Depending on their location and on other factors, chemical-pro-cess-industries (CPI) plants obtain their process water from

a diverse range of sources (see Pro-cess Water Supply — the Big Picture, Chem. Eng., May 2005, pp. 32–34.). The water from most, if not all, of these sources requires some, if not a great deal of, contaminant removal onsite in order to make the water suitable for use. A wide array of contaminant-re-moval technologies is available, so it is important to choose the ones that are most appropriate for the situation at hand. Once the choices are made, certain underlying principles can help get the process design of the treatment unit 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 extremely low (as in the case of highly pure water requiring final polishing before use 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, the contaminants are those defined (for the U.S.) by the Safe Drinking Water Act, whereas for semiconductor rins-ing, anything other than H20 is a con-taminant, and the concentrations of such contaminants must be as close to zero as possible. The uses of water at CPI plants (which range from pe-

troleum refineries to cement mills to breweries) are wide-ranging, so it is not possible to generalize the defini-tion of “contaminant.”

It is virtually impossible to make water free of any and all contaminants. The goal of a treatment process is to reduce the contaminant levels to the extent required by the application. To more 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 system utilizing a combination of technologies that provide the optimal removal to meet the particular, use-specific water quality requirements for the situation at hand.

In general, a water treatment sys-tem consists of three basic components: pretreatment, primary treatment and posttreatment. Pretreatment tech-nologies typically protect the primary treatment technologies from such problems as fouling and chemical deg-radation. The primary technologies bring the water supply to its desired quality level, and the posttreatment technologies are designed to keep the water supply at that quality level during storage and distribution. Ac-cordingly, the key selection must be that of the primary treatment tech-nology or technologies. This decision, in turn, will dictate the selection of

pretreatment technologies. The choice of posttreatment technologies will be dictated by the need for storage and distribution, the instantaneous flow requirements, and water quality maintenance issues.

Table 2 summarizes several water-purification processes with regard to their effectiveness in removing a par-ticular class of contaminants. Because of the wide range of contaminants in each contaminant class, there are cer-tainly exceptions to the effectiveness of a particular treatment process; with that in mind, this table serves as a useful and basically accurate guide.

PRETREATMENTFiltrationFiltration removes suspended solid contaminants mechanically, by use of a porous medium that allows water to pass while retaining the solids. In gen-eral, filters used for water treatment consist of either (a) bed filters, namely, containers that are partially filled with a porous bed of inert particles, or (b) manufactured cartridges or bags, typi-cally constructed of a synthetic porous fabric, and usually designed to process smaller flowrates than a bed filter. Some bed filters or cartridge filters are

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50 CHEMICAL ENGINEERING WWW.CHE.COM MARCH 2006

Process Water Treatment – Challenges and Solutions

TABLE I. WATER CONTAMINANTSClass Typical examplesSuspended solids

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

Dissolved organics

Trihalomethanes, synthetic organic chemicals, humic acids, fulvic acids

Dissolved ionics (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 choicePeter Cartwright, Cartwright Consulting Co.

classified as adsorptive filters, as dis-cussed below. Precoat filters (see below) also serve for water pretreatment.

The unit of measurement used in connection with filtration for contami-nant removal is the micrometer, µm, equivalent to one-millionth of a meter (the smallest particle that can be seen by the unaided eye measures 40 µm).Bed filters. In a bed filter, the water pressure itself provides the force needed for passage through the filter medium or media. Many types of inert, uniformly sized materials can serve as the media. The bed filter shown in Figure 1 employs three: coal, sand and garnet. The flow of water can be either from bottom up or from top down. Typ-ical bed filters can remove suspended solids down to about 10 µm.

An advantage of bed filters is that they can be backwashed to remove entrapped solids, and, therefore, these

filters have very long lifetimes. Obvi-ously, they must be taken offline dur-ing backwashing.Cartridge filters. A typical filter in the form of a cartridge is shown in Figure 2. Cartridge filters are thick-walled tubes measuring typically 2 3/4 in. (7 cm) in outside diameter by 10 or 20 in. (25 or 50 cm) long, and they are constructed of any one of a number of synthetic materials, including rayon, polypropylene, cellulosic polymers, nylon or polytetrafluoroethylene.

