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Since the discovery of a new pathogenic bacterial species in 1976 people have been seeking to blame someone, anyone, for the problems allegedly associated with Plumbing and HVAC systems. Naturally, the designer of that total system is the bull's eye for the accusatory arrows however the plumber who installed it is likewise a candidate. This article hopes to add insight,in the hope that system designers and plumbers will come to grips with the situation and design problems away, if they can.
I will not regurgitate the history of Legionella and problems associated with it, this is not the objective, rather I want to focus on two causative aspects, BIOFILMS and METALLOTHIONEINS. If you want to know about Legionella (where have you been since 1976?) visit my web page for the low down at no charge. It is important for all concerned to understand the problem, for understanding a thing, is the first step in solving or addressing the problems associated with it.
BIOFILMS
This issue has been covered, albeit briefly, in many non-scientific or 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 internals. Let's delve into WHY and HOW it forms as well as its composition.
NEW Pipe Internals
While many surfaces appear smooth and slick as glass to the naked eye, under the microscope there are microscopic hills and valleys depending on the machinery used to manufacture the pipe as well as its age. This is not a condemnation of the manufacturing equipment, it is the nature of the beast. The valleys formed can be as shallow as allowed by 320 grit eletropolishing, about 0.25 microns (um), or as deep as 180 grit allows, 1.5um (See FigureA). The 180 grit is the finished used in pharmaceutical purified water piping. In the Plumbing and HVAC world, who would specify an internal finish of 320 or 180 grit on black iron or copper pipe? The client could not afford it, so we specify or use an "equal", extruded Polypropylene, PVDF or PVC as it is very close to the 180 grit, if code allows, right? I know that GRIT finish is old terminology and that a Profilometer is used to express the surface profile, but most plumbers can understand sandpaper or emery cloth. Why is finish important?
BACTERIA SIZE
Bacteria are small entities, Legionella Bacteria (LB) being about 0.3um by 1.0um, thus one can appreciate that pipe finish can have an impact on deposition of LB within those valleys. Now we get to flow dynamics, the most important issue in LB proliferation. Laminar Sublayer Thickness Before I lose anyone, I ask that you visit a stream or river. Notice how fast the water moves midstream and how very slow it moves close to shore with sections of shore showing absolutely no flow compared to the mainstream. Is it hard to envision the same phenomenon happening within a piping system, albeit on a microscopic scale? This is the Laminar Layer (LL). Table 1 shows that at a flow velocity of 5.0 ft/sec, the maximum that we would want on soft copper pipe used in Potable Hot Water (PHW) systems, the LL for a 2" Sch. 80 pipe would be 60um, at 2 ft/sec it is 158um. So, here we have a liquid layer within a pipe that has little to no flow regardless of the flow at the center of the pipe, and this does not begin to address shut faucets or valves.
The importance of the LL is that NUTRIENTS are deposited therein from themoving torrent above it (See Figure B). Doubt it? Go back to that stream or river and look at the small puddles near the shore. Where does one find dead fish from fast-moving rivers or streams? On shore! The nutrients thus deposited are said to neutralize the surface charge and surface free energy. Copper is a poison, a heavy metal, yet biofilm grows very well on copper pipe. As with nutrients, so it is with bacteria - sooner or later, by chance or Brownian Movement, they will find themselves in this layer. Nutrients? In a PHW loop? Sure, it does not take much to feed bacteria."If only one part per billion [ppb] of organic matter in a 1.0 milliliter (ml) water sample were converted to bacterial bodies (assuming the bacteria to be 20% organic matter and the Specific Gravity of bacteria to be about that of water), approximately 9,500 bacteria, each 1.0 micron in diameter, would be present in a 1.0 milliliter sample" Pittner (1988) (italics mine)
 
BACTERIAL ATTACHMENT
Ok, now we have a stagnant LL and nutrients all we need is one bacteria (not necessarily LB) capable of forming or extruding Extracellular Polymer Substances, EPS. It is not hard to visualize one cell entering this new found paradise, and since most, if not all, bacteria are capable of forming biofilm, and switching on genes to activate HYDROPHOBIC, "water hating", sites on the cell wall. Remember, we are in a "dead zone", the LL thus a starving cell would do well to sit and wait for the water to bring it food rather than spend energy to get its own. Once close to the pipe wall, these bacteria with hydrophobic sites are attracted to the pipe wall and stick to it like a magnet sticks to iron! They are then said to be irreversibly adsorbed. Then they secrete bacterial alginate, thereby cementing themselves to the surface. Once fast to the pipe they begin to extrude the EPS, a tangled mass of polysaccharide fibers that is akin to a spaghetti mass. This EPS functions similarly to the tentacles of a Jellyfish, trapping and holding nutrients within a water slime mass. Once the surface is colonized by this species, other bacteria enter the picture, LB?, as well as protozoans the grazing bacteria eaters. The protozoans are important in that some species are important in LB amplification (population growth).
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