GEPA Capsule Report

United States

Environmental Protection

Agency

Technology Transfer

Office of Research and

Development

Washington DC 20460

EPA/625/R-961009

September 1996

Reverse Osmosis Process
Technology Transfer

Capsule Report

EPA/625/Fi-961009

Reverse Osmosis

Process

September 1996

Center for Environmental Research Information

National Risk Management Research Laboratory

Office of Research and Development

U.S. Environmental Protection Agency

Cincinnati OH 45268

@ Printed on Recycled Paper
Contents

Process Description . . . . . . . . . . . . . . . . . . . 1

Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

Equipment .................................. 2

Operation and Maintenance ..... .4

Failure Analysis.. ........................ 6

References................................. 9

Introduction A failure analysis has been completed for the reverse osmosis (RO)

process. The focus was on process

failures that result in releases of liquids and vapors to the environment.

The reoort includes the followina:

A description of RO anlcoverage of the principles behind

the process.

Applications of RO for treatment of effluent waters from

the metal finishing industry.

Descriptions of equipment and

operating and maintenance

procedures.

Failure analysis that includes

types of failures and causes.

Key questions that can be used

for software development.

A bibliography on RO applications in the metal finishing industry.
Reverse Osmosis

Process

Process Description

In the reverse osmosis (RO) process, water passes through a membrane, leaving behind a solution with

a smaller volume and a higher concentration of solutes. The solutes can

be contaminants or useful chemicals

or reagents, such as copper, nickel,

and chromium compounds, which can

be recycled for further use in metals

plating or other metal finishing processes. The recovered water (penneate) can be recycled or treated

downstream, depending on the quality of the water and the needs of the

plant. As shown in Figure 1, the water that passes through the membrane

is defined as permeate and the concentrated solution left behind is defined as retentaie (or concentrate).

The RO process does not require

thermal energy, only an electrically

driven feed pump. RO processes have

simple flow sheets and a high energy

efficiency. However, RO membranes

can be fouled or damaged. This can

result in holes in the membrane and

passage of the concentrated solution

to clean water, and thus a release to

the environment. In addition, some

membrane materials are susceptible

to attack by oxidizing agents, such as

free chlorine.

Pressurized

wastewater

(dragout)

The flux of component A through

an RO membrane is given by Equation (1):

NA

where

NA=

PA =

DF=

L =

Flux of component A through

the membrane, mass/timelength2.

Permeability of A, mass-length/

time-force.

Driving force of A across the

membrane, either pressure difference or concentration difference, force/length2

 or mass/

length3.

Membrane thickness, length.

At equilibrium, the pressure difference between the two sides of the

RO membrane equals the osmotic

pressure difference. At low solute concentrations, the osmotic pressure ( p)

of a solution is given by Equation (2):

TC = csRT (2)

where

p = Osmotic pressure, force/

length2.

C, = Concentration of solutes in solution, moles/length3.

0 l 0 0 l 0 0 0 0 _ * Concentrate

rn. .A n CJ

7

w w

lo 0 . 00 0

l : Oî 0í m 0 -. r

DD-621

Figure 1. Reverse osmosis process.

Water

(permeate)

(1)

1
R = Ideal Gas Constant, (forcelength)/(mass-temperature).

T = Absolute temperature, “K or “R.

As a mixture is concentrated by

passing water through the membrane,

osmotic pressure of the solution increases, thereby reducing the driving

force for further water passage. An

accurate characterization of the pressure to drive the RO process must be

based on an osmotic pressure computed from the average of the feed

and retentate stream compositions.

The water recovery of an RO process

may be expressed by Equation (3):

REC = (Q#Q,) x 100 (3)

where

REC = Water recovery, %.

QP = Permeate flow rate, length3/

time.

QF = Feed flow rate, IengthVtime.

