1. Introduction
Water for Injection (WFI) serves as a core process medium for the production of sterile pharmaceutical preparations, aseptic equipment cleaning, and raw material compounding in the pharmaceutical industry. Its quality directly determines the safety, efficacy and compliance of pharmaceutical products. Among all quality indicators, microbial limit and bacterial endotoxin are strictly controlled core indicators specified by major pharmacopoeias worldwide, as well as the key and difficult points in the whole-process quality control of WFI preparation systems.
The membrane-based WFI preparation system adopts membrane separation technologies including Reverse Osmosis (RO), Electrodeionization (EDI) and Ultrafiltration (UF) as the core processes. It possesses prominent advantages such as normal-temperature water production, low carbon emission and energy saving, small floor area and high operational efficiency, and has been widely applied in the pharmaceutical industry. At present, ISO, ISPE, WHO and major global pharmacopoeias have fully recognized the compliance of membrane-based preparation processes. In view of the non-thermal bacteriostatic characteristic of normal-temperature membrane water production, stricter specifications and requirements are imposed on the whole-process control of microorganisms and endotoxins.
The pharmaceutical industry has witnessed a gradual transformation of WFI preparation technology from traditional distillation to membrane-based processes. The two technical routes differ significantly in technical characteristics and compliance requirements. The traditional distillation method achieves microbial sterilization and endotoxin degradation through high-temperature distillation, featuring low control difficulty and high operational stability. However, it has obvious drawbacks including high energy consumption, large equipment footprint, low water production efficiency and high operation and maintenance costs. In contrast, the membrane-based process is characterized by normal-temperature operation, environmental friendliness, energy conservation, small equipment occupation and high automation level. Nevertheless, due to the absence of continuous thermal bacteriostasis, the system presents more concealed risks of microbial proliferation, biofilm adhesion and endotoxin residue, leading to higher difficulty in whole-process management and control. In terms of compliance, major pharmacopoeias including Chinese Pharmacopoeia 2025, USP-NF-2025 and EP 12 endorse the legitimacy of membrane-based processes, and mandatorily require enterprises to establish a complete whole-process management system and process verification system for microorganisms and endotoxins. Traditional distillation, as a long-standing mainstream compliant process, does not require additional special control measures.
With the trend of green production and compliance quality improvement in the pharmaceutical industry, membrane-based processes will gradually replace traditional distillation and become the mainstream technical route for WFI preparation. Currently, mainstream membrane systems in the industry are equipped with integrated microbial and endotoxin prevention and control technologies. However, improper process management and imperfect operation and maintenance systems in some manufacturing enterprises still lead to potential risks of water quality non-compliance. Therefore, systematic analysis of the risk characteristics and influence mechanisms of microorganisms and endotoxins in membrane-based WFI systems, sorting out whole-process prevention and control technologies, and conducting risk acceptability evaluation are of important engineering and industrial significance for optimizing equipment design, improving management systems, ensuring water quality safety in pharmaceutical production, and promoting the compliant popularization and application of membrane-based processes. This paper systematically studies the core water quality risks of membrane-based WFI preparation systems, clarifies risk sources, influence mechanisms and prevention strategies, and provides theoretical support and practical guidance for the compliant operation and risk management of membrane water production systems in pharmaceutical enterprises.
2. Analysis on Core Risks and Water Quality Impacts of Membrane-Based WFI Preparation System
Microbial contamination and bacterial endotoxin exceeding standard are the two most critical core water quality risks of membrane-based WFI preparation systems. These two types of risks are highly correlated and synergistic, running through the entire process of raw water pretreatment, purification treatment and terminal filtration, and directly determine whether the effluent quality meets pharmacopoeia standards. Meanwhile, auxiliary risks such as raw water quality fluctuation, detection technology lag and process control defects further aggravate the occurrence probability and impact degree of core risks. Detailed analysis of various risks is presented as follows.
2.1 Core Risk I: Microbial Contamination Risk and Its Impact on Water Quality
2.1.1 Specific Risk Manifestations
Microbial contamination risks cover the entire water production process. Affected by limitations of detection technology, biofilm growth and process management loopholes, such risks are characterized by concealment, suddenness and sustainability. The specific manifestations, causes and potential hazards of each risk are clarified below.
