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Flygt is providing 53 pumps to Munich municipal authorities for use in their major renovation of the city's waste water disposal system. This project is characterized by very high standards of purity for the discharged water and by other environmentally beneficial features.
One of West Germany's biggest, municipal sewage treatment plants became fully operational in September 1989. It's been built by Munich's city authorities as part of an extensive renovation of the city's waste water disposal system. This new plant, which cost DM 565 million, is known as the Munich number two treatment plant and is located just to the north of the city. An extensive expansion and upgrading of the original Munich number one treatment plant, located about 12 km to the south, just within the northern boundary of the city is soon completed.
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Great efforts have also been made to ensure that the new Munich number two plants blends in with the landscape.
For the equipping of both of these plants, the municipal authorities have chosen Flygt submersible pumps. The function of most of these pumps is to pump the waste water along between various phase of the treatment process. All of these pumps are equipped with check valves as a protection against back flow. In the first case, Munich number two, 27 of Flygt's 7000-series propeller pumps are installed and in Munich number one another 26 7000-series propeller pumps. Six of these will be the PL 7100 EX version with speed regulators.
Purer discharge
The new number two plant has the capacity to deal with the waste water generated by a population of one million. But the modernization of Munich's system is not concerned just with expanding capacity: Standards for the finally treated water are also being raised. At the number two plant, for example, water discharged from the plant, into the river Isar, will be 98% pure (in terms of reduced biological oxygen demand and removed solids).
Process refinements
This extremely high level of treatment efficiency is being achieved at Munich number two mainly by two somewhat unusual features of the plant: firstly, an additional clarification process during biological treatment and, secondly, a final sand filter of the type often used in the preparation of drinking water; The sand filter is quite a new feature for a European treatment plant and is only the second one of its kind to be constructed in Germany. The work on the original number one plant involves adding a similar second clarification phase to the plant's biological treatment sequence, again with a view to upgrading the quality of the water finally discharged.
Enviromental consciousness
As well as this concern with the quality of the discharged water, great efforts have also been made to ensure that the new plant itself blends in with the landscape and causes as little environmental disturbance as possible: It's been built in an agricultural area and is surrounded by a screen of trees on three sides. The height of all the buildings and structures in the plant has been kept to a minimum. Several precautions have been taken to avoid the risk of odor polluting the surrounding country-side: The plant is equipped with an extensive system of large diameter pipes containing extractor fans. These have inlet openings at every point where raw or semi-raw sewage is exposed. They extract the sewage vapor and return it to the beginning of the treatment process. In addition, the buildings have double "air lock" doors to contain any vapor inside the building.
Munich´s sloping advantage
The very fortunate feature of Munich's topography, from the point of view of designing and constructing waste water disposal systems, is that the city is built on a slope: Starting from a point just south of the city to a point roughly 50 kilometers directly north of the city, the land, rather smoothly and evenly, drops in height by 100 m. This is why both the treatment plants are located in the north; drainage to them can be effected largely by gravity.
New main channel
A new system of main sewage and drainage channels is built to fully exploit this natural asset. One runs for 12 km from the number one plant to the number two plant. It's constructed from concrete and has three different section profiles: It starts with an oval-shaped interior 2.2 m wide and 2.8 m high, the middle section broadens into a rectangular interior 5.20 m wide and 5 m high and the final section divides into two separate channels, running side-by-side, both of which are 5.2 m wide and 3.5 m high. Another main channel will drain from the northwest of the city directly into the new number two plant.
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The large capacity of the new channel, as it approaches the number two plant is intended to function as an emergency storage sump if the plant's intake capacity becomes overloaded. In addition to this the city is also in the process of constructing a system of very large underground storage basins. This will have a capacity of 700, 000 m3, of which 300,000 m3 has already been completed. The purpose of this system is to retain rain water in the combined sewage and storm water system. From these storage basins the water is later discharged to the number two plant. One of the reasons for constructing this system is that German federal law requires that even storm water drainage systems must be processed in a treatment plant before being discharged.
Because of this reliance on gravity, the channel, when it enters the number two plant, is quite deep. The bar screens which filter out the larger, physical objects from the in-flow, therefore, reach a long way down. The moving mechanical scrapers, which remove these objects from the bar screens, have to make a long journey up and down the screens. For this reason the two screen cleaning systems (one for each inflow channel) required particularly large and powerful motors to drive them. And consequently, this first unit in the mechanical cleaning process required the tallest building in the plant. The physical debris removed at this stage is taken by conveyor belt to containers which are subsequently removed from the plant by trucks. Following the mechanical screening, the inflow is raised to ground level by Archimedes pumps. There are three of these, each with a diameter of 2.5 m. One is a stand-by. They have variable speeds, up to a maximum capacity of 2.5 m3/sec per pump. Thus with two working together at top speed they provide an intake of 5 m'/sec which is the maximum flow through capacity of the plant. This level of inflow is likely to occur only during periods of heavy rain. The normal inflow level is around 3.3 m3/sec.
