membranes separate and remove contaminants · advanced treatment • membrane treatment | 3 figure...
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Fact Sheet
OverviewA membrane is a thin film that treats water by acting as
a selective barrier. It separates materials when a driving
force, typically pressure, is applied across the membrane.
The use of membranes for water treatment has risen
significantly in recent decades. The most common water
treatment membrane processes are microfiltration (MF),
ultrafiltration (UF), nanofiltration (NF), and reverse osmo-
sis (RO). MF and UF are porous membranes, while NF
and RO are nonporous. Tables 1 and 2 compare the four
membrane processes.
Microfiltration And Ultrafiltration MF and UF are typically applied to remove particulate and
microbial contaminants, frequently as an alternative to
rapid sand filtration (Figure 1) (Schendel et al. 2009). The
primary difference between MF and UF is the pore size;
those of MF are 0.1 µm or greater (Mallevialle et al. 1996).
A survey in WRF project #2763 found that approximately
62% of MF/UF plants used surface water as their primary
water source, 41% integrated membranes with existing
processes, while 59% were stand-alone treatment plants
(Adham et al. 2005).
In addition to the particulates listed in Table 1, these pro-
cesses may also remove limited dissolved organics, and
inorganic chemicals such as phosphorus, hardness, and
metals. While pathogen removal is the primary reason for
MF/UF membrane selection by U.S. utilities (Adham et
al. 2005), other cited advantages include using a smaller
Quick Facts
• Pathogen removal is the primary reason most U.S. utilities select microfiltration
and ultrafiltration membranes
• Nanofiltration and reverse osmosis membranes remove dissolved contaminants
such as salts, pesticides and TOC
• The use of membranes for water treatment has risen significantly in
recent decades
ADVANCED TREATMENTMembrane Treatment
Membranes Separate and Remove Contaminants
2 | Advanced Treatment • Membrane Treatment
Table 2. Approximate costs of the four main treatment processes
Membrane Treament Nanofiltration and Reverse Osmosis
Design Flow (mgd) 0.01 0.1 1.0 10 100 0.01 0.1 1.0 10 100
Average Flow (mgd) 0.005 0.03 0.35 4.4 50 0.005 0.03 0.35 4.4 50
Capital Cost ($/gal)1 $18.002 $4.30 $1.60 $1.10 $0.85 $8.253 $1.75 $1.00 $1.00 $0.75
Annual O&M Cost ($/kgal)2 $4.254 $1.10 $0.60 $0.30 $0.25 $5.005 $1.50 $0.90 $0.65 $0.551Capital costs are based on $/gal of treatment plant capacity, excluding pre- and post-treatment processes. 2For example, addition of MF or UF at a treatment facility with a capacity of 10,000 gpd would be expected to cost approximately
$180,000 ($18.00/gal x 10,000 gal = $180,000).3For example, addition of NF or RO at a treatment facility with a capacity of 10,000 gpd would be expected to cost approximately
$82,500 ($8.25/gal x 10,000 gal = $82,500).4Annual O&M costs for an MF or UF system with an average daily flow of 5,000 gallons (5 kgal) would be approximately $7,756
($4.25/kgal x 5 kgal/day x 365 days/year = $7,756).5Annual O&M costs for an NF or RO system with an average daily flow of 5,000 gallons (5 kgal) would be approximately $9,125
($5.00/kgal x 5 kgal/day x 365 days/year = $9,125).Source: Schendel et al. 2009
Table 1. A comparison of the four main membrane treatment processes
Microfiltration Ultrafiltration Nanofiltration Reverse Osmosis
Process Membrane filtration Membrane filtrationMembrane
separation
Membrane
separation
Type Porous Porous Nonporous Nonporous
Needs source water pretreatment
Yes Yes Yes Yes
Primary reason for selection
Pathogen removal Pathogen removal Hardness and organ-
ics removal
Total dissolved sol-
ids (TDS) and mon-
ovalent ion removal
Particulates removed
Suspended solids,
turbidity, some col-
loids, bacteria, and
protozoan cysts
Suspended solids,
turbidity, some
colloids, bacteria,
protozoan cysts and
some viruses
Dissolved con-
taminants such as
salts or salinity,
pesticides, total
organic carbon, and
pathogens
Dissolved con-
taminants such as
salts or salinity,
pesticides, total
organic carbon, and
pathogens
Source: Schendel et al. 2009
Advanced Treatment • Membrane Treatment | 3
Figure 2. Flow diagram for typical NF/RO water separation process
land area for the plant in comparison with conventional
filtration, and lower capital and O&M costs (EPA 2001).
Membrane filtration systems must undergo periodic
direct integrity testing and continuous direct monitor-
ing during operation (EPA 2005). Membrane fouling is a
common challenge, causing these systems to frequently
require source water pretreatment and routine back-
washing. Residuals generated from MF and UF systems
include spent cleaning solutions and spent backwash.
