process water treatment – challenges and solutions

8/10/2019 Process Water Treatment Challenges and Solutions

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Depending on their location andon other factors, chemical-pro-

cess-industries (CPI) plantsobtain their process water from

a diverse range of sources (see Pro-cess Water Supply the Big Picture,Chem. Eng., May 2005, pp. 3234.).The water from most, if not all, of thesesources requires some, if not a greatdeal of, contaminant removal onsite in

order to make the water suitable for

use. A wide array of contaminant-re-moval technologies is available, so itis important to choose the ones thatare most appropriate for the situationat hand. Once the choices are made,certain underlying principles can helpget the process design of the treatmentunit off to a good start.

All water supplies contain contami-nants. The type of contaminant can

vary greatly, and the contaminant con-centration may range from extremelylow (as in the case of highly purewater requiring final polishing beforeuse in semiconductor manufacture), to

very high (as in a typical wastewater

stream that is to be recycled).What constitutes a contaminant

depends entirely on the application.For drinking water, for instance, thecontaminants are those defined (forthe U.S.) by the Safe Drinking Water

Act, whereas for semiconductor rins-ing, anything other than H20 is a con-

taminant, and the concentrations ofsuch contaminants must be as closeto zero as possible. The uses of waterat CPI plants (which range from pe-

troleum refineries to cement mills tobreweries) are wide-ranging, so it is

not possible to generalize the defini-tion of contaminant.

It is virtually impossible to makewater free of any and all contaminants.The goal of a treatment process is toreduce the contaminant levels to theextent required by the application. Tomore easily address that removal, it is

convenient to look at contaminants by

category (Table 1).

Removal technologiesNo single technology effectively re-moves all contaminants. The chal-lenge is to specify and design a systemutilizing a combination of technologiesthat provide the optimal removal to

meet the particular, use-specific waterquality requirements for the situationat hand.

In general, a water treatment sys-tem consists of three basic components:pretreatment, primary treatment andposttreatment. Pretreatment tech-nologies typically protect the primary

treatment technologies from suchproblems as fouling and chemical deg-radation. The primary technologiesbring the water supply to its desiredquality level, and the posttreatmenttechnologies are designed to keepthe water supply at that quality levelduring storage and distribution. Ac-

cordingly, the key selection must bethat of the primary treatment tech-nology or technologies. This decision,in turn, will dictate the selection of

pretreatment technologies. The choiceof posttreatment technologies willbe dictated by the need for storage

and distribution, the instantaneous

flow requirements, and water qualitymaintenance issues.

Table 2 summarizes several water-purification processes with regard totheir effectiveness in removing a par-ticular class of contaminants. Becauseof the wide range of contaminants ineach contaminant class, there are cer-

tainly exceptions to the effectivenessof a particular treatment process; withthat in mind, this table serves as auseful and basically accurate guide.

PRETREATMENTFiltrationFiltration removes suspended solid

contaminants mechanically, by use ofa porous medium that allows water topass while retaining the solids. In gen-eral, filters used for water treatmentconsist of either (a) bed filters, namely,containers that are partially filled witha porous bed of inert particles, or (b)manufactured cartridges or bags, typi-

cally constructed of a synthetic porousfabric, and usually designed to processsmaller flowrates than a bed filter.Some bed filters or cartridge filters are

Engineering Practice

50 CHEMICAL ENGINEERING WWW.CHE.COM MARCH 2006

Process Water Treatment Challenges and Solutions

TABLE I.WATER CONTAMINANTS

Class Typical examples

Suspendedsolids

Dirt, clay, silt, dust,insoluble metal ox-ides and hydroxides,colloidal materials

Dissolvedorganics

Trihalomethanes,synthetic organicchemicals, humicacids, fulvic acids

Dissolvedionics (salts)

Heavy metals, sil-ica, arsenic, nitrates,chlorides, carbon-ates

Microorganisms Bacteria, viruses,protozoan cysts,fungi, algae, molds,yeast cells

Gases Hydrogen sulfide,carbon dioxide,methane, radon

Clean water is needed by chemical-process plants

for a wide range of process-related and auxiliary

uses. Likewise wide is the range of options available

to the plant for cleaning up the raw, incoming water.

