ADDING A PICOTECH® 2 TO A PICOSYSTEM®

By Rick Rechen
 

The following discussion of Picosystem® accessories builds on previous presentations. There isn’t sufficient time to repeat all of the explanations, so if something seems unclear, refer to earlier publications of Water Works. The Picotech® 2 outlet is another way to answer the question about getting sterile water from a Pico System, posed by the user in last months Water Works.

HOW A PICOTECH WORKS

The Picotech® 2 outlet contains an Ultra Violet (UV) Sterilizer with a low ozone (254 nano meter (nm) wavelength) UV lamp. It exposes water from the Picosystem® to high intensity UV radiation. UV radiation at the 254 nm wavelength is very effective at killing bacteria (bugs) and other microorganisms. The way it does this is to break up the DNA of the microorganism. When a microorganism with broken, non-working, DNA tries to reproduce itself and multiply, it can’t. So, when it dies that’s the end of it.

Since the UV light is the last thing that the water sees before it exits the spout, the water should have no living microorganism in it and should be sterile. The only thing that can add back bacteria to the water is contamination from the Picotech® 2 spout. For this reason, the nut connecting the spout to the UV unit must be very tight to keep air from being drawn in. Also, no one should touch the end of the spout. That could easily leave bugs growing there.

It’s very important that the correct lamp be used in the Picotech® 2. There are two kinds of lamps. The 254 nm (low ozone) lamp is effective at killing bacteria. This is the one used in the Picotech® 2. The other lamp, the 185 nm (high ozone) lamp, is good at breaking up organic substances, however carbon dioxide gas, produced from the broken up organics lowers the specific resistance of the water from 18 megohms to around 12 megohms. So, the 185 nm lamp, used in the UV Plus Pump Assembly, must always have a deionizer after it.

WHO MIGHT BENEFIT FROM A PICOTECH 2?

A Picotech® 2 outlet could be provided for a customer intending to use water for preparing buffers or reagents that could be degraded by bacteria. Someone filling incubators where bacterial growth in the water chamber is a problem might also appreciate a source of bacteria free deionized water. Just about any general-purpose application where microorganisms are of concern could be considered. Typical ones are Biochemical Oxygen Demand and Clinical Chemistry. Please note: Even though water from a Picotech® 2 might have no living bacteria in it, there still might be endotoxin.

SPECIFICATIONS FOR WATER FROM A PICOSYSTEM® WITH A PICOTECH® 2

Resistance: At Least 18 meg
TOC: <35 ppb
CFU: <0.10 CFU/ml

WHAT DOES RECIRCULATION DO?

Adding recirculation to a Picosystem® with a Picotech® 2 reduces numbers of bugs growing in the system and level of TOC in the system. But, since water is not recirculated through the Picotech® 2, it doesn’t really do anything to affect its performance. A Picosystem® Plus with a Picotech® 2 will operate with the same specifications except that TOC will be less than 10 ppb.

WHAT ABOUT ADDING 185 NM WAVELENGTH UV TO THE PUMP?

Again, the Picotech 2 attaches to the end of the Picosystem® so that recirculated water doesn’t pass through it. Water is pumped through the 185 nm UV unit in the pump assembly and through the Picosystem® over and over. Multiple passes through 185 nm (high ozone) UV reduce TOC from less than 10 ppb to less than 5 ppb.

WHO MIGHT WANT A COMBINATION OF PICOTECH® 2 AND PLUS PUMP OR UV PLUS PUMP?

Any application that requires chemically pure water and is sensitive to living bacteria in the water might benefit from this. Specifically, many of the same users who would like a Picosystem Plus or a Picosystem UV Plus might want to add a Picotech 2. These include Amino Acid Analysis, GC-MS, DNA sequencing and automated clinical chemistry.
 

WHAT ARE THE BENEFITS OF UV AND RECIRCULATION?

By Rick Rechen

Microorganism control is the primary reason for using ultraviolet and recirculation in purified water systems. A second reason is to reduce TOC levels.

Minimizing microorganism levels in purified water systems involves two related concepts: Prevention and Control.
In order to produce a sterile environment, specific efforts must be carried out, or organics will be present.

ULTRAVIOLET RADIATION

UV (254 nm) acts as a barrier to living organisms carried by a moving stream of water. It does not effect microorganisms attached to piping upstream or downstream of the UV unit.

It has been theorized that for every microorganism floating in the water (killed by UV) there are 10,000 on the surface of the piping (unaffected by UV). At the very point of use (Picotech®, Picopure®), 254 nm UV can produce a sterile product. However, it is more difficult to determine what 245 nm UV achieves “in-line.” The amount, or activity of, endotoxin is not reduced by 254 nm UV. Exposure to high intensity 185 nm UV, over a period of time, will break down endotoxin and other microorganism by-products, and reduce TOC levels.

RECIRCULATION

Recirculation, as part of a synergistic design, promotes lowered microorganism counts. A synergistic design is a system design where the combined action of 2 or more pieces of equipment achieve an even greater effect than that of which each piece of equipment is individually capable. Some of the synergistic pieces that make recirculation effective are:

Smooth Piping and Joints. Smooth interior piping surfaces and joints make it more difficult for microorganisms to adhere to the piping and allow fewer places for microorganisms to hide. Smooth surfaces and crevice free joints allow disinfectant to reach all parts of the system, so that it does a more complete job.

No Dead Legs. A dead leg is defined as an un-recirculated section of piping longer than six times the pipes diameter. That is, a dead leg in a 1/2” diameter piping system would be any non-recirculated section of piping more than 3” long. Dead legs create all sorts of problems in a piping system. The most common problem is allowing an area where microorganisms and endotoxin accumulate.

Closed Loop. It is preferable that once highly purified water enters the distribution system, it does not come in contact with air again. Air recontaminates the water with carbon dioxide and other gasses.

It has been found that most bacteria do not survive and reproduce well in highly purified water. As long as the deionizers are producing water with high resistance (is ion free), CFU counts are low. Once ion breakthrough begins to occur, high CFU counts and endotoxin levels accompany the ions. In a closed-loop you maintain the micro-environment of high resistance with low CFU levels. When you return DI water to a storage tank, with higher nutrient RO water, CFU counts and endotoxin levels increase.

Storage facilities should be sealed, with incoming air being filtered through a pre-sterilized hydrophobic 0.2 micron vent filter. Regular disinfection of the storage tank, or chlorine injection, is a good idea to keep microorganisms and their by-products from accumulating.

Also, remember that opening storage tanks, sticking hands in the stored water, pulling out and replacing level controls are all potential sources for microbial contamination in storage and downstream equipment.

SUMMARY

In-line 185 nm UV in a recirculated system will reduce overall TOC and, more specifically, will break down endotoxin, proteins, enzymes and other microorganism by-products. Application of 254 nm UV at points of use, such as a Picotech 2 or Picopure 2 systems, can produce sterile water.

Continuous recirculation of purified water in a distribution system designed without dead legs and other sources of microbial contamination helps to provide effective control of microorganism growth. On a small scale, the Hydro 17008 pump or UV Plus Pump will perform this task.

Controlling Organics and Bacteria in a Picosystem®

By Keith Shepard

ORGANICS AND MICROORGANISMS ARE DIFFERENT

The category of water-borne impurities known as Organics includes all carbon containing compounds. Bacteria and other microorganisms contain carbon, and are alive, therefore they have their own separate category.

ORGANIC REMOVAL

Filtration: Some organics, such as fragments of plant life, are not dissolved but are in a particle form. These can be removed by filtration. The level of filtration may be: screens, depth media, microfiltration, ultrafiltration or reverse osmosis.

Activated Carbon: The microporous structure within carbon attracts and holds organics by Van der Walls force. Organic adsorption by activated carbon is an effective means for removing a broad range of organics from water. Recirculated deionizer systems, using activated carbon, can produce water with TOC levels from 5-20 ppb.

Ultraviolet radiation: With sufficient intensity and exposure time, most organic bonds between carbon and other elements can be broken with 185 nm UV. Longer exposure times (3-4 times sterilization time), or repeated exposure (such as recirculation), is necessary to achieve low TOC levels. Recirculated deionizer systems using both activated carbon and 185 nm ultraviolet can produce water with TOC levels of 1-5 ppb.

CONTROL OF ORGANICS IN PURIFIED WATER SYSTEMS

Here are some things we can do to help control organics in purified water systems:

1) Avoid using things which add organics. a) Use aged resins - New resin contains a lot of organics as a result of the manufacturing process. b) Pre-rinse RO membranes - new membranes contain a surfactant (organic) that must be rinsed off.

2) Protect the system from the intrusion of organics. a) Good pre-filters to remove organic particles. b) Vent filters on storage tanks.

3) Design the system to protect itself. a) Use continuous recirculation. b) No dead legs. Dead legs are a breeding ground for microorganisms, which increase the organic content of the purified water. c) Use 185 nm UV in a recirculated distribution loop. Multiple passes through the 185 nm UV breaks down many organic substances by photo-oxidation.
 

HOW DOES A PICOSYSTEM® WORK?

By Keith Shepard

INTRODUCTION:

Before discussing how a Picosystem® works, lets discuss what a Picosystem® works on.

Hydro groups contaminates into six general categories: dissolved solids, organics, particulates, microorganisms, microorganism by-products and dissolved gases.

A typical 11218 Picosystem® consists of a pr- filter housing with a 0.2 micron filter cartridge, activated carbon canister and two mixed-bed deionizers. It is designed to purify water by means of a series of treatments, each with a fairly specific purpose. Which contaminant does each treatment step address?

