Environ. Sci. Technol. 2001, 35, 4691-4696
Salinity Effect on Mechanical Dewatering of Sludge with and without Chemical Conditioning I R E N E M . C . L O , * ,† KEITH C. K. LAI,† AND G. H. CHEN‡ Department of Civil Engineering and Department of Chemical Engineering, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong
The salinity levels of wastewater and sludge are relatively high in some coastal cities as they may use saline water for toilet flushing, and as such, the sludge dewaterability can be affected by it. The salinity effect on sludge dewaterability was therefore investigated through experimental testing of specific resistance in filtration (SRF), time to filter (TTF), and final solid content of sludge. SRF and TTF were determined using Buchner funnel tests. The final solid content was estimated by centrifuging the sludge at four levels of rotational speed. Sludge with three salinity levels (5 000, 10 000 and 20 000 ppm) were considered in this study. Coagulants such as alum, iron(II) sulfate, and organic polyelectrolytes were added to the sludge to study the dewaterability of such sludge with chemical conditioning. Experimental results show that doubling the salinity level of the sludge from 10 000 to 20 000 ppm shows not much change in SRF and TTF. Compared with the sludge without chemical conditioning, the addition of the coagulants to the sludge at a salinity level of 5000 ppm drastically reduces its SRF and TTF. However, sludge with and without chemical conditioning at a salinity of 20 000 ppm has similar SRF and TTF. The final solid content of sludge increases almost linearly with salinity. Among the coagulants used in this study, the cationic polyelectrolyte is found to be better in improving sludge dewaterability, while iron(II) sulfate performs slightly better in enhancing the final solid content of the sludge.
Introduction Sludge dewatering is a paramount process in wastewater treatment systems as it reduces the volume of sludge and, consequently, the costs for transporting the sludge to its ultimate disposal site. Moreover, dewatered sludge is generally much easier to handle and transport. In addition, removal of moisture from sludge can increase the energy content for incineration, reduce the requirement for supplemental bulking agents during composting, and decrease the amount of leachate production in landfills. It can also help render the sludge nonputrescible (1-3). Drying bed, lagoon, belt filter press, vacuum filter, and centrifuge are some common sludge dewatering methods. The required capital outlay for a sludge drying bed is the lowest among these methods, but a large area of land is * Corresponding author phone: +852-2358-7157; fax: +8522358-1534; e-mail:
[email protected]. † Department of Civil Engineering. ‡ Department of Chemical Engineering. 10.1021/es010834x CCC: $20.00 Published on Web 11/02/2001
2001 American Chemical Society
required for this option. A sludge lagoon also needs a large area of land for dewatering and may cause odor problems and groundwater contamination. A belt filter press produces a dry sludge cake. It consumes little energy and has reasonable capital and operating costs. The vacuum filter requires minimum maintenance, although it consumes the most energy of these options. A solid bowl centrifuge can also provide a dry sludge cake at low capital cost, although this technology demands a fair amount of maintenance. In general, the mechanical dewatering devices such as the belt filter press, vacuum filter, and centrifuge are commonly employed for sludge dewatering (3). The efficiency of mechanical dewatering depends mainly on the dewaterability of the sludge. It has been found that the cellulose content, pH value, particle charge, organic content, solid concentration, grease content, compressibility coefficient, mechanical strength of the particles, floc density, and floc size of the sludge can affect the sludge dewaterability in one