The quality of drinking water is essential for maintaining good human health. Water provides key elements for humans however with a slight pollution, it can become an avenue of undesirable substances that can have relative impacts on human health (Reyes et al. 2008). This has raised the awareness of consumers and increased their consciousness to have access to safe drinking water. According to Zhang et al. (2008), unsafe water, poor sanitation and hygiene was ranked third amongst other 19 leading risk factors for health burden in Nigeria as well as other developing countries. Thus to protect human health from the negative impacts caused by contamination of drinking water, guideline limits for microbial and chemical parameters are created by different bodies to regulate the standards for drinking water.
In Europe for example, the WHO and the European council directives are responsible for these standards while NAFDAC (National Agency for Food Administration and Control) plays this role in Nigeria with reference to World Health Organisation (WHO) guidelines for drinking water quality.
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Assessment of drinking water quality is essential and a wide range of properties/parameters such as organoleptic properties, perceprtion of chemicals, contextual cues provided by the supply system amongst many other factors can compromise its quality as well as influence perception of the public. Thus to achieve the provision of safe drinking water to the populace of any location, there is need for proper monitoring from source to tap (Sarin et al. 2004).
Not only do we need enough enough water for potable and an industrial use, its quality is also essential. Water should be aesthetically acceptable and palatable. The control of its taste, odour, turbidity and colour results in water which is clear, colourless and without objectionable or unpleasant taste or odour. Other aspects of water quality such as corrosiveness, a tendency to form incrustations and excessive soap consumption should be controlled on the basis of economic considerations because of their effects on the distribution system and/or the intended domestic and industrial use of the water (put reference).
The variation in the quality is multidimensional of which the coloration of water is also part of the quality. The general colour which can be quickly questioned is the red coloration. Red water is principally caused by the release of iron from corroded water pipes, water-sediment scaling and those dissociated from rocks which causes problem in drinking water distribution (put reference). Water sediment scales also deposited on the inner walls of water distribution iron pipes which may re-disolve after treatment as physico-chemical status of the water changes thereby deteriorating the water quality, restricting the flow of water, and may impact human health. Various pipe materials, such as iron, galvanized steel, concrete, PVC, and copper, have been extensively used in the drinking water distribution systems of which some have been phased out. However, sediment scales formed in the inner walls of water distribution systems greatly contribute to the amount of major and trace elements in drinking water which cause problems such as clogging, providing habitat for pathogens, bringing unpleasant color (red water), lowering the pressure resistance, release of several undesirable substances to drinking water, thereby accelerating water quality deterioration and posing threat to human health (Teng et al. 2008).
Scaling occurs when water has high levels of minerals like Ca2+, Mg2+ and Fe, which build-up on pipe surfaces simultaneously to corrosion (Wu et al. 2010) This Ca2+ and Mg2+ are common metallic elements found in the earth's crust thus water source that percolates through soil and rocks can dissolve these mineral and hold them in solution (Website 1). Slight scaling can be considered beneficial in the inside surfaces of metal pipes when it becomes coated with harmless minerals (e.g siderite) slightly that it act as a barrier to corrosion (Sander et al. 1996) however increased levels of scaling can be harmful. Most water distribution pipelines have a buildup of sediment or corrosion scales product inside the conduits and these corrosion scales releases iron into the bulk waters that can re-precipitate forming mound-like features referred to as tubercles (Gerke et al. 2008) which not only increases pumping cost but also impact water quality.
The minerals that combine to form scale are normally present to some degree in the source water and with changes in water parameter such as rise in water temperature and drops in water pressure, the suspended minerals will be released and form a carbonate scale (Hasson et al. 1968) therefore the water composition is important and this will be studied further in relation to the issue in Lagos.
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Although cast iron, especially grey cast iron has been banned in many developed countries (Teng et al. 2008), it has however been widely used for decades and for economic reason it is difficult to substitute them with plastic or stainless steel pipes immediately in developing countries. Thus it is projected that the use of these pipes by drinking water industry in developing countries will still last for some years.
Iju and Adiyan are among the most densely populated area in Lagos State, Nigeria and there is need for good potable water for the inhabitants. Although the cause of coloration is not particularly known, the likely causes of this could be attributed to source of water and the water distribution system hence the water quality, pipe conditions and other hydro geochemical studies is quite important.
