If you’ve ever heard the acronym WFI spoken, know that we’re not talking about wireless technology to connect your computer. WFI stands for water for injection, a sterile water used as a solvent to dilute other solutions for injections to the body. Traditionally, WFI was only produced through the distillation process using water for injection equipment until other methods such as reverse osmosis, electrodeionization, and ultrafiltration were developed.

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Achieving high purity water through different water for injection equipment

Achieving high purity water for injection requires complex equipment that depends on the preferred method to be used. The three common methods for WFI production are multiple effect distillation, vapor compression distillation, and reverse osmosis, electrodeionization, and ultrafiltration. They all use water softening and carbon filtration as a means of scale control and dechlorination of the water source.

Starting with the multiple effect distillation method, your feedwater is preheated before going to a column containing a heat exchanger. As the feedwater is heated, the cyclone separator separates the pure steam which is then used to heat more feedwater.
The condensate of pure steam collected in the condenser that goes to the holding tank is your water for injection.

The second method is vapor compression distillation. Feedwater is preheated before going to the decarbonator which removes non-condensable gases from the feedwater. It then goes to the evaporator tubes where steam-heating coils at the bottom further heat the feedwater to convert it to vapor. A demister removes water droplets from the vapor before the vapor goes to evaporator bank where it condenses, forming distillate. This distillate is collected in a box and pumped through a cooler to produce water for injection.

The third method combines multiple processes such as reverse osmosis, electrodeionization, and ultrafiltration using different water for injection equipment such as filters and membranes. Reverse osmosis uses membranes to remove most of the unwanted molecules and particles. Electrodeionization removes positive and negative ions using electric power and is considered as a polisher after performing reverse osmosis. Ultrafiltration uses ultra-fine membrane filters to further remove contaminants.

Bacteriostatic water or sterile water for injection?

Water for injection, in general, should be pure. Though in some applications, it may be necessary to add antimicrobial agents to minimize the chance of contamination once opened. Bacteriostatic water is sterile water containing 0.9% benzyl alcohol used to dissolve or dilute medications administered through intravenous, intramuscular, or subcutaneous injection.

The container can be re-entered multiple times by sterile needles for injection because benzyl alcohol stops the growth of a possible contaminating bacteria, making it applicable for multi-dose medications. However, not all drugs are compatible with bacteriostatic water. Sterile water is purified water and does not contain any preservative or antimicrobial agent, making it only applicable for single-dose applications.

Standard criteria for qualifying water as WFI

Pharmacopeias develop standards to ensure that water for injection is pure and safe for its intended application. There are differences in the criteria used by Europe, the USA, and Japan Pharmacopoeias for the production of WFI. Europe requires using distillation processes only while the US and Japan allow membrane and other purification processes.

They all have similarities for some criteria such as for conductivity which should be less than 1.3 mS/cm or total organic carbon (TOC) in organic compounds found in aqueous systems, should not be more than 500 ppb. Endotoxins that are considered as pyrogens that can cause fever to patients should be less than 0.25 EU/mL. In contrast, bacteria or total aerobic microbial count should be less than 10 CFU/ 100mL and pH level between 5.0 to 7.0. These ranges in the criteria are considered safe for applications as water for injection.