Environ. Sci. Technol. 1994, 28, 7444-1449
Fast Freeze/Thaw Treatment on Excess Activated Sludges: Floc Structure and Sludge Dewaterability Duu Jong Lee’gt and Yuan Hway MsuS Department of Chemical Engineering, National Taiwan University, Taipei, Taiwan, 10617, Republic of China, and Department of Chemical Engineering, Yuan-Ze Institute of Technology, Taoyuan, Taiwan, 32026, Republic of China
The effects of a fast freezelthaw treatment (with freezing speed about 40 mmih) on activated sludge dewaterability were demonstrated by conducting drying, centrifugal settling, and vacuum filtration tests. After a freeze/thaw treatment, the BOD and COD values of the sludge supernatant were found to raise significantly, the moisture movement resistance and the bound water content of the sludge cake both decreased, the floc volume and sludge compressibility were reduced, and the sludge filterability was improved greatly. The fractal dimension of the original and frozen sludges was measured via the sizedensity relationship. A simple structural interpretation based on fractal geometry was proposed to explain the experimental findings.
Introduction
Sludge dewatering is one of the most difficult problems to wastewater treatments. The water distribution within the slurry is highly restricted by the floc structure and forms the basis for further understanding of many processes (1-5). Freezelthaw processes have been investigated extensively (6-25). It is found that this physical process can significantly improve certain sludge dewatering characteristics and change the floc structure, usually irreversibly, into a more compact form (e.g., see ref 2 5 ) . The suspended particles tend to agglomerate and form larger flocs within which the bound water content is reduced (17-21). In the freezelthaw treatment, the freezing speed is found to be critical, and instant freezing is inadequate (10,26). It is generally accepted that sludge dewatering efficiency will decrease with increasing freezing speed; however, a long freezing time will cause the process to be uneconomic when compared with other conditioningldewateringprocesses (12,27). This constraint attracts attention to cheap natural freezeithaw conditioning in some cold regions (9, 10,13,22-24). Further information about the fast freezei thaw process but still with sufficient effectiveness is desired. When treating activated sludges, the freezing speed usually applied is around 2-30 mmlh (10,15-21). Therefore, in this work, tests with the average freezing speed above these values is referred as a “fast” process. There are various tests employed for estimating sludge dewaterability. Some are drying (2, 3, 5 , 21, 28, 29), centrifugal settling (18,20,21), and filtration (14,30,31). However, the resulting sludge characteristics are not usually discussed based on a more fundamental structural basis. This is partially due to the fact that the floc shape
* E-mail address:
-
[email protected]. + National Taiwan University. 1 Yuan-Ze Institute of Technology. 1444
Environ. Sci. Technol., Vol. 28, No. 8, 1994
Table 1. Properties of Four Activated Sludges. % (wiw)
BOD
COD
SS
Pa
no.
(g/m3)
(g/m3)
(g/m3)
(kg/m3)
(kg/m3)
lr If 2r 2f 3r 3f 417 4f
0.558 0.558 0.690 0.690 0.936 0.936 0.886 0.886
15 1060 22 688 21 401 19 493
84 1620 104 1040 100 646 96 897
17
1450 1459 1452 1436 1448 1455 1457 1423
1005.5 1005.5 1006.0 1006.0 1007.3 1007.3 1007.0 1007.0
20
23 14
Pb
a The BOD, COD, and SS data are for the sludge supernatant and were measured according to EPA (Taiwan) standard methods 501.1, 515.1, and 210.1, respectively. r, original sludges; f, frozen sludges.
