Biofilm formation inside water distribution systems

Biofilm formation inside water distribution systems

A biofilm is a surface deposit of bacteria, other microorganisms, and organic and inorganic materials that accumulate within a slime layer. Biofilms can form on solid and liquid surfaces when nutrients and water are present. Biofilms can form inside drinking water distribution systems and can sometimes cause a number of problems.

Biofilm Formation
Distribution systems are complex environments that can provide many opportunities for biofilm development. This development may occur fairly rapidly or slowly, sometimes over a period of years. However, clean pipes (especially metal pipes) are not initially attractive surfaces for bacteria.

Bacteria are typically the first microorganisms to colonize pipe surfaces. Once enough organic material adheres to the pipe surface — a process referred to as “conditioning” — bacteria can begin to attach.

Once the bacteria reach a critical density, they begin to produce a gelatinous substance that gives biofilms their characteristic slimy nature. This slime layer makes up the majority of the weight and volume of the biofilm. After the slime layer forms, a veritable micro-ecology can flourish. The slime layer helps trap additional organic particles that many bacteria can use for food and energy.

Other microorganisms including viruses, protozoa, algae, fungi, and helminthes may become associated with or entrained within the biofilm. Some protozoa graze on biofilm bacteria creating a food web.

Biofilms provide a number of advantages for attached organisms compared to free-floating (planktonic) organisms. In a low-nutrient environment, it is easier for microbes to let the nutrients come to them rather than to search for the nutrients. The slime layer allows metabolic byproducts or wastes to accumulate, some of which may be used as food by other microorganisms, forming a cooperative ecology.

The biofilm also protects the inhabitants from the effects of disinfectants — biofilm microbes are many times more resistant to disinfection than planktonic microbes. Biofilm thickness is variable but is usually in the range of 50 to 100 microns. As the thickness increases, pieces of biofilms can shear off, allowing for colonization of downstream sections of the system.

Microorganisms can enter the distribution through two main categories:

• Surviving the treatment process
• Recontamination

Most microorganisms found in distribution systems biofilms are also found in the system’s source water. They may survivedue to ineffective treatment such as filter breakthrough or ineffective primary disinfection. However, even effectively treated water contains some bacteria in small numbers. Potable water is not sterile.

Assuming water of good quality enters the distribution system, there are still numerous ways the water can be contaminated, including cross connections and back flow. Leaking pipes, joints, and valves can also allow for the entry of microbes, especially during temporary periods of negative pressure.

Poorly designed or maintained finished water reservoirs and tanks can allow for recontamination if birds and other animals, including humans, have access.

Repairing and replacing distribution system components also allows for the introduction of microorganisms if care is not taken to disinfect repaired or replaced mains and tools introduced into the system, such as mobile cameras.

Some biofilm organisms can also accelerate the corrosion of some types of pipes. Iron-oxidizing bacteria oxidize iron and steel, depositing iron oxides (rust) in raised deposits called tubercles. Sulfur-oxidizing and sulfurreducing bacteria produce sulfuric acid and hydrogen sulfide, respectively, which can cause pitting of pipe surfaces. Corrosion products, such as iron oxide sediments and tubercles, provide additional habitats and attachment sites for other biofilm organisms.