Ro Hot Filtration

Industrial water reuse opportunities and high

temperature compatible membranes

A.-C. Valentin

ABSTRACT

A.-C. Valentin

GE Water & Process Technologies,

1 allee du 1er Mai,

77183 Croissy-Beaubourg,

France

E-mail: [email protected]

Process condensates represent a real value to the industry as it usually contains several

items of potential savings, including but not limited to heat energy and water. In most

cases the condensate has become contaminated with unwanted particles or with product

carryover making it unfit for direct reuse in the process or as boiler makeup water.

Conventional methods use heat exchangers for partial recovery of the heat content to

be followed by ion exchange or reverse osmosis limited to 30408C feed temperature.

By using the Durathermw High Temperature compatible membranes in RO and NF, it is

now possible to process the condensate at temperatures up to 808C thereby maintaining

the calorific value of the stream. Many plants also produce a product using evaporation.

The overheads from the evaporators usually contain a small amount of their product that

must either be recovered by an additional evaporation step or disposed of in a waste

treatment plant. These reverse osmosis or nanofiltration systems allow concentration

of the product, produce high quality water suitable for reuse, and reduce the load on the

waste treatment plant. The treated condensate can then be used for various utility

operations including boiler & process makeup.

Key words | condensate recovery, high temperature, nanofiltration, reverse osmosis,

water reuse

INTRODUCTION

Better managing the environment and the natural

resources will be the main challenge for the 21st century

as those resources become scarce. Oil is becoming scarce

and more expensive to extract. Even if the production is

limited to specific regions, the ability to transport oil

and liquefied gas easily has allowed the globalization

of oil&gas commerce. As a consequence, the difficulties

around oil availability are driving the prices up affecting

every industry on the planet. Water scarcity remains a local

problem: even if it is commonly known that pure water is

becoming scarce at a global level via massive pollution of

rivers or salt water intrusion in aquifers, the price setting of

water remains dependent on the local availability of water.

In an inflationary environment, water & energy

recovery projects become effective tools for industries to

generate savings while benefiting from CO2 emission

trading and governmental incentives. Such projects can

also be used as part of the corporate communication to

highlight the environmental awareness of the given industry.

This article will describe the high-temperature compatible

membranes and how they can be used in industries

generating or consuming hot aqueous streams for water

and energy recovery.

INDUSTRIAL CONDENSATES

Whatever the industry, most industrial processes require

at some level a transfer of heat and water is the most

common transport fluid for this heat both for heating and

doi: 10.2166/ws.2010.083

113 Q IWA Publishing 2010 Water Science & Technology: Water SupplyWSTWS | 10.1 | 2010

for cooling purposes. In the absence of contamination,

water or steam is directly reused for the same heat transfer

function. For example closed cooling circuits are using

water as cooling fluid with constant recycle. In evaporation,

distillation or steam generation, the clean condensates from

the first effects are recycled to the entry of the heating

process in order to recover the calories and optimize the

energy efficiency.

Contaminated streams

When used for heat transfer, steam can be contaminated

by contact with the product or with product-related

equipment. The contaminated condensed steam identified

as condensate becomes a waste stream with an elevated

calorific content.

Concentration processes based on heating, whether

under partial vacuum or at atmospheric pressure, are based

on the evaporation of all or part of the solvent. For aqueous

liquids, the condensed solvent identified as evaporator

condensate, is another hot waste stream generated by

equipments such as evaporators, concentrators, spray

dryers or crystallizers.

Unlike clean steam condensates, the condensates

generated by product evaporation and contaminated con-

densed steam (after product contact) are not suitable for

direct recycle into a boiler and are generally transferred

to the wastewater treatment plant. The use of high-

temperature compatible membranes for the purification of

aqueous condensates and contaminated condensed vapor

allows the recovery of both water and energy.

Typical contaminants

Condensate composition will vary depending on the

generating process, however there are main characteristics,

which apply to all of them. Because they are generated

from the condensation of contaminated water vapor,

condensates are hot. That may seem obvious, but the

available condensate temperature typically varies from

558C to 958C. The second main and common characteristic

is the extremely low content of suspended solids.

The dissolved solids present will then include minerals

and organics.

In the case of contaminated condensed vapor after

product contact, the range of possible contaminants is

broader as it will include any substance present on the

product surface, which is soluble in hot water.

Whatever the generating process, the industrial con-

densates typically have Total Dissolved Solids content

comparable to city water (200400ppm). The organic

content (TOC) which can vary greatly depending on the

substance evaporated, was less than 500ppm in the applica-

tions detailed in this article.