Water recovery is determined by

temperature, operating pressure, and

membrane surface area. Rejection of

contaminants determines permeate

purity, while water recovery primarily

determines the volume reduction of

the feed or the amount of permeate

produced. Generally, for concentration of waters from the metal finishing

industry, greater water recoveries are

desirable to obtain overall greater volume reduction.

Applications

Nickel plating rinsewaters can be

treated with RO with over 90% of the

rinsewater recovered, with suitable

quality for reuse. Plant payback for a

5 cubic meter per hour recovery RO

plant has been estimated at 1.3 years

in the case of 2,000 mg/l nickel in the

feed (Shoeman et al., 1992; Cross

and Evans, 1991). There are at least

150 RO systems operating on various types of nickel baths; most use

cellulose acetate membranes

(Cartwright, 1984).

At least 12 RO systems are operating on various copper sulfate rinses.

These systems use both hollow-fiber

polyamide and cellulose triacetate

membranes and spiral-wound, thinfilm composite types, and offer a

membrane life of 1 to 3 years.

One effective RO system, which is

being used on a zinc sulfate rinse,

employs spiral-wound thin-film composite membranes at a feed rate of

45 gal/hr and at a water recovery of

88%. The membrane concentrate is

further reduced in volume in an evaporator and returned to the process

(Kinman, 1985; Cartwright, 1984).

Approximately five RO systems are

operating on various types of brass

cyanide rinses. Both polyamide and

cellulose triacetate hollow-fiber membrane elements are used.

An RO system is being used after

contact plating on printed circuit

boards. The rinse is fed to a polyamide hollow-fiber membrane element

at the rate of 210 gal/hr. The system

is operating at a water recovery of

about 90%; part of the concentrate is

recycled to the plating bath and the

remainder is routed to the waste treatment system. All of the membrane

permeate is reused as a rinsewater

(Cartwright, 1984).

Cadmium and chromium rinsewaters are also treated with RO. Membrane fouling has been experienced

for the cadmium rinsewaters, but little

fouling has been experienced for chromium rinsewater applications. Preliminary results show that payback for 5

cubic meters per hour RO cadmium/

water and RO chromium/water recovery plants are three and seven years,

respectively (Shoeman et al., 1992).

In other industries, RO is used for

production of potable water from seawater and brines, for water recovery

from landfill leachates, and for concentration of industrial wastewaters

and brines. RO is sometimes used as

a pre-concentrator for evaporators to

lower energy requirements and increase process efficiency. RO has

also found many applications in the

food and dairy industries; it is used in

the food industry to concentrate apple

juice and in the dairy industry to concentrate cheese whey.

Equipment

The module is the housing that contains the membrane. With regard to

failure analyses, module configuration

is important because some types of

modules are more reliable than others. Membrane modules are commercially available in four configurations:

. Plate-and-Frame

. Spiral-Wound

. Hollow-Fiber

. Tubular

Plate-And-Frame Modules

As shown in Figure 2, plate-andframe modules use flat sheet membranes that are layered between

spacers and supports. The supports

also form a flow channel for the permeate water. The feed water flows

across the flat sheets and from one

layer to the next. Recent innovations

have increased the packing densities

for new design of plate-and-frame

modules. Maintenance on plate-andframe modules is possible due to the

nature of their assembly. They offer

high recoveries with their long feed

channels and are used to treat feed

streams that often cause fouling problems. Only recently advanced designs

of plate-and-frame modules capable

of operating up to 25% dissolved solids and operating pressures up to

4500 psia have been placed in operation in Germany (Stanford and

Miller, 1994). This development opens

new opportunities for the use of reverse osmosis for concentration of

metal finishing wastewaters.

Spiral-Wound Modules

Spiral-wound modules use a sandwich of flat sheet membranes and

supports, wrapped spirally around a

collection tube (see Figure 3). The

feed flows in against one end of the

rolled spiral and along one side of the

membrane sandwich. The support layers are designed to minimize pressure drop and allow a high packing

density. Additionally, the spiral-wound

modules can be designed by equipment suppliers to promote turbulence

and therefore increase the mass transfer across the membrane or to provide an uninterrupted flow path to

decrease membrane fouling. Spiralwound modules offer greater packing

densities, but maintenance is difficult.