Here are guidelines for making the right choice

Peter Cartwright, Cartwright Consulting Co.


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filters are available either in bed-typeconfigurations or in cartridge con-structions similar to filter cartridges.

Various types of activated carbon areavailable; the choice depends on thecontaminant to be removed. Although

it is possible to reactivate activatedcarbon, it is usually more cost effectiveto replace the material upon exhaus-tion. Other adsorptive media, such asactivated alumnia, are specific for cer-tain dissolved contaminants such asarsenic and fluoride ions.

Precoat, or diatomaceous earth

(DE), filters. These compact, light-weight filters can remove particles assmall as 0.1 m from water without

the need for prior particle destabiliza-tion (coagulation). The term, precoat,refers to the requirement to feed thefiltering medium (diatomaceous earth)into a stream of water in order to coat

a fine cloth or screen, the septum, withthe medium. Initially, the DE layer isabout 3 mm thick. Additional DE maybe added during the filter run (bodyfeed), and the total medium thicknessmay ultimately reach 10 mm or more.The filters are backwashed, dischargingboth the captured suspended solids and

the diatomaceous earth medium. To

re-initiate the filtering cycle, DE mustfirst be coated onto the septum again.

Chemical pretreatmentNormal water supplies contain certaindissolved salts which, when concen-trated beyond their solubility level,precipitate out and can foul or interfere

with the primary treatment technolo-gies downstream. Perhaps the most no-torious of these salts is calcium carbon-ate, which is in a saturated conditionin virtually all non-seawater supplies.

A traditional water softener (sodiumion exchange) replaces the calcium ionwith sodium to prevent the formation

of scale. Another approach consists ofpH adjustment with an acid, whichconverts the carbonate ion into carbondioxide and water.

For many years, a traditional pre-treatment technology for large volumesof water has been coagulation or floc-culation. Specific chemicals are added

that react with suspended solids inthe water supply, causing them to ag-gregate into large particles to facilitateremoval by settling or filtration.

PRIMARY TREATMENTMembrane technologiesIn membrane technologies, a semi-permeable membrane separates con-taminants from the water by a process

known as crossflow filtration (alsocalled tangential flow filtration). Thebulk solution flows over, and parallelto, the filter surface, while, under pres-sure, a portion of the water is forced

through the membrane to produce apermeate stream. The turbulent flow ofthe feedwater over the membrane sur-face minimizes accumulation of par-ticulate matter there, and facilitatescontinuous operation. Figure 3 furtherexplains crossflow membrane filtrationby contrasting it with conventional fil-

tration in conventional filtration, the

entire solution is pumped through thefilter medium, whereas only a portiongoes through in the crossflow filtrationprocess. Conventional filtration is es-sentially a batch process, while cross-flow filtration is a continuous process.

Microfiltration(MF) involves the re-moval of insoluble contaminants rang-

ing in size from 0.1 to 10 m (1,000 to100,000 angstroms). Figure 4 depictsthe mechanism of crossflow microfil-tration.Ultrafiltration (UF; Figure 5) re-moves materials in the 0.001-to-0.1-m range (10 to 1,000 angstroms).Ultrafiltration is employed to take

out dissolved nonionic contaminants(macromolecules), typically organ-ics, whereas microfiltration is usedmainly to remove suspended solids.Removal properties of UF membranesare usually expressed in terms of mo-lecular weight cutoff (MWCO), whichtypically range from about 1,000 up

to 50,000 Daltons. Typical micro- andultrafiltration membrane polymersinclude polysulfone, cellulosic acetateand polyamide types.

Reverse osmosisis a technique used

mainly to remove salts from water.The membrane rejects over 99% ofsalts content, as well as virtually100% of macromolecules with molecu-lar weights above 100 Daltons. Figure6 illustrates reverse osmosis, whichtypically separates materials lessthan 0.001 m (10 angstroms) in size.

As with ultrafiltration, reverse osmo-sis is used to remove dissolved mate-rials, but its primary application is in

lowering the content of salts or miner-als. Polymers used in RO (as well as innanofiltration; see below) membranesinclude cellulose acetate, cellulose tri-acetate, polyamide and thin-film com-

posite types. These latter membranesare typically composed of a thin film,fabricated by coating one of a numberof amide-type polymers onto a polysul-fone layer.

