Legionella control in cooling towers is the combination of continuous biocide treatment, cleaning and routine culture testing that keeps Legionella below 10 CFU/mL, the level the CDC calls well controlled, in recirculating water that sits inside the bacterium's 25 to 45 °C (77 to 113 °F) growth range. Which result calls for action, which water treatment chemicals hold the count down, and how often must a tower be tested?
Legionella programs draw on the same water treatment chemicals as every cooling tower program (chlorine, bromine, chlorine dioxide and non-oxidizing biocides), but they add numeric action levels, remediation doses and test schedules that rules in New York City, Germany and France now enforce.
Culture bands and action levels define the result, oxidizing residuals and hyperhalogenation answer it, and an accredited culture, monthly in New York City and France, proves it under ASHRAE 188, local codes and EPA registration.
The four figures that frame a cooling tower Legionella program are listed below.
- Below 10 CFU/mL by culture: well controlled (CDC routine testing module, dated 2025-01-03).
- 0.5 to 1.0 ppm free chlorine as Cl2: the continuous residual in the hot return water (Cooling Technology Institute WTB-148, 2008).
- 20 ppm free available oxidant, then 10 ppm held for 24 hours: CDC emergency disinfection.
- Monthly: the Legionella sampling interval in New York City since 2026-05-08.
Every value on this page is checked against primary sources; see our methodology and fact-checking process.
What Is Legionella Control in Cooling Towers and Water Systems?#
Legionella control is a managed program of 4 elements: a continuous oxidizing biocide residual, periodic non-oxidizing or high-halogen treatment, physical cleaning at least once a year, and Legionella culture testing that proves the result. The written plan around the 4 elements is a Legionella water management program, for which the CDC publishes the toolkit "Developing a Legionella Water Management Program" (page dated 2024-03-15).
Water systems in this sense also include building water. Why does a cooling tower need a dedicated program?
Why Does Legionella Grow in Cooling Towers?#
Legionella grows in cooling towers because recirculating water stays in its 25 to 45 °C (77 to 113 °F) growth range for most of the operating season, and bacteria under optimal conditions double every 20 to 30 minutes (Veolia Water Technologies handbook, chapter 26).
Biofilm is where control fails first. Attached bacteria outnumber those in the bulk water by 3 to 4 log units, and usually less than 1 % of all cells grow on a culture plate (review in FEMS Microbiology Ecology). Where does untreated water stand still?
Stagnant water and make-up water are the two other causes. The CDC asks for low-flow runs and dead legs to be flushed at least weekly; the Cooling Technology Institute (CTI) lists reclaimed-water make-up among its hyperhalogenation triggers; New York City fills towers from the municipal supply and requires an approved alternative water source plan for nonpotable make-up. Fill, basin and drift eliminator layouts are covered under how cooling towers work.
What Legionella Levels Are Acceptable in a Cooling Tower?#
An acceptable Legionella level in a cooling tower is a culture result below 10 CFU/mL (CDC), and the rules that set numeric action levels, in New York City, Germany and France, start their first response at or near that value. Each source counts in its own unit, so the tables below keep CFU/mL, cfu/L and KBE/100 mL as published.
CDC Interpretation of Legionella Results: Below 10 CFU/mL#
The CDC reads a cooling tower Legionella culture in 4 bands: below 10 CFU/mL is well controlled, 10 CFU/mL or more may allow growth, a 10 to 100-fold increase means poor control, and a rise of more than 100-fold means the system is out of control (CDC routine testing module, dated 2025-01-03). The bands are guidance, not a legal limit.
The CDC routine testing module defines 4 interpretation bands for cooling towers.
- Below 10 CFU/mL: well controlled.
- 10 CFU/mL or more: growth possible.
- 10 to 100-fold increase: poorly controlled.
- More than 100-fold increase: uncontrolled.
The culture detection limit is about 10 CFU/mL, so "below 10" usually reads "not detected". What warns earlier than a 7 to 14-day culture?
Heterotrophic Plate Count as an Early Warning: 10,000 CFU/mL#
A heterotrophic plate count below 10,000 CFU/mL in the bulk water, with sessile counts below 100,000 CFU/cm², is the Cooling Technology Institute target (WTB-148, July 2008), and New York City uses the same 10,000 CFU/mL line in Table 8-2 to trigger residual checks 3 times a day.
