioned flocculated sludge further confirms the homogeneous mixing of flocculated sludge and lignite. When the wet flocculated sludge and the lignite are observed under ESEM separately, two distinctively different structures can be seen (Fig. 10a and b). However, the structure of the lignite-conditioned flocculated sludge (Fig. 10c) is very similar to that of the flocculated sludge. Therefore, it can be said that the lignite particles are captured within the network structure of the flocculated sludge and are surrounded by the sludge flocs. The homogeneous mixing of the lignite particles within the sludge flocs is also shown in the SEM pictures of the dried samples. The SEM picture of the lignite (Fig. 11a) shows that it forms a discontinuous surface with many channels or voids around the particles. The flocculated sludge on its own (Fig. 11b) forms a continuous surface with no channels or voids. The lignite-conditioned flocculated sludge (Fig. 11c) forms a discontinuous surface similar to the lignite which indicates that the sludge solids are accommodated into the channels or the voids around the lignite particles. The relatively incompressible lignite particles maintain the channels or voids and the sludge can be pushed into these voids during the compression dewatering. This stops the formation of an impermeable thin layer of sludge on top of the filter medium during compression dewatering. As a result, the permeability of the flocculated sludge increases after lignite conditioning.

5.3.

Effect of compression pressure on permeability

The change in permeability with increasing compression pressure is not the same for the lignite, the flocculated sludge and the lignite-conditioned flocculated sludge. The decrease in permeability with increasing compression pressure for the lignite-conditioned flocculated sludge is slightly higher than that of the lignite (Fig. 7a). This is due to the difference in compressibility of the two samples as illustrated by the slopes of the yield stress curves shown in Fig. 5. The slope of the yield stress curves for the lignite-conditioned flocculated sludge cakes is slightly less compared to that of the lignite indicating

Fig. 10 – Environmental Scanning Electron Microscopic (ESEM) images (a) lignite (b) polyelectrolyte-flocculated sludge (c) lignite-conditioned flocculated sludge (40 wt% sludge).

that the sludge increases the compressibility of the solid matrix. Because of this, the change in permeability with increasing compression pressure is slightly higher for the lignite-conditioned flocculated sludge than for the lignite.

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The flocculated sludge has a much flatter yield stress curve compared to the lignite and the lignite-conditioned flocculated sludge cakes. Therefore, a sharp change in permeability with increasing compression pressure could be expected for the flocculated sludge. However, Fig. 7b shows that the flocculated sludge and the high sludge-lignite ratios for ligniteconditioned sludge have similar trends with increasing solid volume fraction.

5.4.

Role of lignite porosity

Mercury intruded (mL/g lignite solids)

The porosity of the compressed cake increases significantly after lignite conditioning. This improvement in cake porosity is mainly due to the channels or the pores created by the lignite particles present in the compressed cake. The channels or the pores are created when the lignite and sludge particles with different particle size distributions are re-arranged to form a cake during compression. The lignite particles have a large internal porosity of approximately 65% by volume. Therefore, it can be speculated that the improvement in permeability after lignite conditioning can also be the result of lignite’s internal porosity. Porosity is normally classified into three types: macro pores (>50 nm), meso pores (2–50 nm) and micro pores (<2 nm) (Webb and Orr, 1997). Fig. 12 shows the porosity volume per unit mass of lignite, which allows a comparison with the raw lignite’s porosity. These results show that the pores which fall in the meso or micro pore range, particularly the pores smaller than 0.5 mm are not affected by the sludge, indicating that these pores do not assist in sludge dewatering. If they were assisting in sludge dewatering, it would be expected that their volume would be reduced due to the accumulation of sludge solids. Therefore, the improvement in permeability after lignite conditioning is rather due to the macro pores on the surface of the lignite particles which are accessible to water flow and the channels or pores that are created around the lignite particles which are formed when the homogeneously mixed flocculated sludge and lignite mixture is compressed during the cake formation stage. The sludge, being a highly compressible material, is pushed into channels or pores and coats the lignite particles.

Fig. 11 – Scanning Electron Microscopic (SEM) images. (a) Lignite (from Scholes (2005)), (b) Flocculated sludge, (c) Lignite-conditioned flocculated sludge (40 wt% sludge).

0.35 0% sludge

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25% sludge 40% sludge

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0.20 0.15 0.10 0.05 0.00 100

10

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0.1

0.01

0.001

Pore diameter (mm) Fig. 12 – The mercury intrusion porosity profile plotted as a function of void volume per mass of lignite solids.

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Therefore, as shown in Fig. 9, a reduction in total pore volume is observed with increasing amount of the sludge in the cake. The coating of the lignite particles with the sludge is also evident in the SEM pictures shown in Fig. 11.

6.

Conclusions

The use of lignite to improve the dewatering performance of flocculated sludge has been discussed in this work. The following conclusion can be made:  The achievable equilibrium moisture removal for the flocculated sludge is greater than the lignite-conditioned flocculated sludge. Therefore, the benefit of lignite conditioning is the substantial increase in the rate of dewatering (of the order of five times) in both the filtration and compression stages.  In the filtration stage, lignite conditioning produces a filter cake of low specific cake resistance compared to the flocculated sludge alone. For the sample containing 40% sludge and 60% lignite (dry basis), the specific cake resistance is approximately twenty seven times lower than that of the flocculated sludge alone. In the compression stage, lignite conditioning maintains the porosity of the filter cake under compression as measured by its permeability. Even though the volume of solids is increased by the addition of lignite, the permeability is increased by a significantly greater factor.  The results from mercury porosimetry, SEM and cake yield stress curves all support the theory that lignite conditioning creates a completely different structure of filter cake to the flocculated sludge. Whereas the flocculated sludge on its own forms a fairly homogeneous structure which is highly compressible, the lignite conditioning creates a porous particulate structure and the sludge solids are accommodated partly within this structure.

Acknowledgements The authors gratefully acknowledge the financial and other support received for this research from the Institute of Sustainable Water Resource (ISWR), Monash University. The authors would also like to acknowledge the assistance of GHD Ltd. and Melbourne Water for providing the digested sludge samples.

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