Water for the populace in this area is being channeled through metallic pipes in which the effect of corrosion and sediment scaling cannot be ignored
Lagos is the largest city in Nigeria and the second largest in Africa. It contains over 40% of Nigeria's manufacturing activities with the highest level of emission of 8000 tons of hazardous waste per year, most of which is discharged directly into the lagoon. It is surrounded by water however the water in the lagoon and ocean surrounding Lagos is not potable and does not meet World Health Organization standards consumption as a result of urban and industrial wastes discharged into the Lagos Lagoon and its fast depletion of its coastal terrain thus are not fit for human consumption (Ebiare and Zejiao, 2010). Hence, in the effort of producing water for the people of Lagos to use and drink, colonialist searched as far as to Iju, a suburb on the outskirts of Lagos , where water was flowing in from River Ogun. It is from this river that water is supplied to Lagos areas (LSWC, 2009)
According to Olayinka and Alo (2004), heavy metals which include lead, copper, zinc, nickel, chromium, Cadmium and Iron are present in most Nigerian rivers in concentrations well above acceptable and permissible levels. However a 1 year study (covering dry season and rainy season) of the the Ogun river in South-West, Nigeria from which iju and adiyan areas of Lagos gets their source of water from was analyzed by Jaji et al. 2007 for physico-chemical and bacteriological parameters as well as heavy metals using standard methods. The values obtained for turbidity, phosphate, iron, total dissolved solids, the manganese content, dissolved oxygen and chloride in both seasons were above the maximum acceptable limit set by the World Health Organization (WHO) for drinking water.
The problem associated with the water distribution of the populace of IJU - ADIYAN area is colored water. This water contains reddish particles as it is being collected from the tap. However this particle settles out as water stands.
The study area falls within longitude 30151 to 30281E and latitudes 60341 to 60431N (Fig. 1a) and falls within the sedimentary and the crystalline terrain of Southwestern Nigeria. It has an extent of 57.33km2 and it is located on the Southern part of Lagos. Thus it is expected that the metallic elements such Fe, Ca and Mg which are found in the earth's crust would accumulate and be held in solution in the source water as it percolates through soil and rocks of this area.
The study area lies within the tropical rain forest region. It is characterized by alternation of dry and wet seasons with an annual rainfall of approximately 1408 mm (57.7 inches). Eighty percent of the annual rainfall (1160 mm) falls during the south west monsoon, (April-October) and the remaining twenty percent (250 mm) falls during the northeast monsoon (November-March). The air is very humid throughout the year, with monthly average temperatures ranging from 28Â°C in July/August to 32Â°C in February/March (Yusuf, 2007). This however will need to be put into consideration as seasonal change can affect water quality and influence the rate of corrosion of water distribution system by impacting many parameters that are critical to pipe corrosion including physical properties of corrosion scale, chemical rates, and biological activity.
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The drainage is moderately integrated, but most of the rivers are relatively small. The drainage pattern is dendritic with the rivers flowing in the southeastern direction. Generally, the small rivers drain into River Ogun and River Majidun (reference the right figure). Source water for the populace of this area is gotten from River ogun (reference figure) and this needs to be put into consideration when evaluating the factors that influence corrosion of distribution systems as water from various sources ten to influence water quality thus speed up corrosion of distribution system.
1.1 AIMS AND OBJECTIVES
The overall aim of this project is to evaluate the issues and impact of scaling in water distribution systems, and to investigate its impact on drinking water quality. This will be achieved by the following key objectives:
To characterize the physical properties and chemical composition of scaling
To identify the factors responsible for their accumulation and deposition.
To examine the difference between scales from dissimilar water distribution systems.
To evaluate the impact of scaling on water quality
To understand reasons for the success and failure of colored water and scale formation control procedures.
To suggest control and mitigation strategies that may be designed and implemented once the chemistry is more completely understood.