PREFILTER

The pre-filter is a critical part of a Picosystem®. Generally, the better the pre-filter, the better the Picosystem® output quality.
A pre-filter can also be a source of contamination. Some membrane pre-filters have a wetting agent (surfactant) which coats the surface. This wetting agent must be rinsed off, by flushing the filter. Surfactant not rinsed from the pre-filter will pass through the Picosystem® and can interfere with some applications.

NOTE: The Polyethersulfone filters that we now get from Parker are naturally hydrophilic. Hydrophilic filters do not require a wetting agent, so there is none to rinse off from the Parker Polyethersulfone filters.

ACTIVATED CARBON

Granular activated carbon is employed to remove chlorine, chloramines and dissolved organics. In doing so, it provides a chlorine-free environment with a lot of nutrient for microorganism growth. Hydro recommends the replacement of carbon tanks every six months to prevent accumulated microorganism and microorganism by-products from overwhelming the rest of the system.

Carbon fines, generated during shipping can also be a problem. If the carbon tank is not flushed to remove these fines, when the tank is installed carbon particulates can pass through the system and contaminate the final product.

MIXED-BED RESINS

New resin contains organics from the manufacturing process. If not removed by several regeneration and exhaustion cycles, which expand and contract the beads (like squeezing a sponge), they can contaminate the product water.

NOW, HOW DOES A PICOSYSTEM® WORK?

Deionization is a process involving the removal of dissolved mineral salts. The dissolved salts are preferentially exchanged for other ions that are already on the resin bead. The ions that are already on the resin beads (from our regeneration) are: Hydrogen (H+) on cation beads and hydroxyl (OH-) on the anion beads. So, mixed-bed resin, with Type I anion resin, is capable of removing all ionized species and producing 18+ Megohm H2O.

CONCLUSION

Tap water contains all sorts of particles, organics, microorganisms, dissolved salts, gasses, etc. The pre-filter removes particulates, activated carbon treats chlorine/chloramines and dissolved organics, deionizers remove dissolved solids (salts) and carbon dioxide. Each treatment step is fairly specific in what it removes and one cannot compensate for another’s failure.

 

Can you make a Picosystem® better?

By Keith Shepard

INTRODUCTION:

We will now discuss equipment that can be added after the Picosystem®, to make the water even better.

In general, we can add ultrafiltration, ultraviolet (254 nm and 185 nm) and recirculation. Sometimes we add a combination of these three things.

Purified water from a standard Picosystem® will have the following specifications:

Resistance - >18 megohm
TOC - <35 ppb
CFU - <10 CFU/ml
Endotoxin - ??

WHAT CAN BE IMPROVED WITH POST-TREATMENT EQUIPMENT?

SPECIFIC RESISTANCE will not be increased by any of the three post treatment steps.

TOC can be improved with the addition of recirculation. Recirculation can be provided by a Plus Pump or UV Plus Pump. The Plus pump will lower TOC levels to <10 ppb. The UV Plus pump (with 185 nm UV) will lower TOC levels to <5 ppb.

To reduce bacteria (CFU), the least expensive way is to add a Picotech® 2 unit, which contains 254 nm UV. The CFU’s will be reduced to <0.1 CFU/ml.

To reduce endotoxin requires the addition of an ultrafilter. The Picopure® 2, with an ultrafilter and 254 nm UV will reduce the endotoxin level to <0.25 eu/ml. This unit will also reduce the CFU’s to <0.1 CFU/ml. This is a combination unit since it contains both UV and Ultrafiltration.

Other combination units would be the Picotech® 2 UV Plus. This unit will reduce both TOC and CFU’s.
A Picopure® 2 UV Plus will lower TOC, CFU’s and endotoxin.

SUMMARY:

Our standard Picosystem® can perform even better with the addition of some accessories. Just determine each customers needs, and then provide the proper system/post treatment equipment to meet those needs.
 

How do you best transport purified water?

By Keith Shepard

INTRODUCTION:

Many materials have been used to transport purified water. Things like glass, stainless steel, tin and all kinds of plastic. This article will discuss different plastic piping materials.

TUBING/PIPING

Water has to be transported from a Picosystem® to the point of use in something. Polyethylene and Polypropylene have proven to be popular choices. Black PE tubing does not admit light. This helps reduce the growth of bacteria and algae.

Black PE tubing and Beta Polypropylene pipe contain ultraviolet inhibitors. These UV inhibitors protect them from becoming brittle due to exposure to ultraviolet light. The ultraviolet light might be from sunlight or fluorescent lights.

Numerous other types of tubing exist. There is tubing made of Nylon, Polypropylene, Teflon and PVC. One type of tubing that we often see in a laboratory is Tygon tubing. Tygon is made of PVC. By itself, PVC is not flexible. To make it into flexible tubing, organic compounds called plasticisers are added. These organic plasticisers leach from the tubing throughout its life and can interfere with many types of research, especially HPLC or GC-MS. Also, bacteria love to eat the plasticisers in Tygon.

It should be noted that new tubing/piping is not sterile. Disinfection of a new tubing/piping system may be required, depending on the application.

FAUCET OUTLET

How to best dispense water from a Picosystem® is a good question. Hydro is one of the few ultrapure water companies that makes faucet assemblies.

Our Picotaps® are constructed from UHMW PE. Picotaps® contain a diaphragm valve and will last for many years. Diaphragm valves are generally cleaner than other types of valves and replacement of the diaphragm is a simple, inexpensive procedure.

TUBING ON FAUCET IS BAD

Many users attach a section of tubing onto the outlet, either to fill large containers or reduce splashing in the sink. Hydro does not recommend this, but instead recommends the use of a hand held faucet or a Flex-tap® faucet. Nonetheless, tubing is sometimes attached to outlets. Rubber is the worst. All sorts of microorganisms like to eat rubber. Sometimes tubing is long enough to lie in the sink. This enables tubing to be contaminated not only with microorganisms from the air and the Picosystem®, but also from anything that is dumped into the sink.

 

WHAT DOES IT DO TO ADD A PUMP TO A TYPICAL 11218 PICOSYSTEM AND WHY WOULD YOU WANT TO DO IT?

By Rick Rechen

As simple as it is, the basic Hydro Picosystem®, consisting of pre-filtration, organic adsorption, two stages of mixed-bed ion exchange, QA light and a Picotap® represents the limits of the technologies it employs. There just isn’t much more you can do to improve it, without changing one or more of the treatment steps or adding something to the system.

One thing that can be added to a Picosystem® is a Plus Pump to recirculate the water.

What would this do?

It does several things, but it may make more sense to describe the changes that recirculation brings about if we first talk about what a Picosystem® does without recirculation.

IMPURITIES THAT PICOSYSTEMS REMOVE

There are six broad categories of impurities in most tap water. These are: Particulates (dirt, sand, silt etc…), Organics (Carbon containing compounds), Dissolved Solids (salts or ions), Microorganisms (bacteria, virus, algae, fungus, mold, etc…), Microorganism by-products (endotoxin, proteins, ammonia, DNA, RNA, RNase, etc…) and Dissolved Gasses (mainly carbon dioxide but also ammonia and oxygen). Picosystems®® are designed to go a long way toward removing four categories of impurities and to keep the other two at very low concentrations. Water from a typical 11218 Picosystem®, that is used and serviced regularly, has certain overall characteristics.

Particulates: It has very few particles (from using a 0.2 micron pre-filter),

Organics: It has a low level of organics (less than 35 ppb TOC ),

Dissolved Solids: It is essentially ion free (at least 18 megohms specific resistance)

Microorganisms: It has a low number of bacteria (<10 CFU/ml) and other microorganisms in the water. (This can vary with use levels.)

Microorganism by-products: Although Microorganism by-products are not covered by Picosystem® specifications, water from a Picosystem® is typically endotoxin-free and RNase-free (but, it may not be).

Dissolved gasses: The anion resin removes carbon dioxide and carbon removes dissolved ammonia so that it is free from dissolved gasses.

WHY USE RECIRCULATION

Limitations of the Picosystem® relate to what happens when water isn’t being used. In fact, water only moves when someone is actually running it out of the Picotap®. So, most of the time water isn’t moving. When water isn’t moving microorganisms tend to multiply in it. Multiplying microorganisms tend to produce endotoxin and other by-products. So, straight pass Picosystem® output is more likely to have higher bacteria counts and endotoxin as well as DNA, proteins and enzymes than water that is kept moving.
In addition, water that passes through carbon one time and sits still tends to have more organics than water that passes through carbon several times and keeps moving. So, water from a Picosystem® tends to have more organics (higher TOC readings) than water that is recirculated. The maximum TOC concentration specified for water from a Picosystem® is 35 ppb. Water from a recirculated Picosystem® (Picosystem® Plus) usually produces TOC readings of less than 10 ppb.

ADDING 185 NM ULTRAVIOLET TO RECIRCULATION

Including 185 nm (high ozone) ultraviolet in the Plus Pump assembly (UV Plus Pump) is not the same as adding 254 nm (low ozone) ultraviolet to the end of a Picosystem®. It doesn’t do much more to reduce microorganism levels than the Plus Pump by itself, but it does reduce organic levels even further. Water from a recirculated Picosystem®® with a 185 nm UV unit (Picosystem® UV Plus) usually produces TOC readings of less than 5 ppb.

WHEN TO ADD RECIRCULATION WITH 185 NM ULTRAVIOLET

Most Hydro customers use the water for applications where the water doesn’t have to be sterile but should have a low number of bacteria. All of these users would benefit from recirculation.

