ro vs uf system

7/29/2019 RO vs UF System

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UF Ap pl ica ti on UF Des cr ip ti on Te ch ni ca l S pe ci fi ca ti on s E PAData

Hollow Fiber Ultra Filtration Membranes

Compared to the Drawbacks ofReverse Osmosis Technology

The market for membrane-based water treatment is booming.

Respected market researchers Frost & Sullivan estimated that reverse osmosis (RO) comprises 45 percent of revenues for the

residential under-the-sink water treatment equipment market and had predicted that this segment would experience a

compounded annual growth rate (CAGR) of 7.7 percent from 2004 through 2008.

The same market research company estimated a CAGR of 12.9 percent for hollow-fiber ultra-filtration (UF) m embrane revenues

through 2010.

Newcomer to residential

For years, RO membrane technology for residential and commercial water treatment has been widely used and accepted. Other

well-known applications for RO include seawater/brackish water desalination and ultrapure water production for industrial

processes.

UF membrane technology only recently has been used for residential water treatment, although it has been proven and accepted

in municipal markets for many years. UF is widely used for municipal drinking water and wastewater treatment as well as in the

dairy, beverage and food production industries.

Here well compare the two membrane technologies to show where they can be used and how they can be combined. Although

both RO and UF have applications in which each can excel, in many instances RO and UF are complementary technologies and

are not competitive at all.

Cross-section of a UF membrane, magnified 500 times.

Difference is in pore sizes

Both RO and UF are processes based on pressure-driven membranes. The fundamental difference between them is the pore


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.

Both types of membranes contain millions of pores per square inch, but those on an RO membrane are much smaller than those

of a UF membrane. RO pore sizes are generally 0.001 micron or smaller; UF pore size ranges from 0.001 to 0.1 micron. RO

technologies use dense non-porous membranes; UF uses asymmetric porous m embranes as a separations barrier (see

accompanying photos).

RO membranes reject dissolved and suspended materials (including m onovalent salts) and permit relatively few s ubstances to

pass. UF membranes remove particles which are bigger than the pore size of the membranes and leave almost all the dissolved

substances in the permeate (product water).

The separation performance of the RO membranes is often described in terms of their rejections of certain minerals (mainly

NaCl). UF membranes are characterized by their nominal pore size or molecular weight cut-off (MWCO) because the separation

is based almost entirely upon the differences between the pore size of the membrane and that of the particles themselves. UF

membranes can also be characterized by their log reduction capacities for bacteria, cysts and viruses.

Table 1: RO and UF technical comparison

Reverse Osmosis (RO) Ultra Filtration (UF)

Rejec ted Subs tances Prac tic ally all sus pended and diss olved

materials above 100 molecular weight

All particles above t he pore s ize of the

membrane

Membrane Pore Hole Size 0.001 micron or less 0.001-0.1 micron

Type of Membrane Dense nonporous Asymmetric porous

Most Common module Type Spiral Wound Hollow fiber

Most Common Operation Mode Cross-flow filtration Dead-end filtration

Typical Pressure 50 - 1,200 psi 5 150 psi

Operating pressures

Due primarily to the difference in relative pore sizes between RO and UF membranes, the required operating pressure for RO is

usually much higher than that of UF. Less pressure is required to force water through the larger pores of UF membranes.

RO is often utilized in the cross-flow filtration mode, which means that only a portion of the feedwater actually passes through themembrane to produce permeate. The rejected m aterials are flushed away in a stream called concentrate or retentate.

UF generally works in dead-end filtration mode. That is, all the water that enters the membrane surface is forced through the

membrane. Some solids and components will stay behind on the membrane while water flows through. See Tables 1 and 2 for

comparisons of RO and UF characteristics.

Different applications

Because of its relatively low operating pressure requirements and water waste, UF membrane technology is preferred for

separation processes where the undesired materials present do not include dissolved solids. In these situations the investment

and operation costs of UF are much lower and the sensitivity of the membrane to many contaminants is also lower.

Some examples of the contaminants for which UF demonstrates greater operational and economic benefits are: bacteria, cyst

and virus reduction; suspended solid removal; and colloidal materials removal from water.

Very often UF is used as RO pretreatment. The longevity and efficient operation of RO membranes often depend upon the quality

of the feedwater.

UF membranes are relatively insensitive to upsets caused by high turbidity or variable raw water quality and can consistently

produce RO feedwater with a turbidity of less than 0.1 NTU (nephelometric turbidity units) and a low silt density index (SDI), often

less than 2.5. UF can also be used as post-treatment for RO applications where bacteriological disinfection is desired. See Table

3 for examples of RO and UF applications.

Table 2: Some characteristics of RO and UF


Total Suspended Solids (TSS) Reduction Yes Yes

Total Dissolved Solids (TDS) Reduction Yes No

Bacteria, Virus and Cyst Reduction Yes with Limitations due to Membrane

Configuration

Yes

Ability to Reduce Inorganic Contaminants (lik e Yes No


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me a s, c em cas, nsec c es

Application for Direct use on Ground and

Surface Water Supplies

No Yes

Fl ow and pres sure drop c harac teris ti cs Low flow/ high pres sure drop High flow/ low pres sure drop

Pre-treatment Requirements Relatively high Relatively low

Membrane sensitivity to iron and hardness Relatively High Relatively Low

Membrane sensitivity to chlorine Depends on membrane Relatively low

Ease of use for point-of-entry (POE)

applications

Low High

Ease of membrane cleaning Low High

Product water volume Low High

Volume of water sent to drain (waste water) High Low

Each can improve the other

Despite the fact that both RO and UF are membrane treatment technologies, the two are quite different in terms of performance

characteristics and claims.

In most applications either one or the other will be most suitable, though in many applications RO and UF complement each other,

and the two technologies can be combined to provide additional benefits to customers:

UF can improve RO performance and longevity when used as pretreatment for RO membranes.

RO can reduce the dissolved substances that UF cannot.

UF can be used on water from variable sources (municipal, ground or surface water supplies) without significant

pretreatment, while RO generally c annot. W hen used before or after RO, UF can add bacteriological claims as an additional

benefit. Relatively few UF membranes can make an independently tested and certified bacteriological purification claim.

(The authors are aware of only one brand of UF membrane in widespread use today that can do this.)

Table 3: RO and UF applications


Residential applications for point-of-use (POU)

drinking water TDS reduction

Yes No

Residential application for point-of-entry (POE)

sediment, turbidity, colloid, bacteria, virus and

cyst reduction (surface and groundwater

treatment)

No Yes

High-Purity water for industrial use us in

microelectronics, food and beverage, power and

pharmaceutical facilities

Yes No

Sea and brackish water desalination Yes No

Oily wastewater treatment No Yes

Reuse of municipal wastewater No Yes

Polishing wastewater UF permeate Yes No

Milk protein concentration No Yes

Whey protein concentration Yes Yes

Dying effluent treatment Yes No

Due to the many differences in membrane types and m embrane qualities currently in use, water treatment professionals should

communicate with membrane manufacturers to determine if the claims being made have been independently tested and certified

by a well-known and trusted organization like the WQA Gold Seal Program or NSF International.

ro vs uf system

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