Water Treatment Technology Optimization on Aggressive Properties in Long-Distance Distribution System

Transcription

1 1 Paper N 0 : IV.03 Water Treatment Technology Optimization on Aggressive Properties in Long-Distance Distribution System Jozef Kriš, Karol Munka Elena Büchlerová, Vladimír Roško, Jozef Dunaj Abstract: The Stakčín water-supply reservoir serves as a water resource for long-distance distribution system with available water capacity of 45 mil.m 3. Surface water from water-supply reservoir is transported by gravitational force to the Stakčín WTP through the steel pipes of DN 1000 mm with the total length of 4471 m. The water is treated by clarification on gallery clarifiers and filtration through the sand rapid filters using 40 % ferric sulfate aqueous solution as a coagulant. Depending on treated water quality the dosage of ferric sulfate was ranged from 4 to 10 mg/l of calcium hydrate in the period from to Water disinfection has been carried out with regard to actual delay of water in pipeline by chlorine-amonisation with dosage of gaseous chlorine with the maximum of 0,4g and 0,2 g of ammonium sulfate. At the site of Ťahanovce housing development, the maximum iron concentrations at particular sampling sites reached the value range from 0,4 to 1,1 mg/l with the mean concentrations ranged from 0,20 to 0,31 mg/l. Keywords: Optimalization, quality of water, water treatment, transportation 1. Introduction The Stakčín water-supply reservoir serves as a water resource for long-distance distribution system with available water capacity of 45 mil.m 3. Surface water from water-supply reservoir is transported by gravitational force to the Stakčín WTP through the steel pipes of DN 1000 mm with the total length of 4471 m. The water is treated by clarification on gallery clarifiers and filtration through the sand rapid filters using 40 % ferric sulfate aqueous solution as a coagulant. Depending on treated water quality the dosage of ferric sulfate was ranged from 4

2 284 Kriš, Munka, Büchlerová, Roško, Dunaj to 10 mg/l of calcium hydrate in the period from to Water disinfection has been carried out with regard to actual delay of water in pipeline by chlorine-amonisation with dosage of gaseous chlorine with the maximum of 0,4g and 0,2 g of ammonium sulfate. The total length of the Starin-Košice long distance distribution system is 134,5 km and it is made of steel pipes with diameter of 1000 mm. The length of distribution system from the Stakčín WTP up to the water divider is 130,03 km. The water reservoirs along the entire distribution system are built at Snina, Humenné, Vranov nad Topľou, Medzianky, Prešov and Košice with the total capacity of 93 thousands m 3. The pipeline at the stretch from Stakčín WTP to Prešov water reservoir has been lined by bituminous coating (length of 98,4 km), the section between the Prešov water reservoir and Košice water divider is without inner coating (length of 31,6 km). The actual delay of water in long-distance distribution system from the Stakčín WTP to Košice water divider is 133 hours with the WTP capacity of 313 l/s at a site below the Košice T2 water reservoir, with capacity of 570/l it is 100 hours and with 990 l/s it is 75 hours. During the operation of long-distance distribution system, there has been observed water quality deterioration by effect of corrosion as a consequence of decreased water consumption and subsequent increase of iron concentrations in a pipeline. Considerably higher concentrations of iron in drinking water have been observed mostly at the end of pipeline in the stretch of the Košice- Ťahanovce water divider. At the site of Ťahanovce housing development, the maximum iron concentrations at particular sampling sites reached the value range from 0,4 to 1,1 mg/l with the mean concentrations ranged from 0,20 to 0,31 mg/l. Since the water quality has not reached a desirable level, the Water Research Institute has carried out operational evaluation of aggressive properties of water (corrosion tests) during a period The work has been performed within the solution of stage task Stability of drinking water during distribution included in VTP Research of drinking water treatability and environmental aspects of watercourses. Regarding the existing water quality status of the Starina-Košice long distance distribution system as well as steel pipe lining by inner bituminous coating, the equipment for monitoring of aggressive properties of water has been installed in the following sampling sites: Stakčín Water Treatment Plant (WTP) Humenné Water Reservoir (WR) (34,8 km) Prešov Water Reservoir (WR) (98,4 km) Košice Water Divider (WD) (130,0 km) 2. Evaluation of the corrosion tests during the period During the above period, six corrosion tests with 30 and 60-day exposition were valuated at each sampling site. Considering the duration of corrosion test, they have included all changes