In one type of construction, the fil-ter is string-wound: the material, in string-like form, is wound around the core in a pattern that creates a tor-tuous path through which the water must flow. Another type of construc-tion, which also forces the water to flow through such a path, is instead fabricated of nonwoven, feltlike ma-terials. A third type contains a mem-

brane consisting of a porous polymer sheet through which the water is forced. This last-named cartridge is the most effective version for remov-ing particles of very small size. The particle-removal rating for cartridge filters ranges from above 100 µm to less than 0.1 µm. Bag filters. These usually consist of bags created from nonwoven fabrics, which are supported within housings. They have filtration characteristics similar to those of cartridge filters.Adsorptive filters. Activated carbon, sometimes referred to as activated charcoal, has an extremely large sur-face area per unit volume because of its enormous number of very fine pores. This material is particularly effective in removing dissolved gases, such as chlorine and most volatile hydrocarbons, as well as many other dissolved organics. Activated carbon

CHEMICAL ENGINEERING WWW.CHE.COM MARCH 2006 51

TABLE II, EFFECTIVENESS OF WATER PURIFICATION PROCESSES

Treatment Process & Technology

Types of contaminantsSuspended Solids

Dissolved Organics

Dissolved Ionics (salts)

Micro-organisms

Gases

Filtration:Bed filtration Very effective NA NA NA NACartridge filtration

Very effective NA NA NA NA

Bag filtration Very effective NA NA NA NAPrecoat (DE) Very effective Partially

effectiveNA NA NA

Adsorption:Activated carbon

NR Very effective

NA NA Partially effective

Membrane technologies:Microfiltration Very effective NA NA Partially

effectiveNA

Ultrafiltration NR Very effective

NA Effective NA

Nanofiltration NR Very effective

Effective Very effective

NA

Reverse Osmosis NR Very effective

Very effective

Very effective

NA

Thermal technologies:Distillation NR Partially

effectiveVery effective

Very effective

NA

Freezing NR NA Very effective

NR NA

Electrical technologies:Electrodialysis NA NA Effective NA NAElectrode-ionization

NA NR Effective NR NA

Chemical technologies:Ion exchange NR NA Very

effectiveNA NA

Ozonation NA Partially effective

Partially effective

Very effective

NA

Use of chlorine compounds

NA NA NA Effective NA

Irradiation:Ultraviolet NA Partially

effectiveNA Effective NA

NA= not applicable NR= not recommended (not cost effective)

Source for Figure 1 and 2: Water Quality Assn.

FIGURE 1. Bed filters are effective for removing suspended solids from water

FIGURE 2. Cartridge filters can be fab-ricated in any of several configurations

filters are available either in bed-type configurations or in cartridge con-structions similar to filter cartridges. Various types of activated carbon are available; the choice depends on the contaminant to be removed. Although it is possible to “reactivate” activated carbon, it is usually more cost effective to replace the material upon exhaus-tion. Other adsorptive media, such as activated alumnia, are specific for cer-tain dissolved contaminants such as arsenic and fluoride ions.Precoat, or diatomaceous earth (DE), filters. These compact, light-weight filters can remove particles as small as 0.1 µm from water without the need for prior particle destabiliza-tion (coagulation). The term, precoat, refers to the requirement to feed the filtering medium (diatomaceous earth) into a stream of water in order to coat a fine cloth or screen, the septum, with the medium. Initially, the DE layer is about 3 mm thick. Additional DE may be added during the filter run (body feed), and the total medium thickness may ultimately reach 10 mm or more. The filters are backwashed, discharging both the captured suspended solids and the diatomaceous earth medium. To re-initiate the filtering cycle, DE must first be coated onto the septum again.

Chemical pretreatmentNormal water supplies contain certain dissolved 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 condition in virtually all non-seawater supplies. A traditional water softener (sodium ion exchange) replaces the calcium ion with sodium to prevent the formation of scale. Another approach consists of pH adjustment with an acid, which converts the carbonate ion into carbon dioxide and water.

For many years, a traditional pre-treatment technology for large volumes of water has been coagulation or floc-culation. Specific chemicals are added that react with suspended solids in the water supply, causing them to ag-gregate into large particles to facilitate removal 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 (also called tangential flow filtration). The bulk solution flows over, and parallel to, the filter surface, while, under pres-sure, a portion of the water is forced through the membrane to produce a permeate stream. The turbulent flow of the feedwater over the membrane sur-face minimizes accumulation of par-ticulate matter there, and facilitates continuous operation. Figure 3 further explains crossflow membrane filtration by contrasting it with conventional fil-tration — in conventional filtration, the entire solution is pumped through the filter medium, whereas only a portion goes through in the crossflow filtration process. 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 µm (1,000 to 100,000 angstroms). Figure 4 depicts the mechanism of crossflow microfil-tration.Ultrafiltration (UF; Figure 5) re-moves materials in the 0.001-to-0.1- µm range (10 to 1,000 angstroms). Ultrafiltration is employed to take out dissolved nonionic contaminants (macromolecules), typically organ-ics, whereas microfiltration is used mainly to remove suspended solids. Removal properties of UF membranes are usually expressed in terms of mo-lecular weight cutoff (MWCO), which typically range from about 1,000 up to 50,000 Daltons. Typical micro- and ultrafiltration membrane polymers include polysulfone, cellulosic acetate and polyamide types.

