The regional water authorities of Lake Zug in Switzerland required the automation of 30 remote stormwater basins and pumping stations and the networking of these to the central wastewater treatment plant. In 2002 Swiss engineering company Chestonag Automation AG won the contract to supply this system.
From wastewater to clear lake
The ARA Schönau plant treats residential and industrial wastewater from an area with approximately 140 000 residents. The dry-weather treatment capacity of the plant is 800 litres per second. In wet weather the maximum treatment capacity is 1600 litres per second, with an additional 1200 litres per second which are mechanically cleaned and discharged into the nearby river.
The 30 stations around Lake Zug collect surface water and wastewater from the surrounding towns in stormwater basins and where possible the surface water from residential areas is separated and led directly into the lake. The stormwater basins have a storage capacity of 100-1000 m³ and the pumping station has a capacity of 20-200 litres per second. From the stormwater basins, the wastewater is pumped to the ARA Schönau plant. A total of 75 kilometres of pipes and tunnels connect the remote stations with the plant. The pipe and tunnel system has a storage capacity of 8000 m³, which is used as a giant stormwater basin. In heavy rains, when the basins fill up quickly, some water is discharged directly into the lake. The discharge outlet is at the top of the basin to ensure that the released water contains the lowest amount of particulate matter.
Realtime remote access
Since the remote stations are unmanned, realtime monitoring and operation of each station from the main scada system at the ARA Schönau plant is crucial. Chestonag resolved this need by establishing an intranet with a data-over-voice network allowing continuous modem communication with the stations. Inputs from a total of 1100 signals enable the controller systems at the stations to monitor smoothly and to regulate water levels and the distribution of water to the central plant.
For maintenance purposes a Cimrex 90D operator terminal is located at each of the 30 stations. The operator terminal portrays the application as a mini scada system, giving the maintenance engineer a complete overview of the entire system.
Realtime values from all 30 stations are available on the local terminal and the look of the application resembles that of the scada system at the ARA Schönau plant. "Being able to see immediately the impact of my actions here on the system as a whole, helps me to make the most efficient adjustments to the systems," says Albin Amgwerd, maintenance engineer of the ARA Schönau wastewater treatment plant.
Technical description of the application
The communication between the Cimrex 90D operator terminal and the Siemens S7-xx-2DP systems uses the Beijer Electronics S7 MPI driver. Each station is connected to the wastewater treatment plant by an infranet modem and analog telephone line. Infranet is a Swiss data-over-voice network enabling data transfer and simultaneous dial-up voice connections over a single telephone line. The data transfer is in ASCII protocol via an RS232 interface on the infranet modem. This system enables all remote stations to be monitored and operated in realtime from the main scada system at the wastewater treatment plant.
CBI-electric: low voltage is the sub-Saharan agent and distributor of the Beijer Electronics range of HMI products.
For more information contact Shaun Loesch, CBI-electric, +27 (0)11 928 2000, [email protected], www.cbi-electric.com
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