Best Practices: Sample Holding Times for Microbial Water Analysis

The Water Testing, Research, and Capacity Strengthening Program (WaterTRACS), supported by the Conrad N. Hilton Foundation, monitors microbial water quality in Ethiopia, Ghana, and Uganda while enhancing the capacity of local laboratories. Researchers often travel long distances to collect samples in rural areas, making holding time a critical issue. This brief reviews the literature on microbial sample holding conditions to guide field logistics.

Sample holding time is defined as the duration a sample can be stored after collection and before analysis without affecting the results. Staying within recommended holding times is critical during water quality testing, particularly for indicator bacteria (e.g., total coliform, fecal coliform, E. coli), as holding time and temperature can affect their survival.

To ensure the most accurate data are generated, microbial water samples should always be analyzed as soon as possible. No universal recommendation exists, but common methods specify analysis within 8 hours for non-potable water samples, and within 30 hours for drinking water samples when stored below 10 °C (APHA, 2018; USEPA, 2016). The World Health Organization recommends analyzing drinking water samples within 6 hours of collection, and up to 24 hours maximum when stored at 4-10°C (WHO, 2003). The risk of exceeding holding times is that microbial concentrations decline (Thapa S. et al., 2020).

Despite recommendations to analyze non-potable or surface water samples within 8 hours, studies such as Pope et al. (2003), Selvakumar et al. (2004), and Thapa et al. (2020) support utilizing holding times longer than 8 hours for microbial analysis when necessary.

The study by Pope et al. (2003) included 24 sites across the United States, 11 laboratories, and four commonly used methods to enumerate E. coli in surface water. They found most E. coli samples can be processed beyond 8 hours while still generating comparable E. coli results, provided the samples are held below 10°C and are not allowed to freeze. Selvakumar et al. (2004) concluded that fecal coliform concentrations in sanitary wastewater and stormwater samples did not change significantly between 7 hours and more than 24 hours holding time when stored at 4 °C. A review by Thapa S. et al. (2020) suggests that most surface water samples analyzed between 8 and 48 hours after collection will generate comparable E. coli results to those analyzed within 8 hours when stored below 10 °C.