1.2 LITERATURE REVIEW
WATER QUALITY PARAMETERS INFLUENCING CORROSION (CHANGE TOPIC)
Several studies have been carried out in the past decades to investigate iron corrosion and to elucidate fundamental mechanisms responsible for iron release from pipes which often causes red water. These studies have proven useful with a drawing conclusion that several water quality parameters such as alkalinity, pH, dissolved oxygen (DO), water flow characteristics, temperature, water treatment practices, application of an inhibitor, and fluctuations in water quality affects iron corrosion (Tang et al. 2006). However for iju-adiyan area of Lagos, the most important and likely factors that need to be put into consideration include parameters pH, alkalinity and dissolved oxygen (what of treatment practice??) all of which will be looked in detail in the next chapter.
Corrosion is a result of oxidation that occurs when metals come in contact with water resulting in the formation of stable solids / tubercules which consist of layers of corrosion product that build up over time to form lumps and mounds over local areas of metal and can further cause the release of iron into solution (Volk et al. 2000).
It is important to understand scale formation and behavior so as to evaluate its impact on water quality. Sediment and corrosion scales grow radially inwards towards the pipe center. These scales are porous deposits with a distinct layered structure compounds usually found in the developed scales comprising of goethite (FeOOH), lepidocrocite (FeOOH), magnetite (Fe3O4), siderite (FeCO3), ferrous hydroxide (Fe(OH) 2), ferric hydroxide (Fe(OH)3), ferrihydrite (5Fe2O3 _ 9H2O), green rusts (e.g. Fe4 IIFe2 III(OH)12(CO3)) and calcium carbonate (CaCO3ÂÂÂ) (Sarin et al. 2004).
According to (Sarin et al. 2004), red water is caused by reduction and dissolution of formed corrosion products (e.g. goethite FeOOH) with lack of oxygen during stagnation. It was proposed that corrosion of iron metal, dissolution of ferrous components of the scale and hydraulic sourcing of particles from the scale are the three main means of iron release into water in a corroded distribution system. Sontheimer et al. 1981 proposed that the formation of siderite (FeCO3) might play an important role in iron corrosion. However, despite the extensive research efforts, adverse impacts of blending different water source on iron release have not been systematically investigated, and the interaction of corroded iron surfaces with water quality are not completely understood.
Chemical changes are not the only thing happening. Also biological activity in pipes which result in biofilms (put reference). The effect of these biofilm either on corrosion promotion or prevention is not completely understood. However, as part of a research investigation to develop red water control strategies, Teng et al. 2008 carried out a study to analyze the effects of biofilms on corrosion scales in cast iron pipes. The study confirmed that biofilms developed in the process of microorganism growth can greatly affect element composition and crystalline phase of corrosion scales. It was revealed that the role biofilm played could be positive or negative and had a relationship with the immersion time. It was observed that the accelerated corrosion in distribution system due to biofilm was aided by two kinds of iron bacteria, namely L. ferriphilum and L. ferrooxidan, which could respire Fe or Fe2+ within 7days and that corrosion inhibition by biofilm after 7 days could be linked with some iron reducing bacteria, which could reduce Fe(III)-Fe(II) and result in corrosion inhibition. The transition from accelerating to inhibiting corrosion was inferred to be probably due to the change of biofilm microbial diversity.
IMPORTANCE OF IRON CORROSION IN RELATION TO WATER QUALITY
It is important to understand the process influencing corrosion of the iron supply pipes. A wide range of metal corrosion exists and this kind of attack varies and is dependent on the material to become corroded, construction of the system, scale and oxide formation as well as hydraulic conditions surrounding the process.
These corrosion forms can range between uniform to intense localized attack. The uniform corrosion brings about the formation of uniform scales on the pipe wall while the localized corrosion can produce growing mounds of corrosion products called tubercules (obrecht and pourbaix 1967). However aqueous corrosion is the most common form of degradation of metal as a result of the electrochemical or chemical interaction with the environment at ambient temperatures.
Electrochemical corrosion of metals which occurs when electrons from atoms at the surface of the metal are transmitted to a suitable electron acceptor or depolarizer (such as oxygen, chlorine or hydrogen ion in aqeous form) in which oxygen is the most common thus causing oxidized iron to either dissolve in the solution or to form iron oxide scales on the metal surface.