First, the risk of detection technology lag. The traditional plate counting method has a long detection cycle of 48 to 72 hours, resulting in severe hysteresis of detection results and inability to realize real-time quality control. Conventional online rapid detection modules are susceptible to interference by impurities such as organic matters and heavy metal ions in water, failing to accurately identify low-concentration microorganisms. Thus, the detection results cannot be used as the basis for water quality release. This risk leads to the failure of early detection of microbial contamination, and unqualified water is likely to flow into pharmaceutical production links, causing potential pharmaceutical contamination risks.
Second, the risk related to biofilm. Biofilms are prone to grow on the inner wall of membrane components and pipelines during long-term operation. Working condition fluctuations such as flow velocity variation and equipment shutdown and restart will cause massive biofilm shedding, resulting in instantaneous excessive microorganisms in water and membrane pore blockage. Conventional disinfection systems only perform post-incident sterilization without real-time biofilm prevention functions, and incomplete equipment cleaning and disinfection further exacerbate biofilm formation. Ultimately, the microbial concentration in water rises sharply, the performance of membrane components is damaged, and the subsequent water treatment efficiency decreases continuously, forming persistent hidden contamination hazards.
Third, the risk of microbial proliferation in process links. Microbial proliferation may occur in each process of pretreatment, purification and terminal filtration, continuously contaminating finished WFI. The main causes include incomplete pretreatment disinfection, damage of membrane components and EDI modules, non-standard integrity testing of terminal ultrafiltration membranes, and failure to drain residual stagnant water in pipelines after equipment shutdown. Long-term management loopholes will lead to continuous microbial proliferation and diffusion, resulting in long-term exceeding of microbial indicators in effluent and non-compliant water quality.
2.1.2 Core Impacts on Water Quality and Production
Microbial contamination has multi-dimensional negative impacts on WFI quality, pharmaceutical quality, medication safety and equipment operation and maintenance, with specific core hazards as follows.
In terms of water quality indicators, microbial proliferation directly causes excessive microbial count and increased Total Organic Carbon (TOC) value in water, and further generates a large number of endotoxins, forming a chain contamination reaction of “microbial contamination leading to endotoxin excess”. In accordance with pharmacopoeia standards, the microbial limit of WFI shall not exceed 10 CFU/100 mL; any excess will directly result in unqualified water quality that cannot be applied to pharmaceutical production.
In terms of pharmaceutical quality and medication safety, WFI with excessive microorganisms used for sterile preparation production and equipment cleaning will significantly increase the risk of pharmaceutical microbial contamination, cause drug efficacy attenuation and substandard purity, easily trigger adverse clinical medication reactions in patients, seriously endanger personal health, and induce pharmaceutical safety incidents.
In terms of equipment operation and maintenance, continuous microbial proliferation in water causes membrane pore blockage and accelerated aging of membrane components, greatly shortening the service life of membrane elements, significantly increasing the frequency and cost of equipment maintenance, and reducing the operational stability of the water production system.
2.2 Core Risk II: Endotoxin Excess Risk and Its Impact on Water Quality
Bacterial endotoxin (pyrogen), a lipopolysaccharide component of the cell wall of Gram-negative bacteria, is the core safety risk of membrane-based WFI systems. Endotoxins mainly originate from microbial contamination in the system, and residual contamination in pipelines and membrane components can also cause endotoxin accumulation. Endotoxins feature high thermal stability and cannot be completely degraded and removed by conventional normal-temperature disinfection methods, posing a major threat to the safety of WFI quality.
2.2.1 Specific Risk Manifestations and Prevention Difficulties
Highly coupled with microbial contamination, endotoxin excess risks are characterized by strong concealment, high residual property and high removal difficulty. The core risk types, generation pathways and prevention difficulties are summarized as follows.
First, microbially derived endotoxin risk. Gram-negative bacteria such as Escherichia coli and Pseudomonas aeruginosa in the system continuously release endotoxins after apoptosis. Meanwhile, during the shedding of biofilms on pipelines and membrane components, a large number of microorganisms wrapped in biofilms die intensively, causing a sharp increase of endotoxin concentration in water.
The core prevention difficulty lies in the small molecular size of endotoxins (1~100 kDa), which enables them to easily penetrate filter membranes with defective integrity, forming superimposed hazards with microbial contamination and showing strong concealment.
Second, equipment residual endotoxin risk. Incomplete daily cleaning and disinfection of membrane components and pipelines lead to the accumulation and residue of microbial debris and endotoxins on the inner wall and dead corners of equipment. In addition, aging and damage of membrane components after long-term operation reduce their endotoxin interception performance, failing to effectively trap free endotoxins in water. The major difficulty is that endotoxins are easily adsorbed on the surface of pipelines and membrane materials to form stubborn residues, which cannot be completely removed by conventional flushing and disinfection processes, resulting in persistent hidden contamination.