3 Water Treatment Technology Optimization on aggressive Properties in long-distance Distribution System 285 in quality of treated water entering the distribution system in connection with changes in quality of raw water flowing into the Stakčín WTP from water-supply reservoir as well as own operation of water treatment technology (dosage of ferric sulfate and calcium hydrate). During the corrosion tests, the long-distance distribution system transported water in volume of about m 3. During the continuous operation it would represent the average water supply of 430 l/s. The value range of selected treated water quality parameters during the corrosion tests was as follows: COD Mn 0,9-1,4 mg/l, ph 7,5-8,1, ACIDITY 1,70-1,95 mmol/l, calcium mg/l, magnesium 4,3-6,1 mg/l, iron lower than 0,09 mg/l, manganese lower than 0,04 mg/l, ammonium ions lower than 0,05 mg/l, nitrites lower than 0,01 mg/l, nitrates 2,9-3,8 mg/l, chlorides 3,2-4,3 mg/l, sulfates 19,2-39,4 mg/l, turbidity lower than 0,5 NTU and water temperature of 3,5-9,3 C. During the first tree corrosion tests ( ) the positive effect of sufficient CaCO 3 saturation of water on corrosion rates was observed in the entire length of pipeline. During the CaCO 3 oversaturation from 0,05 to 0,15 mmol/kg, the saturation indexes have reached values of 0,0 0,25. Corrosion rates for such stabilized water are shown in table 1. Following the above results, water distributed by long-distance distribution system might be evaluated as slightly aggressive (aggressiveness category I) without need to propose anti-corrosion measures. Table 1 Range of corrosion rates in the distribution system during the period Period of corrosion test Range of corrosion rates (µm/year) The water temperature during that period was ranged from 3,5 to 10.1 C with observed increase in the long-distribution system of 0,6 1,3 C. The iron concentrations reached maximum values of 0,15 0,23 mg/l at Pumping Station (PS) Hanušovce (77,6 km) and Section Valve (SV) no. 18 (85,9 km) respectivelly. At the Košice water divider they reached values of 0,15 0,19 mg/l. The dosages of ferric sulfate ranged from 7 to 10 mg/l and calcium hydrate from 3 to 8 mg/l. During the next tree corrosion tests ( ), the corrosion rates were ranged as shown in table 2. Table 2 Range of corrosion rates in long-distance distribution system during Period of corrosion test Range of corrosion rates (µm/year)

4 286 Kriš, Munka, Büchlerová, Roško, Dunaj The decrease in dosage of calcium hydrate to 2-3 mg/l with dosage of ferric sulfate of 6mg/l during the corrosion test resulted in insufficient CaCO 3 saturation of water (- 0,03 mmol/l) in entire length of the long-distance distribution system. During the corrosion test the dosage of calcium hydrate was increased to 4-5mg/l with unchanged dosage of ferric sulfate, e.i. 6 mg/l and it resulted in oversaturation of water at the Stakčín WTP by 0,05mmol/l, while in following parts of distribution system it has continually decreased up to 0,0 mmol/l at the Košice water divider. During the last corrosion test the dosage of calcium hydrate was increased to 5-6 mg/l and ferric sulfate decreased to 4 mg/l that resulted in oversaturation of water to 0,05mmol/l at the Stakčín WTP and it reached balanced values on the level of 0,025 mmol/l in the whole distribution system. The temperature was ranged from 6,2 to 11,2 C and there was observed its increase from 2,2 to 4,5 C. The maximum iron concentrations were observed mostly during the state of insufficient saturation and insufficient CaCO 3 oversaturation, respectively, at PS Hanušovce, SV no. 18 and Košice WD (0,23 0,30 mg/l). As far as treated water stabilization is considered, it is necessary to point out the relations between calcium hydrate and CaCO 3 overasaturation as well as corrosion rates. The results of corrosion tests show that decreas of calcium hydrate towards ferric sulfate dosage results in lower saturation of water and there were observed also higher values of corrosion rates. The dosage of ferric sulfate depends on raw water quality and subsequently calcium hydrate dosage depends on ferric sulfate concentrations with regard to formation of optimum conditions for water treatment and assurance of their quality according to requirements on drinking water quality. For water quality treated at the Stakčín WTP it was necessary to take into account a possibility of water stabilization during the calcium hydrate dosage and thus for reduction of aggressive properties of water, especially during the period , when corrosion rates at the end of system exceeded the limit of 100 µm/year, there was proposed water treatment technology for achievement of calcium-carbonate balance and state of required CaCO 3 oversaturation of water for the purpose to form a protection layer with high affinity to pipe walls. 3. Water treatment technology optimization for assurance of water stabilization The effect of water treatment technology optimization related to water stabilization at the Stakčín WTP was monitored during the period and it was compared with the same period before technology optimization. ( ). During that period four corrosion tests have been evaluated with 30 and 60-day exposition and corrosion devices were placed at the same sites of the long-distance distribution system as during the first corrosion tests ( ).

