Disinfecting drinking water: the methods compared
Chlorine, sodium and calcium hypochlorite, chlorine dioxide, ozone and UV irradiation are permitted for disinfecting drinking water in Germany; that is set out in the German Environment Agency's list under Section 20 of the Drinking Water Ordinance. Hot-water systems can additionally be disinfected thermally. Hydrogen peroxide appears on the list only for oxidation, and silver no longer appears at all.
The methods at a glance
The table shows what the list under Section 20 of the Drinking Water Ordinance and the technical rules lay down for each method. The values apply to water that is supplied as drinking water.
| Method | Purpose per the UBA list | Residual concentration in the pipe network | Reaction products | Addition and concentration | Limits |
|---|---|---|---|---|---|
| Chlorine dioxide | Disinfection | possible | Chlorite, chlorate | Addition up to 0.4 mg/l; thereafter 0.05 to 0.2 mg/l | On-site generation or dosing of a ready-made solution by recognised methods (DVGW W 224) |
| Chlorine | Disinfection | possible | Trihalomethanes | Addition up to 1.2 mg/l free chlorine; thereafter 0.1 to 0.3 mg/l | By-products from organic substances in the water |
| Sodium and calcium hypochlorite | Disinfection | possible | Trihalomethanes, bromate, chlorate | as chlorine | also commonly used for system disinfection |
| Ozone | Disinfection, oxidation | not applicable | Trihalomethanes, bromate | Addition up to 10 mg/l; thereafter up to 0.05 mg/l | at the waterworks; not used as the final stage because of AOC formation |
| UV irradiation | Disinfection method | not applicable | no additives | demonstrated efficacy of at least 400 J/m² | acts only on the water at the point of installation |
| Thermal disinfection | System disinfection of hot water | not applicable | none | at least 70 °C for at least 3 minutes at every outlet | temperature must reach every outlet; risk of scalding |
| Hydrogen peroxide | oxidation only, not disinfection | not applicable | not applicable | Addition up to 17 mg/l; thereafter up to 0.1 mg/l | named for system disinfection in DVGW rules |
| Silver | no longer on the list | not applicable | not applicable | not applicable | formerly only for preservation, as an exception |
Sources: UBA list under Section 20 TrinkwV, as of January 2023 (in German); DVGW, Hambsch 2009 (residual concentration, AOC); LGL Bayern (thermal disinfection). Full list below.
Which methods are permitted in Germany?
Under Section 19(3) of the Drinking Water Ordinance, only substances and methods on the list under Section 20 may be used in treatment. The German Environment Agency keeps this list. Its section for disinfection substances names exactly five substances: calcium hypochlorite, chlorine, chlorine dioxide, sodium hypochlorite and ozone. UV irradiation is listed as a disinfection method if the device has demonstrated an efficacy of at least 400 joules per square metre, tested under DVGW W 294-2 or DIN 19294-1.
Thermal disinfection is not a treatment substance but a method for disinfecting hot-water systems. The technical rules for it are set out in DVGW W 551 and W 551-3.
What do residual concentration and depot effect mean?
A disinfectant with a residual concentration also protects the water on its way through the pipe network. According to a publication of the DVGW Water Technology Centre, chlorine and chlorine dioxide normally achieve effective disinfection and can at the same time maintain a residual concentration in the pipe network. That is why they are often used as the final treatment stage.
UV irradiation and ozone cannot do this. For both, the UBA list notes that the method is not applicable for maintaining disinfection capacity in the pipe network. UV irradiation adds nothing to the water; it acts only on the water flowing past the device.
What by-products form?
Every chemical method has its own reaction products, which the UBA list names explicitly. For chlorine and hypochlorites these are trihalomethanes, and for hypochlorites also bromate and chlorate. For ozone they are trihalomethanes and bromate. For chlorine dioxide they are chlorite and chlorate.
The Drinking Water Ordinance limits these substances in Annex 2: trihalomethanes to 0.050 mg/l in total, bromate to 0.010 mg/l, chlorite to 0.20 mg/l and chlorate to 0.070 mg/l for continuous dosing. The details for chlorine dioxide are on the page Limit values, chlorite and chlorate.
Why hydrogen peroxide and silver are not drinking water disinfectants
Hydrogen peroxide appears on the list under Section 20 only in the section for general treatment substances, with the intended use of oxidation. It is absent from the section on disinfection substances. For disinfecting systems, where the water is afterwards discarded and rinsed, the DVGW codes of practice W 291 and W 551-3 name hydrogen peroxide alongside sodium hypochlorite and chlorine dioxide as proven chemicals.
