Physical, Chemical and Biological Properties of Submerged ...

PHYSICAL, CHEMICAL AND BIOLOGICAL PROPERTIES OF SUBMERGED
MEMBRANE BIOREACTOR AND CONVENTIONAL ACTIVATED SLUDGES
ABSTRACT
Rion P. Merlo, R. Shane Trussell, Slawomir W. Hermanowicz, and David Jenkins
Department of Civil and Environmental Engineering
University of California at Berkeley
hermanowicz@ce.berkeley.edu
A pilot-scale submerged membrane bioreactor (SMBR) and two bench-scale conventional
activated sludge (CAS) reactors were operated on municipal primary effluent over a range of
mean cell residence times (MCRTs) from 2-10 d. The reactors had different turbulence levels.
The root mean square velocity gradient (G) of the SMBR was 632 s -1 ; for the CAS reactors the G
values were 72 s -1 and 250 s -1 . The sludges from all systems were analyzed for particle size
distribution (PSD), colloidal material, extracellular polymer substances (EPS) and filamentous
microorganisms. Capillary suction time (CST) and time to filter (TTF) analyses were also
performed. The SMBR sludges had the highest amount of small particles and higher levels of
colloidal material than the CAS sludges. This was attributed to its higher G value and the use of
a membrane for solids-liquid separation. The SMBR sludge contained the higher levels of total
filamentous organisms attributable largely to its higher nocardioform level. This resulted from
more efficient foam trapping by the SMBR. The normalized CST values of the SMBR sludge
were lower than for the CAS sludges. This was attributed to its lower EPS content. There was no
significant difference between the normalized TTF values of the SMBR and the CAS sludges.
This was attributed to the offsetting effects of colloidal material and EPS contents.
KEYWORDS
Submerged membrane bioreactor (SMBR), particle size distribution (PSD), extracellular
polymeric substances (EPS), filamentous microorganisms, nocardioform organisms, capillary
suction time (CST), time to filter (TTF)
BACKGROUND
The membrane bioreactor (MBR) process uses a membrane (either an ultrafilter or a microfilter)
to perform solids-liquid separation in place of the gravity clarifier used in typical conventional
activated sludge (CAS) systems. The membrane can be submerged in the reactor and operated
under vacuum, (submerged MBR, SMBR), or external to the reactor and operated under
pressure, (external MBR, EMBR). The MBR process can achieve high biochemical oxygen
demand (BOD) removal efficiencies (>95%) and virtually complete total suspended solids (TSS)
removals when treating domestic wastewater (Cote et al., 1998, Fan et al., 1996, Rosenberger et
al., 2002, Yamamoto et al., 1989) making it an attractive treatment option when water
reclamation or stringent effluent discharge requirements exist. MBRs are capable of all of the
unit processes of biological nutrient removal (BNR) including nitrification (Fan et al., 2000,

Trouve et al., 1994), denitrification (Adham et al., 2000, Cote et al., 1997, Murakami et al.,
2000) and enhanced biological phosphorus removal (EBPR) (Adam et al., 2002).
In the SMBR, membrane fouling is controlled by using coarse bubble aeration of the membrane
fibers to provide the cross-flow needed to prevent solids accumulation (Ueda et al., 1997). The
use of the coarse bubble aeration for fouling control and a membrane for solids-liquid separation
may influence the SMBR sludge properties. An understanding of sludge properties is needed to
predict the behaviour of waste SMBR sludge thickening, digestion and dewatering processes.
In this study, the physical, chemical and biological characteristics of CAS and SMBR sludges
grown on municipal wastewater were evaluated. Properties evaluated were: particle size
distribution (PSD), colloidal material content, extracellular polymeric substances (EPS)
concentration, filamentous bacteria content, capillary suction time (CST) and time to filter
(TTF).
MATERIALS AND METHODS
Wastewater characteristics
All reactors were fed with primary effluent from the Southeast Water Pollution Control Plant
(SEP), San Francisco, CA. The SMBR pilot plant was fed continuously through a line that took
primary effluent directly from the SEP primary clarifier effluent channel. The CAS reactors were
fed in the same way during the 10-d MCRT testing period. At all other times, the CAS reactors
were fed from a tank that was filled and emptied daily to minimize seeding the reactors with
filamentous organisms that grow on piping walls (Gabb et al., 1989). Reactor influent
characteristics are presented in Table 1.
Measurement of turbulence
The root mean square velocity gradient (G) has been used as a measure of turbulence in several
activated sludge studies (Das et al., 1993, Parker 1970). The G value for the SMBR and CAS
reactors was calculated from:
G ≅
Q air Hγ w
Vµ
(1)
where: G = root mean square velocity, s -1
Q air = air flow, m 3 /s
H = depth of water column, m
γ w = specific weight of water, N/m 3
µ = viscosity of water, Pa⋅s
V = reactor volume, m 3

