The Importance of High Purity Water
Achieving high purity ASTM water is crucial for industrial processes that demand the highest standards of water purity. Whether you’re working in pharmaceuticals, semiconductors, or any industry where water quality is non-negotiable, understanding how to achieve high purity water is essential.
When it comes to high purity water standards, it is typically split into four types based on ASTM water specifications, with Type 1 being the highest purity.
This article will provide an overview of the four types of high purity water and provide and discuss the five key methods used to achieve different levels of purity.
The Four Different Types of ASTM High Purity Water
The ASTM (American Society for Testing and Materials) defines four types of water, each with specific purity requirements:
- Type 1 Reagent Water – The purest form, used in sensitive analytical procedures like atomic absorption and gas chromatography. Requires a resistivity of at least 18 MΩ-cm at 25°C and TOC levels of less than 50 ppb.
- Type 2 Reagent Water – Used for general laboratory purposes, including microbiological media preparation. Requires a resistivity of at least 1 MΩ-cm at 25°C, with TOC levels typically below 50 ppb.
- Type 3 Reagent Water – Suitable for less critical applications like rinsing glassware and filling autoclaves. Requires a resistivity of at least 4 MΩ-cm at 25°C, with TOC levels expected to be less than 200 ppb.
- Type 4 Reagent Water – Used in non-critical tasks such as manual glassware washing. Requires a minimum resistivity of 0.2 MΩ-cm at 25°C, with no specific TOC limit.
TOC is Often the Main Challenge in Achieving Type 1, 2, and 3 Water
Total Organic Carbon (TOC) in water represents the collective amount of carbon found in various organic molecules dissolved in the water. These organic molecules can include a wide range of substances, such as natural organic matter (NOM), industrial pollutants, and even biological materials like bacteria and algae. It is an important parameter in water quality analysis, particularly in industries such as pharmaceuticals and semiconductor manufacturing, where TOC can cause problems.
TOC is typically measured by oxidizing the organic carbon in a water sample and then quantifying the amount of carbon dioxide produced. This is often done using methods like high-temperature combustion or UV-persulfate oxidation followed by non-dispersive infrared (NDIR) detection.
Typical TOC values in feedwater can vary. In the United States, there isn’t a specific Maximum Contaminant Level (MCL) set by the Environmental Protection Agency (EPA) for Total Organic Carbon (TOC) in municipal water sources. However, TOC is regulated indirectly under the Disinfectants and Disinfection Byproducts (DBPs) Rule because TOC can react with disinfectants, such as chlorine, to form harmful disinfection byproducts like trihalomethanes (THMs) and haloacetic acids (HAAs), which are regulated. TOC levels in water are influenced by a complex interplay of environmental factors, source water characteristics, and water treatment and distribution processes, but TOC values will typically range from 0.5 to 3 mg/L.
ASTM Type 1 and 2 water call for a TOC value of less than 50 parts per billion and 200 ppb for Type 3, which is significantly less than typical feedwater values of 0.5 to 3 mg/L (0.5 mg/L equals 500 ppb and 3 mg/L equals 3,000 ppb). So let’s say your feedwater has a TOC value of 1 mg/L (or 1,000 ppb) and you need Type 2 water, which calls for less than 50 ppb. That is a 20x reduction in TOC just to meet the high end of the TOC spec.
How to Remove TOC From Water
There are five key methods to reduce TOC in a water system.
Removing TOC with Granular Activated Carbon (GAC)
Granular Activated Carbon (GAC) is highly effective at adsorbing organic molecules, including those that contribute to TOC, due to its large surface area and porous structure. GAC is particularly effective at removing large, non-polar organic molecules, which are more likely to adhere to the carbon surface. It can also adsorb smaller, more polar compounds, though the efficiency may vary depending on the specific characteristics of the GAC and the water chemistry. It also doubles as a chlorine/chloramine removal method that is required before an RO system.
Removing TOC with Reverse Osmosis (RO)
An RO system uses a semipermeable membrane to remove a wide range of contaminants. Organic molecules, especially those with larger molecular weights or those that are not highly polar, are rejected by the RO membrane. In addition to size exclusion, RO membranes can also reject organic molecules based on their charge. Many organic compounds in water are negatively charged, and the negatively charged surface of the RO membrane repels these molecules, enhancing rejection and reducing TOC levels.