AVAILABLE CROSS-FLOW TECHNOLOGIES

Membrane cross flow filtration covers a wide range of

selectivity from the suspended solid removal with micro-

filtration (MF), to the demineralization applications using

Reverse Osmosis (RO).

The purification of condensates requires the removal of

dissolved salts and organics: reverse osmosis and nano-

filtration are the two filtration technologies that will allow

remove charged and uncharged dissolved species.

Membrane materials can be either polymeric or

inorganic. Non-polymeric membranes exist in a variety of

inert materials (ceramic, stainless steel or carbon), which

can be used up to very high temperatures. They are

covering the MF and ultrafiltration (UF) ranges, with

selected membrane as tight as 5,000 Dalton. However this

is not sufficient for the removal of dissolved minerals.

Polymeric membranes

The different polymeric membranes can be split into two

different structure groups.

The homogenous membranes: they consist of one singlepolymer cast on a non-woven backing material which

provides the mechanical resistance.

The composite membranes, also called TFC (Thin-FilmComposite) or TFM (Thin-Film Membrane), they are

made in two-layer or three-layer designs. The thin

skin layer is polymerized in situ on a polyethersulfone

UF membrane, cast on a backing. The three-layer

design has two thin film membranes on top of the UF

membrane. The three-layer design provides an extremely

114 A.-C. Valentin | High-temperature compatible membranes for industrial water reuse Water Science & Technology: Water SupplyWSTWS | 10.1 | 2010

smooth surface compared to the two-layer design as

shown on Figure 1. The Duratherm RO and NF

membranes are built using three-layer polymers, whereas

the most common RO and NF membranes available

on the market for water applications are built with

the two-layer design.

Membrane element configuration

So the membrane used is an assembly (superposition) of

polymers cast on a non-woven support fabric. This mem-

brane can be assembled in different module configuration:

spiral-wound tubular hollow fiber plate & frame

Tubular and hollow fibers are common configurations

for UF applications because they can tolerate suspended

solids. Main applications are surface water filtration and

wastewater treatment.

Plate & frame being the least compact configuration,

it is used for small flow rate applications with a very

important amount of suspended solids mostly with UF and

RO membranes. Well established industrial examples are:

Kubota membrane bioreactor, Rochem High pressure

systems for landfill leachate, Novasep Pleiade for electro-

deposition paint recovery.

The spiral-wound configuration was developed in the

1980s for water desalination. Being the most compact, it

was also economical and all other applications and

industries tried to converge to this configuration. The

element construction consist in a number of different

components organized around a permeate collection tube.

The semi-permeable membrane is pleated and glued in the

shape of envelopes, with the thin-film membrane on the

outside. A permeate carrier is placed inside the envelope

and will drive the permeate stream along the spiral into the

permeate tube. The feed spacer material, which creates the

flow channels for the feed and concentrate streams, is

placed between the different membrane envelopes. This

assembly is glued and rolled around the central tube. The

outside shell of the spiral assembly can be in different

materials: fiberglass reinforced resin for industrial appli-

cations, or polypropylene cage for sanitary applications as

the most common (Figure 2).

Because a spiral-wound element is a complex assembly

of many different components, it is important to not only

verify the resistance of the membrane, but also of the entire

module, including the different components. So if the

backing material in polyester provides good temperature

resistance, central tube, anti-telescoping device and

interconnector should be selected in high temperature

compatible materials such as polysulfone instead of the

standard PVC or ABS (Acrylonitrile Butadiene Styrene).

The glue, which seals the membrane envelopes and

maintains the different layers together (permeate carrier,

feed spacer, membrane sheet) also needs to be selected

for high temperature stability: special formulation of

polyurethane and epoxy can meet those requirements.

Figure 1 | 2-layer versus 3-layer membranes.


HIGH TEMPERATURE OPERATION

Operating parameters

The spiral-wound RO elements designed for high tempera-

ture operation will be built with components and materials

stable up to about 1008C. However during operation,

the membrane module will not be exposed to static hot

water, but to a combination of fluid velocity, pressure and

aggressive pH during cleanings. The limitations for those

parameters are usually defined on the membrane element

literature considering 508C as the absolute maximum. When

considering the Durathermw membrane products, those

parameters quantified as pressure drop, operating pressure

and pH ranges had to be adjusted to those extreme

temperatures. Indeed plastic materials tend to soften at

elevated temperatures and therefore the risk of membrane

compaction related to operating pressure, as well as the risk

of telescoping, related to pressure drop need to be taken

into account and reflected through stricter limitations.

