Drinking water quality

(the text below is based on the chapter Drinking water quality published in yearbook Prague Environment 2005)

 

In 2004 Prague was supplied for 11 months from two water treatment plants (Želivka, Káraný), for one months from three water treatment plants – Želivka, Káraný, and Podolí.

Since the end of 2002 the Water Treatment Plant Podolí has been serving as a sparse source for Prague. In 2004 the Water Treatment Plant Podolí was under regular operation in March when water produced in the plant was distributed to consumers. For the rest of the year water from this plant was not delivered for distribution.

In 2004 drinking water quality was evaluated pursuant to the Decree of the Ministry of Health No. 376/2000 Code, establishing requirements for drinking water quality and scope and frequencies of drinking water control in the period from January to April. On 1 May 2004 the amended version of the Decree under the name of the Decree 252/2004 Code, establishing public health requirements for drinking water and hot water quality and scope and frequencies of drinking water control (further here under as the Decree) became effective. The Decree is the executive regulation to the Act No. 258/2001 Code on public health protection. The aforementioned regulations are in accord with the EU requirements for drinking water.

In 2004 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. (for the control of drinking as well waste water and for sampling) have been merged into the Department of Water Quality Control. The laboratories for drinking water of 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 drinking water quality monitoring for 2004, both for the water treatment plants and the distribution network, was developed 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 2004 the laboratories of the company of PVK, a. s. monitored drinking water quality in total of 5,779 samples in with total number of determinations 154,639. Of that number 4,219 samples were taken from the Prague’s distribution network where 120,696 determinations were carried out. For the Želivka Water Treatment Plant 997 samples (19,799 determinations) of treated water, for the Káraný Water Treatment Plant 446 samples (11,269 determinations), and for the Podolí Water Treatment Plant 137 samples (2,875 determinations) of drinking water were processed, respectively. Of the total number of analyses of drinking water performed 1.40 % were not in compliance with the Decree.

 

Drinking Water Treatment Plant Želivka

The Drinking Water Treatment Plant Želivka is the most up-to-date and largest water treatment plant serving Prague. The drinking water is transported through a shaft influent conduit 2.64 m in diameter and 51.97 km long. In 2004 the share of the Želivka Plant supply of the total drinking water supply to the City was 75.7 %. The Želivka Water Treatment Plant also supplies drinking water to areas of the Central Bohemia Region and Vysočina Region.

With its maximum peak output of 6,900 l.s^-1 of drinking water and output in 2004, of approx. 103.3 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.

In 2004 treated water quality met limit values as established in the applicable legislation. The only troublesome parameter was microscopic image of treated water in the course of the spring and autumn circulation. These natural effects in the water management reservoir require reinforced technology measures in critical nodes, both in the water treatment technology line and in the course of treated water distribution in order to deliver water of required quality to customers. Because of the raw water source nature (surface water) the mineral content is very low and water, even if treated with final alkalizing, does not have the optimum calcium-carbonate equilibrium. According to the TNV 75 7221 water was classified of the second degree of aggressiveness concerning corrosion of metallic pipelines, which means the water is medium aggressive. The Želivka’s water corrosiveness is reduced by addition of optimized dose of hydrated calcium oxide to final adjustment of the pH of water treated to 8 to 8.5, the current legislation allows.

Since 2001 PVK, a. s. has been purposefully monitoring the family of triazine herbicides. Namely following the flood situations, alarming concentration values nearing the limit value for drinking water of these were determined in raw water. In tributaries to the water reservoir concentration values of these compounds several times exceeding the limit value (100 ng.l^-1 for respective pesticide, 500 ng.l^-1 for sum of the pesticides) were repeatedly found. In the Švihov Dam Lake the companies of PVK, a. s. and Povodí Vltavy, s. p. carry out jointly monitoring concerning basic chemical and microbiological parameters and on the basis of findings of triazine-based herbicide occurrence the joint monitoring programme was expanded to cover these compounds since 2004.

Water from buffer water reservoirs is led through shaft mains into the water reservoir in Jesenice, and from this reservoir, after being chlorinated to required level, it is distributed across Prague.

 

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. This groundwater features balanced contents of ions, which in positive way affects organoleptic properties of water.

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 through three pump water mains of identical length 23 km.

In 2004 quality of the water from the Plant Káraný met limit values of all indicators monitored according to the valid legislation. Organic pollutants monitored in compliance with the valid legislation have been permanently below limit of determination. Concerning corrosion the water is close to calcium-carbonate equilibrium having very little corrosion effects on metallic pipelines. According to the TNV 75 7221 water was classified at the brink of 1^st and 2^dn category of aggressiveness, which means the water is slightly to medium aggressive. The Act on Water (No. 254/2001 Code) deals with protective zones of water sources in the form of general protection. Therefore the operator shall provide for control monitoring of quality of abstracted water and raw water in the Jizera River, including other check localities of the area concerned. In the first half of 2004 the mathematic model of the whole catchment area Káraný was completed. Then primary partial outcomes of the model were processed. One of those was the optimization of locations and number of parameters at monitored profiles and of the artificial recharge operations. Furthermore, targeted “monitoring of nitrates” at abstraction series of bank recharge was started.