Hollow-Fiber Modules

As shown in Figure 4, hollow-fiber

modules consist of small diameter

membrane fibers bundled within cylindrical pressure vessels. The fibers

are pressurized from the outside. The

permeate flows to the interior bore or

lumen of the fiber and down the length

of the fiber to the product header.

Fibers can also be pressurized from

the inside, but greater mechanical

strength of the fibers is necessary to

prevent fiber rupture. By feeding on

2
Concentrated

solution

4-

Permeate 1-t-^-

(uaal I

water)

Wastewater 4

(dragout)

DD-837

Figure 2. Plate-and-frame reverse osmosis module.

Feed

\

Feed -

Membrane

Permeate

MM-9

Figure 3. Spiral-wound module.

3
Retentate outlet

Fiber bundle plug

Hollow fiber

i

Carbon steel shell

Liquid feed

MM-10

Figure 4. Hollow-fiber module.

the shell side of the fibers, a lower

pressure drop is encountered down

the bore of the fiber since the permeate flow rate is less than the feed

flow rate. Hollow-fiber modules offer

the greatest packing densities of the

configurations described.

Tubular Modules

Tubular modules have membranes

supported within the inner part of

tubes. The operator can easily service feed and permeate channels to

remove fouling layers. Tubular modules are somewhat resistant to fouling when operated with a turbulent

feed flow. This is accomplished with

F Permeate

larger flow channels than those used

with hollow-fiber and spiral-wound

modules. The drawbacks of tubular

modules are their high energy requirements for pumping large volumes of

water, high capital costs, and low

membrane surface area per unit volume of module (see Figure 5 ).

Operation And

Maintenance

To maintain membrane performance and extend membrane life,

pretreatment chemicals may be necessary, depending on the characteristics of the wastewater. In addition,

chemicals may be required to achieve

clean water specifications. Filtering

wastewater may be necessary to remove suspended solids before wastewater is fed to the RO modules.

Membrane performance can be enhanced by control of pH, removal of

certain dissolved species and colloidal materials such as clays and oils,

and dissolved or suspended organits. In any RO system, depending on

the capacity and size of modules, a

number of parallel modules may be

needed.

Membrane fouling can result from

the formation of a fouling layer on the

membrane surface, or from internal

changes of the membrane material.

Both forms of fouling can cause membrane permeability to decline. Scaling

is a form of fouling that occurs when

dissolved species are concentrated

in excess of their solubility limit.

Chemical agents can be added to

slow the formation of precipitates.

Acidification is used to prevent the

formation of carbonates of low solubility, such as magnesium carbonate.

An ion exchanger is sometimes used

to trade cations of low solubility salts

for cations that are more soluble, for

example, sodium sulfate may be

traded for calcium sulfate.

Prevention of biological growth is

necessary to prevent damage to the

membrane. Biological growth can be

inhibited with chlorination, but some

RO membranes are chlorine sensitive, so water must be dechlorinated

before entering the RO module. Other

disinfectants are ozone, formaldehyde,

ultraviolet light, copper sulfate, and

sodium bisulfate. A schematic of an

RO system with four modules in parallel, chemical pretreatment, and an

up-front filtration step is shown in Figure 6.

Staging RO Systems

RO can be used as a one or twostage process, depending on requirements for purity of the water removed

(permeate). In the two stage process,

the permeate from the first stage is

“polished” by the second, producing

a higher purity water than is possible

with one stage alone. As indicated in

Table 1, solute concentration in the

permeate may be reduced from about

500 ppm for one stage to 6 ppm in

two stages. The flow diagram for the

two-stage RO process is shown in

Figure 7.

4
Shell

Membrane Baffle Header cover

Feed

t

Retentate

+

DD-595

Figure 5. Tubular module.

