I received an e-mail from an industry leader in water conservation asking about a recent column on Legionnaires Disease. The column mentioned that water conservation efforts were contributing to Legionella incidents without explaining how water treatment chemicals (Chlorine) dissipates in the municipal and building water systems.
He asked me to explain how this happens, so I thought it would be best to explain it to everyone in this column. When low-flow fixtures and water conservation efforts are enacted, the flow of water is reduced, but the water mains must still be sized to handle fire flows. The reduction in flow causes the water stay in the piping system three to four times longer than prior to 1992, when the energy policy act was enacted.
The slower water flow allows water treatment chemicals to dissipate to lower levels by the time the water gets to the remote portions of the water distribution systems. This phenomenon is known chlorine dissipation or some people are calling it “aged water” or “stagnant water.”
The water treatment process
The water treatment process draws source water in from a well, lake, stream or river. A coagulant is added to remove dirt and other particles suspended in water. Chemicals, such as aluminum and iron salts, are added to the raw water to form tiny sticky particles called “floc” known as a flocculation process. Flocculation acts as a magnet and attracts other dirt particles.
The combined weight of the dirt and flocculants become heavy enough to sink to the bottom of settling basins during the clarification process. During the clarification process, the heavy particles (flocculants) settle out in settling basins and are removed and the clear water moves on to sand filtration beds. Filtration occurs when the water passes through layers filters, that consist of some made of layers of sand, gravel and activated carbon, which help remove even smaller particles.
The next step is the disinfection process, where a small measured amount of chlorine, monochlorine or Chlorine Dioxide is added to kill any bacteria, amoebas and other organic pathogens in the water system. After the Chlorine or other water treatment chemical is added, the water is transferred to a holding tank or basin to provide ample contact time for disinfection to occur. Most municipal water systems have large storage tanks, where treated water is stored until it is needed. When water is needed it is pumped through pipes to homes and businesses in the community. Distribution tanks and pumping stations throughout the system provide storage of water and stable pressure to enhance reliability of water service and provide ample volume for fire protection.
The most common disinfectant or water treatment chemical is Chlorine. Other water treatment chemicals used less often are monochlorines and chlorine dioxide. Monochlorines are like the distance runner with a single shot rifle hunting bacteria, they dissipate at a slower rate than Chlorine, so the chlorine residuals are there for a long period. But, they oxidize as a slow pace so they can only kill bacteria at a slow rate. Monochlorines can be overwhelmed if there is a disturbance like water main construction, water hammer or a fire flow event in the piping system that causes a release of a large dose of scale or biofilm containing a lot of legionella bacteria.
Chlorine Dioxide is like the sprinter of the water treatment chemicals carrying an “uzi” machine gun to attack bacteria. The Chlorine Dioxide oxidizes rapidly and will run out of ammunition that is used to fight bacteria faster than Monochlorine and Chlorine. Chlorine Dioxide, however, will oxidize (deplete its ammunition) rapidly and be ineffective over long periods of time because it dissipates much faster than other water treatment chemicals.
Chlorine has been the most popular water treatment chemical over the years and it is like a middle distance runner of water treatment chemicals with an assault rifle. It is fast and very effective at killing bacteria and other contaminants. The things that affect the dissipation rate of all of these water treatment chemicals are the PH of the water, the number of contaminants present, the piping material, the temperature and time.
Chlorine dissipation rate
Chlorine typically must be above 0.5 parts per million (ppm) of free chlorine in order to be effective against Legionella bacteria and other organisms in the water supply. You should try to maintain a minimum level of 1 ppm of free chlorine in all parts of the building water system in order to have a safety factor for killing bacteria in the water system. When the water leaves the water treatment plant, the Chlorine levels are generally between 2 ppm and 4 ppm.
The quality of the water, piping material, temperature and flow rate (which is a factor of time) all combine with the water treatment chemicals and cause it to react and oxidize as long as it can until it gets to the point it can no longer be effective at fighting bacteria. This is referred to as the rate of dissipation.
Prior to1992 water conservation efforts, the rate of dissipation from 4 ppm to 0.5 ppm could occur as quick as 1-½ days in galvanized piping systems. The rate of dissipation from 4 ppm to 0.5 ppm could take 10 to 12 days in PVC or lined cast iron piping systems.