In 2004 the share of the Káraný Plant of the City total drinking water supply accounted for 23.3 %, the Plant produced approximately 31.8 mil. m³ high quality water (close to groundwater in its characteristic).

 

Drinking Water Treatment Plant Podolí

As already stated in the introduction since the end of 2002 the Drinking Water Treatment Plant Podolí has been serving as a spare source to Prague. The Plant is regularly maintained in such shape to be able to start the drinking water production any time as the need may be.

The Plant produced water to the distribution network for the period of one month in 2004, in 8. 3.–8. 4. 2004, when raw water quality was very good in terms of its treatability. Quality of produced water complied with all requirements of the Decree. It may stated that raw water quality in 2004 was within usual values and it was treatable to obtain drinking water as laboratory tests proved.

Concerning corrosion the water is close to calcium-carbonate equilibrium having very little corrosion effects on metallic pipelines. According to the TNV 75 7221 water was classified at the brink of 1^st and 2^dn category of aggressiveness, which means the water is slightly to medium aggressive.

Since 2001 triazine – based herbicides have been systematically monitored in raw water and in major tributaries to the Vltava River (Sázava River, Berounka River), including water from the Vltava River cascade (profile Zbraslav). Till summer 2002 (before the plant was set aside into the stand-by regime) increased concentration of respective triazines (most of all atrazine) in treated water were dealt with by mixing the water treated with water from other sources so no limit value at consumer was exceeded in the distribution network. In case of emergency start-up of the treatment plant as a spare source, if increased concentrations of pesticides are found, this regime of „source mixing“ shall be applied again.

In 2004 the share of the Water Treatment Plant Podolí in the drinking water supply to Prague was 1.0%, total volume of drinking water produced in one month of operation was approx. 1.3 mil. m³.

Fig. Comparison of water treatment plants and public water supply systems on the basis of selected parameter

 

Source: PVK, a. s.

 

Water Supply System Network

In the course of drinking water distribution quality has been changing due to:

• effects of materials in contact with drinking water (secondary increased iron content due to corrosion);
• in relation to the drop in water consumption residence time of water in the distribution network (hereinafter as the DN) has been prolonging, flow velocity is decreased (decrease in final volume of Cl₂ Ţ potential for microbiological non-compliance);
• high failure rate of the distribution network;
• handling operations caused by reconstruction of water mains;
• in the period of increased bioseston in raw water in the case of the Želivka Water Dam doses of ozone and Cl₂ are increased at the plant outlet. Therefore an increase of chlorination by-products (THMs) at consumer is found in the DN. Neither respective trihalogen methane derivatives nor the total sum of THMs exceeded permitted limit values established in legislation in 2004;
• due to the prolonged residence time of water it was necessary to provide for additional-chlorination of determined DN sections in order to ensure microbiological innocuousness. Except for stable locations of additional disinfection (Cl₂, NaClO) it is possible on the basis of accidents found to provide ad-hoc disinfection of a target section of the DN by means of a mobile battery unit;
• analogically to the treatment plants all accumulation facilities and pumping stations of the DN were regularly sanitised with water quality control following the cleaning;
• for the sake of water quality improvement in the DN the “Sludge Removing Code for Major Distribution Mains” has been applied since 2002. Besides target flushing of local troublesome areas of the DN are carried out;
• In areas of the Prague’s DN where limit values of iron are permanently exceeded (due to corrosion of pipe material) the Public Health Authority issued a time-limited exemption for the limit value of this parameter. Till the exemption expiration the situation shall be fixed either by reconstruction of the existing pipeline or by the replacement of the pipeline system. Concerning water quality these areas under exemption have been favoured in planned repairs and/or investments.

The reasons given here above have caused that approximately 1.5% increase of non-compliant parameters determined compared to percentage of non-compliant analyses at the outlet from the treatment plants.

The Public Health Authority of the Capital City of Prague controls quality of drinking water in the distribution network on a regular basis. In 2004 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. Results of water quality at consumers tested have been handed over in an electronic form to the National Monitoring Programme of the Public Health Authorities (software PiVo), this duty is established by the Act No. 258/00 Code. Results of control radiological analyses of treated water at the treatment plants are, in compliance with the requirement of the valid legislation, handed over to the State Institute of Nuclear Safety.