Spent cleaning solutions are generally acidic and
require neutralization. Spent backwash may be recycled
to the process or discharged to a sanitary sewer or
receiving stream.
Nanofiltration and Reverse OsmosisNF and RO are membrane separation technologies that
divide contaminants based on differences in solubility and
diffusivity (Mallevialle et al. 1996). A feed pressure forces
water through a membrane, increasing the dissolved
contaminant on one side of the membrane. The primary
difference between the two is the size of contaminants
that can be removed. NF membranes are typically used to
Figure 1. Flow diagram for typical MF/UF water treatment process
Rapid Mix Flocculation/Sedimentation
Microfiltration/Ultrafiltration
Coagulant
Source: Schendel et al. 2009
Storage
Feed Pump(Pressure Systems)
Filtrate Pump(Vacuum Systems)
Concentrate to Waste
Permeate
pH AdjustmentAcid/Antiscalant
Source: Schendel et al. 2009
Storage
Nanofiltration/Reverse Osmosis
Feed Pump
4 | Advanced Treatment • Membrane Treatment
ReferencesAdham, S., K. Chiu, K. Gramith, and J. Oppenheimer. 2005.
Development of a Microfiltration and Ultrafiltration
Knowledge Base. Project #2763. Denver, Colo.:
AwwaRF.
Antony, A., and G. Leslie. 2014. Protocol for Evaluating
Chemical Pretreatment for High Pressure Membranes.
Project #4249. Denver, Colo.: AwwaRF.
Drewes, J. E., C. Bellona, P. Xu, G. L. Amy, G. Filteau, and
G. Oelker. 2008. Comparing Nanofiltration and Reverse
Osmosis for Treating Recycled Water. Project #3012.
Denver, Colo.: AwwaRF.
EPA (U.S. Environmental Protection Agency). 2001. Low-
Pressure Membrane Filtration for Pathogen Removal:
Application, Implementation, and Regulatory Issues.
EPA 815-C-01-001. Washington, D.C.: EPA Office of
Water. Accessed June 10, 2016. nepis.epa.gov/Exe/
ZyPDF.cgi?Dockey=P10056FL.txt.
———. 2015a. Water Treatability Database. “Membrane
Filtration.” Accessed June 3, 2016. https://iaspub.
epa.gov/tdb/pages/treatment/treatmentOverview.
do?treatmentProcessId=510273414
———. 2015b. Water Treatability Database. “Membrane
Separation.” Accessed June 3, 2016. https://iaspub.
epa.gov/tdb/pages/treatment/treatmentOverview.
do?treatmentProcessId=-2103528007
Hofman, J. A. M. H., A. J. Gijsbertsen, and E. Cornelissen.
2006. Nanofiltration Retention Models for Organic
Contaminants. Project #2945. Denver, Colo.: AwwaRF.
Mallevialle, J., P.E. Odendaal, and M.R. Wiesner, eds. 1996.
Water Treatment Membrane Processes. Project #826.
New York: McGraw-Hill.
Schendel, D. B., Z. K. Chowdhury, C. P. Hill, S. Summers, E.
Towler, R. Balaji, R. S. Raucher, and J. Cromwell. 2009.
Decision Tool to Help Utilities Develop Simultaneous
Compliance Strategies. Project #3115. Denver, Colo.:
Water Research Foundation.
Last updated October 2016
remove hardness and organics, such as DBP precursors.
RO membranes are typically used to remove TDS and
monovalent ions such as seawater desalting and fluoride
and chloride removal (Schendel et al. 2009).
In addition, both processes remove salts, salinity,
pesticides, total organic carbon, and pathogens, as
well as many trace organic contaminants, including
some of emerging concern from recycled water or
other challenged water supplies (Drewes et al. 2008,
Hofman et al. 2006).
Typical NF and RO water separation processes include
three basic flow streams: (1) the feed, (2) the permeate or
product, and (3) the concentrate or water streams. These
treatment processes generally consist of multiple stages,
wherein the concentrate from the prior stage becomes
the feed for the subsequent stage. The permeate from
each stage is blended together for the final product
stream. The concentrate from the final stage is usually
wasted (Figure 2).
Like porous membranes, NF and RO systems require
pretreatment to prevent membrane fouling. For surface
waters, pretreatment may be extensive (Antony and
Leslie 2014). Other considerations with these membranes
include concentrate disposal, spent chemical cleaning
solutions, post-treatment pH and/or alkalinity adjustment,
corrosion of cement-mortar linings in distribution piping,
and low alkalinity waters leading to increased corrosion of
iron, lead and copper.
Concentrate disposal, highly regulated by government
agencies, typically involves a high-volume, high total
dissolved solids waste stream. Discharge requires a large
body of water, wastewater treatment plant, or deep well
injection. Acidic spent chemical cleaning solutions require
neutralization. An orthophosphate-based corrosion inhib-
itor can help minimize the potential for increased met-
als release.
Pressure-driven membrane treatment processes improve operating efficiency and offer lower capital and O&M costs compared with conventional treatment.