The Wagner diagram (Wagner 2001) provides guidelines

regarding operating pressure depending on the application

temperature that reflect the experiences gained using such

elements (Figure 3).

Permeability and rejection

The evolution of water transport at increasing temperature

is commonly incorporated in reverse osmosis projection

programs, using the published temperature correction

factors as in the Winflows program developed by GE

Water & Process Technologies.

Those factors developed for the normalization at 258C

of operational data from RO systems indicate that at 508C

the flux of water is about twice the flux at 258C. However

fresh water used in industry, whether coming from a local

well or from the city network, is rarely as warm as 258C,

and if considering a more realistic fresh water temperature

of 108C, the ratio of fluxes exceeds 3. So operating an RO

system at high temperature allows a remarkable increase

in water flux.

Operating a spiral-wound RO membrane at fluxes

as high as 100 l/m2h on contaminated water presents

however a serious risk of fouling because of the local

concentration on the surface of the membrane. In order to

keep that risk under control and to maximize the element

lifetime, the membrane elements should not be used above

Figure 3 | Wagner diagram.

Figure 2 | Spiral-wound element construction.


33 l/m2h as indicated on the GE Duratherm Excel

specification sheet (2008). In order not to exceed this

maximum flux, the operating pressure will be reduced.

The salt passage through the membrane is quantified

with the B-value, which is the specific salt permeability

through the membrane measured at 1 bar. The B-value

increases also with temperature as described by Snow

(1996): at constant pressure, NaCl transmission doubles

at 708C compared to 258C. So in case an RO could be

operated at 708C at the same pressure than at 258C, the

RO permeate would then have a better permeate quality

at 708C compared to 258C because the water flux

increases faster with temperature than the salt flux

(B-value). However because there is a limitation on the

water flux due to fouling risk, the RO will be operated at

about 50% of the 258C operating pressure in order to

maintain the flux below 33 l/m2h. Therefore the

increased salt flux will be diluted in a relatively constant

water flux, leading to increased salt concentration as

described in Figure 4.

Therefore a RO system will produce a higher salinity

permeate when operated at high temperature, while

keeping the permeate flow rate constant.

The temperature will not affect the rejection of large

organics and rejection will remain above 95%. However

the rejection of small organics which is very dependent on

the operating pressure, will degrade as operating tempera-

ture increases. That is why the treatment of condensates

with important quantity of volatile organics will require

innovative system designs in order to optimize the overall

total organic carbon (TOC) rejection.

System design

The purification of condensates for reuse can be performed

via RO technology in 2 different ways:

standard temperature (T , 408C) high temperature (408C , T , 808C)

Even if the standard RO membranes are given for

operating temperature up to 508C, the standard RO

machines are commonly designed for continuous operation

up to 35408C especially when they incorporate plastic

tubing for the low pressure piping. The standard tempera-

ture RO will therefore requiring cooling the condensate

prior to the RO and if necessary re-heating the RO

permeate. The RO machine will not require any particular

feature, but the system will require a heat exchanger.

High temperature RO equipment will require upgraded

components:

all stainless steel piping high temperature resistant instrumentation & sensors stainless steel pressure vessels high temperature resistant membranes

The pump will require high temperature materials,

but the operating pressure will be less than for a standard

temperature system. When including the membrane

elements, the difference in capital investment between a

standard RO and a high temperature compatible RO for the

same design is estimated between 20 and 50% depending on

the flow rate. However when TOC consists mainly in small

organics including volatile, complex and therefore more

expensive system designs might be necessary in order to

achieve high rejection on TOC.

CASE STUDIES

Water scarcity and oil rising prices are strong motivations

for industries to look at recycling the contaminated

condensates. The following examples are case studies for

existing systems using the Durathermw High temperature

membrane elements manufactured by GE Water & Process

Technologies. They are located in factories in the rubber,

dairy and beverage industries.Figure 4 | Salt rejection at high temperature for high rejection (HR) and high flow (HF)

RO membranes.


Tire condensate

In the manufacturing of car tires, the tire assembly made of

different rubber layers reinforced with metal is submitted

to steam during the vulcanization process. This curing

process generates both a clean condensed vapor and a

condensate contaminated with organics from contact with

the bladder and inorganics from the concrete storage sump.

The clean condensed vapor is recycled in the boiler, but the

contaminated condensate is discharged to the sewer.