1 ‘Tube

Permeate water

Wastewater

(dragout)

Pump

DD-838

Figure 6. Reverse osmosis system.

t

Concentrated solution

for recycle (retentate)

Pump

Clean water

(permeate)
Table 1. Reverse Osmosis: One- and Two-Stage Processes, Water Recovery, and Purity

Configuration Water Recovery,% Water purity, ppm

RO-one stage 77 500

RO-two stage 77 6

Prefiltered and

treated metal

finishing industry

wastewaters

(dragout)

DD-592

1st stage RO

Concentrated

l------ sohJt’on

1 st stage

permeate

2nd stage RO

Figure 7. Two-stage reverse osmosis process.

Failure Analysis

A failure analysis is presented below for the RO process when used to

treat waters from the metal finishing

industry. As shown below, the failures are categorized as to probability

of occurrence (high, moderate, and

low). To our knowledge there are no

published data that further quantify

the frequency of occurrence of these

failures.

High Probability

Relief Valves (Liquid)

Liquid relief valves are included in

RO (and other processes) to protect

the piping from overpressure. Overpressure frequently occurs during

startups, shutdowns, and upsets.

Overpressures can result from control valves failing in the closed position, and from the plugging of valves,

Clean

water

piping, and membrane modules.

Plate-and-frame and tubular modules

are not as susceptible to plugging as

hollow-fiber and spiral-wound modules.

Seals

Seal or o-ring failures may occur in

the membrane feed pump, chemical

feed pump, or the air compressor that

delivers instrument air to instruments

6
and control valves. Possible causes

of seal failures include overheating

and mechanical stress. Visual inspection can confirm spraying or leaking

of wastewater at the pumps or compressor.

Valves and Pipe Fittings

These failures are more prevalent

in older plants than in newer ones.

Causes include mechanical stress,

improper maintenance procedures,

and freezing during cold weather. Visual observations can confirm leaks

of wastewater or chemicals from valve

stems and fittings.

Miscellaneous Spills During

Daily Operations

Spills of chemicals or wastewater

frequently occur when tanks are replenished or when the system is shut

down for maintenance. For RO systems, chemical spills can include acids, bases, phosphates, and chlorine.

Relief Valves (Vapor)

Storage and run down tanks are

equipped with vapor relief valves to

maintain a constant pressure. These

valves release contaminated vapors

to the atmosphere as tank levels (and

tank pressures) increase. These releases are small, but they can occur

frequently.

Moderate Probability

Tank Overflows

Tank overflows can result in significant releases of wastewaters or

chemicals to the environment. They

occur mostly during startups, shutdowns, and plant upsets.

Membrane Failures

Holes may develop in the membrane material, allowing wastewater

to escape to contaminate the clean

water permeate. The potting material

that attaches the membrane material

to the module housing may also fail

and result in contamination of the

clean water permeate. If the upstream

filters fail, solids can escape and damage the membrane. And the membrane can be defective when it is

delivered from the supplier. In addition, corrosive chemicals, such as

chlorine, can attack some types of

membranes, though some membrane

materials are more durable than others. For example, ceramics are more

durable than polymer membranes. An

indication of membrane failure is a

sudden reduction in pressure drop

across the membrane.

Low Probability

Tank Ruptures

A tank can rupture, possibly because of mechanical failure or freeze

damage. Though this type of failure

is rare, a rupture can result in the

release of a large quantity of wastewater or chemicals to the environment.

Piping Ruptures

Piping is typically strong and not

likely to rupture. Possible causes of

rupture include mechanical stress,

freezing, and improper maintenance

procedures. Large leaks are possible

with this type of failure.