The viscosity and the specific weight of sludge were assumed to be the same as water at 20ºC
(10 -3 Pa⋅s and 9800 N/m 3 , respectively). For the intermittently-aerated SMBR membrane tank,
Q air was the air flow during aeration.
Reactor Operation
The reactors were operated at several MCRTs in the range of 2-10 d. Data collection for a given
MCRT commenced only after at least 3 MCRT values at the target MCRT had elapsed. The
dissolved oxygen (DO) concentration was always ≥2 mg/L in all reactors. For the SMBR, the
MLSS concentration was held constant at 8 ± 2 g/L and the hydraulic retention time (HRT) was
varied from 1.1 to 3.6 h. The HRT of the CAS reactors was held constant at 7.1 h, and the MLSS
concentrations were varied from 0.7-3.0 g/L.
SMBR pilot plant
The SMBR was a pilot-scale ZenoGem ® (Zenon Environmental Services Inc., Burlington, ONT,
Canada), submerged membrane system (Figure 1) containing one full-scale membrane module
(60.4 m 2 surface area Zenon OCP ultrafilter, 0.035 µm nominal pore size). Solids are retained on
the outside of the membrane, and permeate passes through the membrane into the interior of the
hollow fibers. The SMBR consisted of an aeration tank and a membrane tank. Influent
wastewater was pumped through a 3-mm screen to the aeration tank (757 L working volume),
which was aerated through membrane diffusers fed with either compressed air or pure oxygen at
a flow rate of 85 L/min. The membrane was submerged in the membrane tank (833 L working
volume). Mixed liquor was pumped from the bottom of the aeration tank to the bottom of the
membrane tank using either a high-shear centrifugal pump (Teel Model 1P70, W.W. Grainger
Inc., Northbrook, IL) or a low-shear, lobe pump (Boerger LLC, Minneapolis, MN). When using
the centrifugal pump, the SMBR is said to be under “higher shearing conditions” (SMBR H);
when the lobe pump was in service, the SMBR is said to be under “lower shearing conditions”
(SMBR L). Return mixed liquor flowed by gravity from the surface of the membrane tank back
to the aeration tank surface. A vacuum pump downstream of the membrane provided transmembrane
pressure to maintain a constant permeate flux of 30 L/m 2 ⋅h. Intermittent (10 s on/10 s
off) coarse bubble aeration at a flow of 850 L/min (G = 632 s -1 ) was used to control membrane
fouling. The permeate vacuum pump was automatically turned off for 30 s every 9 min. A
portion of the membrane permeate could be returned to the membrane tank so that HRT could be
controlled without changing either the tank volume or membrane flux. Waste SMBR sludge was
removed continuously from the membrane tank using a peristaltic pump.
Bench-scale CAS reactors
The CAS reactors consisted of an aeration basin (10-L working volume) and a gravity clarifier
(4-L working volume). Mixed liquor flowed by gravity from an aeration basin surface overflow
to the gravity clarifier. Secondary effluent flowed over an unbaffled weir; settled sludge was
continuously pumped back to the aeration basin. Waste sludge was removed from the aeration
basin once per day at the 10, 5 and 4-d MCRT conditions and more frequently at the 3 and 2-d
MCRT conditions. CAS A (G = 250 s -1 ) used coarse bubble aeration at a flow rate of 4.7 L/min.
CAS B (G = 72 s -1 ) used fine bubble aeration at a flow rate of 0.5 L/min.

