An Approach to Estimating Water Quality Changes in Water Distribution Systems Using Fault Tree Analysis
An Approach to Estimating Water Quality Changes in Water Distribution Systems Using Fault Tree...
Tchórzewska-Cieślak, Barbara;Pietrucha-Urbanik, Katarzyna;Papciak, Dorota
resources Article An Approach to Estimating Water Quality Changes in Water Distribution Systems Using Fault Tree Analysis 1 1 , 2 Barbara Tchórzewska-Cieslak ´ , Katarzyna Pietrucha-Urbanik * and Dorota Papciak Department of Water Supply and Sewerage Systems, Faculty of Civil, Environmental Engineering and Architecture, Rzeszow University of Technology, Al. Powstancow Warszawy 6, 35-959 Rzeszow, Poland; firstname.lastname@example.org Department of Water Puriﬁcation and Protection, Faculty of Civil, Environmental Engineering and Architecture, Rzeszow University of Technology, Al. Powstancow Warszawy 6, 35-959 Rzeszow, Poland; email@example.com * Correspondence: firstname.lastname@example.org; Tel.: +48-17-865-1703 Received: 13 August 2019; Accepted: 24 September 2019; Published: 27 September 2019 Abstract: Given that a consequence of a lack of stability of the water in a distribution system is increased susceptibility to secondary contamination and, hence, a threat to consumer health, in the work detailed here we assessed the risk of such a system experiencing quality changes relating to the biological and chemical stability of water intended for drinking. Utilizing real operational data from a water treatment station, the presented analysis of the stability was performed based on the fault tree method. If they are to protect their critical-status water supply infrastructure, water supply companies should redouble their eorts to distribute stable water free of potentially corrosive properties. To that end, suggestions are made on the safeguarding of water distribution systems, with a view to ensuring the safety of operation and the long-term durability of pipes. Keywords: water supply system; corrosion aggressiveness of water; safety of water system functioning; biological and chemical water stability; fault tree analysis 1. Introduction Thanks to the development of treatment technology, the quality of water directed to the supply network can meet strictly deﬁned standards and recommendations. However, the water supply intended for customers must be of an adequate quality not only at the place and time of entry into the network but also at the point of reception [1,2]. This is important as the transfer of water to recipients may often entail a deterioration in quality, e.g., as harmful substances are released by the material from which the network is made , as bioﬁlms or other deposits ﬁrst form and then become detached, and as the release of other compounds take place [4,5]. According to Van der Kooij, to maintain water stability, pressure of a minimum of 2 bar should be provided . In this regard, it is the impact of changes in hydraulic conditions, such as ﬂow and velocity, that is key . In general, irregularity of the water supply may lead to a physicochemical and microbiological destabilization of the pipeline material and result in a growth of deposits [8,9]. The level of secondary pollution is aected by the amount and chemical composition of the sludge, the type and number of microorganisms present in the bioﬁlm, and the degree of corrosive aggressiveness of the water [10,11]. Consequences may then include releases of sediments, substances, or microorganisms accumulating over decades [12,13], with the result that aesthetic problems or even health hazards arise, i.e., in connection with turbidity and color changes, as well as impacts on the tap water taste and smell [14,15]. As has been noted, qualitative changes in the composition of water during transfer reﬂect not only the technical condition of the network itself, but also the aforementioned lack of stability in physical, biological, and/or chemical terms of the Resources 2019, 8, 162; doi:10.3390/resources8040162 www.mdpi.com/journal/resources Resources 2019, 8, 162 2 of 11 water leaving a waterworks [16–18]. Even after treatment, water brings into the distribution system a physical load (of suspended particles in the so-called disperse phase), microorganisms, and organic and inorganic nutrients [19,20]. Under certain circumstances, such water may be prone to secondary pollution sucient to change parameters in a way that may compromise its safety, thus posing a threat to consumers [21,22]. A water supply system (WSS) is safe when it is able to carry out its functions even in the face of undesirable events of various kinds . The consumer is the primary entity to which the concept of safety applies [24,25]. A risk function is in turn deﬁned as the probability of the occurrence of an undesirable event (the cause) that may result in a real threat to the health or life of water consumers (the eect) [26–28]. Thus, as we seek to analyze the functioning of a water distribution subsystem in terms of the safety of consumers, in this paper, we ﬁrst and foremost consider the resistance to such undesirable events of various types through the fault tree analysis (FTA) method. The FTA method involves decomposing an event, e.g., object damage, into elements of a cause-and-eect chain. At the base of the damage-tree are the elementary events that may be the cause of the event which is at the top of the tree. The FTA method, according to the International Electrotechnical Commission guidelines, should include a deﬁnition of the analysis, the functional characteristics of the system, the tree structure, and logical analysis of the interaction between the elements of the tree . It is important to adopt working assumptions regarding the operating conditions of the system, simplify the description of processes, and adopt indicators to evaluate individual elements or their speciﬁc state in the event of an adverse event, as well as their identiﬁcation. There are two main types: the static FTA, which determines the set of causes of the event at the top; and the probabilistic FTA, which additionally allows us to determine the probability . The advantage of the FTA method is the ability to analyze any path in the tree. It is also a technique that allows us to draw conclusions on the basis of the deﬁnition of the problem. In the case of an uncertain knowledge base, it is also possible to use fuzzy FTA analysis. Extensive probabilistic characteristics for FTA, also in fuzzy form, can be found in other works . FTA analyses in dynamic and