is highly irregular, and several attempts at developing a detailed structural model have failed (32, 33). Recently, based on the floc size-density relationship constructed via the free-settling tests (32,34-38), fractal geometry was found to be an adequate tool in describing the floc structure (37,38). The fractal concept developed by Mandelbrot (39) provides a method for describing many objects “made of parts similar to the whole in some way” (40) with the fractal dimension as an index for measuring quantitatively how the particles fill the space. For activated sludge flocs, the fractal dimension is found to range from 1.45 to 2.07 (37). It is believed by the authors that the fractal concept is potentially very useful in further studies on sludge transport/bioreaction processes. In this paper, the effects of a fast freezelthaw process on activated sludge dewaterability (demonstrated by results of drying, centrifugal settling,and vacuum filtration tests) are described. Possible explanation for the experimental findings based on the floc structure information is discussed. Experimental Section
Sludge Samples. Activated sludge samples were taken from the wastewater treatment plant in the Hsinpu Fiber Plant, Far Eastern Textile Ltd., Hsinchu, Taiwan, and were tested within 2 h after sampling. The weight percent of the solid phase was measured by weighing and drying. The bulk sludge density was measured by a digital density meter with accuracy up to 0.1 kg/m3,and the solid density was measured using a pycnometer. The BOD,COD,and SS data of the sludge supernatant were obtained according toEPA (Taiwan) standardmethods 501.1,515.1,and 210.1, respectively. Since the sludges studied in this work all behaved similarly, only four samples are chosen for further discussions. The characteristics of these sludges are listed in Table 1. Experimental Methods. ( a )Freeze/ Thaw Treatment. A pool of waterlethylene glycol mixture (50150 wiw),whose temperature was set at -15 “C and maintained by 0013-936X/94/0928-1444$04.50/0
0 1994 Americar
Chemical Society
circulating the refrigerant through an immersed coil, was employed as the freezing pool. The sludge sample was placed in a PP plastic bottle 12 cm in both in diameter and height. When an experiment started, 5-10 sample bottles were instantly immersed in the freezing pool, and ice crystals could be observed to form gradually from the bottle surface into the core region. Since the conduction heat transfer becomes more difficult as the ice layer grows, the freezing speed will not be a constant. However, the effect is compensated for to some extent due to the bottle-shape effect. The time needed for complete freezing of the sample was about 1.5 h, which gives an average freezing speed of about 40 mm/h. As mentioned above, this freezing speed is a 'fast" freeze/ thaw process. Following the freezing process, the frozen samples were thawed under room temperature for another 18 h. The original and the treated sludges are referred as the original sludge and the frozen sludge, respectively. (b)Measurementofsludge Dewaterability. Adigitally controlled centrifuge (Kuhota 2100) with an arm length of 13.5 cm and a rotational speed of W O O 0 rpm was used in the centrifugal settling tests. The testing time was set as 1h since it is found that the sediment height will keep unchanged within about 0.5 h. The sediment height was then measured and referred as the equilibrium height of sludge. Four tubes of a slurry with an initial height of 9 cm were centrifuged simultaneously, and the equilibrium heights were recorded and averaged under various rotational speeds. The equilibrium height data versus rotational speed data were utilized for estimating the sludge compressibility. A constant temperature/bumidity dryingapparatus was employed for the drying test. The drying temperature was kept at 40 "C, and an electronic balance connected to a personal computer automatically recorded the sample weight during an experiment. A typical testing time was about 24 h for the system to attain an equilibrium state. The residue water content remaining in the cake was determined by further drying the cake a t 102 "C. The samples utilized in a drying test were prepared first by vacuum filtering the sludge into a wet cake to remove most of the free moisture. Then, the wet cake was shaped intoadiskwithfixedthickness(lrnm)andarea(liPm2). The sample weight was kept a t approximately 11g. A vacuum filtration system was installed and described elsewhere (41). The filtrate weight was also recorded as a function of time by an electronic balance and a personal computer. The pressure difference was fixed at 62 cmHg, andthefilterareawas1.03 X 10-3m2. Standardprocedures were applied to determine the average specific resistance of cake (31). (c) Size-Density Relationship. A pool (10 cm in diameter and 50 cm in height) was used for constructing the size-density relationship (35). Before testing, a mechanical stirrer with a rotational speed of 60 rpm was employed for suspending the sludge. A JVC camera equipped with a closeup lens was used for recording all experiments. The floc diameter normal to the vertical direction and the terminal velocity were measured by replaying the tape. From the free-settlingdata and by assuming a constant correction factor for the nonspherical shape of flocs, the effective density can be calculated according to the modified Stoke's law as stated in ref 35.
'ti
Microphotographs 01 activated sludge flocs with a magniflcation of 249X. Sample no. 2 (a) original sludge; (b) frozen sludge. Flgure 1.