Particle size distribution (PSD)
A sludge sample (5 mL for CAS reactors and 1 mL for the SMBR) was removed from each
reactor with a wide-mouth pipette and placed in 1 L of particle-free SMBR effluent. The diluted
sample was gently mixed with a magnetic stir bar to disperse the sludge evenly. The mounting
and Methylene Blue staining procedures were adapted from Parker (1970). Samples were viewed
under a light microscope at 200x magnification for the SMBR sludge, 100x for the CAS A
sludge and 50x for the CAS B sludge. Digital photographs were taken of 20 random fields and
stored as TIF files (2048 by 1536 pixels, 24 bits/pixel) for image analysis. Each image was
converted into an 8 bit gray image by extracting the green component of the original RGB color
image then further scaled down to 1024 by 768 pixels to enhance contrast. The particles
deposited on the filter appeared as dark objects on a lighter background. A suitable image
threshold was selected from the maximum of the second derivative of the grayscale image
histogram. After selecting a threshold, the selected objects were eroded and dilated to separate
individual particles, and their geometric features were measured. Only particles completely
visible in each image were counted and analyzed. Image processing was performed using Mocha
software (Jandel Scientific, San Rafael, CA). Initial file conversion employed a procedure
written in IDL (Research Systems Inc., Boulder, CO). Frequency distributions were reported as
number of particles, and characteristic length was reported as the square root of the projected
area. Other definitions of characteristic length (such as geometric mean of maximum and
minimum lengths, feret diameter) were also considered. They yielded identical trends. The lower
particle size detection limit of the PSD analysis was 2 µm for CAS B sludge and 1 µm for the
SMBR and CAS A sludges.
Colloidal material
Particles smaller than the detection limit of the PSD analysis (colloidal material) were assessed
by turbidity measurements. Mixed liquor from each reactor was centrifuged for 2 min at 1000g
and supernatant turbidity was measured using a Hach 2100N Turbidimeter (Hach Company,
Loveland, CO) following Wilen et al. (2000). This method measured the turbidity due to
particles with diameters of approximately

the SMP concentration and the centrifuge supernatant of the sample after CER addition
represented the sum of the SMP and EPS concentrations. The difference between these
measurements was the EPS concentrations. Bovine serum albumin (BSA) was used as a protein
standard, and dextrose was used as a carbohydrate standard.
Total filamentous microorganisms
The filament counting procedure of Pitt and Jenkins (1990) was modified to include all rather
than only extended filamentous organisms. Intersections were counted on a Gram-stained
preparation. Separate counts for nocardioform bacteria and for all other filamentous
microorganisms were conducted. Results were expressed as “number of intersections/gTSS”.
Capillary suction time (CST) and time to filter (TTF)
CST was determined using a CST apparatus (Triton Electronics, Dunmow, Essex, UK) following
Standard Methods, Method 2710G (APHA et al., 1992). TTF was determined by Standard
Methods, Method 2710H (APHA et al., 1992). A 90-mm Buchner funnel was used to measure
the time required to filter 100 mL of a 200 mL sample through Whatman No. 1 paper. CST and
TTF values were both normalized to MLSS concentration and expressed in units of s⋅L/gTSS.
TSS, and chemical oxygen demand(COD)
TSS and total COD were measured by Standard Methods, Methods 2540D and 5220D,
respectively (APHA et al., 1992). Soluble COD was determined on samples filtered through a
0.45 µm filter (Standard Methods, Method 5220D (APHA et al., 1992)).
RESULTS
Reactor Performance
Effluent characteristics from each reactor are shown in Table 2. The median SMBR effluent total
CODs (18-32 mg/L) were much lower than for the CAS systems (65-86 mg/L) because the
SMBR effluent, which is membrane filtered, contained undetectable (

detected the presence of an abundance of small particles (Choo and Lee 1998, Cicek et al., 1999,
Luxmy et al., 2000, Shimizu et al., 1996, Wisniewski and Grasmick 1998, Wisniewski et al.,
2000, Zhang et al., 1997). This likely results from the efficient retention by the membrane of the
small particles produced by the high shear conditions in MBRs (in our SMBR the G value was
632 s -1 ).
The CAS B sludge (G = 72 s -1 ) had the typical bimodal PSD of activated sludge with peaks at 2-
4 µm and 20-40 µm (Figures 2a and 2c). The highly-sheared CAS A sludge (G= 250 s -1 ) did not
exhibit the bimodal distribution; its PSD was similar to those of the SMBR sludges (Figure 2d).
Colloidal material content for all sludges is shown in Table 3. The SMBR sludges contained
colloidal material levels (25-83 NTU) that were approximately one order of magnitude higher
than either of the CAS sludges (2.6-5.9 NTU). There was no significant difference in the
colloidal material content of the CAS A sludge (3.2-4.4 NTU) and the CAS B sludge (2.6-5.9
NTU). The large difference in the colloidal material content of the SMBR and CAS sludges can
be attributed to the difference in solids-liquid separation method used in these two reactor types.
In the CAS reactors, the gravity settling used for solids separation does not retain colloidal
material efficiently so it escapes in the secondary effluent. In the SMBR, the membrane pores are
small enough to retain significant amounts of colloidal material so that most colloids (either
those present in the influent that remain un-flocculated or those produced by floc dispersion)
remain in the mixed liquor.
Chemical Properties
The variation with MCRT of the total EPS, protein EPS and carbohydrate EPS contents of all
sludges are shown in Figure 3. The EPS content (total, protein and carbohydrate) of the SMBR
sludges was consistently lower than that of the CAS sludges. This difference increased as the
MCRT decreased and was especially noticeable at the 3 and 2 d MCRTs (Figure 3). In the CAS
sludges the increase in EPS content at the lower MCRTs was due to increases in both
carbohydrate and protein EPS; in the SMBR sludges the carbohydrate EPS varied much more
with MCRT than the protein EPS (Figures 3b and 3c). Liao et al. (2001), working with synthetic
wastewater, found no relationship between MCRT (4-20 d) and total EPS although the
carbohydrate content of the EPS increased at MCRT

