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DESALINATION
ELSEVIER
Desalination 167 (2004) 1-11
www.elsevier.com/locate/desal
Comparison of membrane options for water reuse
and reclamation
Pierre C6t6", Michel Masini, Diana Mourato
ZENON Environmental Inc., 3239 Dundas Street West, Oakville, Ontario L6M 4B2, Canada
Tel. +1 (905) 465-3030 Ext. 3080; Fax +1 (905) 465 3050; email: [email protected]
Received 3 February 2004; accepted 12 February 2004
Abstract
The reuse of effluents for irrigation and indirect potable water uses is rapidly developing as an alternative to
seawater desalination. This paper explores two membrane-based options available to treat sewage for water reuse,
tertiary filtration (TF) of the effluent from a conventional activated sludge (CAS) process and an integrated membrane
bioreactor (MBR). These options are compared from technical, performance and cost points of view using ZeeWeed®
immersed membranes. The analysis shows that an integrated MBR is less expensive than the CAS-TF option. The
total life cycle costs for the treatment of sewage to a quality suitable for irrigation reuse or for feeding reverse
osmosis decrease from 0.405/m3to 0.205/m3as plant size increases to 75,000 m3/d.It is also shown that the incremental
life cycle cost to treat sewage to indirect potable water reuse standards (i.e. by ultrafiltration and reverse osmosis)
is only 39% of the cost of seawater desalination.
Keywords: Membrane bioreactor; Water reuse; Ultrafiltration
1. I n t r a d u c t i o n
Public concerns over health and the environment, combined with the increased requirement
for municipalities to reuse wastewater have
created a need to treat effluents to a higher level.
For water reuse, contaminants that require treat*Corresponding author.
ment over and above what is provided by conventional biological treatment include suspended
solids, microbial contaminants, nutrients, trace
dissolved contaminants (e.g. endocrine disruptors), and in certain cases, dissolved salts.
Ultrafiltration (UF) membranes are flexible
water treatment tools that can be used in a number
of process configurations to meet the advanced
Presented at the EuroMed 2004 conference on Desalination Strategies in South Mediterranean Countries: Cooperation
between Mediterranean Countries of Europe and the Southern Rim of the Mediterranean. Sponsored by the European
Desalination Society and Office National de l'Eau Potable, Marrakech, Morocco, 30 May-2 June, 2004.
0011-9164/04/$- See front matter © 2004 Elsevier B.V. All rights reserved
doi;lO.1016/j.desal.2004.06.105
2
P. Cttd et al./Desalination 167 (2004) 1-11
effluent treatment objectives listed above. Immersed membranes, when used by themselves, are
limited to the removal of particulate and colloidal
contaminants; however, they can be combined
with biological or chemical treatment to remove
dissolved contaminants. Furthermore, they represent the ideal pretreatment to reverse osmosis.
The main objective of this paper is to compare
treatment options available for water reuse from
the point of view of water quality and cost. The
two primary options, shown in Fig. 1, are conventional activated sludge (CAS) followed by tertiary
filtration (TF) and integrated membrane bioreactor
(MBR). In this paper, these options are discussed
for plants ranging in size between 3,800 m3/d
(1 MGD) and 76,000 m3/d (20 MGD).
(a) Clarifier
(b)
Aeration
tank
Clarifier
MF/UF
MBR
Fig. 1. Primary options to treat sewage for water reuse:
(a) conventional activated sludge + tertiary membrane
filtration (CAS-TF); (b) membrane bioreator (MBR).
2. Design and costing models
CapdetWorks©was used for the design and cost
estimation of conventional activated sludge plants.
CapdetWorks e is a preliminary design and costing
program available from Hydromantis, Inc.
(www.Hydromantis.com).
WTCost e was used for the design and cost
estimation of reverse osmosis plants. WTCost~ is
a preliminary design and costing program for
membrane process plants developed with the
support of the US Bureau of Reclamation, available from [email protected].
ZeeCost © was used for the design and cost
estimation of the immersed membrane systems
used as tertiary filtration with the conventional
activated sludge plants and for the membrane
bioreactor plants. ZeeCost e is ZENON's proprietary design and costing program for ZeeWeed®
immersed membrane systems.