Others are not concerned with bacteria, but would like to be sure that organics remain low. Typical applications: High Performance Liquid Chromatography (HPLC), Ion Chromatography, Gas Chromatograph-Mass Spectrometry (GC-MS), Amino Acid analysis).
Adding a UV Plus Pump to a Picosystem® could meet that requirement. To get the full effect for microbial control it is necessary to have a fully recirculated Picotap so that there is no water standing anywhere in the system.

MOST USERS WOULD BENEFIT FROM RECIRCULATION

Most Hydro customers could benefit from adding recirculation to a straight pass Picosystem® (to make a Picosystem® Plus) and a recirculated Picotap® faucet because water quality would improve and be more stable than it is with a single pass Picosystem® alone.
 

WHY ARE PICOSYSTEMS & QA’S SO EXPENSIVE?

By Keith Shepard

INTRODUCTION:

The short answer might be because of the quality materials we use and the excellent service we provide. But, a better answer might be, expensive in comparison to what?

A Hydro Picosystem® normally produces purified water at about 1/3 the cost of prepared bottled water, water from a cartridge type system or distillation. Here in Durham, NC, the average Picosystem®® will produce about 340 gallons with tap water feed. If the exchange cost is $477.00, then the cost per gallon of ultrapure water is $1.40. This is for purified water, on demand, at 1/2 gpm with plain old tap water feeding the system. A cartridge system has to have pretreated feed and bottled water is not “on demand.” The same arguments apply to distilled water.

If the comparison is to equipment from other companies (that have tanks and filter housings that look like ours) our price may be higher, for what seems to be equivalent equipment. Now is the time to discuss the quality of material we use and the way we process it.

CARBON:

Hydro selects activated carbon that is manufactured from a metallurgical grade of bituminous coal with low ash content and a high degree of abrasion resistance. The activated carbon has a good distribution of large and small pore sizes to remove the broad spectrum of organics from water. Many competitors use carbon that is primarily designed for removal of chlorine and taste and odor compounds. Hydro selects activated carbon based on its ability to remove TOC in water.

New carbon has a lot of fines to be flushed from it. Before you can flush the fines from new carbon it has to be hydrated (soaked with water). This hydration process can take several hours, but it is an important step in preparing carbon to go into our tanks. Our tanks are cleaned and sanitized before carbon is put into them.

RESIN:

The primary consideration in the selection of resin is its ability to produce the best possible water quality. This not only includes the ability of mixed resin to produce 18 megohm water, but the integrity of the resin to prevent degradation of the water by introducing organic extractables and particulates.

Cation resin is made both as strong acid and weak acid cation resin. For demineralization, strong acid cation resin is almost always used.

Anion resin is made as both strong base and weak base resin. Strong base resin is also made as Type I and Type II. Some competitors prefer to use Type II strong base anion resins because they cost less and can be regenerated less expensively. Type II resins are less resistant to high temperature, oxidation, and physical attrition. Type I, strong base, anion resin is required for the production of ultra high purity water. Only strong base, Type I, anion resin will remove weakly ionized carbon dioxide and silica. This is why Hydro uses strong base Type I anion resin, even though some of the associated costs may be higher.
 

Why isn’t the pH of my purified water 7?

By Rick Rechen

INTRODUCTION:

“Why doesn’t the pH read 7.0 from the purified water system?

WHERE DO WE GET PH?

The term, pH,is a mathematical expression for the concentration of Hydrogen ions in solution, and is commonly used to describe how acidic or basic water is. A pH of less than 7 indicates that there are more Hydrogen ions than Hydroxide ions and is acidic. A pH greater than 7 indicates that there are more Hydroxide ions than Hydrogen ions and is basic. A pH of 7 indicates that there are equal numbers of Hydrogen and Hydroxide ions and is neutral.

RESISTANCE OF DI WATER

Water by itself does not conduct electricity very well and has a high resistance to electrical flow. One way in which the quality of deionized water is expressed is by its Specific Resistance. Good quality deionized water has a specific resistance of at least 10 million ohms (megohm). The more ions present in water, the lower the specific resistance. This idea figures into the measurement of pH in D I water.

MEASUREMENT OF pH

Measurement of pH, by means of a pH meter, involves two electrodes. One electrode is a reference electrode which establishes the ionic strength of the solution. It does so under the assumption that Chloride ion (Cl-) represents the bulk of the ions present in the solution and uses a Chloride (also known as Calomel) electrode to establish the reference point. The second electrode, a Hydrogen ion electrode, measures the concentration of Hydrogen ion. The electrical potential difference between the two electrodes is translated by the meter into a number called pH.

pH OF DI WATER

There are two problems in directly measuring the pH of deionized water.
 

THE FIRST PROBLEM-WHY IS THE PH SO ACIDIC (OR BASIC)?

There are no Chloride ions in deionized water greater than 10 megohms. So, the pH meter reference electrode is unable to set up an accurate reference point from which to measure pH. Therefore, whenever pH meter electrodes are placed, in freshly drawn deionized water, the meter provides a reading which is incorrect. The reading obtained will depend upon the type of electrode connected to the meter. Dual electrodes usually give a pH reading of around 5.8. A combination electrode usually gives a reading of 8.5. They will do this simultaneously in the same beaker of water.

To measure the pH of deionized water it is necessary to add a crystal or two of Potassium Chloride to the water. After this has been done, pH will be around 6.5-6.8.

THE SECOND PROBLEM-WHY ISN’T THE pH 7.0?

Just as soon as deionized water is placed in a beaker, it starts to absorb Carbon Dioxide (CO2) from the atmosphere. Carbon Dioxide dissolves in water as Carbonic Acid (H2CO3). The small amount of Carbonic Acid from dissolved Carbon Dioxide will cause the pH to shift from 7.0 to 6.5-6.8.

HOW CAN THE USER KNOW THAT THE pH OF D I WATER IS 7.0?

With typical laboratory pH meters, in open beakers, it is not possible to obtain a pH of 7.0 in deionized water. Adding Chloride ion to the water will give only an indication that the pH might have been 7.0 before the water was placed in the beaker. However, it is possible to answer the question of pH indirectly.

Deionized water (>10 megohm specific resistance) is essentially ion free. (If it were not, because dissolved ions conduct electricity, the resistance would be less than 10 megohm.) If the specific resistance is found to be greater than 10 megohm, then there are not enough ions present to conduct electricity. So, there is also no excessive concentration of Hydrogen or Hydroxide ions. Therefore, the pH must be 7.0 or neutral. Conversely, if there were enough excess Hydrogen or Hydroxide ions to shift pH to the acid or base sides of the scale, the specific resistance would be less than 10 megohm. This is the best we can do, at this time, to answer this question.
 

Why did I get only half the normal amount
from my DI system?

By Keith Shepard
 

Let’s look at some things that effect deionizer capacity.
Deionizer capacity is a function of three things:

1) Feed water quality
2) Feed water flow-rate
3) Regeneration state of the resins

Feed water quality is constantly changing. There are two types of “tap water”, water from deep wells and surface water from rivers and reservoirs. Well water is usually more constant in quality than surface waters. It contains less organics and more inorganics. Surface waters vary considerably from place to place, time of the year, sometimes from hour to hour. Basing deionizer life on one water sample is not ideal. When deionizer life is in question, we recommend taking water samples weekly until the cause is determined.

Another problem associated with feed-water is organic acids. Organic acids can blind the exchange sites on resin beads and reduce the quality and quantity that a deionizer produces. We cannot test for organic acids in the lab but you can test a tank in the field. Remove a suspect tank and connecting it to a water supply where you are not having a problem. If the quality comes back up, organic acids are probably the problem. If organic acids are a problem, the usual treatment is to add ultrafiltration before the deionizers.
 

Flow-rate can affect deionizer capacity. Deionizers have both minimum and maximum flow-rates. The maximum flow-rate is about four gpm per cubic foot of resin. A flow rate faster than this may not allow time for complete deionization. A fast flow rate will show-up as ionic break-through, or exhaustion, before all exchange sites on the resin beads are used-up. This translates into less than expected throughput from the deionizer.

The minimum flow-rate is ½ gpm per square foot of resin. This would be 1/10 (.1) gpm for 18” deionizers. Slower flow will “channel” through the resin bed, so the entire resin bed does not get used. The tank appears exhausted, but only about half of the resin in the tank has been used. Is your first thought, when you have a customer with an “early exhaustion” problem, to put in a Kent water meter? This can be a mistake because the ¾” Kent water meter (the only one available until recently) is not accurate below ¼ gpm. The meter will not read water flowing very slowly. In the customers mind this would confirm his suspicion, that the DI tank only gave half of its rated capacity.

We now have a ½” Kent water meter (P/N 27250) that is accurate down to 0.125 gpm (1/8 gpm). This meter may help us to identify low flow problems, but it is possible that a slow leak would still not be detected.
 

The first thing to do in a suspected low flow problem is to look for a slow leak somewhere in the system, a very small solenoid valve, or faucet that is left open to flow really slow etc. One fix for this problem may be to install a recirculation pump so that the flow rate through the resin tanks is always above the minimum required flow rate.
 

The regeneration state can also effect deionizer capacity. For example, lets look at the regeneration of a water softener. A water softener has only cation resin and is generally regenerated with salt (sodium chloride). A water softener has a capacity of 30,000 grains per cubic foot of resin, when regenerated with 15 pounds of salt per cubic foot of resin. The same softener has only 20,000 grains capacity per cubic foot, when regenerated with 5 pounds of salt per cubic foot. Now, if you had tap water with 8 grains per gallon hardness the first softener could treat 3750 gallons of water before exhaustion. The second softener could treat only 2500 gallons of water before exhaustion. They both have one cubic foot of media, but the first was regenerated with more chemical, therefore giving more water before exhaustion.

