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Paper N0: IV.10
The Reliability Assessment of Water Treatment Subsystem of Wrocław City Izabela Zimoch
Absract: The proper Water Treatment Plant (WTS) operation influences on reliable operation of the whole Water Supply System (WSS). The treatment processes of drinking and communal water take place in a technological consisted of the parallel-connected facilities of the same type. Such a technological series structure guarantees that in case of shutting down of a device, the system would proper operate. During emergency periods water may be directed to the rest of operating facilities, without any risk of WTP operation failure. Such operations are connected with the changes of technological parameters of unit process during water treatment or with the decrease of the rate of treated water flow. The above mentioned cases are not situation of full subsystem failure. However, significant decrease of water quantity may influence on WTP operation. More important water supply systems in the world are designed and evaluated, just after the usage of the above mentioned theory achievements. Reliability theory is very important while projecting evaluation of WTP supply systems. It prevents from elements failure caused by eventual mining damage. It may decline the risk of supplying the noxious water. In the paper the practical usage of probabilistic evaluation model of water treatment plant technical operation on the example of Water Treatment Plant “Mokry Dwor” and “Na Grobli”, which are water supply systems for Wrocław city. The evaluation based on the real exploitation data in the years 2001-2004 and published data, includes the determination of reliability index estimators of water treatment facilities and the stationary availability index of water intake subsystem of Wrocław city. Keywords: Water Supply System, Water Treatment Plant, reliability theory,
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1.
Introduction
The reliability of Water Treatment Plants (WTP) is a feature based on water production (in given service conditions) of proper quality and quantity dependently on buyers needs. The variation of water quality is random function describing set of chemical, physical and bacteriologic features of raw and treated water. The abovementioned variation influences on such features of water production subsystem (WPS), (Zimoch, Wieczysty, 2001): • complicated engineering-technological structure, • random character of factors influencing subsystem operation, • the lack of distinct limit between efficiency and inefficiency or partial efficiency, • occurrence of various types of failures such as: full, not full, sudden, gradual, dependent, independent, detectable, undetectable and difficult to detect etc., • the progress in automation and control of subsystem operation. The paper was written within the framework of realization of research project KBN 5T07E 044 25 under the title “Determination of reliability operation model of water supply system (WSS) in the aspect of secondary water contamination in a water pipe network”.
2.
Water Supply System in Wrocław
Wroclaw, the capital of Lower Silesia is the city situated on the Odra River in the southernwest of Poland. The population of Wroclaw is about 700 thousands of inhabitants. It is on the biggest and most important cultural and scientific centers of Poland. There are a lot of industrial branches (electro machinery, chemical, typography etc.). Such strongly developed place requires large amount of water to meet needs of its inhabitants and industry. The beginnings of Wroclaw water pipes are connected with the privilege of the Prince Henry IV the Right given to the inhabitants on 31 January 1272. This privilege let people to take water from the Odra River to supply water pipes and moats and to dump sewage. From the XV century some information appears about next water taps i.e. buildings used to take water: The Grate Scooper next to Dolne Młyny, the Maciej Scooper, the Cat Scooper and the House of Pump. Water was distributed by wooden water pipes set under the ground at the depth of 4-5 feet. In that way water was supplied to the city until 1871. In this year, the municipal water works “Na Grobli” started to operate. The water-tower and the system of cast iron pipes, equipped with gates and hydrants. The plant supplied water to the city from the Odra River. Water was pumped through sand filters, and after treatment it was directed to the water-tower at the height 38 m, and then gravitationally by a pipe ∅ 762 mm to the municipal water supply system. The daily production was 11000m3. At present water supply system of Wroclaw consist of two subsystems- water supply subsystem (WSS) and water distribution subsystem (WDS). The water supply systems in Wroclaw take water from the Oława River, which is supplied by switching water system from the Nysa Kłodzka River. The distribution water subsystem is connected with two big water
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treatment plant WTP: “Na Grobli” and “Mokry Dwor”, which are separate technological lines. The „Na Grobli” plant treats infiltrated water, the „Mokry Dwor” plant – surface water, and the „Leśnica” plant – underground water. The primary assignment realized by water treatment plants is to maintain continuity and reliability of supplying water of standardized quality and required pressure. Plants continuously supply water to meet city demand. So far, during exploitation of Wroclaw water supply system occur two failure situations. One – in July 1997 during the flood. Then technological devices had been flooded and plants stopped supplying water. And the second – during manganese catastrophe in 1906, when quality of water rapidly made worse, because of abundance of manganese compounds (caused by content of iron). Water treatment plants have total production capacity at the level of 300 000 m3/d, what is a double value of water consumption. Pumping clean water by plants and urban zonal pumping station ensure essential pressure in water-pipe network. The plants are successively modernized. Priority of investments is improving water quality. During realization much is made with regard to conditions of the environment protection. There are chosen technologies that minimize number of produced sediments and interference in environment. In 2002 it was initiated two-stage process of water treatment. It was started irrigating water-bearing area with water cleaned in the “Mokry Dwor” plant, what will let make intensification of natural way of cleaning water in wet ground areas. Ecosystem of water-bearing areas is place of rigorous protection. In treatment plants all devices and technological process system are built with at least 100% of reserve. That ensures holding constant supply during emergency situations. Reliability of water supply to the city may be threatened only in case of unusual events. However, possessing two big treatment plants even in those cases decreases effects of potential threat. Total length of Wroclaw water-pipe network is 1,800 km. This network has a ring system. It consist of about 200 km of main network, 1,200 km of distribution network and about 30 thousands of terminals of length 390 km. Distribution subsystem has ring system. Owing to this there exists possibility to deliver water to specified place with different “waterways”, what matter while cut-off fragments of the network caused by failure or modernization.