While the cost estimations with these programs
allow comparing options and drawing conclusions
on a relative basis, the absolute precision of the
cost estimations is considered to be +25%.
3. Treatment configurations
Process flow diagrams for the CAS-TF option
are presented in Fig. 2. Two levels of screening
are provided, a coarse screen similar to a CAS
plant, and a fine screen to protect the immersed
membranes from accumulation of trash and hair.
For small plants (<19,000 m3/d - 5 MGD), there
is no primary clarification and the thickened
sludge is digested aerobically (Fig. 2a). For large
plants (>19,000 m3/d- 5 MGD), a primary clarification step was added, and the thickened sludge
is digested anaerobically (Fig. 2b).
Process flow diagrams for the MBR option are
presented in Fig. 3. The inclusion of primary
clarification and aerobic or anaerobic digestion
as a function of flow rate is identical to the CASTF case. Also, for all but very small plants the
membranes are immersed into separate tanks from
the main bioreactors. This provides more flexibility
in isolating membranes for cleaning or maintenance without handling.
4. Process conditions
Typical sewage concentrations were used as
input to the models in order to design the biological treatment steps and estimate sludge production (Table 1). Simulations were run for average
flows of 3,800 m3/d (1 MGD), 19,000 m3/d
P. C6tO et al. /Desalination 167 (2004) 1-11
(a)
3
ReJect
Sewage )//----~
Treated
effluent
ioreactor (
Coarse
Screen
Secondary
clarifier
T
Sludgerecycle
q
Dewatering
Dewatered
sludge
Fine
screen
MF/UF
Excess sludge
Aerobic
digester
(b)
Reject
Sewage
'@-D"
-V
, #/t
Coarse
Screen
Bioreactor
Primary
clarifier
I
sludge
Dewatering
Treated
effluent
Secondary
clarifier
Sludge recycle
Fine
screen
MF/UF
Excess sludge
Anaerobic
digester
Fig. 2. Process flow diagram for the conventional activated sludge tertiary filtration (CAS-TF) option: (a) small plants;
(b) large plants.
Table 1
Raw sewage characteristics
Parameter
Concentration, mg/L
BOD
SS
TKN
TP
240
240
40
8
,
(5 MGD), 38,000 m3/d (10 MGD) and 76,000 m3/d
(20 MGD). Typical peaking factors of two times
average flow were used for all plants.
The process conditions used to design the
plants are summarized in Table 2. For the conventional activated sludge system, typical values
for North American design were selected by using
most of the default values suggested by
CapdetWorks. The same sludge age (SRT) was
used for CAS and MBR, but hydraulic retention
times and mixed liquor concentrations were
significantly different.
The membrane system used for the MBR is
ZENON's ZeeWeed ® 500d [1]. Membranes
cassettes containing 48 modules of 31.6 m 2 each
were arranged into separate membrane tanks (2,
6, 8 and 12 tanks for the four flow rates simulated).
The continuous mixed liquor re-circulation flow
rate between the membrane tanks and the main
bioreactor was set at 5 ×Qa~eto de-concentrate the
membrane tank and provide for nitrification/
denitrifieation. The design was based on an
average flux of 20 L/m2/h. A set of blowers for
4
P C6t~ et al./Desalination 167 (2004) 1-11
(a)
Sludgerecycle
Sewage
I Bioreactor
P
,,.,oarse
¢IF/UF
Screen ~
Excesssludge
Screeningsolids
Dewateredsludge[
Aerobic I
digester
Dewatering ~
Primary
cladfier
(b)
Sludgerecycle
I' 'If
.//
Sewage
_ ~ Treated
effluent
Bioreactor I
Coarse
Screen
~
MFAJF
Excesssludge
Screeningsolids
Dewateredsludge
1~ Treated
effluent
Dewatering[
-[Anaerobic [
"1 digester
|
~
,
Fig. 3. Process flow diagramfor the membranebioreactoroption(MBR): (a) smallplants; (b) large plants.