Electrochemical corrosion involves two half-cell reactions; an oxidation reaction at the anode and a reduction reaction at the cathode. For iron corroding in water with a near neutral pH, these half cell reactions can be represented as:
Anode reaction: 2Fe => 2Fe2+ + 4e- (1)
Cathode reaction: O2 + 2H2O + 4e- => 4OH- (2)
Although thermodynamically feasible, proton reduction is dominant only at acidic (pH < 4) conditions (Montgomery, 1985). However, some studies have documented H2 gas formation coupled with iron metal oxidation under abiotic, near-neutral pH, conditions (Baylis, 1926; Daniels et al., 1987; Lorowitz et al., 1992).
Besides proton reduction, it was postulated that the anodic dissolution of iron can also be balanced by cathodic reduction of Fe(III) oxides in the rust layer (Evans, 1965; Evans and Taylor, 1972). (ALL HAS BEEN PLAGIARIZED)
Based on understanding of iron and steel pipes covered with corrosion scales, literatures conclude that corrosion scales are not protective towards further corrosion of underlying metal in aqueous conditions yet a mechanism which explains continued corrosion process is still not available.
STRUCTURE AND COMOSTITION OF CORROSION SCALES IN IRON PIPES
Knowledge about structure and the composition of the scales are important parameters which can determine both the amount and the rate of soluble iron release to water thus its understanding is essential. Although findings of Gerke et al (2008) shows that different scale sample collected from the same distribution system could have different compositon, Corrosion scales originate and grow radially inwards from the pipe wall to the pipe centre of a water distribution system and are best understood as porous deposits of Fe (III) phases, Fe3O4 and Y - Fe2O3 with a shell like dense layer near the top of the scales and a high concentration of readily soluble Fe (II) content present inside the scales.
A microscopy study of these scales as reported by Sarin et al. 2004 revealed that three distinct structural regions which comprise of a top surface layer at the scale-water interface, a dense, shell-like layer just below the top surface that covers the porous core and provides structural integrity to the scale and a porous interior which contains both fluid and solid that forms the core of the scale (Fig 1).
The top surface layer of the scale, at the water-scale interface is loosely held, while the shell-like layer that enveloped the porous interior core of the corrosion scale was dense. The dense layer has been analyzed to comprise iron containing phases which are magnetite and geotite (Sarin et al. 2001). Compounds usually found in the developed scales (Fig 1) include goethite (FeOOH), lepidocrocite (FeOOH), magnetite (Fe3O4), siderite (FeCO3), ferrous hydroxide (Fe(OH) 2), ferric hydroxide (Fe(OH)3), ferrihydrite (5Fe2O3 _ 9H2O), green rusts (e.g. Fe4 IIFe2 III(OH)12(CO3)) and calcium carbonate (CaCO3) .
Inside the scale, close to the pipe wall, lower oxidation is expected for phases having iron however the composition of water flowing past the pipe surfaces could alter the characteristics of corrosion scales thus leading to the the formation of Fe (II) solids such as siderite and ferrous hydroxide. Under reduced conditions, Ferrous hydroxide is formed however in the presence of Carbonic species, siderite (feCO3) is the stable ferrous solid.
The presence of siderite as corrosion product was documented by Benjamin et al. (1996) even though the role it plays is not understood complete as some literatures (Blengino et al. 1995 and Simpson& Melendres, 1996) conclude that siderite is the key to forming a protective scale in distribution system while Ikeda et al. (1984) proved under the same condition that siderite formation destroys the protective nature of other iron oxide films.
Fig 1: Schematic of a scale in a cast iron water distribution pipe (after Sontheimer et al. 1981).
Green rusts which are hydrated ferrous-ferric compounds having Cl- or SO42- or CO32â€‘ anions are transient compounds formed during the partial oxidation of ferrous hydroxide in the presence of other anions (Genim and Refait, 1997) and act as similarly to siderite and form a dense protective film (AWWARF & DVGW, 1996). However given the mineralogy of the water in Lagos Nigeria, these corrosion products are anticipated to be present in the corrosion scales of the study area.
FACTORS RESPONSIBLE FOR CORROSSION IN WATER DISTRIBUTION SYSTEM
Corrosion in drinking water distribution system could be catalyzed by a number of factors either single handedly or combined. Based on the degree of these factors, even newly installed piping systems are liable of showing corrosive characteristics within a very short period of time however I am dealing with old system which are certainly corroded.