Biological Properties
The SMBR sludges always contained about one order of magnitude more total filamentous
organisms than the CAS sludges (Table 4). This difference could be just about completely
accounted for by the presence of much higher nocardioform organism levels in the SMBR
sludge. Of the total filamentous organism levels in the SMBR, >80% were nocardioforms while
in the CAS sludges an average of >80% were filamentous organisms other than nocardioforms.
High nocardioform levels in activated sludge require the presence of features that trap floating
solids (including nocardioform foams) (Jenkins et al., 2003). Membrane separation of solids in
SMBRs provided extremely efficient foam trapping while in the CAS units there was a free flow
of surface material into the secondary effluent.
Capillary Suction Time (CST)
The normalized CSTs of the CAS sludges were higher than those of the SMBR sludges at all
MCRTs tested (Figure 4). The normalized CSTs of both sludges increased at approximately the
same constant rate over the MCRT ranges from 10 to 3d (CAS) and from 5 to 2 d (SMBR).
Between a 3 and a 2 d MCRT the normalized CST of the CAS sludges increased dramatically.
The normalized CSTs of both the CAS and SMBR sludges seem to be linearly related to the total
EPS content of the sludges (Figure 5). There was not a similar positive correlation between the
normalized CST of the sludges and their colloidal material contents. If colloidal material were
important in determining normalized CST then one would expect that the normalized CSTs of
the SMBR sludges would be higher than those of the CAS sludges because the SMBR sludges
contained much higher colloidal material levels than the CAS sludges (Table 3). In fact the
opposite occurs. Figure 5 shows that the normalized CSTs for the SMBR sludges were lower
than most of the CSTs for the CAS sludges.
The CST test measures the time that it takes for water to move a given distance through a
Whatman No. 17 chromatographic paper from a sludge sample under capillary suction. The
relationships discussed above could be explained as follows “The EPS acts to bind the water to
the sludge particles but the colloidal particles in the sludge do not interfere with the rate of the
water movement through the filter paper pores.” Houghton et al., (2001), working with
municipal activated sludges from multiple CAS plants, found a similar trend of normalized CST
with total EPS concentration. The Houghton et al. (2001) work did not evaluate the effect of
colloidal material on normalized CST. In another study, Houghton and Stephenson (2002)
measured EPS and particle size in anaerobic digester sludges and concluded it was the higher
concentration of EPS and not the amount of small particles that increased the normalized CST
values.
Time to Filter (TTF)
The normalized TTF results (Table 5) did not show a significant trend with MCRT. A single
factor ANOVA analysis (α=0.05) of the data did not detect a significant difference between the
normalized TTF values of the SMBR and CAS sludges for any condition tested. The TTF test
measures the time required to filter (under suction) a given volume of water through a cake of
the sludge solids retained on a filter paper. Several workers have shown that high levels of

smaller particles decrease filtration rate (measured by the specific resistance to filtration, SRF)
(Higgins and Novak 1997, Karr and Keinath 1978, Lawler et al., 1986, Mikkelsen and Keiding
2002). Other workers showed that EPS content had an important influence on sludge filterability
(Kang et al., 1989, Pere et al., 1993). We propose that both the EPS and colloidal material
content of the sludge affect TTF. EPS binds water and makes it more difficult to remove by
suction. Colloidal material blocks pores and makes it more difficult for water to pass through the
cake (and the filter paper) into the filtrate. We propose that the normalized TTFs of the SMBR
and CAS sludges did not differ because the effects of their colloidal material and EPS contents
on normalized TTF offset each other (Table 6).
CONCLUSIONS
1. SMBR effluent total COD concentrations were much lower than CAS effluent total COD
concentrations at all MCRTs tested because of the absence of TSS in the SMBR effluent
and because the SMBR effluent contained much lower soluble CODs than the CAS
effluents, possibly because of the retention of some soluble and colloidal organic material
by the membrane.
2. At a 5-d MCRT, the SMBR and the high-shear CAS sludges had similar PSDs with high
numbers of small particles (