Table 2
Designprocess conditionsfor the sewagetreatmentplants
Unit process
Parameter
Coarse screen
Primary clarifier
Fine screen
Bioreactor
Size, mm
Loadingrate, rn/h
Size, mm
HRT, h
MLSS, g/L
SRT, d
Loadingrate, m/h
Averageflux, L/m2/h
DischargeTSS, g/L
Secondaryclarifier
ZeeWeed®filtration
Gravity thickener
Sludgedigestion
HRT, d
Smallplants (<19,000m3/d)
CAS-TF
MBR
10
10
No
No
2
2
23
6.5
3
10
15
15
1.4
No
22
20
60
60
Aerobic
Anaerobic
15
15
Largeplants (>19,000 m3/d)
CAS-TF
MBR
10
10
1.7
1.7
2
2
12
3.6
3
10
15
15
1.4
No
22
20
60
60
Aerobic
Anaerobic
35
35
P. Crtd et al. / Desalination 167 (2004) 1-11
membrane scouring was provided independently
from the biological process blowers and was sized
to provide an average of 0.26 m3/h/m2 of membrane surface area.
The membrane system used for the CAS
tertiary filtration is ZENON's ZeeWeed®1000 [2].
Membrane cassettes containing 48 modules of
37 m 2each were arranged into separate membrane
tanks (2, 6, 8 and 12 tanks for the four flow rates
simulated). The ZeeWeed® 1000 is operated as a
dead-end filtration system with backpulses and
de-concentration every 20 min. It was assumed
that the backwash reject water was returned to
the head of the plant. The design was based on an
average flux of 22 L/mVh. The scouring aeration
for the ZeeWeed ® 1000 is an average of
0.02 mVh/m2 of membrane surface area, applied
only during the backwashing sequence.
Energy consumption for each of the two membranes systems included suction pumps assuming
an average trans-membrane pressure of 35 kPa,
and scouring aeration as described above.
5. Treatment efficiency comparison
In this section, an analysis of the major
differences between the two processes, conventional activated sludge - - tertiary filtration (CASTF) and membrane bioreactor (MBR) is presented
for key water quality parameters.
5.1. Suspended solids/Silt Density Index
For both the CAS-TF and the MBR, the ultrafiltration membrane allows reducing suspended
solids to below detection limit [3]. The membrane
step also provides a physical barrier against upsets
of the biological process that results from a poorly
settle-able sludge. The barrier effect is reflected
in the Silt Density Index (SDI) parameter, which
is typically below 3 [4]. Key challenges to maintain treated water quality over time include
preventing against re-growth on the permeate side
of the membrane and maintaining a high level of
membrane integrity. The first challenge is met by
5
using pre-chlorination (TF only) or chlorine and/
or acid in the backpulse water (both TF and MBR).
The second challenge is met by conducting regular
bubble-point integrity tests and repairing defects
as needed.
5.2. Microbes
Microbial pathogens are particles that are
rejected by the membranes, like suspended solids.
Parasites and bacteria are much larger than the
pore size of UF/MF membranes and are rejected
by a sieving mechanism. Viruses may be smaller
than the membrane pore size but are normally
totally rejected because they are associated with
suspended solids. As pointed out above, membrane integrity must be maintained over time.
5.3. Chemical oxygen demand~synthetic organic
chemicals
The MBR process, when compared to CAS,
1) runs at a higher MLSS concentration, 2) has a
longer and better controlled SRT, and 3) is less
susceptible to upsets; this leads to the development
of a more diversified biomass and results in better
biodegradation of soluble organic compounds [5].
Therefore, in general, the MBR is a better solution
than CAS-TF to remove COD and SOCs.
5.4. Nutrients
The MBR process, because it works at a high
MLSS concentration and it eliminates the loss of
slow growing nitrifiers to the clarifier weir, has
demonstrated superior performance for nitrification, even at very low temperature [6]. One issue
is to make sure that the retum sludge from the
membrane tank, which may have a high DO concentmtion, does not impact denitrification; technical solutions exist to deal with this issue and it
has been showed that total nitrogen (TN) concentrations smaller than 10 and 3 mg/L can be achieved
in cold and warm water, respectively [7,8]. In
general, the CAS-TF process is unable to achieve
the same level of TN removal because the CAS
6
P C6t~ et al./Desalination 167 (2004) 1-11
and the TF processes do not work synergistically.
