Updated on 6/24/19
New York City building owners should consider six key safety precautions in complying with health commissioner Dr. Mary T. Bassett’s August 6th order to disinfect their cooling towers:
1. Perform “online” rather than “offline” disinfection unless full physical cleaning is absolutely necessary at the time. Cooling towers can be disinfected “online” or “offline.” The online procedure involves only chemical disinfection while the cooling tower is operating. The offline procedure involves shutting the cooling tower down and scrubbing it physically in addition to disinfecting it with chemicals. A full offline disinfection and cleaning is more thorough but is expensive and presents some serious risks, as discussed below.
Although the online method does not address the physical cleaning of the cooling tower, it is effective in killing Legionella and is much safer, cheaper, and quicker. It is also less corrosive. A typical online disinfection basically amounts to maintaining a free chlorine concentration of approximately 5 parts per million (ppm) for at least 6 hours. More information on the method can be found in the Cooling Technology Institute’s publication Legionellosis Guideline: Best Practices for Control of Legionella, which can be downloaded free at Cooling Technology Org.
If you use the offline method:
2. Perform online disinfection of the cooling tower prior to cleaning it physically. Otherwise, the Legionella risk could get worse before it gets better. Physically cleaning a cooling tower without disinfecting it chemically beforehand is like aggressively sweeping a dirty garage floor–you might eventually get it clean but in the short term you’ll inhale a lot of dust. Hence precaution #4 below.
3. Provide personal protective equipment for the workers. Respirators will reduce the risk of inhaling Legionella bacteria. Boots, protective clothing, and goggles will help protect their eyes and skin from chemicals.
4. Close building outdoor air intakes within the vicinity of the cooling tower. The Centers for Disease Control and Prevention (CDC) has recommended closing intakes within 30 meters.
5. Do it properly. Check with organizations such as the Cooling Tower Institute and the American Society of Heating, Refrigerating and Air-Conditioning Engineers for their latest recommendations. Follow instructions on chemical product labels. Keep records of the procedure, including the chemicals used, the time the chemicals were added to the system, and chlorine and pH test results. Be sure to follow all EPA and other applicable regulations and observe safety precautions. Utilizing a highly qualified and experienced water treatment specialist is the best way to ensure proper protocol is followed.
6. With either method – online or offline – understand the long-term limitations of the procedure. As pointed out in “Five Steps New York City Building Owners can take to Reduce the Risk of Legionnaires’ Disease,” studies have shown that Legionella bacteria may reemerge within only days after fully disinfecting a cooling tower. A program of regular water treatment, maintenance, and inspections provides better protection than periodic disinfection. The same applies to plumbing systems, hot tubs, and other water systems that can harbor and transmit Legionella — the best protection is to implement a water management plan per ANSI/ASHRAE Standard 188.
Update: In 2016, both New York City and New York State established permanent laws for registering and maintaining cooling towers to reduce the risk of Legionella. New York State requires hospitals and nursing homes to maintain all water systems, not just cooling towers, to minimize Legionella.
Do you have additional safety recommendations? Please comment below.
Based on my limited reading on the subject , it is my understanding that the Water management plan per ANSI/ASHRAE Standard 188-2015 does not lay emphasis on regular sampling and laboratory analysis to ascertain LBC counts. While the steps mentioned above are of utmost importance to mitigate risk factors , ultimately the efficacy of either a bio flushing on-line/offline program and/or routine chemical and BCDMH cum isothiazoline program can only be ascertained by scheduled sampling and analysis . It is true that LBC testing may offer varied results from a single site at multiple locations even from samples taken simultaneously,however this factor does not in any way reduce the importance of water sampling and analysis on a regular basis .
Mahesh
Mahesh: Although ANSI/ASHRAE Standard 188-2015 does not require testing for Legionella, it does require validation that the water management plan is controlling the hazard (Legionella). Testing for Legionella, if done properly, provides the most direct feedback on Legionella control measures.
