Pretreatment Unit Operations

gxpand jv t .com Journal of Validation technology [Spring 2010] 37

Pharmaceutical Water System Fundamentals.William V. Collentro]

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“Pharmaceutical Water System Fundamentals” dis-cusses technical justification, design considerations, operation, maintenance, compliance, and validation for pharmaceutical water systems. The primary objec-tive of this column is to provide a basic summary of the function, selection, design consideration, proper operation, preventative maintenance, and regulatory expectations associated with the individual unit oper-ations employed in pharmaceutical water systems.

Reader comments, questions, and suggestions are needed to help us fulfill our objective for this column. Please send your comments and suggestions to col-umn coordinator William V. Collentro at [email protected] or to journal coordinating editor Susan Haigney at [email protected].

KEY POINTSThe following key points are discussed in this article:

•Pretreatment components are unit operations employed prior to removal of ionic material. Pri-mary types of pretreatment units include multi-media filtration, activated carbon units, and water softening units

•Multimedia filtration removes particulate matter from raw water by passing raw water through inert material layers of anthracite and sand of decreasing particle size. Design, operation, maintenance, and validation considerations for multimedia filtration units are discussed

•Activated carbon removes residual disinfecting agent and reduces the concentration of naturally occurring organic matter (NOM) from raw water by means of surface reaction with activated carbon. Design, operation, maintenance, and validation consider-ations for activated carbon units are discussed

•Water softening removes multivalent cations such as

magnesium, calcium, and iron from feed water by means of ion exchange. Design, operation, main-tenance, and validation considerations for water softening units are discussed

•Other pretreatment unit operations or support acces-sories include pumps; heat exchangers; chemical injection of disinfecting agent, reducing agent, or acid; inline ultraviolet sanitization units; inline ultra-violet chlorine/chloramines destruct units; organic scavengers; storage tanks; and cartridge filtration systems.

INTRODUCTIONThis paper continues a discussion on subject matter first presented in “Pharmaceutical Water System Fundamen-tals: Impurities in Raw Water,” published in the Journal of Validation Technology, Volume 16, No. 1, Winter 2010.

Pretreatment components are unit operations employed prior to removal of ionic material. Primary types of pretreatment units include multimedia filtra-tion, activated carbon units, and water softening units. A discussion of each component is presented. Design, operation, maintenance, and validation considerations are discussed. Operating data are included as appropriate to reinforce critical topics.

MULTIMEDIA FILTRATIONMultimedia filtration removes particulate matter from raw water. It is generally the first component in a phar-maceutical water purification system; although, disin-fecting agent injection or a raw-water break tank may be positioned upstream.

Design ConsiderationsMultimedia filtration units are vertical cylindrical col-umns containing two distinct “layers” of material. The

Pretreatment Unit Operations

ABOUT THE AUTHORWilliam V. Collentro is a senior consultant and founder of Water Consulting Specialists, Inc., Doylestown, PA (www.waterconsultingspecialists.com) and has more than 40 years experience in water purification. He may be reached at [email protected].

William V. Collentro

38 Journal of Validation technology [Spring 2010] i v t home.com

Pharmaceutical Water System Fundamentals.

lower level of material is referred to as “support media,” consisting of graduated size gravel. This layer supports the filter media layer. The filter media also consists of graduated levels of material. However, this material is layered in the reverse physical size configuration as the support media with the coarsest material at the top and finest filter material at the bottom. Layering of the filter media produces the multimedia filtration process that can provide highly effective removal of particulate mat-ter. As water passes down through the filter layers, larger particles are removed at the top of the bed with smaller particles removed toward the bottom of the media. The course filter media is generally anthracite and the finest filter media is generally sand.

The column diameter for a multimedia filtration unit should be selected such that the face velocity through the bed is about six gallons per minute (gpm) per square foot of cross sectional bed area. Lower face velocities may produce “channeling,” a process associated with non-uni-form flow over the entire cross section of the filter bed. Channeling decreases both the ability to remove smaller particles as well as the physical amount of material that can be removed prior to “breakthrough” of particulate matter into the product water stream. Periodic backwash is required to remove entrapped particulate matter. The backwash flow rate should be about 12-15 gpm per square foot of cross sectional bed area. Because feed water tem-perature, particularly for water from a surface source, may vary with seasons, provisions for regulating the backwash flow rate must be included. Excessive backwash flow rates are undesirable because impingement of the upper layers of filter media can occur on the sides and top of the filter column resulting in production of “fractured” material or “fines.”

