Knowledge gaps

Water companies have to deal with two main problems: regulatory compliance failures and discolouration events, both of which can lead to penalisation and loss of confidence by customers, as well as concerns regarding potential health impacts. While discolouration can be detected by human eyes and prompts customers to complain, Fe and Mn compliance failures are assessed using analytical methods, because they cannot be detected by sight. Currently, many drinking water companies identify regions with high-risk of discolouration/Fe and Mn failures by either selecting areas in the network with high Fe and Mn concentrations from their random sampling, or by using customer complaints data due to water discolouration. These methods can be ineffective for two reasons. First, the large sizes of water supply zones (WSZs) make it difficult and expensive to monitor Fe and Mn concentrations. With about 315,000 km of water mains across England and Wales, it will be impossible to sample every node in large WSZs. Hence, regions which have high Fe and Mn concentrations that are not sampled will not be detected. Secondly, studies in the United Kingdom have shown that only 30% of customers that experienced discoloured water actually complained (Ewan & Williams, 1986). Similarly, a study conducted by Roseth and Rock (2003) in Melbourne, Australia, indicated that only 15% of customers who experienced water discolouration complained. These studies show that certain regions in WSZs with high discolouration risk/Fe and Mn accumulation potential will go undetected because complaints are not reported. Using Fe and Mn concentrations and customer complaints to identify regions with high-risk of discolouration or failures of Fe and Mn is desirable. However, a model that can predict the risk of Fe and Mn accumulation potential in every region in WDNs and indicate the causes of this risk will be more beneficial.

Hydraulic distance from source of water supply is a very important variable that influences Fe and Mn accumulation which has not been investigated thus far. It is the distance taken for water to travel from a source of water supply to a given node within a WDN. In general, the further water travels through WDNs, the more chlorine dissipates. This increases microbial growth, which increases biological oxidation of Fe and Mn, and subsequently increases Fe and Mn accumulation potential.

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A number of discolouration risk assessment tools have been developed by researchers. They include the Particle Sediment Model (PSM) (Wu et al., 2003), Discolouration Risk Analysis Tool (DRAT) (Boxall & Husband, 2005), Resuspension Potential Method (RPM) (Vreeburg, Schaap, & van Dijk, 2004a), Discolouration Risk Modelling approach (DRM) (Dewis & Randall-Smith, 2005), Discolouration Propensity Model (DPM) (McClymont et al., 2011), and Pressure-dependent Analysis (PDA) model (Seyoum & Tanyimboh, 2014). Most of these models deal exclusively with the risk of discolouration based on physical/hydraulic variables such as water velocity, turbidity, shear stress, water age, turbopherosis, and pipe material. Very little research has been conducted on Fe and Mn accumulation on the inner surface of pipe walls. More importantly, none of the developed models are able to predict discolouration/Fe and Mn accumulation potential for every node in WSZs using chemical, biological, and hydraulic/physical variables.

Fe and Mn may be present in different complex species and compounds as well as in solution or particulate forms. They may also be loosely present within the water mains, or adhered to the inner surface of pipe walls. In this research, Fe and Mn were studied from the point of view of accumulation rather than from discolouration. The accumulation of Fe and Mn on the inner surface of pipe walls is influenced by the following factors:

(a) chemical reactions

(b) microbiological activity, and

(c) physical or hydraulic processes within the network.

Studies in this area have thus far mainly focused on the following issues:

1. Discolouration, with little research carried out to understand what factors affect accumulation and compliance problems.

2. The above mentioned factors have been investigated by researchers independently, rather than in combination.

Figure 1.1 The Fe and Mn accumulation potential model

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To date, researchers have only studied these factors partially or separately, but never in combination. Clearly, there is a limitation in their attempt to unravel the complex process of accumulation. In this research, the focus was on studying Fe and Mn accumulation potential holistically rather than independently. However, in order to study the factors that influence Fe and Mn accumulation potential holistically and estimate the combined effect on deposition dynamics, it is imperative to understand their influence both individually and in association. Here, the challenge is to correlate Fe and Mn accumulation potential with the relevant chemical, biological, and physical/hydraulic variables. A diagram of the developed model showing how all the variables were correlated with Fe and Mn accumulation potential is presented in Fig. 1.1. Chapters 2, 4, 5, and 6 discuss the effect of each of the variables on the accumulation of Fe and Mn in detail.

As mentioned above, this study took a holistic approach in investigating the factors that influence the accumulation of Fe and Mn particles from post-treatment to customers’ taps through WDNs. This approach is important because continuous deposition due to these factors will lead to compliance failures, and eventually result in discolouration during hydraulic events such as opening of fire hydrants during fire extinguishing exercises and increased flow caused by increased water consumption. Mitigating the former problem will clearly mitigate the latter, as the two problems are directly linked. The ability of the developed models in this research to predict and indicate the causes of high Fe and Mn accumulation potential at the node level make them a unique and practical tool to guide drinking water companies in managing discolouration. These models can help in the maintenance of water mains, the development of cleaning protocols, and the development of operational and management strategies for water distribution at the national and international levels.