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Conditioning of alum sludge with polymer and gypsum
Colloids and Surfaces A: Physicochemical and Engineering Aspects 194 (2001) 213– 220 www.elsevier.com/locate/colsurfa
Conditioning of alum sludge with polymer and gypsum Y.Q. Zhao a,*, D.H. Bache b a
En6ironmental Engineering Research Centre, School of Ci6il Engineering, Queen’s Uni6ersity Belfast, Belfast BT7 1NN, UK b Department of Ci6il Engineering, Uni6ersity of Strathclyde, 107 Rottenrow, Glasgow G4 0NG, UK Received 23 February 2001; accepted 18 May 2001
Abstract In this study, gypsum was introduced into alum sludge as a skeleton builder in combination with a polymer. The selection of gypsum lies in its common use as an amendment for ameliorating alkaline soils and may enhance the potential disposal of alum sludge to land. Preliminary work describes the impact of gypsum on the net sludge solids yield (YN), a factor which is used to express sludge filterability. Experiments demonstrated that gypsum could be used in association with a polymer to improve sludge filterability. The purpose of the skeleton builder is to form a permeable and rigid lattice structure that can remain porous under high pressure. By examining the polymer residual in the bulk solution and comparing data between the cases of gypsum and glass-microsphere (as a non-shrinking skeletal material) it was evident that the gypsum possesses a polymer demand indicating an interaction between them. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Alum sludge; Conditioning; Dewatering; Gypsum; Polymer; Structure
1. Introduction Although chemical conditioning using organic polymers of sludge is a dominating method in sludge treatment, the effective use of polymers as a conditioning agent represents a critical aspect of sludge dewatering and remains the subject of much research. Here, the term ‘effective use’ includes both ‘single use’ and use in combination with other materials. In the literature, some researchers focus on physical or combination conditioning methods with a view to altering the sludge * Corresponding author. Tel.: + 44-28-90335606; fax: + 4428-90663754. E-mail address: [email protected] (Y.Q. Zhao).
compressibility in the cause of improving the dewatering efficiency. This is because there is evidence to demonstrate that one effect of the chemical used in sludge conditioning is to make the sludge more compressible [1,2]. A highly compressible sludge will deform under positive pressure during the compression step of filtration (following cake growth), thus reducing the size of the micro-passages through which water is expressed. Sorensen and Hansen [3] indicated that the extreme compressibility of a sludge cake resulted in a filtrate flow which was independent of the applied pressure. In practice, there exists a tendency to increase the dewatering pressure or to prolong the dewatering time. From tests on a digested sewage sludge, La Heij [2] reported that
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increases in the mechanical pressure barely affected the void ratio of the sludge cake and it was found that larger applied pressures were offset by lower permeability. Therefore, it is reasonable to believe that the goal of conditioning should include both the reduction of the specific resistance and the diminution of the cake compressibility. Chemical conditioning of sludge improves sludge filterability by flocculating small gel-like sludge particles into larger aggregates with less affinity for water. Physical conditioners, such as, fly ash, cement kiln dust, quicklime, hydrated lime, fine coal and bagasse, are generally inert materials and mostly the wastes from industry. These are referred to as ‘skeleton builders’ because when added to a sludge, they can form a permeable and rigid lattice structure that can remain porous under high pressure [4]. From the literature, Benitze et al. [4] investigated sewage sludge conditioning with optimised combinations of a polymer and, respectively, three kinds of skeleton builders prior to filter pressing. Experimental results showed that fly ash, cement kiln dust and bagasse were successful skeleton builders, but significantly increased the solids content of the final cake. For example, when fly ash was used, the optimum conditioning strategy was based on a polymer dose of 1.1% (w/v) and the fly ash addition of 151% (w/v). This combination treatment increased the net sludge solid yield (YN) by 580% when compared with conditioning just using the polymer. Zouboulis and Guitonas [5] reported their pilot-scale experimental results for biological sludge conditioned with fly ash and dewatered by filtration press. It was found that around 5% w/v addition of fly ash could result in an increase of the final cake solids content from 11.6 to 49.5%. Zall et al. [6] investigated the effect of skeleton builders (industrial hydrated lime and fly ash) on the conditioning of an oily sludge. By examining the sludge compressibility they pointed out that the raw sludge and polymer conditioned sludges were each very compressible. Conditioning with skeleton builders greatly reduced the compressibility and yielded a more rigid and incompressible structure, capable of maintaining high porosity during high pressure filtration. It was also noted during filter pressing,
that polymer conditioned sludge cake experienced an expansion, or ‘spongy rebound’, when the filter press was opened. However, skeleton-conditioned sludge held its compressed shape. In the present study, gypsum (CaSO4 ·2H2O) was selected as skeleton builder to be used in combination with a polymer for conditioning an alum sludge. Although, there is no evidence in literature to show the use of gypsum in this type of application, gypsum has some advantages. First, in the agricultural context, gypsum is commonly used for ameliorating alkaline soils that occur in arid or semiarid regions. Its function is replacement of Na+ by Ca2 + in the cationic exchange site of the soil colloids. Enough gypsum could remove most of the carbonate ion and can cause the pH to be lowered. Gypsum is also used as a source of calcium or sulphur in soils if and when specific needs for calcium or sulphur of fertiliser are identified. Miller [7] verified that gypsum could improve the infiltration characteristics of soil. In a similar vein, Chartres et al. [8] observed pore collapse in an Australian soil when the added gypsum was completely leached from the soil. Second, addition of gypsum to alum sludge increases the potential application of alum sludge to land as a disposal route. Traditionally, alum sludge is directly discharged or landfilled because it is relatively inert, providing marginal, if any, benefits to soil fertility. Compared with sewage sludge, an alum sludge is relatively clean with respect to heavy metals and organics, and poses few environmental risks [9]. Groups of experiments in this study were designed to assess the influence of gypsum on alum sludge filterability, this being expressed in terms of the net sludge solids yield (YN) [10]. Emphasis was placed on the interaction between polymer and gypsum by direct measurement of polymer residual in bulk solution of this combined condition. In order to provide comparative data, a non-shrinking material (glass microspheres) was used in parallel tests. Finally, sludge compressibility (s) was estimated using a high pressure cell shown schematically in Fig. 1. Sludge compressibility (s) [11] was defined in terms of the specific resistance to filtration (SRF) in the form:
Y.Q. Zhao, D.H. Bache / Colloids and Surfaces A: Physicochem. Eng. Aspects 194 (2001) 213–220
SRFi Pt,i = SRF0 Pt,0
s
(1)
Terms Pt, i, Pt,0 refer to the applied pressure and a reference pressure, respectively. SRFi and SRF0 are the corresponding values of the specific resistance. Parameter s can be obtained from a log– log plot of SRFi /SRF0 against the pressure ratio.