4. Close building outdoor air intakes within the vicinity of the cooling tower. The Centers for Disease Control and Prevention (CDC) has recommended closing intakes within 30 meters.
Does the CDC mean permanently ?
We have a situation where a smaller tower from an annex is less than 10 meters from the Main air intake to our building (36 story, air intakes on 3rd & 35th flrs., tower in question is next to 3rd flr. intake)
John: No, the 30 meters pertains to the time during which the tower is being cleaned. That’s because Legionella can be released into the air when biofilms and other deposits are disburbed during the cleaning process. However, if feasible it would be best to permanently site cooling towers more than 30 meters from intakes.
Hello Matt,
As you know I am pushing for the removal of matrices that can protect microbes from disinfection. https://youtu.be/C3_MycQnwFA & other public venues. In my view, it is critical that these be removed prior to disinfection-decontamination. In my opinion, these deposits are very frequent in commercial & institutional buildings.
If one wants to verify if they are present then all that needs to be done is insert a camera with a plumber. If not then acid clean.
These deposits are frequently calcium carbonate which can be removed simply by adding citric acid at a rate of about 10%. First, measure the volume of the system by adding table salt. I usually go with 1 to 10 kg depending on the size of the tower. Do a chloride test prior to addition, circulate in every parts of the system including stand-bys, circulate for approximately 3x times the required duration to circulate the volume of the system. Usually about 15-30 minutes is enough. Make sure the blow down is closed during this procedure. If this is too long a duration then do a pre-bleed to lower the conductivity prior to adding the salt. Do a chloride test after.
%acidity can be measured by a simple drop test with phenolphtalein such as the ones sold by AquaPhoenix (training video on youtube) & Taylor. If it goes down below 6% then more acid needs to be added. When the %acidity is stable during 3 consecutive measurements 15 minutes apart then the acid clean is over & flushing begins. Most often, the acid clean will release a considerable amount of sediments. If this is the case the system will need to be flushed a couple of times offline or a couple of hours online.
When you inject the biocide you always want to co-inject a biodispersant. It can be a specialised molecule such as BCP-1015 as sold by AMSA or it can be a commodity product such as d-lemonene or GEEB.
You can verify that the biodispersion is actually doing something by taking ATP meter measurements. That measurement can also be made during the acid clean.
If the system is in stainless and it is a first decontamination then it is possible to use non-oxydizing technologies such as DNPA and/or glutaraldehyde. Both act very quickly and you will get over 6 logs disinfection within a few hours. An Aqucar formulation is available from Dow that combines the biocide with a compatible biodispersant. IF an oxidizing biocide is applied such as sodium hypochlorite it is wise to inject a high concentration of triazole prior to starting the disinfection. As a matter of fact, it is useful to add this also prior to the acid clean. this will protect the expensive copper tubes of the heat exchanger.
If the deposits include iron oxidation by-products than a stronger acid such as phosphoric acid 85% need to be used. The liquid can be injected quickly by using a venturi type of devices such as those sold at Home Depot for pool owners. Aim for 10% w/w again.
If you want Matt I would be delighted to give you names at the above cies to help you with using these products as I would be writing-reviewing a procedure.
thanks.
Thank you, Philippe! I appreciate the information and can follow up with you directly later.
As I submitted earlier, until formal standards are set why not use swimming pool water quality standards that are already in place or at least use them for a guideline..
Hi Nelson. I’m not sure I understand what you mean. Are you suggesting that specific disinfectant concentrations be required for cooling tower water similar to standards and laws for pools and spas?
Presently can find no standards for use of a respirator in an environment when LD bacterium might be aerosolized and present a danger to workers. 3M has a particulate respirator N95 (model 9211-/37193) that is recommended for the prevention of tuberculosis transmission. Their helpline says the mask would also be effective against LD insofar as the two are both transmitted similarly. The packaging says it has a 95% filtration efficiency against sold and liquid aerosols that do not contain oil.
