Bacterial survival versus pH for drinking water microbiology Escherichia coli (E. coli) bacteria die off rapidly in water when pH is greater than approximately:

Introduction / Context:
E. coli is an indicator organism for fecal contamination. Its survival is influenced by environmental conditions such as temperature, disinfectant residual, and pH. Extreme pH levels are hostile to bacterial viability.

Given Data / Assumptions:

• Typical drinking water pH ranges from about 6.5 to 8.5.
• Question asks for a threshold above which E. coli die off quickly.

Concept / Approach:
Most enteric bacteria prefer near-neutral pH. At elevated pH (strongly alkaline), cell membranes and enzymatic processes are disrupted, causing rapid inactivation. Empirical guidance in waterworks practice recognizes pH above roughly 9.5 as strongly bactericidal (though relying solely on pH control is not a standard disinfection method).

Step-by-Step Solution:

Verification / Alternative check:
Operational experience shows microbial counts drop rapidly in highly alkaline water; however, utilities still apply disinfectants for reliable pathogen control.

Why Other Options Are Wrong:

• 5.5 and 6.5 remain within tolerable ranges for many bacteria.
• 7.5 is close to neutral and supportive of survival.
• 8.5 is only mildly alkaline; not reliably lethal without disinfectant.

Common Pitfalls:
Assuming pH alone is adequate for disinfection. Chlorination or other disinfectants are required for public health protection.

Final Answer:
9.5