3.
The “Mokry Dwor” Water Treatment Plant
The “Mokry Dwor” WTP has started to operate on 15 September 1982. The Plant was designed for the production of 200 000m3, at present the amount of water is 70,000m3 a day. Water is supplied from the Oława River in Czechnica by two pipes ∅ 1200. Nowadays the plant has two functions: treats water to irrigate water bearing area (not full technological process) and completes water production for the city (full technological process).Water intake from water bearing area supplies the second water treatment plant WTP “Na Grobli”
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Zimoch
Water in the Oława River is contaminated both mechanically (turbidity, suspension), chemically (iron, manganese and ammonium compounds) and biologically (bacteria, diatoms, plankton, blue-green algae) and is characterized by often changes of concentration. To tread water properly according includes the following unit processes: coagulation, filtration, disinfection by ozone, and final disinfection by chlorine and its compound (chlorine dioxide). Raw water comes into a fast-mixing chamber. A coagulant (PAC-10WA) is dosed to the pipe before the chamber with the use of pumps Milton-Roy Maxroy B145 at a delivery 900l/h, automatically steered by a SCD system. The SCD steering system keeps the same coagulant concentration. The solution is supplied to the pipes by static mixers. Then, water is directed to slow-mixing chambers, where flocculation process takes place. Then water goes to setting tanks. There are 12 slow-mixing chambers and 12 three-level setting tanks, where contaminations settle at the bottom. The setting tanks are equipped with drift fenders of ZICKERT system, which work automatically according to a given operation scheme. In this system sludge is scraped to the outlet and then to sewage ponds. From the settling tank, water flows to rapid filters. There are 24 sand-gravel, rapid, gravitational filters at a efficiency 400m3/h. Only 12 of them are in use. The proper filtration process requires rinsing every 24-72 hour. The filters are rinsed by air using two blowers of DR700T type at a delivery 3000 Nm3/h and by water in counter-current using two pumps IFV 400 DDTL (delivery 2250 Nm3/h). Rapid filters washings are led to two Dorra settling tanks. Over sediments water is pumped to the beginning of the whole technological system i.e. before fast-mixing chamber at the delivery 400 Nm3/h. Sediment is pumped to the sediment ponds using Sarlin pumps (efficiency 200 m3/h). After filtration, clean water flows to two ozone tanks, where ozone and sodium hydroxide is added (to correct the pH).The “Mokry Dwor” WTP has the installation of ozone production consisted of: a main air filter, a filter sponge, a compressor VM125 12 of capacity Q=400÷960 Nm3/min (3 pieces), a cooling water and freon bed (2 pieces), two absorbing driers, three ozone generators at a capacity 11kg O3/h and 4 pumps to cool. Because ozone works locally, to prevent distributed water from bacteriological contamination water is finally disinfected using chlorine and chlorine dioxide. These compounds are added before a clean water tank. Chlorine water is produced at the treatment plant and dosed using chlorinators. Disinfection agent dosing are let automatically by concentration analyzer of chlorine or chlorine dioxide. After disinfection water from the clean water tank is delivered by two pipes ∅ 1200 with pumps of II degree pumping station. The present capacity of clean water tank equals 45,000 m3. The equipment of the II pumping station consist of 10 pumps including 4 pumps WAFA 50P17 (Q=1650 m3/h, Hp=5.2 m) and 6 pumps type ABS Z22 (Q=2000 m3/h, Hp= 60m). Moreover, there are four anti-hammer water tanks at capacity 5÷6 m3 and pressure 10 atm, which are to stabilize water distribution subsystem operation. The II degree pumping station works automatically and is steered by required pressure in distribution system. Except for technological line of water production, there are other objects and facilities at the “Mokry Dwor” WTP connected with sediment from settling tanks, rapid filters and sewage from a chemical building.