Above that line, NYC Table 8-2 adds a Legionella sample when the count is not under control within 3 days, with a retest after 48 hours. The CTI treats total bacteria above 100,000 CFU/mL within 24 hours of a hyperhalogenation as an emergency trigger. UK guidance calls aerobic counts above 10⁵ cfu/mL an action level for a shot dose (HSE HSG274 Part 1, 2024, Table 1.10). Dip slides, ATP tests and plate counts as methods are covered under microbiological monitoring.
Legionella Action Levels by Jurisdiction: New York City, Germany and France#
Legionella action levels differ by jurisdiction: New York City acts from 10 CFU/mL and demands full remediation within 48 hours at 1,000 CFU/mL, Germany's 42. BImSchV sets test values of 5 and 50 CFU/mL and an action value of 500 CFU/mL for cooling towers, and France stops the spray at 100,000 UFC/L.
Three sets of rules and one UK guidance document publish numeric Legionella action levels for cooling towers; the table keeps each in its published unit.
| Jurisdiction and instrument | Unit | First level | Second level | Action level |
|---|---|---|---|---|
| New York City, 24 RCNY 8, Table 8-1 | CFU/mL | Level 2: 10 to below 100 (disinfect within 24 h, retest in 3 to 7 days) | Level 3: 100 to below 1,000 (plus inspection for cleaning) | Level 4: 1,000 or more (full remediation within 48 h, notify DOHMH within 24 h) |
| Germany, 42. BImSchV, Anlage 1: cooling towers (Kühltürme) | KBE/100 mL (CFU/mL) | Prüfwert 1: 500 (5) | Prüfwert 2: 5,000 (50) | Maßnahmenwert: 50,000 (500) |
| Germany, 42. BImSchV, Anlage 1: evaporative cooling systems | KBE/100 mL (CFU/mL) | Prüfwert 1: 100 (1) | Prüfwert 2: 1,000 (10) | Maßnahmenwert: 10,000 (100) |
| France, arrêté of 14 December 2013 (ICPE 2921), art. 26 | UFC/L L. pneumophila | target below 1,000 | n/a | 100,000 or more: stop dispersion, inform the inspection immediately |
| UK, HSE HSG274 Part 1 (2nd edition, 2024), Table 1.11 (guidance) | cfu/L | 100 or less: maintain | above 100 to 1,000: reassess, resample | above 1,000 or persistent: immediate action, shot dose, resample within 48 h |
| CDC (guidance, not a rule) | CFU/mL | below 10: well controlled | 10 or more: may allow growth | rise above 100-fold: uncontrolled |
Values as published in each instrument; NYC Level 1 is below 10 CFU/mL. CFU/mL equivalents for Germany as given by the regulation's units (100 mL basis).
Germany sets lower values for evaporative cooling systems than for cooling towers, and each plant derives a colony-count reference value from at least 6 consecutive laboratory tests, with an interim maximum of 10,000 KBE/mL (42. BImSchV § 4).
France also requires make-up water with L. pneumophila below the quantification limit and suspended matter below 10 mg/L (arrêté of 14 December 2013, art. 28).
Which Water Treatment Chemicals Control Legionella?#
Legionella in cooling towers is controlled by 2 classes of water treatment chemicals: oxidizing biocides (sodium hypochlorite, activated bromine, BCDMH and chlorine dioxide) held as a continuous residual, and non-oxidizing biocides (DBNPA, isothiazolinone and glutaraldehyde) dosed as timed shots.
The 8 chemicals in the table carry a cooling-tower or building-water Legionella figure with its basis; monochloramine is the building-water row.
