Prague drinks from the Vltava. Podolská water plant uses trout, activated carbon and UV radiation
Drinking water from here goes to several Prague reservoirs. But what happens before one can drink without fear? Seznam Zprávy is accompanied by Ladislav Herčík, manager of water treatment operations, who has been working here for ten years, but has been working at the water plant in Kárané since 1986. Ondrej Gandžala, head of the water treatment plant, and Tomáš Mrázek, spokesman for Prague Waterworks and Sewerage, are also going on a tour of the water plant.
Once machines, today empty
The huge interior space of the first hall will surprise you. Strangely, there is almost nothing here, except for two aquariums with trout at the entrance. “There used to be big engines and long shafts because they drove the pumps down below the hatches. Today, the entire pumping system is in the basement,” says Ladislav Herčík. We are standing in a raw water pumping station. When viewed from the embankment, it is the large building on the left, which was built in the 1930s and is one of the oldest in the area.
Unseen pumps under the floor transport raw water from the Vltava to the highest, fourth floor, and then it flows through the entire technology under the influence of gravity. On the opposite front wall is a large diagram that simplifies the entire local water treatment technology. What is shown graphically on the wall, we will also go through physically today. We will look everywhere, including a beautiful vantage point from which Vyšehrad and the more distant Prague Castle can be seen almost side by side.
What the diagram on the wall shows
Using pumps, the raw water from the Vltava reaches the fourth floor, where it receives the first stage of treatment – that is, chlorine oxidizers and clarifiers, where, for example, ferric sulfate coagulants are dosed. In this phase, about ninety percent of the impurities are removed from the water and the treated water flows to the second stage, which is filtration.
Until the reconstruction that ended a year and a quarter ago, there were only sand filters, today behind them is technology with granulated activated carbon. “Because today there are problems with pesticides and odors in almost all surface waters,” explains Herčík.
Podolska water plant
The buildings are constructed in a neoclassical style, with a 45-meter high water tower in the middle.
The façade is decorated with carved sandstone buildings that represent allegories for the Vltava River and its important tributaries – Vydra, Otava, Blanica, Malša, Berounka, Sázava, Podblanicka Blanica, Želivka, Lužnica and Nežárka.
According to the scheme, another modern piece of equipment follows with ultraviolet radiation, which eliminates any unwanted life in the water. Only then, just before being pumped into the network, does it head for final chlorination.
The last signs bear the names – Karlov, Flora, Zelená liška, Laurová… “We pump water from here in two directions. On the one hand, to the reservoir on Flora, which is a discharge height of about one hundred meters, and the other direction is through the pipeline under the Vltava River to its left bank on Laurová – to Bruska. That’s about seventy meters of discharge height,” says the operations manager.
Hard water flows from the east
The water from Podolí therefore goes to reservoirs, not directly to street rows. When and where do they mix together with other resources? “The system is quite complicated in Prague. Water is mixed in different ways, for example on Flora there are chambers from the Kárané water plant and chambers from us in Podolí,” explains Herčík. He said that he personally did not recognize the differences in the taste of water from individual sources, but he admits that some people may have a milder taste.
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According to him, the difference is rather that the water from Podolí and the Želivka reservoir is treated surface water, while the water from Kárané to the east of Prague is groundwater. This means a significant difference in hardness. “In the case of Káraného, harder water has a negative impact on the precipitation of limescale in kettles and washing machines. On the contrary, it has beneficial effects for the organism, because minerals such as calcium and magnesium enter the human body with it,” he says.
According to him, whoever wants to support the body and not the washing machine should live on the east side of Prague, as 100% Kárná water is only in Horní Počernice. Everywhere else is mixed. As Ondrej Ganjala adds, on the Prague Waterworks and Sewerage website it is beautifully processed how water is mixed in the metropolis, including its composition and hardness.
Trout and cooler bags
At the entrance to the giant hall there are two aquariums with trout, with a transparent but cloudy brown finish. One has a capital letter L, the other P. No code – they simply indicate the left and right aquariums. “The trout here monitor possible toxicity in the raw water,” describes Herčík. In a similar way, canaries were once used in coal mines, which, thanks to their natural sensitivity to the influence of their surroundings, “checked” the presence of even low concentrations of poisonous gases.
According to Herčík, there are only coarse and fine combs between the Vltava and these aquariums, i.e. devices that rid the water of mechanical impurities. If it was after a heavy rain, the water is said to be more cloudy. “The water in the aquarium corresponds exactly to the state of the water in the river. After heavy rains, trout tend to be much harder to see than today,” confirms Herčík and subsequently confirms the presence of opaque bags floating under the surface among the fish.