A tire factory producing 17,000 pieces daily generates

17m3/h of 708C condensate, with about 50% clean con-

densed vapor and 50% contaminated condensate with

minerals and organics including ketones and aromatics

with an average TOC of 30ppm, with spikes to 300ppm.

The pilot study confirmed that NF was necessary in front of

the RO to remove the large organics, which fouled the RO

membrane. The NF/RO system installed provides an 85%

recovery of the condensatewhile achieving . 99% rejection

for all minerals (Table 1).

The system set-up is outlined on Figure 5. Because the

steam produced is not only used for the curing process, the

amount of treated condensate is not sufficient as boiler feed

and make-up water is added between the 2 membrane steps.

The feed stream to the RO has a temperature of 458C and

therefore does not require high temperature membrane, but

a full stainless steel RO installation is necessary. Now

treated by the Reverse Osmosis, the boiler feed water has a

significant lower salinity compared to the previous city

water after zeolite softener, allowing a significant increase

in boiler cycles, therefore further contributing to a reduction

of the plant fresh water consumption.

The results

The high-temperature system installed to treat the curing

condensate allowed the factory to:

increase its return to the boiler house from 40% to 60% eliminate the sewer cost of sending 8m3/h of contami-

nated condensate to the drain

reduce the discharge temperature to the sewer for thetotal plant by 33%

Table 1 | Tire condensate composition and purification

Contaminant Condensate content NF rejection RO rejection

Total hardness 50100ppm .80% .95%

Iron 15ppm .90% .95%

Copper 15ppm .90% .95%

Sodium 15ppm Variable .95%

Silica 0.55ppm 1020% .95%

Sulfate 15ppm .80% .95%

Chloride 15ppm Variable .95%

TDS 70100ppm Variable .95%

TOC 30300ppm Variable .95%

Figure 5 | Tire condensate recovery system.


increase 5 times in boiler cycles substantially reducingthe cost of raw and demineralized water and the

associated chemical water treatment cost

improve the quality of boiler feed water allowing forimproved operation and lower maintenance cost at the

boiler house.

In the first 24 months of operation the system has

enabled a reduction of gas consumption of approximately

52,000Mcf (million cubic feet) in a year or 10% energy

savings for the plant. With current gas prices at $9.00 per

Mcf this equates to $468,000 in fuel cost savings.

The consequent simple payback for the system was less

than 2 years.

Dairy condensate

Cow milk naturally contains about 88% of water. Any dairy

product, whether yogurt or hard cheese, has a higher dry

matter content than milk and therefore any dairy plant will

have significant waste water volumes generated by the

concentration of fats, proteins and sugars from milk.

Because the volumes of wastewater are important, the

use of reverse osmosis on dairy condensates was already

described by the International Dairy Federation in1988

(IDF 1988) but the described systems did require cooling

before treatment. Depending on the product evaporated,

the condensate composition varies significantly as reported

in IDF (1988) in Table 2. Therefore the achievable permeate

TOC will depend on the raw material evaporated, and more

difficult products such as acid whey will require more

complex system designs.

A dairy factory processing 850 million liters of milk

per year for the production of butter and milk powder

generates more than 2,000m3/d of wastewater, mainly

coming from the evaporation of milk into powder. This

evaporator condensate is collected at 658C.

The RO unit is operated at 658C continuously and

delivers a 90% recovery producing 1,800m3/d of 658C

permeate. After a ClO2 dosage to prevent any microbiolo-

gical activity, the permeate is used hot for boiler feed and

hot Clean-In-Place (CIP). The heat of the remaining

permeate is recovered via heat exchangers including milk

heater, before being used in cold applications such as

process water, cold CIP or cooling. Considering the

important BOD content of such condensate, the high

temperature operation of the RO unit prevents also

important biological growth. When operating an RO at

low temperature, significant amounts of sanitizers (chlorine,

peracetic acid) have to be added.

The results

The implementation of the high temperature RO system to

treat dairy allowed this site to:

become self-sufficient in water except during exceptionalshut-downs

reduce by 66% the volume of wastewater discharged tothe sewer

recover energy either from direct reuse of hot ROpermeate or via heat exchangers for milk pre-heating.

The savings generated were almost equally distributed

between water savings, energy recovery and wastewater

cost reduction (Table 3) as reported by Envirowise (2003).

Considering the investment cost of the RO system, the

payback period was only 9 months.

Even though a high temperature RO system costs

between 20 and 50% more than a standard unit, the

expenses of heat exchangers (CAPEX) and higher CIP

frequency (OPEX) can partly offset this difference. However

the final argument for choosing a high temperature RO

operation is the potential energy savings.