Several factors influence corrosion of water distribution system include water quality and composition, disinfectant residuals, pipe age, scale formation, temperature, flow conditions, biological activity, and corrosion inhibitors. However amongst these several factors, water quality and characteristics (dissolved oxygen (DO) concentration, solution pH, Total alkalinity, suspended solids, organic matter, buffer intensity, biological factors, and temperature (Vik et al., 1996).
Although the effects which source water can have on metal corrosion is not completely understood however once the scales are formed in the walls of the pipe, further corrosion leads to the release of ions into the bulk water.
These water quality factors which can influence corrosion of iron / steel pipe in distribution is explained below as understood from past research.
EFFECTS OF DISSOLVED OXYGEN
The rate of corrosion of clean iron surfaces generally increases as the concentration of oxidants such as oxygen increases (Sarin et al. 2004). Uhlig et al. 1955 recorded a linear relationship between DO concentration and corrosion rate when 165 ppm of CaCl2 was observed in distilled water at ordinary temperatures in neutral or near neutral water.
Initially, increase in oxygen concentration accelerates the rate of metal corrosion, however, increase in the amount of DO to an excess (> 12 mg/L) in water and maintaining flowing conditions can reduce the amount of iron release from corroded iron pipes as a result of exposure to strong oxidizing condition resulting in the oxidation of the ferrous oxide film to one which has greater protective value and can act as diffusion barrier. This effect was examined in an experiment carried out by (Sarin et al. 2004) in which pipe releasing high quantities of iron, the same pipe section was filled with water with an initial DO of 9.7 mg/L, and placed inside the anoxic chamber.
Although increase in DO concentrations can decrease the amount of iron release when corrosion scales cover the metal surface in the sense that high DO conditions favor oxidation of ferrous iron to ferric iron and this precipitation of the soluble ferrous ions as ferric oxides on the scale surface can decrease iron release, it is however not an advisable mitigation technique as higher concentrations of DO could possibly lead to breakdown of the oxide film barrier, which can result in catastrophic rates of localized corrosion if high DO concentration is maintained in water (BOOK).
EFFECTS OF pH
When pH is acidic, it favours the breakdown of protective oxides on iron pipes. Although corrosion develops faster and is more severe as pH decreases, it is independent of pH when it ranges between 4 and 10 (Fig 2) simply because corrosion at this range is controlled by the rate at which the oxygen reacts with absorbed atomic hydrogen thus depolarizing the surface and allowing reduction reaction to continue.
Although the effect of pH on iron release from corroded pipes has not been systematically studied, experiments carried out by Eliassen et al., 1956 with steel pipes containing flowing water showed that pH between 5.5 to 9.0 had little influence on long-term corrosion rates at a particular flow velocity.
Increment of pH above 10 was observed to decrease long term corrosion rate (Fig 2) as a result of increase in the rate of reaction of oxygen with Fe (OH) (hydrated FeO) in the oxide layer to form more protective FeO.
This effect was observed in an experiment carried out by Baylis (1926) where alkaline pH was found to produce impervious iron membrane on the scale which acted as a corrosion protection to the iron metal. Based on this observation, it could be concluded that the formation of an impervious membrane at high pH may decrease iron release.
This was however supported by the works of Becket 1998 and Clement 2002 in which it was observed that increase in pH of water from 7.6 to 9.5 reduced the amount of iron released from 1.5mg/L to less than 0.5mg/L.
On the contrary, pH within the acid region (<4) significantly increases the corrosion rate (BOOK) by providing a plentiful supply of hydrogen ion thus indicating that the corrosion rate no longer depends entirely on the depolarization by oxygen but on a combination of both factors (hydrogen evolution and depolarization).
Fig 2: Corrosion of steel and aluminum as a function of pH at the same temperature (22oC) (Website 2)
EFFECTS OF TEMPERATURE
The effects of temperature as a water quality parameter should not be overlooked when considering the factors that may influence corrosion of iron pipes in drinking water distribution systems.
Although only a few studies have examined the role of different temperatures effects on iron corrosion of distribution system, studies of distribution systems infer that manifestations of corrosion is less severe in colder winter months; for example, lower iron concentrations and corrosion rates and fewer customer complaints of red water were observed in winter.
