Drinking water disinfection is the inactivation of Giardia, viruses and Cryptosporidium before the first customer, followed by a residual of at least 0.2 mg/L (ppm) entering distribution (40 CFR 141.72). A filtered surface-water plant must reach 3-log Giardia and 4-log virus removal and inactivation, so which of the 8 disinfectants delivers that credit, and which one holds the residual?
Disinfectants are the one family of water treatment chemicals that every US public water system needs in some form.
Four chlorine sources, chlorine dioxide, ozone, monochloramine and UV share the work; CT tables set the contact, feed arithmetic sets the pump, and byproduct limits, NSF/ANSI/CAN 60, the WHO and the EU set the ceilings.
The four figures that frame a disinfection program are listed below.
- 3-log Giardia, 4-log virus: the floor for filtered surface water (40 CFR 141.72).
- 0.2 mg/L entering distribution: not lower for more than 4 hours (40 CFR 141.72).
- 4.0 mg/L as Cl2: the MRDL for chlorine and chloramines, running annual average (40 CFR 141.65).
- About 18 times: chloramine's CT versus free chlorine, 3-log Giardia, pH 7, 10 °C (50 °F) (calculated from 40 CFR 141.74).
Every value on this page is checked against primary sources (eCFR, EPA, WHO, NSF listings, AWWA, EUR-Lex); see our methodology and fact-checking process.
What Is Drinking Water Disinfection and Why Must Every Public Water System Disinfect?#
Every public water system disinfects because Safe Drinking Water Act section 1412(b)(8) requires disinfection as a treatment technique, always for surface water and, as necessary, for ground water. Over 148,000 public water systems serve 90 % of Americans (EPA).
Disinfection is measured as CT: the residual C in mg/L before or at the first customer times the contact time T in minutes (40 CFR 141.2). The WHO rates free chlorine as not effective against Cryptosporidium, so filtration, ozone, chlorine dioxide or UV carry that credit.
How Much Inactivation Do the Rules Require? Log Credits for Giardia, Viruses and Cryptosporidium#
Filtered surface water needs 3-log Giardia and 4-log virus removal plus inactivation, unfiltered systems need the same every day except one day per month, and ground water that must be treated needs at least 4-log virus treatment before the first customer. Cryptosporidium follows the Long Term 2 bins in oocysts/L.
US rules set log credits by source and filtration, and EPA Region 8 policy shows how much credit is left for disinfection after each filter type.
| Requirement | Value | Instrument |
|---|---|---|
| Filtered surface water, total | 3-log Giardia, 4-log virus (removal + inactivation) | 40 CFR 141.72 |
| Unfiltered surface water | 3-log Giardia, 4-log virus every day except one day per month | 40 CFR 141.72 |
| Ground water (where treatment is required) | at least 4-log virus before or at the first customer | 40 CFR 141.403 (Ground Water Rule, 2006) |
| Cryptosporidium | 2-log removal (SWTR); LT2 conventional plants: Bin 2 +1, Bin 3 +2, Bin 4 +2.5 log | EPA SWTR fact sheet; Subpart W |
| LT2 bins | Bin 1 < 0.075; Bin 2 0.075 to < 1.0; Bin 3 1.0 to < 3.0; Bin 4 ≥ 3.0 oocysts/L | Subpart W |
| Unfiltered Cryptosporidium | ≥ 2-log inactivation at a mean ≤ 0.01 oocysts/L; ≥ 3-log above | Subpart W |
| Credit left after conventional / direct / slow sand / DE filtration | 0.5 / 1.0 / 1.0 / 1.0-log Giardia; 2.0 / 3.0 / 2.0 / 3.0-log virus | EPA Region 8 SWTR policy |
| Credit left after RO/NF / UF/MF | 0-log Giardia; 1.0 / 4.0-log virus | EPA Region 8 SWTR policy |
| Entry-point residual | not below 0.2 mg/L for more than 4 hours | 40 CFR 141.72 |
| Distribution residual | undetectable in no more than 5 % of monthly samples, 2 consecutive months; HPC ≤ 500/mL counts as detectable | 40 CFR 141.72 |
| MRDL | 4.0 mg/L as Cl2 (chlorine, chloramines); 0.8 mg/L as ClO2 | 40 CFR 141.65 |
Region 8 credits are a regional EPA policy table; the state primacy agency sets the credit for each plant.