Both the MBR and CAS-TF processes can
achieve very high removal of total phosphorus,
either biologically [9] or chemically [7] through
complete removal of the phosphorus associated
with suspended solids (either bacteria or colloidal
inorganic particles). Effluent levels of 0.2 mg/L
(biologically) or 0.1 mg/L (chemically) can be
achieved consistently.
to 9.5 h in large plants; for large plants, the addition of the primary clarifiers is not compensated
by the reduction of the biological tanks HRT, and
the total HRT increases with flow rate as a result
of using anaerobic digester. Overall, the MBR
plants HRT is 75% (small plants) to 50% (large
plants) smaller than those of the CAS-TF plants.
Fig. 5 shows the total surface area occupied
by the plant as a function of flow rate. The total
surface area was estimated based on the total
process tanks footprint (proportional to flow rate)
and the area required for roads, parking, laboratory, etc. (not directly proportional to flow rate).
The land required for an MBR plant is about half
of that required for a CAS-TF plant.
6. Plant size comparison
MBR plants are much smaller than CAS-TF
plants. This is shown by comparing the total
hydraulic retention time (HRT) in Fig. 4 and the
total plant surface area in Fig. 5. The total HRT
(the volume of all tanks divided by the average
flow rate) for the CAS-TF plants is 28 h in small
plants, decreasing to 20 h for the larger plants;
HRT decreases with flow rate as the net impact of
adding primary clarifiers and reducing the
biological tanks HRT in larger plants is positive
regardless of anaerobic digester. For MBR plants,
the total HRT increases from 7.5 h in small plants
7. Capital costs
The total capital costs are presented in Fig. 6
for 3 plants, CAS, CAS-TF and MBR, including
the following components for all unit processes
shown in Figs. 2 and 3:
• Direct costs (equipment for all unit processes,
mobilization, site preparation, site electrical,
30
25
l
~
i
v
a
t
e
d
Sludge- TertiaryFiltration
I
20
Ill
10
MembraneBioreaetor
0
10,000
20,000
30,000
40,000 50,000
Flow rate (m3/d)
Fig. 4. Hydraulic retention time.
60,000
70,000 80,000
P C6td et al./Desalination 167 (2004) 1-11
7
1.8 T
!
0.2 ~IJ
o~0
T - - - -
10,000
20,000
30,000
40,000
50,000
60,000
70,000
80,000
Flow rate (m3/d)
Fig. 5. Total plant surface area.
(a)
(b)
2,000-
1,200.
~
1,50O
r.
1,000
g00
"~ 1,000
g 600
50o
,.,41--CA8
.'O..MBR
,,,at-CAS.TI7
40(I
200
0
10,00O 20,0O0 30,000 40,000 50,000 60,000 70,000 80,00O
Flow rate, m3/d
0
CAS
MBR
CAS-TF
Fig. 6. Total capital costs: (a) total capital costs; (b) capital costs breakdown (38,000 mVd).
yard piping, instrumentation and control, laboratory and administration buildings)
Indirect costs (legal fees, engineering design,
inspection, contingency and miscellaneous)
Land cost
The capital costs of CAS plants are shown as
a reference in Fig. 6a. For all plant sizes considered, MBR plants are less expensive than CAS
plants. This is because the savings associated with
eliminating secondary clarifiers, reducing the size
of the aeration tanks and reducing footprint are
larger than the added costs for the membrane
system and the fine screen. The capital costs of
CAS-TF plants are higher that the costs of CAS
or MBR plants. The cost of a tertiary filtration
membrane system is basically added to that of a
CAS plant, without eliminating anything.