| Chemical | Class | Where used | Legionella-relevant figure with basis | Limit or caution |
|---|---|---|---|---|
| Sodium hypochlorite | oxidizing (chlorine) | cooling towers | 0.5 to 1.0 ppm free as Cl2 in the hot return, continuous (CTI); remediation 20 ppm, then 10 ppm for 24 h (CDC) | pH below 8.0 during remediation; HOCl predominates below pH 7.6 |
| Sodium bromide (activated bromine) | oxidizing (bromine) | cooling towers | 1.45 mg NaBr per mg/L Cl2 (1:1 molar, calculated); HOBr predominates below pH 8.7 | bromine remediation at pH below 8.5 (CDC) |
| BCDMH | oxidizing (bromine) | cooling towers | 1 ppm bromine for at least 4 h | no organics or other biocides in the feeder |
| Chlorine dioxide | oxidizing | cooling towers; building water | efficacy largely unaffected from pH 6 to 9; building water 0.1 to 0.5 mg/L total ClO2 at the tap | volatile over towers; chlorite monitored daily in buildings |
| Monochloramine | oxidizing (combined chlorine) | building water | 1.0 to 4.0 mg/L as Cl2 fed; 1 to 2 mg/L residual | Cl2:NH3 3:1 to 5:1; pH 7.5 to 9 |
| DBNPA | non-oxidizing | cooling towers (Legionella remediation listed) | under 1 to 24 ppm as active (EPA RED label range) | deactivated by H2S |
| Isothiazolinone (CMIT/MIT) | non-oxidizing | cooling towers | 100 ppm as product for 4 h (Dober, supplier figure) | degraded by sulfides, amines and thiols |
| Glutaraldehyde | non-oxidizing | cooling towers | slug 40 to 200 ppm as product (20 to 100 mg/L as active, 50 % product) | at most 200 ppm as product per day (label) |
Label, supplier or guidance figures with the basis shown; the registered label sets the legal dose.
Continuous Oxidizing Biocide Residuals: Chlorine, Bromine and Chlorine Dioxide#
The Cooling Technology Institute recommends a continuous free halogen residual of 0.5 to 1.0 ppm as Cl2 in the hot return water, or at least 1.0 to 2.0 ppm for at least 1 hour each day where the oxidizer is fed intermittently (WTB-148, 2008). UK guidance asks for 0.5 to 1 mg/L free chlorine or 1 to 2 mg/L bromine (HSG274 Part 1, para 1.103).
All three halogen sources belong to the wider family of oxidants and oxidizing biocides. pH decides how much of the residual works: hypochlorous acid predominates below pH 7.6 and is up to 80 times more effective than the hypochlorite ion (Veolia Water Technologies handbook, chapter 27). OSHA states that chlorine above 0.5 ppm may prevent bacterial growth at pH below 8.0. Above pH 8, HSE guidance asks for a 3 to 4-fold higher chlorine residual or bromine or chlorine dioxide (HSG274 Part 1, paras 1.101 to 1.102).
Bromine covers the alkaline range. Sodium bromide is activated by hypochlorous acid (NaBr + HOCl → HOBr + NaCl), and hypobromous acid predominates below pH 8.7. A DPD reading multiplied by 2.25 gives bromine as Br2.
Chlorine dioxide is pH-independent, does not react with ammonia and penetrates biofilm, but part of it volatilizes over the tower. Activated bromide and BCDMH are compared under bromine water treatment.
Non-Oxidizing Biocides for Legionella: Contact Time and Alternation#
Non-oxidizing biocides support Legionella control as timed shots with a contact time of typically at least 4 hours, and HSE guidance alternates 2 biocides with different kill mechanisms (HSG274 Part 1, 2024).
Label and supplier figures carry their basis. DBNPA runs from under 1 to 24 ppm as active (EPA RED label range), acts fastest and is deactivated by H2S. Glutaraldehyde is slug-fed at 40 to 200 ppm as product (20 to 100 mg/L as active for a 50 % product), at most 200 ppm as product per day. Isothiazolinone is shot at 100 ppm as product for 4 hours (supplier figure).
Program design, feed points and oxidizer selection belong to cooling tower biocides. For testing, the Legionella sample is taken immediately before a non-oxidizer shot (HSE).
Hyperhalogenation and Remediation Doses After a High Legionella Result#
Remediation after a high Legionella result climbs a dose ladder: CTI hyperhalogenation holds at least 5 ppm free halogen for 6 hours, New York City's corrective step holds 5 to 10 ppm for at least 1 hour, and CDC emergency disinfection starts at 20 ppm free available oxidant and holds 10 ppm for at least 24 hours. The CDC sets the pH below 8.0 for chlorine and below 8.5 for bromine.