Primary vs Secondary Disinfection: Inactivation Before the First Customer, Residual in the Pipes#
Primary disinfection delivers the log inactivation before the first customer; secondary disinfection keeps at least 0.2 mg/L entering distribution and a detectable residual in at least 95 % of monthly samples (40 CFR 141.72). One chemical can do both, or primary disinfection with ozone can hand over to chloramine.
The WHO's 3 chlorine application points and the residual role are listed below.
- Pre-chlorination: before treatment.
- Primary chlorination: after treatment, where the basin earns CT.
- Booster chlorination: in distribution.
- Residual: the WHO target of 0.2 to 0.5 mg/L free chlorine to the point of delivery.
There is no federal Legionella limit; EPA assumes control when Giardia and viruses are inactivated as the SWTR requires.
Which 8 Disinfectants Do Utilities Use? Chlorine, Other Oxidants and UV#
Utilities use 8 disinfectants in 3 groups: 4 chlorine sources (chlorine gas, sodium hypochlorite, calcium hypochlorite, TCCA), 3 other chemicals (chlorine dioxide, ozone, monochloramine) and UV, with chlorine the most common. More than one in five Americans drink chloraminated water (EPA).
The chemical groups belong to the family of oxidants and oxidizing biocides, which also covers bromine and peroxygens. The table compares role, CT at 10 °C (50 °F), residual and NSF/ANSI/CAN 60 status.
| Disinfectant | Group / role | 3-log Giardia CT, 10 °C (50 °F) | Residual | NSF/ANSI/CAN 60 (NSF listings, 2026-10-21) | Product standard | Main limit or byproduct |
|---|---|---|---|---|---|---|
| Chlorine gas | chlorine source; primary + secondary | 104 (C ≤ 0.4 mg/L, pH 7) to 112 (1.0 mg/L, pH 7) mg-min/L | yes (free chlorine) | 38 companies, 58 products, 106 product-function rows; MUL 30 mg/L as product on 102 rows | ANSI/AWWA B301-24; EN 937:2016 | MRDL 4.0 mg/L as Cl2; RMP 2,500 lb / PSM 1,500 lb |
| Sodium hypochlorite | chlorine source; primary + secondary | as free chlorine | yes | 178 companies, 768 products, 1,551 product-function rows; most common MUL 84 mg/L as product, then 40 and 105 mg/L | ANSI/AWWA B300-24; EN 901:2024 | chlorate and bromate in stored product; TTHM, HAA5 |
| Calcium hypochlorite | chlorine source; small systems | as free chlorine | yes | 39 companies, 237 products, 243 product-function rows; most common MUL 15 mg/L as product | ANSI/AWWA B300-24; EN 900 | TTHM, HAA5 |
| Trichloroisocyanuric acid (TCCA) | tablet chlorine source; small systems | as free chlorine | yes | 19 companies, 49 products, 50 product-function rows; most common MUL 30 mg/L as product | no AWWA or drinking-water EN identified | WHO cyanuric acid guideline value 40 mg/L |
| Chlorine dioxide | non-chlorinating oxidant; primary | 23 mg-min/L | no lasting residual (WHO) | "Chlorine Dioxide, Liquid": 5 companies, 10 products; precursor sodium chlorite: 41 companies, 144 products, most common MUL 22 mg/L as product | precursor ANSI/AWWA B303-24; EN 12671:2016 | MRDL 0.8 mg/L as ClO2; chlorite MCL 1.0 mg/L |
| Ozone | oxidant; primary | 1.4 mg-min/L | no | no listing | EN 1278:2026 | bromate MCL 0.010 mg/L |
| Monochloramine | combined chlorine; secondary | 1,850 mg-min/L | yes (most stable) | no own listing (precursors: ammonium sulfate, ammonia, hypochlorite, chlorine) | precursors ANSI/AWWA B302-23, B305-22, B306-22 | MRDL 4.0 mg/L as Cl2; nitrification; NDMA |
| UV disinfection | non-chemical; primary | 11 mJ/cm2 (dose, not CT) | no | not applicable (reactor validation) | not applicable | needs chlorine or chloramine after it |
CT values: 40 CFR 141.74 and EPA Disinfection Profiling and Benchmarking guidance (2022), Tables B-1 to B-8; NSF maximum use levels (MULs) are as product and product-specific.
