J. Environ. Eng. Manage., 18(4), 257-260 (2008)
PERFORMANCE OF NEW GENERATION CERAMIC MEMBRANES USING
HYBRID COAGULATION PRETREATMENT
S. Geno Lehman,* Samer Adham and Li Liu
National Technology Group
MWH Americas, Inc.
Arcadia, CA 91007, USA
Key Words: Microfiltraion, ceramic membrane, pretreatment, coagulation, surface water
ABSTRACT
Historically, the use of microfiltration/ultrafiltration for drinking water and wastewater
treatment has been almost exclusively focused on polymeric membranes. New generation ceramic
membranes have been recently introduced in the market, which possess unique advantages over
currently available polymeric membranes. Ceramic membranes are mechanically superior to
polymeric membranes and are more resistant to severe chemical and thermal environments. As a
result, ceramic membranes can perform more efficiently than existing polymeric membranes under
more rigorous operation conditions. Ceramic membranes are able to operate at higher filtration flux,
higher feedwater recoveries, have more efficient backwash operation using high pressures, operate at
extended backwash intervals and have low chemical cleaning requirements, while experiencing long
membrane life without breakage. This paper presents results of one of the first pilot studies in the
U.S. designed to investigate the application of new generation ceramic membranes for surface water
treatment.
INTRODUCTION
The installation of microfiltration (MF) and ultrafiltration (UF) facilities for municipal water and
wastewater treatment has dramatically increased over
the past decade [1]. Technical innovations by researchers and manufactures have brought new MF/UF
membrane materials, modules and systems to the
market every year.
Historically, the use of MF/UF for drinking water and wastewater treatment has been almost exclusively focused on polymeric membranes [2]. Polymeric membranes have been known to often have integrity problems regarding the module assembly (potting) or material stability. Most polymeric membrane
systems operate well within a fairly neutral to slightly
acidic pH range, with prolonged extreme acidic or alkaline conditions posing potential problems [3]. Furthermore, exposure to extreme oxidant conditions created by chlorine and ozone can cause degradation of
polymeric membranes [4]. As a result, innovative
MF/UF membranes are currently being developed to
improve pore distribution, mechanical stability, and
chemical stability, using optimized polymeric formulations or alternative materials (i.e., ceramics, steel,
*Corresponding author
Email: Geno.E.Lehman@us.mwhglobal.com
polytetrafluoroethylene, etc.).
APPROACH
New generation ceramic membranes have been
recently introduced to the U.S. market. These membrane products exhibit unique advantages over currently available polymeric membranes, due to improvements in materials and design. Ceramic membranes are mechanically superior to polymeric membranes and more resistant to severe chemical environments (chemical and thermal), allowing ceramic
membranes perform more efficiently than existing
polymeric membranes under rigorous operating conditions [5-7].
Ceramic membranes are able to operate at higher
filtration fluxes, experience higher feedwater recoveries, operate with more efficient backwash operations
using high pressures, operate at extended backwash
intervals, and have low chemical cleaning requirements, while experiencing long membrane life without
breakage. They are becoming cost competitive with
conventional polymeric membranes for water and
wastewater applications and have a strong potential
for drinking water treatment. However, limited cost
J. Environ. Eng. Manage., 18(4), 257-260 (2008)
258
information exists for application of the ceramic membrane in the U.S. Currently, MWH Americas, Inc. is
conducting a cost evaluation for the application of ceramic membranes compared to existing polymeric
membrane system, designed for large-scale applications.
This paper discusses results of pilot studies designed to investigate the application of new generation
ceramic membranes for water and wastewater treatment, focusing on specific properties described above.
Few pilot studies with ceramic membranes have been
performed - most of which have occurred outside of
the U.S. To the authors’ knowledge, this study is the
first pilot study performed in the U.S. to evaluate ceramic membranes using full-scale membrane modules
to treat surface water.
Compressed air
In-line
coagulation
Raw water
2-stage mixing
Feed pump
BW water
tank
Ceramic
membrane
module
Filtrate
Backwash waste water
tank
Fig. 1. Schematic of the ceramic membrane pilot unit.
MATERIALS AND METHODS
1. Pilot System
A schematic diagram of the ceramic MF pilot
system (Metawater Co., Ltd., Japan) is shown in Fig.
1. The pilot system mainly consists of two parts: the
coagulation/flocculation unit and the membrane module with backwash facilities. Coagulation and flocculation can be performed either in a two-stage mixing
tank or in-line, using a static mixer and the feed pump
to evenly distribute the dosed coagulant in the feed
pipe.