Removing TOC with Ion Exchange
To achieve the resistivity specifications of ASTM Type 1, 2, and 3 water, you will need to employ mixed bed ion exchange tanks. The strong base anion resin inside the mixed bed is positively charged and designed to attract and exchange negatively charged ions (anions) in the water. Many organic compounds that contribute to TOC, such as humic acids, fulvic acids, and other dissolved organic materials, carry a negative charge and will therefore be removed by the strong base anion resin. If the organic compound carries a neutral or positive charge, then the strong base anion resin in the mixed bed will not be effective at removing it.
Another consideration with ion exchange resins is choosing the right service provider for your resins. Ion exchange resins, particularly if new or degraded, can leach organic compounds into the water, potentially increasing TOC levels and being detrimental to your ASTM water system.
Removing TOC with Ultraviolet Light (UV)
Ultraviolet (UV) light is commonly used to reduce TOC in water. UV light with a wavelength of 185 nanometers (nm) is particularly effective at reducing TOC. It works by breaking down organic molecules through photolysis, where the high-energy UV light cleaves chemical bonds, resulting in smaller, oxidized compounds that can be further converted into carbon dioxide and water. In some systems, UV light is used in combination with other oxidizing agents, such as hydrogen peroxide (H₂O₂) or ozone (O₃), to enhance the reduction of TOC. This combination leads to the formation of highly reactive hydroxyl radicals (·OH).
For Type 1 water systems that require resistivity values greater than 18 meg-ohm, the 185nm UV poses a further challenge because the TOC that converts to CO2 will then convert to carbonic acid in water, which will reduce the resistivity value in the water. To address this, another set of mixed bed tanks needs to be placed after the 185nm UV system to bring the resistivity back up to meet spec. For Type 2 and 3 systems, the resistivity spec is low enough that the CO2/carbonic acid in the water will not affect the resistivity value enough to be of concern.
Reducing TOC with Distribution Piping and Layout
For reducing TOC values in a high-purity water system, the best piping materials are those that minimize the potential for organic contamination and resist microbial growth. PTFE, PVDF, electropolished stainless steel, polypropylene (PP), and high-density polyethylene (HDPE) are among the top choices. These materials are commonly used in industries where maintaining low TOC levels is critical, such as pharmaceuticals, semiconductors, and biotechnology. The choice of material depends on the specific requirements of the system, including chemical compatibility, temperature, and pressure conditions.
Furthermore, proper piping arrangement in a high-purity water system is crucial to minimizing bacterial growth which is important because bacteria growth relates to increased TOC values. Factors such as avoiding dead legs, ensuring consistent flow, maintaining temperature control, and designing for easy maintenance and sanitization all contribute to reducing the risk of bacterial contamination and increased TOC values.
Commonly Used Methods to Achieve High Purity Water
For achieving Type 1 or 2 water, all five of these methods are typically employed together to provide a comprehensive water treatment setup. In contrast, for Type 3 water, only a few methods may be necessary. The choice of methods depends largely on the quality of the feed water.
- Type 1 and Type 2 Reagent Water: All five methods, Granular Activated Carbon, Reverse Osmosis, Ion Exchange, Ultraviolet Light, along with recirculation and the use of smooth piping such as PVDF, are used to meet Type 1 and Type 2 high purity water requirements.
- Type 3 Reagent Water: Granular Activated Carbon and Ion Exchange are often enough to meet Type 3 requirements. However, sometimes a UV system is required to meet this spec depending on the feed water quality.
- Type 4 Reagent Water: Oftentimes, Ion Exchange alone or a double pass RO, can meet Type 4 requirements.
Common Water Treatment Process for High Purity Water
Achieving ASTM high purity water is a complex yet essential process for industries where water quality is critical. By understanding the different types of water defined by ASTM standards and employing a combination of the five key treatment methods, you can ensure that your water meets the specific purity requirements for your application. Whether you’re targeting Type 1, 2, or 3 water, the right approach will depend on your feedwater quality and the demands of your industry. The diagram below provides an example of common water treatment process flows for high purity water.
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