1. Chlorine Gas: Cylinders, Ton Containers and Vacuum Chlorinators#
Chlorine gas (Cl2, CAS 7782-50-5) comes in 150-lb (68 kg) cylinders and 1-ton containers and is fed through vacuum chlorinators as primary and secondary disinfectant. It hydrolyzes to HOCl and HCl, lowering pH and alkalinity, and needs 104 mg-min/L for 3-log Giardia at 10 °C, pH 7 (40 CFR 141.74). Almost all listings carry a MUL of 30 mg/L as product (NSF listings, 2026-10-21). Gas feed is described under chlorinators and hypochlorite generation systems.
2. Sodium Hypochlorite: The Most-Listed Chlorine Source#
Sodium hypochlorite (NaOCl, CAS 7681-52-9) is the most-listed chlorine source: 178 companies, 768 products and 1,551 product-function rows (NSF listings, 2026-10-21). Strength is sold in trade percent: 12.5 trade % equals 125 g/L available chlorine, 11.23 wt % NaOCl and 1.04 lb Cl2 per gallon at a density of 1.169 (Olin sodium hypochlorite handbook). The most common MUL is 84 mg/L as product, with most listings between about 40 and 210 mg/L.
3. Calcium Hypochlorite for Small Systems#
Calcium hypochlorite (Ca(OCl)2, CAS 7778-54-3) is a dry source with 65 to 70 % available chlorine, used most often at very small systems (EPA). Once dissolved it follows the free-chlorine CT tables. NSF/ANSI/CAN 60 lists 39 companies and 237 products, most commonly at 15 mg/L as product (NSF listings, 2026-10-21).
4. Trichloroisocyanuric Acid (TCCA) Tablets#
Trichloroisocyanuric acid (TCCA, CAS 87-90-1) is a tablet source for small systems that releases free chlorine and cyanuric acid. NSF/ANSI/CAN 60 lists 19 companies and 49 products, most commonly at 30 mg/L as product (NSF listings, 2026-10-21); the WHO guideline value for cyanuric acid is 40 mg/L.
5. Chlorine Dioxide: Generators, Low CT and the Chlorite Ceiling#
Chlorine dioxide (ClO2, CAS 10049-04-4) is generated on site and dosed at 0.07 to 2.0 mg/L as ClO2, with a 3-log Giardia CT of 23 mg-min/L at 10 °C (40 CFR 141.74). Up to 70 % of the dose becomes chlorite, so about 1.4 mg/L keeps chlorite below the 1.0 mg/L MCL (calculated, 1.0 / 0.7); Health Canada caps the feed dose at 1.2 mg/L. High-purity ClO2 forms no THMs, HAAs or bromate, and gives no lasting residual (WHO). The OxyChem chlorine dioxide manual gives 1.34 lb sodium chlorite plus 0.5 lb chlorine per lb ClO2. Yields are compared under chlorine dioxide generators.
6. Ozone: Lowest CT, On-Site Generation and the Bromate Limit#
Ozone (O3, CAS 10028-15-6) has the lowest CT, 1.4 mg-min/L for 3-log Giardia at 10 °C (40 CFR 141.74), and earns 2-log Cryptosporidium credit at 20 mg-min/L (40 CFR 141.720). Air-fed generators make 0.5 to 3.0 % ozone by weight, oxygen-fed 2 to 4 times more, at a dew point of -60 °C (-76 °F) (EPA). Ozone must be followed by chlorine or chloramine. Bromate (MCL 0.010 mg/L) falls 30 to 50 % per pH unit lowered from above 7.5 (Health Canada). Feed gas and off-gas destruction belong to ozone generation systems.