The Reliability Assessment of Water Treatment Subsystem of Wrocław City
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The “Na Grobli” Water Treatment Plant
The plant has been started to operate on 31 July 1871. The plant was designed for the production of 120 000m3/d, to supply water for 240 thousands inhabitants of Wroclaw. Nowadays it produces about 70 thousand m3/d. It treats infiltration water. According to development plan of WTP Wroclaw, which goal is improving quality of service, there is forecast of soon modernization the “Na Grobli” WTP. After modernization efficiency of the plant will be 90 000. m3/d. The plant is supplied by two pumping plants of infiltration water. They scoop water from water-bearing areas, supplied by water initially cleaned after process of coagulation and sedimentation in the “Mokry Dwor” WTP. The basic function of supplying the WTP fulfils a pumping plant Świątniki with efficiency of about 84,000 m3/d, supplied with underground water from water intake Radwanice with efficiency 17 000 m3/d. The WTP is also supplied by a pumping plant Bierzany with efficiency of about 12 000 m3/d. In the case of disable the Radwanice pumping plant resulting from occurred failures, efficiency of „Na Grobli” WTS will decrease to 72 000 m3/d. It comes from fact that pumping stations Świątniki and Bierzany takes water from the same water-bearing area. Water to the plant is pressed inside two waterpipes φ 850 and one water-pipe φ1000. Taken water has attributes of underground water – elevated content of iron, manganese, carbon dioxide. It is characterized by stable quality with regard to physicochemical and microbiological meaning. Water treatment is done in processes of: preaeration, water stabilization, filtration and disinfection. After WTP modernization water treatment technology will be enlarged of surface coagulation on accelerated filters, intermediate ozonization and sorption an active carbon. In the technological system will be working two intermediate pumping stations. The first treatment process will stay unchanged. Raw water will be still aerated in an iron remover, consisting of 48 aerating towers. Those devices are functionally divided into 12 sections, consisting of 4 towers each. Next, through intermediate pumping station water will be pass onto sand filters, with possibility of simultaneous coagulation in a filtration deposit. Technological string will be equipped with 16 acceleration filters. After process of filtration with the aid of intermediate pumping station water will flow through ozonization chambers and carbon filters (12 pieces including 4 reserve ones) to clean water tanks. Final stage of water treatment, before entering into the distribution network, will be disinfection with chlorine dioxide and/or chlorine. In the water treatment technological system was designed water stabilization using sodium hydroxide solution.. That process will be two-stage, before sand filters and after carbon filters.
5.
Reliability of technical operation of water production subsystem
Water treatment plants are renewable objects of the WSS. To estimate reliability of their operating it is commonly used two-parameter method, where reliability measure is any
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combination pair of two elements coming from a three-element set {K,Tn,TP}, (K-availability factor, Tn- mean renewal time, Tp- mean operation time). Availability factor K determine probability that the WTP at any moment t, enough far from start of operation and in given exploitation conditions will be available; it means will do its task, which is water production for inhabitants and industrial of required quality and indispensable amount. Mean renewal time Tn is defined as expected value of object failure time E(Ton) , while expected value E(Top) of operation time Tp is determined by mean object operation time between following failures. Renewal and operation time are random function and create fluxes of failures and renewal. In practice it is assumed that fluxes are the simplest Poisson fluxes being characterized by: stationary and no time sequence. For WTP objects, when known is only number and failure removing time, availability factor presents: K=
µ λ+µ
5.1
where: µ –failure intensity [1/h], λ –repair intensity [1/h]. It is complex factor taking into consideration failure rate as well as object renewal ability (Wieczysty and others, 2001). Determining reliability level of WTP (or other objects) there are used reliability diagrams made basing on engineer solutions of a given station. Exploitation connections between particular technical objects of WSS present reliability structures as: serial, parallel or threshold. These structures present reciprocal connection of WTP elements from a point of view of its failure results on the whole system operation. The paper tries to determine operation reliability of urban water supply subsystem of Wroclaw containing plants “Mokry Dwor” and “Na Grobli”. Calculation was done basing on structural reliability diagrams of an existing water treatment technological string in the “Mokry Dwor” WTP and assumed designing solutions of the “Na Grobli” WTP