way or another (4). The floc size and density are found to be the major factors affecting dewaterability. A sludge with higher floc density and larger floc size is much easier to dewater and settle (5). Particle agglomeration is the main mechanism that increases floc size and density. There are two mechanisms that promote particle agglomeration: coagulation and flocculation (1, 6, 7). Most sludge particles in nature are negatively charged, and their surfaces are surrounded by cations that form an electrical double layer. This electrical double layer creates a repulsive force among the particles and prevents them from agglomerating into large floc (3). Coagulation is therefore required to neutralize the surface charge of the colloidal material or the sludge particles in order to minimize the repulsive forces. As coagulation can disrupt the electrical double layer of the particles, the coagulated sludge becomes tightly packed and is able to release more bound water from the sludge to the bulk solution (2). When polyelectrolytes are used as flocculants, flocculation can occur by bridging the colloidal particles through long polymer chains without disrupting the electrical double layer of the particles. Thus, flocculated sludge is loosely packed and therefore allows more water drainage (1). Most sewage treatment plants add chemicals to sludge for improving sludge dewaterability. These chemicals include alum, iron(III) chloride, iron(II) sulfate, and organic polyelectrolytes. They are used to coagulate and flocculate the particles and to improve the dewaterability of the sludge (1, 6). In addition, salinity is also believed to have a significant effect on sludge dewaterability (8-10). Research to date in this area focuses only on the effect of salinity on the physical characteristics of sludge floc or on its effect on a sewage treatment system (8, 9, 11, 12). The direct effect of salinity on the mechanical dewatering of sludge has not yet been reported. The effect of salinity on sludge dewatering is both academically and practically important. In Hong Kong and other coastal areas, seawater is used in municipal flushing systems. In such areas, the salinity content in sludge should be higher than the conventional sludge. We therefore focus on the effect of salinity on mechanical dewatering of sludge and the dewaterability of sludge with chemical conditioning in this study.
Experimental Methods Sludge Collection and Its Characteristics. Biological sludge was collected from sludge digestion tanks of the Tai Po sewage treatment plant in Hong Kong. This plant treats municipal and industrial sewage generated from the Tai Po New Town VOL. 35, NO. 23, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Characteristics of Sewage Sludge Generated from Tai Po Treatment Works in Hong Konga parameters
sewage sludge
moisture content (%)
97.3
pH total solids (ppm)
6.7 27,360
volatile solids (ppm)
15 300
VS to TS (%) suspended solids (ppm)
55.8 18,400
salinity (ppm)
11,200
ammonia (ppm)
5.53
nitrate (ppm) phosphate (ppm)
4.36 8.47
total phosphate (ppm)
267.0
organic nitrogen (ppm) potassium (ppm)
993.0 224.5
nickel (ppm) zinc (ppm) copper (ppm) lead (ppm) cadmium (ppm) chromium (ppm)
6.99 32.6 20.7 19.7 3.01 18.2
a
methods used drying in oven at 103-105 °C for 24 h pH meter drying in oven at 103-105 °C for 24 h drying in oven at 103-105 °C for 24 h and burning in furnace at 500 ( 50 °C for 1 h applying vacuum filtration and drying in oven at 103-105 °C for 24 h electrical conductivity method distillation and phenate method ion chromatography vanadomolybdophosphoric acid colorimetric sulfuric acid-nitric acid digestion and vanadomolybdophosphoric acid colorimetric method semi-micro-Kjeldahl method microwave-assisted digestion and inductively coupled plasma method same as above same as above same as above same as above same as above same as above
Note: All data are done in triplicate and expressed on a wet basis.