Full-scale monolith ceramic membrane elements
were used in this study, having dimensions of 1.5 m in
length and 0.18 m in diameter, shown in Fig. 2. The
active membrane surface area for a single pressuredriven element is 25 m2. The ceramic membrane operates as a pressurized membrane configuration in a direct filtration (dead-end) mode. Each ceramic membrane element utilizes 2,000 inside/out channels (filtration cells), in which raw water is filtered through a
thin ceramic membrane separation layer (0.1 µm
nominal pore size). The filtrate outflows from the
element through water collection slits, via internal water collection cells. At the completion of each filtration phase, the membrane is backwashed at high pressure of up to 503 kPa. The membrane is chlorine resistant and could be operated at pH ranged from 2 to 12.
The ceramic membrane surface is hydrophilic and has
a slightly negative surface charge.
2. Water Quality
California State Project Water (SPW) was used
for all experiments in this study. Raw SPW was supplied by the Agua de Lejos Water Treatment Plant,
operated by the City of Upland’s Water Facilities Authority (Upland, CA). The source water is characterized by moderate hardness and alkalinity, moderate
total organic carbon (TOC) and low turbidity. The raw
Fig. 2. Full-scale ceramic membrane element (courtesy
of METAWATER Co., Ltd.).
water alkalinity was approximately 75 mg L-1 as
CaCO3. Turbidity varied from 0.9 to 20 NTU while
the TOC values were observed between 2.8 and 5.4
mg L-1. Raw water pH was consistently 7.5 and temperature varied from 11 to 26 °C during the study.
RESULTS AND DISCUSSION
Extensive pilot studies were conducted by MWH
in the U.S. using a full-size element (1.5 m in length
and 0.18 m in diameter) with surface water to demonstrate the performance of ceramic membranes with respect to operating flux, backwash efficiency, and long
term fouling trends. All tests were performed at an
applied flux of 40.7 LMD (L m-2 d-1) with in-line coagulation (direct coagulation), unless specified otherwise. Selected results are presented below.
1. Pretreatment with Coagulant
A 2-stage coagulation/flocculation test established the baseline performance for comparison to inline coagulation. The retention time in the mixing
zone was 10 min and the retention time for floc formation and growth in the second stage of the tank was
10 min at slower mixing velocities. Polyaluminium
chloride (PACl, General Chemical) was used as the
coagulant at a dose of 2 mg L-1 as Al, and the flux was
held at 40.7 LMD.
Lehman et al.: New Generation Ceramic Membranes
1.0
Normalized specific f lux @ 20 °C
Normalized specific f lux @ 20 °C
1.0
259
0.8
0.6
In-line coag.,2 mg L-1-Al
Flocculated, 2 mg L-1-Al
0.4
0.2
0.8
0.6
20. 4 LMD( TMP = 1. 4 kPa)
40. 7 LMD( TMP = 8. 3 kPa)
61. 1 LMD( TMP = 33. 8 kPa)
0.4
0.2
Initial specific flux = 130 LMD kPa-1
0.0
0.0
0
10
20
30
40
50
0
10
20
30
Time (h)
Results shown in Fig. 3 demonstrated that with
flocculated water, the specific flux declined to 78% of
the initial water flux after 40 h of operation. In contrast, when using in-line (direct) coagulation with the
same coagulant dose with 20 s of retention time, 91%
of initial specific flux was maintained after 40 h of
operation, which showed improved performance compared to 2-stage coagulation with the same dose of
PACl.
3. Impact of Applied Flux
Fig. 4. Performance of
increasing flux.
50
60
ceramic
membrane
with
1
Normalized specific f lux
Fig. 3. Comparison of pretreatment configuration for
ceramic membrane.
40
Time (h)
0.8
0.6
0.4
2-h BW
4- h BW
6- h BW
0.2
0
The ceramic membrane demonstrated stable performance when operated at fluxes ranging from 20.4
to 61.1 LMD (50 to 150 gfd), as shown in Fig. 4. The
specific flux declined to 94, 83, and 84% of the initial
specific flux after 40 h of operation, for 20.4, 40.7,
and 61.1 LMD, respectively. As expected, the membrane fouled less during a cycle (between each backwash) when operated at lower flow rates. The transmembrane pressure (TMP) increase per cycle was
around 1.4 kPa at 20.4 LMD, while the TMP increases
for 40.7 and 61.1 LMD were 8.3 and 33.8 kPa per cycle,
respectively. The recovery of the membrane system increased as it is operated at higher flux. The recoveries
were 97.3, 98.6 and 99.1%, respectively when the filtration fluxes were 20.4, 40.7, and 61.1 LMD.