A similar phenomenon could happen in the regeneration of mixed-bed resins. This is why at Hydro we use a “cook book” approach to regeneration. Every batch of resin is regenerated the same way. The chemicals used are constantly scrutinized for quality and are precisely measured into each regeneration batch. Also, resin from each batch is tested for capacity, and records are kept on file. Each tank filled also has to pass a quality test before being shipped.
 

As you can see this last item is the one that we, at Hydro, have the most control over. It is actually the least likely cause of a premature exhaustion problem. You will usually find that the problem is related to feed-water quality, flow-rate or tank exchange procedures.

We have not talked about tank exchange procedures yet, but they could include air in the tanks, wrong flow direction, carbon and resin tanks switched, a bad cell light, exhausted polisher or worker/polisher mix-up.
 

WHY DOES THE WATER FLOW SO SLOWLY?
 

By Keith Shepard

INTRODUCTION:

This question can have several answers. The system may be brand-new, or recently exchanged. The system may have been operating just fine for several months but now the flow rate has slowed, or stopped. The system may be a tap-fed Picosystem®, or it may be on a pretreated feed source. It may be a single pass system, or recirculated, with a Picotech®2 or Picopure®2.
I will lump the reasons for low flow under three categories. The three categories are: Supply, Plugging and Mechanical.

SUPPLY

I would recommend that you first check the water supply to the Hydro system. If the system is on tap water, it is possible that there is no (or low) tap water pressure. A valve upstream may have been closed. The water supply may have been cut-off to the whole building. A well pump may have failed.

If the system has a pretreated feed source, there could be some of the same problems, and some other problems. The storage tank may be out of water. There may be a problem with the distribution pump.

PLUGGING

If you have supply water, then the next thing to check for is the Hydro Picosystem®. Some components may be plugged. Plugged also applies to crimped, in the case of tubing. Start with the first component and work your way through the system until you fInd what is plugged, or where the tubing is crimped. The most likely item will be the pre-filter or carbon tank. Tubing can get crimped when someone in the lab moves things around near the Picosystem®. If the system contains a Picopure®2, the ultrafilter could be plugged.

MECHANICAL

If you do not find anything plugged or crimped, you may have a mechanical problem. Two or three possible mechanical problems are:  1) A problem with the pressure regulator, 2) a problem with the faucet, or 3) a problem with the pump.

You should have found that there was a problem with the pressure regulator while checking the filter for plugging. The faucet valve would be another thing to check. It is possible that something is under the diaphragm or the faucet stem is not working properly. The knob may be slipping, or threads may be stripped.

A third mechanical problem could be with the recirculation pump. If the pump is before the system, is it the correct size, is it running? Is the pump actually pumping? If the pump is after the system, it can draw water away from the faucet so that there will be low flow at the faucet. It has been found that it is usually better to have the pump before the system.
 

WHY SHOULD I EXCHANGE THIS SYSTEM?

By Keith Shepard

Why should I service my Hydro system every six months even though the light is on, or I haven’t used much water?

The essence of research is to gather valid data from scientific studies that can be duplicated later (or in a different lab); under the same controlled conditions. To duplicate the same controlled conditions you need the same strength, quality and purity of chemicals as in the original experiment. In many cases, the most used chemical is purified water. The purified water may be used to wash or rinse the glassware used in the experiment, as well as to mix or dilute other chemicals. It may also be the main ingredient. This is the reason that the consistency of the purified water is critical to research.

Chemicals, of guaranteed quality, are purchased in fairly small quantities, are expensive and may have a limited shelf-life. Water from a Hydro Picosystem® is no different. It is a chemical that comes with guaranteed consistency, is fairly expensive and has a limited shelf-life. Unlike most other chemicals used in the lab, the Hydro purified water system can make water on-demand, in fairly large quantities, from either tap water or pretreated water. It also has a very economical cost per gallon, as compared to most other chemicals, and it can be used to make excellent coffee.

How can we guarantee the quality of the purified water from a Hydro Picosystem®? From over 34 years of experience we know the quality that each type of Hydro Picosystem® is capable of producing. We have verified these findings with expensive and continued testing. This testing and experience has given us the confidence to guarantee the quality of our purified water, if the six month Quality Assurance Service is performed. So, even if the light is on, or they haven’t used much water, the six month Quality Assurance Service is required for Hydro to guarantee the expected water quality.
 

UV Replacement Frequency Explained

By Keith Shepard

HOW IT WORKS

Energy from a UV lamp destroys the deoxyribonucleic acid (DNA) and prevents microorganisms from reproducing. UV lamps can be made to emit only 254 nm wavelength UV radiation, or both 185 and 254. The 185 nm bulbs are used for TOC reduction. The 185 nm wavelength first promotes the conversion of dissolved oxygen to ozone. Ozone is then changed by the 254 nm wavelength UV light to hydroxyl radicals. These hydroxyl radicals then oxidize organic molecules in the water. With sufficient contact time and/or recirculation, the organics are oxidized to CO2 and water.

UV DOSAGE

Disinfection effectiveness depends on the UV system dosage. The UV dosage must also take into consideration the transmission qualities of the source water. The total intensity that the UV lamp emits is measured in microwatts at a specific wavelength. By combining the total lamp intensity and the residence time, a UV dosage can be derived. This dosage is expressed in microwatt-seconds per square centimeter (mW-sec/cm2). Most UV bulbs are manufactured to be able to provide a dosage of 30,000 mW-sec/cm2. This 30,000 mW-sec/cm2 is what the bulb is rated to produce at the end of 8-9,000 hours (about a year). This means that when new, UV bulbs produce approximately 60,000 mW-sec/cm2. The reason that lamps are rated in this manner is key to understanding why we, at Hydro, recommend a six month replacement of the UV bulb, and annual replacement of the quartz jacket.

LAMP LIFE

Unlike a regular light bulb, UV lamp performance deteriorates with time. Their life span is limited. According to one expert, they should be replaced every 9 to 12 months for maximum effectiveness. The reason is scale build-up and Solorization – a condition in which the quartz discolors. This discoloration decreases the intensity of the UV light. Solorization affects both the lamp material and the quartz jacket. The effective UV intensity decreases approximately 40-60 percent over 9,000 hours (just over one year) of operation.
At this point, I would like you to refer to Hydro Technical Bulletin #1 “Is 18 Meg-ohm Good Enough”, and #3 “Quality Assurance: The Key to Consistency”. If a customer has a purified water system, he/she wants consistency from day to day. To assure consistency, the exchange frequency of some items is time dependent (Bulletin #3). The UV bulb and jacket along with the pre-filter and carbon tank are such items. As is pointed out in Bulletin #1, there are six categories of contaminants in tap water. A standard Picosystem will address four of these items. A customer who’s Picosystem contains a UV unit is, obviously, trying to achieve even better water quality. His/Her system now addresses five of the six contaminant categories. It only seems reasonable that our recommendation on maintenance should be consistent with maintaining the customer’s water system in a manner that will not allow a change in quality. Only with a recommendation of six month bulb and annual quartz jacket replacement can we feel confident in achieving this goal.

Ion Exchange Explained

By Keith Shepard

PRINCIPLES OF DEIONIZATION

Deionization is the process of removing dissolved, ionizable solids from water using the principles of ion exchange. Ion exchange is a reversible process. The reversing of the process is called regeneration. Ion exchange works for two reasons:

1. Resins prefer “higher valence” ions such as calcium over lower valence ions, like hydrogen or carbon dioxide.
    

2. Resins are influenced by the mass of ions present.

In a softener, the ion exchange process essentially exchanges the minerals calcium and magnesium (also iron and manganese) for sodium. A water softener requires only one type of resin because it exchanges only cations.

Mixed-Bed Deionization is similar, but more complicated because it involves the removal of cations and anions from the water.
All dissolved minerals are composed of both a metallic part (a positively charged cation) and a non-metallic part (a negatively charged anion). Mixed-Bed deionizers require two resins because they exchange both cations and anions. No single resin can exchange both cations and anions. Ion exchange depends on the tiny electrical charges in which like particles repel and unlike particles attract each other, like a magnet. Cation exchange resin is chemically formulated to attract positive ions, and anion exchange resin is formulated to attract negative ions.

MONO-BED DEIONIZATION

Before progressing to mixed-bed deionizers, lets try to understand how a mono-bed deionizer works. In a two-column unit the cation exchange resin is in one pressure vessel and the anion exchange resin in another. Water first passes through the cation tank, then the anion tank. You need to understand this principle before learning how a Mixed-Bed deionizer functions. Here is what happens when the cation resin is in one tank and the anion resin is in another tank.

As water passes down through the cation tank it encounters millions of resin beads, each of which contain a large number of negatively charged exchange sites in the pores and microscopic paths of its structure. When the resin is in the regenerated state, each exchange site is occupied by a positively charged hydrogen ion (H+). As the positively charged cations in the water contact the beads, they are attracted to the negative exchange sites. Since they have a stronger positive charge than the hydrogen ions they drive off the hydrogen ions and attach to the exchange sites. The displaced hydrogen ions pass down through the resin bed with the water.

Because the hydrogen ions are acidic, water from the cation tank is a stream of dilute mineral acid.

At the same time anions, such as sulfates, chlorides and other elements pass through the cation tank unchanged. Now everything is piped to the anion tank.