A summary of the types and causes

of failures and the associated questions for later software development

are presented in Table 2.
Table 2. Failure Analyses for Reverse Osmosis System

Failure Cause(s)

High Probability

Questions for Software

Development

Relief valves (liquid)

Seals

Valves and pipe fittings

Miscellaneous spills

during daily operations

Relief valves (vapor)

Tank overflows

Membrane module

failures

Tank ruptures

Piping ruptures

- Overpressures during startups, upsets, and shutdowns

- Key control valves failing in

closed position.

- Plugging of valves, piping, and

membrane modules due to buildup

of solids. Hollow-fiber and spiral

membrane modules are most

susceptible to fouling.

What is the expected quantity of leaks through the

liquid relief valves (gallons)? What is the disposition of

these leaks (i.e., Do they go to a capture system,

process sewer, or are they lost directly to the environment)?

- Overheating

- Mechanical stress

- Abrasive wear

What is the expected quantity of leaks through seals

(gallons)? What is the disposition of these leaks?

- Mechanical stress What is the expected quantity of leaks through

- Improper maintenance procedures valves and pipe fittings (gallons)? What is the

- Freezing disposition of these leaks?

- Spills during filling of tanks (due to What is the expected quantity of leaks from spills

faulty gages and equipment and (gallons)? (Base on plant experience and

mistakes by operators). Spills can operating records). What is the disposition of these

include pretreatment chemicals spills?

(such as acids, bases, and phosphates).

- Faulty maintenance procedures

- Increases in tank levels

- Changes in ambient temperature

What is the expected quantity of leaks through vapor

relief valves (standard cubic feemour)? What is the

disposition of these leaks?

Moderate Probability

- Occur mostly during unstable

conditions (during startups and

shutdowns). Overflows can

include pretreatment chemicals

(such as acids, bases, and phosphates).

- Membrane defective

- Module potting material defective

- Presence of corrosive chemicals

- Presence of solids

What is the expected quantity of tank overflows

(gallons)? (Base on plant experience and records).

What is the disposition of these overflows?

What is the expected quantity of leaks through membrane

modules (gallons)? What is the disposition of these leaks?

Low Probability

- Mechanical failures

- Freezing

What is the expected quantity of releases due to tank

failures (gallons)? (Be sure to include the concentrated

waste if it is stored onsite). What is the disposition of

these releases?

- Mechanical failures

- Freezing

What is the expected quantity of losses due to pipe

ruptures (gallons)? What is the disposition of these

losses?

DD-839
References Shoeman, J. J. et al., “Evaluation

of Reverse Osmosis for ElecSuggested Reading

Cartwright, P. S., “An Update on

Reverse Osmosis for Metal Finishing,” Plating and Surface

Finishing, April 1984, pp 62-

66.

troplating Effluent Treatment,”

Water Science and Technology, 25:lO (1992) pp 79 93.

Stanford, P. T., and K. A. Miller,

“Cleanup of Hazardous Waste

Using an Advanced Reverse

Osmosis System,” paper presented at Emerging Technologies in Hazardous Waste

Management VI, Atlanta, Georgia, September 1994.

1.

2*

3.

Ho, W. S. and K. K. Sirkar,

Membrane Handbook, Van

Nostrand Reinhold, New York

(1992).

Cross, J. R. and P. A. Evans, “Recycling Rinse Waters and Recovering Metals,” Metal

Finishing, 15:7, July 1991.

Kinman, R. N. et al., “Reverse Osmosis Membrane Fouling,”

Metal Finishing, November

1985, pp 53-55.

4.

5.

Amjad, Z., Reverse Osmosis:

Membrane Technology, Water

Chemistw, and industrial Ap

plications, Van Nostrand

Reinhold, New York (1993).

Eisenberg, T. N. and E. J.

Middlebrooks, Reverse Osmosis Treatment of Drinking Water, Butterworths Publishers,

Boston, MA (1986).

Belfort, G., Synthetic Membrane Processes, Academic

Press, Inc., Orlando, FL (1984).

Porter, M. C., Handbook of Industrial Membrane Technology,

Noyes Publications, Park

Ridge, NJ (1990).

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