Reverse osmosis is a technique used mainly to remove salts from water. The membrane rejects over 99% of salts content, as well as virtually 100% of macromolecules with molecu-lar weights above 100 Daltons. Figure 6 illustrates reverse osmosis, which typically separates materials less than 0.001 µm (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 in nanofiltration; see below) membranes include cellulose acetate, cellulose tri-acetate, polyamide and thin-film com-posite types. These latter membranes are typically composed of a thin film, fabricated by coating one of a number of amide-type polymers onto a polysul-fone layer.Nanofiltration (NF) can be consid-ered “loose” reverse osmosis (RO). It rejects dissolved ionic contaminants but to a lesser degree than RO. NF membranes reject multivalent salts to a higher degree than monovalent salts (for example, 98% rejection ver-sus 20%). These membranes have MWCOs for macromolecules below 500 Daltons. Nanofiltration is illus-trated in Figure 7.

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

In general, strong oxidants will break organic bonds, and since all mi-croorganisms are organic, AOTs will inactivate them. With sufficient oxida-tive power, it is theoretically possible to break down all organic compounds into carbon dioxide, water and any ad-ditional elements that may have been present in the original compound (such as chlorine). AOTs include ultraviolet irradiation, ozonation, and hydrogen peroxide addition.Ultraviolet irradiation is a tech-nology that emits light energy at a

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52 CHEMICAL ENGINEERING WWW.CHE.COM MARCH 2006

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

FIGURE 4. Microfiltration’s role con-sists of the removal of suspended solids

wavelength peaking at about 254 nanometers (nm). At that wavelength, many microorganisms are inactivated, including most bacteria, some viruses and Cryptosporidium parvum, a patho-genic protozoan cyst. In addition to the 254 nm wavelength for microorgan-ism inactivation, 185-nm is optimal for breaking organic bonds; however, approximately four times the power or contact time is required for doing so.

Although relatively inexpensive to purchase and operate, ultraviolet ir-radiation systems have the following disadvantages:• Since the light travels in straight

lines, suspended solids or any other obstruction will reduce this technol-ogy’s effectiveness

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

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

Ozonation. Ozone is one the most powerful oxidants available, second only to fluorine. It is produced as a result of an electrical discharge in oxygen or air. Technically, ozone is an allotrope of oxygen with the chemi-cal formula O3. It will revert back to oxygen in approximately 20 minutes in ultrapure water. Ozone is the pun-gent odor prevalent in the air near lightning strikes, and often noticeable around copying machines where elec-trical discharges occur.

Although only slightly soluble in water, dissolved ozone is extremely effective in inactivating microorgan-isms; it also oxidizes certain water contaminants such as iron, manga-nese and hydrogen sulfide, and it is partially effective in breaking down dissolved organic contaminants.

Ozone is not without its drawbacks. It requires dry air, as discussed below; because it is short-lived, it has to be prepared onsite; and it requires very inert materials of construction. Ozone also reacts with bromine in water sup-plies to form bromate ions, suspected carcinogens.

Ozone is produced, onsite, by two methods: ultraviolet irradiation and corona discharge. The first of these methods employs 185-nm ultraviolet energy reacting with dissolved oxygen in the water. Whereas the concentra-tion of ozone produced in this matter is not sufficient for microorganism inactivation, it can be used for oxida-tion of inorganic and organic contami-nants. Corona discharge, the approach more widely used (especially in large installations), utilizes the creation of a spark across a dielectric to produce gaseous ozone from either air or oxy-gen. The gas is then introduced into the 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 dissolved ozone, sufficient to inactive microor-ganisms and accomplish the other performance objectives of ozonation.

