A comprehensive review reveals how biofilms and poor maintenance can undermine filtration systems, challenging the assumption that water dispensers are always a safer choice than tap water.
Study: Microbiological quality of drinking water from water dispensers. Image credit: PeopleImages/Shutterstock.com
Despite their claim, water dispensers (WDs) are not always free from microbial contamination and, in many cases, may harbor microbial loads comparable to those found in tap water, raising concerns about public health and the need for better maintenance. Recently, researchers conducted a comprehensive review to evaluate microbial contamination in water dispenser machines and examine their potential health implications. This review has been published in AIMS Microbiology.
This review analyzed published literature to assess microbial contamination in WDs, water quality, common contaminants, and health implications. The authors obtained all relevant documents from PubMed, EBSCO, and Google Scholar, synthesizing evidence from approximately 70 studies conducted across multiple countries and settings.
Are bottled waters or water dispensers safer than tap water?
Access to safe drinking water is a fundamental human right and essential for public health. Despite rigorous treatment of public water systems, including coagulation, flocculation, sedimentation, filtration, and disinfection, consumers increasingly turn to alternatives like bottled water and WD machines, seeking perceived health benefits, convenience, and better taste.
WDs, commonly known as water coolers, dispense drinking water and are classified as point-of-use (POU) systems and bottled WDs. POU systems are directly connected to tap water, and bottled WDs use replaceable bottles. Typically, WD machines incorporate filtration technologies like reverse osmosis, carbon filtration, and UV sterilization to purify water and remove unwanted tastes or odors.
Despite these filtration efforts, many studies reviewed have reported sporadic contamination within WD systems, raising concerns about microbial contamination originating from the dispensers themselves. Such contamination may increase the risk of waterborne disease, particularly for vulnerable populations. Across comparative studies, WDs were frequently found to be more contaminated than the tap water supplying them, and water quality often deteriorates over time with regular use.
Compared with tap water samples, scientists have detected approximately 1,2 times more bacterial contamination in carbon filters in POU devices. These bacterial contaminants tended to regrow in WDs even after treatment and flushing. Although most research recommends regular maintenance and cleaning, microbes frequently recur within days, challenging the assumption that well-maintained WDs are consistently microbe-free.
Researchers identified biofilms as the primary contamination source, defined as structured microbial communities that build up in machines and continuously release planktonic cells and by-products into the water. Multiple international studies conducted in Turkey, Thailand, Iran, Canada, Italy, and Malaysia found that WDs were, in many cases, more contaminated than their tap water sources. These studies observed that heterotrophic plate counts and aerobic bacteria were higher in dispensers than in tap water. Pseudomonas aeruginosa has also been detected in WD systems.
Taken together, WDs are not necessarily safer than tap water, and their safety depends strongly on system design, maintenance practices, and frequency of use, with regular maintenance remaining essential to reduce microbial colonization and regrowth.
Assessing the quality of drinking water
The U.S. Environmental Protection Agency (EPA) uses microbial indicators, including Fecal Indicator Bacteria (FIB) and coliforms, to assess drinking water quality and regulatory compliance. Total coliforms are used as indicator organisms that may signal fecal contamination or system integrity failures, and their presence can suggest the possible presence of pathogens, such as Salmonella, Shigella, and Vibrio cholerae. Municipal water districts use coliforms to measure treatment efficiency and distribution system integrity.
Approximately 20 % of U.S. water samples and 80 % of Malaysian water vending machines (WVMs) were reported to be contaminated with coliforms due to filtration failures. Pseudomonas aeruginosa was included as a water quality indicator due to its serious health implications, particularly for immunocompromised individuals.
Heterotrophic plate count (HPC) bacteria, classified as oligotrophic or opportunistic microorganisms, indicate the overall microbiological quality of drinking water. The EPA recommended that HPC bacteria not exceed 500 CFU/mL, primarily to limit interference with coliform detection, though the World Health Organization (WHO) stated coliform testing was a better indicator. Despite ongoing debate over their reliability as direct health-risk indicators, the EPA and water regulatory bodies worldwide continue to use HPCs as supplementary indicators of microbial water quality.
Health implications of WD contamination
Studies conducted in Sweden and other Nordic countries have linked drinking water contamination to gastrointestinal illnesses and waterborne outbreaks. Research on soda fountain machines estimated that 17 % of bacteria, including Chryseobacterium meningosepticum, Klebsiella, Staphylococcus, Stenotrophomonas, Candida, and Serratia, were opportunists that may cause gastrointestinal infections, particularly in immunocompromised individuals.
Colonized biofilms in plumbing fixtures can serve as reservoirs for microorganisms associated with endocarditis, folliculitis, keratitis, cystic fibrosis, osteomyelitis, and septicemia. Opportunistic pathogens generally cause infection when host immunity is compromised.
Waterborne bacteria pose elevated health risks to children, the elderly, and immunosuppressed individuals. Previous research indicated that 23 % of Malaysian WVMs harbor Pseudomonas aeruginosa, which colonizes WDs persistently via biofilm formation. Contaminated drinking water dispensers have also been implicated as potential sources of Pseudomonas fluorescens in hospital settings.
Multiple studies have indicated that HPC bacteria may be more harmful than previously thought under certain exposure conditions. Researchers have reported elevated HPCs in WDs, including 62 % of Iranian samples, 100 % of U.S. samples, with 73 % above 500 CFU/mL, 32 % of U.S. WVMs, 84 % of UK WVMs, and 87 % of Brazilian samples exceeding the 500 CFU/mL threshold.
HPCs above the recommended thresholds indicate that WDs can serve as potential breeding grounds for bacteria, including pathogenic microorganisms that may severely impact public health, particularly when monitoring and maintenance are inadequate.
Recommendations to improve drinking water quality
To improve water quality and mitigate microbial contamination, the authors recommend implementing measures that inhibit biofilm formation, such as internal system lines made from biofilm-resistant materials or coatings containing nanoparticles that discourage bacterial attachment and growth.
Adoption of novel technologies should be accompanied by compliance with regulatory safety standards, long-term stability testing, and certification to confirm that no harmful nanoparticle migration occurs during operation. Additionally, periodic disinfection protocols, such as hydrogen peroxide application, could enable short-term but effective reduction in microbial populations in microfiltered dispensers.
Regular maintenance, routine cleaning, and user hygiene practices are critical in maintaining water quality. Without consistent upkeep, even systems equipped with advanced filtration technologies are prone to contamination, highlighting the authors’ conclusion that technological upgrades alone are insufficient without sustained oversight and maintenance.
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