development and the Tai Po Industrial Estate and has a capacity of 94 300 m3 of sewage per day. Sludge from the Tai Po sewage treatment plant is activated sludge and has undergone thickening and anaerobic digestion processes. During the thickening process, a certain amount of anionic polyelectrolyte is added to the sludge for coagulation and flocculation (13). The collected sludge samples were stored in a refrigerator at 4 °C. They were discarded after 1 month. Another set of sludge samples was then collected and characterized. The sludge characteristics in terms of moisture content, suspended solids, salinity, volatile solids, nitrate, ammonia, organic nitrogen, phosphate, and certain heavy metal contents were determined based on standard methods (14). Table 1 provides a brief description of the standard methods used in this study and the characteristics of the biological sludge. As expected, the sludge samples contained high levels of salinity with an average of 11 200 ppm and total solids of 27 360 ppm. Sludge Preparation and Chemical Dosage. In this study, the salinity of sludge was tested at three levels (5000, 10 000 and 20 000 ppm, which corresponds to half of, equal to, and twice the average value of the salinity in the sludge samples, respectively). In preparing the sludge with a salinity of 5000 ppm, 800 mL of original sludge was centrifuged (Allegra 6 Centrifuge, Beckman) at 1000 rpm equivalent to 228 G for 30 min to separate the sludge into two layers: the supernatant and the concentrated sludge (15). The supernatant was replaced with equal amounts of deionized water. Then the salinity of the 800 mL of centrifuged sludge was measured with a salinity meter (YSI 85/10 FT). The amount of salt required to adjust 1000 mL of the sludge to 5000 ppm was calculated and dissolved in 200 mL of deionized water. Finally, 200 mL of salty water was mixed with the 800 mL of centrifuged sludge to produce 1000 mL of sludge with 5000 4692
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ppm salinity. The other two sludges with 10 000 and 20 000 ppm salinities were prepared by mixing 200 mL of salt solution with the correct amount of salt with 800 mL of the original sludge. The sludge mixing was obtained by using a blade impeller with 6.5 cm diameter (Heidolph RZR1) at 60 rpm for 5 min (16). The mixture was maintained in a static state for 15 min to reach equilibrium before any tests were performed (8). The mixing procedure so employed would somewhat affect the floc size and properties, which in turn may influence the dewatering characteristics. Thus, care was taken to ensure that the same procedure was applied to all the samples. The solid content of the sludge at different salinity levels with and without chemical additions was fixed at 2.5%. Since chemicals such as alum, iron(II) sulfate (copperas), and organic polyelectrolytes are commonly used for promoting sludge coagulation and flocculation (17), the salinity effect on mechanical dewatering processes for sludge conditioned by those chemicals was studied. The other reason for the selection of iron(II) and alum is due to the fact that these two chemicals have been short-listed for the primary treatment of wastewater in Hong Kong. For each type of chemical, two dosages were used: one at the typical value used for activated sludge (17) and the other at double the typical value. It is important to note that the dosages used in this study might not be the optimal amount as this is not the objective of this study. The selection of second dose as twice the amount of the first dose is to ensure that the amount is beyond but not too much away from the typical value. Table 2 shows the characteristics, dosages, and sources of the chemicals. After adding the desired chemical dosage into sludge, it was first mixed at 220 rpm (0.6-0.9 m/s) for 5 min using a blade impeller, and then the sludge was stirred at 50 rpm for another 5 min. Finally, it was left in a static state for 20 min (3, 18). SRF and TTF Measurements. The dewaterability of sludge was measured by the Buchner funnel test, one of the most common methods used in dewaterabilty measurement (2), and expressed in terms of specific resistance in filtration, SRF (17), and time to filter, TTF (14). The theoretical basis for this method is available elsewhere (14, 17, 19). Figure 1 shows the experimental setup of the Buchner funnel test. In each test, 200 mL of sludge was poured into the funnel fitted with a filter paper (Whatman No. 1). After 2 min of gravitational drainage, a vacuum of 50 kPa was applied. Then the volume of the filtrate collected at different times was recorded until no additional water flowed out of the sludge (2). TTF is defined as the time required to collect 100 mL of filtrate, and SRF is calculated by using the following equation (17):
SRF )
2bPA2 µc
(1)
where P is the pressure of filtration, N/m2; A is the filtration area, m2; µ is the filtrate viscosity, N (s)/m2; c is the weight of solids/unit volume of filtrate, kg/m3 ) 1/Ci/(100 - Ci) Cf/(100 - Cf); Ci is the initial moisture content, %; Cf is the final moisture content, %; b is the slope determined from the t/vol vs vol plot; vol is the volume of filtrate, m3; and t is the filtration time, s. Final Solid Content Measurement. Besides the dewaterability of sludge, the final solid content of the sludge after mechanical dewatering was also estimated using a centrifugal settling test (16, 20). In each test, 50 mL of sludge was poured into a 100-mL centrifuge glass tube (AMK Glass AST 9240), and then it was rotated in the digitally controlled centrifuge (Sorvall RC-3C Plus with HB-6000A rotor) for 30 min. Four levels of rotational speeds were employed: 1000, 2000, 3000 and 4000 rpm equivalent to 268, 1072, 2413 and 4289 G,
TABLE 2. Types and Dosage of Chemicals for Sludge Coagulation and Flocculation chemical
chem composition
aluma (catalogue no. 22761-7) copperasb (lot no. 2115A) cationic polymerc (catalogue no. 21743)
Al2(SO4)3‚18H2O FeSO4‚7H2O poly(acrylamide/2-methacryloxyethltrimethylammonium bromide) poly(acrylamide/acrylic acid) polyacrylamide
anionic polymerc (catalogue no. 18545) nonionic polymerc (catalogue no. 02806) a
Aldrich Chemical Co., Inc.