It is well known that the specific resistance to filtration has been used as a criterion in judging conditioner effectiveness when the sludge solids remains relatively constant. In the case of skeleton builders, large amounts of conditioner solids are added to the sludge and a new sludge of higher solids content is produced. Although the SRF of new conditioned sludge might be lower, and the removal rate of total solids by filtration higher, the removal rate of the original sludge solids would not necessarily be higher. Therefore, if large amounts of conditioner solid are added, another index is required to express the filterability. Rebhun et al. [10] demonstrated that sludge solids yield (Y) could be used for expressing the filterability. Y was related mathematically to SRF and could be calculated directly from the Buchner test [12]. The filterability can be expressed directly as the rate of sludge solids filtered per unit area and unit time. So Y could be defined as: Y=
CtV tA
Fig. 1. Schematic diagram of high pressure cell.
in which Y is the sludge dry solids yield (kg m − 2 h − 1) and V, t and A are in turn the filtration volume, time and area, respectively. Ct is the total (original solids+ conditioner solids) solids concentration. As is well known [12], SRF may be calculated from the slope of the linear region of a t/V versus V plot in accord with the standard filtration equation: t v SRF CtV vRm = + V 2PtA 2 PtA
2. Net sludge solids yield (YN)
(2)
215
(3)
where, Pt and v refer, respectively, the total pressure drop in Buchner test and the dynamic viscosity of the filtrate. Rm is the filter medium resistance. By neglecting the medium resistance in Eq. (3), substitution into Eq. (2) leads to: Y=
2PtCt 1 vt SRF
(4)
In order to express the net sludge solids yield, a correction factor () may be introduced, i.e. =
Cs Ct
(5)
where, Cs refers to original sludge solids concentration. From this, YN can be expressed as: YN =
2PtCt 1 vt SRF
1/2
(6)
3. Experimental materials and methods
3.1. Experimental materials 3.1.1. Sludge and polymer The alum sludge used in this study was collected from the sludge holding tank of a waterworks treating a low-turbidity coloured water coagulated with aluminium sulphate. Properties of this alum sludge are listed in Table 1. Magnafloc LT25, an anionic organic polymer (Allied Colloids UK Ltd, now CIBA Speciality Chemicals Ltd), was used for conditioning. This was prepared as a 0.01% stock solution using nanopure water and used after 24 h. The stock solution was made up every 48 h. Table 2 showed the basic characteristics of LT25.
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Table 1 Properties of alum sludge used in this studya Suspended solids (mg l−1)
pH
SRF (×1012 m kg−1)
CST (s)
Viscosity (mm2 s−1) (filtrate)
6400–6800
6.6–7.3
63.2–133.4
61.1–84.0
1.0052–1.0325
a
CST: capillary suction time.
3.1.2. Glass-microsphere and gypsum 100 mm diameter glass-microspheres were used as a non-shrinking skeletal material. The purpose was to compare their performance with gypsum. In order to investigate the influence of particle size of skeleton materials to the conditioning, glass-microspheres with the particle size of 63 and 212 mm were also used in the conditioning tests. Red gypsum (supplied by Tioxide Europe Ltd.) with specific gravity of 1.4– 1.6 g cc − 1 was selected as a skeleton material for the use in the trials. The particle sizes of the gypsum were determined as 4– 20 mm using a Galai CIS-100 particle size analyser. 3.2. Experimental methods 3.2.1. Combined conditioning procedure The skeleton builder (i.e. glass-microspheres or gypsum) was first added to a 200 ml sludge sample. The dose of skeleton builder was expressed as a percentage of the alum sludge dry solids concentration. Doses in the range of 20– 150% were examined. After several seconds of rapid mixing to ensure dispersion, the polymer was added in the concentration range 0– 20 mg l − 1. Following polymer addition, the sludge was subjected to 30 s of rapid mixing followed by 1 min slow mixing to promote flocculation. It was noted that for the sludge examined the optimum dose of the single polymer conditioning is 15.0 mg l − 1, which was evaluated by modified SRF [13]. To estimate YN, conditioned sludge samples were subjected for filtration tests. They were performed on 100 ml sludge samples in a Buchner funnel using standard procedures. In these tests, the pressure differential was maintained at 800 mm Hg and Whatman 1 c filter paper was used as the filter medium. Following the Buchner test, SRF was calculated and YN was determined using Eq. (6).
3.2.2. Sludge compressibility test A high pressure cell used in this study was shown schematically in Fig. 1. A positive pressure was applied to the sludge sample by an air pressure driven piston. Two Whatman 1c filter papers wetted with distilled water were placed at the base of the pressure cell to hold the sludge sample. 100 ml sludge samples of raw and conditioned sludge were subjected for the test in five pressure differentials in the range from 1.5 to 7.6 bar. At each applied pressure, the filtrate volumes and the filtration time were recorded and the SRF was calculated. After this, the compressibility coefficient (s) was thus estimated using Eq. (1). 3.2.3. Measurement of polymer residual Measurements of polymer residual in supernatant of conditioned sludge samples were undertaken using high performance liquid chromatography (HPLC) based on size exclusion chromatography (SEC) as described in Keenan et al. [14]. As a general rule, all measurements were repeated three times and an average value is reported.