fuzzy form, are especially useful in the case of complex technical systems, in which the analysis of failure scenarios is a dicult process because it requires the examination of numerous cause–eect relationships. Undoubtedly, a water supply network is such a complex technical system. With very extensive damage trees, especially with dynamic gates , it is necessary to use specialized software and in some cases, use simulation techniques, such as the Monte Carlo method . Speciﬁcally in this article, we consider factors aecting the biological and chemical stability of supplied water and discuss how secondary pollution may arise within a supply network. The operational data forming the basis for such considerations relate to the functioning of a real-world supply system controlled by a municipal water company. Technical documentation and information obtained from its managers and services were also referred to. Treated water collected from a clean water tank prior to ﬁnal disinfection was analyzed, while statistical processing involved Statsoft software , as supported by the ReliaSoft BlockSim comprehensive platform . Above all, the research described here has sought to present a methodology that may be utilized in assessing the risk of changes in water quality in a distribution subsystem. 2. Materials and Methods 2.1. Characteristics of the Research Object The city selected for study is supplied by a shore-edge water intake of a capacity of 90,000 m /day. Water from there is treated at two water treatment plants (WTPs) that are independent, though operating on the basis of the same technology, with pre-ozonization, coagulation supported by a synthetic ﬂocculant (at low temperatures), sedimentation in horizontal settling tanks, and ﬁltration on fast ﬁlters. In the case of WTP , the latter are sand ﬁlters, while at WTP they are of anthracite I II Resources 2019, 8, 162 3 of 11 plus sand, with indirect ozonation and biologically active ﬁltration (BAF). The intermediate ozone Resources 2019, 8, x FOR PEER REVIEW 3 of 10 chambers are completely separate for WTP and WTP . Water after ozonization goes to four chambers I II with carbon ﬁlters, and from there to clean water reservoirs speciﬁc to WTP and WTP . I II goes to four chambers with carbon filters, and from there to clean water reservoirs specific to WTPI The treated water meets the quality requirements set for water intended for human and WTPII . consumption [37,38], while the quantity produced each day is around 37,000 m —enough to meet the The treated water meets the quality requirements set for water intended for human consumption customers’ needs in full. More speciﬁcally, the system under consideration consists of the following: [37,38], while the quantity produced each day is around 37,000 m —enough to meet the customers’ an emergency downhole shot of a capacity of 240 m /day; needs in full. More specifically, the system under consideration consists of the following: local water intake of a capacity of 450 m /day; an emergency downhole shot of a capacity of 240 m /day; 36 water pumping stations (hydrophores); local water intake of a capacity of 450 m /day; 11 clean water equalizing tanks of a total capacity of 35,000 m ; 36 water pumping stations (hydrophores); 190 public wells. 11 clean water equalizing tanks of a total capacity of 35,000 m ; 190 public wells. The water supply network of a total length of ca. 900 km comprises 49 km of the main network, The water supply network of a total length of ca. 900 km comprises 49 km of the main network, 529 km of the distribution network, and 322 km of household connections. The main network is made 529 km of the distribution network, and 322 km of household connections. The main network is made of cast iron and steel pipes, while the distribution network is of cast iron, steel, polythene (PE), and of cast iron and steel pipes, while the distribution network is of cast iron, steel, polythene (PE), and polyvinyl chloride (PVC) pipes. Household connections are mainly made of galvanized steel, PE, PVC, polyvinyl chloride (PVC) pipes. Household connections are mainly made of galvanized steel, PE, and cast iron. The analyzed water supply network consists of four main buses transporting water PVC, and cast iron. The analyzed water supply network consists of four main buses transporting treated at the second-degree pumping station. water treated at the second-degree pumping station. 2.2. Methods 2.2. Methods 2.2.1. The Concept of the Fault Tree Method 2.2.1. The Concept of the Fault Tree Method Fault Tree Analysis (FTA) presents graphically the relationship between events that aect the occurrence of a speciﬁc undesirable event, called the “peak event” [39,40]. The tree makes reference to Fault Tree Analysis (FTA) presents graphically the relationship between events that affect the so-called functors (logic gates), deﬁning the logical product of events and the logical sum of events as occurrence of a specific undesirable event, called the “peak event” [39,40]. The tree makes reference shown in Figure 1. to so-called functors (logic gates), defining the logical product of events and the logical sum of events as shown in Figure 1. (a) (b) Figure 1. Alternative gate designations (a) AND gate (and)—logical product, (b) OR gate (or)—logical sum Figu accor re ding 1. Alternative g to . ate designations a) AND gate (and)—logical product, b) OR gate (or)—logical sum according to . The gates for the assessment of ﬁtness are as follows : The gates for the assessment of fitness are as follows : OR gate—an event above the gateway occurs if at least one event below occurs (an exit event occurs OR when gate— any an eve of the nt a input bove events the gate occur). way occurs if at least one event below occurs (an exit event occurs when any of the input events occur). AND gate—an event above the gateway occurs if all events below occur (an exit event occurs AND gate—an event above the gateway occurs if all events below occur (an exit event occurs when all input events occur). when all input events occur). If w , w ,..., w are the events, the given element is in a state of eciency or disability and saves 1 2 If w1, w2,..., wn are the events, the given element is in a state of efficiency or disability and saves with the zero vector of the ones according to EN 61025 : with the zero vector of the ones according to EN 61025 : 1 whenthe structure is operational 1whenthestructureisoperational w , w ,::: , w input events = , (1) 2 n 1 w1, w2,..., wn — input events = , (1) 0