defr Glossar

Drinking water treatment

Drinking water treatment
Fig. 12: Drinking water treatment

1 Turbidity in raw water 15 Ozone in air or oxygen (O2)
2 SAK 254 (UV absorption) in raw water 16 SAK 254(UV absorption) after activated carbon
3 Colour (Hazen) of raw water 17 Turbidity before disinfection
4 Oil traces (HC) in raw water 18 Concentration of chlorine dioxide (ClO2)
5 Turbidity for dosing flocculant 19 Turbidity in treated water
6 SAK 254 (UV absorption) in raw water 20 SAK 254 (UV absorption) in treated water
7 Colour (Hazen) of raw water 21 pH value in raw water / treated water
8 Turbidity after rapid filtration 22 Conductivity in raw water / treated water
9 Turbidity after filter back wash 23 Redox potential in raw water / treated water
10 Turbidity before ultra or membrane filtration
11 Turbidity after ultra or membrane filtration 24 Dissolved oxygen in raw water / treated water
12 Turbidity after sand filter
13 Turbidity after filter back wash 25 Concentration of free chlorine, chlorine dioxide (ClO2) and ozone (O3) in water
14 Ozone generator monitoring

All three measuring principles applied by SIGRIST are used in the treatment of drinking water:

turbidity, absorption and fluorescence

The criteria of interest for a given raw water depends on its origin. In the case of surface water, the turbidity value is used to control the amount of flocculent added. Where spring water is tapped, the turbidity and oil trace measurements decide whether the water will be used or discarded. In addition to solids, dissolved organic carbon can be present and can be measured with SAK (UV 254 nm absorption). Any yellow coloration in the raw water is determined by absorption measurement. Generally around 380-430 nm to suit local requirements.

In the case of raw water laden with impurities, flocculation/sedimentation is the crucial treatment step for eliminating solids. So the turbidimeter operating between the settling tank and sand filtration is a key contributor to optimal plant operation.

At the sand and membrane filters, turbidity measurement not only monitors the filtration step, but also helps optimize the sand cleaning process.

Finally, UV absorption measurement handles several different functions. It is used following the ozone generators to check the concentration of the generated ozone in the gas phase and following the residual ozone decomposer to detect the residual content (ozone measurement). At the outlet of the activated carbon filters and prior to the sterilization by UV-radiation and disinfectants, turbidity and the SAK/DOC value is monitored. Before the water is fed into the water network, turbidity, the SAK/DOC value, the disinfectant and the pH value are again monitored. The border between them is fuzzy and varies depending on which author you read.

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