4. Results Fig. 2 shows the results concerning the addition of 100 mm glass-microspheres at polymer doses of 5.0, 15.0 and 20.0 mg l − 1, respectively. In the cases of polymer dose at both 5.0 and 15.0 mg l − 1, it can be seen from Fig. 2 that YN passes through a small maximum, indicating a minor improvement in the filterability. Maximum values occur at the glass-microspheres addition 60% DS (DS refers original sludge dry solids) with 5.0 mg l − 1 polymer dose and at 20% DS with a 15.0 mg l − 1 polymer dose. A reason why YN passes through a maximum value was given by Rebhun
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Table 2 Properties of polymer LT25 used in this study Type
Molecular weight
Charge (% by weight)
Concentration (%)
Manufacturer
Anionic
(10–15)×106
15–30
0.01
CIBA Speciality Chemicals Ltd
et al. [10]. Although the total solids yield increases with decreasing of resistance of filtration, there comes a point when the decrease in the original sludge solids fraction outweighs the increase in yield of total solids. At a polymer dose of 20.0 mg l − 1, YN decreases continuously with the microsphere addition. It reflects an increase in the resistance to filtration. Generally, it is evident that the yield is more sensitive to the polymer dose than to the microsphere addition. Further investigations of the effects of glass-microspheres size on conditioning were performed. Sludge samples were conditioned by a combination of polymer and different sizes of glass-microsphere, i.e. 63, 100 and 212 mm. From Fig. 3, it is evident that increased particle size can reduce the SRF and improve the filterability by increasing YN. However, beyond 100 mm there is relatively little benefit. Fig. 4 illustrates the effects of gypsum addition to the sludge conditioning with three polymer doses, respectively. When the polymer dose is 5.0 mg l − 1, the addition of gypsum (at 60% DS) leads a maximum YN. When the polymer dose is 15.0 mg l − 1, 20% DS gypsum addition produces the maximum YN. The salient feature of Fig. 4 is the case when the polymer dose is 20.0 mg l − 1, in which it is seen that increasing gypsum addition can lead to significant improvements in the yield. The trends shown in Figs. 2 and 4 show similarities for polymer doses of 5.0 and 15.0 mg l − 1 in response to increasing amounts of skeletal material. It suggests that the subtle changes in the yield represent real effects. Gypsum has a slightly reduced capacity to improve the sludge filterability when compared with the glass-microspheres (considered as a non-shrinking material). Compared with the behaviour of gypsum addition at polymer dose of 20.0 mg l − 1 (see Fig. 4), the effects in the cases of polymer doses of 5.0 and 15.0 mg l − 1 are relative minor. In addition,
at the particular dose of 20.0 mg l − 1additions of glass-microspheres and gypsum produce quite different effects on YN (see Figs. 2 and 4). In order to examine the controlling factors behind this result, polymer residuals in supernatant were measured. Fig. 5 shows data corresponding to a polymer dose of 20.0 mg l − 1. It can be seen that the polymer residual associated with the glass-microspheres (2.45 mg l − 1) (Fig. 5) is about the same as that in single polymer conditioned sludge (2.66 mg l − 1). This reflects a lack of interaction between the polymer and the glass-microspheres. However, it is evident that the presence of gypsum creates an additional polymer demand (presumably by adsorption) because of the lower polymer residual (0.77 mg l − 1) (see Fig. 5). Therefore, it is believed that the addition of glass-microspheres at the 20.0 mg l − 1 dose cannot improve the sludge filterability because the filterability is mainly affected by the excess polymer; this has been identified as a cause of clogging in a filter medium [15]. On the other hand, it appears that gypsum does improve the sludge filterability as the result of its consumption of excess polymer,
Fig. 2. Effect of the 100 mm glass-microspheres addition to the sludge conditioning, with polymer dose at 5.0, 15.0 and 20.0 mg l − 1 (error bars denote SDs).
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Fig. 3. Effect of particle size of glass-microsphere on conditioning.
and the reduction of its potential to promote clogging at the filter medium. In view of the above, additional tests were conducted to examine the gypsum-polymer interaction in further detail. In this group of dual conditioning tests, the order of the polymer and gypsum addition was altered. Table 3 summarised the test procedures and the results. It is seen that the order of addition causes significant changes in the SRF and YN. Adding the gypsum after polymer leads relatively poor dewaterability. Whereas when gypsum was added before the polymer, the polymer appears to promote flocculation in both the sludge and the gypsum. When polymer is added first, this reduces the opportunity for gypsum to gain a better structure. Three sludge samples, including, raw sludge; polymer-conditioned sludge (at dose of 5.0 mg
Fig. 4. Effect of the gypsum addition to the sludge conditioning with polymer dose at 5.0, 15.0 and 20.0 mg l − 1 (error bars denote SDs).
Fig. 5. Polymer residuals in polymer and dual added conditioned sludge samples (polymer dose of 20.0 mg l − 1, glasssphere and gypsum doses of 60% DS).
l − 1); and dual conditioned sludge (and 5.0 mg l − 1 polymer plus 60% DS gypsum) were subjected to compressibility tests using high pressure cell as shown in Fig. 1. The results were displayed in Fig. 6. It is evident that the compressibility of the raw and polymer (alone) conditioned sludge are rather similar (about s= 0.8). A small reduction (Ds = 0.2) of sludge compressibility occurred when gypsum was added. This might be the reason that skeleton builders could cause a marked improvement of dewatering capacity by produce a more incompressible cake structure as described by Zall et al. [6]. Table 3 Result of tests for the effects of additional manners on sludge conditioning (raw sludge SRF = 67.9×1012 m kg−1)
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5. Discussion
Fig. 6. Results of compressibility tests.