This observation was also observed in an experiment carried out by (Laurie et al.) to observe the effects of different temperature on the corrosion rate of iron. In the study, samples that were held at 5Â°C had higher iron concentration, more tuberculation, and increased weight loss compared to samples at 20Â°C or 25Â°C. This is due to variations in the composition and morphology of scale formed at each temperature. Temperature cycling did not significantly impact iron corrosion compared to different constant temperatures. Overall, the observed differences in corrosion with temperature were relatively small, although larger differences may be found in other systems.
Another laboratory study to examine the role of different temperatures in distribution system corrosion found decreased weight loss for iron samples held at 13°C versus 20°C (Fiksdal, 1995).
Parameters that are critical to pipe corrosion include including dissolved oxygen solubility, solution viscosity, diffusion rates, activity coefficients, enthalpies of reaction, compound solubility, oxidation rates, and biological activity. Each of these factors can affect the rate of iron corrosion, the composition and properties of scale built up inside pipes, and aspects of corrosion by-product release. These parameters however can be impacted by temperature (laurie et al 2001).
Considering the fact that pipes in a distribution system are almost always buried, and the temperature of the surrounding soil remains relatively constant. However, the water temperature within a pipe can change throughout the year due to seasonal variations of the water source. Thus, a pipe may exhibit different corrosion behavior in cold seasons against the hot seasons.
While the role of temperature is speculative at this time, unless utilities are alerted to its possible importance and at least consider its effects, improved understanding of corrosion phenomena may be limited.
EFFECTS OF PIPE AGE
The service age (duration in which the pipe has been installed for) could also influence the corrosion rate of a distribution system. As proposed by Laura phd "both iron concentration and the rate of corrosion increase with time when a pipe is first exposed to water, but then gradually both are reduced as the scale builds up" (put reference). Experiment carried out by Al-Jasser, (2007) to view the effect of pipe service age on the effective wall decay constants in cast iron pipes showed that this is an important factor which should not be neglected when considering factors that influence corrosion.
EFFECTS OF BIOLOGICAL / MICROBIAL ACTIVITIES
Iron corrosion and biological activity are interrelated in many water distribution systems thus their role cannot be overlooked. Microbes have been found in various distribution system and have also been observed in iron tubercules. This growth of bacteria on the pipe wall can serve as corrosion prevention (Chapter 1 laura) and on the other hand speed up corrosion. The growth of these bacteria on the pipe wall (biofilms) can promote corrosion by inducing the formation of corrosion cells as a result of the consequence of aerobic respiratory activity within biofilms that leads to the establishment of local cathodic and anodic regions on the steel surface, which promotes electron flow (Dubiel et al. 2002). (32 pdf needs further readin for understandin)
In evaluation of the effects biofilm can have on corrosion scale of cast iron pipes, Teng et al. (2008) carried out an experiment using twelve cast iron coupons (length * width * thickness = 20 cm * 2 cm * 0.5 cm) which were were disinfected preliminarily and immersed in a covered 2 L glass bottle filled with drinking water from Tsinghua University with groundwater as source water. His results proved that biofilm had a great influence on the elemental composition and crystalline phase of corrosion scales. It was observed that corrosion rate could be accelerated or inhibited as a result of iron bacteria and iron reducing bacteria (IRB), respectively.
Biological / microbial activity effects on iron corrosion/water could however be positive or negative but the negative effects seems to overshadow the positive. Some of the negative influences bacteria can have on water quality include its effect on iron speciation by reducing Fe3+ or oxidizing Fe2+, consumption oxygen, cause localized pH gradients, and production of corrosive metabolites such as H2S or iron phosphide all of which enhances iron corrosion in water distribution system.
Considering the characteristics of water of the populace of this area and the factors that influence corrosion, the cause of red water is as a result or iron particles from corroded pipes and equipments. This is however facilitated by the service age of these service pipes as well as water characteristics which is influenced by the factors mentioned in the previous chapter (Chapter 3).
(Talk about the effects of all the parameters u added in chapter 3)
IMPACT OF CORROSION SCALES ON WATER QUALITY
The presence of iron in water is not considered health problem. In fact, small concentrations are essential to human health. However, high concentrations of iron may give the water an unpleasant metallic taste while still being safe to drink. Iron will cause reddish-brown staining of laundry, porcelain, dishes, utensils and even glassware. Soaps and detergents do not remove these stains, and use of chlorine bleach and alkaline builders (such as sodium and carbonate) may intensify the stains.
