Xylem Water Solutions - Computerized wastewater management takes Flygt in swedish municipality.

Computerized wastewater management takes Flygt in swedish municipality.

The city of Trollhättan, in western Sweden, was ready to invest in a major upgrade of most of its sewage pumping stations. Engineers, managers and local elected officials were in agreement: it was time to reduce maintenance visits, increase oversight capabilities, improve the working environment and - not least - see to it that the investment would quickly pay for itself in lower operating costs.

"I am convinced we can recoup the costs (of upgrading Trollhättan's pump stations) in less than three and a half years," says Water and Sewage Treatment Manager Ronald Svensson.

ITT Flygt, which had over the years supplied the submersible pumps and related equipment for 37 of Trollhättan's 38 pumping stations, was the natural choice as a partner in the upgrade. Working with the city's engineers, led by Water and Sewage Treatment Manager Ronald Svensson, Flygt helped shape a package of solutions to meet all the specified needs, including flush valves to prevent sludge accumulation in pumping station sumps, and Flygt's MacTec monitoring and control system for centralized operation. "I've promised the city's technical committee that this upgrade will pay for itself in three and a half years," Svensson says.

The upgrade, including a large transfer pipe and two new pump stations, will make it possible to close down a small treatment plant at the southern end of the city, better utilizing the capacity of the main plant to the north.

The right pump in the right place
Flygt's PEAS (Pump system Economy AnalysiS) computer system was used to optimize functions in many of the stations, ensuring that pump capacity matches the requirements of each site. Computer loggers were placed in several stations to gather data about flow rates and pump activity. It was found that pump capacity in some stations was dimensioned to handle situations that arise only every few years, leading to excessive energy costs for large pumps moving small volumes. In some stations, two pumps were found to be adequate where three had been installed. In others, a smaller third pump was installed to complement two large units. The small pump takes care of normal flow, while the capacity is in place to handle exceptional events. The PEAS system calculated the exact payback time for the investment in the third pump.

In addition to energy consumption calculations, PEAS can be used for pump and pipe dimensioning on both new and existing networks, and for present-value analysis of investments and operating costs for different combinations of pipe and pump systems.

Flush valves for cleaner sumps
In 28 of the stations, pump housings were replaced and Flygt flush valves installed to prevent sludge build-up in the sumps. At the beginning of each pump cycle the flush valve is open, sending a powerful jet of water into the sump to create a swirling motion that re-suspends the sludge. After some 20 to 40 seconds, the fully automatic control valve closes and the contents of the sump, now thoroughly mixed, are pumped out of the station toward the treatment plant. When the pump shuts off, the valve returns to its open position to await the next pump start. Operated by changes in over and under-pressure, the technically simple flush valve is designed to operate reliably in the demanding environment of a sewage station.

Trollhättan's 38 pump stations are individually equipped to meet local pumping requirements. The basic components of the Flygt Auto-CleanseTM system are shown in this illustration. Designed to keep sewage stations clean automatically, the system includes a control unit, a flush valve to resuspend sewage deposits before pumping, and the patented NevaClog impeller to minimize stoppages even when handling especially contaminated sewage.

This complete evacuation of sludge eliminates much of the odor that can cause problems in and around pumping stations, as well as reducing the risk of poisonous and corrosive hydrogen sulfide gas build-up. Just as important, routine maintenance of the station is sharply reduced. No longer do crews need to be sent out on a regular basis for the unpleasant job of flushing loose a layer of grease and sewage solids that have built up on the sump walls and bottom.

Total control
With the help of the Flygt company MacTec, a PC-based control and monitoring system has been installed throughout Trollhättan's water works and pump station network. Crews used to visit each station at least once a week to check that all was in order; now, site visits are necessary only when the computerized monitoring system identifies a problem. Almost all maintenance now takes place during normal working hours, and expensive middle-of-the-night emergency repairs are all but eliminated.

When pumping begins, the flush valve is open, setting in motion a jet stream of water that thoroughly mixes any deposits that have accumulated since the last pump cycle. After about thirty seconds, the flush valve closes and the pump can empty the sump. When the pump stops the valve opens automatically, ready for the next pumping cycle.

The MacTec units in each station continually track all pump activity, and if any function deviates from normal values, an alarm is sent to the central computer. Operators can see what is happening at all of the stations from the comfort of a desk. In fact, using a portable PC and modem, the duty operator can keep an eye on things from any place with a phone connection.

And not only are the stations monitored from a distance; it's also possible to carry almost any operation from the central computer. Flow rates and pump operations displayed on the computer screen quickly alert the operator if problems arise. It's then possible to start or stop a pump, run the pump backwards to dislodge blockage, or simply start an alternate pump until a crew can be sent out to make repairs.

"We have total control over every station now," says Svensson. "We know from second to second what's going on and whether there are any malfunctions."

The MacTec control units can be programmed for just about any situation that the operators feel is needed. One of the requirements that the Trollhättan upgrade was intended to meet involved measuring overflow rates. Strict new national standards require Swedish municipalities to report the amount of sewage that overflows when treatment plants and pumping stations are overloaded or suffer breakdowns.

Trollhättan's engineers hung sensors at exactly the level that water begins to flow out of the overflow pipe. The sensors tell the computer how long the excess is exiting the station, and even how full the pipe is. With the size of the pipe programmed in, the computer can calculate the total overflow volume. A battery back-up keeps the MacTec system operating long enough for repairs to be made."This gives us some pretty reliable figures to report to the pollution control authorities," Svensson says.

MacTec even improves safety at the pumping stations. When the lights in the station building have been on for 15 minutes, a local alarm is sounded. The worker at the station simply presses a reset button, indicating that all is well. If the worker is for any reason unable to respond, an emergency alarm is sent to the central computer. It's now possible to send out a single worker for most above-sump maintenance.