It is useful to discover whether the skeleton materials themselves, especially the gypsum, have a role in sludge conditioning and dewatering. For this reason, a conditioning test with the addition of gypsum (without polymer) was carried out. Fig. 7 shows the result. It is evident that addition of gypsum by itself has little effect on the improvement of sludge filterability (comparing Fig. 7 with Fig. 4). This is because gypsum is not a flocculent. It does not have the ability to flocculate the sludge particles as is the case of the polymer. Its beneficial effects as described above must be in association with a polymer.
Fig. 7. Showing effect on sludge filterability by gypsum addition (sludge solids concentration of 6.4 –6.8 g l − 1).
Within this study, the primary purpose was to explore whether the alum sludge filterability (in terms of YN) was affected by the skeleton builders in combination with a polymer. Gypsum was selected as skeleton builder in this study because it was the most commonly used amendment for ameliorating alkaline soils. In order to provide comparative data, a non-shrinking material glass microspheres were also used. The starting point of experimental investigation follows an alum sludge conditioning procedure described for a single polymer conditioning [15]. Groups of experiment described in this paper demonstrated that the addition of glass-microsphere had only a marginal effect on YN (see Fig. 2). Residual polymer measurement shows that there was no interaction between glass-microsphere and the polymer. In contrast, experiments demonstrated that gypsum had a significant effect on YN jointly with a polymer dose of 20.0 mg l − 1. It is noted that this particular dose refers to a state of overdose (the optimum dose was 15.0 mg l − 1 as gauged by the modified SRF). Below 20.0 mg l − 1 the effects of gypsum addition are minor (see Fig. 4). Further tests indicate that gypsum poses a polymer demand (see Fig. 5), showing an interaction between gypsum and the polymer. The experiments based on the addition manner of gypsum (see Table 3) and gypsum addition without polymer (shown in Fig. 7) confirm the above view. Due to the lack of the interaction with polymer, the addition of gypsum does not produce a beneficial sludge structure. Further investigations of sludge compressibility show that the involvement of the gypsum can decrease the sludge compressibility of s from 0.83 to 0.65 (see Fig. 6), implying that it might help to build up a more porous, permeable and rigid lattice structure, which retains solid particles and allows the water to be transmitted. Measurement of the polymer residuals [14] provides valuable supporting information for discerning the interaction between polymer and skeleton builders. The beneficial effect on YN described above must be associated with polymer. This view differs from some results in literature, such as, Smith et al. [16], Nelson and Brattlof [17] and
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Zouboulis and Guitonas [5], in which they only added the skeleton materials to obtain a significant effect on sludge dewaterability. However, it is noted that when skeleton materials were used it is not uncommon to add up to 5 kg conditioners solids for each 1 kg sludge, as pointed by Smith et al. [16]. This arouses the suspicion of that the improvement of dewaterability may be tied in the total change of components subjected for dewatering because such additions of conditioner render the sludge the minor consituent. Nevertheless, the results about the involvement of gypsum in sludge conditioning derived from this study are at least encouraging. The importance of this study, if any, lies in the potential application of dewatered alum sludge to the land use. As is known, land application of sludge is not a new concept. Sewage and lime sludges have been applied to land for long time. Both sludges improve soil fertility and are used as soil amendments in agriculture and land reclamation. However, currently, alum sludge is directly discharged or landfilled because it is relatively inert, providing marginal, if any, benefits to soil fertility. Therefore, research work dwelling the potential application of alum sludge is highly desirable. For example, it is noted in literature that attention is paid to the disposal of alum sludge in spite of the limited information, e.g. Geertsema et al. [9] carried out 30 months investigation into the effects of the land use of alum sludge. Measurements at the original field site included soil analysis, soil water monitoring, groundwater monitoring, and analysis of tissues from pine needles. From their study, it was concluded that an alum sludge may be applied to forest lands at loading rates of at least 1.5– 2.5% by dry weight with no long-term adverse effect being observed. Overall, the involvement of gypsum in alum sludge treatment will be another possible approach to enhance its disposal. However, more work needs to be carried out.
6. Conclusions
It has been demonstrated that the gypsum can be used as skeletal builder in alum sludge con-
ditioning to improve its filterability. The presence of gypsum helps to build up a more porous, permeable and rigid lattice structure of sludge cake. However, this beneficial effect must be associated with use of a polymer. Enhanced alum sludge filterability lies in the interaction between gypsum and polymer since gypsum is demonstrated to possess polymer demand. It is considered that application of gypsum in alum sludge treatment may enhance its potential for disposal to land.
Acknowledgements The authors thank West of Scotland Water for the provision of material and access to a number of water treatment works.
