Drinking water quality
(the text below is based on the chapter Drinking water quality published in yearbook Prague Environment 2004)
In August 2002 the Water Treatment Plant Podolí was shutdown and the decision was taken to leave it as a sparse source for Prague then. The plant is regularly maintained to be at such shape to be able, anytime as the need may be, to supply drinking water. In 2003 the Water Treatment Plant Podolí was under regular operation in February and October when water produced in the plant was distributed to consumers. For the rest of the year water from this plant was not delivered fro distribution.
In 2003 drinking water quality was evaluated pursuant to the Decree of the Ministry of Health No. 376/2000 Code, establishing requirements for drinking water and scope and frequencies of drinking water control (further here under as the Decree). The Decree is the executive regulation to the Act on public health protection. The aforementioned regulations are in accord with the EU requirements for drinking water.
In 2003 the drinking water production process was monitored by laboratories at respective treatment plants within the scope of indicators inevitable from the technology standpoint. Analyses in the full scope of requirements of the Decree were carried out at the Department of Laboratory Control Prague (OLK Praha). Since 2001 all laboratories of the company of PVK, a. s. have been merged into the Department of Water Quality Control. The OLK Prague, OLK Káraný, and OLK Želivka are accredited, in compliance with the Czech Standard EN ISO/IEC 17025, including the sampling, by the Czech Institute for Accreditation (ČIA), certificate No. 1247; 1247,1; and 1247,2.
The programme of water quality monitoring both for the water treatment plants and the distribution network, was developed for the year 2003 in accordance with requirements of the applicable legislation and those of the Public Health Authority of the Capital City of Prague and the Regional Public Health Authority of the Central Bohemia Region, respectively and following needs of respective water treatment plants.
In 2003 laboratories of the company of PVK, a. s. monitored drinking water quality in total of 7,709 samples in with total number of determinations 82,721. Of that number 5,513 samples were taken from the Prague’s distribution network where 66,972 determinations were carried out. For the Želivka Water Treatment Plant 883 samples (6,138 determinations) of treated water, for the Káraný Water Treatment Plant 1,027 samples (7,583 determinations), and for the Podolí Water Treatment Plant 286 samples (2,028 determinations) of drinking water were processed, respectively. Of the total number of analyses of drinking water performed 1.22 % were not in compliance with the Decree.
Drinking Water Treatment Plant Želivka
With its maximum peak output of 6,900 l.s-1 of drinking water and output in 2003, of approx. 108.7 million m3 per year the Drinking Water Treatment Plant Želivka belongs to the largest water treatment plants in Europe and is the largest one in the Czech Republic.
Technology Procedure of the Drinking Water Production in the Drinking Water Treatment Plant Želivka
The water abstraction by means of storey system enables to take raw water of the best quality from various depth of the Švihov Dam Lake. The water treatment is carried out in single stage coagulation filtration where aluminium sulphate is employed as the main coagulant with potential for the pH adjustment by sulphuric acid. Then water is filtered through sand percolating filters and final adjustment of the pH of water treated is made by hydrated calcium oxide. Ozonisation is an important step in the water treatment, which substantially affects organoleptic properties of water and reduces precursors of by-products of chlorination. Feeding chlorine into water at the outlet from the treatment plant provides for health innocuousness of drinking water.
Water from buffer water reservoirs is led through shaft mains into the water reservoir in Jesenice, and from this reservoir it is distributed across Prague.
In spring 2003 raw water quality was deteriorated due to affects of the 2002 floods in which water from the reservoir was discharged through the dam crown overflow.
As a consequence a shortcut steams occurred in the reservoir making raw water quality in the spring season of 2003 worse, namely in parameters concerning nutrients, in the parameter of microscopic image, and last but not least an increased content of pesticides was found. Yet employing of thoroughly managed treatment technology the raw water was treated to give drinking water. Quality of the water produced was compliant with the valid legislation for the whole year.
