General problem statement

Fe is a naturally occurring element that is found in certain rocks and soils, and it constitutes approximately 5% by weight of the earth’s crust. It is the fifth most abundant element in the earth’s crust (Gschneidner, 1996). Unsurprisingly, a study conducted by Boxall, Skipworth and Saul (2003) on flushing samples collected in the UK identified Fe and Mn as the first and second most common water contaminants, respectively, irrespective of the pipe material in WDNs. A related study by Slaats (2002) showed that gradual accumulation of Fe and Mn particles in WDNs is the most common cause of water discolouration. Although elements such as silicon, calcium (Ca), Al, and Cu as well as organic compounds, can also cause water discolouration, they are not as prevalent as Fe and Mn (Teasdale, O’Halloran, Doolan, & Hamilton, 2007).

1.2.1 Health and aesthetic problems

Fe and Mn in drinking water have long been considered to cause only aesthetic problems; that is, they are secondary contaminants that have little or no adverse health effects. In fact, low concentrations of Mn and Fe are known to be essential for human health (Swistock, Sharpe, & Robillard, 2001). Although only low concentrations of Fe and Mn enter WDNs

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after the treatment process, years of accumulation in distribution systems, as well as periodic re-release in significant quantities, and other adsorbed compounds associated with the deposits can result in more than the traditional aesthetic issues. For example, Wasserman et al. (2006) investigated the relationship between increased concentrations of Mn in drinking water and reduced intellectual functions of children.

High concentrations of Mn and Fe in WDNs can also give water an unpleasant medicinal or metallic taste (Swistock et al., 2001). Studies have attributed red-brown, yellow, yellow- brown, and brown colours of drinking water to corrosion of Fe (Yarra Valley Water, 1998). Black water has been attributed to excess concentrations of Mn and biofilms stripping (Sly, Hodgkinson, & Arunpairojana, 1990; Yarra Valley Water, 1998). This discoloured water could also lead to similarly coloured stains on laundry and porcelain, thereby prompting numerous customer complaints. Vegetables cooked with Fe-contaminated water become dark and look unappetizing, and Fe or Mn bacteria can cause black-brown slimy masses inside toilet tanks (Herman, 1996).

In response to these known issues, water companies have adopted expensive and sophisticated risk-based management systems for monitoring discolouration in WDNs. However, despite their efforts to comply with drinking water standards, they continue to receive customer complaints related to water quality. In this research, a customer complaint is defined as a record of a customer complaining directly to the water company regarding incidents such as discolouration, metallic taste, or slime. These complaints significantly undermine customers’ confidence in water companies. An analysis of customer complaints related to the quality of water supplied by a UK water company over a five-year period showed that 34% and 7% of the complaints are related to discolouration and other aesthetic problems, respectively (Cook, Boxall, Hall, & Styan, 2005). Customers evaluate water quality by taste, sight, and smell. However, most substances that can be evaluated by human senses are secondary contaminants, and are often harmless (Department of Human Services & Department of Natural Resources and Environment, 2000). In fact, some of the highest health risks of water are attributed to substances that cannot be perceived by human senses (for example, bacteria and dissolved organic compounds). Although customer complaints are a good indicator of water quality, using them alone can be misleading, as not all customers complain. Nevertheless, they can be

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very useful for predicting discolouration/Fe and Mn accumulation potential in WDNs when used in combination with other chemical, biological, and physical variables.

1.2.2 Compliance problems

High concentrations of Fe and Mn in WDNs can lead to compliance failures. The DWI has set the maximum concentration levels (MCLs) of Fe and Mn in drinking water to 200 and 50 µg/L, respectively. In general, water companies set post-treatment targets of Fe and Mn to approximately 3% of their respective MCLs. They do so to reduce the concentrations of Fe and Mn entering WDNs, thereby leading to reduced deposition. However, irrespective of how effective water is treated, very low concentrations of Fe and Mn may still enter the network from water treatment plants and gradually accumulate on the inner surface of pipe walls within WDNs. During hydraulic events, such as high flows created by bursts in water mains or high diurnal consumption of drinking water, these accumulated particles may be dislodged from the pipe walls, cause discolouration, and subsequently end up at customers’ taps.

1.2.3 Financial losses

In April 2010, the Water Services Regulation Authority in England and Wales (Ofwat) introduced the Service Incentive Mechanism (SIM). This mechanism rates the performance of water companies based on customer satisfaction, and either rewards or penalises them. In view of this, it has become extremely important for water companies to reduce the number of customer complaints caused by drinking water discolouration. Water companies also receive fines from the DWI if the concentrations of Fe and Mn exceed their respective MCLs.

The deposits of Fe and Mn in WDNs can clog pipelines and decrease water pressure, thereby requiring more energy to pump water through the network. Furthermore, these deposits can increase pumping and rehabilitation costs (Vreeburg & Boxall, 2007). Moreover, the corrosion of iron pipes is an important chemical process in water discolouration. For this reason, several water companies have spent substantial amount of money replacing iron pipes with polyvinyl chloride (PVC) pipes, with the aim of decreasing discolouration in WDNs. However, customers still experience some discolouration in areas that are entirely networked with PVC pipes, although they do not corrode over time as they do not react with air and water (Vreeburg, 2007). A study by

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Cerrato, Reyes, Alvarado and Dietrich (2006) indicated that this observation could be attributed to the deposition dynamics in PVC pipes. They observed that the Mn deposits on the walls of PVC pipes were loose because of their exceptionally smooth walls; as a result, were subjected to sloughing and discolouration under smaller shear forces than in iron pipes.

In a related study, Cook (2007) investigated plastic, asbestos cement, cast iron, epoxy lined, and cement and bitumen lined pipes, and observed no correlation between customer complaints related to water discolouration and these pipe types. Boxall et al. (2003) reported that, irrespective of the pipes used in WDNs, Fe and Mn were the first and second most common water contaminants, respectively. This result indicates that there are other factors in addition to pipe material that cause Fe and Mn particles to accumulate in WDNs. 1.2.4 Modelling difficulties

The processes influencing the accumulation and release of Fe and Mn in WDNs are highly complex, unpredictable, not fully understood, and difficult to model mathematically. The concentrations of Fe and Mn frequently change with time and space as water moves from the treatment plant to customers. The variability of source materials, hydraulics, biological and chemical reactions that occur within a network contribute towards creating a very complex environment that is difficult to understand.

Moreover, increased treatment costs, increased pumping and rehabilitation costs, fines, and sophisticated risk-based management systems are costing water companies significant amount of money. There is therefore an urgent need for water companies to not only gain a practical understanding of the processes and mechanisms that lead to compliance failures and discolouration, but also devise a comprehensive strategy to deal with such events. Water companies worldwide are urgently looking for solutions to prevent the above- described problems. Furthermore, there is also a strong need for a model that can predict the risk of Fe and Mn accumulation potential based on not only physical and hydraulic variables, but in combination with chemical variables and variables that influence biological processes.

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