References [1] J.T. Novak, J.H. O’Brien, J. WPCF 47 (1975) 2397 –2410. [2] E.J. La Heij, Ph.D. Thesis. Technical University Eindhoven, Netherlands, (1994). [3] P.B. Sorensen, J.A. Hansen, Water Sci. Technol. 28 (1993) 133 – 143. [4] J. Benitez, A. Rodriguez, A. Suarez, Water Res. 28 (1994) 2067 – 2073. [5] A.I. Zouboulis, A. Guitonas, Fresenius Environ. Bull. 4 (1995) 387 – 392. [6] J. Zall, N. Galil, M. Rehbum, J. WPCF 59 (1987) 699 –706. [7] W.P. Miller, J. Soil Sci. Soc. Am. 51 (1987) 1314 –1320. [8] C.J. Chartres, R.S. Greene, G.W. Ford, P. Rengasamy, J. Soil Res. 23 (1985) 467 – 479. [9] W.S. Geertsema, W.R. Knocke, J.T. Novak, D. Dove, J. AWWA 86 (1994) 64 – 74. [10] M. Rebhun, J. Zall, N. Galil, J. WPCF 61 (1989) 52 –54. [11] P.C. Carman, J. Soc. Chem. Ind. 53 (1934) 159T & 301T. [12] P. Coackley, B.R.S. Jones, Sew. Ind. Wastes, August (1956) 963 – 975. [13] Y.Q. Zhao, Ph.D. Thesis, University of Strathclyde, Glasgow, UK, (1999). [14] H.E. Keenan, E.N. Papavasilopoulos, D.H. Bache, Water Res. 32 (1998) 3173 – 3176. [15] Y.Q. Zhao, E.N. Papavasilopoulos, D.H. Bache, Filtration Sep. 35 (1998) 947 – 950. [16] J E. Smith et al., Proc. 27th Ind. Waste Conf., Purdue Univ. Ext. Ser. (1972). [17] R.F. Nelson, B.D. Brattlof, J. WPCF 51 (1979) 1024 –1031.
Conditioning of alum sludge with polymer and gypsum Y.Q. Zhao a,*, D.H. Bache b a
En6ironmental Engineering Research Centre, School of Ci6il Engineering, Queen’s Uni6ersity Belfast, Belfast BT7 1NN, UK b Department of Ci6il Engineering, Uni6ersity of Strathclyde, 107 Rottenrow, Glasgow G4 0NG, UK Received 23 February 2001; accepted 18 May 2001
Abstract In this study, gypsum was introduced into alum sludge as a skeleton builder in combination with a polymer. The selection of gypsum lies in its common use as an amendment for ameliorating alkaline soils and may enhance the potential disposal of alum sludge to land. Preliminary work describes the impact of gypsum on the net sludge solids yield (YN), a factor which is used to express sludge filterability. Experiments demonstrated that gypsum could be used in association with a polymer to improve sludge filterability. The purpose of the skeleton builder is to form a permeable and rigid lattice structure that can remain porous under high pressure. By examining the polymer residual in the bulk solution and comparing data between the cases of gypsum and glass-microsphere (as a non-shrinking skeletal material) it was evident that the gypsum possesses a polymer demand indicating an interaction between them. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Alum sludge; Conditioning; Dewatering; Gypsum; Polymer; Structure
1. Introduction Although chemical conditioning using organic polymers of sludge is a dominating method in sludge treatment, the effective use of polymers as a conditioning agent represents a critical aspect of sludge dewatering and remains the subject of much research. Here, the term ‘effective use’ includes both ‘single use’ and use in combination with other materials. In the literature, some researchers focus on physical or combination conditioning methods with a view to altering the sludge * Corresponding author. Tel.: + 44-28-90335606; fax: + 4428-90663754. E-mail address: [email protected] (Y.Q. Zhao).
compressibility in the cause of improving the dewatering efficiency. This is because there is evidence to demonstrate that one effect of the chemical used in sludge conditioning is to make the sludge more compressible [1,2]. A highly compressible sludge will deform under positive pressure during the compression step of filtration (following cake growth), thus reducing the size of the micro-passages through which water is expressed. Sorensen and Hansen [3] indicated that the extreme compressibility of a sludge cake resulted in a filtrate flow which was independent of the applied pressure. In practice, there exists a tendency to increase the dewatering pressure or to prolong the dewatering time. From tests on a digested sewage sludge, La Heij [2] reported that
0927-7757/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S0927-7757(01)00788-9
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increases in the mechanical pressure barely affected the void ratio of the sludge cake and it was found that larger applied pressures were offset by lower permeability. Therefore, it is reasonable to believe that the goal of conditioning should include both the reduction of the specific resistance and the diminution of the cake compressibility. Chemical conditioning of sludge improves sludge filterability by flocculating small gel-like sludge particles into larger aggregates with less affinity for water. Physical conditioners, such as, fly ash, cement kiln dust, quicklime, hydrated lime, fine coal and bagasse, are generally inert materials and mostly the wastes from industry. These are referred to as ‘skeleton builders’ because when added to a sludge, they can form a permeable and rigid lattice structure that can remain porous under high pressure [4]. From the literature, Benitze et al. [4] investigated sewage sludge conditioning with optimised combinations of a polymer and, respectively, three kinds of skeleton builders prior to filter pressing. Experimental results showed that fly ash, cement kiln dust and bagasse were successful skeleton builders, but significantly increased the solids content of the final cake. For example, when fly ash was used, the optimum conditioning strategy was based on a polymer dose of 1.1% (w/v) and the fly ash addition of 151% (w/v). This combination treatment increased the net sludge solid yield (YN) by 580% when compared with conditioning just using the polymer. Zouboulis and Guitonas [5] reported their pilot-scale experimental results for biological sludge conditioned with fly ash and dewatered by filtration press. It was found that around 5% w/v addition of fly ash could result in an increase of the final cake solids content from 11.6 to 49.5%. Zall et al. [6] investigated the effect of skeleton builders (industrial hydrated lime and fly ash) on the conditioning of an oily sludge. By examining the sludge compressibility they pointed out that the raw sludge and polymer conditioned sludges were each very compressible. Conditioning with skeleton builders greatly reduced the compressibility and yielded a more rigid and incompressible structure, capable of maintaining high porosity during high pressure filtration. It was also noted during filter pressing,
