At our present-day “drinking water factory”, water is pumped in from the Vltava River, passed through coarse and fine screens, and then driven up by pumps into “break tanks” on the upper floors. These allow the stream of water to stabilise, so that the water then flows out of them calmly and smoothly and does not stir up contaminants in the next stage. The first stage of treatment – the clarifiers – removes most of the contamination from the raw water. Clarifiers are large cylindrical tanks with a conical bottom, and we add a coagulating agent to help clumps of “floc” form around any fine contaminant particles, so they can be removed (a process called “flocculation”). A “floc cloud” precipitates in the clarifiers, which is taken off. The almost clean water then pours across an overflow weir and flows on to the next treatment process, which is filtration. Water flows into the filtration hall from the adjacent building through a 70 m long pipe bridge, which you can’t miss when visiting the waterworks. Before going into the next separation stage, its pH is also adjusted by adding lime water (calcium hydroxide solution).
First, the water is filtered through sand, which has been here for several decades. This is silica sand, which is very durable; it only needs to be cleaned once every day or two with a strong jet of pressurised air and water, which washes out the collected contaminants. After that, the sand filter is ready to do its job again. The water then flows on to filters filled with granular activated carbon, which is used to remove any traces of fertilizers or pesticides that the river carries with it, as well as residues of pharmaceutical products and other contaminants.
When the Želivka Water Treatment Plant was built in the 1970s, a proposal was put forward to close the Podolí Waterworks. At that time it was seen as just too old and unreliable. Only in the late 1980s did it become clear that it would actually be better to keep “Podolí” in operation, and so finally in 1992 an extensive refurbishment project began. The plant’s process equipment was brought up to date without interrupting its operation, and the old filtration building was converted into premises for the Prague Waterworks Museum, which moved here in 1997 from its former home on Národní třída.
During its last operational upgrade, between 2014 and 2016, four of the plant’s nine clarifiers were refurbished. In 2019–2021 its filtration process was also renovated, with granular activated carbon filtration replacing one row of sand filters. UV treatment was also added to its disinfection stage, which is completed by gaseous chlorination.
... the most fundamental way of treating water – simulating a natural process – has remained unchanged for centuries? Water was already filtered through sand in the original waterworks, and is still filtered through sand today.
If you’d like to learn more, continue to the other parts of the treatment plant...