ANION EXCHANGE PROCESS

The anion exchange process is similar to the cation exchange process. Strong base anion resin is made of beads which have positive exchange sites, which in the regenerated state are occupied by negative hydroxide ions (OH-). As the negatively charged anions contact the beads, the same attraction-repulsion process takes place and the negative hydroxide ions are dislodged and replaced by the stronger negative anions.

The hydroxide ions (OH-) pass down through the anion resin and are discharged from the tank. At the same time, the hydrogen ions (H+) from the cation tank have passed through the anion resin unchanged. They now join with the hydroxide ions to form HOH, or H2O, or water.

MIXED-BED DEIONIZATION

If the cation-anion (mono-bed) exchange process could be repeated many times, the efficiency of ion exchange would improve remarkably. Since no exchange process is 100 percent efficient, successive ion exchanges would be an improvement in purity with each cycle. This is exactly what happens when cation and anion resins are mixed together in a Mixed-Bed Deionizer. As water passes through the mixed-bed deionizer it has millions of chances to contact a cation resin bead, then an anion, then another cation, another anion, and so on. With each exchange, the purity of the water improves because more ions are removed and held by the resin beads. As water passes down through the resin, the cation resin will remove cations, releasing hydrogen into the water, and the anion resin will remove anions, releasing hydroxide into the water. Released hydrogen ions will ionically bond with released hydroxide ions to form a new water molecule.

The Unique and Unusual Properties of Water

Charles Riley, Jr. PE

INTRODUCTION

To the average person, water is a common and ordinary substance which is often taken for granted, that is until a drought threatens crops and drinking water supplies, or a severe flood destroys life and property. Most people do not understand that without water, and its unique and unusual properties, life, as we know it on earth, would not exist.

The abundance of water is apparent. It is also the only substance naturally present on earth simultaneously in three distinct states or forms– solid, liquid, and gas. On a cold winter day, snow and ice cover a field, while water flows in a nearby stream and gaseous clouds float overhead.

FORMS OF MATTER

All substances exist in three distinct forms (solid, liquid, and gas). The form of a substance, at a given time, is a function of temperature and pressure. A solid is defined as matter with rigidity and definite shape, having a crystalline internal structure. Solids tend to resist external forces. Solids can be converted to liquids by heating. The freezing point temperature of pure water is 0° C, at one atmosphere pressure. At temperatures below the freezing point, water exists as a solid - ice.
 

A liquid, in contrast to a solid, lacks rigidity and has no definite shape. It has a definite volume and conforms to the shape of the container in which it is stored. External forces will cause a liquid to flow. Water is a liquid between the freezing point temperature (0° C) and the boiling point temperature (100° C), at one atmosphere pressure. Liquids can be converted to the gaseous phase by heating beyond the boiling point temperature.

A gas has neither rigidity nor definite volume. A gas conforms to the shape and volume of the vessel containing it. A gas greatly expands and contracts with changes in temperature and pressure and has the ability to readily diffuse into other gases.

BOILING AND FREEZING POINTS

Water has unusually high boiling and freezing point temperatures, compared to other compounds with similar molecular structure. All other compounds with similar molecular structure are gases at ordinary temperatures. However, due to the polar nature of the water molecule, and hydrogen bonding, the boiling point of water is a remarkable 100° C and the freezing point is a remarkable 0° C. The boiling point of the most similar substance (Hydrogen Sulfide) is –60° C and the freezing point is –84° C.

FREEZING

Generally, substances contract (become denser) with a decrease in temperature, and water is no exception. Between 4° C and the freezing point at 0° C, an amazing thing happens, water gradually expands becoming less dense. Since the density of ice is less than that of liquid water, ice floats on water. About one-eleventh of the liquid volume is added at freezing.

It is very significant that ice expands and floats on water. The consequences of this action can be seen in broken water lines in the winter and potholes in the roads. In fact, the freezing and thawing action of water is largely responsible for the fracturing of rock and the formation of soils. Also, consider the consequences if lakes and streams froze from the bottom to the top – aquatic life would not even exist, and climate and weather patterns would be altered drastically.

HEAT CAPACITY

Another remarkable property of water is its extremely high capacity to absorb heat, without a significant increase in temperature. For example, the summer sun at the beach will increase the temperature of the sand to the point that it is too hot to walk on; however, the water is cool to the touch. Both the sand and the water absorb the same amount of heat energy, but the temperature of the sand is higher than the water temperature. The high heat capacity of water makes it a good coolant to use in condensers and automobile radiators to keep engines from over-heating. Water can absorb about five times the amount of heat of sand for an equivalent increase in temperature.

The moderate climate in coastal areas is the result of the absorbing of huge amounts of solar heat energy by water during the day and the slow release of heat energy during the night. Inland areas, away from the coast, typically experience much wider temperature extremes. The vast oceans on earth (about 75 percent of the surface area) are responsible for tempering the climate on earth, permitting life to exist.

UNIVERSAL SOLVENT

A solvent is a substance capable of dissolving another substance (solute) to form a homogeneous mixture (solution) at the molecular level. The highly polar nature of water makes it an excellent solvent, especially for other polar compounds – salts, alcohols, carboxyl compounds, etc. More substances dissolve in water than in any other solvent. More than half of the known elements can be found in water, some in high concentrations, and others only in trace amounts. The ability of water to dissolve a substance depends on the chemical composition, chemical bond strengths of the elements, temperature, and pH. Non-polar compounds including most hydrocarbons are difficult to dissolve in water and dissolve in low or trace amounts. For example, oils tend to float on the surface of water.

SURFACE TENSION

Hydrogen bonding is attributed to the ability of water to adhere to or “wet” most surfaces; such substances are said to be hydrophilic (water-loving). Other substances such as oils, fats, waxes, and many synthetics (polypropylene, etc.) will not “wet” with water; these substances are hydrophobic (water-fearing). Membrane filter cartridges for water filtration, made from hydrophobic polymers and with sub-micron pore sizes (< 1 micron), must be manufactured with wetting agents to lower the surface tension of the water to allow the water to “wet” the pores of the filter. Once the pores are filled with water, water will stay in the pores due to the surface tension; this is termed capillary action. Capillary action is responsible for the movement of water through soils, blood through blood vessels, and water carrying nutrients through the roots of plants.

MEDIUM OF LIFE

Water is an essential ingredient for the existence of life, as we know it. This explains the recent interest in discovering water in other parts of the universe. All known biochemical processes occur in aqueous environments. The composition of most living things is 70 to 80 percent water, by weight.

Water plays a significant role in the process of photosynthesis. In photosynthesis plants utilize radiant energy from the sun to convert two inorganic substances, water and carbon dioxide, into carbohydrates and oxygen. Photosynthesis is the most basic and significant chemical reaction on earth. It supplies the primary nutrients, directly or indirectly, for all living organisms and is the primary source of atmospheric oxygen.

CONCLUSION

To the average person, water is an ordinary substance, often taken for granted. Even though the cause of these unique and unusual properties is explainable, at the atomic level, water is truly a remarkable substance. From our examination of these properties it is evident that water is essential for life, as we know it, to exist on earth. Water is the mediator of chemical and biochemical processes. Water shapes our natural environment and even mediates our climate and weather. Without water we would not be here.

Reagent Grade Water

Charles Riley, Jr. PE

What is Reagent Grade Water?

Reagent grade water is water suitable for use in methods of analysis and testing which will not interfere with the specificity, accuracy, and precision of the procedure. Emphasis is on the specific chemistry of the procedure and the use of the water rather than how the water is prepared.

What are applications for reagent grade water?

• Glassware washing and rinsing*

• Chemical reagents

• Dilutions

• Buffer solutions

• Media preparation

• Tissue culture

• Microbiology

• Spectrophotometric analysis

• Chromatography

• Electrophoresis

• Microscopy

• Molecular biology

• HPLC Standards

• Trace chemical analysis

• Immunology

• Hematology

• Histology

• Qualitative analysis

• Urinalysis

* Proper glassware washing and rinsing is critical to prevent contamination which could interfere with testing procedures. This is a common problem in some critical applications with insufficient quality control.

In instrumental analytical methods, pure water is used for blanks, preparing standards, and sample dilutions.
 

Who establishes reagent grade water specifications?

Based on the definition of reagent grade water, the user of the water should establish the appropriate specifications for the intended application. However, guidelines have been established for more routine applications by such organizations as the American Chemical Society (ACS), College of American Pathologists (CAP), National Committee for Clinical Laboratory Standards (NCCLS), and the American Society for Testing and Materials (ASTM).

Specifications:

Type I water must be produced on demand without storage to meet resistivity specifications. Methods of production include deionization, distillation with polishing deionization, or reverse osmosis with polishing deionization.

The specified method of testing the resistivity of Type I water is a temperature compensated (25°C), in-line monitor.The specified method of testing for silicate is a spectrometric method based on the reduction of silicate-molybdate complex.

The pH of Type I and Type II water cannot be measured electrometrically because the water is unstable; the water is devoid of dissolved solids and therefore unbuffered. The slightest amount of carbon dioxide or ammonia will radically shift measured pH.

Type II water is prepared by distillation or deionization. Type III water is prepared by deionization or reverse osmosis.

Type I water must be produced on demand and cannot be stored.

Type II water must be prepared by distillation with the appropriate pretreatment; this water is generally sterile, pyrogen-free, storage recontaminates the water.

What is an integrated reagent grade water system?

An integrated water system is a system that takes advantage of the effectiveness of the various treatment technologies for specific contaminates and integrates these technologies in a synergetic manner to achieve a very high level of overall performance.
System walk-through:

• Pretreatment with reverse osmosis improves the overall water quality as well as the economy of operation.
   

• Complete recirculation helps to control bacteria growth and provides consistent quality.
   

