Evaluating aerobic endospores as indicators of intrusion in distribution systems

Aerobic endospores are naturally found in soil, easy to measure, and more resistant to chlorine than bacterial indicators are. The objective of this study was to assess whether aerobic endospores could be used as an indicator of intrusion in a full‐scale distribution system. The background aerobic endospore concentration in distributed water was low (average of 0.13 cfu/100 mL). Pipe deposits and biofilm sloughing were not found to be significant sources of endospores; concentrations in 8 of 10 spot‐flushing samples were not significantly different from those in distributed water. Only 5 of 71 water samples collected during flushing after main repairs showed endospore concentrations that were higher than they were during spotflushing. These observations suggest that aerobic endospores can be used as indicators of intrusion in distribution systems and could be useful to assess the adequacy of maintenance practices, identify areas of improvement, and manage incidents with risks of intrusion.