There are other design factors that should be considered in addition to proper column diameter sizing. The column volume above the support and filter media should be at least 50% of the combined volume of support and filter media. This volume, referred to as “freeboard,” allows room for bed expansion during the backwash operation. Inadequate freeboard will result in impingement of filter media on column surfaces during backwash. The column should be equipped with a lower distribution system that provides a pressure drop of at least 5-7 psid (pounds per square inch differential). The pressure drop coupled with proper distributor physical arrangement will produce uniform flow distribution over the entire horizontal cross section of the filter media layers. The lower distributor pro-vides flow distribution. While a simple upper distributor is suggested, this distributor does not significantly contribute to the flow characteristics through the filter media.

Column material of construction may be fiberglass reinforced vinyl ester (or polyester) for lower flow appli-cations. Larger flow rates require steel columns with interior lining. The filtration media is very abrasive. During periodic backwash the filter media will expand. During this expansion, the filter media will provide a “sandpaper-like” action on the interior walls of the filter vessel. For steel vessels with simple sprayed- or backed-on lining material, erosion will occur. Further, corrosion of the exposed steel surface will provide ongoing iron oxidation, introducing particulate matter to the water in the area of the finest filter material (most abrasive). Fiberglass reinforced vinyl ester/polyester columns are “standard” straight side height. Media volume must be adjusted to provide the indicated minimum 50% free-board volume. Distributors for fiberglass reinforced vinyl ester/polyester columns are generally of PVC, CPVC, or other plastic material construction. It is suggested that 316L Stainless Steel distributors be considered for large diameter, lined steel vessels.

Multimedia filtration units should be equipped with appropriate valves. For fiberglass vinyl ester/polyester columns, multiport valves may be considered in lieu of individual valves. However, top-mounted multiport valves with top inlet “riser” tube and top outlet are strong-ly discouraged because desired distribution cannot be achieved. Side mounted multiport valves with top inlet and bottom outlet provide a technically superior alterna-tive to top-mounted valves. Individual pneumatically operated diaphragm valves should be considered for lined-steel column units. The valves should be positive acting, air-to-open, spring-to-close. A typical multime-dia filtration unit with individual valve configuration is shown in Figure 1.

Other design considerations include the following:•Units should be provided with manual inlet and

outlet isolation valves. The use of ball-type valves is strongly discouraged. Diaphragm valves are preferred

•Feed water and product water sampling valves should be provided. Again, diaphragm-type manual valves are preferred

•Feed water and product water pressure gauges should be provided. Liquid-filled gauges with minimum three-inch face diameter are desirable. Sanitary pres-sure gauges with custom adapters, shown in Figure 2, provide desired accuracy and minimum dead leg for microbial growth. The diaphragm at the base of a sanitary pressure gauge eliminates the lengthy dead leg associated with classical pressure gauges also shown in Figure 2


William V. Collentro, Coordinator.

Figure 1: Typical multimedia filtration unit.

•A feed water flow rate meter is required to determine operating, backwash, and post backwash rinse flow rates. Variable area flow rate meters and turbine meters are appropriate for this application

•The use of a pressure relief valve positioned on the top inlet piping of the column should be considered. This eliminates the possibility of column failure on high pressure

•Access manways and “handholes” should be con-sidered for lined-steel columns. This will facilitate media removal as well as access to the lower distri-bution system

•The waste line (to drain with air break) from the multimedia filtration unit should contain a transpar-ent section of tubing or site glass for observation of entrapped material during the backwash and post backwash rinse-to-drain operations.

Operating Considerations A multimedia filtration unit designed and operated as discussed previously should be capable of removing par-ticles with a size of 10 microns and larger. The human

eye has the ability to detect particles of approximately 40 microns and larger. However, removal of particles with a size of one micron and smaller, including heavy molecular weight naturally occurring organic material (NOM), NOM complexed with colloidal material, and larger colloidal material can be achieved if the multime-dia filter is operated in a “ripened” condition. Figure 3 provides a graph demonstrating the ripening process. While a common practice in municipal applications, this process is seldom employed for multimedia units for pharmaceutical applications. As particles accumulate in a multimedia filtration unit after backwash, the entire filter media “bed” begins to tighten. The entrapped material allows smaller particles to be removed. It may take hours or even days for the multimedia filtration unit to achieve this ripened condition after backwash. Obviously it is desirable to delay backwash, operating in the ripened condition, but avoid breakthrough of particulate matter. Experience indicates that breakthrough of particulate matter will not occur until the delta P through the unit is 7–11 psid greater than the post backwash delta P. As indicated, most multimedia filtration units operate in an



Figure 2: Sanitary pressure gauge with adapters.

unripened state. As further indicated, the use of accurate pressure gauges is required to employ this highly desir-able condition.