History and future of the water plant
- 1921 – approval of construction
- 1924 to 1929 – construction of a new water plant according to the project of architect Antonín Engel
- 1932 – expansion of technology, output 640 l/s
- 1942 – intensification of technology, output increased to 900 l/s
- 50s to 60s of the 20th century – construction to the present form, output 2,100 l/s
- 1992 to 2002 – extensive reconstruction during full operation, max. output 2,500 l/s
- 2002 – floods and termination of operation
- 2014 to 2017 – reconstruction of half of the clarifiers, from 2019 further modernization of the water plant, addition of technologies
- 2020 – preparatory study of addition of new technologies, semi-operational verification project
- 2021 – restoration of regular water supplies
- 2021 to 2023 – Semi-Operational Validation of New Technologies (Continuation and Completion of Phase 1)
- 2023 – final design of the addition of technology at the study level (2nd phase of pre-project preparation of the 2nd stage of reconstruction)
“The water in the river is warm for the trout, so they have cooler bags there. Each contains two liters of frozen water that gradually melts. Over there in the freezer, we’re chilling another one for them,” he points out. Back in July, the water in the Prague part of the Vltava was around fourteen degrees, in August around sixteen. “When water is released from the dam, the river is colder. When more water flows from Sázava and Berounka, it is warmer again,” explains Herčík. It is said that trout have never been belly up here – so there has been no toxic accident on the Vltava that would have killed them.
Rafts roar on the fourth floor
We take the elevator to the fourth floor. “There are mentioned chlorine oxidizers and at the same time clarifiers,” reminds Ladislav Herčík. Clink, we’re exiting. Another door opens and the sound of water, like a raft, echoes through the space just below the vaulted roof. It even looks that way – there are rafts in concrete channels covered with mesh on top.
Five hundred liters of water per second flows through the pumps into the waterworks system – and you can hear it. “From that, there are approximately 420 to 430 liters of treated water. The rest is so-called technological water polluted by what the Vltava brought, some of it gets into the sewage system. And the sewage sludge goes to the sewage treatment plant,” adds Ondřej Ganjala.
The entire visible space under the roof is filled with bridges and walkways. “After the reconstruction of the construction and technological parts, all the necessary materials are made of stainless steel or composite, so that subsequent maintenance is minimal,” maintains Herčík. We are currently standing at the clarifiers, which are special water tanks.
“We have four reconstructed, but for the output of five hundred liters of raw water it is enough to have two in operation. The other two are in reserve,” describes the operations manager. Water stays in them for two to two and a half hours.
Path at height through the bridge
After the first stage of purification, the water reaches the neighboring “old filtration building” through the so-called pipe bridge, which is clearly visible even from outside the embankment – it is a bridge at the height between the two main buildings, long with long meters. We walk along a dominating green tube with a diameter that even a sprawling bear could crawl through.
In the so-called old filtration building, again monumentally conceived, three separate “galleries” with sand filters functioned even before the reconstruction. “Now only the first gallery is sand, from there the water flows to the filtration with granulated activated carbon. Only then is it the final water, which goes to UV radiation and subsequent chlorination,” Herčík points out, adding that new pumps and blowers are working here. To give you an idea – the interior space is not divided by partitions and, according to Herčík, is roughly one hundred by one hundred meters.
The layer of sand in the reservoir is about thirty meters high. According to the operations manager, the sand is still original, it can be here for decades, because it is quartz, i.e. very hard and durable, and it is washed regularly – once a day or every two days as needed.
It’s not the service, it’s the repairmen
Men in overalls pass by between the tanks. “They are not operational workers. They are in charge of repairing the inflow channel leaks for the six filters of the second gallery, which are now shut down,” explains Herčík.
But where are the people who take care of all this? And from where? Most of the traffic apparently takes place in automatic driving mode. “We have a control room where the foreman sits, or his deputy or the engineer, who monitors the entire water treatment system from start to finish. Only the occasional addition of chemicals is done manually,” says the manager of the water treatment plant Ganjala. Alright, let’s go there.
In continuous operation at the Podolská vodárna, there are alternating shifts, each of which consists of a foreman and three machinists. Thanks to automatic management, only four people manage the entire huge area. We arrive at the control room, in a large room with a long counter on which there are twelve monitors in working groups of three. On the wall behind us are more built-in modern panels.
Now it’s all guarded by one man, and from a layman’s point of view, the most interesting thing is behind his back – a wall of alarm clocks, lights and other original mechanisms, left here even after the reconstruction out of nostalgia – their appearance would easily fit in Captain Nemo’s submarine. Here is an interesting piece of information: it takes about half a day from the time water is pumped from the river near the port of Podolsk to the drinking water draining into the reservoirs. It takes us much less time to walk through the Podolská waterworks.
Once more trout, this time on the grill
On leaving, the talk turns again to the trout in the local aquariums. “We know from the fishermen at the research institute that we gradually lose our sensitivity to water quality, so we have to replace them once a year,” explains Ladislav Herčík. Some of the retired fish are headed for dissection and subsequent analyzes to find out what they absorbed during that year. “For example, heavy metals that are common in surface water today,” he clarifies.
Right now – in mid-September – they are waiting for the results of the analysis. They will be available for the first time since the reactivation of the water plant in June last year. A tasty bonus for machinists is somewhat related to this. “Experts told us that releasing other trout into the Vltava means condemning them to death because they are fed pellets all year round. They wouldn’t support themselves in nature,” says Herčík. Once a year there is a planned barbecue of aquarium trout from Podolí.
And now, just for fun, go up to the terrace – the view is definitely great! “We can still go to the museum, which is on the premises,” reminds spokesman Tomáš Mrázek. So here we go, it’s interesting, but there’s no room for that today. Maybe sometime next time.