Third, detection and control lag risk. The mainstream Limulus Amebocyte Lysate (LAL) test in the industry only supports offline sampling detection without real-time online monitoring capability, and insufficient monitoring points fail to realize full-time and full-coverage water quality monitoring. This problem leads to delayed discovery and intervention of endotoxin excess, easily causing continuous production and wide diffusion of contaminated water.
Fourth, raw water-borne endotoxin risk. Raw water naturally contains Gram-negative bacteria with basic endotoxin load. Abnormal residual chlorine concentration and raw water quality fluctuation induce massive microbial proliferation in raw water, indirectly increasing the influent endotoxin content. The prevention difficulty lies in the dynamic uncertainty of raw water quality and unstable operating load of the pretreatment system, making it hard to accurately predict and control the source endotoxin risks in advance.
2.2.2 Impacts on Water Quality and Medication Safety
Endotoxin excess breaks through the bottom line of WFI water quality safety, not only causing unqualified water quality, but also inducing severe clinical medication safety risks, with specific impacts as follows.
In terms of water quality compliance, endotoxin excess completely undermines the safety and compliance of WFI. In accordance with global pharmacopoeia standards, the endotoxin limit of WFI is ≤0.25 EU/mL; any excess will directly result in unqualified water quality prohibited for pharmaceutical production.
In terms of clinical medication safety, sterile preparations produced with endotoxin-excessive WFI will cause pyrogen reactions in patients after entering the human body, including fever, chills, vomiting and other adverse symptoms. In severe cases, shock and organ damage may occur, easily triggering major pharmaceutical safety accidents, endangering patients’ lives and health, and bringing serious compliance risks and economic losses to enterprises.
2.3 Auxiliary Superimposed Risks
In addition to the two core risks, auxiliary risks including raw water quality fluctuation and defective detection and process control do not directly cause water quality non-compliance, but continuously aggravate microbial and endotoxin contamination risks and form risk superimposition effects, with specific influences as follows.
On the one hand, raw water quality fluctuation risks. Increased microbial load, abnormal residual chlorine concentration (<0.3 mg/L or >0.5 mg/L) and excessive turbidity of raw water will greatly increase the operating load of the pretreatment system, easily induce contamination, blockage and damage of membrane components, accelerate microbial proliferation in the whole system, and indirectly cause endotoxin accumulation and excess, thus amplifying core contamination risks.
On the other hand, detection and process control risks. Most current water production systems have imperfect online detection systems, and offline detection has inherent hysteresis that cannot support real-time quality control. Meanwhile, some enterprises lack standardized whole-process management mechanisms with delayed risk investigation and intervention response, failing to identify early trace contamination hazards. Minor risks continue to accumulate and expand, eventually leading to systematic water quality non-compliance problems.
3. Core Risk Control Technology System for Membrane-Based WFI Preparation System
Aiming at the two core risks of microbial contamination and endotoxin excess in membrane water production systems, and combined with the technical characteristics of normal-temperature operation and absence of continuous thermal bacteriostasis of membrane processes, the industry has established a full-process closed-loop management system featuring source prevention, process interception, real-time early warning and emergency disposal. Refined technical means including process optimization, precise filtration, intelligent monitoring and standardized disinfection are adopted to realize accurate prevention and control of core risks, ensuring long-term compliant effluent quality in line with pharmacopoeia requirements.
3.1 Full-Process Control Technology for Microbial Contamination Risks
Targeting the pain points of microbial proliferation, biofilm formation and detection lag, the industry adopts a combined prevention and control strategy of “prevention + interception + monitoring + disposal” to inhibit microbial proliferation at the source, block contamination diffusion in the process, warn risks in real time and dispose of hidden dangers rapidly. The full-process control measures and technical parameters are specified as follows.
At the source prevention stage, pretreatment process upgrading, automatic equipment backwashing, periodic chemical disinfection and clean environment management of water production areas are adopted to inhibit microbial growth fundamentally. The core process adopts nanofiltration coupled with medium-pressure ultraviolet pretreatment technology, with a 48-hour automatic backwashing cycle, strictly controlling the raw water microbial load ≤10 CFU/mL to build a solid source water quality defense line.