Distillery condensate

The spirits are commonly produced by distillation of

sweet liquor prepared with fruits, vegetables or grains.

The distillation bottoms called spent mash are treated as

a waste. In many distilleries, this mash is also concentrated

via evaporation in order to reduce the final waste volume.

The condensate from the spent mash evaporation from

Table 2 | Dairy products characteristics

Product pH value COD(mg/l) BOD5 (mg/l)

Skim milk 5.98.0 1488 1168

UF-whey permeate 6.87.8 5286 4156

Sweet whey 5.48.6 34389 28256

Acid whey 3.25.6 2161,053 132928


whisky production has been evaluated for high-temperature

RO treatment and reuse in the process.

A distillery producing 45 million litres of spirit per year

is generating 25m3/h of 80908C condensate from spent

mash concentration. A pilot study was performed to verify

that the treated permeate was suitable for reuse inside the

production process. The analysis of the feed and permeate is

detailed in Table 4.

The results

The full-scale system is delivering more than 95% reduction

of Total Dissolved Solids, and a 90% reduction of TOC.

Because the present organics are small, mainly volatile, the

system design had to be optimized to provide the necessary

quality for reuse into the production process.

Since the freshwaterused so far for theproductionprocess

is at 88C, and considering that the condensate available at

758C is being entirely reused in the production process, the

energy savings alone should represent about 1 million USD

annually, based on 0.8$/m3 for fuel.

The return on investment will need to be confirmed after

2 years of operation. However water reuse was the main

motivation for the end customer, and the energy recovery

was originally seen as a bonus feature. Therefore the return

on investment is estimated so far around 6 months.

CONCLUSION

The purification of hot condensates is achievable up to 808C

for continuous operation using Durathermw membrane

elements. The achieved permeate quality will depend

on the feed stream composition (minerals and organics)

and innovative system designs are requested for organic

reduction of 90% and more when volatiles are present.

The opportunities for reuse of the treated condensate

need to be selected based on the flow rate, the calorific

content and the residual contamination of the permeate.

In the case studies described, the projects initiated with

a site survey for water and wastewater. Such preliminary

studies are unique opportunities to map the water uses

inside a factory across production, utilities and wastewater

treatment. The outsourcing of utilities such as steam and

demineralized water production is a hindrance to such large

scope reuse/recovery projects as the capital investment and

the savings will not affect the same budgets.

The savings generated by any condensate recovery

project are based upon water reuse, energy recovery and

wastewater minimization. The access to all those costs is

essential in order to evaluate the economical feasibility of

these projects. Depending on the industry, the condensate

recovery project may bring significant additional benefits

including product recovery, improved boiler efficiency,

reduction of chemical consumption, improved product

quality, but also public incentives for reduction of fresh

water consumption and CO2 emission trading.

REFERENCES

Dairy profits from zero water use 2003 Envirowise CS404.

Duratherm Excel Series Product Fact Sheet 2008 AM-

FspwDurathermExcel_EN, December 2008.

Snow, M. 1996New techniques for extreme conditions: high temperature

reverse osmosis and nanofiltration.Desalination 105, 5761.

The Quality, Treatment and Use of Condensate and Reverse

Osmosis Permeates 1988 Bulletin of the International Dairy

Federation no 232/1988, Brussels, Belgium.

Wagner, J. 2001 Membrane Filtration Handbook, 2nd edition,

Revision 2, Osmonics, Minnetonka, USA.

Table 4 | Distillery condensate composition

Unit Feed Permeate

pH 7.1 N.A

Conductivity mS/cm 453 9

m-alkalinity ppm CaCO3 172 4.2

Sulfate ppm ,0.1 ,0.1

Chloride ppm 1.4 ,0.6

Sodium ppm 131 2.1

Total hardness ppm CaCO3 1 ,0.1

Total suspended solids mg/l 11.1 2.6

Total organic carbon ppm as C 322 35

Table 3 | Effective cost comparison

Savings per year ($/year) Capital costs ($)

Water reuse 736,000

Energy savings 800,000

Wastewater reduction 716,000

Total 2252,000

High temperature RO 1650,000


Industrial water reuse opportunities and high temperature compatible membranes&?tpacr=1;IntroductionIndustrial condensatesContaminated streamsTypical contaminants

Available cross-flow technologiesPolymeric membranesMembrane element configuration

High temperature operationOperating parametersPermeability and rejectionSystem design

Case studiesTire condensateDairy condensateDistillery condensate

ConclusionReferences

Documents

Ro Hot Filtration