For unlined cast iron piping systems, it takes about 4-½ days for the chlorine to dissipate from 4 ppm down to 0.5 parts per million. Given the recent water flow rate reductions and fact we must maintain the water mains sizes to accommodate periodic fire flow rates from fire hydrants, the amount of time that water is held in the water mains and distribution piping is exceeding the time required to maintain water treatment chemical levels that will be effective at fighting off contaminants like Legionella, Cryptosporidium, E.coli and many other organic contaminants.
Since 1992, shower flow rates have gone from 5 to 7 gallons per minute (gpm) down to 1.75 gpm to 2 gpm. Water closet flow rates have gone from 3.5 to 5 gallons per flush (gpf) down to 1.28 gpf to 1 gpf. Previously, water would take about three to four days to flow from the water treatment plant to the farthest outlet. Now, on average it takes about 12 to 16 days. This is why many water utilities are switching to monochlorines, which are not as effective when there are large doses of scale and bacteria introduced to the piping system.
Minimum disinfectant levels to effectively control Legionella bacteria
Chlorine is the most common disinfectant used by municipal water departments for treatment and disinfection of domestic water distribution systems. The water utilities have a goal to produce domestic water or drinking water that has a minimum residual disinfectant level between two to four parts per million for Chlorine or Monochlorine. For Chlorine Dioxide, they will generally have a maximum of 800 parts per billion.
The minimum level of Chlorine Dioxide must be monitored because it dissipates faster than Chlorine and Monochlorine. As the water flows through water distribution mains the Chlorine reacts with the contaminants in the water and the walls of the pipe and the heat in the environment and the Chlorine will dissipate as it reacts with these conditions. Chlorine will also dissipate as time passes when the water sits still, and faster as the temperature rises.
The heat effect on the Chlorine dissipation rate is why hot water systems at temperatures below 124°F are more susceptible to bacteria growth than cold water systems. Disinfectant levels should be monitored at the ends of water distribution systems to assure disinfection levels are being maintained. If a facility is located in a remote portion of a water distribution system, it is possible that the disinfectant levels could be non-existent or below the minimums recommended to control bacteria and pathogens in the water system. The increase in water conservation efforts and reduction in water flows has caused a decrease in disinfection chemicals in some remote water distribution systems.
OSHA design recommendations
The Occupational Safety and Health Administration (OSHA) gives advice on controlling Legionella in building water systems. OSHA misquotes the type of mixing valves that should be used at delivery points. hey need to clarify the language to address “temperature limit stops” and “temperature limiting valves” instead of “pressure-independent, thermostatic valves.” The plumbing codes clearly outline what is acceptable for scald protection.
OSHA also states that point-of-use water heaters can eliminate stagnation of hot water in infrequently used lines. This claim is not scientifically based. There is no evidence that point-of-use or tankless water heaters perform any better than any other type of water heater with respect to controlling Legionella bacteria growth. Actually, it is more likely that certain point-of-use water heaters do not raise the hot water temperature to a disinfecting temperature.
I am aware of instantaneous/tankless water heaters that have tested positive for Legionella bacteria and other pathogens in several outbreaks that I have investigated. These point-of-use water heaters only raise the hot water temperature to the usage temperature instead of to a higher disinfecting temperature. Some instantaneous steam water heater models raise the temperature to disinfecting temperatures and then use a mixing valve to blend the hot water temperature back down to a stable temperature for distribution. Other types of heaters work in a similar way, but the most common heaters do not have excess capacity for disinfection temperatures.
OSHA’s advice on controlling Legionella also implies that tanks can cause Legionnaires’ Disease. It should be noted that it is most commonly storage temperatures not tank size that allow legionella bacteria to grow. There is no scientific evidence of tanks causing Legionella to grow when storage temperatures are maintained properly and tanks are recirculated and disinfected periodically if required. Domestic hot water should be stored at a minimum of 140°F and delivered above the maximum temperature at which legionella grows and multiplies. ASHRAE Guideline 12 has recommended a couple of degrees safety factor and a minimum hot water distribution temperature of 124°F to all fixtures.