• Primary treatment with mixed-bed deionizers provide ultimate demineralization; resin selection is an important consideration to maintain quality (use Type I anion to remove weakly ionized species like silica and to reduce TOC extractables).
   

• Ultrafiltration removes microorganisms, pyrogens, and colloids.
   

• Ultraviolet (254nm) at the point-of-use provides sterile water and prevents retrograde growth.
   

• Ultraviolet (185nm) in the recirculation loop reduces TOC to less than 10 PPB.

What is water system validation?

Validation is the process of establishing documented evidence that provides a high degree of assurance that the system consistently produces water meeting the established specifications. Written protocols establish installation and operational qualifications; process performance qualifications; sampling locations and frequency; testing methods; instrumentation calibration, etc.

High Purity Water System Design

Charles Riley, Jr. PE
 

Introduction

Water is an exceptionally aggressive solvent that attacks most of the substances it contacts. More substances dissolve in water than any other solvent. Most of the known elements can be found dissolved in water, some in high concentrations and others only in trace amounts. As water moves through the natural hydrologic cycle, it dissolves substances it contacts. Contaminants include atmospheric gases (oxygen, nitrogen, and carbon dioxide), dissolved minerals and organic substances, and suspended colloidal matter. Water also provides an ideal environment for the growth of bacteria and other microorganisms if the necessary nutrients and conditions for growth exist.
 

Depending on the type and concentration of contaminants, most natural waters are not suitable for potable use much less for most research and industrial applications. Most all municipalities and other purveyors of potable water provide some level of water treatment to make the water suitable for consumption. The U.S. Environmental Protection Agency has established legally enforceable National Primary Drinking Water Regulations (NPDWR) for public water systems. These regulations are published on the U.S. EPA web site (www.epa.gov).
 

Most high-purity water systems provide additional treatment to potable water to remove residual contaminants to meet the water quality specifications for the given application. Reagent grade water (RGW) specifications have been established by such organizations as the National Committee for Clinical Laboratory Standards (NCCLS) and the American Society for Testing and Materials (ASTM). The US Pharmacopoeia (USP) establishes specifications for compendial water used in the manufacturing of drug products. The two major compendial water types are USP purified water and USP water for injection. Many industries have established unique water quality standards specific for their use.
 

Contaminants in Water

To design a high purity water system, the specific contaminants in the source water must be identified and measured. The NCCLS classifies water contaminants based on six general categories: dissolved solids (inorganics), dissolved organics, dissolved gases, particulate matter, microorganisms, and endotoxins (bacterial by-products that are toxic in injectable drug products).

Dissolved Solids

Total dissolved solids (TDS) include all the ionized inorganic salts in solution. Dissolved salts ionize into their respective cations and anions in water and contribute to electrical conductivity. The approximate TDS concentration in water can be determined by measuring the electrical conductivity or resistivity. In pure water, a relatively small number of water molecules ionize into hydrogen and hydroxyl ions; therefore, pure water is a relatively poor conductor of electrical current. The theoretical resistivity of pure water is about 18.2 megohm-cm (18,200,000 ohm-cm). In contrast, most potable waters have resistivities ranging from about 10,000 ohm-cm to 1,000 ohm-cm. The relationships between resistivity, conductivity, and approximate TDS are shown in the following table.

It should be noted that resistivity in units of megohm-cm is the reciprocal of conductivity in units of microSiemens/cm (?S/cm).
 

Common cations (positive charged ions) in potable water include sodium (Na+), potassium (K+), calcium (Ca++), magnesium (Mg ++), ferrous iron (Fe++), and aluminum (Al+++). Trace amounts of heavy metals such as lead, zinc, and copper can also be present. Total hardness includes the calcium and magnesium salts; the main concern with hardness is scale formation in hot water heaters, distribution piping, stills, and reverse osmosis membranes. Ion-exchange softening is generally used for treatment of water to boilers, stills, and reverse osmosis systems to prevent hardness scaling. Softening removes most of the cations in water replacing them with sodium ions in a selective ion-exchange process.
 

At high pH ranges (> 7.0 pH) and in the presence of oxidizing agents such as dissolved oxygen or chlorine, ferrous iron readily oxidizes to insoluble ferric iron (Fe+++) which can foul ion-exchange resins and RO membranes. Treatment methods for removal of iron include manganese greensand and Pyrolox. These manganese oxide medias convert soluble iron and manganese to insoluble oxides, then filters them from the water. Softening can also be effective in some circumstances.

Common anions (negative charged ions) in potable water include chloride (Cl-), sulfate (SO--), nitrate (NO3-), carbonate (CO3--), bicarbonate (HCO3-), etc. Total alkalinity includes carbonate, bicarbonate, and hydroxyl ions. If present with hardness cations and some heavy metals, alkalinity contributes to scale formation.
 

Silica (SiO2) is one of the most common elements on earth, and it is very common in natural waters. Silica concentrations in water can create scaling problem in boilers, stills, reverse osmosis membranes, and cooling water systems. Silica is usually present in two forms: ionic silica (reactive) as SiO2 complex, and colloidal (non-reactive) particles. Ionic silica is weakly ionized, but it can be removed by ion-exchange, reverse osmosis, or distillation. Colloidal silica is present in sub-micron particles (less than 0.1 microns) and can be removed by ultrafiltration, reverse osmosis, or distillation.

TDS removal is accomplished using such technologies as ion-exchange demineralization, electrodeionization (EDI), reverse osmosis, or distillation.

Dissolved Organics

Dissolved organics in water occur from the natural degradation of vegetation and animal wastes and as pollution from synthetic compounds such as pesticides and other chemicals in industrial discharges. Naturally occurring contaminants include compounds such as tannins (humic and fulvic acids). These compounds cause color in water and can foul ion-exchange resins and RO membranes. Free chlorine can react with some of these compounds and form trihalomethanes (THM) which are determined to be carcenogenic (cancer causing).

Many synthetic organic compounds are used in industry and agriculture and can be found in natural water from industrial discharges or by leaching and runoff from soils. Many of these compounds have significant health consequences and are regulated by the EPA.

Total organic carbon (TOC) is a direct, quantitative measure of the amount of oxidizable, carbon-based organic matter in water. TOC concentrations in potable water typically range from about 5 to 20 PPM.

Adsorption with activated carbon is the most effective means for removing most dissolved organic contaminants. Reverse osmosis is also effective for removing organic compounds larger than about 150 molecular weight (MW) in size; TOC rejections of greater than 95 percent can be achieved with RO membranes. The use of 185 nm ultraviolet is an effective method for further reducing trace dissolved organic contamination in high purity water.

Dissolved Gases

Dissolved gases include carbon dioxide, oxygen, nitrogen, and hydrogen sulfide. Carbon dioxide (CO2) is moderately soluble in water and can absorb in water from the atmosphere; however, most of the carbon dioxide in natural waters comes from the carbonates dissolved in water. Carbon dioxide reacts with water forming carbonic acid. Carbonic acid weakly ionizes forming bicarbonates and carbonates. The distribution of carbon dioxide, bicarbonates, and carbonates is a function of the pH of the water. Carbon dioxide reduces the pH of water and is responsible for corrosion in water lines and boilers. Although weakly ionized, carbon dioxide can be removed by ion-exchange and de-aeration methods.

The solubility of atmospheric gases in water is directly proportional to the partial pressure of the gas above the solution; this is known as Henry’s Law. Accordingly, oxygen and nitrogen gas dissolve in water to saturation levels. Temperature and TDS content can also affect solubility. Nitrogen is an inert gas, but dissolved oxygen is a strong oxidizing agent. Dissolved oxygen is responsible for corrosion in water lines, boilers, and heat exchangers. Dissolved gases can be removed using various de-aeration methods including vacuum degasifiers and gas transfer membrane contactors. Oxygen can also be removed by an ion-exchange process or by chemical scavenger agents.

Chlorine gas (added for disinfection) reacts with water to form hypochlorite ion (ClO-) and hypochlorous acid (HClO); the relative amounts of each depend on the pH of the water. Hypochlorus acid is the more effective disinfectant and it is formed at pH values less than 7.0. Free chlorine is also known to react with residual organic compounds to form trihalomethanes (THMs). Many purveyors of potable water are now adding ammonia gas with chlorine to form monochloramines (NH2CL). Chloramines are not as effective as free chlorine for disinfection, but they minimize THM formation and are more stable and longer lasting than free chlorine. Chlorine and chloramines are effectively removed with granular activated carbon or by injection of chemical reducing agents such as sodium bisulfite.

Hydrogen sulfide (H2S) is primarily found in well water supplies where anaerobic conditions or bacterial action reduce sulfate to sulfide. Hydrogen sulfide has a characteristic rotten egg smell. Various oxidation methods such as chlorine, ozone, or oxidizing filters can remove hydrogen sulfide.

Particulate Matter

Suspended particulate matter in water may be inorganic or organic and includes colloidal silica, fine silt, organic acids, microorganisms, and other discrete dispersed matter. Generally, the small size prevents rapid settling, or the particles are dispersed by electrostatic surface charges. Turbidity is the term used to define this type of contamination.

Turbidity is measured using light scattering optical methods such as a Nephelometer. Turbidity is not an absolute measure of the concentration of particles, but it is a relative measure based on standard stabilized solutions of various suspensions such as formazin. Turbidity is removed using various filtration methods including multimedia filters, ultrafilters, membrane filters, and reverse osmosis membranes.

Silt Density Index (SDI) is another measure of particulate matter, and the measured value reflects the rate of plugging of a 0.45 micron membrane filter disc by particles in the source water. The test is used to correlate the level of suspended solids in water that tends to foul reverse osmosis membranes. Most RO membrane manufacturers specify SDI values of less than 3.0 in the feed water.

