Interview with Riccarda Engi from City of Zürich’s Water Supply/Wasserversorgung
The city of Zürich has a fantastic water supply network. The Water Network research team had a wonderful opportunity to interview Ms. Engi Riccarda - public relations officer at the Zürich Water supply office.
In this interview, she talks about the various treatment methods applied before supplying drinking water and the monitoring steps followed to check the quality of the drinking water. In the attached document the description of the Lake Treatment plant at Lengg can be found.
Q1. The major source of water supply in Zurich being the Zurich lake, what are the treatment steps being carried out before supplying the lake water as drinking water to the residents of Zurich?
Answer: There are 5 treatment steps until we reach drinking water.
The untreated water is taken from Lake Zurich. The abstraction point is 600 meters from the bank at a depth of 30 meters. At this depth, the water is a cool 4 to 8 degrees Celsius and of good quality throughout the year. From here it flows through a pipe to the suction well of the pumping station. Pumps lift the water from this well through a pressure pipe into the Lengg Lake Waterplant located 50 m higher where it gets cleaned throughout 5 treatment steps. This process is inspired by nature.
- First step: Ozonation, Ozone kills bacteria and germs
- Second step: Rapid Filter ( gravel, quartz sand). The rapid filter provides the first purification, a filtering out of particles.
- Third step: Intermediary Ozonation
- Fourth step: Activated Carbon Filter. Ozone remaining in the water after the intermediate ozonation is transformed back into oxygen. If the water is contaminated with organic substances such as petrol, phenol or solvents, as the result of an accident, these substances are held back through absorption.
- Fifth step: Slow sand filter: They take over the fine purification at the end of the treatment chain and also hold back germs. That is why the water flows here ten times more slowly than in the rapid filters. The filter bed is made up of lake sand of various grain sizes.
After the treatment chain, we have drinking water. The drinking water is pumped into the 21 reservoirs located in higher areas around Zurich. From the reservoir, the water flows into the distribution network to the households.
Q2. When groundwater is used as the source, how are the treatment methods different?
Answer: We don’ t treat the groundwater at all. We pump the groundwater from a depth of 25 meters. So the water passes through the ground. That means the water already has been cleaned through the natural filter in the ground so we just need to examine the water. That is what we do with online monitoring and biomonitoring. We pump the groundwater directly into the reservoirs.
Q3. How are the polluters polluting the groundwater dealt with? What are the quality control measures implemented?
Answer: We already examine the lake water. Then we examine the water between the treatment steps and also in the reservoirs. We examine the water regularly with online monitoring such as biomonitoring ( using trouts and daphnia). We own different laboratories where we examine the water:
-Microbiology (germs, bacteria)
-Limnology ( Algae, plankton)
-Chemistry ( chemical materials, metals)
-Microelements ( organic substances within the nanogram/l range)
Q4. In the last 30 years, what trends have you identified in terms of quality and threats to the quality of drinking water and to the ecosystem in Zurich?
Answer: We never had a big incident to deal with. But what a major incident would be lack of electricity because without electricity we wouldn’t be able to run a pipe or a treatment step. So a lack of electricity over several days would indicate a major problem. That’s why we run emergency power generators to run the pipes of the groundwater plant. That way we are able to pump at least groundwater into the distribution network.
Q5. How do you integrate your testing results and what kind of efforts are made to tackle the water quality threats?
Answer: We always are up to date with all the different parameters to examine. Our laboratories are able to examine different quality threats within the nanogram/l range.
Q6. How do you maintain the piping and other water supply infrastructure to ensure longevity? Are regular checks carried out?
Answer: It’s very important to maintain the distribution network. We have 1’ 540 km of pipes only with drinking water. Most of the money we earn goes back into the maintenance of the network. We renew each year at least 2% that means circa 30 to 35 km of pipes. We also keep the water plants in good shape and redevelop the network and also keep up our laboratory.
Q7. Most Drinking Water Utilities use Chlorine for purification of the water source. However, you use an alternative technology by using Ozone. Is this effective? What is the situation here that makes the use of chlorine disinfection unnecessary?
Answer: We don’ t use chlorine since 1992. We use ozone. One reason why we don’t need to use chlorine is that the quality of the lake got much better for the last 40 years ( also the wastewater treatment) . And also with the process of the treatment steps inspired by nature (with ozone and the activated carbon filter) it’s not necessary to use chlorine.
Watch our Video Interview with Riccarda about the City of Zurich Water Utility