The primary disadvantage of the corona discharge approach is that if there is moisture in the air (dew point greater than –40 °C) nitric acid is formed, which is difficult to eliminate and will cause severe corrosion prob-lems. This factor precludes the appli-cation of corona discharge ozonation for 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, hydrogen peroxide is widely employed by itself for 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 illustrates a typical water distillation unit. Dur-ing the production of ultrapure water,

distillation is very effective in remov-ing suspended solids and dissolved salts, as well as most microorganisms. Distillation also provides disinfec-tion of the water. Its shortcomings in-clude high-energy utilization and high maintenance costs.Electrodialysis. Electrodialysis is an electrochemical membrane-separation process in which ions (under d.c. volt-age) are transferred through a pair of ion-selective membranes, from a water stream into a more concentrated stream for discharge. Electrodialysis removes only the ionic solutes.Ion exchange (deionization). De-ionization is a term casually given to the use of ion exchange resins for the removal of ionic contaminants. Cation resins adsorb cationic contaminants (positive charge) and exchange them for hydrogen ions; anion exchange resins adsorb anionic contaminants (negative charge) and replace them with hydroxyl ions. The hydrogen and hydroxyl ions combine to form water; hence the term, “deionization.” The resins require regeneration with acid and caustic solutions.

When anion and cation resins are mixed together in a single bed, an ar-rangement known as “mixed bed” ion exchange, they produce the highest purity of water (from an ionic stand-point) commercially available. If the water is run through individual tanks each containing one kind of resin, this known as “two-bed” ion exchange; it produces a water quality comparable from an ionic standoint to that of re-verse osmosis. On the other hand, ion exchange 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 combination of electrodialysis and mixed-bed ion exchange, wherein ion exchange res-ins are encapsulated between layers of electrodialysis membranes. This technology is capable of producing the same quality of ionic purity as mixed bed 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

advantages over the latter are that it does not require regeneration.

Electrodeionization is a continuous process. And it appears to be fairly proficient 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 are not also present).Thermal destruction. Organics in water can also be broken down through the application of a number of thermal technologies, usually combining heat with a catalyst. However, these tech-nologies are site-specific and, because of their high capital and operating costs, are usually employed in the treatment of heavily polluted liquids such as those at U.S. Environmental Protection Agency (EPA) Superfund sites, rather than in treating water supplies for use within a process plant.Bioremediation. This technology is used almost exclusively for the reduc-tion of BOD (Biochemical Oxygen De-mand). It is primarily used for waste-water treatment, and only rarely for treatment of incoming water. Biore-mediation employs bacteria which feed on the organics and break them down during their normal metabolic processes. For a recent review of re-lated technology, see Biological Waste-water Treatment, Chem. Eng., October 2005, pp. 44–51.

An interesting combination of old and new technologies consists of membrane bioreactors (MBR), the incorporation of microfiltration or ultrafiltration with bioremediation to continuously filter the treated water to remove dissolved macromolecules and/or suspended sol-ids. As part of the bioremediation pro-cess, MBR is used almost exclusively for wastewater treatment.

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

It is well understood that the higher the water purity, the more aggressive the water is. Although many of the com-mon materials of construction are inert to “high purity” water, ultrahigh-purity water (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 technologies are the same as (or very similar to) pri-mary treatment technologies; however, as they are required only to remove a relatively small quantity of contami-nants, polishing systems usually re-quire less resins or fewer membrane elements 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 use of the purified water as it leaves the distribution loop, it is treated with a submicron cartridge filter, ultrafiltra-tion membrane, or another appropri-ate type of treatment technology.

DisinfectionDisinfection, which is used in several types of chemical process plants, con-sists of inactivating microorganisms or removing them from water supplies. Microorganisms that are commonly found in water supplies include bacte-ria, viruses, protozoan cysts, algae and fungi. Within these broad classes may be hundreds of thousands of different species, each with its own response in the face of specific inactivation technol-ogies. Among these can be variations in individual microorganisms’ suscepti-bility to high temperatures, particular UV wavelength, oxidation potential, exposure time, and so on, as discussed in more detail below.

Microorganisms that affect human health are known as patho-gens. Immuno-compromised indi-viduals, such as the very old, the very young, AIDS patients and people undergoing certain kinds of chemotherapy, may react adversely to low concentrations of microor-ganisms that do not affect the “nor-mal” population. The point is that virtually all water supplies into which 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 chemical disinfectants today utilize oxidizing agents. A strong reducing agent, form-aldehyde, is very effective and has been used in the past with excellent results; however, it is now prohibited in most industrialized countries be-cause the vapors have been found to be hazardous to human health.

Of the oxidizing chemicals, the halogens are most commonly used, with the exception of fluorine, which is much too dangerous and difficult to handle. Chlorine is the most popular in the U.S., where chlorinated water supplies have been prevalent for the last 100 years. The form of chlorine most commonly used for municipal drinking water treatment is gaseous chlorine; for residential and other small applications, chlorine is either delivered in the form of liquid sodium hypochlorite or solid pellets of calcium hypochlorite. The chemical species of interest here is the hypochlorite ion, which will inactivate almost all patho-genic microorganisms.