Microorganisms

Most bacteria found in purified water systems are Pseudomonas species. These bacteria are generally plant pathogens found in soil and water, but a few species are known to be human pathogens. They are highly motile (flagellated), live in an aerobic environment, and oxidize glucose for nutrients (heterotrophic). They are rod shaped single cell microorganisms about 0.5 microns in diameter and about 3 to 5 microns in length. They are opportunistic and can adapt and survive under severe conditions of extremely low concentrations of organic substrates such as in purified water systems. The slimy polysaccharide cell wall of the bacteria promotes adhesion to surfaces and biofilming occurs rapidly on any contact surface. The polysaccharide coating also traps nutrients and protects the cell from disinfectants such as chlorine.

Bacteria are quantified in terms of colony forming units per volume of water (CFU/ml or CFU/100 ml). Bacteria testing involves filtering a known volume of water through a sterile 0.45 micron membrane filter disc, incubating the filter disc on a nutrient pad at a standard temperature (35oC), and counting the colonies formed on the filter disc after the prescribed incubation time (48 hours). Each colony is assumed to have grown from a single cell. This is termed the “standard plate count” method.

Since bacteria replicate rapidly under ideal conditions for growth, control of bacteria in a purified water system is one of the most difficult challenges. The best strategies include the following: provide complete recirculation at turbulent velocities (3 fps); use 254 nm ultraviolet in the loop; eliminate any piping dead-legs; use sanitary piping with low surface roughness; operate at sanitizing temperatures; or frequently flush and sanitize with hot water, ozone, or other chemical disinfectants. Even with these measures, biofilming can occur in ambient temperature systems and compromise the bacteriological quality of the water. An inexpensive, yet effective measure for controlling bacteria at the point of use is to install sterile 0.2 micron membrane filter capsules on the water faucets or outlets.

Endotoxins

Endotoxins are the polysaccharide compounds from the cell wall of certain bacteria such as those found in purified water systems. They are termed pyrogenic because they induce a fever response when injected in warm-blooded mammals and can even cause shock and death. They have two major components: a hydrophilic (water soluble) polysaccharide chain attached to a hydrophobic (insoluble in water) lipid (fatty) group. The hydrophobic portion causes endotoxins to aggregate together in vesicles ranging in size from about 20,000 daltons to millions of daltons. One molecular weight (MW) is about one dalton. Based on this, endotoxins can be removed using ultrafilter membranes in the 10,000 dalton pore size range or by using reverse osmosis membranes. Properly designed distillation systems can also remove endotoxins.

Endotoxins are quantified in terms of Endotoxin Units per milliliter (EU/ml) using the Limulus Ameobocyte Lysate test (LAL). EU’s are assigned by comparison with a USP reference endotoxin standard. Endotoxins react with the LAL (purified extract of the blood of the horseshoe crab) causing a turbid or clotting reaction that permits quantification to extremely low levels (about 0.001 EU/ml). The USP water for injection specification limit for endotoxin is less than 0.25 EU/ml.

Water Quality Specifications

Reagent Grade Water (RGW)

RGW is defined as water suitable for use in a specified procedure such that it does not interfere with the specificity, accuracy, and precision of the procedure. In addition, the water quality must meet the specifications established for the application. This definition applies to any high purity water application.

CAP, NCCLS, and ASTM have established RGW specifications for uses ranging from general laboratory to specific clinical laboratory applications. General laboratory applications include glassware washing and rinsing, chemical reagent and buffer solution preparation, making blanks and standards for calibrating analytical instrumentation, culture media, etc. Clinical laboratory applications include procedures in bacteriology, immunology, hematology, histology, etc. The NCCLS reagent grade water specifications are shown in Table 1. The ASTM reagent grade water specifications are shown in Table 2.

Some applications may have “special” requirements beyond RGW specifications. For example, high performance liquid chromatography (HPLC) may require water with a maximum absorbence of a specified wavelength of ultraviolet light. Special “HPLC” grade water systems are offered by some companies, and some suppliers offer “HPLC” grade bottled water.

National Committee for Clinical Laboratory Standards (NCCLS)

The NCCLS specifies three grades of RGW (Types I, II, and III). The NCCLS does not specify the acceptable methods of water purification for producing RGW; however, it does state that any method or combination of methods is acceptable as long as the product water meets the applicable specifications. Type I water is the highest quality and is generally used in more critical applications such as trace element analysis, automated analyzer systems, reagent and buffer solution preparation, etc. Type II water is used in general clinical methods including immunology, hematology, etc. Type III water is used for some qualitative procedures, glassware washing, etc.

The NCCLS specifies water sampling and testing methods for the parameters listed. Type I water quality must be measured using an inline resistivity sensor to avoid the problems associated with rapid absorption of atmospheric carbon dioxide into the deionized water. Rapid absorption of even small amounts of carbon dioxide into the water sample causes a significant drop in the resistivity of the water.

In addition, general water purification system design and maintenance guidelines are offered by NCCLS. It is suggested to use inert materials of construction to prevent leaching of inorganic and organic contaminants. Systems should be designed with complete recirculation avoiding dead-legs (stagnant areas). Outlet designs should minimize dead spaces and use non-leaching seal materials. It is generally not preferred to store and distribute pure water (Type I water can not be stored and remain Type I quality); it is suggested to produce final product water quality on demand at the point-of-use. Systems designed to store and distribute Types II and III water should be provided with measures to protect the chemical and microbial water quality (recirculation with 254 nm ultraviolet, sealed tanks with 0.2 micron hydrophobic vent filters, etc.). Sanitization of the system is recommended at least semi-annually or as necessary for quality control.

NCCLS, Type I water systems must include granular activated carbon treatment for organics and chlorine removal, mixed-bed deionization to meet resistivity and silica specifications, and 0.2 micron post-filtration for bacteria and particle control. Type II water can generally be produced by distillation, deionization, or reverse osmosis with polishing deionization or electrodeionization (EDI). Reverse osmosis technology is capable of providing Type III reagent grade water depending on the feed water quality and the design and operation of the reverse osmosis system.

Table 1
National Committee for Clinical Laboratory Standards
Reagent Grade Water Specifications

The American Society for Testing and Materials (ASTM)

The ASTM establishes specifications for Types I, II, III, and IV reagent grade water (D1193-99e1) as shown in Table 2. In addition, the water quality is further classified as Type A, Type B, or Type C depending on the applicable bacteriological and endotoxin quality. Type I water is the highest quality and is generally used for the most critical applications – trace element analysis, HPLC, reagent preparation, etc. The ASTM further specifies that Type I water is produced by mixed-bed deionization with suitable pretreatment (distillation or other equal process that can produce water with a maximum conductivity of 20 uS/cm) and post filtration with 0.2 micron membrane filters. Type I water quality can not be maintained in storage and must be produced on demand at the point-of-use. Resistivity can only be measured using inline resistivity monitoring equipment.

Type II reagent grade water is produced by distillation with suitable pretreatment (reverse osmosis or deionization) and, depending on the design of the storage tank, is generally sterile and endotoxin-free. This grade of water is suitable for preparing culture media, microbiology, bacteriology, etc. Care must be taken in the design of the storage tank and the distribution system to prevent bacterial contamination.

Type III reagent grade water is produced by distillation, deionization, reverse osmosis, electrodeionization, or a combination of these technologies, followed by post-filtration with a 0.45 micron membrane filter. This grade of water is generally suitable for preparing various reagents, qualitative analysis, etc. Design of storage tanks and distribution systems is critical to prevent contamination.

Type IV reagent grade water is produced by any of the primary treatment methods (distillation, deionization, electrodialysis, or reverse osmosis) or a combination of these methods. This water quality is generally used for glassware washing, cooling applications, etc.

Table 2
American Society for Testing and Materials
Reagent Grade Water Specifications

Pharmaceutical Grade Water

The United States Pharmacopoeia (USP 25) is the accepted guide (compendium) for producing pharmaceutical products in the US. The USP specifies standards of quality, purity, packaging, and labeling for many pharmaceutical products and the ingredients used in the manufacture of these products. The guide specifies two grades of bulk ingredient water used in the preparation of compendial dosage forms, USP purified water (PW) and USP water for injection (WFI).

The US Food and Drug Administration (FDA) provides a regulatory /enforcement function in the pharmaceutical industry. The FDA has established guidelines known as the Current Good Manufacturing Practices (CGMP) to regulate the industry. The CGMPs are essentially guidelines used by industry to establish comprehensive quality management plans to insure the safety of drug products. The CGMPs provide some guidelines that affect the design and operation of USP grade water systems. In addition, water systems must be validated systems; validation is a documented process that provides assurance that the system will remain in control and consistently provide the specified water quality.

