waterrf.org

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.