modernization. Water treatment process in the WTP are done in independent technological strings (described above), which from reliability point of view forms a serial structure. In every mentioned stage of the water production, unit treatment processes are realized on technological systems, forming so called threshold or parallel structure of objects operation. The above analysis allows creating global calculation reliability diagrams of the plants. For created reliability structures of the „Mokry Dwor” plant, it was performed calculation basing on exploitation data for observation period in years 2001÷2004. Failure time analysis of particular elements was performed basing on the failure records of the objects in “Failure logs”. The records contain information concerning technical objects failures i.e. type, number, date and hour of occurrence and failure time, as well as effects on WTP operation. Moreover, they contained information of scheduled engineer inspections and preventive maintenances of technological devices. This information allows determining reliability characteristics for particular objects and classifying them. In case of the “Na Grobli” WTP reliability analysis was performed basing on design solutions of its
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modernization. Fundamental reliability factor was assumed basing literature data (Kwietniewski and others 1993; Wieczysty and others, 2001;Zimoch, Wieczysty, 2001;). Obtained analysis results are presented in tables 1 and 2. Table 1 Values of reliability parameters of technological system of the “Mokry Dwor” WTP The “Mokry Dwor” WTP arrangement
Availability factor K
Primary pumping station
0.999999369
Coagulation
0.999999504
Filtration
0.999853158
Ozonization
0.996771189
Water disinfection system
0.999998691
Secondary pumping station
0.999999942
Table 2 Values of reliability parameters of technological system of the “Na Grobli” WTP The “Na Grobli” WTP arrangement
Wskaźniki gotowości K
Primary pumping station
0.996869566
Aeration
1.0
Circulating pump on FP
0.999912354
Contact filtration
0.999998874
Circulating pump
0.999912354
Ozonization
0.999964486
Carbon filters
0.999999919
Water disinfection system
0.999999629
Secondary pumping station
0.999990958
Performed reliability calculation give final result in a form of determined a availability factor of water treatment plants of Wroclaw: K (" MokryDwor "WTP ) = 0.996622335
5.2
K (" NaGrobli "WTP ) = 0.996648849
5.3
K (WSSWr ) = K (" MokryDwor "WTP ) × K (" NaGrobli "WTP ) = 0.993282503
5.4
K (WSSWr = 0.993282503 > K W = 0,991328855
5.5
where: KW – required reliability factor for water supply subsystem for settlement over 500 thousands inhabitants having two independent water supply systems (Wieczysty and others, 2001).
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6.
Summary
Reliability of engineer operation the “Mokry Dwor” WTP was determined basing on exploitation data as reliability factor is K=0.996622335>KW=0.995654988. It satisfy reliability conditions, what means that the plant at any moment with probability over 0.99 will operate correctly by producing required amount of water and of adequate quality. The most unreliable element of the “Mokry Dwor” WTP is water ozonization system. It is very important element of technological string, because on its correct operation depends final high quality of water passed to the distribution network, what is guaranteed by Municipal Water and Sewage Company in Wrocław. Reliability of that arrangement is 0.996771189. Calculations have shown that high engineer reliability characterize coagulation and secondary pumping station. It comes from high reservation of elements in this arrangement, what is essential for operation reliability of engineer systems. Analysis of design solution of the “Na Grobli” WTP modernization has shown that engineer solutions of technological water treatment system satisfies reliability standards. The most failure elements arrangement of the “Na Grobli” WTP is a raw water pumping station, what is caused by a little number of reserve elements and a fact that pumping stations are fed from the same water-bearing region. Performed probabilistic estimation has shown in addition that the WSS of Wroclaw satisfies required reliability level.
References Kwietniewski M. and others.(1993). Niezawodność wodociągów i kanalizacji. Arkady, Warszawa 1993, pp 20-31. Wieczysty A. and others (2001). Metody oceny i podnoszenia niezawodności działania komunalnych systemów zaopatrzenia w wodę.ISBN 83-915874-0-1. Komitet Inżynierii Środowiska PAN, Krakow 2001. Zimoch I., Wieczysty A.(2001). Analiza porównawcza metod określania niezawodności stacji uzdatniania wody na przykładzie Dobczyc. Metody oceny i podnoszenia niezawodności działania komunalnych systemów zaopatrzenia w wodę.ISBN 83-915874-0-1. Komitet Inżynierii Środowiska PAN, Krakow 2001, pp 259-282 Archival materials of MPWiK Ltd. Wrocław. Dzienniki Awarii - Failure logs. Wrocław 2001-2004. Author Izabela Zimoch D. Eng: Silesian University of Technology, Institute of Water and Sewage Engineering St. Konarskiego18, 44-100 Gliwice ,Polan,
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