What is primary disinfection

Disinfection of drinking water

The Drinking Water Ordinance requires:
  • Number of coliforms, Escherichia coli and enterococci 0/100 ml,
  • Limit value for the number of colonies 100 / ml at the tap - no abnormal change (as an indicator parameter)
  • Colony count 20 / ml in the disinfected water immediately after processing
  • Detection of free chlorine at least 0.1 mg / l if disinfection is based on chlorine
  • or at least 0.05 mg / l of chlorine dioxide after the preparation has been completed.

The following procedures are common for drinking water:

  • Heating, fine filters, oxidizing agents, UV radiation and addition of metal ions.
  • The main oxidizing agents used are the halogens chlorine and chlorine dioxide, rarely bromine and iodine. Chlorine most often, bromine occasionally for pool water treatment, iodine for small systems.
  • Ozone is generally used more as a highly effective oxidizing agent because of its higher cost.
  • Hydrogen peroxide is a strong oxidizing agent, but has only a low disinfecting effect and is therefore not permitted for disinfection.
  • When disinfecting with metal ions, only the silvering process can be used under certain conditions.
  • Although copper ions have a good effect in killing algae, they are less effective in disinfecting.
  • Galvanizing a steel pipe (house connection lines) also slowly kills the germs without it being usable as a disinfection process.

The microbiological quality of the drinking water must be continuously monitored!

The procedures that are permitted for drinking water treatment in Germany are described in the notification from the Federal Environment Agency “List of treatment materials and disinfection processes in accordance with Section 11 of the 2001 Drinking Water Ordinance; 6th amendment; Status: November 2006 ”defined and described.

You can find the list at www.umweltbundesamt.de/


Chlorination process - In practice, the following are mainly used:

  • Chlorine gas,
  • Chlorine dioxide,
  • Sodium hypochlorite, known in liquid form as sodium hypochlorite,
  • Calcium hypochlorite, solid in tablet form.

Tab. 1: Active chlorine content of the common chlorine compounds

Chlorination is still the most common disinfection process for the central water supply, as both the costs of the necessary equipment and the running operating costs are low, and the effectiveness of the disinfection can be checked by simply verifying the chlorine content, in some cases only using the redox voltage can and a given depot effect prevents recontamination in the distribution network when carried out properly.

Chlorination for the oxidation of inorganic and organic substances in water is no longer permitted, and the addition of large amounts of chlorine during disinfection should be avoided if possible, as there is a risk of the formation of carcinogenic trihalomethanes (THM) (limit value 0.025 mg / l). The THM also form the typical bath water odor and thus lead to noticeable quality losses in drinking water due to odor formation.

The typical THM are:

  • - bromoform (CHBr3)
  • - Dibromochloromethane - CHClBr2
  • - dichlorobromomethane - CHBrCl2
  • - Chloroform (trichloromethane) - CHCl3

The permissible addition is therefore based on the Drinking Water Ordinance for chlorine, sodium, calcium and magnesium hypochlorite as well as chlorinated lime 1.2 mg / l of free chlorine have been limited, only in exceptional cases where there is otherwise a hygienic hazard are allowed up to 6 mg / l.

In the submitted Pure water may only be used after processing maximum 0.3 mg / l of free chlorine or in exceptional cases up to 0.6 mg / l can be detected. Some waterworks have therefore switched to chlorine dioxide, which does not produce such haloforms when added, or to other disinfection processes.

Safe disinfection is achieved if

1. The disinfectant is thoroughly mixed with the water.

2. The disinfectant has had a sufficiently long exposure time.

3. a chlorine content of 0.1 mg / l of free chlorine is continuously detected, with chlorine dioxide 0.05 mg / l of ClO2 .

Illustration: Scheme of a vacuum chlorine gas dosing system


Chlorine gas (DIN 19606 and 19607):

Properties - Cl2 is a yellow-greenish gas that is very toxic. Table 2 shows the effect of chlorine gas at various concentrations in 1 m3 Air indicated on humans. Chlorine gas is about 2.5 times heavier than air.

Tab. 2: Effects of different chlorine content ml / m3 Air on people


ml of chlorine gas in 1 m3 air

permissible chlorine content in air. during 8 working hours

0.5 - 1.58 mg / l

noticeable odor


Irritation in the throat

from 15

to cough

from 30

Maximum value for a short stay


Dangerous, even with a short stay


Quickly fatal


Chlorine dioxide ClO2  

  • Orange gas, 2.5 times more oxidizing than chlorine
  • Advantage: there are no by-products such as chlorophenol, it does not react with ammonium, no carcinogenic substances
  • Chlorine dioxide is explosive, no delivery in containers,
  • is made at the place of use from sodium chlorite and chlorine or sodium chlorite and hydrochloric acid
  • Solution of Cl2 in the water and as an 8% solution added to the treated water
  • Sodium chlorite must not get into the water
  • Chlorine dioxide process is increasingly used in larger WVU because of better handling of the basic materials and because no undesirable by-products (haloforms) are created in waters with organic pollution (OW, UF, contaminated GW)
Illustration: Scheme of a chlorine dioxide system
Chlorine dioxide is available in appropriate commercial packs for disinfecting system components. Packaging for use in dosing is also offered so that it can also be used for temporary disinfection.

Sodium hypochlorite  

  • Corrosive and highly toxic solution (lye)
  • Easy to use, is delivered in containers, can be added using simple dosing pumps
  • Disadvantage: short shelf life, higher costs, decomposition due to light, temperature, impurities, also rapid decomposition with traces of iron and metals
  • Use for the disinfection of system parts, with small DHW,
Figure: Scheme of a dosing system for sodium hypochlorite

UV disinfection  



With regard to the use of ozonation, it should be noted that this leads to an increased formation of biodegradable substances and therefore cannot be used as the last processing stage. As a rule, a downstream filter stage that operates biologically is required. Restrictions on the use of ozone can result from the formation of bromate if the water to be disinfected has increased bromide content.

The decisive advantage of UV irradiation compared to the use of chemical disinfectants is that it is a disinfection process that is practically free of by-products. The prerequisite for use is that the disinfected water is biologically stable, as no residual disinfection capacity can be maintained after disinfection is complete. The UV irradiation system must be designed for the respective water quality.

Table 3: Areas of application and boundary conditions to be observed for the use of disinfectants and methods

Disinfectant /


Added amount

Maximum concentrations after processing


or worksheets

Chlorine and
Chlorine compound

- pH <8
- ammonium <0.1 mg / l
- DOC ≤ 2.5 mg / l 2)

- 1.2 mg / l Cl2
(6.0 mg / l Cl2) 1)

- max. 0.3 mg / l Cl2
- min.0.1 mg / l Cl2 (6.0 mg / l Cl2) 1)

- THM and other organochlorine compounds
- biodegradable substances

W 229, W 295,
W 296 and
W 623

Chlorine dioxide

- entire pH range
- DOC ≤ 2.5 mg / l 2)

- 0.4 mg / l ClO2

- max.0.2 mg / l ClO2
- min.0.05 mg / l ClO2

- chlorite
- biodegradable substances

W 224 and
W 624


- entire pH range

- 10 mg / l O3

- 0.05 mg / l O3

- bromate
- increased formation of biodegradable substances

W 225 and
W 625

UV irradiation

- according to approval (test certificate),
- biologically stable water




W 293 and
W 294

Decoction 3)

Emergency measure





1 - permissible if the disinfection cannot be ensured otherwise, or if the disinfection is temporarily impaired by ammonium
2 - Orientation value due to limit values ​​for by-products
3 - bubbly boiling