b
mol wt 666 278 50 000
dosage (ppm) 162.5, 325 135, 270 3.5, 7.0
>10 000 000 5 000 000-6 000 000
0.625, 1.25 0.625, 1.25
Riedel-deHae¨ n. c Polysciences, Inc.
final solid content ) VFSSc VFSSc + [VFS(1 - Sc) - (V - V∞)Fw]
× 100% (2)
where V is the volume of sludge sample, mL; FS is the density of sludge, g/mL; Sc is the initial solid content of sludge; V∞ is the sediment volume at infinite speed, mL; and Fw is the density of water, g/mL.
Results and Discussion
FIGURE 1. Experimental setup of Buchner funnel test.
FIGURE 2. Plot of sediment volume against the reciprocal of centrifugal force. respectively. It was found that 30 min of centrifugation was sufficient for the sediment volume to reach a stable value. The theoretical basis of this method is that the sediment volume increases linearly with the reciprocal of the centrifugal acceleration. The final sediment volume of the sludge corresponding to an infinite rotational speed is the y-intercept of the straight line. Figure 2 shows one of the typical data sets for the plot of sediment volume vs reciprocal of speed, 1/xRCA. By knowing the initial solid content of the sludge, the final solid content of the sediment under infinite rotational speed can be calculated based on the following equation:
Sludge Dewaterability. Figure 3 shows the SRF of sludge under three different salinity levels. Without chemical addition, the SRF of sludge decreases with increasing salinity. There is about 30% reduction in SRF by doubling the salinity level. This trend is consistent with the theoretical analysis of Derjaugin, Landau, Verwey, and Overbeek (called DLVO) theory (8-10). According to the DLVO theory, increasing the salt concentration in the sludge can compress the thickness of double layers and reduce the surface charge of particles, thereby promoting particle agglomeration. In addition, the presence of salt in the sludge might cause a salting out effect, which in turn helps release bound water from the flocs to the bulk solution. With the addition of chemicals to low salinity sludge, the SRF is significantly reduced, probably because the chemicals added are typical coagulants that have proved efficient in improving the dewaterability of sludge and the chemical dosage used in this study was sufficient for compressing the double layer thickness. However, in the high salinity (20 000 ppm) sludge, the effect of chemical coagulants on SRF is less significant. This finding reveals a fact that the higher the salt content of sludge, the lower the chemical dosage that is required to meet a desired SRF. As shown in Figure 3, it is also found that the SRF of sludge conditioned by organic polyelectrolytes is generally lower than that conditioned by alum and iron(II) sulfate. This means that organic polyelectrolytes are more effective than inorganic chemicals in the improvement of sludge dewaterability. This is probably because the flocs induced by organic polyelectrolytes are larger and more loosely packed than the flocs by inorganic chemicals (1). This results in more void spaces and allowing more water drainage. Among the three organic polyelectrolytes used in this study, cationic polyelectrolyte shows the best performance, while anionic polyelectrolyte shows the least. As sludge particles have a negative charge in nature, the polymer chains of cationic polyelectrolytes are in favor of catching the negatively charged sludge particles through a coagulation-bridging phenomenon (3). For the nonionic and anionic polyelectrolytes, particle agglomeration can occur through interparticle bridging mechanisms (3). However, the repulsive force between the negative surface charge of the sludge particles and the anionic polyelectrolyte makes it difficult for the polymer chain to catch sludge particles. A comparison of the alum and iron(II) sulfate in Figure 3 shows that alum provides lower SRF and better sludge dewaterability. In principle, iron(II) sulfate and alum dissociate into Fe2+, SO42-, and Al(H2O)63+, SO42-, respectively, when dissolved in water, thereby neuVOL. 35, NO. 23, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 3. SRF vs different salinity levels of sludge conditioned by chemicals at typical dosage.
