The highs and lows of high-purity water

High-purity water means something totally different to different industries. Today‘s municipal drinking water, regulated by the Safe Drinking Water Act and its amendments, might be called high-purity water when compared with drinking water consumed by humans over the past thousands of years.

Municipal drinking water is regulated by the US Environmental Protection Agency (EPA) primary standards to single-digit parts per billion (ppb) or micrograms per milliliter (μg/mL) levels for certain metals and organic compounds, to less than 1 nephelometric turbidity unit (NTU) for clarity, and to less than 500 milligrams per liter (mg/L) (secondary standard) for total dissolved solids (TDS).

The following are some applications that may use high-purity water:

?Bottled drinking water. Some people don抰 consider municipal drinking water to be high enough purity water. The growth of the bottled water industry is testimony to how many people feel that perceived, or actual, higher purity is better when it comes to potable water.

Bottled water manufacturers may use membranes, distillation, ozonation, ultraviolet (UV) irradiation and/or other technologies to produce drinking water that, in comparison, has relatively few contaminants. Some may consider this to be high-purity water for the drinking water industry.

Pharmaceutical industry. Many industries purchase municipal drinking water as raw water (raw material) and purify it to end-use specifications. The pharmaceutical industry is an example.

The pharmaceutical industry must start out with raw water that meets regulated drinking water standards. The water treatment systems within a pharmaceutical plant then purify the raw water to produce compendial waters, which include any water intended to be used for final drug dosage forms, including Sterile Purified Water, Sterile Water For Injection, Sterile Bacteriostatic Water For Injection, Sterile Water for Irrigation (e.g. used to rinse, or irrigate, internal organs during surgery), and Sterile Water for Inhalation.

Sterile Purified Water and certain Sterile Water for Inhalation products are made using water processed in a pharmaceutical water treatment plant called purified water (PW). For PW, the US Pharmacopoeia (USP) requires a conductivity limit of 0.6-4.7 mS/cm, determined by a three-stage process that accounts for the temperature and pH of the water; a total organic carbon or total oxidizable carbon (TOC) limit of 500 ppb (mg/L); and a bacterial count limit of100 colony forming units per milliliter (cfu/mL).

Sterile Water For Injection, Sterile Bacteriostatic Water For Injection, Sterile Water for Irrigation, and certain Sterile Water for Inhalation products are produced from water processed in a pharmaceutical water treatment plant called Water For Injection (WFI). For WFI, the USP requires a conductivity limit of 0.6-4.7 mS/cm (same three-stage process as for PW); a TOC limit of 500 ppb (mg/L); an endotoxin limit of 0.25 endotoxin units (EU); and a bacterial count limit of 10 cfu/100 mL.

So to a municipal drinking water technician, high-purity water may mean potable water with a TDS of less than 500 mg/L, a turbidity of less than 0.5 or 1.0 NTU (depending on the filtration method), and all other regulated parameters in limits, while to a pharmaceutical water treatment technician high-purity water means drinking water that has had considerably more contaminants removed.

?Boiler feedwater. Not only is high-purity water a relative term between industries, it is a relative term within an industry. For example, in the power generation industry, boiler feedwater is high-purity water. The purity requirements, however, depend on the pressure of the boiler. The lower the boiler pressure, the lower the purity requirements.

The American Society of Mechanical Engineers (ASME) has put together tables for the quality of boiler feedwater at various drum operating pressure. For a 300-pounds-per-square-inch (psi) boiler, the feedwater should have less than 100 ppb of dissolved iron, less than 50 ppb of dissolved copper, and less than 1,000 ppb of nonvolatile TOC. For a 2,000-psi boiler, the iron level should be less than 10 ppb, the copper level less than 10 ppb and the nonvolatile TOC less than 200 ppb.

In the power generation industry, contaminants like silica and oxygen are important, where they weren抰 in the other waters discussed so far. The level of allowable dissolved oxygen in boiler feedwater may be less than 5 ppb. The allowable level for silica may be less than 20 ppb.

?Semiconductor rinse water. Some of the highest purity water will be found in a semiconductor manufacturing plant. In the semiconductor industry, the water produced is generally called ultra-pure water (UPW). UPW, which is being produced at many facilities today, has a resistivity of about 18.2 megohm-cm, TOC concentration of 500 parts per trillion (ppt) and ppt levels of essentially all inorganic species.

While each UPW system is unique and includes different treatment steps and treatment equipment, some generalizations may be made. Figure 1 provides a process flow diagram of the equipment components for a sample UPW system. Figure 1 is not intended to illustrate the best? or the latest? process flow; it illustrates the majority of common treatment steps. The number and types of treatment steps vary with each UPW system. The location of certain pieces of equipment will differ for different UPW systems.

A UPW system may be divided into subsections, including:

1. Municipal water treatment. In Figure 1, the untreated raw water is assumed to be from a surface water source. The raw water is processed at a municipal drinking water plant. The finished water from the municipal drinking water plant becomes the raw water to the plant.