This is not a full face type protection, simply a disposable mouth/nose respirator that we carry for onetime usage.
Any comments on the effectiveness of this mask ? Any suggestions on that may provide better protection? (knowing the full face, cartridge types of masks are more effective)
Robert: I don’t know enough about respirators to offer a recommendation. See Valerie Hensley’s comment about N-95 or better. Also, keep in mind that effective online hyperchlorination prior to physical cleaning may provide sufficient protection. Thanks
As my memory serves CDC used to say 25 ppm for 4 hours, but even that does not guarantee to penetrate the sludges (potential protozoa) in the tower sumps. The bacteria harbors in the sludge and even though you may eliminate LDB in the bulk water that day it will likely be back within a 5-10 days after.
Frank, I agree that one should expect any cooling tower disinfection procedure to provide only short term Legionella control. Thanks
Good basic advice.
I would add that if you are to utilize a specialist to perform your cleaning and disinfection works, undertake due diligence first, check they are well qualified, experienced and competent as the article says but also set up a contract of exactly what you want them to do. Ensure you know what is involved in the procedure to ensure the contractor does it correctly.
Done use cost as a guide as to how good they are, sometimes the most expensive is not the best, just the most expensive. Having your expectations set out at the start will ensure a thorough job is complete.
Finally, keep records of everything you are doing.
Good advice, Haydn. Thank you!
Disinfectant must be flushed through dead legs so they do not reinoculate the system. Common examples include equalizer piping and free cooling coils which are temporarily valved off.
The standard maintenance protocol should include flushing of dead legs. This can be automated in most cases.
Thanks for your comment, Sarah. I would appreciate more information about auto flushing of equilizer lines. Although I have some experience with it, I’d like to know how you recommend doing it. Could you email me directly about it?
Very sensible advice – clear and concise, too. Great job Matt!
Thanks Vera!
closing intakes within 30 meters? Shouldn’t it be in feet? This is for the US.
I probably quoted the 30-meter reference verbatin. 100 feet. Thanks
A biodispersant should be added to the Chlorine as part of the disinfection process. Chlorine in the absence of biodispersant may not effectively penetrate the biofilm where Legionella bacteria propagate.
Thanks for sharing your expertise, Bern!
I have been involved in chlorination of cooling towers since 1985.
I suggest Routine Cleaning & Chlorination Procedure For Cooling Towers
This procedure should be undertaken in the Spring and Autumn of each year.
Routine Cleaning and disinfection should normally be carried out in the following order :
a) Pre-chlorination
The system water should be disinfected using an oxidising biocide such as chlorine, bromine or chlorine dioxide to minimise health risks to the cleaning staff. This is done by adding either sodium hypochlorite solution or chloroisocyanurate compounds available as rapid release tablets to achieve a measured residual of 5 mg/l free chlorine. Sodium hypochlorite solutions typically contain 10 – 12 % available chlorine and rapid release tablets 50 – 55 % available chlorine. Such products should be handled with care and according to instructions given by the supplier. A biodispersant should also be used to dislodge biofilms and enhance the effectiveness of the chlorination.
The chlorinated water containing 5 mg/l free chlorine should be circulated through the system for a period of 5 hours with the fan off, maintaining a minimum of 5 mg/l free chlorine at all times. However, if the system pH value is greater than 8.0, the measured residual will need to be in the range 15 – 20 mg/l free chlorine in order to achieve the required disinfection level. An alternative procedure to provide more effective use of chlorine is to introduce a heavy bleed off for several hours to both reduce the pH of the system water and its chlorine demand, before carrying out disinfection. The system should then be de-chlorinated and drained.
