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Presented at the 2nd Membrane Science and Technology Conference of Visegrad Countries (PERMEA),Polanica Zdroj, Poland, 1822 September 2005.

Desalination 198 (2006) 4146

Reverse osmosis in water treatment for boilers

Pavel uda, Petr Pospil*, Jaroslava TenglerovHradec Krlov Branch, MEGA a.s. Praha, Veverkova 1343, 500 02 Hradec Krlov, Czech Republic

Tel. / Fax: +420 (498) 500-393; email: [email protected], www.mega.cz

Received 3 November 2005; Accepted 22 December 2005

Abstract

Intense treatment of feed water solves high demands on a demineralized water quality in the processes involvinghot water/steam boilers. There are various demands on water quality parameters its hardness, alkalinity, pH value,carbon dioxide and oxygen content, etc. according to the type of boiler and its working pressure. Collectively,efficient demineralization and/or softening are always inevitable. Every desalination water treatment unit consists

of a standard pre-treatment part and a demineralization part. Arrangement of the pre-treatment part depends on thekind of water source (well, surface, or tap water) and its individual analysis. This part of a water treatment unit isessential to protect the plant. Thereafter, the demineralization part of a water treatment unit is chosen and designedto meet product water quality demands. There are two basic methods used for brackish water desalination in theCzech Republic: reverse osmosis and ion exchangers. An application is presented of the Mega Companys reverseosmosis units in the area of water treatment for boilers. It describes some installations and their properties sourcewater quality, pre-treatment techniques, and product quality. A brief comparison of the reverse osmosis process andion exchangers is also presented.

Keywords: Reverse osmosis; Boilers; Ion exchange; Demineralized water; Brackish water

1. Introduction

Water treatment serves as supplementary tech-nology in all industrial applications. The qualityof technological water or wastewater plays a very

*Corresponding author.

important role in industrial processes and even inlocal ecology. Thus, a question of proper watertreatment with respect to economical or environ-mental aspects is discussed. An application ofadvanced membrane processes enters the indus-trial playground nowadays.

0011-9164/06/$ See front matter 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.desal.2006.09.007


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P. uda et al. / Desalination 198 (2006) 414642

This paper describes an application of reverseosmosis (RO) in the area of water treatment for

boilers and cooling systems. Two examples ofRO water treatment units built by Mega are

presented. A brief comparison of the RO processand ion exchange (IE) process is also given.

Demands for boiler water quality are differentfor various types and working pressures of

boilers. There is also a question of usage of waterin boiler circuits: water can be used as circulationwater or feed water to cover sludge blow-off andsurface blow-down losses. Generally, clear andcolourless water must be assured, without sus-

pended solids, oils and aggressive chemicals.Other parameters of product water are lowcontent of hardness, alkalinity, carbon dioxide,oxygen and SiO2, and pH value above 8.5 [1].

This paper deals only with treatment of brack-ish water. There are some applications of ROtechnology in seawater treatment around theworld [2,3], successfully competing with thermaldesalination processes [4,5]. River water treat-ment by RO technology is combined withultrafiltration in newer installations instead of

clarification and sand filtration processes [6,7].Furthermore, the application of membrane pro-cess pre-treatment and the RO process for puri-fication and reuse of secondary treated effluenthave been installed to the cover fresh waterrequirements of power stations in Australia [8].

2. Water treatment

Water treatment units with desalination can be

divided into a pre-treatment part, desalination partand post-treatment part. Pre-treatment of thesource water plays a significant role in efficiency,the economical aspects and life-span of a desali-nation unit. When the investment costs of a newwater treatment unit are calculated, the price of

pre-treatment is nearly constant irrespective ofdesalination method applied.

The set-up of the pre-treatment depends on thetype of source water. In greater capacities surface

water can be treated; in small capacities it is wellor tap water which is usually treated. The aim ofthe desalination and post-treatment parts of watertreatment plants is to reach a composition of thewater according to regulations for the type of

boiler and its working pressure. The followingmethods can used to produce water of standard

parameters for boilers from brackish waters in theCzech Republic:C IE filters (softening, decarbonization),C one-pass RO followed by IE demineralization,C oneppass RO followed by electro-deionization

(EDI),C two-pass RO.

IE filters are a conventional method for waterdemineralization, softening and decarbonization.It seems now that their disadvantages (e.g., highconsumption of regeneration chemicals togetherwith production of salty wastewaters) pre-dominate over their advantages (e.g., low invest-ment cost). RO, a well established process forwater demineralization, can be introduced here

due to its advantages. By using two-pass RO or acombination of RO with a complementary desali-nation process, it is possible to obtain productwater with conductivity less than 0.1 S/cm.

3. Comparison of desalination methods

A comparison can be drawn among desali-nation methods in connection with various atti-tudes. Generally, the most important are:C

investment costs,C operational costs, which can be roughly sortout on costs of electric energy, chemicals andoperators,

C ecological impact of proposed technology.