A commonly used first step for the treatment of surface water is to coagulate small suspended particles into larger, more readily settleable or filterable solids by the injection of aluminum or iron salts. Next, a long-chain ?typically cationic ?polymer is injected to bring together (flocculate) the coagulated particles and the coagulated/flocculated suspended materials are then allowed to settle out in a sedimentation step.

Lime may also be added to soften hard water. Downstream of the clarification step there typically will be a filtration step using sand or multimedia filtration to polish the clarified water.‘

For microbiological control, chlorination or chloramination (chlorine plus ammonia) is usually required. Other treatment steps may also be required.

2. Pretreatment. The term "pretreatment" is commonly used when there is a membrane water-treatment step in a water-treatment scheme. Pretreatment includes all of the treatment steps ahead of the membrane-treatment step. These steps are primarily required to protect the membrane units from scaling with sparingly soluble salts, fouling with living or nonliving suspended particles, or chemical attack by pH, oxidizing agents or other dissolved contaminants.

Pretreatment equipment may include media filtration for bulk suspended solids removal. This step removes corrosion products, biofilm mass, and any other bulk suspended solids contamination from the city water lines.

Downstream of the media filtration step, frequently a polishing step will be included for suspended solids removal, most commonly using a 1-5 micron cartridge filtration system. Other polishing technologies are sometimes used? filtration, microfiltration and even ultrafiltration.

Sodium-cycle cation exchange softening may be present in order to remove potentially scale-forming cations such as calcium, magnesium, barium and strontium. If ion exchange softening is not used, then many pretreated feedwaters required acid injection to minimize cellulose acetate membrane damage and/or to control carbonate scaling. Acid does not typically control noncarbonate scaling, so the addition of a scale-inhibiting chemical may be needed.

The commonly used polyamide thin-film composite membranes are susceptible to damage by oxidizing agents such as chlorine. Oxidizing compounds must, therefore, be removed prior to feedwater introduction into the membrane unit. Activated carbon filtration or sulfite ion injection (usually as sodium sulfite or sodium metabisulfite) are most commonly used for dechlorination. Additionally, activated carbon will remove certain organic molecules from a feedwater.

3. Membrane treatment. Membrane treatment refers to several water treatment technologies that use a membrane to separate contaminants from water. Reverse osmosis (RO) is the most common membrane used in the production of water for the semiconductor industry because:

?RO membranes provide extremely high rejections of dissolved ions, such as sodium, calcium, magnesium, copper, lead, chloride, bicarbonate, phosphate and sulfate.

?RO membranes also reject most uncharged, dissolved organic and silica compounds very well.

?RO membranes can virtually completely reject suspended contaminants.

They cannot, however, reject everything: Dissolved gases and many volatile organic compounds can pass through the membrane.

RO units are typically in a double-pass configuration consisting of two RO membrane units in series. Permeate (filtered water) from the first RO unit is sent to the second RO unit to be filtered again.

It is not uncommon for the permeate from a double-pass RO unit to have a resistivity reading up to 0.5-4 megohm-cm, with well less than 1 mg/L of organic and silica contaminants. The more contaminants removed in the membrane treatment step, the lower the loading on the polishing steps.

4. Volatiles removal. The removal of dissolved oxygen, carbon dioxide, other gases and volatile organic compounds is a necessary treatment step for many UPW systems. The removal of these volatile contaminants is accomplished to acceptable levels in vacuum degasifiers and in membrane degasification units.

Some volatile organic compounds and dissolved gases (e.g. carbon dioxide) can ionize (form ions). The removal of volatiles that can form ions reduces the loading on downstream polishing equipment.

In addition to removing ions, downstream ion exchange units remove volatile organic compounds, which reduces the loading to the ion exchange units.

5. Polishing. The relatively low level of contaminants that are not removed in the membrane treatment and volatiles removal steps are polished down to acceptable levels in the polishing steps. Ionic, organic and silica contaminants are removed in typically two stages of ion exchange in series. The first ion exchange subsystem is commonly called the primary beds and the second is commonly called the polishing beds.

Organic compounds ?measured as TOC ?that are found downstream of the RO membrane units may be subjected to TOC-reduction UV irradiation, typically from 185-nanometer wavelength low-pressure or medium-pressure units. TOC reduction UV wavelengths convert most uncharged organic compounds, which downstream ion exchange units may not be able to remove, into ionic compounds that can be effectively removed by the downstream primary ion exchange units.

TOC compounds that exit the primary ion exchange units are typically subjected to TOC-reducing UV irradiation to break them into ionized compounds to be removed by the polishing ion exchange units. The vast majority of all living suspended particles (mainly bacteria) that enter any UV unit are inactivated. Downstream filters with a pore size of less than 0.45 micron remove most of the inactivated bacterial bodies. The final filter prior to distribution typically has a pore size of less than 0.2 micron.

High-purity water is a relative term that means different things to different industries. The allowable levels of dissolved and suspended contaminant define high-purity water. The highest purity water is typically found in a semiconductor manufacturing plant.

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