Hot water temperatures to all other fixtures will be higher and it can be mixed down to a safe temperature using the proper code-compliant, tempering valve to limit the hot water temperature coming from the fixture outlets for scald protection. OSHA recommends the hot-water tank be drained periodically to remove scale and sediment, and flushed and cleaned with a chlorine solution, if possible. Many water heater manufacturers recommend a mild acid solution be circulated through the heater to de-lime the heater as required in the maintenance instructions. The tank must be thoroughly rinsed to remove excess acid and chlorine before reuse.
OSHA recommends eliminating dead legs when possible, or installing heat tracing to maintain 122°F in the lines. I disagree with heat tracing in general because it allows stagnant water. The better solution would be to use a circulated hot water systems with the entire loop kept above 124°F.
OSHA recommends removing rubber or silicone gaskets which provide a surface for biofilm and nutrients for the Legionella bacteria. Again, I disagree. If the water is maintained with the proper water treatment chemical levels and/or at the proper temperatures to prohibit growth, legionella will not be a problem with gasket materials.
OSHA recommends frequent flushing of these lines to reduce growth. I fully agree here and would like to see automatic flushing systems for schools, sporting facilities, seasonal resorts and other building types where the building could sit unoccupied for long periods of time. I also believe domestic hot-water recirculation pumps should run continuously and they should be excluded from energy conservation measures that shut them off in off peak hours.
OSHA recommends raising the water-heater temperature to control Legionella bacteria growth. They also recommend periodically raising the hot water temperature to pasteurize the hot water system by raising the water-heater temperature to a minimum of 158°F for 24 hours and then flushing each outlet for 20 minutes. It is important to flush all taps with the hot water because stagnant areas can "re-seed" the system. Exercise caution to avoid serious burns from the high water temperatures used in the heat-and-flush or Pasteurization process. This heat and flush method of Legionella control should be done under controlled conditions to prevent scalding.
OSHA recommends periodic chlorination of the system storage tanks to produce 10 ppm free residual chlorine and flushing of all taps until a distinct odor of chlorine is evident is another means of control. Chlorine monitors and In-line chlorinators can be installed on the building water service lines. It should be noted that chlorine is quite corrosive and can shorten the service life of plumbing components if the chlorine levels are too high. Control of the pH is extremely important to ensure that there is adequate residual chlorine in the system.
Alternative means to control Legionella growth include the use of metal ions such as copper or silver (which have a biocidal effect) in solution. Ozonization injects ozone into the water. Ultraviolet (UV) radiation also kills microorganisms. Commercial, in-line UV systems are effective and can be installed on incoming water lines or on recirculating systems, but stagnant zones may diminish the effectiveness of this treatment. Scale buildup on the UV lamp surface can rapidly reduce light intensity and requires frequent maintenance to ensure effective operation.
OSHA recommends domestic cold water systems maintaining cold-water lines below 20 degrees 68°F to limit the potential for amplification of the bacteria. Elevated levels of Legionella have been measured in ice machines in in many building types including hospitals. Ice machines are often implicated in Legionnaires Disease outbreaks because the cold-water lines supplying the ice machines are typically long flexible copper pipe coiled behind the ice machine to allow for the ice machine to be removed for cleaning and maintenance.
ASHRAE 188 Standard
As you may have heard ASHRAE has recently published the ASHRAE 188 Legionellosis: Risk control for building water systems. At the recent ICC Hearings, I proposed adoption of the ASHRAE standard for the design and installation of building water systems in the plumbing and mechanical codes. Not surprisingly, the standard was turned down for adoption because there were many people that did not know what is in the new standard and there was the fear of the unknown. It will be proposed again in future code cycles and by then everyone should have a better understanding of what the standard covers. Until then it is still the industry standard for the control of Legionellosis associated with building water systems.
As water and energy conservation efforts continue to reduce water flows and redirect water for reuse and reclaimed water systems we will undoubtedly see an increase in contamination of our potable water/drinking water supplies. We need to be aware of the delicate balance between reduced water flows and increased bacteria growth from stagnant water and reclaimed or reuse water systems.
Ron George, CPD, is president of Plumb-Tech Design & Consulting Services LLC. He can be reached at: office 734-322-0225; cell phone 755-1908; and website www.Plumb-TechLLC.com.