7. Monochloramine (Chloramine): The Secondary Disinfectant#
Monochloramine (NH2Cl, CAS 10599-90-3) is the secondary disinfectant formed at 4.5:1 to 5:1 Cl2 to NH3-N by weight, chosen for a stable residual. It needs 1,850 mg-min/L for 3-log Giardia at 10 °C against 104 for free chlorine (40 CFR 141.74), so the WHO advises against primary use. The WHO guideline value is 3 mg/L; supplies typically carry 0.5 to 2 mg/L.
8. UV Light: Primary Credit Without a Residual#
UV earns primary credit by dose, 11 mJ/cm2 for 3-log Giardia, 12 for 3-log Cryptosporidium and 186 for 4-log virus (40 CFR 141.720), but leaves no residual. Credit requires validated conditions for at least 95 % of the water each month. Chlorine goes downstream of the reactor, since chlorine with iron and manganese fouls the sleeves (EPA UV toolkit).
How Is a Drinking Water Disinfection Program Designed and Dosed?#
A disinfection program is designed in 4 steps: fix the application points, prove the CT, calculate the feed rate, and choose the disinfectant pair.
Where Are Disinfectants Added? Pre-Oxidation, Primary and Booster Points#
Disinfectants are added at up to 4 points: raw water for pre-oxidation, the contact basin for primary disinfection, the entry point for the residual, and booster stations in distribution. The first 3 are the WHO's points; the entry point is the US compliance point. The points in flow order are listed below.
- Pre-oxidation, which the WHO advises avoiding where possible to limit byproducts.
- Primary disinfection in the clearwell or contactor, where T10 and the residual set CT.
- Entry point, where the residual may not stay below 0.2 mg/L for more than 4 hours.
- Booster chlorination, where water age consumes the residual.
UV and membrane plants add chlorine or monochloramine downstream, since neither leaves a residual.
How Is CT Calculated for Disinfection? A Worked Clearwell Example#
CT equals the residual C times T10, the time in which 10 % of the water passes the basin; the log credit is 3 x CTcalc / CT99.9 for Giardia and 4 x CTcalc / CT99.99 for viruses (EPA Disinfection Profiling and Benchmarking guidance, 2022). T10 comes from a tracer test.
All temperature and pH rows are tabulated under CT values for disinfection.
Worked example, calculated. Inputs: free chlorine 1.0 mg/L as Cl2, T10 45 min, pH 7.0, 10 °C (50 °F).
- CTcalc = 1.0 x 45 = 45 mg-min/L; Giardia 3 x 45 / 112 = 1.21-log.
- Viruses: 45 mg-min/L is 7.5 times the 6 mg-min/L CT99.99.
- Monochloramine, same inputs: 3 x 45 / 1,850 = 0.07-log Giardia.
- Ozone 0.3 mg/L for 5 min: 3 x 1.5 / 1.4 = 3.2-log Giardia.
The CT each disinfectant needs at 10 °C shows why ozone, chlorine dioxide and UV carry Cryptosporidium credit and chloramine does not carry primary duty.
| Disinfectant | 3-log Giardia | 4-log virus | Cryptosporidium | Source |
|---|---|---|---|---|
| Free chlorine | 104 (C ≤ 0.4 mg/L, pH 7.0); 112 (1.0 mg/L, pH 7.0); 149 (C ≤ 0.4 mg/L, pH 8.0) mg-min/L | 6 mg-min/L (pH 6 to 9) | not effective (WHO) | 40 CFR 141.74; EPA Tables B-1, B-2 |
| Monochloramine | 1,850 mg-min/L | 1,491 mg-min/L (only if chlorine is added before ammonia) | no credit | EPA Tables B-7, B-8 |
| Chlorine dioxide | 23 mg-min/L | 25.1 mg-min/L | 2-log 553; 3-log 830 mg-min/L | 40 CFR 141.74, 141.720; Irish EPA |
| Ozone | 1.4 mg-min/L (1.43 in EPA guidance) | not stated here | 1-log 9.9; 2-log 20; 3-log 30 mg-min/L | 40 CFR 141.74, 141.720 |
| UV (dose, not CT) | 11 mJ/cm2 | 186 mJ/cm2 | 3-log 12 mJ/cm2 | 40 CFR 141.720 |
Cold water decides the design: free-chlorine CT at 1.0 mg/L and pH 7.0 rises from 112 mg-min/L at 10 °C to 210 at 0.5 °C (33 °F). A basin's credit can be checked in the CT value calculator.