The duration of the backwash operation should be at least 15-20 minutes. However, the duration should be long enough to allow complete removal of entrapped particulate matter. The transparent section of tubing or site glass, discussed earlier, should be used to verify the absence of particulate matter prior to termination of the backwash cycle. If necessary, the duration of the backwash cycle should be extended.

Subsequent to unit backwash, it is desirable to include a “settle” time of approximately five minutes between the end of the backwash cycle and start of the rinse-to-drain cycle. This step allows the filter media to settle in desired defined “layers” by gravity.

Subsequent to the settle cycle, a rinse-to-drain cycle (in the normal operating flow direction) should be per-formed. This operation should be executed at the normal operating flow rate for the unit. It removes particulate matter that may have been introduced by unfiltered back-wash water as well as “stray” particulate matter from the

filter bed. The duration of this operation is generally approximately 10-15 minutes.

Proper operation of a multimedia filtration unit should be verified by periodic sampling and analysis. Turbid-ity, total suspended solids, and color analyses should be considered. Sampling prior to backwash, subsequent to backwash, and during operation is appropriate.

MaintenanceThe following multimedia maintenance items are appropriate:

•Feed water and product water pressure gauges should be calibrated once every 6-12 months. Feed water flow indicators should be calibrated annually

•The visible section of the interior of lined-steel col-umns should be inspected annually. Further, filter media volume (level inside the column) should be verified annually

•Filter media and support media should be replaced every five years. For fiberglass column units, dis-tributors should be replaced subsequent to old media removal but prior to installation of new media. For



Figure 3: Ripening of a multimedia filtration unit.

lined-steel column units lower distributors should be thoroughly inspected (and replaced if applicable)

•Lined-steel column access port gaskets should be replaced every time an access port is removed

•Multiport valves should be “rebuilt” every two years

•Diaphragms in diaphragm valves should be replaced every two years.

ValidationThe following items should be considered during valida-tion of a multimedia filtration unit:

•Support media—certificate of origin•Filtration media—certificate of origin and analysis•Column data—manufacturer, pressure rating, test

pressure, temperature rating, diameter, straight side height, overall height, drawing, and materials of construction

•Valves—manufacturer, model number, serial number, material(s) of construction, size, and type

•Pressure gauges—manufacturer, model number, serial number, materials of construction, accuracy, range, pressure increments, face diameter, material certifica-tion, and certificate of calibration

•Relief valve—manufacturer, model number, seri-al number, size, relief pressure, and materials of construction

•Flow rate meter—manufacturer, type, model number, serial number, size, range, accuracy, and materials of construction

•Operating flow rate—record and verify•Backwash flow rate—record and verify•Backwash duration—record and verify•Settle time duration—record and verify•Post backwash rinse-to-drain operation flow rate—

record and verify•Post backwash rinse-to-drain operation duration—

record and verify•Valve sequence—operation, backwash, settle,

rinse-to-drain.It should be noted that the suggested validation items

apply, as appropriate, to other pretreatment unit opera-tions employing a column and media, such as activated carbon units and water softening units.

ACTIVATED CARBON UNITSActivated carbon can provide two functions within a pretreatment system. Activated carbon removes residual disinfecting agent present in raw water. It is important to consider the function of activated carbon for feed water from a ground water source and feed water from a surface source or ground water source influenced by a surface water source. As discussed in Part I of this series of articles, raw water from a surface source or ground water source influenced by a surface water source will contain NOM. One of the functions on an activated carbon unit is to reduce the concentration of NOM. For systems employing downstream reverse osmosis (RO) as a primary ion removal step, NOM, if not removed, will result in fouling of membranes. Fouling can result



in loss of RO unit product water flow rate. Perhaps of greater importance, NOM will accumulate on RO membrane surfaces directly above a layer on the RO membrane surface containing bacteria and bacterial endotoxins, shown in Figure 4. The organic material provides nutrients for microbial proliferation. This will result in increased RO product water total viable bacte-ria levels. For the limited number of pharmaceutical water systems employing ion exchange as a primary ion removal technique, NOM, if not removed/reduced, will foul anion (negative ion removal resin) resin. Organic fouling of anion resin results in a reduction of exchange capacity. The fouling may be irreversible, requiring replacement of anion resin.