Four sources set halogen levels for Legionella remediation, from routine hyperhalogenation to emergency disinfection.
| Step | Source | Trigger | Residual and basis | Hold time | pH condition |
|---|---|---|---|---|---|
| Hyperhalogenation | CTI WTB-148 (2008) | total bacteria above 100,000 CFU/mL, Legionella above 100 CFU/mL, reclaimed-water make-up | at least 5 ppm free halogen | at least 6 h | n/a |
| Summer hyperhalogenation | NYC 24 RCNY 8 | once a year, 1 July to 31 August | at least 5 ppm free halogen at 2 locations | at least 6 h | n/a; Legionella sample 3 to 31 days after |
| Corrective hyperhalogenation | NYC 24 RCNY 8 | Table 8-1 levels | 5 to 10 ppm free halogen | at least 1 h | "effective pH" |
| Pre-cleaning disinfection | HSE HSG274 Part 1 (2024), Table 1.7 | before cleaning | 5, 25 or 50 mg/L free chlorine throughout the system | 5 h, 2 h or 1 h | n/a |
| Emergency disinfection | CDC cooling towers module | uncontrolled result | initial at least 20 ppm free available oxidant, then 10 ppm | at least 24 h; after refill at least 10 ppm for 1 h | below 8.0 (chlorine), below 8.5 (bromine) |
| Emergency disinfection | CTI WTB-148 (2008) | Legionella above 1,000 CFU/mL; suspected cases; total bacteria above 100,000 CFU/mL within 24 h of hyperhalogenation | 25 to 50 ppm free halogen, then 10 ppm | 24 h, drain, refill, repeat; 10 ppm for 1 h after mechanical cleaning | n/a |
ppm equals mg/L in water. Residuals are measured, not calculated; chlorine demand raises the dose needed.
Worked example (calculated): the initial CDC dose for a small tower, with every input labelled.
- System volume: 50 m³ (13,210 gal).
- Target: 20 mg/L free available chlorine as Cl2; demand ignored.
- Available chlorine: 50 m³ x 20 g/m³ = 1,000 g = 1.0 kg (2.2 lb).
- Product: 12.5 trade % sodium hypochlorite = 125 g/L available chlorine (1.04 lb Cl2 per gal).
- Volume: 1,000 g / 125 g/L = 8.0 L (2.1 gal).
Chlorine demand raises the real dose. Other volumes and strengths run in the chemical dosage and feed rate calculator.
New galvanized towers add a conflict: they are passivated at no more than 1.0 mg/L free halogen as Cl2 to prevent white rust, while startup disinfection may need up to 10 mg/L for 24 hours (Association of Water Technologies).
Cleaning, Dead Legs and Standby Towers#
Cleaning removes the biofilm and deposits that a biocide residual cannot reach, and the minimum frequency is set by the rule that applies: at least once a year offline under CDC guidance and French ICPE 2921 rules, and at least twice a year in New York City. UK guidance gives typical intervals of 3 to 12 months by environment (HSG274 Part 1, para 1.87).
Four stagnation and cleaning controls appear in the CDC cooling towers module and NYC Chapter 8.
- Weekly flushing of low-flow runs and dead legs (CDC).
- Circulation 3 times a week for towers in wet standby under 5 days (CDC).
- Offline cleaning and disinfection at least once a year (CDC).
- Cleaning whenever monitoring indicates, and no less than twice a year (NYC).
Procedures are covered under cooling tower cleaning and disinfection chemicals.
Legionella Control in Building Water Systems: Chlorine Dioxide and Monochloramine#
Building water systems use supplemental chlorine dioxide or monochloramine for Legionella control, with total chlorine dioxide at 0.1 to 0.5 mg/L at the tap or monochloramine fed at 1.0 to 4.0 mg/L as Cl2 to hold 1 to 2 mg/L in premise plumbing (EPA, Legionella technologies document, September 2016).
Chlorine dioxide needs daily chlorine dioxide and chlorite tests; in one study, hot-water loops lost about 88 % of the residual.
Monochloramine runs at a Cl2:NH3 ratio of 3:1 to 5:1 and pH 7.5 to 9. The US DOE puts ozonation and ionization behind a life-cycle cost evaluation in cooling-tower retrofits, and New York City does not let a non-chemical device replace the chemical biocide unless approved.
How Is Legionella Testing Done for Cooling Towers?#
Legionella testing for cooling towers is a laboratory culture of a water sample, reported in CFU/mL (or cfu/L in Europe), taken on a fixed schedule and before biocide shots, with PCR as a faster screen that counts genomic units instead of live colonies.