4. Extended Backwash Interval
Figure 5 shows that the backwash interval can be
extended to 6 h when the membrane is operated at
40.7 LMD, which pushed the total water recovery to
99.5%. The membrane demonstrated similar fouling
trends at backwash intervals of 2, 4, and 6 h. The hydraulic backwashes were efficient to recover the
membrane permeability to the same level, even when
the build-up of TMP in the extended filtration cycles
(6 h) was much higher than the short filtration cycles
(2 h).
5. Performance with Alternative Coagulants
0
10
20
30
40
50
Time (h)
Fig. 5. Performance of ceramic membrane at extended
backwash intervals.
For the California SPW used in this study, the
authors found that the ceramic membrane worked
compatibly with four types of coagulants tested during
2-d screening tests at a flux of 40.7 LMD. The comparison of different coagulants was based on a common metal molar dose of 0.12 mM (i.e., 3.1 mg L-1 -Al
of PACl, PAX-XL19, Alum and 6.5 mg L-1-Fe of ferric chloride). The coagulation was performed in-line
with a retention time of 40 s. As shown in Fig. 6, the
specific flux declined to 92, 91, 83, and 76% of the
initial flux after 40 h for PAX-XL19, ferric, PACl and
Alum, respectively. PAX-XL19 and ferric recovered
the flux decline to the greatest extent, while PACl and
Alum still worked well with the METAWATER
membrane.
6. Regulatory Approval
In February 2006, the METAWATER ceramic
membrane was granted conditional acceptance by the
California Department of Public Health (CDPH) as an
alternative filtration technology, based on third-party
pilot testing performed by MWH. The CDPH Water
Treatment Committee credited the METAWATER ce-
J. Environ. Eng. Manage., 18(4), 257-260 (2008)
260
Normalized specific f lux @ 20 °C
1.0
removal, corresponding to a maximum system operation flux of 81.4 LMD (200 gfd) and maximum
TMP of 379 kPa (55 psi).
0.8
FUTURE WORK
0.6
3.1 mg L-1-Al of alum
3.1 mg L-1-Al of PACl
3.1 mg L-1-Al of PAX-XL19
6.5 mg L-1-Fe of ferric
0.4
0.2
0.0
0
10
20
30
40
50
Time (h)
Fig. 6. Performance of ceramic
alternative coagulants.
membrane
with
ramic membrane with 4-log Cryptosporidium, Third
4-log Giardia, and 1-log virus removal, corresponding
to a maximum system operation flux and maximum
TMP of 81.4 LMD and 379 kPa, respectively.
CONCLUSIONS
Third party pilot testing of the METAWATER
ceramic membrane is essential to generate performance and subsequently cost data to be evaluated by the
industry. The following specific conclusions were obtained from this study:
‧ In-line direct coagulation dramatically reduced
membrane fouling and provided enhanced organics
removal in a compact footprint, by eliminating the
need for flocculation and clarification. Membrane
performance was slightly enhanced with using direct coagulation compared to flocculated feed.
‧ The ceramic membrane has demonstrated stable
performance when operated at fluxes ranging from
20.4 to 61.1 LMD.
‧ The ceramic membrane worked compatibly with
several types of coagulants, when tested on the
same surface water. Both aluminum-based (Alum,
PACl, aluminum chlorohydrate, etc.) and ferricbased coagulants were able to control membrane
fouling, when applied on the same molar basis.
‧ The backwash interval could be extended to 6 h
when the membrane was operated at 40.7 LMD,
which increased the total water recovery to 99.5%.
The membrane has shown similar fouling trends at
backwash intervals of 2 and 4 h. The hydraulic
backwashes effectively recovered the membrane
permeability to the same level, even when the
build-up of TMP in the extended filtration cycles
(6 h) was much higher than the short filtration cycles (2 h).
‧ The METAWATER ceramic membrane was recently granted conditional acceptance by CDPH as
an alternative filtration technology with 4-log
Cryptosporidium, 4-log Giardia, and 1-log virus
Pilot studies are underway in Southern California to demonstrate the application of new generation
ceramic membranes to treat secondary wastewater effluent for reuse applications and seawater as pretreatment to desalination processes. Due to the robust
nature of the ceramic membrane, more aggressive pretreatment techniques can be used, including ozonation
before the membrane as well as conventional pretreatments (i.e., enhanced coagulants, powdered activated carbon, etc.).
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Discussions of this paper may appear in the discussion section of a future issue. All discussions should
be submitted to the Editor-in-Chief within six months
of publication.
Manuscript Received: November 4, 2007
Revision Received: April 1, 2008
and Accepted: April 15, 2008