As said above, investment costs of pre-treatmentare practically identical regardless of desalinationmethod, and this can be also said about itsoperational costs. Thus, a comparison of desali-nation methods is of interest. When the criterion


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P. uda et al. / Desalination 198 (2006) 4146 43

Table 1Comparison of chemicals costs for two technology arrangements (tap water, TDS 730 ppm; production of boiler water,8 m3/h)

Process order

RO IE IE ROCons. (kg/m3) Unit price (/kg) Cost (/m3) Cons. (kg/m3) Unit price (/kg) Cost (/m3)

38% H2SO4 for RO 1.060 0.113 0.120 0.093 0.113 0.011

100% NaCl for IE 0.008 0.267 0.002 1.564 0.267 0.417Cost on 1 m3 of product 0.122 0.428

for choosing the right desalination technology isvolume of treated water and its salinity, then:C RO is better in treatment of large capacities

and waters with higher levels of TDS.C IE filters are better in small capacities and

waters with low TDS.

This contention is based on experience and oncalculation of costs [9]. It can be seen from thefollowing example where the chemicals costshave been calculated for RO and IE softening that

the ordination of joint technologies in desali-nation is also important.

RO units are still more expensive for treatmentof both large and small capacities, but total ope-rational costs are lower than chemicals costs of IEfilters, but no word about their environmentalimpact. The consumption and cost of chemicalsin the Czech Republic are compared for arelatively small capacity in Table 1. This is thecase of a potential customer who has had an old

water treatment plant with softening by IE toproduce boiler water. Because of the need forproduct quality, they required a RO unit as asecond step to the IE unit (see Fig. 1a). Thechemical costs were calculated for this demandand compared with the chemical costs oftechnology where the order of desalinationmethods is reversed (see Fig. 1b). It seems betterfrom the point of view of the environment todecrease the TDS of tap water on the RO unit

Fig. 1. Technology set-up: (a) demanded by customer,(b) proposed by Mega.

with a relatively higher electrical energy cost andthen remove the remaining calcium and mag-nesium content on an IE softening filter with avery small capacity and with low consumption ofchemicals. Nevertheless, the investment costswere more important in this case than the opera-tional costs with a negative environmental impactof softening by IE; thus, the technology wasdesigned according to the original demand of thecustomer by a competitor.

The advantages of the set-up shown at Fig. 1b

compared to 1a are:C smaller osmotic pressure of feed water for RO,

which means low pump pressure and thereforelow operational costs,

C lower alkalinity of the product,C lower concentration of dissolved solids (TDS)

in product,C lower concentration of dissolved solids (TDS)

in RO concentrate, and a small volume ofwastewater from regeneration of the IE filter.


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4. Reverse osmosis application

Application of the IE desalination process for

water treatment for boilers is widely spread in theCzech Republic. Low mineralized well or surfacewater is commonly treated so the IE processsometimes seems economical [9]. The ecologicalimpact of IE is overcome by the investment costsof new desalination processes, although they are

becoming less expensive. Even the operationcosts of new desalination processes are lowerwith new types of low-pressure membranes (inthe case of RO) and low electrical resistance

membranes (in the case of electrodialysis andelectrodeionization processes) introduced into themarket.

When new boiler rooms are built or someolder ones are reconstructed, it can be observedthat RO is a promising alternative. IE deminerali-zation is also sometimes used as a supplementarysecond stage of demineralization after RO.

Mega has designed two RO units for appli-cations in water treatment for boilers in recentyears [10]. Both technologies serve as a sub-stitute of IE desalination units treating well water.In the first application the production ofdemineralized water was 2.1 m3/h; the secondapplication produced 60 m3/h of demiwater.

The first application in Saint-Gobain Vertex(Litomyl, Czech Republic) was designed in1999. The simple set-up of technology is shownin Fig. 2, which gives a diagrammatic repre-sentation of the system. Well water enters thetreatment plant through the ball valves with a

bypass. Then a protective cartridge filtrationfollows. Before well water enters the RO unit, pHis adjusted to the proper value by an addition ofH2SO4 solution. The acid dosing is controlledaccording to the pH value measured by thethrough-flow pH detector placed behind thedosing. The RO unit (Fig. 3) divides the feedwater into the permeate stream and the con-centrate stream. The concentrate stream is drainedoff and the permeate stream passes into the

Fig. 2. Technology set-up at Saint-Gobain Vertex,Litomyl.

Fig. 3. RO unit in Saint-Gobain Vertex, Litomyl.

aeration tower where CO2 is removed. The pH ofpermeate is adjusted to the proper value by anaddition of NaOH solution and residual ions areremoved in a IE softening filter.

The RO unit pressure vessels are filled withnine Hydranautics ESPA2-4040 membranemodules in 32 + 31 configuration. Theworking pressure of the unit is 13 bar.

