Pisárky water treatment plant

Water treatment plant in Brno - Pisárky
Throughout its history, the city of Brno has struggled with the problem of water supplies. The high living density, which is amongst the highest in the Czech Republic, was very disadvantageous for water supplies and, along with fires, destroying whole districts, this called for constant handling of this problem. The water treatment plant in Brno-Pisárky was built between 1869 and 1872 based on a project of Thomas Docwry from  London.

At that time, raw water was taken upstream the weir in Kamenný mlýn and it was treated in three open biological filers with a total surface area of 2,940 m2. In 1884, a covered filter was added, and yet another was constructed in 1889 and then in 1893. Increased abstraction necessitated the construction of two additional open filters in 1901. The water treatment plant also included two settling tanks on the left side of the Kohoutove road, ensuring a 4-5 day retention capacity and a pumping station that was used as a steam pumping station until 1923.

However, the present water treatment plant in Brno-Pisárky looks very different. Since its commissioning in 1872, it has undergone several reconstructions and refurbishments, which have increased its capacity, and the old equipment has been replaced with new, more modern  equipment.

Water treatment plant in Pisárky after reconstruction
The last reconstruction of the water treatment plant in Pisárky took place between 1986 and 1995. The reconstruction of the water treatment plant achieved increased reliability of the operation of plants and pumping stations at the water treatment plant, improved quality of treated water by introducing more efficient technological processes, installation of new equipment and reconstruction of existing equipment, increased capacity of the water treatment plants up to the permitted raw water abstraction, i.e. 700 l/s, increased storage of produced water, increased pump delivery.

The social-economic changes after the year 1989 and a major drop in potable water consumption compared to the expected rise were reflected in the reconstruction of the water treatment plant.  The extent of the reconstruction was modified and the original plan was implemented only partially.

Technical process of raw water treatment

The technical process treating raw water abstracted from the Svratka river upstream the weir in Kamenný mlýn consists in chemical clarification using ferric sulphate (Prefloc) as the main coagulant with subsequent two-stage separation of formed flocs. The whole process can be divided into the following technological stages:

  • Homogenisation and aggregation – taking place in reconstructed equipment
  • Settling in open horizontal settling tanks along with homogenisation and aggregation is the first stage of separation. A total of four settling tanks are available and each is divided into fours zones. Three scraping units in all settling tanks ensure removing of settled sludge from the bottoms of the tanks into sludge pits and to the municipal sewerage system, following adequate thickening
  • Treated water filtration in open rapid filers with granulated active carbon medium represents the second stage of flocs separation
  • Sanitary treatment of the treated water is the last stage of the treatment process

The technological process of water treatment is a gravity system. Construction of the homogenisation and aggregation units along with the performed reconstruction of the settling tanks with plug flow ensures perfect mixing of the dosed solution of ferric sulphate (Prefloc)  with the treated water. Furthermore, higher efficiency of the 1st stage of aggregation was achieved and thanks to dividing of each settling tank into four identical sections it was also possible to achieve higher efficiency of the last aggregation stage. At the same time, the total retention time upstream the inlet to the settling tanks was extended, which enhanced the total clarification effect. By optimising the operating doses of Prefloc it was possible to considerably improve the settleability of flocs and thus the separation effect of the settling tanks.

Filters with granulated active carbon medium installed at the Pisárky water treatment plant were the first in the Czech Republic being used as the second stage of separation ensuring chemical clarification of surface water. The design of converting the original sand filters with fine medium, known as the „American-type“ filters into filters with granulated active carbon medium using the sorption capacity to significantly improve the organoleptic characteristics of treated water to and to separate the residual flocs from the sedimentation process, was developed based on experience of the French company Lyonnaise des Eaux DUMEZ with a similar type of filters used for the same purpose. The possibility of installing these filters in Pisárky was verified using a model plant and the results obtained from the technological processes at water treatment plants were compared with the results achieved by French water treatment plants using filters with granulated active carbon medium for two-stage flocs separation.  The operating values provided by several French water treatment plants and their comparison with long-term values that are achieved by the Pisárky water treatment plant prove that even permanent operation of the Svratka water treatment achieves comparable results, which means that the quality of final water significantly improves compared to the quality achieved before the reconstruction. The optimum performance of the water treatment plant following the reconstruction is determined mainly by the optimum capacity of the filters, which is 670 l/s, the peak output is 750 l/s on condition of a standard quality of raw water and the expected separation effect.

Once the Vír regional water supply system was put into operation, the Pisárky plant has started being used as a stand-by in operable condition for emergency situations. The correctness of this conception was confirmed during the flood events in July 1997, when the abstraction from the Březové nad Svitavou spring area was limited for a certain time.

Quality of water produced by the Pisárky water treatment plant
The Svratka River is the source of surface water for the Pisárky water treatment plant. The quality of raw water in the river fluctuates strongly depending on the season, frequency and rain precipitation, problems are also caused by excessive algae growth during summer months. What is also unfavourable is the location of the abstraction point downstream the  Brno lake, which is intensively used for recreational purposes. The lake significantly contributes to the occurrence of organoleptic and other problems.

The original water treatment technologies, designed to address the previous, substantially better quality of raw water, could not ensure such water treatment that could meet all the stringent parameters prescribed for potable water. The completed reconstruction of the water treatment plant and changes in the water treatment technology have considerably  improved the quality of water. In particular, the organoleptic characteristics have significantly improved (taste, odour, colour and turbidity). The content of organic substances ranges below the minimum measurability limit permitted by the potable water standard. Likewise, all the other monitored parametres do not exceed the standardised limit values. However, the recommended water temperature, i.e. 8 – 12 oC cannot be achieved as it depends on the temperature of raw water in the Svratka River and the water treatment process cannot basically affect the temperature.

Nevertheless, the risk factor of a potential emergency contamination of raw water in the Svratka River by substances, the removal of which cannot even be ensured by a new water treatment plant technology, persists. The Svratka River flowing in parallel with heavy trafficked roads and roads crossing the water course upstream the abstraction point in the districts of Bystrc, Komín and Jundrov is highly sensitive, e.g. to oil products from traffic accidents, road washing and excessive turbulence during heavy rain events.