FIGURE 4. SRF vs different salinity levels of sludge conditioned by chemicals at higher dosage. tralizing the surface charges and compressing the electrical double layers of particles. Since alum can further undergo hydrolysis and polymerization for producing polymers with a long chain, particle agglomeration is then promoted by both bridging mechanisms and charge neutralization (25). For this reason, it is not surprising to find that alum is more effective than iron(II) sulfate in improving sludge dewaterability. Figure 4 shows that doubling the dosage of chemicals does not provide further improvement to SRF. This is particularly true when the sludge has a higher salinity level
(20 000 ppm). Comparing the SRF values of sludge at a salinity of 20 000 ppm with and without chemical addition shows that the SRF values all merge to about 2.0 × 1013 m/kg. This further supports the finding shown in Figure 3 of the effect of salinity on sludge dewaterability being similar to that of chemical coagulants when the sludge reaches a high salinity level. In summary, the use of chemical coagulants in improving SRF is more effective for sludge with low salinity levels. In high salinity sludge, salt performs similar to and behaves very much like a chemical coagulant. To compare the dewaterability of our sludge with salt content with conventional sludge at almost zero salinity, the SRF of sludge samples without salt obtained from different sources is plotted in Figure 4. The source, chemical coagulants used, and properties of these samples are given in Table 3. Samples A and D are chemical sludges obtained from a primary clarifier, while samples B and C are biological sludges produced after the sludge digestion process. The SRF values of samples B-D are similar to that of the present sludge conditioned by chemical coagulants. This comparison supports our findings that the effect of salinity is less significant if sludge with low levels of salinity has undergone chemical conditioning. Although samples A and D are similar, their SRF values are very different. Perhaps this is due to the types of anionic polyelectrolytes used and the different sources of the sludge. The data shown in Figure 4 illustrate the range of SRF of sludge obtained from difference sources and can be used for references, but direct comparison is rather difficult as sludge dewaterability is a function of the raw waste characteristics and the types of treatment processes used in a treatment system. The experimental data on the TTF of the sludge at different salinity levels with and without chemical conditioning is shown in Figure 5. The TTF decreases with salinity when there is no chemical addition. TTF increases slightly with salinity in the presence of chemicals may be explained by the slight increase of SRF in the range of salinity between 5000 and 10 000 ppm. Therefore, we conclude that the TTF results are consistent with the SRF results (refer to Figure 3). The TTF of the sludge conditioned with organic polyelectrolytes is generally better than that of the sludge conditioned with inorganic coagulants, while cationic and nonionic polyelectrolytes have better performance than anionic polyelectrolyte. Since TTF is comparably easier to measure than SRF, it can be used as a preliminary check to illustrate the general trend of sludge dewaterability prior to testing SRF. Final Solid Content. The SRF reveals the relative ease of sludge to be mechanically dewatered, while the final solid
TABLE 3. Sludge Generated from Other Water or Sewage Treatment Plants symbol
source
A
primary clarifier of waterworks
B
wastewater treatment plant at Pakmaya, Izmit
C
Bedok sewage treatment plant in Singapore clarifier of Burncrooks water treatment works
D
F G
9
ref
comment
22 alum sludge derived from treatment of low 3.85 mg/L anionic turbidity colored water polyelectrolyte (Magnaflocs LT25) 15 mg/L cationic 18 anaerobic digested sludge produced from sewage polyelectrolyte treatment plant that mainly treats sewage from (Zetag-57) baker’s yeast production plant 12% of lime 23 aerobic digested sludge containing 3.5% oil content
3.85 mg/L anionic polyelectrolyte (Flocmiser 50) wastewater treatment plant of without chemical the Hsinpu Fiber Plant in Taiwan treatment municipal wastewater treatment without chemical plant in Japan treatment municipal wastewater treatment without chemical plant in Japan treatment
E
4694
chem conditioning
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24 alum sludge derived from low turbidity, colored water of upland catchment 27 activated sludge with fibers inside; final solid content was measured by centrifugal settling test 20 activated sludge; final solid content was measured by drying method 20 activated sludge; final solid content was measured by centrifugal settling test
FIGURE 5. TTF vs different salinity levels of sludge conditioned by chemicals at typical dosage.