A second function of activated carbon is the removal of residual disinfecting agent. If not removed, residual disinfecting agent will chemically react with downstream reverse osmosis membranes (thin-film composite poly-amide-type employed for pharmaceutical applications) resulting in rapid membrane failure. The NOM in raw water from a surface source or ground water influenced by a surface water source will react with residual chlo-rine disinfecting agent producing carcinogenic products. Both trihalomethanes (TTHMs) and haloacetic acids (HAA5) are regulated disinfection byproducts by the US Environmental Protection Agency (EPA) “National Primary Drinking Water Regulations–Disinfectants and Disinfection Byproducts Rule” (DBPR) (1). To reduce the concentration of undesirable disinfection byproducts, many municipalities inject ammonia at the point of distribution, producing chloramines. The predominate chloramine compound in raw water is monochloramine (2). Removal of monochloramine by activated carbon occurs as follows:

NH2Cl +C* + 2H2O → NH3 +H3O+ + Cl- + CO*

2NH2Cl + CO* + H2O → N2 + 2H3O+ + 2Cl- + C*

Where C* indicates the activated carbon surface and CO* represents a surface oxide on the activated carbon surface. The rate of the first reaction is greater than that of the second reaction (3). The reactions are presented to demonstrate the fact that monochloramine is removed in a manner different than that of chlorine (hypochlorite ion); produces ammonia (NH3), which is a “reactive” gas that will pass through a RO membrane; and that the kinetics of the reaction are not the same as that for chlorine removal.

Raw water from a ground water source or surface water source may employ chlorine for disinfection if

the municipality can demonstrate the ability to meet the indicated EPA DBPR limits. Chlorine is removed from activated carbon, as follows:

C* + 2Cl2 = 2H2O → 4HCl + CO2

C* + H2O + HOCl → CO* + H3O+ + Cl-

Note that ammonia gas is not produced by this equation.

Design ConsiderationsAn activated carbon unit designed for chlorine removal with or without the presence of NOM can be selected based on a face velocity of approximately 3 gpm per square foot of column cross sectional bed area and a volumetric flow of approximately 1 gpm per cubic foot of activated carbon media. Activated carbon media should be acid rinsed to remove naturally present heavy multivalent cations such as barium, aluminum, and strontium. Subsequent to acid rinse by the supplier, the activated carbon should be treated with a base such that pre-shipment product water pH is approximately pH of 7. Activated carbon media should be replaced annually for feed water that does not contain NOM or every six months if the feed water contains NOM.

As indicated, the rate of activated carbon removal of monochloramine is much slower than that of chlorine. The rate of reaction also decreases with increasing pH and concentration of NOM (4). Proper activated carbon unit design for monochloramine removal requires a face veloc-ity ≤ 3 gpm per square foot of cross sectional bed area and a volumetric flow rate ≤ 0.50 - 0.75 gpm per cubic foot of media (5). The indicated design conditions assume that catalytic activated carbon is employed (6). Activated carbon media replacement must be performed every six months. Activated carbon unit free and total chlorine monitoring should be performed frequently to verify that chloramine “breakthrough” has not occurred.

Because activated carbon units remove residual dis-infecting agent and the carbon bed provides a relatively warm, dark, wet area with abundant carbonaceous mate-rial, microbial proliferation will occur within the unit. In fact, the highest pretreatment system total viable bacteria levels will be noted in activated carbon unit product water samples. Periodic ambient temperature backwash is required to reduce product water bacteria levels. The use of periodic hot water sanitization at 80-90°C for a two-hour period provides excellent microbial control (7). The use of steam for periodic hot water sanitiza-tion is not suggested. The backwash flow rate should be



Figure 4: Layering of material on RO membrane.

about 4.5 to 5.0 gpm per square foot of cross sectional bed area. Because feed water temperature, particularly for water from a surface source, may vary with seasons, provisions for regulating the backwash flow rate must be included. Excessive backwash flow rate is undesirable because impingement of the relatively fragile activated carbon media will occur on the sides and top of the filter column resulting in production of “carbon fines.”

In addition to proper column diameter sizing, there are other design factors that should be considered. An activated carbon bed depth of 48 inches is suggested, particularly for chloramine removal. The column volume above the support and filter media should be at least 75% of the activated carbon volume to allow room for bed expansion during the backwash operation. Column selection and distributor design should be similar to that for multimedia filtration units. For hot water sanitizable units, column material of construction must be either 316L Stainless Steel or preferably high temperature rub-ber lined, carbon steel.

Hot water sanitizable activated carbon units should be equipped with 316L Stainless Steel “face” piping/tub-ing and diaphragm-type valves. Further, for hot water sanitizable units, the use of a dedicated tank, recircula-tion pump, and heat exchanger should be considered for recirculation of hot water, avoiding pressure and tem-perature issues associated with heating and expansion of a “solid” system containing water. Figure 5 depicts an activated carbon unit with suggested hot water sanitiza-tion accessories.