How Much Chlorine to Feed? Dose, Chlorine Demand and Feed Rate#
The chlorine dose equals demand plus target residual: 0.5 mg/L demand and a 1.0 mg/L free residual need 1.5 mg/L as Cl2. Feed follows lb/day = mg/L x MGD x 8.34 / purity, or kg/d = mg/L x m3/d / 1,000.
Demand is measured by ASTM D1291 and read from the breakpoint curve; see chlorine demand and breakpoint chlorination.
Worked example, calculated.
- Chlorine gas: 1.2 MGD (4,542 m3/d) x 1.5 mg/L x 8.34 = 15 lb/day (6.8 kg/d) as Cl2.
- Hypochlorite: 10 MGD (37,850 m3/d) x 2.0 mg/L x 8.34 = 166.8 lb/d (75.7 kg/d) as Cl2; at 1.04 lb Cl2/gal (12.5 trade %) about 160 gal/d (606 L/d).
Treating 12.5 trade % as 12.5 wt % (1.18 lb/gal) sets the pump about 12 % low.
Pump settings run in the chemical dosage and feed rate calculator.
Choosing the Primary and Secondary Disinfectant Pair#
The pair is chosen on 4 criteria: credit left after filtration, cold-water CT, byproducts and water age. The CT check should use the lowest temperature and highest pH the plant sees. The 4 common pairs are listed below.
- Free chlorine + free chlorine: one chemical; THMs and HAAs rise with contact time.
- Free chlorine + monochloramine: chlorine earns CT; NDMA reaches up to 0.16 µg/L in chloraminated distribution (WHO).
- Ozone or chlorine dioxide + chloramine: Cryptosporidium credit; bromate or chlorite limits the dose.
- UV + chlorine or chloramine: dose credit, residual from the chemical.
Chlorine carries no Cryptosporidium credit, so LT2 Bin 3 and 4 plants need at least 1-log from ozone, chlorine dioxide, UV, membranes or another toolbox option.
How Are Disinfectant Residuals Monitored and Controlled?#
Residuals are controlled by continuous entry-point measurement, distribution samples and pH control at the contact basin.
Entry-Point and Distribution Chlorine Residual Monitoring#
The entry-point residual is measured continuously and may not stay below 0.2 mg/L for more than 4 hours; if the analyzer fails, grab samples every 4 hours may replace it for up to 5 working days. Online analyzers are verified by grab sample at least every 5 days.
Distribution samples prove the secondary residual, reported in 3 forms of chlorine residual in mg/L as Cl2, listed below.
- Free chlorine: Cl2 + HOCl + OCl-.
- Combined chlorine: chloramines.
- Total chlorine: free plus combined.
Texas requires 0.2 mg/L free chlorine or 0.5 mg/L chloramine as total chlorine in distribution.
pH, Temperature and Chlorination Chemistry: The HOCl Fraction#
Hypochlorous acid is about 100 times more effective than hypochlorite ion and makes up about 90 % of free chlorine at pH 6.5, 50 % at 7.5 and about 10 % at 8.5 (25 °C), so 3-log Giardia CT rises from 79 mg-min/L at pH 6 to 234 at pH 9. The WHO sets pH below 8.0 for effective chlorination.
The table links pH, the HOCl share and CT at 1.0 mg/L free chlorine and 10 °C (50 °F).
| pH | HOCl share of free chlorine (25 °C) | CT 3-log Giardia, mg-min/L |
|---|---|---|
| 6.0 or below | not stated | 79 |
| 6.5 | about 90 % | 94 |
| 7.0 | not stated | 112 |
| 7.5 | about 50 % (62 % at 5 °C) | 134 |
| 8.0 | not stated | 162 |
| 8.5 | about 10 % (calculated from pKa 7.54) | 195 |
| 9.0 | not stated | 234 |
HOCl shares: DuPont FilmTec chlorination and dechlorination manual, pKa 7.54 (Morris 1966); CT: EPA guidance Table B-1.
Speciation and reactions are explained under water chlorination chemistry.