Results and Discussions General. Microphotographs of the original (sampleno. 2r) and frozen sludge (sample no. 2 0 are shown in Figure 1. Clearly the floc structure of the frozen sludge is much more compact than the original sludge (with darker interior). It is also noted from Table 1 that the freeze/ thawtreatmentwillcause theBOD andCOD of thesludge supernatant to increase significantly. The solid density and the hulk density are immaterial for the original and the frozen sludges. Visual observation suggests that the compact frozen sludge flocs possess better gravitational settling and that the final sediment height is less than for the original sludge. However, it is also noted that the frozen sludge is quite fragile and that weak agitation intensity can tear the flocs into microflocs with smaller sizes. The strength of these microflon isstrong enough to sustain their structureunder higher agitation intensity. It seems likethecompad frozen flociscomposedof manyweaklylinkedmicroflocsofmuch stronger interior strength. Centrifugal Settling Tests. The results for the centrifugal settling tests are summarized in Figure 2. The initial heightsofthe frozensludgearelower thantheinitial height of the original sludge. When the rotational speed increases, the sediment heights for both the original and frozen sludges decreases, with a higher decreasing rate for Envhon. Sei. Technol.. VoI. 28. NO.8. 1994
1445
- 0.4
I
I
'
I
'
'
I
'
No. original frozen
- 0.5 - 0.6
e
I
l 2 3
a . *
4
.
0 r]
0 0
-Os7
m -0 -0.8
- n9 No. original frozen
- 1.0 0
-1.1
0 0
1
1.6
2
1.8
2
2.2
2.6
2.4
2.8
log n
ti' (1 03mi Flgure 2. Experimental results for centrifugal settling tests; activated sludges.
Figure 3. Log hlho versus log Q ; centrifugal settling tests; activated sludges.
the original sludge, which suggests a larger sludge compressibility. The equilibrium sludge height has been found to be a linear function of N - l , from which the bound water content can be estimated (18, 20, 21). The curved relationship shown in Figure 2 indicates that one cannot use these centrifugal settling data to estimate bound water content. A more quantitative description on the sludge compressibility can be found from the relation proposed in ref 42 or in
Table 2. Experimental Results for Activated Sludges" no.
0
lr If 2r 2f 3r 3f 4r 4f
0.175 0.112 0.184 0.106 0.188
0.130 0.172 0.102
d Wldt (g/min)
Wt,
(10l1rn/kg)
S
RC
D
NA NA
NA NA
NA NA
0.0127 0.0232 0.0107 0.0174 0.0120 0.0166
0.46 0.27 0.51 0.25 0.45 0.25
63.4 0.86 22.1 0.76 49.8 0.33
-1.48 -1.51 -1.41 -1.43 -1.44 -1.47 -1.45 -1.42
0.96 0.92 0.94 0.89 0.98 0.93 0.93 0.90
1.52 1.49 1.59 1.57 1.56 1.53 1.55 1.58
aau
RC is the regressioncoefficientof linear regressionfor evaluating slopes and the fractal dimension D. NA, not applicable. 1.0
where h, is the equilibrium sediment height at infinite rotational speed limit and /3 is a measure of sludge compressibility. From eq 1,log h versus log Q data will be linear with the slope -2/3. Figure 3 is the log-log plot for the data shown in Figure 2. Clear linearity exists in all cases, and the /3 values can be evaluated via linear regression analysis with a regression coefficient greater than 0.99. The best-fitted 0values are listed in Table 2. The fast freeze/thaw process used, therefore, reduced the sludge compressibility by more than
I
I
I
I
2
4
6
0.5
3 Q
40%.
Within experimental ranges, the sediment height for the frozen sludge is less than the height for the original sludge, though the latter possessesa higher compresibility. Drying Tests. The data for the drying tests are summarized in Figure 4. The drying curves for the original sludges are located to the right of the frozen sludges. The lower drying rate for the original sludge indicates a higher moisture movement resistance for the free water in an original sludge cake. When a porous media is dried under a constant temperature/humidity environment, the drying curve can be roughly distinguished into a constant-rate period, the (first and the second) falling-rate period@), and an equilibrium stage (43). The transition between the constant-rate period and the falling-rate period is usually interpreted as a signal of the exhaustion of the free water, 1446
Envlron. Scl. Technol., Vol. 28, No. 8, 1994
0 0
8
t ,lo4
Flgure 4. Experimental results for drying tests; activated sludge. Sludge thickness is 1 mm, and the drying area Is 0.01 m2.
and the remaining water content is classified as the bound water (2, 3, 21). The transition points can be easily located in all tests in Figure 4, and the dimensionlessweight for the transition points, W ~ ; Sare , listed in Table 2. All Wt, data for the
50
1
N o . original frozen
I
40
1 2 3
3 3 3
0
8
E x"
0; 30 3 0 0 0
4
a
0
w
.