At the process interception stage, a dual-stage RO + redundant terminal UF combined filtration process, full-system sanitary pipeline design, stable flow velocity and shutdown protection mechanism are applied to realize full microbial interception. The core process parameters include RO membrane microbial interception rate ≥99.8%, terminal UF membrane pore size of 0.01 μm, and stable system operating flow velocity of 1.0~1.5 m/s. This process can intercept 100% of bacteria in water, effectively inhibit pipeline biofilm formation, and completely block the cross-process spread of microorganisms.
At the detection and early warning stage, a graded alert limit and action limit management system is established, and Process Analytical Technology (PAT) is deeply integrated to realize intelligent monitoring. The industry’s general control standards set the microbial alert limit at 5 CFU/100 mL and action limit at 10 CFU/100 mL, which fully comply with pharmacopoeia thresholds. This system realizes early warning of microbial risks and effectively avoids sudden water quality excess.
At the emergency disposal stage, supporting mechanisms including automatic over-standard shutdown alarm, full-system in-depth disinfection, post-disinfection water quality verification and whole-process data retention and traceability are equipped. The standard in-depth system disinfection duration is no less than 60 minutes, focusing on targeted cleaning and disinfection of membrane components and pipeline dead corners, which can rapidly eliminate over-standard hidden dangers, complete pollution traceability analysis, and continuously optimize prevention and control schemes.
3.1.1 Special Application of Process Analytical Technology (PAT)
As the core technology for whole-process quality control in the pharmaceutical industry, PAT realizes accurate process regulation and advanced risk early warning through real-time collection and analysis of key production parameters, effectively solving the hysteresis defect of traditional offline detection and ensuring the stability and consistency of water quality. Composed of an online sensing module, data analysis module and closed-loop control module, PAT is applied in membrane-based water production systems as follows.
First, real-time parameter monitoring. High-precision online sensors collect key process parameters including raw water turbidity, residual chlorine, RO membrane flux and EDI conductivity in real time, and upload data to the PLC control system synchronously, realizing dynamic synchronous monitoring of process parameters and water quality indicators, fully grasping system operating conditions, and providing accurate data support for risk prediction.
Second, real-time risk early warning. A correlation model between process parameters and microbial/endotoxin risks is constructed based on big data algorithms. Abnormal fluctuations of operating parameters will trigger an audible and visual alarm automatically, with targeted intervention suggestions pushed synchronously, realizing advanced prediction and pre-intervention of water quality risks and avoiding passive disposal.
Third, process closed-loop control. The PAT system is deeply linked with equipment disinfection, automatic backwashing, shutdown protection and other functional modules. When water quality indicators approach the alert limit, the system automatically starts intervention procedures and adjusts operating parameters in real time, forming a closed-loop management of “monitoring-warning-intervention-recovery”, and fundamentally avoiding water quality excess at the process level.
Fourth, data traceability and optimization. The system retains full-scale monitoring data and equipment operation records. Big data analysis is adopted to sort out risk rules and process deficiencies, continuously optimize process parameters and prevention strategies, support accurate pollution traceability, and improve the stability of system risk prevention and control.
3.2 Full-Process Control Technology for Endotoxin Excess Risks
In view of the core characteristics of endotoxins including microbial derivation, high thermal stability, easy adsorption and residue, and strong penetration, a four-dimensional prevention and control system of “source control + precise interception + residue removal + special detection” is adopted to comprehensively block the generation, accumulation and excess of endotoxins. The full-process control technologies and core parameters are detailed as follows.
In the source control link, raw water pretreatment management is strengthened, and periodic high-temperature and chemical composite disinfection is implemented for the system to reduce endotoxin generation fundamentally. By stabilizing raw water quality load and precisely regulating pretreatment processes, the proliferation of Gram-negative bacteria in the system is inhibited, reducing endotoxin release caused by microbial apoptosis, effectively lowering the initial endotoxin load of the system, and avoiding sudden endotoxin surge risks.
In the precise interception link, special endotoxin interception membranes and dual-stage series RO filtration processes are adopted, with regular membrane integrity testing implemented. The core technical parameter is that the terminal UF membrane with 0.01 μm pore size achieves an overall endotoxin interception rate ≥99.9%. Regular bubble point integrity verification of filter elements ensures stable filtration performance of membrane components, completely intercepts free endotoxins in water, and prevents endotoxin penetration and leakage caused by membrane component damage.
In the residue removal link, a combined process of targeted professional cleaning of membrane components, high-flow circulating flushing of pipelines and targeted chemical disinfection is adopted to eliminate stubborn equipment residues. Continuous high-flow water flushing removes endotoxin residues adsorbed on the surface of pipelines and membrane components, and targeted chemical disinfection thoroughly cleans accumulated pollutants in equipment dead corners, avoiding persistent endotoxin excess caused by long-term accumulation.