that polymer conditioned sludge cake experienced an expansion, or ‘spongy rebound’, when the filter press was opened. However, skeleton-conditioned sludge held its compressed shape. In the present study, gypsum (CaSO4 ·2H2O) was selected as skeleton builder to be used in combination with a polymer for conditioning an alum sludge. Although, there is no evidence in literature to show the use of gypsum in this type of application, gypsum has some advantages. First, in the agricultural context, gypsum is commonly used for ameliorating alkaline soils that occur in arid or semiarid regions. Its function is replacement of Na+ by Ca2 + in the cationic exchange site of the soil colloids. Enough gypsum could remove most of the carbonate ion and can cause the pH to be lowered. Gypsum is also used as a source of calcium or sulphur in soils if and when specific needs for calcium or sulphur of fertiliser are identified. Miller [7] verified that gypsum could improve the infiltration characteristics of soil. In a similar vein, Chartres et al. [8] observed pore collapse in an Australian soil when the added gypsum was completely leached from the soil. Second, addition of gypsum to alum sludge increases the potential application of alum sludge to land as a disposal route. Traditionally, alum sludge is directly discharged or landfilled because it is relatively inert, providing marginal, if any, benefits to soil fertility. Compared with sewage sludge, an alum sludge is relatively clean with respect to heavy metals and organics, and poses few environmental risks [9]. Groups of experiments in this study were designed to assess the influence of gypsum on alum sludge filterability, this being expressed in terms of the net sludge solids yield (YN) [10]. Emphasis was placed on the interaction between polymer and gypsum by direct measurement of polymer residual in bulk solution of this combined condition. In order to provide comparative data, a non-shrinking material (glass microspheres) was used in parallel tests. Finally, sludge compressibility (s) was estimated using a high pressure cell shown schematically in Fig. 1. Sludge compressibility (s) [11] was defined in terms of the specific resistance to filtration (SRF) in the form:
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SRFi Pt,i = SRF0 Pt,0
s
(1)
Terms Pt, i, Pt,0 refer to the applied pressure and a reference pressure, respectively. SRFi and SRF0 are the corresponding values of the specific resistance. Parameter s can be obtained from a log– log plot of SRFi /SRF0 against the pressure ratio.
It is well known that the specific resistance to filtration has been used as a criterion in judging conditioner effectiveness when the sludge solids remains relatively constant. In the case of skeleton builders, large amounts of conditioner solids are added to the sludge and a new sludge of higher solids content is produced. Although the SRF of new conditioned sludge might be lower, and the removal rate of total solids by filtration higher, the removal rate of the original sludge solids would not necessarily be higher. Therefore, if large amounts of conditioner solid are added, another index is required to express the filterability. Rebhun et al. [10] demonstrated that sludge solids yield (Y) could be used for expressing the filterability. Y was related mathematically to SRF and could be calculated directly from the Buchner test [12]. The filterability can be expressed directly as the rate of sludge solids filtered per unit area and unit time. So Y could be defined as: Y=
CtV tA
Fig. 1. Schematic diagram of high pressure cell.
in which Y is the sludge dry solids yield (kg m − 2 h − 1) and V, t and A are in turn the filtration volume, time and area, respectively. Ct is the total (original solids+ conditioner solids) solids concentration. As is well known [12], SRF may be calculated from the slope of the linear region of a t/V versus V plot in accord with the standard filtration equation: t v SRF CtV vRm = + V 2PtA 2 PtA
2. Net sludge solids yield (YN)
(2)
215
(3)
where, Pt and v refer, respectively, the total pressure drop in Buchner test and the dynamic viscosity of the filtrate. Rm is the filter medium resistance. By neglecting the medium resistance in Eq. (3), substitution into Eq. (2) leads to: Y=
2PtCt 1 vt SRF
(4)
In order to express the net sludge solids yield, a correction factor () may be introduced, i.e. =
Cs Ct
(5)
where, Cs refers to original sludge solids concentration. From this, YN can be expressed as: YN =
2PtCt 1 vt SRF
1/2
(6)
3. Experimental materials and methods
3.1. Experimental materials 3.1.1. Sludge and polymer The alum sludge used in this study was collected from the sludge holding tank of a waterworks treating a low-turbidity coloured water coagulated with aluminium sulphate. Properties of this alum sludge are listed in Table 1. Magnafloc LT25, an anionic organic polymer (Allied Colloids UK Ltd, now CIBA Speciality Chemicals Ltd), was used for conditioning. This was prepared as a 0.01% stock solution using nanopure water and used after 24 h. The stock solution was made up every 48 h. Table 2 showed the basic characteristics of LT25.
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Table 1 Properties of alum sludge used in this studya Suspended solids (mg l−1)
pH
SRF (×1012 m kg−1)
CST (s)
Viscosity (mm2 s−1) (filtrate)
6400–6800
6.6–7.3
63.2–133.4
61.1–84.0
1.0052–1.0325
a
CST: capillary suction time.
3.1.2. Glass-microsphere and gypsum 100 mm diameter glass-microspheres were used as a non-shrinking skeletal material. The purpose was to compare their performance with gypsum. In order to investigate the influence of particle size of skeleton materials to the conditioning, glass-microspheres with the particle size of 63 and 212 mm were also used in the conditioning tests. Red gypsum (supplied by Tioxide Europe Ltd.) with specific gravity of 1.4– 1.6 g cc − 1 was selected as a skeleton material for the use in the trials. The particle sizes of the gypsum were determined as 4– 20 mm using a Galai CIS-100 particle size analyser. 3.2. Experimental methods 3.2.1. Combined conditioning procedure The skeleton builder (i.e. glass-microspheres or gypsum) was first added to a 200 ml sludge sample. The dose of skeleton builder was expressed as a percentage of the alum sludge dry solids concentration. Doses in the range of 20– 150% were examined. After several seconds of rapid mixing to ensure dispersion, the polymer was added in the concentration range 0– 20 mg l − 1. Following polymer addition, the sludge was subjected to 30 s of rapid mixing followed by 1 min slow mixing to promote flocculation. It was noted that for the sludge examined the optimum dose of the single polymer conditioning is 15.0 mg l − 1, which was evaluated by modified SRF [13]. To estimate YN, conditioned sludge samples were subjected for filtration tests. They were performed on 100 ml sludge samples in a Buchner funnel using standard procedures. In these tests, the pressure differential was maintained at 800 mm Hg and Whatman 1 c filter paper was used as the filter medium. Following the Buchner test, SRF was calculated and YN was determined using Eq. (6).