Approved Test Methods for Chlorine, Chloramine, ClO2 and Ozone Residuals#
Approved methods are Standard Methods 4500-Cl for chlorine and chloramine, 4500-ClO2 D and E or EPA 327.0 for chlorine dioxide, and indigo 4500-O3 B for ozone (40 CFR 141.131). The 4 method groups are listed below.
- Chlorine: SM 4500-Cl D, E, F, G, H and I.
- Field kits: DPD kits where the State approves them.
- Chlorine dioxide: 4500-ClO2 D or E, EPA 327.0 Rev 1.1; chlorite daily at the distribution entrance plus a monthly 3-sample set.
- Ozone: 4500-O3 B; bromate monthly per ozone plant.
What Goes Wrong in Drinking Water Disinfection?#
Disinfection fails in 4 ways: residual decay, byproducts, degraded stored product and mishandling.
Residual Loss, Water Age and Nitrification in Chloraminated Systems#
Nitrification affects about two thirds of medium and large US chloraminating systems (1996 survey), and over 90 % of nitrifying samples held less than 2 mg/L (Wilczak et al. 1996). Health Canada recommends about 2 mg/L leaving the plant and above 1.5 mg/L everywhere. Ratio control is covered under chloramination. Blending with free-chlorinated water causes uncontrolled breakpoint, and 43 % of surveyed utilities saw more rubber degradation.
Disinfection Byproducts from Each Disinfectant#
US byproduct limits are TTHM 0.080 mg/L and HAA5 0.060 mg/L as LRAA, bromate 0.010 mg/L and chlorite 1.0 mg/L (40 CFR 141.64). The WHO groups byproducts by disinfectant, listed below.
- Chlorine: THMs, HAAs, haloacetonitriles; chlorate from hypochlorite.
- Chloramine: haloacetonitriles, cyanogen chloride, NDMA.
- Ozone: bromate, aldehydes, bromoform.
- Chlorine dioxide: chlorite, chlorate, chloride.
Lower pH lowers THMs but raises HAAs. Formation and control of disinfection byproducts sit on their own page.
The WHO principle is that disinfection is never compromised to meet a byproduct value; the Stage 1 best available technology is enhanced coagulation, enhanced softening or GAC10.
Precursor removal by enhanced coagulation comes before any change of disinfectant.
Hypochlorite Decay, Chlorate and Bromate in Stored Product#
Sodium hypochlorite loses half its strength in about 100 days at 15 %, 220 days at 10 % and 790 days at 5 % (WHO, citing AWWA M20), and chlorate builds as it decays. The WHO advises buying fresh, storing cool and dark, using within about a month and never adding new product to old. Dosed at its NSF MUL, hypochlorite adds a mean 2.5 µg/L bromate (range 0.10 to 26) against a single-product allowance of 3.3 µg/L (Health Canada).
Chlorine Handling Incidents and EPA RMP and OSHA PSM Thresholds#
Hypochlorite mixed with acid releases chlorine gas; NaOCl/HCl mix-ups caused 24 % of 72 analyzed unloading accidents (INERIS, 2025). Chlorine thresholds are 2,500 lb (1,134 kg) under EPA RMP and 1,500 lb (680 kg) under OSHA PSM; chlorine dioxide 1,000 lb under both; ozone 100 lb under PSM only. Program elements are set out under EPA RMP and OSHA PSM. Titanium and tantalum are the only metals suited to hypochlorite-wetted parts (Olin handbook).
Which Rules Govern Drinking Water Disinfection?#
Four rule sets apply: US treatment rules, US disinfectant and byproduct rules, NSF, AWWA and EN product standards, and the WHO and EU texts.
Surface Water Treatment Rules, the Ground Water Rule and the Safe Drinking Water Act#
The Surface Water Treatment Rule of 1989-06-29 (54 FR 27486) set the 3-log Giardia and 4-log virus floor; the IESWTR (1998-12-16), LT1ESWTR (2002-01-14) and LT2ESWTR (2006-01-05) added Cryptosporidium control.
Bins and toolbox credits are detailed under the surface water treatment rules.
The Ground Water Rule (71 FR 65574, 2006-11-08) requires at least 4-log virus treatment where treatment is required.