The increased content of nitrates is the permanent issues of the Želivka River catchment basin. The water produced is not suitable for the preparation of food of suckling babies. In the last decade the nitrate concentration was fluctuating around the average value of 30–35 mg.l-1.
Drinking Water Treatment Plant Káraný
The Drinking Water Treatment Plant Káraný, as the only one plant, produces and supplies groundwater to the City that features excellent quality parameters resulting in beneficial biogenic properties.
In Káraný the drinking groundwater is acquired from three systems: natural groundwater recharge, artificial groundwater recharge, and artesian water sources (water of extraordinary quality collected from 7 artesian wells 60–80 m deep). Iron is removed from the artesian water by aeration and sand percolation. The water is, after the compulsory chlorinating, pumped to Prague.
In 2003 the quality control of water from Plant Káraný carried out in a strict manner showed values of all indicators monitored below limits established in the Decree. In order to maintain permanently the quality of drinking water collected the protection of water sources and related monitoring are the inevitable prerequisites. The optimisation of source protection, monitoring and water management on the catchment area Káraný – these are the expected outcomes of a mathematic model of the whole catchment area. The mathematic model was ordered by the administrator company of Pražská vodohospodářská společnost, a. s. in half of 2003 and the model shall be utilised by both the administrator company and the operating company of Pražské vodovody a kanalizace, a. s.
In 2003 the share of the Káraný Plant of the City total drinking water supply accounted for 22.3 %, the Plant produced almost 31.8 mil. m3 high quality water (in characteristic close to groundwater).
Drinking Water Treatment Plant Podolí
The current system of water treatment consists in pre-treatment of raw water in clarifiers where liquid iron sulphate is fed as the flocculant. Under worse conditions to the clarification auxiliary flocculant PRAESTOL is added. In the clarifiers raw water up to 95 % impurities are removed and the pre-treated water pH is adjusted by an alkali addition and then filtered through sand percolating filters. Treated water is further disinfected by chlorine.
The plant produced water to the distribution network in February and October in 2003 when raw water quality was very good in terms of treatability. Quality of produced water complied with all requirements of the Decree. It may stated that raw water quality in 2003 was within usual values as before the deluge and it is treatable to obtain drinking water as laboratory tests proved.
In 2003 the share of the Water Treatment Plant Podolí in the drinking water supply to Prague was 1.5 %, total volume of drinking water produced in two months of operation was approx. 2.2 mil. m3.
Water Supply System Network
Indicators of iron, colour, opacity, and first of all residual free chlorine at the end consumer were among the troublesome indicators in the distribution network in 2003. Iron, colour, and opacity, which occurred at limit exceeding concentrations in the distribution network, have been generated in secondary processes in the water supplied especially in end mains and interstitial mains. The increased iron content (and related increase in colour and opacity), which occurred in the distribution network, was party caused by corrosion of metallic pipelines without any internal surface coating combined with a very low velocity of water flow in the mains, and furthermore manipulations in the distribution network related to both planned repairs and emergency ones. When such occurrence was identified appropriate rectification in the given area was carried out immediately (flushings, sludge separation, etc.) with subsequent control of its effectiveness. In 2003 a system of regular sludge removal of the major distribution mains was introduced, which should contribute to improved water quality at end consumer. There are twenty-two operating opacity meters also to monitor drinking water quality in the distribution network; three of them are mounted at the outlet from the Water Treatment Plant Podolí, others are deployed at crucial points of the distribution network. The opacity meters are on-line connected to the Central Control Room of the PVK, a. s., which enable continuous monitoring of drinking water quality and potentially flexible action in the case of a need.
The Public Health Authority of the Capital City of Prague controls quality of drinking water in the distribution network on a regular basis. In 2003 no serious fluctuations in water quality were found in samples monitored within the super-control activities of the Public Health Authority of the Capital City of Prague.
Fig. B2.11 Comparison of water treatment plants and public water supply systems on the basis of selected parameter
Source: PVK, a. s.