Factors affecting bulk to total bacteria ratio in drinking water distribution systems

Area of Science:

• Environmental Microbiology
• Water Quality Management
• Public Health

Background:

• Bacteria in drinking water systems proliferate in both bulk water and biofilms.
• Understanding the interplay between bulk water bacteria and biofilms is crucial for effective water distribution system management.
• Previous research often analyzed bulk water and biofilm bacteria separately.

Purpose of the Study:

• To investigate the impact of chlorine concentration and residence time on the ratio of culturable bacteria in biofilms versus bulk water.
• To determine the conditions under which bulk water bacteria may dominate over biofilm bacteria in drinking water distribution systems.

Main Methods:

• Experimental evaluation of culturable bacteria in bulk water and biofilms under varying chlorine residuals (0-0.7 mg/L) and residence times (up to 48 hours).
• Quantification of bacteria in both phases to calculate the bulk-to-total bacteria ratio.

Main Results:

• Without chlorine, 81% of bacteria were in bulk water; biofilms contained only 19%.
• Increasing chlorine to 0.2-0.7 mg/L significantly reduced bulk water bacteria percentages (28-37%).
• With 0.2 mg/L chlorine, longer residence times (up to 48h) increased bulk water bacteria presence (up to 88%).

Conclusions:

• Biofilm dominance in drinking water distribution systems is condition-dependent.
• Low chlorine residuals and longer residence times can lead to bulk water bacteria becoming the dominant population.
• Findings suggest that bulk water bacteria may be more prevalent in certain parts of distribution systems with limited chlorine.