A Growing Problem: Legionella in Water Systems
Our Specifier's Notebook from September explores why eliminating Legionella from water systems can prove to be a daunting task.
Since the discovery of a new pathogenic bacterial species in 1976, people have been seeking to blame someone, anyone, for the Legionella problems associated with plumbing and HVAC systems. Naturally, the designer of that total system is in the bullseye for the accusatory arrows. It is important for the consulting engineer to understand the problem and some of its causative aspects. Two of the most interesting-and difficult-issues involve biofilms and Metallothioneins.
Formation of biofilms
The presence of biofilms has been covered in many trade articles without a focused explanation of its significance. A biofilm is much more than a 'slimy' feel in the sump or debris wiped off by a 'swab' from plumbing.
The most important issue in biofilm and Legionella bacteria (LB) proliferation is flow dynamics . Just like any river, every length of pipe has a laminar layer (LL): an area at the surface where water flow slows to nearly a standstill. At a flow velocity of 5.0 ft./sec., the LL for a 2-in. Schedule-80 pipe would be 60 micrometers (um); at 2 ft./sec. it is 158 um. Compare this to LB, which are about 0.3 um by 1.0 um.
The significance of the LL is that nutrients are deposited therein from the moving torrent above it. With a stagnant, nutrient-laden LL, all that is needed is one bacteria-not necessarily LB-capable of forming or extruding extracellular polymer substances (EPS), which most bacteria have the capability to do.
The EPS, a tangle of polysaccharide fibers akin to a spaghetti mass, traps and holds nutrients within a mass often called a 'slime city.' The slime city becomes an interactive cooperative, attracting different bacteria and protozoans within its walled enclosure, all working together for mutual benefit. The EPS and the watery slime binds them together, with the height limited only by the LL and available nutrients.
Biofilms form through colonization by different bacteria, thus the species with the largest population of initial cells will dominate, followed by a cycle of colonization by the largest bacteria or those that eat the most. Eventually the population within a biofilm will grow to a stage where either a massive die-off will occur (unlikely) or a 'bloom' will happen. The bloom will release the contents of a biofilm into the water flow via erosion, sloughing, biological releases or chemical changes.
The formation of biofilm places a formidable task before any biocide, whether oxidizing or nonoxidizing. Before it can reach the inhabitants within, any biocide must have a means to infiltrate the nonmoving LL. Also, if it cannot penetrate the slime proper, it cannot effect a kill. This is why lab tests of various biocides are often doomed to failure: they do not represent the real world. To infer that any treatment is effective on LB based on flask tests of free swimming lab-grown organisms is misleading.
As for heavy-metal attacks-copper/silver technology for potable supplies and organo-tin or copper for open circulating systems-the EPS has the ability to tie up heavy metals and protect the inhabitants within. This is why EPS is useful in sewage-treatment plants.
Also, Metallothioneins (MT) , which are cell constituents, can tie up, extract and remove heavy metals before they can have an adverse effect on the cell proper. All cells have MT manufacturing capabilities, thus when a heavy metal is used to kill, only those cells with low MT generating capacity are killed, effecting a process of natural selection. Eventually the bacterial population that remains is 'immune' to the dosage of that metal, with some bacteria having the ability to transfer that 'immune trait' to other cells.
Putting knowledge to use
The proliferation of bacteria, and specifically Legionella, in water systems can be a daunting problem for the consulting engineer. Unfortunately, the additional issues surrounding biofilms and MT just exacerbate this problem. Any effective treatment program will combine what is known about the LB with a knowledge of how they live within a plumbing or HVAC system.
For the author's advice on treatment options for potable-water and open-circulating systems, click here .