1

Regular cleaning and maintenance of the distribution system are essential to protecting water quality. In particular, a successful unidirectional flushing program will not only maintain acceptable water quality throughout the distribution system but also prevent water quality degradation that often occurs between the treatment plant and the customer’s tap.

BIOFILM GROWTH AND REMOVAL

Distribution systems consist of water mains, storage tanks, hydrants, pumps, and other assorted appurtenances designed to help deliver water to consumers. Microorganisms can enter the system through various means, including water main breaks, maintenance and construction, and storage tanks vented to the atmosphere. Vented tanks allow water levels to rise and fall, but also permit easy entry of airborne bacteria, viruses, and fungal spores in the tank.

Virtually every water distribution system provides a suitable habitat for the growth of microorganisms, with favorable temperatures and pH conditions combined with the presence of natural organic matter (NOM) and nutrients essential for microbial growth. As colonies of microorganisms grow, they can become attached to the surfaces of storage tanks and distribution system piping, and biofilm begins to form. Unlined cast-iron pipe provides an ideal surface for the formation and growth of biofims. Corrosion of the interior surface of the iron pipe consumes free chlorine (intended for the inactivation of the microorganisms), and pitted areas are convenient homes for microorganisms to grow.

Biofilms can range from a few organisms scattered along a pipe section to layers of organisms several hundred microns thick. Control of biofilm growth is essential to maintaining the health of the distribution system. Chlorine residual alone is not effective in eliminating biofilm growth in iron pipe systems; periodic removal of the biofilms is required.

The occurrence of biofilms can be reduced either by mechanical cleaning (pigging or swabbing) or flushing. Pigging is effective in removing biofilms but is expensive and can cause leaks in or damage to older, deteriorated systems. When increased hydraulic resistance in pipes is caused by sediment, loose biofilm, or other mobile obstructions, hydraulic performance can be restored through flushing, but mechanical cleaning is generally better suited to restoring hydraulic performance.