The water supply system in Zurich supplies approx. 850,000 people in the city of Zurich and in 67 nearby municipalities with high-quality drinking water at all times. On average, 151,200 m3 of water are consumed each day, the entire water supply network is 1,542 km long (Figures: 2010).
Wasserversorgung Zürich (Zurich Water Supply) has redesigned the pre-oxidation and post-oxidation steps at its Limmat water treatment plant, which is used for artificial groundwater recharging, both technically and chemically. The objective is to replace the chlorine component of the chlorine/chlorine dioxide mixture that is used for this, which has been obtained from liquid chlorine to date.
"To increase the environmental safety of the water treatment system, we wanted to replace the process of obtaining chlorine from liquid chlorine and instead use the less dangerous method of obtaining it from brine", explains Walter Brechbühl, project leader for process engineering/plant construction at Wasserversorgung Zürich. That was why they advertised for bids for the provision of an electrolytic chlorination system. The call for tender specified that the system must use the tubular cell process. Three suppliers expressed an interest in the call for tender, although only two actually submitted bids in the end, one of which was Rheno Umwelttechnik AG from Schlieren.
The project leader compared the bids meticulously. "The most important factor was the technical solution", that took top priority in making the final decision. This included the plant concept, the chemical processes used, the functionality, warranty/maintenance details, the completeness and transparency of the offer as well as the availability. Other criteria were the price, the references and the evaluation of the supplier.
The outcome of the evaluation procedure saw both the supplier Rheno and the OSEC®-B electrolytic chlorination system from Evoqua Water Technologies were selected as the winners. The product characteristics: that it adhered to the specified process quality data (thresholds), especially over a prolonged period (process reproducibility), were especially decisive. Other plus points were the functionality, the predicted reliability and operating life of the system and the quality of the service provided by the supplier. To reassure themselves of this, the decision makers at Wasserversorgung Zürich visited one of the more than 1400 reference plants in Switzerland located in Basel as well as the factory where the electrolytic chlorination systems are made in Günzburg.
The OSEC-B systems have been in operation in Zürich since mid-2007. Installation and commissioning was completed fast and without any problems. The two OSEC-B systems are used alternately, so that there is always one system available as a back-up in case of an emergency. Since then, Wasserversorgung Zürich has installed two more electrolytic chlorination systems for the Moos lake water works for the pre-oxidation of the treated water. The production capacity in Moos is even higher, with each system having a capacity of 2660 g/h chlorine.
THREE GOOD REASONS TO USE OSEC-B ELECTROLYTIC CHLORINATION SYSTEMS:
- Robust, mature systems engineering; secure, permanent supply thanks to the use of two systems in parallel
- Economical due to the inexpensive raw material (salt) and low energy
- High-quality service from the supplier (maintenance service, availability, spare parts service, competent personnel)