Public water supply is distributed through water pipe network, which affects the quality of water that gets to the consumers when the integrity of the pipe distribution network is compromised. Hence, this study aimed at investigating and modelling water quality deterioration in the distribution network of Kaduna Metropolis. In order to achieve this water samples were obtained from selected points along the distribution network of Kaduna metropolis for water quality analysis. The parameters analyzed were residual chlorine concentration, pH, turbidity and dissolved oxygen. Linear regression equations obtained from the relationship between pairs of parameters were used in a MATLAB/SIMULINK environment to model the change in the various water quality parameters along the network. The results clearly showed that the quality of water produced at the treatment plant is below standard water specifications (Nigerian Standard for Drinking Water Quality, 2007) with turbidity and pH having average values of 9.5 NTU and 5.7 respectively; this quality deteriorates significantly along the network. Residual chlorine concentration was below 0.35mg/l at all points in the network and ranged from 0 to 0.3 mg/l; turbidity levels varied between 2.6 to 10.6 NTU were observed at the treatment plant and reservoirs/booster stations; pH and dissolved oxygen levels varied slightly between 5.4 to 7.4 and 0.4 to 0.7 mg/l along the network. Leakages were also observed along the network. On an average, the model was 77% accurate in predicting values of water quality parameters along the network. Also significant differences observed between the water quality values at Kaduna North Water Works and the selected sampling points. This indicates that the quality of water deteriorates along the network and is not safe for human consumption. Improvement of treatment processes and rehabilitation of the water distribution network were suggested to improve quality of water produced and distributed to the consumers in Kaduna metropolis.

1.1       Preamble
Safe drinking water and basic sanitation are extremely important to the preservation of human health, especially among children. Water-related diseases are the most common cause of illness and death among the poor of developing countries (World Water Council, 2005). The World Health Organization (WHO) has observed that about 80 percent of diseases in the world are water related. Currently, about 20% of the world‟s population lacks access to safe drinking water and more than 5 million people die annually from illness associated with safe drinking water or inadequate sanitation. If everyone had safe drinking water and adequate sanitation services, there would be 200 million fewer cases of diarrhoea and 2.1 million fewer deaths caused by diarrhoea illness each year (Hunt et al., 2001). In addition to these „direct health‟ effects of inadequate water supply provision, there is an additional cost in time and energy expended in carrying water from the supply to the family dwelling.

Improving water supplies has been a high priority activity for most developing country governments, donor agencies and communities for many years now so as to achieve the Millennium Development Goal 7 target 10 (halving by 2015, the proportion of people without sustainable access to safe water and basic sanitation), with the reference year of 1990. In order to achieve this goal, the federal and the state governments of Nigeria have step up efforts in expanding
the various water corporations‟ distribution networks, which are charged with the responsibilities of delivery potable water to the populace. In spite of these recent efforts, water and sanitation coverage rates in Nigeria are among the lowest in the world. According to WHO/UNICEF (2006) drinking water coverage in Nigeria fell from 49 percent in 1990 to 48 percent in 2004 as against the expected 65 percent coverage.

A large number of those with access to the public water systems in Nigeria are not completely free from water borne diseases, because the quality of the water that eventually get to the consumers may not be guaranteed. Although protected water sources and modern, well-maintained drinking water treatment plants can provide water adequate for human consumption; ageing, stressed or poorly maintained distribution systems can cause the quality of piped drinking water to deteriorate below acceptable levels and pose serious health risks to its consumers (Lee and Schwab, 2005).

Biofilms, which are coatings of organic and inorganic materials in water pipe distribution systems, has generated health concerns because they harbour, protect and allow the proliferation of several bacteria pathogens, including Legionella and Mycobacterium avium complex (MAC). Bacteria growth in biofilms is affected by several factors, including water temperature, type of disinfectant and residual concentration, biodegradable organic carbon level, degree of pipe corrosion and treatment/distribution system characteristics. Hence, Lahlou (2002) noted that the water quality of a drinking water system might be acceptable when the water leaves a treatment plant. However, a variety
of physical, chemical, and biological transformations can happen once the water enters and travels through a distribution system, which can result to objectionable taste and odour and the risk of gastrointestinal illnesses. This situation was confirmed by Frederick (2007) who reported that microorganisms in municipal drinking water supplies have led to several outbreaks of water-borne diseases in the United States. For instance, cryptosporidium in Milwaukee‟s water supply resulted in some 400, 000 serious illnesses and 50 deaths in the spring of 1993. In 1983, contaminated drinking water in Luzerne County, Pennsylvania caused an outbreak of giardiasis-a common diarrhoea that left 6, 000 people ill and forced 75, 000 others to obtain more expensive alternative sources of drinking water.