Nanofiltration (NF) can be consid-ered loose reverse osmosis (RO). Itrejects dissolved ionic contaminants

but to a lesser degree than RO. NF

membranes reject multivalent saltsto a higher degree than monovalentsalts (for example, 98% rejection ver-sus 20%). These membranes haveMWCOs for macromolecules below500 Daltons. Nanofiltration is illus-trated in Figure 7.

Advanced oxidation methodsAdvanced oxidation technologies(AOTs) cover a number of primarytreatment technologies designed to re-move dissolved organic contaminantsas well as microorganisms.

In general, strong oxidants willbreak organic bonds, and since all mi-

croorganisms are organic, AOTs willinactivate them. With sufficient oxida-tive power, it is theoretically possibleto break down all organic compoundsinto carbon dioxide, water and any ad-ditional elements that may have beenpresent in the original compound (suchas chlorine). AOTs include ultraviolet

irradiation, ozonation, and hydrogenperoxide addition.Ultraviolet irradiation is a tech-nology that emits light energy at a



FIGURE 3. Membrane technologies em-ploy crossflow, not conventional, filtration

FIGURE 4. Microfiltrations role con-

sists of the removal of suspended solids


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wavelength peaking at about 254

nanometers (nm). At that wavelength,many microorganisms are inactivated,including most bacteria, some virusesand Cryptosporidium parvum, a patho-genic protozoan cyst. In addition to the254 nm wavelength for microorgan-ism inactivation, 185-nm is optimalfor breaking organic bonds; however,

approximately four times the power orcontact time is required for doing so.

Although relatively inexpensive to

purchase and operate, ultraviolet ir-radiation systems have the followingdisadvantages: Since the light travels in straight

lines, suspended solids or any other

obstruction will reduce this technol-ogys effectiveness

The lamps can become coated withsuspended solids or organic depos-its, which reduce the transmission

Over time, UV lamps lose their abil-ity to generate sufficient energy andmust be replaced

Ozonation. Ozone is one the most

powerful oxidants available, secondonly to fluorine. It is produced as aresult of an electrical discharge inoxygen or air. Technically, ozone is anallotrope of oxygen with the chemi-cal formula O3. It will revert back tooxygen in approximately 20 minutesin ultrapure water. Ozone is the pun-

gent odor prevalent in the air nearlightning strikes, and often noticeablearound copying machines where elec-trical discharges occur.

Although only slightly soluble inwater, dissolved ozone is extremelyeffective in inactivating microorgan-isms; it also oxidizes certain water

contaminants such as iron, manga-nese and hydrogen sulfide, and it ispartially effective in breaking downdissolved organic contaminants.

Ozone is not without its drawbacks.It requires dry air, as discussed below;because it is short-lived, it has to beprepared onsite; and it requires very

inert materials of construction. Ozonealso reacts with bromine in water sup-plies to form bromate ions, suspectedcarcinogens.

Ozone is produced, onsite, by two

methods: ultraviolet irradiation andcorona discharge. The first of thesemethods employs 185-nm ultravioletenergy reacting with dissolved oxygenin the water. Whereas the concentra-tion of ozone produced in this matteris not sufficient for microorganisminactivation, it can be used for oxida-

tion of inorganic and organic contami-nants. Corona discharge, the approachmore widely used (especially in large

installations), utilizes the creation ofa spark across a dielectric to producegaseous ozone from either air or oxy-gen. The gas is then introduced intothe water stream, either through the

use of a venturi or by means of a dif-fuser submerged in the water. This ap-proach produces relatively high (typi-cally 4%) concentrations of dissolvedozone, sufficient to inactive microor-ganisms and accomplish the otherperformance objectives of ozonation.

