Surface Water as Drinking Water Resource – Excursion to the NaToxAq Partner Organization St. Galler Stadtwerke

by Carina Schönsee (ESR9)

On 7^th August Regiane (ESR 10) and I went to visit the NaToxAq Partner Organization St.Galler Stadtwerke (SGS) in St.Gallen (Switzerland). We were invited by Rolf Stettler, head of the Water Quality Assurance section, who kindly took the day to show us around and answer all our questions.

To start off the day we went to SGS’s main water abstraction site in Frasnacht, directly at the shore of beautiful Lake Constance. Water from Lake Constance is used as drinking water resource for about 4.5 million people from the adjacent countries Switzerland, Austria and Germany and is the main source for the city of St.Gallen. Raw water is abstracted at a lake depth of 60 m and already has a very high quality to start with. Lake Constance shows a very stable stratification with year-round constant water temperatures of about 5°C in the hypolymnion. Thus, the water abstraction site is hardly affected by chemical pollution or microbial growth. Nevertheless, the water purification process still combines several steps including ozonation and filtration which we discussed in all details. It was very interesting to compare the treatment procedure with other treatment plants we were able to visit on other occasions or in our home countries. After purification, the drinking water is pumped from the lake shore treatment facilities to reservoirs in St.Gallen from where it is further distributed. To reach all areas the water has to travel several hundreds of kilometers in length and almost 400 m in height! In the afternoon we visited two of the city's drinking water reservoirs, one on the very top of a small mountain with very nice views back to Lake Constance where we started our tour in the morning. The reservoirs are not only important for easy distribution but also guarantee constant supply and can bypass shortages for a limited time. The dimensions of such a distribution network were very impressive to see and we learned about all the aspects that have to be considered in treatment and distribution, from the importance of keeping low levels of assimilable organic carbon (AOC) throughout the whole distribution network to problems arising from low withdrawal patterns or dead-ends in the system. Water quality monitoring is a very important key process for a water supplier and already starts at the drinking water resource. Algae blooms have been a previously observed problem in Lake Constance, mainly in the 1970s, and the lake is highly influenced by increasing temperatures. In recent years, the invasion of certain mussel species is also closely monitored as the efficiency of the abstraction procedure can be affected when pipes or filters are blocked by extreme growth of these species. Regular monitoring and risk assessment allows the water supplier to act fast and adequately in case any threats to water quality or process operation occur.

We further talked about our PhD projects in more detail and discussed which results could be of interest for a water supplier relying on surface water as drinking water resource. Due to the lake characteristics, natural toxins from both cyanobacteria and plants are so far no threat to raw water quality as any compound in the lake is extremely diluted or washed out in a short time by the Rhine River flowing through Lake Constance in the epilimnion. Anyway, it was definitely interesting to learn about the occurrence and monitoring of other micropollutants in Lake Constance and the potential scenarios how and to what extent compounds like natural toxins could become a problem for a water supplier providing drinking water for a big population.