Legionella Culture Versus PCR: Detection Limits and Turnaround#
Culture is the reference method for Legionella testing, with a detection limit of about 10 CFU/mL and results in 7 to 14 days, while NAAT/PCR reports genomic units, not CFU, within 2 to 48 hours and serves mainly as a negative screen (CDC).
UK laboratories culture to BS EN ISO 11731:2017 with a detection limit of 100 cfu/L or lower (at least 30 mL plated) and incubate for 7 to 10 days (HSG274 Part 1, paras 1.135 to 1.137).
Legionella Water Testing: Sample Volume, Timing and Retests#
A Legionella water sample from a cooling tower is typically 250 mL, taken before a non-oxidizing biocide shot, and a retest follows at least 48 hours after the system returns to normal operation (CDC).
Five sampling rules come from the CDC, HSE, French and NYC texts.
- Volume: 250 mL is typically sufficient (CDC).
- Timing: immediately before a non-oxidizer shot (HSE).
- Biocide gap: at least 48 hours after a biocide injection (France).
- Retest: at least 48 hours after normal operation resumes (CDC).
- Post-treatment sample: 3 to 31 days after the NYC summer hyperhalogenation.
How Often Is Legionella Testing Required?#
Legionella testing frequency depends on the jurisdiction: New York City requires a culture at least every month (no more than 31 days apart) since 2026-05-08, France at least monthly, Germany a laboratory test at least every 3 months, and HSE at least quarterly for open systems.
The table compares the culture interval and the routine checks around it.
| Jurisdiction | Legionella culture | Other microbial or chemical checks | Instrument and date |
|---|---|---|---|
| New York City | at least monthly, no more than 31 days apart, ELAP laboratory (previously every 90 days) | pH, temperature, conductivity and biocide indicators at least 3 times a week; bacteriological indicator weekly | 24 RCNY 8-05(f), effective 2026-05-08 |
| New York State | at intervals not exceeding 90 days while in use and within 14 days of seasonal start-up, ELAP laboratory | inspection before seasonal start-up and at least every 90 days | 10 NYCRR 4-1.4(b)(2) and 4-1.8(a) |
| Germany | laboratory colony count and Legionella at least every 3 months; 6-monthly after 2 years below Prüfwert 1 (one sample 1 June to 31 August) | internal checks at least every 2 weeks | 42. BImSchV § 4 |
| France | L. pneumophila at least monthly by NF T90-431 (April 2006), at least 48 h after a biocide injection; results to the inspection within 30 days | risk analysis reviewed at least yearly | arrêté of 14 December 2013 (ICPE 2921) |
| UK | open systems at least quarterly; detection limit no more than 100 legionella per litre | conductivity, pH, oxidizing biocide and microbial count weekly | HSE web guidance; HSG274 Part 1 (2024), Table 1.9 |
| CDC (US guidance) | per water management program; retest at least 48 h after normal operation | n/a | CDC routine testing module (2025-01-03) |
New York State outside the city keeps a 90-day maximum.
Legionella Testing Laboratories and Test Kits#
Regulated Legionella results come from accredited laboratories: New York City requires a laboratory approved under the New York State ELAP program, and HSE expects a UKAS-accredited ISO 17025 laboratory culturing to BS EN ISO 11731:2017.
Field test kits do not replace the accredited culture these rules require; a kit result can prompt a laboratory sample but does not count as the scheduled result.
Which Rules Govern Legionella Control in Cooling Towers?#
Legionella rules for cooling towers come from 3 levels: consensus standards such as ANSI/ASHRAE Standard 188-2021, local codes such as New York City's Chapter 8 and national rules in Germany and France, and US EPA pesticide registration of the biocides. US drinking-water rules set no Legionella limit; EPA treats it as controlled when Surface Water Treatment Rule requirements for Giardia and viruses are met.
ASHRAE 188 and Legionella Water Management Programs#
ANSI/ASHRAE Standard 188-2021, Legionellosis: Risk Management for Building Water Systems, sets the minimum risk-management requirements under which a cooling tower gets a written water management program, with ASHRAE Guideline 12-2023 as its companion guidance. The CDC water management program toolkit (page dated 2024-03-15) is the free US template. Program content is covered under ASHRAE 188 and Legionella water management programs.