The breakdown of capital costs between direct,
indirect and land costs is shown in Fig. 6b for a
8
P cot~ et al./Desalination 167 (2004) 1-11
38,000 mVd (10 MGD) plant size. Direct costs
represent about 2/3 of total capital costs. The cost
of land, which represents 13.6% and 12.1% for
the CAS and CAS-TF plants, shrinks to 7.3% for
the MBR plants; this fraction is indeed sensitive
to land unit cost ($150/m 2 in this case).
For labor, it was assumed that the use of membranes did not reduce labor requirement for the
conventional part of the plant; however, there is
evidence that a fully automate MBR plant requires
less labor than a CAS plant. For materials, a membrane life of 8 years was assumed. For energy, a
relatively high unit cost of $0.10/kWh was used.
8. Operation and maintenance costs
The total operation and maintenance (O&M)
costs are presented in Fig. 7 for the 3 plants CAS,
CAS-TF and MBR, including the following components for all unit processes shown in Figs. 1
and 2:
• Labour
• Materials (renewal of equipment, membrane
replacement)
• Energy
• Chemicals (membrane cleaning)
O&M costs of the plants including membranes
(MBR and CAS-TF) are higher than the CAS plant
by 20-30% for all flows (Fig. 7a). The O&M cost
of a MBR plant is slightly higher than the CASTF plant because the MBR membranes system
requires a higher scouring aeration rate and the
membrane replacement cost is higher.
Fig. 7b shows that all four categories of O&M
costs are higher for the MBR system. However,
these were based on conservative assumptions.
9. Total life cycle costs
The total life cycle costs presented in Fig. 8
were generated using a PV factor of 14.32
(20 years, 6% interest rate and 2.5% inflation rate).
They vary from $0.45/m 3 for small plants to
$0.20/m 3 for large plants. Total life cycle costs
are smallest for the CAS plants, followed by MBR
and CAS-TF plant. The premium for a membranefiltered wastewater over CAS is 5-20% and
increases with plant size.
10. Water reuse vs. desalination
In this section, the cost of water reuse is compared to the cost of seawater desalination. To
produce water of equivalent quality, a reverse
osmosis (RO) step was added to the process flow
diagrams shown in Fig. 2. For this case, RO is
needed to remove dissolved organics carbon and
residual nutrients such a nitrate. For desalination,
(b)
(a) 0.30...................................................................................
~ 0.20,
~ 0 1 '5
o 0.10
~O
-.~. CAS
0,05..... ~ MBR.................................................
,It
8
©
, ,ooo
Flow rate(m3/d)
CAS
MBR
Fig. 7. Operation and maintenancecosts: (a) total O&M costs; (b) O&M cost breakdown(38,000 m3/d).
CAS-TF
P. Ctt6 et al. / Desalination 167 (2004) 1-11
0.50~ .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
9
Antiscalant
Ca)
0,4~
o,o,
o.35
Secondaryeffluentfrom
conventionalactivatedsludge
o.3o
o
0.2~
~
0.15
"'*- ¢~
OdO- - -
..,+..~.~
..,~CA~-TF
,
0.0(~
0
MF/UF
i
,
,
10,000 20,000 30,000 40,000 :50,000 60,000 70,000 80,000
F l o w rat~ (mVd)
Fig. 8. Total life cycle costs.
09)
Open
seawater
Antiscalant
~
•1 Multimedia ~
filtration
~
RO
FeC13
it was assumed that surface seawater (TDS of
35,000 mg/L) was pretreated by coagulation and
multi-media filtration prior to reverse osmosis.
The process flow diagrams are shown in Fig. 9
and the conditions used for the two processes are
compared in Table 3.
The total costs estimated for producing RO
water from secondary effluent and from seawater
are compared in Table 4 for 38,000 m3/d (10 MGD)
plants. The costs in column A do not include the
cost associated with conventional activated sludge
as it was assume that sewage would be treated to
that level for discharge; for simplicity, the cost
for tertiary filtration estimated above were used
as pretreatment cost for RO. It was assumed that
the concentrate from both plants could be disposed
of at no cost.