Four plants in one
The rest of treatment process stream is divided into three sequences: first mechanical, followed by a two-phases biological process and finally the sand filter. The plant contains four of each of the units necessary for these stages. Mr. Ellersdorfer, manager for mechanical systems in the Water Section of the city's civil engineering department, explained that the configuration of the plant is, in effect, four separate treatment process streams running in parallel.
From the Archimedes pumps, the waste water goes first to sedimentation tanks, where, owing to the reduced velocity of flow, the coarser sand is deposited. The sand is removed from the bottom of the tanks by mechanical scrapers. It is then, also, eventually removed from the plant in containers. Next in the mechanical phase is the primary clarifier unit. This also relies on sedimentation, this time in a round settling basin. The material deposited in this first clarifier is the heaviest sludge from the in-flowing waste 27 P-pumps are installed in the Munich number 2 plant. water. The floor of the basin is scraped by a whole series of plow-like blades attached to a diametric rotating arm. The action of these blades pushes the deposited sludge, in a spiral motion, into central cavity in the basin. From here it is pumped out, eventually to one of the three anaerobic digesters.
Maximizing resources
Sludge from all four of the clarifier units in each process stream is routed to these digesters. This material remains on average 27 days in the digester, where it generates considerable quantities of gas. This is stored in a gas tank at the plant and used to drive generators, both those which produce the compressed air used in the aeration tanks and electrical generators. These gas-driven generators produce 60% of electrical current consumed in the plant. This represents a saving in energy costs of DM 3 million per annum. In addition, the cooling water from the generator motors is used to provide heat for some of the treatment processes. There are also plans to use the sludge, after it has been processed in the digesters, as a power station fuel.
Two-phase activated sludge process
The waste water is then pumped onto the biological phase of the treatment process. Here the new Munich plant is again unusual in that there are two serial, activated sludge processes. In the first process the population of micro-organisms is predominantly of the type that breaks down the most easily digestible organic material. From this first aeration basin the waste water flows into a secondary clarifier, which operates in the same way as the one described above. Then comes a second contact tank, this time with a micro-organism population which mainly breaks down ammonia. Both these tanks have air diffusers at the bottom, to aerate the activated sludge mass in order to create a good environment for the organisms, to encourage them to breed and consume the organic content of the waste water.
Additional clarifying
At the new Munich plant there then follows a further two clarification processes. The first, in a smaller clarifier, separates out carbon compounds. The second, in a larger clarifier, removes ammonia compounds. This additional clarification during the biological phase is an usually advanced feature and it is also incorporated in the original Munich treatment plant. FINAL SAND FILTER Finally, the plant's treatment flow passes into the sand filter. This has been designed in the form of twelve long, narrow tanks, two groups of six beside each other. These have tightly fitting, fiber-glass lids to prevent freezing in winter. Inside each tank, the sand is arranged in graded layers, with the coarsest at the top and the finest at the bottom. Each tank has an operating cycle of 12 hours in operation followed by 12 hours out of the process flow during which cleaning, by flushing with air and clean water, takes place. When it emerges from the sand filter, the treated water has a purity rate of 4 ppm (parts per million), which can be compared with the 10 ppm rate required by German federal law.
The pump configuration
Flygt's pumps at the Munich number two plant are located in groups at four separate pumping stations. The first three stations pump between various stages in the treatment process and the last is an emergency storm water pumping station. The first process station pumps the flow from the first activated sludge tanks to the secondary clarifiers. It contains four 45 kW PL 7060 pumps and two 22 kW PL 7050 pumps. The second pumping station is located between the second activated sludge tanks and the final clarifiers. The nine pumps here are of the following models: six 63 kW PL 7060's, two 37 kW PL 7050's and one 45 kW PL 7060. The third process station pumps between the final clarifiers and the sand filter and here there are six 63 kW PL 7080's. Finally, at the discharge point a further six Flygt pumps stand ready to assist: Another six 63 kW PL7080's units are located in what is in effect a storm water pumping station. This will be used in the event that the surface level of the river Isar rises above the plant's discharge outlet. Mr. Ellersdorfer estimates that the frequency of this occurrence will be something like once in ten years.
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