0
+
0 C
20
10
0
0
20 40 60 80 100 120 140 160 180 200
0
0.2
0.4
0.8
0.6
1.0
1.2
1.4
t,s
Flgure 5. Experimental results for vacuum filtration tests. Pressure m2. difference is 62 cmHg, and the filter area is 1.03 X
frozen sludges are less than the original sludges. The results demonstrate that there exists a larger portion of moisture within the original sludgewhich is attached firmly onto the flocs than the frozen ones, Le., the effect of freeze/ thaw treatment releases some bound water in the sludges to the bulk solution, which is consistent with the conclusion drawn in ref 21. The freeze/thaw process increases the drying rate for the constant-rate period by about 60% and reduces the bound water content by almost one-half, which suggests a dramatic structural change of flocs, as observed in microphotographs (15; Figure 1). Vacuum Filtration Tests. The data from filtration tests are summarized in Figure 5. Under the same pressure difference (62 cmHg),the filtrate flow rate from the frozen sludge is much higher than that from the original sludge, Le., the sludge dewaterability is improved significantly. The filtration data can be used to calculate the average specific resistance of the cakes. The results are also listed in Table 2 for comparison. The data for the frozen sludges show about 2 orders of magnitude reduction in resistance of the original sludges. Actually, the frozen sludges can be dewatered almost completely via simple gravitational filtration. An original sludge is highly compressible, and the local porosity is a strong function of compressive pressure (31, 44, 45), which can produce relatively high resistance to filtration (31). The decrease of sludge compressibility and floc volume might both be responsible for the dramatic reduction in the cake resistance. Size-Density Relationship. For each sludge sample, more than several hundreds of free-settling tests were conducted, and only cases where the individual floc can fall free of the disturbance of other flocs were measured. The relationship between floc diameter and the effective density are demonstrated in Figure 6, with closed symbols for original sludges and open symbols for frozen sludges. The size-density data for all original and frozen sludges locate on a single curve. The floc density is close to the water density when the floc size is large, and it increases quickly with decreasing floc diameter. The effective floc density ranges from about 0.2 to as high as 100 kg/m3,
Dp qlO*rn Flgure 6. Ap versus Dpplot. The floc traveling distance for terminal velocity is 5 cm.
1.6 1.4
a 4
-ul
0.6 0.2
0 -0.2
-0.4
No. original frozen
l 2 3 4
a
,
0 C
-0.6 + 0 . 0 - 0.8 -10 1 " 1 1 .- ' 1 " 1 -3.4 -3.2 -3.0 -2.8 -2.6 -24 -22 -2.0 -1.8
'
'
logDp Flgure 7. Log Ap versus log 4 plot. Ap is in kg/m3, and DDis in m.
