Water for Injection (WFI) Preparation By Membrane May Become Possible

Water is one of the most widely used and consumed excipients in pharmaceutical production.

It is applied throughout the entire manufacturing process, including API production, separation and purification, formulation, and equipment cleaning etc. The quality of pharmaceutical water has a direct impact on the quality of drug products, thereby affecting the life safety of patients. Therefore, the pharmacopoeias of various countries have clear requirements and regulations for pharmaceutical water and WFI, as shown in Table 1 below.

Country/Region	Quality Standard Version	Definition of WFI
China	The Chinese Pharmacopoeia (2020 Edition)	WFI is obtained by distillation of purified water, which can be used as a solvent or diluent for injectables, eye drops etc. and for washing containers.
the United States	The United States Pharmacopeia (USP) (Current Edition)	WFI is used for the production of parenteral preparations and other preparations requiring bacterial endotoxin control, as well as for cleaning certain equipment and components in contact with parenteral preparations
Japan	The Japanese Pharmacopoeia(18th Edition)	WFI is water specially treated (e.g., by ion exchange or reverse osmosis(RO) or purified water that is prepared by distillation, RO, or ultrafiltration.
Europe	The European Pharmacopoeia(11th Edition)	WFI is obtained from drinking water or Purified Water through distillation or by a purification process proven to be no less effective than distillation."

Table 1 Quality standards for pharmaceutical water and definition of WFI in various countries

Comparison of WFI Preparation Methods

Distillation is commonly used to prepare WFI across different countries, as shown in Table 2. 

In 2017, the European Pharmacopoeia revised monograph 0169 on WFI, marking a significant change in the production of WFI. Since its implementation, the EU has allowed the use of methods equivalent to distillation for the production of WFI, such as RO with membrane separation and electrodeionization (EDI) combined with ultrafiltration.

Annex 3 of WHO TR 1025, Production of WFI by Means Other than Distillation, also states that WFI can be prepared through distillation or other alternative methods as the final step. These methods include deionization, electrodeionization(EDI), nanofiltration, ultrafiltration, softening, rust removal, prefiltration, degassing, UV treatment, and single-pass or double-pass RO systems combined with other technologies. Currently, Europe, the United States, and Japan all permit to use drinking water as source water for the preparation of WFI through either distillation or non-distillation methods.

Country/Region	Source Water	Purification process
China	Purified water	Distillation
the United States	Drinking water	Distillation or a purification process proven to be superior to distillation in removing chemical substances and microorganisms
Japan	Drinking water/Purified water	Distillation, RO, or ultrafiltration
Europe	Drinking water/Purified water	Distillation or a purification process proven to be no less effective than distillation

Table 2 Comparison of preparation processes of WFI in various countries

Non-Distillation Methods for Preparing WFI (Membrane Separation Technology)

The  European Pharmacopoeia introduced the category of Highly Purified Water (HPW), allowing non-distillation methods (e.g., RO combined with ultrafiltration or deionization technologies) to produce HPW. Its application was limited to cases where WFI was not required for preparation but strict microbial control in water was necessary. After years of practical experience, the European Union implemented regulations in 2017, permitting the production of WFI using non-distillation methods, such as membrane separation via RO or electrodeionization(EDI) combined with ultrafiltration. Since then, HPW has been phased out.

With advancements in equipment, facilities, and technological capabilities in recent years, other regions and countries may also consider introducing HPW as a transitional category, gradually relaxing restrictions on the purification process for WFI to align with international standards.

Figure 1 Schematic diagram of the pharmaceutical water preparation system

Cobetter 6kD hollow fiber endotoxin removal effectiveness

Testing temperature 25°C	Filtration time	Endotoxin Concentration (EU/mL)
		Concentrate End	Permeate End
	Beginning	10000	<0.02
	after 1h	10000	<0.02
	after 3h	10000	<0.02
	after 5h	10000	<0.02

Table 3 Endotoxin challenge test results (Test product: Cobetter 6kD hollow fiber membrane column)

As shown in Table 3, ultrafiltration was performed using the Cobetter 6kD hollow fiber membrane column with endotoxin solution of 10,000 EU/mL. The endotoxin concentration in the permeate end was detected to be <0.02 EU/mL, which preliminarily indicates compliance with the standard of <0.25 EU/mL for WFI. In conjunction with the revision of the definition of WFI in General Rule 0261 of the Chinese Pharmacopoeia(2025 edition), this opens up the possibility of using non-distillation methods, such as membrane technology, to prepare WFI.

Table 5 Cobetter hollow fiber products