In the special detection link, the offline detection scheme is optimized, and a standardized graded alert and action limit management system for endotoxins is established. The industry’s general detection standards include offline detection accuracy ≤0.03 EU/mL and conventional detection cycle of 1~2 hours, with an endotoxin alert limit of 0.12 EU/mL and action limit of 0.25 EU/mL. This system realizes dynamic and accurate monitoring and early intervention of endotoxins, ensuring that effluent indicators are stably below pharmacopoeia limits.
4. System Core Risk Acceptability Evaluation
The two core risks of microbial contamination and endotoxin excess in membrane-based WFI preparation systems can be effectively controlled through the above full-process and multi-dimensional standardized prevention and control technology system. The system reduces the risk occurrence probability to an extremely low level, and limits the water quality impact within the acceptable thresholds specified by pharmacopoeia specifications and industrial standards. There are no major uncontrollable water quality safety hazards, fully meeting the compliance and safety requirements of sterile production in the pharmaceutical industry.
It should be clarified that the acceptability of core risks is dynamic rather than permanent. Pharmaceutical enterprises shall establish a regular dynamic risk assessment mechanism. Risk assessment shall be restarted timely and prevention and control strategies and operation schemes shall be optimized synchronously under conditions including significant raw water quality fluctuation, major equipment and process transformation, failure of prevention and control technologies, and drastic adjustment of production conditions. Meanwhile, with the continuous iteration of membrane separation technology, intelligent detection technology and PAT process control technology, the microbial and endotoxin prevention and control system will be continuously optimized to reduce the water quality risk level of the system and ensure the long-term stable, compliant and efficient operation of membrane-based water production systems.
5. Conclusion and Prospect
5.1 Research Conclusions
This paper systematically studies the core water quality risks and prevention and control systems of membrane-based WFI preparation systems, clarifies the risk characteristics, coupling mechanisms and control key points of microbial contamination and endotoxin excess, and completes risk acceptability evaluation based on industrial universal prevention and control technologies. The core conclusions are summarized as follows:
First, microbial contamination and endotoxin excess are the core controllable risks of membrane-based WFI preparation systems. The two risks are highly coupled and synergistic, and microbial proliferation is the core inducement of endotoxin excess. Running through the entire water production process, inadequate management and control will directly lead to non-compliant water quality, induce pharmaceutical quality problems and clinical medication safety hazards, and constitute the key and difficult points of compliance control for membrane water production systems.
Second, the industry’s current full-process prevention and control system of “source prevention, process interception, detection early warning and emergency disposal” can specifically solve core problems such as microbial proliferation, biofilm formation, endotoxin residue and detection lag. Refined technical means including precise membrane filtration, composite disinfection technology, PAT intelligent management and graded early warning mechanism can effectively inhibit microbial proliferation, efficiently intercept endotoxins, eliminate equipment residual contamination, and realize systematic prevention and control of core risks.
Third, under the premise of strictly implementing standardized prevention and control processes, standardizing equipment operation and maintenance, and establishing a dynamic risk assessment mechanism, the microbial and endotoxin risks of membrane-based WFI systems are within the industrially acceptable range without major safety hazards. The effluent quality can stably meet the standards of major domestic and foreign pharmacopoeias, possessing good industrial application feasibility and compliance.
5.2 Industrial Prospect
In the future, membrane-based WFI preparation technology will develop towards intellectualization, precision and advanced risk prevention. The industry will focus on breaking through the technical bottlenecks of online rapid detection of microorganisms and endotoxins, and deepen the full-scenario application of PAT to completely solve the hysteresis problem of traditional offline detection. Meanwhile, continuous optimization of membrane component structure design and upgrading of disinfection and cleaning processes will be carried out to strengthen the long-term prevention of biofilms and removal capacity of stubborn endotoxin residues, improving the accuracy and timeliness of risk prevention and control.
In addition, the industry needs to further improve the standardized operation, maintenance and training system, promote pharmaceutical enterprises to standardize the implementation of equipment operation, process management and risk assessment systems, and build an integrated quality control system of “equipment technology + process management + dynamic assessment”. Continuous improvement of the operational stability and compliance of membrane-based WFI preparation systems will provide core water quality guarantee for the high-quality, green and safe development of the pharmaceutical industry.