5 Water Treatment Technology Optimization on aggressive Properties in long-distance Distribution System 287 Table 3 The mean dosages of lime and ferric sulfate during particular comparative corrosion tests Period of corrosion tests: Period of corrosion tests: Corrosion test Mean dosage of lime [mg/l] Mean dosage of ferric sulfate [mg/l] Corrosion test Mean dosage of lime [mg/l] Mean dosage of ferric sulfate [mg/l] ,6 7, ,4 7, ,7 6, ,2 7, ,2 5, ,5 5, ,6 5, ,2 4,3 In compared period of the first corrosion tests, lime dosages were raged from 2 to 6 mg/l and ferric sulfate from 5 to 10 mg/l. However, such dosages were insufficient regarding achievement of needed CaCO 3 oversaturation of water, what resulted in higher values of corrosion rates. If the CaCO 3 oversaturation of water with a mean dosage of lime and ferric sulfate was ranged between 0,125 and 0,05 mmol/l, it means that water in longdistance distribution system was saturated insufficiently or insufficiently oversaturated by CaCo 3, respectivelly, and optimally it reached only low limit of range values included in STS (Slovak Technical Standard) (0,5 0,10 mmol/l). After water treatment technology optimization and also in view of water stabilization the values of water oversaturation reached the range between 0,025 mmol/l and 0,10 mmol/l, while the values of 0,025 mmol/l were determined only in particular cases. While the mean dosage of lime during the corrosion tests with the mean dosage of ferric sulfate of 5,0-7,8 mg/l were ranged from 2,6 to 4,6 mg/l, the mean dosage of lime during the further corrosion tests with the mean ferric sulfate dosage of 4,3 7,6 mg/l were ranged from 4,4 to 6,2 mg/l. A comparison of lime dosage for corresponding periods of corrosion tests shows that lime dosages were increased 1,3 2,3 times after the optimization. The increase of lime dosage calculated to ferric sulfate dosage represented 0,27-0,55 mg of Ca(OH)/mg Fe 2 (SO 4 ) 3, and higher values were observed especially during the summer period. The increase of lime dosage resulted in decrease of corrosion rates as well as decrease of iron concentration in particular sampling sites along the distribution system. Figures 1 4 show effects of water treatment technology optimization for selected periods of compared corrosion tests at the Stakčín WTP considering water stabilization on the courses of water oversaturation by CaCO 3, saturation indexes and iron concentrations in the long-distance distribution system.

6 288 Kriš, Munka, Büchlerová, Roško, Dunaj 1997/08/ /08/24 CaCO 3 [mmol/l] 0,10 0,05 0,00-0,05-0,10-0, distance [km] Figure 1 Process of water oversaturation by CaCO 3 in the long-distance distribution system for period of compared corrosion tests. 1997/04/ /03/09 I S 0,4 0,3 0,2 0, distance [km] Figure 2 Course of saturation indexes in the long-distance distribution system for selected period of compared corrosion tests c(fe) [mg/l] 0,20 0,15 0,10 0,05 0, distance [km] Figure 3 Iron concentration in the long-distance distribution system during the selected period of compared corrosion tests