A particularly important feature of chlorination is that it persists (leaves a residual) in the water supply to en-sure long-term disinfection properties. On the other hand, chlorine reacts with certain organic compounds in the water (naturally occurring or oth-erwise) to form a family of suspected carcinogens known as trihalometh-anes. In the past 20 years or so, many municipalities (but not many process plants) have included ammonia with chlorine in their disinfection process

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54 CHEMICAL ENGINEERING WWW.CHE.COM MARCH 2006

Source: Water Quality Assn.

FIGURE 8. While it may be a far cry from a petroleum-refinery hydrocar-bon splitter, this distillation vessel is appropriate for primary treatment during water purification

in order to produce chloramines which do not form trihalomethane byprod-ucts. A disadvantage of this approach is that chloramines are not as stable as chlorine alone.

As stated earlier, ozone is the most powerful oxidant available on a prac-tical basis. Because of its outstanding oxidation characteristics, it is an ex-cellent disinfectant; and in spite of its drawbacks cited earlier, it is the most widely used municipal disinfectant in Europe.Radiation. As a disinfection technol-ogy, ultraviolet irradiation is perhaps the most widely used technology for non-municipal applications, including chemical process plants. It has sig-nificant limitations, however, in terms of efficacy when compared to ozone. As with ozone, ultraviolet irradiation leaves no residual.Mechanical. It is possible to physi-cally filter out microorganisms using membrane technologies. In particular,

microfiltration with its sub-micron pore sizes will readily remove the protozoan cysts and the larger sized bacteria, whereas, ultrafiltration is generally effective with the rest of the classes, depending upon its pore size.

A complication, however, is the fact that because bacteria are viable, grow-ing microorganisms, they will “grow through” membranes. This is a contro-versial issue; however, it is widely ac-cepted that when water remains stag-nant on a membrane surface, bacteria will, 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 layers from disinfectants and cleaners. Bio-film also tends to collect suspended solids and other fouling materials. Another important consideration with mechanical treatment is that the mi-croorganisms are physically removed but not inactivated.

SYSTEM DESIGN REQUIREMENTSSuccessful implementation of water treatment technologies requires in-corporating the right combination of them into a total system that produces the appropriate water quality with the greatest reliability and at the lowest possible capital and operating costs.

The most important initial param-eters to consider are: • Feedwater quality• Treated-water quality requirements• Volume or flowrate requirementsAs regards feedwater quality, a thor-ough water analysis is essential. That analysis should include those parameters which influence the technology selection, such as sus-pended solids, dissolved organics, microorganism concentration and water temperature (the last-named is particularly relevant with respect to the use of membrane-based water treatment). Feedwater quality most

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significantly affects the selection of pretreatment technologies.

To determine the required treated-water quality, on the other hand, the engineer must have a complete understanding of the effect of water quality upon the intended use (for in-stance, product synthesis, or rinsing). In many cases, guidance is available from industry standards. Some of these may be mandated, whereas oth-ers are simply recommended.

As for the total volume or flowrate, the total daily volume requirement generally determines the production rate required for the primary treat-ment technologies. The instanta-neous flow rate (gallons or liters per minute) and duration (minutes) typi-cally dictate the size of the distribu-tion system (storage tank capacity and pump flowrate).

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

size limitations of the system haz-ardous waste considerations, and the relevant regulations and the re-quired permits.

ConclusionsThere are a myriad of excellent tech-nology choices to treat water for uses in chemical processing facilities. While far from an exhaustive review of all the technology choices, this ar-ticle has presented a number of the technologies that are most relevant for 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 the technologies in the right combination to produce the optimum results for his or her particular requirements. Of course, there are a number of skilled and experienced consulting engineers capable of providing help.

In closing, it is fitting to point out

that regardless of the plant’s purity requirements for its process water, the source of that water (prior to treatment as outlined above) can readily be wastewater from within the 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 schemes outlined above. ■

Edited by Nicholas P. Chopey

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Engineering Practice

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 Kennedylaan 94, 2343 GT Oegstgeest, Netherlands; Phone: 31-71-5154417. Email: cartwrightconsul@cs.com), in 1980. He specializes in both marketing and technical consulting, in high-technology separa-tion processes for water purification, wastewater treatment, and food and chemical processing ap-plications. A 2001 recipient of the Award of Merit from the Water Quality Assn., he has written dozens of articles and several book chapters, lec-tured around the world, and been awarded three patents for water purification. He holds a 1961 bachelor’s degree in chemical engineering from the University of Minnesota.