The primary disadvantage of the

corona discharge approach is that if

there is moisture in the air (dew pointgreater than 40 C) nitric acid isformed, which is difficult to eliminateand will cause severe corrosion prob-lems. This factor precludes the appli-cation of corona discharge ozonationfor small applications.Treatment with hydrogen perox-

ide. The oxidizing capabilities of tech-nologies such as ultraviolet irradia-tion and ozonation can be enhanced(especially for organics destruction)with the addition of hydrogen perox-ide. Available as a liquid, hydrogenperoxide is widely employed by itselffor the disinfection of ultrapure water

systems, such as those in the semicon-ductor and pharmaceutical industries.It breaks down into oxygen and water,and thus does not leave any other re-siduals in the system.Other primary technologies

Distillation.When distillation is em-ployed for primary treatment, all the

nonvolatile contaminants are theoret-ically left behind. Figure 8 illustratesa typical water distillation unit. Dur-ing the production of ultrapure water,

distillation is very effective in remov-

ing suspended solids and dissolvedsalts, as well as most microorganisms.Distillation also provides disinfec-tion of the water. Its shortcomings in-clude high-energy utilization and highmaintenance costs.

Electrodialysis. Electrodialysis is anelectrochemical membrane-separation

process in which ions (under d.c. volt-age) are transferred through a pairof ion-selective membranes, from a

water stream into a more concentratedstream for discharge. Electrodialysisremoves only the ionic solutes.

Ion exchange (deionization). De-ionization is a term casually given to

the use of ion exchange resins for theremoval of ionic contaminants. Cationresins adsorb cationic contaminants(positive charge) and exchange themfor hydrogen ions; anion exchangeresins adsorb anionic contaminants(negative charge) and replace themwith hydroxyl ions. The hydrogen and

hydroxyl ions combine to form water;

hence the term, deionization. Theresins require regeneration with acidand caustic solutions.

When anion and cation resins aremixed together in a single bed, an ar-rangement known as mixed bed ionexchange, they produce the highestpurity of water (from an ionic stand-

point) commercially available. If thewater is run through individual tankseach containing one kind of resin, thisknown as two-bed ion exchange; itproduces a water quality comparablefrom an ionic standoint to that of re-

verse osmosis. On the other hand, ionexchange technology is not effective in

removing suspended solids, microor-ganisms or dissolved organics.

Electrodeionization (EDI).Electro-deionization, (or continuous deioniza-tion, CDI), is basically a combinationof electrodialysis and mixed-bed ionexchange, wherein ion exchange res-ins are encapsulated between layers

of electrodialysis membranes. Thistechnology is capable of producing thesame quality of ionic purity as mixedbed ion exchange resins. Its primary

CHEMICAL ENGINEERING WWW.CHE.COM MARCH 2006 53

FIGURE 5. Ultrafiltration can hold back

suspended solids and macromolecules

FIGURE 6. Reverse osmosis retains

solids, macromolecules and salts

FIGURE 7. Nanofiltration works better

on multivalent salts than on monovalent


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advantages over the latter are that itdoes not require regeneration.

Electrodeionization is a continuous

process. And it appears to be fairlyproficient at removing dissolved or-ganics and microorganisms (although

it is not a cost-effective treatment op-tion for either of those two contami-nant categories if dissolved ionics arenot also present).Thermal destruction. Organics inwater can also be broken down throughthe application of a number of thermaltechnologies, usually combining heat

with a catalyst. However, these tech-nologies are site-specific and, because oftheir high capital and operating costs,

are usually employed in the treatmentof heavily polluted liquids such asthose at U.S. Environmental Protection

Agency (EPA) Superfund sites, ratherthan in treating water supplies for use

within a process plant.Bioremediation. This technology isused almost exclusively for the reduc-tion of BOD (Biochemical Oxygen De-mand). It is primarily used for waste-water treatment, and only rarely fortreatment of incoming water. Biore-mediation employs bacteria which

feed on the organics and break them

down during their normal metabolicprocesses. For a recent review of re-lated technology, see Biological Waste-water Treatment, Chem. Eng., October2005, pp. 4451.

An interesting combination of old andnew technologies consists of membranebioreactors (MBR), the incorporation of

microfiltration or ultrafiltration withbioremediation to continuously filterthe treated water to remove dissolvedmacromolecules and/or suspended sol-ids. As part of the bioremediation pro-cess, MBR is used almost exclusivelyfor wastewater treatment.