3.2.2. Sludge compressibility test A high pressure cell used in this study was shown schematically in Fig. 1. A positive pressure was applied to the sludge sample by an air pressure driven piston. Two Whatman 1c filter papers wetted with distilled water were placed at the base of the pressure cell to hold the sludge sample. 100 ml sludge samples of raw and conditioned sludge were subjected for the test in five pressure differentials in the range from 1.5 to 7.6 bar. At each applied pressure, the filtrate volumes and the filtration time were recorded and the SRF was calculated. After this, the compressibility coefficient (s) was thus estimated using Eq. (1). 3.2.3. Measurement of polymer residual Measurements of polymer residual in supernatant of conditioned sludge samples were undertaken using high performance liquid chromatography (HPLC) based on size exclusion chromatography (SEC) as described in Keenan et al. [14]. As a general rule, all measurements were repeated three times and an average value is reported.
4. Results Fig. 2 shows the results concerning the addition of 100 mm glass-microspheres at polymer doses of 5.0, 15.0 and 20.0 mg l − 1, respectively. In the cases of polymer dose at both 5.0 and 15.0 mg l − 1, it can be seen from Fig. 2 that YN passes through a small maximum, indicating a minor improvement in the filterability. Maximum values occur at the glass-microspheres addition 60% DS (DS refers original sludge dry solids) with 5.0 mg l − 1 polymer dose and at 20% DS with a 15.0 mg l − 1 polymer dose. A reason why YN passes through a maximum value was given by Rebhun
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Table 2 Properties of polymer LT25 used in this study Type
Molecular weight
Charge (% by weight)
Concentration (%)
Manufacturer
Anionic
(10–15)×106
15–30
0.01
CIBA Speciality Chemicals Ltd
et al. [10]. Although the total solids yield increases with decreasing of resistance of filtration, there comes a point when the decrease in the original sludge solids fraction outweighs the increase in yield of total solids. At a polymer dose of 20.0 mg l − 1, YN decreases continuously with the microsphere addition. It reflects an increase in the resistance to filtration. Generally, it is evident that the yield is more sensitive to the polymer dose than to the microsphere addition. Further investigations of the effects of glass-microspheres size on conditioning were performed. Sludge samples were conditioned by a combination of polymer and different sizes of glass-microsphere, i.e. 63, 100 and 212 mm. From Fig. 3, it is evident that increased particle size can reduce the SRF and improve the filterability by increasing YN. However, beyond 100 mm there is relatively little benefit. Fig. 4 illustrates the effects of gypsum addition to the sludge conditioning with three polymer doses, respectively. When the polymer dose is 5.0 mg l − 1, the addition of gypsum (at 60% DS) leads a maximum YN. When the polymer dose is 15.0 mg l − 1, 20% DS gypsum addition produces the maximum YN. The salient feature of Fig. 4 is the case when the polymer dose is 20.0 mg l − 1, in which it is seen that increasing gypsum addition can lead to significant improvements in the yield. The trends shown in Figs. 2 and 4 show similarities for polymer doses of 5.0 and 15.0 mg l − 1 in response to increasing amounts of skeletal material. It suggests that the subtle changes in the yield represent real effects. Gypsum has a slightly reduced capacity to improve the sludge filterability when compared with the glass-microspheres (considered as a non-shrinking material). Compared with the behaviour of gypsum addition at polymer dose of 20.0 mg l − 1 (see Fig. 4), the effects in the cases of polymer doses of 5.0 and 15.0 mg l − 1 are relative minor. In addition,
at the particular dose of 20.0 mg l − 1additions of glass-microspheres and gypsum produce quite different effects on YN (see Figs. 2 and 4). In order to examine the controlling factors behind this result, polymer residuals in supernatant were measured. Fig. 5 shows data corresponding to a polymer dose of 20.0 mg l − 1. It can be seen that the polymer residual associated with the glass-microspheres (2.45 mg l − 1) (Fig. 5) is about the same as that in single polymer conditioned sludge (2.66 mg l − 1). This reflects a lack of interaction between the polymer and the glass-microspheres. However, it is evident that the presence of gypsum creates an additional polymer demand (presumably by adsorption) because of the lower polymer residual (0.77 mg l − 1) (see Fig. 5). Therefore, it is believed that the addition of glass-microspheres at the 20.0 mg l − 1 dose cannot improve the sludge filterability because the filterability is mainly affected by the excess polymer; this has been identified as a cause of clogging in a filter medium [15]. On the other hand, it appears that gypsum does improve the sludge filterability as the result of its consumption of excess polymer,
Fig. 2. Effect of the 100 mm glass-microspheres addition to the sludge conditioning, with polymer dose at 5.0, 15.0 and 20.0 mg l − 1 (error bars denote SDs).
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Fig. 3. Effect of particle size of glass-microsphere on conditioning.
and the reduction of its potential to promote clogging at the filter medium. In view of the above, additional tests were conducted to examine the gypsum-polymer interaction in further detail. In this group of dual conditioning tests, the order of the polymer and gypsum addition was altered. Table 3 summarised the test procedures and the results. It is seen that the order of addition causes significant changes in the SRF and YN. Adding the gypsum after polymer leads relatively poor dewaterability. Whereas when gypsum was added before the polymer, the polymer appears to promote flocculation in both the sludge and the gypsum. When polymer is added first, this reduces the opportunity for gypsum to gain a better structure. Three sludge samples, including, raw sludge; polymer-conditioned sludge (at dose of 5.0 mg
Fig. 4. Effect of the gypsum addition to the sludge conditioning with polymer dose at 5.0, 15.0 and 20.0 mg l − 1 (error bars denote SDs).