b) Cleaning
Manual cleaning operations can then be carried out, with all accessible areas of the tower etc being adequately cleaned. Where practicable, the packs should be removed at least once a year and preferably every six months. If this is not practicable, it may be necessary to apply supplementary strategies such as side-stream filtration, increased monitoring etc. Accessible areas of the tower and its pack should be adequately washed but cleaning methods that create excessive spray, for example high-pressure water jetting, should be avoided. If this is not possible, the operation should be carried out when the building is unoccupied or, in the case of permanently occupied buildings, windows in the vicinity should be closed, air inlets blanked off and the area that is being water-jetted should be tented. The area should be isolated and consideration also given to other occupied premises in the immediate areas as well as members of the public who may be in the vicinity during cleaning.
Cleaning staff who carry out water-jetting should wear suitable respiratory protective equipment such as a positive-pressure respirator with full facepiece or a hood and blouse. Staff who use this equipment should be adequately trained and the equipment properly maintained.
Adherent scale or other deposits on the tower and distribution system that have not been removed by the above method can be dissolved using chemical descalants carefully chosen to avoid damage to the fabric of the system. If this is not possible, routine inspection and testing of water quality should be particularly thorough.
Finally, the system should be sluiced out until the water going to drain is clear.
c) Post – Chlorination
On completion of the cleaning operation, the system should be refilled and chlorinated to maintain a minimum level of 5 mg/l of free residual chlorine for a period of 5 hours with the fan off. This needs to be checked hourly to ensure that a concentration of 5 mg/l is present for the total period. Again, the use of a biodispersant will enhance the effectiveness of this chlorination. If the system volume is greater than 5 m3, the water should be de-chlorinated, drained, flushed and refilled with fresh water and dosed with the appropriate start-up level of treatment chemicals, including the biocides.
While the maintenance of a continuous minimum residual of 5 mg/l of free chlorine for a minimum period of 5 hours is considered the best practice, if the downtime to conduct such a lengthy operation is not available, some compromise may be necessary. Under such circumstances it may be acceptable to shorten the pre- and post-chlorination times and to increase the free chlorine level, e.g. 50mg/l for 1 hour or 25 mg/l for 2 hours. This should only be done if the operators are trained in this process because, at these levels, there is a greater risk of damaging the fabric of the system. The system should then be de-chlorinated, drained, flushed and refilled with fresh water and dosed with the appropriate start-up level of treatment chemicals, including the biocides.
Before water containing high-residual free chlorine is discharged to drain, it should be de-chlorinated. The usual procedure is to add sodium thiosulphate, sodium sulphite or sodium bisulphite as a neutraliser. The level of free chlorine is determined by testing and the quantity of sodium salt then is calculated.
Emergency Cleaning & Chlorination Procedure For Cooling Towers
Emergency cleaning and disinfection should normally be carried out in the following order:
a) Pre-chlorination
The system water will be disinfected using an oxidising biocide such as chlorine, bromine or chlorine dioxide by adding either sodium hypochlorite solution or chloroisocyanurate compounds available as rapid release tablets to achieve a measured residual of 50 mg/l free chlorine. A biodispersant should also be used to dislodge biofilms and enhance the effectiveness of the chlorination.
The chlorinated water containing 50 mg/l free chlorine must be circulated through the system for a period of 6 hours with the fan off, maintaining a minimum of 20 mg/l free chlorine at all times.
After 5 hours the system will be de-chlorinated and the system drained.
b) Cleaning
Following the pre-chlorination all accessible parts, both internally and externally, will be cleaned.
To minimise any aerosol creation all surfaces will be scraped and washed down by hand with a disinfectant solution containing free residual chlorine at a level of at least 30 mg/l. Particular attention should be paid to the surfaces of the fill pack, within the pond, balance pipes, filters, drift eliminators and spray bars / water distribution system. Blocked spray nozzles should be removed and cleaned. The pond may be cleaned using a wet vacuum cleaner. The cooling tower may be hosed down to remove the smaller remnants in the tower. This should be continued until the water going to drain is clear and the sediment in the sump has been removed.
