Biogenic Compounds and ATP Measurement as Indicators for Assessing Operational Risk and Biological Stability of Tap Water

This study assessed the quality and physicochemical and biological stability of tap water in the Rzeszow distribution system during the 2023–2024 autumn−winter season. Samples were collected at the water treatment plant and from taps in selected public buildings. The results showed that the water met drinking water standards and was chemically and physically stable (Langelier indexSaturation Index from −0.5 to 0.5; turbidity <0.8 NTU), minimising the risk of scale formation and operational problems. However, complete biological stability was not achieved, as the concentrations of both nitrogen (N > 0.2 mg/L) and phosphorus (>0.01 mg/L) exceeded the recommended thresholds, only biodegradable organic carbon remained low (BDOC < 0.25 mg C/L). This underscores that maintaining full biological stability throughout a distribution system remains a highly challenging task due to nutrient availability. The absence of indicator bacteria (Escherichia coli, coliform bacteria) does not guarantee complete safety, as increases in total bacterial counts were observed in indoor systems. The results underscore the importance of regular and comprehensive monitoring—including ATP and phosphorus measurements—to ensure water safety and protect public health.