Other design considerations are similar to those pre-sented for multimedia filtration units.

Operating Considerations Operating considerations should include the following:

•An activated carbon unit designed and operated should be capable of removing all residual disinfect-ing agents and removing greater than 50-60% of NOM from the feed water. Obviously, it is desirable to periodically backwash units to remove bacteria. It is suggested that the backwash frequency be limited to 1-2 times per week. Excessive backwash can result in transfer of denser, activated carbon-containing residual disinfecting agent and NOM from the top of the bed to the bottom of the bed. This could accelerate “breakthrough” of residual disinfecting agent and/or NOM, requiring frequent activated carbon media replacement

•The duration of the backwash operation should be about 15-20 minutes. However, the duration should be long enough to allow complete removal of activated carbon fines. The transparent section of tubing or site glass, discussed previously, should be used to verify the absence of activated carbon fines prior to termination of the backwash cycle

•Subsequent to unit backwash, it is desirable to allow a five-minute settle time period between the end of the backwash cycle and start of the rinse-to-drain cycle. This step allows the media to settle

•Subsequent to the settle cycle, a rinse-to-drain cycle (in the normal operating flow direction) should be performed. This operation should be executed at the normal operating flow rate for the unit. This step hydraulically compresses the relatively light activated carbon media back to the lower section



Figure 5: Process flow diagram. Activated carbon unit hot water sanitization components.

of the column. The duration of this operation is generally approximately 10-15 minutes

•Proper operation of an activated carbon unit should be verified by periodic sampling and analysis. Feed water and product water total organic carbon (TOC), free chlorine, total chlorine, total suspended sol-ids, and total viable bacteria analysis should be considered

•Maintenance items are similar to those for a mul-timedia filtration unit with the exception of media replacement frequency. Validation considerations, including documentation, are also similar to that of a multimedia filtration unit.

WATER-SOFTENING UNITSWater-softening units remove multivalent cations from feed water replacing the ions with sodium. The multiva-lent cations, such as magnesium, calcium, and iron will form insoluble compounds in the concentrating stages of a reverse osmosis unit. The resulting scale will reduce RO product water flow and purity. Chemical cleaning can be used to remove some scalents. However, scales formed by trace concentrations of aluminum, barium, and strontium may result in compounds that cannot be removed from the RO membranes requiring replacement. The use of water softening as a pretreatment unit opera-tion in a system employing RO for primary ion removal is critical to successful system operation.

For the limited number of pharmaceutical systems employing ion exchange resin (two-bed or mixed bed) as the primary ion removal process, water softening should not be employed as a pretreatment technique. For ion exchange applications, the attraction of ions to exchange sites is a function of “charge density,” a func-tion of molecular weight and charge. Higher molecular weight ions and ions with multiple electronic charge (valence) have a greater attraction to an ion exchange site than lighter molecular weight monovalent ions. The Table provides a summary of the affinity of vari-ous cations for ion exchange sites (8). A water softener converts the heavy molecular weight multivalent cations to monovalent light molecular weight sodium. This is highly undesirable and would adversely affect both the product water quality and volume of water processed between regeneration cycles with acid and caustic for ion exchange units.

Water softening is an ion exchange process with typi-cal reaction, as follows:

Ca++ + R-Na+ ↔ R-Ca++ + Na+

Calcium ion present in feed water is attracted to the ion exchange resin site and displaces sodium ion from the resin site. Calcium has a molecular weight of 40 Daltons and a +2 charge. Sodium has a molecular weight of 23 and a +1 charge. Subsequently, the calcium ion has a



Table: Relative attraction of various cations to exchange sites.

Cation Selectivity coefficient vs. H3O+

Lithium (Li+) 0.8

Sodium (Na+) 2.0

Hydronium (H3O+) 1.0

Potassium (K+) 3.0

Ammonium (NH4+) 3.0

Magnesium (Mg++) 26

Calcium (Ca++) 42

Source: Rohm and Haas, 1965 (9).

much greater attraction to the resin site than sodium. Product water from the softener will contain sodium as the positive ion (cation) electronically balanced with negative anions (anions) such as chloride, bicarbonate, sulfate, etc.