Overview of technology at Saint-GobainVertex:C treatment of well water, TDS 535 ppmC pre-treatment: cartridge filtration, acid dosingC RO unit: permeate flow of 2.1 m3/h, recovery

66%C post-treatment: aeration to remove CO2, IE

demineralization, alkalization, and thermaldegasification

The composition of the waters in Saint-GobainVertex is shown in Table 2, Col. A. The mainions in the well water are calcium and


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P. uda et al. / Desalination 198 (2006) 4146 45

Table 2Comparison of water composition, Saint-Gobain Vertex,Litomyl, Czech Republic

Indicator Col. A Col. B Col. C

Well water IE RO + IE

Ca2+ (mg l!1) 124.0 1.0 0.38Mg2+ (mg l!1) 4.7 0.1 0.03

Na+ (mg l!1) 4.6 154.5 2.56K+ (mg l!1) 2.7 2.7 0.07

NH4+ (mg l!1) < 0.05 < 0.05 < 0.05

HCO3! (mg l!1) 243.4 243.4 6.1

SO42! (mg l!1) 70.0 70.0 0.4

Cl

!

(mg l

!1

) 31.0 31.0 < 0.2NO3! (mg l!1) 44.2 44.2 < 3.0

SiO2 (mg l!1) 5.0 5.0 < 0.1

pH 7.45 7.45 7.0TDS (mg l!1) 535.0 557.0 12.0Conductivity

(S/cm!1)827.0 768.0 13.4

bicarbonate. Water produced by previous tech-nology (col. B) was characterized by low hard-ness but an increased content of sodium and

bicarbonate. It led to frequent surface water blow-downs. With the new technology, the blow-off isreduced to a minimum so heating energy is alsosaved. The composition of boiler water treated bynew technology is presented in Col. C.

A second application of the Mega RO unit forproduction of water for boilers is installed inPerla, st nad Orlic, Czech Republic. It was

built in 2004. A simple set-up of the technologyis shown in Fig. 4 which gives a diagrammatic

representation of system.Well water enters the treatment plant throughthe ball valves with a bypass. Then the pH isadjusted to the proper value by an addition ofH2SO4 solution. The acid dosing is controlledaccording to the pH value measured by thethrough-flow pH detector placed behind thedosing. The RO unit (Fig. 5) divides the feedwater into the permeate stream and the con-centrate stream. The concentrate stream is drained

Fig. 4. Technology set-up at Saint-Gobain Vertex,Litomyl (Perla a.s. sti nad Orlic).

Fig. 5. RO unit in Perla, st nad Orlic.

off and the permeate stream passes into theaeration tower where CO2 is removed. The pH of

permeate is adjusted to the proper value by anaddition of NaOH solution.The RO unit pressure vessels are filled with 30

Hydranautics ESPA2-7 membrane modules in a45 + 25 configuration. Working pressure of theunit is 12 bar.

Overview of technology at Perla:C treatment of well water, TDS 508 ppmC pre-treatment: acid dosingC two RO units: permeate flow 230 m3/h,

recovery 75%C after-treatment: aeration to remove CO2, par-

tial alkalization, thermal degasification

5. Conclusions

RO is very promising technology in thepreparation of water for boilers. Its applicationallows reduction of operating costs and theintroduction of a higher level of automation of the


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water treatment process. It also improves eco-logical indices of power plants by reducing theamount and salinity of wastewaters. RO, followed

by IE, can be used to prepare completely mineral-free water.

When the decision between RO and IE fordemineralization of water is made, then it isnecessary to consider the capacity of the plant,feed water salinity and requirements of productquality. When the feed water has medium or highmineralization and the capacity of the watertreatment plant is planned to be large, then it isadvantageous to install RO technology. When the

feed water has low mineralization, the capacity ofthe plant is small and product water does not needto have low TDS and conductivity, it is advan-tageous to install IE softening. IE deminerali-zation is not good in any case due to the highconsumption of regeneration chemicals and

production of great volumes of waste water fromregeneration.

The future of water demineralization probablybelongs to combination of RO and EDI where the

chemical cost and production of wastewater arelowered to a minimum. EDI, which combines theadvantages of electrodialysis and IE resins, can,in combination with RO, produce ultra-pure waterof very high quality.

References

[1] Voda a pra pro tepeln energetick zazen s

pracovnm tlakem pry do 8 MPa [Water and steamfor hot water and steam boilers with nominal steam

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(2005) 247252.[3] K. Ohta, H. Kaneda, M. Hirai, K. Kikuchi, Y.

Murayama, S. Yamada, N. Sato, S. Masumi and E.Nishiyama, Desalination, 56 (1985) 367379.

[4] M.A. Darwish and N.M. Al-Najem, Appl. ThermalEng., 20 (2000) 399416.

[5] N.M. Wade and M.R. Hornsby, Desalination, 40(1982) 245257.

[6] M. Clever, F. Jordt, R. Knauf, N. Rbiger, M.Rdebusch and R. Hilker-Schebel, Desalination, 131(2000) 325336.

[7] T. Manth, J. Frenzel and A. van Vlerken, Desali-nation, 118 (1998) 255262.

[8] M. Masson and G. Deans, Desalination, 106 (1996)1115.

[9] P. uda, prava pdavn vody pro kotle reverznosmzou. Lecture, CHEO Conference, Czech Re-

public, 2000 (in Czech).[10] P. Pospil, Industrial application of reverse osmosis.

Presented at MegaKemira Workshop: Membranesand Membrane Processes by MEGA, Nov Bor,2003.

osmosis inversa tratamiento

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