The capital costs for a plant producing water
from seawater are about 50% higher than the costs
Fig. 9. Process flow diagrams for comparison of water
reuse and seawater desalination: (a) Water reuse; (b) Seawater desalination.
of a plant reusing secondary sewage. Both the pretreatment costs and RO cost are higher. In the case
of pretreatment, this is due to the difference in
recovery (75% for secondary effluent; 50% for
seawater), which results in a larger seawater
system. The capital cost for the seawater RO
process is higher than for the secondary effluent
RO as it is operating at a much higher pressure,
lower permeate flux, lower recovery, and must be
made of materials that resist corrosion in seawater.
Similarly, the O&M costs for producing RO
water from seawater are about 3 times higher than
the cost of reusing secondary sewage. The higher
pretreatment costs are due to chemicals, con-
Table 3
Design process conditions for the reverse osmosis plants
Unit process
Coagulation
Pretreamaent
Anti-scalant addition
Reverse osmosis
Parameter
FeC13dose, mg/L
Dose, mg/L
Stages, number
Recovery, %
Flux, L/m2/h
Feed pressure, bar (psi)
Water reuse
No
MBR or CAS-TF effluent
2
2
75
20
13.6 (200)
Desalination
5
Multimedia filtration
5
2
50
13
68 (1000)
10
P. C6td et al. /Desalination 167 (2004) 1-11
Table 4
Costs of producing water from secondary effluent and
from seawater for a 38,000 m3/d (10 MGD) plant
Component
A: from CAS B: from
effÊuent
seawater
Capital costs, $/ma/d
Pretreatment 161
RO
321
Total
482
Total life cycle costs, $/m3
Capital
0.07
O&M
0.21
Total
0.28
Ratio
(B/A)
238
492
730
1.48
1.53
1.51
0.10
0.60
0.70
1.51
2.86
2.50
tinuous dosage o f a coagulant and higher dosage
o f anti-scalant. The higher RO costs are due
primarily to energy (the operating pressure is five
times higher and the feed flow is 1.5 times higher),
but also to membrane replacement.
The total life cycle costs for producing RO
water from secondary effluent and seawater are
0.285/m 3 and 0.705/m 3, respectively, a ratio of 2.55.
The data presented in Table 4 are in agreement
with seawater RO cost data presented by Glueckstern and Priel [10], if one takes into account the
cost o f electricity used in each study (0.045S/kWh
used by Glueckstern and Priel, 0.10S/kWh used
in this study).
11. Conclusions
Two membrane options to reuse water were
presented, conventional activated sludge followed
by tertiary filtration (CAS-TF) and integrated
membrane bioreactor (MBR). Both treatment
trains provide equivalent effluent quality from the
point of view of suspended solids and Silt Density
Index and are suitable to feed a reverse osmosis
(RO) polishing step. However, the MBR is
superior for the removal of organic contaminants
and nutrients (nitrogen and phosphorus) because
the membranes work in synergy with the bioreactor.
Overall, the hydraulic retention time of the
MBR-based plants is 75% (plants <20,000 ma/d)
to 50% (plants >20,000 m3/d) smaller than those
of the CAS-TF-based plants. The land required
for an MBR plant is about half o f that required
for a CAS-TF plant.
The capital costs o f MBR-based plants are
lower than CAS-TF-based plants, but their O&M
costs are slightly higher. Overall, total life cycle
costs o f the two membrane options are comparable, and are 5-20% higher than conventional
activated sludge plants, the difference increasing
with size.
The costs to treat sewage to indirect potable
reuse standards are only a fraction of the costs to
desalinate seawater. When total life cycle costs
are considered, the cost o f treating secondary effluent by membrane filtration and RO is 0.285/m 3,
as compared to 0.705/m 3for seawater desalination
for a capacity of 38,000 m3/d (10 MGD).
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introduces new reinforced hollow fiber membrane
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immersed membrane for water filtration, Proc. IWA
2nd World Water Congress, Berlin, Germany,
October 15-18, 2001.
[3] D. Thompson, C. Schneider and M. Murphy,
Immersed membrane bioreactors for water reuse:
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Honolulu, Hawaii, January 29-31, 2003.
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[101 Glueckstem and M. Priel, Comparative cost of
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