Ion exchange would not be a viable process if the resin could not be regenerated. Note that the symbol between reactants and products in the equation represents an equilibrium reaction. During operation, with the major-ity of resin sites in the sodium form, the equilibrium is strongly driven to the right side of the equation. How-ever, eventually resin sites are converted to the calcium (or other multivalent cation) state. The resin will be incapable of further multivalent cation removal result-ing in “breakthrough” of calcium and other multivalent cations. Regeneration is performed with salt, a sodium chloride solution. During the regeneration process the sodium ion concentration is orders of magnitude greater than the concentration of multivalent cations, resulting in the following:

Na+ + Cl- + R-Ca++ → Ca++ + R-Na+ + Cl-

Ion exchange sites are converted back to the sodium form with exchanged calcium removed and diverted to waste. Subsequent to regeneration and rinse, the water softener ion exchange bed is placed in operation and the indicated cycle repeated. The “dose” of salt sug-gested for conversation from the calcium form back to the sodium form is 15 pounds per cubic foot of cation exchange resin. The salt is introduced over a 30- to 45-minute regeneration time period at a flow rate of 0.5 to 3.0 gpm/cubic foot of cation resin and sodium chloride concentration of approximately 10%, consistent with the resin manufacturer’s recommendation. The regeneration

time may be adjusted to obtain the optimum value for a specific unit, cation resin, and feed water analytical profile.

The regeneration cycle for common concurrent units employs backwash, salt solution introduction, displace-ment rinse, and fast rinse. The backwash operation is performed at a flow rate of approximately 6 gpm per square foot of cross sectional bed area and time period of 10-20 minutes. As indicated previously, regenerant salt solution is introduced over a time period of 30-45 minutes. The displacement rinse (“slow” rinse) is gen-erally conducted at the same flow rate and time period as the regenerant salt introduction step. Final rinse is performed at the operating flow rate for a time period of 15-30 minutes, a function of the feed water total hardness and regeneration parameters.

Design ConsiderationsThe column diameter for a water-softening unit should be selected such that the face velocity through the bed is approximately 7 gpm per square foot of cross sectional bed area. The maximum suggested face velocity should not exceed 10 gpm per square foot of cross sectional bed area. In addition to proper column diameter siz-ing there are other design factors that should be con-sidered. The column volume above the support and filter media should be at least 50% of the resin volume. Other column parameters, including distributor design, are similar to those described for multimedia filtration units. Column material of construction is similar to the description for multimedia filtration units. Hot water sanitization of water softening units is possible but sel-dom employed. The use of stainless steel columns for hot water sanitization is discouraged because of chlo-ride stress and pitting attack concerns. It is suggested



that rubber-lined steel vessels be employed for hot water sanitization application.

Cation resin will become fouled with iron over a peri-od of time. Iron fouling reduces available resin exchange sites because it is not removed during regeneration with salt solution. While use of regenerant salt with an “iron removal” additive and iron removal chemical treatment techniques are not available, it is suggested that the use of either technique is inappropriate for pharmaceutical water purification applications. Regenerant salt with an “iron removal” additive introduces a “Foreign Sub-stance/Impurity” as defined in the “General Notices” section of the United States Pharmacopeia. Tests necessary to verify removal of the additive are difficult to deter-mine since the nature and chemical composition are unknown and often proprietary. Chemical cleaning of cation resin is also inappropriate, because the nature of the cleaning chemicals is unknown. Further, cation resin decomposition products, safety issues, and main-tenance intensive nature of the process are all concerns. It is strongly suggested that samples of softener cation resin be obtained every 6-12 months for analysis. If iron fouling is indicated, a periodic resin replacement program should be initiated. Iron can rapidly foul downstream RO membranes. The cost of cation resin is inexpensive compared to the cost of downstream RO membrane replacement. Finally, iron provides an excellent nutrient for several species of pathogens.

The regeneration time period for a water-softening unit is approximately two to three hours, so two units are generally employed. Because stagnant water provides a location for bacteria to replicate, series operation of water softening units should be considered. Generally, this mode of operation designates one unit as the “lead” unit and the other as a “polisher.” The lead unit is regenerated based on the volume of water processed or indication of product water hardness (online analyzer or “grab” samples). The polishing unit is regenerated based on elapsed time. When either unit is being regenerated the other unit remains in “service” to provide an uninter-rupted flow of softened water.

Other design considerations are similar to those indi-cated for multimedia filters and activated carbon units.

Operating Considerations Operating consideration should include the following:

•Periodic sanitization and cleaning of the salt stor-age tank

•Periodic sample collection of the regenerant salt solution concentration to ensure proper conversion of resin back to the sodium form

•Periodic examination of the waste water during the regeneration steps to verify the absence of cation resin “fines”

•Annual or semi-annual collection of cation resin samples for analysis of resin characteristics includ-ing iron fouling

•Verification of the salt volume and weight introduced during the regeneration cycle

•Proper operation of a water-softening unit should be verified by product and feed water periodic sampling and analysis. Total suspended solids, total hardness as calcium carbonate, total iron, and total viable bacteria are analyses that should be considered.