to be a more rapid increase of normalized CST in the CAS sludge than in the SMBR
sludge.
7. The normalized CST values of both sludges were positively and linearly related to the
total EPS content of the sludges. No such correlation existed between normalized CST
and the colloidal material content of the sludges. In fact, the sludges with the highest
colloidal material levels have the lowest normalized CST values. These results suggest
that EPS was important in determining the value of the normalized CST but that colloidal
material content was not.
8. There did not appear to be any difference in normalized TTF for any of the sludges
tested. It is postulated that this was because of the offsetting effects of the high colloidal
material/low EPS contents of the SMBR sludge and the low colloidal material/high EPS
contents of the CAS sludges.
ACKNOWLEDGEMENTS
The project was funded in part by the Water Environment Research Foundation (WERF) project
No. 01-CTS-19-UR. The authors thank Zenon Environmental Services, Inc. for the pilot SMBR
and technical support; the City and County of San Francisco SEP- Engineering, Operations,
Maintenance and Laboratory staff for support and technical assistance; and Ms. Eileen Deng for
research assistance. At the time this work was conducted, Rion Merlo and Shane Trussell were
doctoral candidates, Slawomir Hermanowicz was Associate Professor and David Jenkins was
Professor in the Graduate School, all at the Department of Civil and Environmental Engineering,
University of California at Berkeley.

TABLES AND FIGURES
Table 1. Influent characteristics.
Analyte Units Median Range
Total Alkalinity mg CaCO 3 /L 188 89 - 248
Ammonia mg N/L 27 14 - 33
COD mg/L 345 78 - 695
TKN mg N/L 30 20 - 34
TSS mg/L 99 57 - 242
Table 2.
Effluent characteristics of CAS and SMBR reactors.
Soluble COD,
mg/L
TSS, mg/L Total COD, mg/L
Ammonia, mg N/L
MCRT,
d Median Range Median Range Median Range Median Range
Reactor
CAS A 5 35 9 - 230 84 51 - >165 54 35 - 71 0.5 0.0 - 12
4 18 7 - 31 76 59 - 99 55 47 - 69 1.5 0.5 - 14.
3 19 9 - 38 81 58 - 98 51 44 - 111 1.4 0.8 - 6.8
2 13 10 - 15 78 77 - 78 52 50 - 53 6.9 3.7 - 10
-
CAS B 10 31 9 - 134 83 48 - >165 49 34 - 116 0.1 0.0 - 0.8
5 21 9 - 76 65 45 - 126 47 38 - 62 0.4 0.0 - 3.1
4 22 12 - 83 78 65 - 140 54 42 - 66 1.5 0.8 - 13
3 28 15 - 67 86 53 - 128 44 36 - 72 2.1 0.8 - 6.2
2 15 10 - 17 80 76 - 83 52 51 - 54 8.2 6.5 - 9.9
SMBR H 10 ND 23 17 - 45 23 17 - 45 0.1 0.0 - 0.5
5 ND 24 10 - 30 24 10 - 30 0.2 0.0 - 1.6
4 ND 24 19 - 39 24 19 - 39 0.5 0.4 - 0.8
3 ND 22 19 - 27 22 19 - 27 0.5 0.2 - 0.5
2 ND 32 25 - 35 32 25 - 35 7.7 0.4 - 13
SMBR L 10 ND 18 17 - 22 18 17 - 22 0.3 0.1 - 0.5
5 ND --- --- ---
4 ND 24 20 - 32 24 20 - 32 0.6 0.4 - 3.3
3 ND 23 21 - 29 23 21 - 29 0.6 0.4 - 0.7
ND = non-detectable; detection limit = 2 mg/L