POSTTREATMENTPolishing treatmentPolishing of water supplies is em-ployed to remove trace contaminantsthat may have entered the water sup-ply during storage and distribution.

It is well understood that the higherthe water purity, the more aggressive

the water is. Although many of the com-mon materials of construction are inertto high purity water, ultrahigh-puritywater (characterized by, for instance,

18 megohm resistance) is so aggres-sive that no matter what materials are

used for its storage and distribution,the water quality will eventually dete-riorate, and polishing is required.

In general, polishing technologiesare the same as (or very similar to) pri-

mary treatment technologies; however,as they are required only to remove arelatively small quantity of contami-nants, polishing systems usually re-quire less resins or fewer membraneelements to return the water to its op-timal quality level. On the other hand,the distribution system may operate at

a much higher flowrate, particularly in

the case of recirculation systems.One component of the polishing sys-

tem may consist of point-of-use (POU)treatment: just prior to the actual useof the purified water as it leaves thedistribution loop, it is treated with asubmicron cartridge filter, ultrafiltra-tion membrane, or another appropri-

ate type of treatment technology.

DisinfectionDisinfection, which is used in severaltypes of chemical process plants, con-sists of inactivating microorganismsor removing them from water supplies.Microorganisms that are commonly

found in water supplies include bacte-ria, viruses, protozoan cysts, algae andfungi. Within these broad classes maybe hundreds of thousands of differentspecies, each with its own response inthe face of specific inactivation technol-ogies. Among these can be variations inindividual microorganisms suscepti-

bility to high temperatures, particularUV wavelength, oxidation potential,exposure time, and so on, as discussedin more detail below.

Microorganisms that affecthuman health are known as patho-

gens. Immuno-compromised indi-viduals, such as the very old, thevery young, AIDS patients and

people undergoing certain kinds ofchemotherapy, may react adverselyto low concentrations of microor-ganisms that do not affect the nor-mal population. The point is that

virtually all water supplies intowhich humans come into contact(including those supplies in pro-

cess plants) must be disinfected.Basically, three distinctive types of

treatment are used to disinfect water

supplies: chemical, radiation and me-chanical.Chemical. In general, all chemicaldisinfectants today utilize oxidizingagents. A strong reducing agent, form-

aldehyde, is very effective and hasbeen used in the past with excellentresults; however, it is now prohibitedin most industrialized countries be-cause the vapors have been found tobe hazardous to human health.

Of the oxidizing chemicals, thehalogens are most commonly used,

with the exception of fluorine, which

is much too dangerous and difficult tohandle. Chlorine is the most popularin the U.S., where chlorinated watersupplies have been prevalent for thelast 100 years. The form of chlorinemost commonly used for municipaldrinking water treatment is gaseouschlorine; for residential and other

small applications, chlorine is eitherdelivered in the form of liquid sodiumhypochlorite or solid pellets of calciumhypochlorite. The chemical species ofinterest here is the hypochlorite ion,which will inactivate almost all patho-genic microorganisms.

A particularly important feature of

chlorination is that it persists (leavesa residual) in the water supply to en-sure long-term disinfection properties.On the other hand, chlorine reactswith certain organic compounds inthe water (naturally occurring or oth-erwise) to form a family of suspectedcarcinogens known as trihalometh-

anes. In the past 20 years or so, manymunicipalities (but not many processplants) have included ammonia withchlorine in their disinfection process



Source: Water Quality Assn.

FIGURE 8. While it may be a far cryfrom a petroleum-refinery hydrocar-bon splitter, this distillation vesselis appropriate for primary treatmentduring water purification


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in order to produce chloramines whichdo not form trihalomethane byprod-ucts. A disadvantage of this approach

is that chloramines are not as stableas chlorine alone.

As stated earlier, ozone is the most

powerful oxidant available on a prac-tical basis. Because of its outstandingoxidation characteristics, it is an ex-cellent disinfectant; and in spite of itsdrawbacks cited earlier, it is the mostwidely used municipal disinfectant inEurope.

Radiation.As a disinfection technol-

ogy, ultraviolet irradiation is perhapsthe most widely used technology fornon-municipal applications, including

chemical process plants. It has sig-nificant limitations, however, in termsof efficacy when compared to ozone.