FIGURE 6. Change of final solid content of sludge with salinity in the presence of chemical coagulants at typical dosage.
content increases with increasing salinity content in sludge. The final solid content of sludge increases by an average of 4-9% when the sludge salinity is doubled from 10 000 to 20 000 ppm. In contrast to SRF as shown in Figure 3, the influence of chemical coagulants on the final solid content of sludge is highly related to its salinity. Such an observation suggests that the salting out effect might prevail in determining the final solid content and the bound water of sludge. This is because in biological sludge, the amount of bound water is closely associated with microorganisms and the osmotic effect through the membrane of microorganisms can drain the water out. As shown in Figures 6 and 7, among different chemicals, iron(II) sulfate gives the highest final solid content while anionic and cationic polyelectrolytes do not affect the final solid concentration too much. This implies that the salinity effects are more important in enhancing the solid content. The reason why the sludge conditioned by iron(II) sulfate achieves higher solid content is unknown. It certainly requires further study. Besides, it is interesting to note that all the chemicals used in this study have some positive effects in enhancing the final solid content of sludge except for the nonionic polymer. The insignificant effect of the nonionic polymer on the final solid content is not due to the low dosage because when the dosage was doubled, the final solid content remained almost the same (refer to Figures 6 and 7). This indicates that the charge groups of the polyelectrolytes might have some influences on the final solid content (1). As shown in Figure 7, the final solid contents of the sludge obtained in this study are higher than those of the activated sludges generated from the municipal wastewater plant in Japan (sludges F and G) and similar to those of activated sludge from a fiber plant in Taiwan. The nature and properties of sludges E-G are listed in Table 3. It is necessary to point out that sludges E-G have not had any chemical conditioning. Therefore, the lower final solid contents of sludges F and G are somewhat expected. Since sludge E came from a fiber processing plant, its higher solid content is perhaps due to the presence of fibers that cause some degree of agglomeration of colloidal particles.
Acknowledgments This research work was supported in part by ECWW 98/99, EG09 from the Environment and Conservation Fund, Hong Kong. The authors also thank Mr. Chen Xue Ming and Mr. S. T. Lui for their technical support and Ms. Frances Chuen for permission to use the centrifuge in the Department of Biochemistry.
Literature Cited
FIGURE 7. Change of final solid content of sludge with salinity in the presence of chemical coagulants at higher dosage. content reflects the degree of slurry in sludge after mechanical pressing or centrifugation. It is easier to handle and transport a sludge cake if it has a higher solid content. The term final solid content in this study is defined as the solid content of sludge under infinite rotational speed in a centrifuge. It is related to the bound water content of sludge. Sludge with higher final solid content has lower bound water content (20). Figures 6 and 7 delineate the final solid content of sludge at different salinity levels. When the salinity of sludge reaches 20 000 ppm, the final solid content is all over 27% even in sludge that has not undergone any chemical conditioning. In sludge conditioned by iron(II) sulfate, the final solid content can be as high as 34%. Generally, the final solid
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Received for review April 9, 2001. Revised manuscript received August 27, 2001. Accepted September 5, 2001. ES010834X