Every treatment rule traces back to the Safe Drinking Water Act.
MRDLs, Profiling and the Disinfectants and Disinfection Byproducts Rules#
MRDLs are 4.0 mg/L as Cl2 for chlorine and chloramines and 0.8 mg/L as ClO2 (40 CFR 141.65); they are not MCLs. Stage 1 (1998-12-16) covers systems adding a chemical disinfectant; Stage 2 (2006-01-04) those using one other than UV. Schedules are listed under the disinfectants and disinfection byproducts rules. Profiling starts at TTHM 0.064 or HAA5 0.048 mg/L; EPA had no revision proposal as of 2026-10-20.
NSF/ANSI/CAN 60 Certification and AWWA and EN Product Standards#
NSF/ANSI/CAN 60 certifies each product with a maximum use level as product, totaled across all application points, and most US states require it (NSF survey of ASDWA members).
How MULs are read is explained under NSF/ANSI/CAN 60. Ozone and monochloramine carry no listing of their own.
AWWA covers hypochlorites (B300-24), liquid chlorine (B301-24), sodium chlorite (B303-24) and the ammonia sources (B302-23, B305-22, B306-22); B300 requires a FIFRA-registered product for disinfection. The EN set includes EN 901:2024, EN 937:2016, EN 12671:2016 and EN 1278:2026.
Editions are listed under AWWA B-series standards.
WHO Guidelines and the EU Drinking Water Directive#
The WHO Guidelines, fourth edition incorporating the first to third addenda (17 June 2026), set free chlorine at 5 mg/L and chlorite and chlorate at a provisional 0.7 mg/L, with no value for ClO2.
Values are on the WHO guidelines for drinking-water quality page. Effective chlorination means 0.5 mg/L free chlorine after 30 minutes at pH below 8.0.
Directive (EU) 2020/2184 sets chlorate and chlorite at 0.25 mg/L each (0.70 mg/L with chlorine dioxide) from 12 January 2026, total THMs at 100 µg/L and bromate at 10 µg/L.
Article 12 duties are covered under the EU Drinking Water Directive 2020/2184. Active chlorine from sodium hypochlorite is approved for PT1 to PT5 under EU BPR Regulation (EU) 2017/1273 (2019-01-01 to 2028-12-31); drinking water is PT5.
How Does Disinfection Fit into the Full Drinking Water Treatment Program?#
Disinfection is the last barrier after filtration, which must hold combined filter effluent at 0.3 NTU or less in 95 % of monthly measurements (40 CFR 141.173).
The full set of drinking water treatment chemicals is mapped on the hub.
Orthophosphate shares the entry point, as part of corrosion control treatment for lead and copper.
Disinfectants as Oxidants: Iron and Manganese Removal, Taste and Odor#
Chlorine dioxide oxidizes manganese at 2.45 parts per part Mn above pH 7, and ozone oxidizes Mn(II) in 30 to 60 seconds.
Doses are covered under iron and manganese removal chemistry, and geosmin needs taste and odor control with carbon.
Disinfection Beyond the Plant: Water Mains, Wastewater Disinfection and Other Uses#
The same chemicals disinfect new mains and wastewater effluent under separate rules; field dechlorination follows AWWA C655-25.
New pipe belongs to water main disinfection, and effluent to wastewater disinfection, where peracetic acid entered US use at Frankfort, Kentucky, in 2005.
Do all public water systems have to disinfect?#
Yes for surface water under the Safe Drinking Water Act, and ground water needing treatment must reach 4-log virus (Ground Water Rule).
Does UV disinfection leave a residual in the water?#
No: the WHO advises chlorine or monochloramine after UV.
Is chloramine a weaker disinfectant than chlorine?#
Yes: it needs about 18 times the CT of free chlorine at pH 7 and 10 °C, so it serves as the secondary disinfectant.
Who Supplies Disinfection Chemicals and Runs Disinfection Programs?#
Producers and distributors supply them; US municipal water uses 628 million kg of chlorine a year, about 5 % of US production (EPA estimate, 2022).
Bid models are compared under water treatment chemical suppliers, and operating support comes from a water treatment program provider.