Note:
a) The area will be isolated and consideration also given to other occupied premises in the immediate areas as well as members of the public who may be in the vicinity during cleaning.
b) Water jetting using high pressure hoses will not be used.
c) Personnel involved in cleaning will be protected by the use of fully pressurised respirators.
d) No persons other than those wearing fully pressurised respirators will be allowed in the immediate vicinity of the cooling towers.
c) Post – Chlorination
On completion of the cleaning operation, the system should be refilled and chlorinated to maintain a minimum level of 20 mg/l of free residual chlorine for a period of 6 hours with the fan off. This needs to be checked hourly to ensure that a concentration of 5 mg/l is present for the total period. Again, the use of a biodispersant will enhance the effectiveness of this chlorination. The water should be de-chlorinated, drained, flushed and refilled with fresh water and dosed with the appropriate start-up level of treatment chemicals, including the biocides.
Before water containing high-residual free chlorine is discharged to drain, it should be de-chlorinated. The usual procedure is to add sodium thiosulphate, sodium sulphite or sodium bisulphite as a neutraliser. The level of free chlorine is determined by testing and the quantity of sodium salt then is calculated.
Barry: Thanks for taking the time to give such detailed recommendations
Firstly – condolences regarding the New York outbreak and all those affected.
Please check out UK guidance document from the Health & Safety Executive – HSG 274 Part 1 – available on-line as a pdf.
Although we still have outbreaks in the UK – this document is probably one of the best available.
Regards
Stuart W (UK)
[email protected]
Thanks for letting us know, Stuart
The reason one would not use swimming pool standards are several. First, no one is going to be immersed in the cooling tower water. Second, most pools are tested and treated more than once a day for safety purposes. This is not going to happen with a tower. Third, the level of chlorine set in a pool would be very corrosive in a tower system. Forth, pool testing does not look at the actual count of bacteria, as suggested by CTI.
Good points Jim. Thanks
I couldn’t agree more with your statement “A program of regular water treatment, maintenance, and inspections provides better protection than periodic disinfection.” Regular water treatment which includes corrosion inhibitors and biocides is the only long term solution to control legionella. Periodic cleaning can be used to remove built up dirt and debris settled in the base of the cooling tower removed by the scrubbing action of the dirty air through the cooling tower. It is more an aesthetic “looks dirty” approach rather than a long term “disinfection”.
Another maintenance item often overlooked is the positioning of the drift eliminators in the cooling towers. If these are correctly fitted, they should prevent aerosols containing the legionella bacteria from escaping the cooling tower in the exhaust air stream, thereby removing the source of the infection even if the bacteria is in the bulk of cooling tower water.
Michael: I agree about the drift eliminators. We include them on a list of items to check during regular inspections.
A N-95 or better respirator must be worn.
Thank you for your comment, Valerie. Is it required by law for cooling tower cleanings in Quebec?
A long term regular water treatment, maintenance, and inspections program is definitely necessary for Legionella control in cooling tower in the long run. Keep in mind, concentrations of the disinfectant and contact time are the two factors for effective disinfection. For Emergency Disinfection after a outbreak, achieving 25 to 50 ppm of
free residual halogen for shock treatment and maintaining 10 ppm free residual halogen for 24 hours is recommended by OSHA guideline.
Thanks Richard!
The NYC law and NY state regulation requires that biofilm be controlled.
The new NYC law states:
28-317.2 of this code.
1. Each inspection shall include an evaluation of the cooling tower and associated
equipment for the presence of organic material, biofilm, algae and other visible contaminants.
New York State Regulations:
4.5 Inspection and Certification
(1) Each inspection shall include an evaluation of:
(i) the cooling tower and associated equipment for the presence of organic material, biofilm, algae, and other visible contaminants;
Chlorine dioxide is the only biocide that will effectively remove biofilm. Chlorine will not. A use of both should be considered in order not to miss this critical area. Legionella hides out in the biofilm.