New York City Cooling Tower Rules (24 RCNY Chapter 8)#
New York City's Health Code Chapter 8 (24 RCNY 8) is the most detailed cooling tower Legionella rule in the US: monthly cultures from an ELAP-approved laboratory since 2026-05-08, automated daily treatment with a registered oxidizing primary biocide, cleaning at least twice a year and a summer hyperhalogenation between 1 July and 31 August.
The monthly rule followed the 2025 Harlem cluster of 118 cases, 92 hospitalizations and 7 deaths, which ended on 2025-08-29. The FY2027 budget adds US$13 million and 56 inspectors in total (NYC Health press release, May 2026); records are kept for at least 3 years.
EPA Registration and Efficacy Testing of Legionella Biocides#
Every cooling tower biocide sold in the US needs an EPA registration under FIFRA (40 CFR 152.15), and EPA's final guidance of August 2024 lets a product claim Legionella remediation only with a mean reduction of at least 5.0 log (99.999 %), and routine maintenance with 3 log (99.9 %). The tests use L. pneumophila ATCC 33152 at 30 ± 1 °C (86 ± 1.8 °F, calculated), control counts of 10⁶ to 10⁷ CFU/mL and 3 batches.
Cooling-tower products are non-public-health antimicrobial pesticides; more than 4,000 antimicrobial products are registered with EPA. Label claims are covered under FIFRA registration of water treatment biocides.
Legionella Rules in Europe: Germany, France, the UK and Spain#
Germany and France regulate cooling-tower Legionella by statute: Germany's 42. BImSchV sets test and action values in KBE/100 mL with laboratory tests at least every 3 months, and France's arrêté of 14 December 2013 for ICPE category 2921 requires monthly L. pneumophila sampling by NF T90-431 and a target below 1,000 UFC/L. The UK works through HSE guidance HSG274 Part 1 (second edition, 2024); Spain regulates under Real Decreto 487/2022 (in force 2023-01-02), amended by Real Decreto 614/2024 of 2 July 2024.
Biocide products fall under the EU Biocidal Products Regulation as PT11 (preservatives for liquid-cooling and processing systems) and PT12 (slimicides). Active chlorine released from sodium hypochlorite is approved for PT1 to PT5 (Implementing Regulation (EU) 2017/1273, 2019-01-01 to 2028-12-31); its PT11 and PT12 uses are still under evaluation in the EU review programme, with no decision as of 2026-10-22.
How Does Legionella Control Fit into the Full Cooling Tower Water Treatment Program?#
Legionella control is one of the microbiological duties of a cooling tower water treatment program, which also controls scale, corrosion and blowdown, and the same controller and test schedule serve all of them.
Chemicals, programs and control limits for the whole system sit on the cooling tower water treatment hub.
Legionella Control Versus General Biocide and Biofilm Control#
General biocide programs aim at slime and heat-transfer loss, while Legionella control adds a pathogen-specific culture result, so a tower can meet its 10,000 CFU/mL plate-count target and still need a Legionella test.
Oxidizing and non-oxidizing chemistries are compared under biocides for water treatment, and formation and removal of attached growth under biofilm in industrial water systems.
Testing, Cleaning and Blowdown in the Same Program#
Legionella testing runs on the same calendar as the routine water tests, pH, conductivity and biocide residual at least 3 times a week in New York City, and blowdown control keeps the chemistry inside the range the biocide needs.
Ranges and methods sit under cooling tower water testing and control ranges; bleed-off rates are covered under cooling tower blowdown.
Can you get Legionella from a cooling tower?#
Public-health rules treat cooling towers as a Legionella exposure source: CDC places them at least 25 ft (7.6 m) from building air intakes, and New York City moved to monthly testing after the 2025 Harlem cluster of 118 cases.
Is Legionella testing mandatory?#
Legionella testing of cooling towers is mandatory where a local or national rule says so, for example monthly in New York City since 2026-05-08, monthly in France under ICPE 2921 and at least every 3 months in Germany under the 42. BImSchV.
What kills Legionella in water?#
Oxidizing biocides such as chlorine and bromine kill Legionella in cooling water when held as a free residual at the right pH, and EPA-registered products claiming remediation must show a mean 5-log reduction in EPA's August 2024 test.
Who Runs Legionella Control Programs for Cooling Towers?#
Legionella control programs are run by the site's water treatment program provider together with an accredited laboratory and, in New York City, a certified pesticide applicator for the biocide work.
How to compare a water treatment program provider is covered on the services page.