Table 3. Colloidal material content of CAS and SMBR sludges.
Turbidity, NTU
Reactor MCRT, d Median Range
CAS A 5 3.2 1.9 -3.8
4 4.4 3.2 -5.1
3 4.0 1.6 -5.4
2 3.3 3.2 -3.3
CAS B 5 4.7 --- ---
4 5.9 2.1 -13
3 5.0 3.3 -6.7
2 2.6 2.2 -3.1
SMBR H 5 83 68 -84
4 35 --- ---
3 26 24 -28
2 50 40 -56
SMBR L 10 73 72 -98
5 29 23 35
4 29 8.9 -58
3 25 --- ---
Table 4. Filamentous microorganism levels in CAS and SMBR sludges.
CAS A CAS B SMBR H SMBR L
Filamentous organisms, log(intersections/gTSS)
Nocardioform organisms 1.1x10 5 5.7x10 5 1.5x10 7 1.5x10 7
Other filamentous organisms 7.0x10 5 1.6x10 6 3.1x10 6 2.8x10 6
Total filamentous organisms 8.1x10 5 2.2x10 6 1.9x10 7 1.7x10 7
Fraction of total filamentous organisms, %
Nocardioform organisms 13 26 83 84
Other filamentous organisms 87 74 17 16

Table 5. Normalized TTF values for CAS and SMBR sludges.
Normalized TTF, s⋅L/gTSS
Reactor MCRT, d Median Range
CAS A 5 46 18 - 55
4 24 16 - 43
3 31 20 - 33
2 26 25 - 35
CAS B 10 15 9 - 55
5 15 14 - 15
4 19 10 - 58
3 39 30 - 56
2 36 29 - 51
SMBR H 10 30 23 - 97
5 35 28 - 44
4 23 21 - 25
3 27 22 - 28
2 44 40 - 47
SMBR L 10 60 57 - 60
5 11 11 - 14
4 27 21 - 35
3 37 31 - 45
Table 6. Offsetting effects of EPS and colloidal material on TTF.
Colloidal Material
Total EPS
Sludge Type Content, NTU Effect on TTF Content, mg/gVSS Effect on TTF
CAS Low (1.6-13) Small High (94-346) Large
SMBR High (8.9-98) Large Low (79-130) Small

Figure 1.
SMBR pilot plant schematic.
Influent
3-mm Screen
3-Way Valve
Effluent
Membrane Tank
WAS
Aerobic Tank
Compressed
Air/Pure Oxygen
High- or Low-
Shear Pump
Blower

Figure 2. PSD based on number frequency at 5-d MCRT for: a. all sludges; b. SMBR
sludges; c. CAS sludges; and d. CAS A and SMBR sludges.
a.
CAS A CAS B SMBR H SMBR L b.
SMBR H SMBR L
0.5
0.5
0.4
0.4
Frequency
0.3
0.2
0.3
0.2
c.
0.1
0.0
1-2
2-4
4-6
6-8
8-10
10-20
20-40
CAS A
CAS B
40-60
60-80
80-100
100-1000
d.
0.1
0.0
1-2
2-4
4-6
6-8
8-10
10-20
20-40
40-60
60-80
80-100
100-1000
CAS A SMBR H SMBR L
0.5
0.5
0.4
0.4
0.3
0.3
0.2
0.2
0.1
0.0
1-2
2-4
4-6
6-8
Characteristic Length, µm
8-10
10-20
20-40
40-60
60-80
80-100
100-1000
0.1
0.0
1-2
2-4
4-6
6-8
Characteristic Length, µm
8-10
10-20
20-40
40-60
60-80
80-100
100-1000

Figure 3.
a.
Median values for: a. total EPS; b. protein EPS and; c. carbohydrate EPS for
CAS and SMBR sludges
CAS A CAS B SMBR H SMBR L
350
300
250
200
150
100
50
0
0 1 2 3 4 5 6 7 8 9 10
MCRT, d
b.
CAS A CAS B SMBR H SMBR H
300
250
200
150
100
50
0
0 1 2 3 4 5 6 7 8 9 10
MCRT, d
c.
CAS A CAS B SMBR H SMBR H
70
60
50
40
30
20
10
0
0 1 2 3 4 5 6 7 8 9 10
MCRT, d

Figure 4.
Median normalized CST as a function of MCRT for CAS and SMBR
sludges.
CAS A CAS B SMBR H SMBR L
30
25
20
15
10
5
0
0 1 2 3 4 5 6 7 8 9 10
MCRT, d
Figure 5.
30
25
Relationship between the normalized CST and total EPS for all sludges.
CAS SMBR
R 2 = 0.83
20
15
10
5
0
0 50 100 150 200 250 300 350
Total EPS, mg/gVSS

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