Thanks for your comment, Louis. Water treaters can weigh in here about methods.
What certifications is NYC looking for in proper disinfection companies. I.e: why can’t I just do the disinfection in house, test levels, fill out a log, and call it disinfected. I know there is a list of companies on the NYC website but nothing states as to why I must use one of those companies.
If you would like to use a company other than one that’s on the list, I suggest you ask the city for approval based on the vendor’s qualifications. You can also ask for permission to do it yourself but I think it’s unlikely that will get approved since there would be no third party verification.
We have been on disinfection and treatment against Legionella in open/closed cooling towers and other systems since 1996 in Spain and Portugal. Spain has mandatory regulation since 1998 to prevente Legionella in cooling towers, humidifcation systmes, hot water…and as far as we know is one of the most restricted in Europe
From our experience I would say that key points to be considered to make an effective open cooling tower disinfection are:
– Continue Ph control (in case of clorination 6,8-7,4)
– Materials protection in order to avoid corrosion on galvanized and copper parts (that them after will feed the bacteria)
– Eliminate biofouling
– Dissasembly of all cooling parts including cooling pad (that most of times should be replaced)
– Blow down for total volume water of the system at least two times
– Control of incoming water
– Not to use a chlorine based product for treatment after disinfection if this was made using chorination (better use a non oxidizing biocide such as silver salts or others)
This should be managed by experienced companies
regards/jmperez@doyaambiental.com
Jose: Thank you for your recommendations.
While ASHRAE, CTI and OSHA have recommendations, the only ones that are mandatory are the State of NY/City of New York edicts. Being that you have to inhale a mist of water containing Legionella to get the disease, what do you recommend as an online cooling tower water treatment to totally prevent Legionella in circulating cooling tower water?
Larry, it may vary with the cooling tower but I suggest starting with CTI’s recommendation of circulating 5+ ppm chlorine for at least six hours. Although the procedure may not “totally prevent Legionella” in the circulating water, it is usually quite effective.
Is it recommended to have a residual of chlorine of 2 mg/L at all times? I know this may damage the cooling tower parts. Is there any alternative for continuous disinfection or it should be punctual? (5 mg/L per 6 hours)
Hi Daniela. Every cooling tower and situation is different so the best solutions will vary. I agree that continuous treatment at 2 mg/L free chlorine will likely be impractical because of damage to the cooling tower. Circulating at 5 mg/L for six hours is typically in response to a Legionella finding or other event rather than recommended for regular treatment. I suggest you contact an experienced water treatment company to find the best regimen for your cooling tower. They will likely recommend automated treatment that doses at certain times or based on ORP.
If the LB will reemerge in towers only a few days after a complete online disinfection, is it prudent to feed small doses of chlorine or bromine daily, as long as the system isn’t corroding? I mean as opposed to a M,W,F schedule with oxidizing biocides and T,T,S with non-oxidizers? If you can maintain .2 – .5 ppm of free chlorine at all times and keep the basins clean, will the LB still be an issue?
Thanks for your question, Jason. Some guidelines have recommended a continuous halogen residual but since each cooling tower and situation is different, I think the microbial treatment details should be left up to the water treatment professional and owner provided that the program is validated with periodic Legionella tests. I cannot say that every tower should have daily chlorine or bromine dosing.
Is there a technology that could treat the water in the cooling towers without chemical intervention on a continuous basis, without the use of chemicals?
Yes, Jack, non-chemical technologies are available for cooling water treatment. Some are used in conjunction with biocide treatment. Any technology should be thoroughly evaluated before use. For any cooling tower system, no matter what the treatment method, Legionella control can be best evaluated by testing for Legionella periodically.
yes non-chemical treatments are available and is used in NYC.
You may want to refer to Wellan Synergy for this, there are a few applications and studies on it with NYC.