As with ozone, ultraviolet irradiationleaves no residual.

Mechanical. It is possible to physi-cally filter out microorganisms usingmembrane technologies. In particular,

microfiltration with its sub-micronpore sizes will readily remove theprotozoan cysts and the larger sized

bacteria, whereas, ultrafiltration isgenerally effective with the rest of theclasses, depending upon its pore size.

A complication, however, is the factthat because bacteria are viable, grow-ing microorganisms, they will growthrough membranes. This is a contro-

versial issue; however, it is widely ac-cepted that when water remains stag-nant on a membrane surface, bacteriawill, in just a matter of hours, have col-

onized on the permeate (downstream)side. During the process of coloniza-tion, biofilm layers are formed, which

shield the bacteria beneath the layersfrom disinfectants and cleaners. Bio-film also tends to collect suspendedsolids and other fouling materials.

Another important consideration with

mechanical treatment is that the mi-croorganisms are physically removedbut not inactivated.

SYSTEM DESIGNREQUIREMENTSSuccessful implementation of water

treatment technologies requires in-corporating the right combination ofthem into a total system that produces

the appropriate water quality with thegreatest reliability and at the lowestpossible capital and operating costs.

The most important initial param-eters to consider are: Feedwater quality Treated-water quality requirements Volume or flowrate requirements

As regards feedwater quality, a thor-ough water analysis is essential.That analysis should include those

parameters which influence thetechnology selection, such as sus-pended solids, dissolved organics,microorganism concentration andwater temperature (the last-named

is particularly relevant with respectto the use of membrane-based watertreatment). Feedwater quality most

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significantly affects the selection ofpretreatment technologies.

To determine the required treated-

water quality, on the other hand,the engineer must have a completeunderstanding of the effect of water

quality upon the intended use (for in-stance, product synthesis, or rinsing).In many cases, guidance is availablefrom industry standards. Some ofthese may be mandated, whereas oth-ers are simply recommended.

As for the total volume or flowrate,the total daily volume requirement

generally determines the productionrate required for the primary treat-ment technologies. The instanta-

neous flow rate (gallons or liters perminute) and duration (minutes) typi-cally dictate the size of the distribu-tion system (storage tank capacityand pump flowrate).

There are a number of other fac-tors that may influence treatmentsystem design, including physical

size limitations of the system haz-ardous waste considerations, andthe relevant regulations and the re-

quired permits.

ConclusionsThere are a myriad of excellent tech-nology choices to treat water for usesin chemical processing facilities.While far from an exhaustive reviewof all the technology choices, this ar-ticle has presented a number of thetechnologies that are most relevantfor process plants, along with some

of the engineering considerations as-sociated with total water-treatment-system design. It is incumbent on the

design engineer to be able to select thetechnologies in the right combinationto produce the optimum results forhis or her particular requirements. Ofcourse, there are a number of skilled

and experienced consulting engineerscapable of providing help.

In closing, it is fitting to point out

that regardless of the plants purityrequirements for its process water,the source of that water (prior to

treatment as outlined above) canreadily be wastewater from withinthe same facility, or even from a

municipal sewage plant. This fact,which often surprises the non-spe-cialist, serves a testimonial to the ef-fectiveness of the treatment schemesoutlined above.

Edited by Nicholas P. Chopey

Circle 16 on p. 72 or go to adlinks.che.com/5827-16


AuthorPeter Cartwright, P.E.,formed Cartwright Con-sulting Co. (U.S. Office 8324 16th Ave. South,Minneapolis, MN 55425-1742; Phone: 952-854-4911. European Office President Kennedylaan94, 2343 GT Oegstgeest, Netherlands; Phone:

31-71-5154417. Email: [email protected]),in 1980. He specializes in both marketing andtechnical consulting, in high-technology separa-tion processes for water purification, wastewatertreatment, and food and chemical processing ap-plications. A 2001 recipient of the Award of Meritfrom the Water Quality Assn., he has writtendozens of articles and several book chapters, lec-tured around the world, and been awarded threepatents for water purification. He holds a 1961bachelors degree in chemical engineering fromthe University of Minnesota.

process water treatment – challenges and solutions

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