MaintenanceThe following water softening unit maintenance items are appropriate:

•Feed water and product water pressure gauges should be calibrated once every 6-12 months. Feed water flow indicators should be calibrated annually

•A stainless steel mesh-type resin trap should be posi-tioned downstream of a water softening unit. The trap should be inspected frequently to verify that the “whole” resin beads are not present. Resin “fines” should be flushed from the trap periodically

•The volume of cation resin in a water softener should be visually determined at least once each year

•Lined steel column access port gaskets should be replaced every time an access port is removed

•Multiport valves should be “rebuilt” every two years

•Diaphragms in diaphragm valves should be replaced every two years.

ValidationThe following items should be considered during valida-tion of a water-softening unit:

•Cation exchange resin—certificate of origin•Cation exchange resin—meets US Food and Drug

Administration requirements for application•Column data—manufacturer, pressure rating, test

pressure, temperature rating, diameter, straight side height, overall height, drawing, and materials of construction

•Valves—manufacturer, model number, serial num-ber, material(s) of construction, size, and type

•Pressure gauges—manufacturer, model number, serial number, materials of construction, accuracy, range, pressure increments, face diameter, material certification, and certificate of calibration

•Relief valve—manufacturer, model number, seri-



al number, size, relief pressure, and materials of construction

•Flow rate meter—manufacturer, type, model num-ber, serial number, size, range, accuracy, and materi-als of construction

•Operating flow rate—record and verify•Backwash flow rate—record and verify•Backwash duration—record and verify•Regenerant salt introduction time—record and

verify•Regenerant salt concentration—sample, record, and

verify•Regenerant salt volume—record and verify•Regenerant salt purity—record, sample, and verify•Displacement (slow) rinse time—record and

verify•Displacement rinse flow rate—record and verify•Fast rinse time—record and verify•Fast rinse flow rate—record and verify•Valve sequence—operation, backwash, settle,

rinse-to-drain.

OTHER PRETREATMENT UNIT OPERATIONOther potential pretreatment unit operations or support accessories for pretreatment equipment are presented. A brief summary and comments are provided.

PumpsPumps may be used for repressurization of a tank, increasing raw water pressure to ensure flow through all pretreatment components, and/or recirculation of water through the pretreatment components to avoid stagnant conditions. While microbial control is a pri-mary consideration for recirculation, diffusion of mate-rial, by concentration difference, in stagnant activated carbon units and water softeners is a concern. Diffusion may result in migration of contaminants to the lower regions of an activated carbon unit or water softening unit resulting in premature “breakthrough” of material. Pumps should be selected such that surfaces in contact with water are stainless steel or non-contaminating mate-rial. Pump motors should be of a totally enclosed fan cooled (TEFC) type. Variable frequency drives (VFDs) are recommended for pump motors to allow flow variation for backwash and regeneration operations.

Heat ExchangersHeat exchangers may be used for heating or cooling appli-cations. Cooling of recirculating pretreated water will reduce microbial growth particularly for systems with raw water from a surface source during summer months.

Heating may be employed for hot water sanitization of activated carbon units and/or water softening units. Heat exchangers of shell and tube type are preferred. Surface in contact with water should be stainless steel with non-chemical eluting gaskets.

Chemical Injection of Disinfecting AgentChemical injection of disinfecting agent may be required if the feed water to a facility has inadequate residual disinfecting agent or elevated total viable bacteria levels (>500 cfu/ml). Chemical injection of a liquid sanitizing agent such as sodium hypochlorite may be appropriate. A metering pump for injection with contact baffled cham-ber/tank providing approximately 20-minute contact time is suggested. Online monitoring for residual disinfecting agent subsequent to storage should be considered.

Chemical Injection of Reducing AgentChemical injection of reducing agent such as sodium bisulfite may be employed in lieu of activated carbon units for ground water supplies not influenced by a surface water supply and with low TOC concentra-tion. A positive displacement pump, chemical storage tank, injection device with mixing capability, and post injection online oxidation-reduction potential (ORP) monitoring system are required. Reducing agents can introduce significant amounts of bacteria if the system is not designed properly or if freshly prepared reducing agent is not used.