which covers the density range reported previously (46, 47). Since the density of sludge floc is a strong function of the floc size, the arguments about the individual floc density without the floc size information seem meaningless (46, 47). The data in Figure 6 are reported in Figure 7 on a loglog scale. It is clear that a linear relation, such as those reported previously, exists (34-38). The slope of the bestfitted line for all data is about -1.45, with a regression coefficient larger than 0.985. This value is also within the data range for activated sludges (37). The slopes, S, for linearly regressing each set of data in Figure 7 are listed in Table 2. These values are all close to the one obtained by regressing all data (-1.45) and might actually be a constant value. Define the mean of the slope data as Envlron. Scl. Technol., Vol. 28, No. 8, 1994
1447
and the hypothesis Sa, = Si, i = 1 ... 8, is tested by the t-distribution (48). Within a significance level of 0.05, the hypothesis is accepted, Le., the density-size relation for all sludge samples possess the same slope. This result is somewhat surprising since the common understanding is that the structure of a frozen sludge floc will be more compact than the original one. Floc Structure. A simple structural interpretation based on fractal geometry is proposed in this section to explain the experimental findings. An activated sludge floc is proposed to be a threedimensional bacteria matrix kept together by metal ions and extracellular polymer (49-51)and is identified as a fractal (37). For a fractal of radius R formed by n primary particles of radius Ro, the number-radius relation can be expressed as a power-law function as n = C1(R/Ro)D(40), where D is the fractal dimension and C1 is a proportionality constant. If the primary particle density is pp and the void portion of the fractal is fully occupied with a fluid of density p ~ , the fractal effective density can be stated as
(3) The slope of the log-log plot of the effective density and the fractal radius is D - 3. If a fractal is divided into m microflocs of equal size with the primary particle number nlm, since one of the major features of a fractal is the self-similarity property, Le., the fractal characteristics will be the same regardless of the fractal size, the following relation holds for each microfloc: (4) where R , is the microfloc radius. Under such circumstances, the volume ratio for the sum of microflocs to the original floc is
(5) From eq 5, it is clear that the volume ratio is decreased with increasing m since D is less than 3. From the linearity shown in Figure 7 and the slopes listed in Table 2, the fractal dimensions can be calculated and are listed in the last column of Table 2. It is clear that the D values are all around 1.55, indicating a rather loose structure for the activated sludge flocs. The unchanged fractal dimension after a fast freeze/thaw treatment demonstrates that the basic features of how the primary particles occupy the space within the original floc and the frozen microflocs are the same. Based on the above discussions, it is proposed that fast freeze/thaw treatments (with freezing speed at about 40 mm/h) will destroy (or weaken) the intermicrofloc but not the intramicrofloc binding strength. The resulting floc is therefore more compact but quite fragile and can be easily divided into many small microflocs with the same fractal characters. During the tearup action, the sediment volume is reduced according to eq 5, and some extracellular polymer and interstitial water between these microflocs 1448
Environ. Scl. Technol., Vol. 28, No. 8 , 1994
will be released, which decreases the bound water content and increases the BOD/COD values. The microflocs possess higher density and less compressibility. The moisture movement through channels in a filtration cake between these microflocs is therefore easier than that in an original sludge, which causes a higher drying rate and a better filterability.
Conclusions The effects of a fast freeze/thaw treatment on activated sludge dewaterability are demonstrated by conducting drying tests, centrifugal settling tests, and vacuum filtration tests. After a freeze/thaw treatment, the BOD and COD values of the supernatant are raised significantly, the moisture movement resistance and the bound water content of the sludge cake both decrease, the floc volume and the sludge compressibility are reduced, and the sludge filterability is improved greatly. The fractal dimension for the excess activated sludges, original or treated with the freeze/thaw process, is measured by the size-density relationship and is found to be about 1.55, which is independent of the freeze/thaw process. A simple structural interpretation based on fractal geometry is proposed to explain the experimental findings.
Acknowledgments The authors are grateful to HsinPu Fiber Plant, Far Eastern Co., Hsinchu, Taiwan, for providing the activated sludge samples. The authors also thank Mr. G. W. Chen for help in preparing the manuscript. This work is supported by the National Science Council, ROC, Project NSC82-0402-E002-322.
Notations proportional constants fractal dimension floc diameter (m) coefficient in eq 1 (Pa-p) gravitational acceleration (m/s2) sediment height (m) slurry initial height (m) sediment height under infinite rotational speed (m) aggregate number produced from a floc rotational speed (revertantdmin) primary particle number in a floc floc radius (m) arm length (m) aggregate radius (m) primary particle radius (m) mean slope defined in eq 2 slope for sample i terminal velocity (m/s) original sludge floc volume (m3) total volume of aggregates (m3) weight of sample (kg) dimensionless weight at transition point initial weight of sample (kg) equilibrium sample weight (kg) Greek Letters averaged specific resistance of cake (m/kg) ffav B measure of compressibility I.L liquid viscosity (Pa-s)
Pb
Pf
PL PP PS
n
slurry density (kg/m3) floc density (kg/m3) liquid density (kg/m3) primary particle density (kg/m3) solid density (kg/m3) angular velocity (Us)
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Abstract published in Advance ACS Abstracts, June 1, 1994.
Environ. Sci. Technol., Vol. 28. No. 8, 1994 1449