USP Purified Water (PW)

USP purified water is an ingredient in many pharmaceutical products. It is not applicable for the manufacture of parenteral (injectable) drugs and some other drug products. The USP specifies the following requirements for purified water:

• Produced by a suitable process – reverse osmosis, deionization, or distillation
• Produced from “drinking water” – meets EPA potable water standards
• Contains no added substances
• Meets the requirements of the Conductivity Test – less than 2.1 ?S/cm at 25oC, and 6.6 pH (varies depending on temperature and pH)
• Meets the requirements of the TOC Test – less than 500 PPB
• To control bacteria growth, USP suggests a microbial action limit of 100 CFU/ml

The FDA stipulates the following additional requirements for purified water:

• Free of “objectionable organisms” that can infect or grow in the product – the burden is on the manufacturer to determine the nature of this problem and establish applicable specifications
• Conformance with CGMPs

USP Water for Injection (WFI)

USP water for injection (WFI) and sterile water for injection are used in the manufacture of parenteral drugs and other products such as ophthalmic and inhalation products. The USP specifies the following requirements for water for injection:

• Meets all of the requirements for purified water
• Produced by distillation or reverse osmosis
• Meets the requirements of the Bacterial Endotoxin Test – less than 0.25 EU/ml
• Suggested microbial action limit of 10 CFU/100ml
• Produced, stored, and distributed under conditions designed to prevent production of endotoxins

The FDA stipulates the following additional requirements for WFI:

• Reverse osmosis as a final means of production is discouraged, but if it is used, double-pass reverse osmosis is suggested
• Conformance with CGMPs

CGMP Compliance Issues

Critical process parameters are those parameters that directly affect water quality and include conductivity, TOC, sanitizing temperature, etc. Because microbial quality and endotoxin quality cannot be monitored in real time, monitoring of parameters that affect these are considered critical and may include process temperature, UV intensity, ozone concentration, recirculation flow rate, loop pressure, etc. Instruments that measure these parameters are considered critical instruments, and these instruments must be properly calibrated on a regular basis and documentation must be maintained.

Regulatory compliance issues have more to do with establishing system design specifications and properly documenting the validation of the system. This includes verifying installation, operational, and performance qualifications; establishing sampling and testing protocols; materials verification documentation; establishing sanitization protocols; record keeping; etc.

The critical concern in the design of pharmaceutical water systems is to avoid designs that could compromise bacterial and endotoxin quality. CGMP design strategies to control bacterial and endotoxin quality include the following:

• Provide continuous recirculation at turbulent flows (3 fps in the return line)

• Eliminate piping dead-legs and use zero dead-leg outlet valves

• Establish standard operating procedures (SOP) for frequent draining, flushing, and sanitizing

• Maintain positive system pressure

• Adhere to accepted back flow prevention practice

• Design for complete system drain ability (WFI systems)

• Provide smooth, inert surfaces and joints (180 grit minimum)

• Store and distribute WFI quality water at 80o to 90oC

General Design Considerations

It is important to establish the appropriate water quality specifications for the given application. If specific regulatory or industry standards do not exist, the user is responsible for establishing the applicable water quality specifications.

It is generally impractical to design individual water systems to provide specific water quality for each application in a large facility. If the most critical application requires the bulk of the water usage, it is common to design a system to meet these requirements and supply this quality of water to all applications. However, if the critical application requires minimal water usage in comparison to other less critical applications, providing point-of-use polishing systems to meet the needs of the most critical applications may be the most practical option. Economic considerations generally influence the system design.

The design of the water system may also be influenced by the applicable water quality specifications. For example, ASTM and USP specify the method of production for the various types of water quality.

A water system designed to provide ASTM, Type I reagent grade water for a small laboratory consists of a number of integrated components. A pre-filter cartridge (0.2 to 0.5 micron pore size rating) provides for removal of particulate matter in the feed water. A granular activated carbon filter removes chlorine and organics. A primary mixed-bed deionizer provides TDS removal to a maximum of 20 uS/cm conductivity, and a polishing mixed-bed deionizer provides final product water quality with a conductivity of 0.055 uS/cm. In addition, the system may include an in-line conductivity sensor and monitor for final water quality, a dispensing faucet with a 0.2 micron post-filter capsule, and a recirculation pump (Figure 1). A sterile 0.2 micron, absolute rated membrane filter cartridge can produce bacteria-free water.

Optional components can include a 185 nm ultraviolet system installed before the primary deionizer to provide for trace organics removal (TOC less than 10 PPB), an ultrafilter membrane for endotoxin removal, and a 254 nm ultraviolet unit for bacteria control.

For greatly improved water quality and economy of operation (especially for higher capacity systems), a reverse osmosis system can be used in lieu of the primary deionizer unit. Reverse osmosis is a membrane separation process that filters water through a membrane with pore sizes less than 0.001 micron and can provide greater than 98 percent rejection of TDS in the feed water. In addition, reverse osmosis systems reject a high percentage of organics, colloids, bacteria, and pyrogens. The throughput of the polishing mixed-bed deionizers can be increased by as much as 6 to 10 times or more (depending on the feed water quality) by using reverse osmosis for primary water treatment. Reverse osmosis membranes do not reject dissolved carbon dioxide in the feed water, and this is an additional load on the polishing mixed-bed deionizer. For many small laboratory systems the capital cost payback can be as short as a few months.

Large central systems designed to provide ASTM, Type I reagent grade water generally include a reverse osmosis system for primary treatment, storage tank with water distribution pump station, polishing mixed-bed deionization, 0.2 micron post-filter, and a recirculated piping system (Figure 2). An optional 254 nm ultraviolet system may be installed after the post-filter for better control of bacteriological quality.

Applicable pretreatment for reverse osmosis systems depends on the feed water quality and may include a turbidity filter, carbon filter for chlorine removal, softening system for hardness removal, and a 5 micron disposable cartridge pre-filter Pretreatment is critical to optimize the performance and the life of the reverse osmosis membranes.

One of the most difficult problems in the design of large central water systems to provide ASTM, Type I water quality is maintaining Type I water quality at every point-of-use location in a complex water distribution system. The design of the water distribution system is critical to maintaining water quality. Low velocity or stagnant areas in the piping system, leaching of contaminants from the piping materials, bacteriological activity, and intrusion of carbon dioxide in the water can rapidly degrade water quality.

Another common design is to provide a central system producing ASTM, Type III water quality or NCCLS, Type II water quality and install polishing mixed-bed deionization systems at critical points-of-use where Type I water quality is required.

ASTM, Type III and Type IV reagent grade water must be produced by distillation, mixed-bed deionization, or electrodeionization (EDI) to meet the strict sodium and chloride specifications. A typical system may consist of reverse osmosis for primary treatment (with applicable pretreatment equipment) followed by mixed-bed deionization or EDI, water storage tank, booster pump station, and a recirculated piping system. To consistently meet the resistivity specification for ASTM, Type III water, polishing deionization may be required after the storage tank. Alternatively, a nitrogen blanketing system on the storage tank to prevent the absorption of carbon dioxide may be required to prevent degradation of the resistivity of the stored water. Applications with minimal water usage requirements are often served with exchangeable mixed-bed deionizers, even though the water quality may be better than needed.

Other integrated system design options can be used, but a thorough knowledge of the feed water quality, water quality objectives, and water system process design is needed to provide a suitable system.

Common practice in high purity water system design for pharmaceutical and biotechnology applications is to store the final product water (PW or WFI) before distribution to the points-of-use. This practice facilitates frequent sanitization of the system or allows for storage and distribution of the water at sanitizing temperatures (90oC) which is common in WFI systems.

Many combinations of unit processes can be used to produce USP purified water. Final treatment options include distillation, mixed-bed deionization, double-pass reverse osmosis, reverse osmosis followed by polishing mixed-bed deionization, and reverse osmosis followed by EDI.

The most common USP purified water system design consists of an RO system for primary water treatment (with applicable pretreatment equipment), polishing mixed-bed deionization, 0.2 micron post-filter, 254nm ultraviolet system, USP water storage tank, booster pump station, and recirculated distribution piping system (Figure 3).

Other common process design configurations include the following:

• Pretreatment, RO primary treatment, EDI polishing treatment, 0.2 micron post-filter, 254nm ultraviolet, storage, and distribution
   

• Pretreatment, RO primary treatment, storage, booster pump, distillation (vapor compression or multi-effect), storage, and distribution

In systems designed for hot water or steam sanitization, the tank, booster pump, heat exchanger, and piping system are constructed of sanitary, 316L stainless steel. The storage tanks used in these systems are ASME Code pressure vessels constructed of polished, 316L stainless steel and rated for vacuum and pressure applications at temperatures above the maximum operating temperature of the system. These pressure vessels are also equipped with sanitary fittings, 0.2 micron absolute rated hydrophobic vent filter, pressure/vacuum rupture disc, internal spray ball on the recirculation return line, level and temperature controls, and insulation.

Other design considerations include sizing of equipment items to meet capacity and peak usage demands, utility requirements, space availability and access, instrumentation and control requirements, maintenance requirements, and future needs. Budget considerations may also influence system design options. In some cases, the cost of the high purity water system can be a significant percentage of total process system cost. Other significant costs could include system validation, quality control, and wastewater treatment.

References

1. National Committee for Clinical Laboratory Standards, NCCLS C3-A2, “Preparation and Testing of Reagent Water in the Clinical Laboratory; Approved Guideline”, Third Edition, Vol. 17 No. 18, October 1997.
2. American Society for Testing and Materials, ASTM D1193-99e1, “Standard Specification for Reagent Grade Water”, Annual Book of ASTM Standards, Volume 11.01, American Society for Testing and Materials, Philadelphia, PA (2003).
3. Meltzer, Theodore H., High Purity Water Preparation for the Semiconductor, Pharmaceutical, and Power Industries, Tall Oaks Publishing, Inc., Littleton, CO (1993).
4. International Society for Pharmaceutical Engineers, Baseline Pharmaceutical Engineering Guides for New and Renovated Facilities, Water and Steam Systems, Vol. 4, First Edition, January 2001.
5. Winstead, Martha. Reagent Water: How, When, and Why?. The American Society of Medical Technologists, Houston, TX (1967).
6. Murphy, Daniel B., and Viateur Rousseau. Foundations of College Chemistry. New York: The Ronald Press Company (1969).