7 Water Treatment Technology Optimization on aggressive Properties in long-distance Distribution System 289 During the corrosion tests after treatment technology optimization ( ) the iron concentration reached maximum value of 0,17 mg/l (Košice water divider ). During all other samplings the iron concentrations were ranged to 0,10 mg/l at the end of longdistance distribution system. WD Košice II X.1997 II X.1999 c(f e) [m g/l] 0,30 0,25 0,20 0,15 0,10 0,05 0, Figure 4 Iron concentration at the end of distribution system at the Košice Water Divider for selected period of compared corrosion tests Corrosion rates after the treatment technology optimization have decreased during compared periods by % ( ), % ( ), 5-45 % ( ) and for the last corrosion test by % ( ). The range of corrosion rates for particular sampling sites during the corrosion tests and are shown in the table 4. Table 4 Range of corrosion rates before ( ) and after ( ) water treatment technology optimization at the Stakčín WTP Sampling site Corrosion rates [µm/year] Corrosion rates [µm/year] Stakčín WTP Humenné WR Prešov WR Košice WD The most significant decrease of corrosion rates have been observed at the end of long-distance distribution system, where decrease represents % compared to values before water treatment technology optimization and the corrosion rates were ranged from 10 to 54 µm a year.

8 290 Kriš, Munka, Büchlerová, Roško, Dunaj 4. Conclusion Based on evaluation of water treatment technology optimization at the Stakčín WTP and considering water stabilization it can be stated that these measures had considerable effect on decrease of aggressive properties of distributed water, what resulted in significant decrease of the iron concentrations as well as corrosion rates in the long-distance distribution system. Following the categories of aggressiveness, the water distributed by the Starina-Košice system was classified into the 1st category of aggressiveness (moderate aggressiveness of water). Acknowledgements The article was written on the base of the support of granted research project VEGA 1/0324/03 and KEGA 3/1140/03 that has been searched on Department of Sanitary Engineering, Faculty of Civil Engineering, STU Bratislava. References Barloková, D., Ilavský, J. (2002): UV Radiation in Water Treatment. In: Proceedings of Conference on Actual Problems of the Water Management of Municipalities, Kočovce, September 2002, pp Büchlerová, E., Munka, K. (1999): Drinking water stability during its distribution, Final report, WRI Bratislava, November Ilavský, J., Barloková, D. (2004): Antimony in Water and Possibilities for its Removal. In: Proceedings of Conference on Actual Problems of the Water Management of Municipalities, Luhačovice, May 2004, pp Košč, J. (1997): Existing experiences from the East Slovakia Water Supply System operation. Proceeding of seminar: Topical issues of water supply biology, Prague 1997 Kriš, J. (1997): Hydraulic analysis of the Starina-Košice long-distance distribution system, KZI SvF STU, Bratislava, December 1997 Munka K. et al. (1998): Operational evaluation of aggressive properties of water distributed by the Starina-Košice long-distance distribution system, WRI Bratislava, September 1998 Tothová, K. (2002): Mathematical Modeling of Water Quality Changes. In: Proceeding of International Conference "Drinking Water", Trencianske Teplice X. 2002, p VVaK š.p. Košice: Basic documents on water quality in the Starina-Košice feeder for period VVaK š.p. Košice, Operation of VVS, Vranov n. T.-laboratory WTP Stakčín: Basic documents on water quality in the Starina-Košice feeder for period VVaK š.p. Košice, Operation of VVS, Vranov n. T.-laboratory WTP Stakčín: Basic documents on water quality in the Starina-Košice feeder for period Authors Prof. Ing. Jozef Kriš, PhD.: Faculty of Civil Engineering of the Slovak University of Technology, Radlinského 11, Bratislava, Slovakia, Tel.: Fax: , Ing. Karol Munka, PhD.: Water Research Institute, Nábrežie Arm. gen. L. Svobodu 5, Bratislava, Slovakia, Tel: Ing. Elena Büchlerová, PhD.: Water Research Institute, Nábrežie Arm. gen. L. Svobodu 5, Bratislava, Slovakia, Tel: RNDr. Vladimír Roško: East Slovakia Water Company, joint-stock company, Košice, Stakčín WTP, Slovakia Ing. Jozef Dunaj: East Slovakia Water Company, joint-stock company, Košice, Stakčín WTP, Slovakia