The integrity of water distribution networks in Nigeria and in particular in Kaduna State is highly questionable, as it is characterized with leaky pipe joints, pipe breaks, corrosive pipes, intermittent supply etc. which may impact negatively on water quality through contamination before it gets to the consumer. But unfortunately, most people used to assume that if water entering into a distribution system were of high quality, its quality would still be good at the tap (AWWA, 2013). Therefore, people consume tap water without any doubt of its quality. This situation can be very dangerous if actually the quality of water that gets to the consumer has been compromised.

Although water is a scarce commodity, its quality should be assured no matter how small the quantity may be. Thus, the goal of a water treatment and
distribution network is to deliver water in sufficient quantities and of good quality where and when it is needed at the required pressure.

First, this quality can be expressed in terms of the water being physically and chemically free from carcinogens and contaminants that impart colour, taste and odour. Secondly, water quality can be expressed as water devoid of pathogenic microorganisms. This suggests that ideally, treatment processes at water treatment plants as well as pipes and storage facilities of a good drinking water distribution network should constitute a network of uncontrolled chemical and biological reactors capable of withstanding significant variations to maintain water quality. But unfortunately, even before these pipes are networked to form a distribution system, their sterility may not be guaranteed during storage and transportation to construction sites and they may have been left outdoors for months or years leading to contamination potentially caused by a variety of animals, plants and microbiological life entering it (Kofi, 2012).

Consequently, after installation, if such mains are not properly “purified” and flushed, then although treated water may meet the required quality criteria when it leaves the treatment head-works and travels through such a distribution network, the quality will deteriorate. In cases where these distribution mains are properly disinfected and flushed after installation, as water continues to run through for some number of years, there may be sediment build up (which may lead to encrustation), corrosion and subsequent leaching of pipe materials, formation of biofilms etc. These occurrences normally compromise the quality
of the distributed water. Generally, some indicators of water quality deterioration in distribution networks include (Kofi, 2012);

1. Loss of disinfectant residual

2. Corrosion of iron pipes

3. Dissolution of Pb and Cu from pipe walls

4. Biofilm formation

5. Occurrence of compounds that confer poor taste and odour

6. Formation of disinfection by-products (some of which are carcinogens). These compounds are normally products of reactions between organic and/or inorganic soluble compounds and disinfectants.

7. Increased turbidity caused by particulate re-suspension.

To attain microbiological safety, potable water should be free of pathogenic microorganisms, and this is achieved through purification by a specific treatment step called disinfection (Kofi, 2012).

Generally, disinfection can be defined as the inactivation of pathogenic microorganisms and is purposed to eliminate any microbiological risks of water-related diseases. The use of Ultra Violet (U.V) light, ozonation and chlorination are all forms of disinfection, but chlorination is usually preferred and employed in most drinking water systems because of its efficiency and durability (leaves adequate residuals) and it is relatively cheap (Mays, 2000).

Thus, to reduce the microbiological risk of potable water, it should have adequate residual chlorine to ensure the bacteriological safety of the water. In
view of this, the World Health Organization (W.H.O) recommends a chlorine residual concentration of 0.5mg/l in final water whereas the Nigerian Standard for Drinking Water Quality (NSDWQ) recommends a concentration not less than 0.35mg/l at the point of consumption. To ensure that potable water is bacteriologically safe at the taps, a chlorine residual concentration between 0.20 to 0.5 mg/l should be maintained in the treated water (WHO, 1997).

However, the physical, chemical and microbiological transformations (existing pipe material/age, water age, biofilm formation, encrustation etc.) occurring within and along a distribution network may defeat this purpose by “using up” the chlorine thereby enhancing its decay. This decay may occur within the bulk of the water (bulk decay) or as a result of the interaction between the water and pipe walls (wall decay). Therefore, to ensure that adequate chlorine residuals persist in distribution lines till consumption, studies must be done to find out how these transformations occurring along a particular distribution network contribute to the bulk and wall reactions.

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