Fig. 5. Polymer residuals in polymer and dual added conditioned sludge samples (polymer dose of 20.0 mg l − 1, glasssphere and gypsum doses of 60% DS).
l − 1); and dual conditioned sludge (and 5.0 mg l − 1 polymer plus 60% DS gypsum) were subjected to compressibility tests using high pressure cell as shown in Fig. 1. The results were displayed in Fig. 6. It is evident that the compressibility of the raw and polymer (alone) conditioned sludge are rather similar (about s= 0.8). A small reduction (Ds = 0.2) of sludge compressibility occurred when gypsum was added. This might be the reason that skeleton builders could cause a marked improvement of dewatering capacity by produce a more incompressible cake structure as described by Zall et al. [6]. Table 3 Result of tests for the effects of additional manners on sludge conditioning (raw sludge SRF = 67.9×1012 m kg−1)
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5. Discussion
Fig. 6. Results of compressibility tests.
It is useful to discover whether the skeleton materials themselves, especially the gypsum, have a role in sludge conditioning and dewatering. For this reason, a conditioning test with the addition of gypsum (without polymer) was carried out. Fig. 7 shows the result. It is evident that addition of gypsum by itself has little effect on the improvement of sludge filterability (comparing Fig. 7 with Fig. 4). This is because gypsum is not a flocculent. It does not have the ability to flocculate the sludge particles as is the case of the polymer. Its beneficial effects as described above must be in association with a polymer.
Fig. 7. Showing effect on sludge filterability by gypsum addition (sludge solids concentration of 6.4 –6.8 g l − 1).
Within this study, the primary purpose was to explore whether the alum sludge filterability (in terms of YN) was affected by the skeleton builders in combination with a polymer. Gypsum was selected as skeleton builder in this study because it was the most commonly used amendment for ameliorating alkaline soils. In order to provide comparative data, a non-shrinking material glass microspheres were also used. The starting point of experimental investigation follows an alum sludge conditioning procedure described for a single polymer conditioning [15]. Groups of experiment described in this paper demonstrated that the addition of glass-microsphere had only a marginal effect on YN (see Fig. 2). Residual polymer measurement shows that there was no interaction between glass-microsphere and the polymer. In contrast, experiments demonstrated that gypsum had a significant effect on YN jointly with a polymer dose of 20.0 mg l − 1. It is noted that this particular dose refers to a state of overdose (the optimum dose was 15.0 mg l − 1 as gauged by the modified SRF). Below 20.0 mg l − 1 the effects of gypsum addition are minor (see Fig. 4). Further tests indicate that gypsum poses a polymer demand (see Fig. 5), showing an interaction between gypsum and the polymer. The experiments based on the addition manner of gypsum (see Table 3) and gypsum addition without polymer (shown in Fig. 7) confirm the above view. Due to the lack of the interaction with polymer, the addition of gypsum does not produce a beneficial sludge structure. Further investigations of sludge compressibility show that the involvement of the gypsum can decrease the sludge compressibility of s from 0.83 to 0.65 (see Fig. 6), implying that it might help to build up a more porous, permeable and rigid lattice structure, which retains solid particles and allows the water to be transmitted. Measurement of the polymer residuals [14] provides valuable supporting information for discerning the interaction between polymer and skeleton builders. The beneficial effect on YN described above must be associated with polymer. This view differs from some results in literature, such as, Smith et al. [16], Nelson and Brattlof [17] and
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Zouboulis and Guitonas [5], in which they only added the skeleton materials to obtain a significant effect on sludge dewaterability. However, it is noted that when skeleton materials were used it is not uncommon to add up to 5 kg conditioners solids for each 1 kg sludge, as pointed by Smith et al. [16]. This arouses the suspicion of that the improvement of dewaterability may be tied in the total change of components subjected for dewatering because such additions of conditioner render the sludge the minor consituent. Nevertheless, the results about the involvement of gypsum in sludge conditioning derived from this study are at least encouraging. The importance of this study, if any, lies in the potential application of dewatered alum sludge to the land use. As is known, land application of sludge is not a new concept. Sewage and lime sludges have been applied to land for long time. Both sludges improve soil fertility and are used as soil amendments in agriculture and land reclamation. However, currently, alum sludge is directly discharged or landfilled because it is relatively inert, providing marginal, if any, benefits to soil fertility. Therefore, research work dwelling the potential application of alum sludge is highly desirable. For example, it is noted in literature that attention is paid to the disposal of alum sludge in spite of the limited information, e.g. Geertsema et al. [9] carried out 30 months investigation into the effects of the land use of alum sludge. Measurements at the original field site included soil analysis, soil water monitoring, groundwater monitoring, and analysis of tissues from pine needles. From their study, it was concluded that an alum sludge may be applied to forest lands at loading rates of at least 1.5– 2.5% by dry weight with no long-term adverse effect being observed. Overall, the involvement of gypsum in alum sludge treatment will be another possible approach to enhance its disposal. However, more work needs to be carried out.
6. Conclusions
It has been demonstrated that the gypsum can be used as skeletal builder in alum sludge con-
ditioning to improve its filterability. The presence of gypsum helps to build up a more porous, permeable and rigid lattice structure of sludge cake. However, this beneficial effect must be associated with use of a polymer. Enhanced alum sludge filterability lies in the interaction between gypsum and polymer since gypsum is demonstrated to possess polymer demand. It is considered that application of gypsum in alum sludge treatment may enhance its potential for disposal to land.
Acknowledgements The authors thank West of Scotland Water for the provision of material and access to a number of water treatment works.
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