Silver no longer appears on the current list; it was removed by 2019 at the latest. Earlier versions permitted silver only for preserving stored water, for non-systematic use as an exception. Guideline No. 5 on ship hygiene likewise assigns silver preparations only to preservation.
Which method suits which application?
| Application | What the rules provide | |
|---|---|---|
| Building installation with Legionella | Investigation of the cause and risk assessment under Section 51 TrinkwV; measures include temperature control under DVGW W 551, thermal or chemical system disinfection, and, for high test results, immediate measures such as point-of-use filters | Legionella |
| Drinking water on ships | Guideline No. 5 permits sodium hypochlorite, calcium hypochlorite and chlorine dioxide | Motorhome and boat |
| Water tank in motorhome and caravan | DIN 2001-2: drinking water supply from non-stationary systems | Cleaning the water tank |
| Drinking water hose at fairs and construction sites | DVGW guidance: flush; if problems occur, system disinfection under W 551-3 and W 291 by a specialist firm | Drinking water hose |
| Dispensing system | Cleaning and disinfection under DIN 6650-6 and the guidance of the German Social Accident Insurance Institution for the foodstuffs and catering industry (BGN) | Dispensing system and beer line |
| Treatment unit at the dentist | Flushing under KRINKO guidance (Commission for Hospital Hygiene and Infection Prevention at the Robert Koch Institute); if colony counts are high, disinfection in consultation with the manufacturer | Treatment unit |
Where does biostream® ZERO fit into this comparison?
biostream® ZERO is a concentrate based on chlorine dioxide. The user prepares it from two components; after 3 hours the concentrate is ready. It disinfects tanks, hoses, pipes, dispensing systems and the water lines of treatment units, without a dedicated plant for generating chlorine dioxide. The dosing calculator works out the quantity for an application.
Efficacy tested. In 1,620 individual tests at the Institute of Food Science and Human Nutrition of Leibniz University Hannover (led by Dr. rer. nat. H.-D. Werlein), biostream® ZERO was shown to have an efficacy of 99.9 % against germs occurring in drinking water, at a contact time of 5 minutes.
Two head-to-head comparisons look deeper into individual questions: chlorine dioxide or chlorine and chlorine dioxide or UV disinfection.
Frequently asked questions on the comparison
Which disinfectants are permitted for drinking water?
The German Environment Agency's list under Section 20 of the Drinking Water Ordinance names calcium hypochlorite, chlorine, chlorine dioxide, sodium hypochlorite and ozone for disinfection. UV irradiation is added as a disinfection method. No other substances may be used to disinfect drinking water.
What is better for drinking water: chlorine or chlorine dioxide?
Both appear on the list for disinfection and can maintain a residual concentration in the pipe network. They differ in their by-products: for chlorine the list names trihalomethanes, for chlorine dioxide chlorite and chlorate. The detailed comparison is under chlorine dioxide or chlorine.
Does a UV system help against Legionella in a building installation?
A UV system disinfects only the water flowing past the device. It cannot build up a residual concentration in the pipe network; the UBA list states that the method is not applicable for this. Germs that persist in pipes behind the device are not reached by the UV irradiation.
Is thermal disinfection enough?
It works if at least 70°C is present at every outlet for at least 3 minutes. That is how the Bavarian State Office for Health and Food Safety (LGL Bayern) describes it. If the hot water does not reach a particular point, for example a rarely used pipe, that point remains untreated. Thermal disinfection does not cover cold water.
May I use hydrogen peroxide for the water tank?
As a drinking water disinfectant, the list under Section 20 does not name hydrogen peroxide, only for oxidation. For disinfecting tanks and pipes, after which the water is discarded and flushed, DVGW rules name hydrogen peroxide as a proven chemical. Chlorine dioxide is listed in both cases.
Sources
- German Environment Agency: list of permitted treatment substances and disinfection methods under Section 20 TrinkwV, as of January 2023 (in German)
- Drinking Water Ordinance, Section 19 (in German) and Annex 2 (in German)
- DVGW Water Technology Centre, Hambsch: necessity and alternatives to final disinfection with chlorine or chlorine dioxide (2009, in German)
- DVGW: UV disinfection devices (in German)
- Bavarian State Office for Health and Food Safety: Legionella (in German)
- DVGW W 551-3: cleaning and disinfection of drinking water installations (in German)
This overview does not replace legal advice. The version of the Drinking Water Ordinance and the list under Section 20 in force at any given time is authoritative.
Read more on drinking water disinfection
Use biocides safely. Always read the label and product information before use. Material compatibility applies when the recommended dosage and contact time are observed.