Chemical Injection of AcidChemical injection of acid may be required within the pretreatment system. Injection of acid may be required prior to an activated carbon unit to reduce the feed water pH to a value ≤ 8.0. Monochloramine removal at pH values > 8.0 requires significant contact time. Many raw feed water supplies for large older cities exhibit pH values >8.0 in an attempt to eliminate corrosion of lead pipe or lead soldered copper fittings in residential and commercial property.

Inline Ultraviolet Sanitization UnitsInline ultraviolet sanitization units are employed in pre-treatment systems generally positioned downstream of activated carbon units. The inline ultraviolet (UV) units employ UV light at a wave length of 254 nanometers and radiation intensity of about 30,000 to 35,000 microwatt-seconds per square centimeter to control bacteria. It is very important to indicate that UV deactivates bacteria by attacking its DNA. However, the bacteria are still meta-bolically active and may become reactivated if exposed



to light. Inline UV units can provide a one- to two-log reduction in bacteria levels if properly selected, installed, and maintained.

Inline Ultraviolet Chlorine And Chloramine Destruct UnitsInline ultraviolet chlorine and chloramine destruct units can be used to remove residual disinfecting agents. The units operate at a wavelength of 185 nano-meters and ultraviolet radiation intensity approxi-mately 5–15 times that used for disinfection. The technology can provide an alternative to activated carbon for certain applications. It should be indicated that the TOC level associated with NOM will also be reduced by 185 nanometer UV. Unfortunately NOM reduction results in a competitive reaction within the stainless steel chamber containing the UV lamps in quartz sleeves. It is strongly suggested that system design and unit selection be performed by the UV manufacturer.

Organic ScavengersOrganic scavengers may be used for pretreatment to both RO units and deionization units if the feed water TOC levels from NOM are excessive. The units con-sist of a column containing either macroporous sty-renic anion resin or gellular acrylic resin. The anion resin will remove NOM that exhibits a slight negative charge. Multiple step regeneration using a caustic brine solution generally followed by hydrochloric acid introduction is required. Chemical handling and safety concerns generally limit the use of this technology.

Storage TanksStorage tanks may be employed at various points in a pretreatment system. As indicated earlier, baffled tanks may be used with sodium hypochlorite injec-tion systems to provide contact time for bacteria destruction. Systems may employ raw water break tanks to provide a definitive “air break” between a domestic supply to a facility and the pretreatment system. The tank will also provide an excellent point for recirculating pretreated water return flow when the downstream primary ion removal system does not require pretreated water. Finally, the feed water piping size to a facility or drain capacity at a facility may not be capable of supporting the backwash flow rate or drain flow rate. Tanks may be used to store

backwash water and to collect backwash water for gradual discharge to drain.

Cartridge Filtration SystemsCartridge filtration systems may be employed in pretreat-ment systems. It is suggested that the number and loca-tion of the cartridge filtration system be carefully evalu-ated. Systems positioned downstream of an activated carbon unit or in a pretreatment recirculation system with activated carbon unit will provide a surface area for accumulation and replication of bacteria.

REFERENCES1. US EPA, “National Primary Drinking Water Regula-

tions: Stage 2 Disinfectants and Disinfection Byproducts; Final Rule,” 40 CFR Parts 9, 141, and 142, Federal Register 71:2:388, 2006.

2. Collentro, W. V., Pharmaceutical Water–System Design, Op-eration, and Validation, informa healthcare, New York, NY, pg. 38, 1999.

3. Glaze, W.H., Chemical Oxidation, Water Quality and Treat-ment–A Handbook of Community Water Systems, 4th Edition, American Water Works Association, Denver, CO, pp 747-779, Publishing Office: McGraw-Hill, Inc., 1990.

4. Fairey, J.L., Speitel, G.E., and Katz, L.E., “Monochloramine Destruction by GAC – Effect of Activated Carbon Type and Source Water Characteristics,” Journal of the American Water Works Association, Volume 99, No. 7, July 2007.

5. Collentro, W. V., Pharmaceutical Water–System Design, Opera-tion, and Validation, Second Edition, informa healthcare, New York, NY, Chapter 3, 2010.

6. Calgon Carbon Corporation, “Centaur 12x40 Granular Activated Carbon,” Product Bulletin LC-765-02/98, 1998.



9. Rohm & Haas Company, “The Deionization of Water, Part I: The Hydrogen Cycle Operation,” Amber-Hi-Lites, No.86, Philadelphia, PA, 1965.

GENERAL REFERENCECollentro, William V., Pharmaceutical Water, System Design,

Operation, and Validation, Interpharm Press, Buffalo Grove, IL, 1999. JVT

Documents

Pretreatment Unit Operations