LiquiTech Blog

Understanding Heterotrophic Plate Count (HPC) Bacteria in Water Systems 

Heterotrophic Plate Count (HPC) bacteria are naturally occurring organisms commonly found in water systems. While their presence is not unusual, HPC bacteria are not regulated or routinely monitored as contaminants in drinking water due to their high variability and typically elevated concentrations. 

HPC bacteria and human health 

The presence of HPC bacteria in drinking water is a common occurrence. However, it is important to note that these bacteria do not pose direct hygienic (disease-related) significance. Extensive research conducted by experts in the field consistently fails to establish any correlation between HPC bacteria concentrations in drinking water and threats to human health. These findings provide strong evidence that consuming water with HPC bacteria does not jeopardize our well-being. 

Nonetheless, the detection of HPC bacteria in a drinking water system can serve as an indicator of potential underlying issues. These issues may include inadequate treatment, poor maintenance of the distribution network, or elevated contamination of water bearing equipment. It is crucial to ensure the proper monitoring of water quality and take prompt corrective actions when necessary. While HPC bacteria themselves are not harmful to humans, changes in HPC concentrations may indicate changes in water quality. Therefore, it is essential to remain vigilant in evaluating complex microbial systems. 

HPC bacteria and Legionella 

Heterotrophic plate count bacteria and Legionella are bacteria that are commonly found in water sources. Legionella is a genus of bacteria that can cause Legionnaires’ disease, a severe form of pneumonia. HPC bacteria, on the other hand, are a group of bacteria that are commonly used as indicators of water quality. Though high levels of HPC in water systems can serve as an indicator of elevated microbial activity, it is not an indicator for Legionella itself. Monitoring and controlling the levels of HPC bacteria can help understand changes in water quality that potentially could impact the risk of Legionella contamination and subsequent health risks. 

Role of HPC monitoring in water treatment 

HPC monitoring plays a vital role in evaluating the effectiveness of municipal water treatment processes. Water providers strive to achieve HPC levels below 500 colony forming units per milliliter (cfu/mL) after treatment to minimize any potential interference with culture-based coliform testing, ensuring accurate and reliable results. With advancements in testing methods, these innovative approaches have successfully mitigated elevated HPC concentrations, thus eliminating any potential impact on coliform test outcomes.  

Variability of HPC concentrations 

HPC concentrations vary significantly across different water systems. Comparisons between various sources, such as New Jersey drinking water systems, dental water lines, hospital hot water, and Michigan lakes, highlight the diverse ranges in HPC presence. 

Water Source  HPC (cfu/mL)  Reference  
NJ drinking water system  320 to 1,000,000,000  LeChevallier et al., 1987  
Michigan lakes  3,000 to >100,000  Jones et al., 1991  
Tuscon, AZ tap water  >3,000  Pepper et al., 2004; Chaidez and Gerba, 2004  
Apartment building  300–300,000  Bagh et al., 2004  
Administration building hot water  24,700–144,000  Sheffer et al., 2005  
Hospital hot water  60,000  Sheffer et al., 2005  
Dental water lines  >3,000  Rice et al., 2006  
Research building  16,000 (average)  Siebel et al., 2008  
New university building  >10,000   Nguyen et al., 2008; Nguyen et al., 2012  
Hospital hot water  8,000–27,000  Zhang et al., 2009  
Hospital hot water  2,900 (average)  Duda et al., 2014  
New office building  1–100,000 cfu/cm2 (biofilm)  Inkinen et al., 2014  
Commercial buildings 3 – 2,100,000 Pierre et al., 2019 

Case Study: Managing HPC bacteria challenges 

A case study from a California hospital in 2016 demonstrates the practical management of HPC bacteria. While preparing to open a new patient tower, the hospital found bacteria levels exceeding state and federal EPA limits in the building water system, necessitating intervention. By implementing copper-silver ionization to continuously disinfect the water system and sediment filtration to remove incoming sediment, the hospital successfully controlled bacterial levels and reduced corrosion damage to the plumbing system. 

Heterotrophic Plate Count Bacteria,HPC Bacteria

While HPC bacteria are common in water systems, they do not pose a significant health threat nor serve as reliable indicators of other waterborne pathogens. Routine HPC monitoring primarily serves to track changes in water quality and treatment efficacy, rather than for health risk assessments. 


Allen 2004: Heterotrophic plate count bacteria—what is their significance in drinking water?   

Duda 2015: Lack of correlation between Legionella colonization and microbial population quantification using heterotrophic plate count and adenosine triphosphate bioluminescence measurement   

Pierre 2019: Water Quality as a Predictor of Legionella Positivity of Building Water Systems   

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