Iron deposits will build up in pipelines, pressure tanks, water heaters and water softeners. This reduces the available quantity and pressure of the water supply. In addition, Iron accumulation becomes an economic problem when water supply or water softening equipment must be replaced or when associated increases in energy costs from pumping water through constricted pipes as a result of mounds forming on the pipe wall and reducing the cross sectional area of the pipes.
When pipes in the distribution system are corroded, some of the metal from the pipes enters the drinking water however asides coloration of water there are some other impacts corrosion scales and iron release can have on water quality.
CHLORINE DECAY IN DRINKING WATER
Chlorine is one of the most commonly used disinfectants for water disinfection and maintenance of water quality in a transmission and distribution system is a relatively cheap, stable and effective disinfectant which is applied to treated water for the deactivation of most microorganisms.
According to Al-Jasser, (2007), a free chlorine residual in excess of 0.2 mg/l must be maintained in the distribution system thus reducing the likelihood of further contamination. However few literatures have shown that the concentration of this free chlorine residual can disappear in a water distribution system as result of reaction with the pipe wall or corrosion products (Hallam et al, (2007) and DiGiano and Zhang (2005)).
This temporal and spatial consumption of chlorine is caused by chemical reactions of the chlorine with water constituents and with the tubercles formed on the pipe wall, as well as reaction with the pipe wall material itself (Castro and Neves, 2003). According to Zhang et al. (1992), the disappearance of chlorine could be attributed to the presence of corrosion by-products and unremoved metallic compounds such as iron (ferrous ions) and manganese all of which are among the constituents of water that react with chlorine and lead to its disappearance.
ASSESSMENT OF CONTROL OPTIONS
"The consumer's perception of the quality of water containing suspended iron is dependent upon the properties of the iron particles which are again an important function of water chemistry" (33 pdf). Appearance of drinking water in a colored form ranging from yellow to red signifies the presence of suspended particulate iron. However based on previous literatures on the causes of red water as a result of iron in drinking water, red water in IJU-ADIYAN area of Lagos could be likened to two main source which could either be from the source water or distribution system materials.
Having reviewed corrosion scale formation, characteristics and structure as well as the major factors which can influence the corrosion of iron pipes and lead to the formation of corrosion scales and tubercules which in turn causes the release of iron into water in drinking water distribution system, it is essential to propose feasible solutions to the control the release of iron into water of the populace of this area. Few strategies can be employed in the reduction / prevention of colored water caused as a result of distribution system material and this depends on the state of the piping material. In cases of new installed pipes, strategy should be aimed at corrosion prevention of distribution system (Dehgani et al. 2010) however, in already corroded systems like that of the study area, inhibiting the release of iron from corrosion scales into the water by promoting a denser scale microstructure as well as controlling the form and properties of iron particles will be the best applicable method for the reduction of corrosion products into the water system.
Corrosion control monitors the acidity, alkalinity, and other water qualities that affect pipes and equipment used to transport water with the aim of preventing the possibility of iron leaching into water supply of any area. This chapter aims at assessing the various control options and adopting the most suitable and feasible for the control of red water in iju - adiyan area of Lagos.
pH and Alkalinity adjustment
Alkalinity which is the measure of the ability of a solution to neutralize acids to the equivalence point of carbonate or bicarbonate can play a vital role in influencing the rate of corrosion in water distribution systems.
In drinking water this capacity is attributable to bases such as HCO3- CO32- and OH- as well as to species often present in small concentrations such as silicates and phosphates. While in practice, the total alkalinity is determined by titration of known volume of sample with a standard solution of strong acid to a pH value in the approximate range of 4 to 5
Total Alkalinity = [HCO3-] + 2 [CO32-] + [OH-] - [H-]
Although the effects of alkalinity on iron uptake is not entirely understood, literatures gives information that the greater alkalinity, the slower the rate of corrosion of water distribution system. This observation corresponds to Sontheimer reasoning that higher buffer intensity would lower iron release (Sontheimer in Sarin et al. 2004).
Furthermore, an experiment carried out by Pisigan and Singley (1987) showed that mild steel corrosion rate decreased with increasing buffer intensity at a constant alkalinity of 100 mg/L as CaCO3 in the pH range 6 to 9. However, no decrease in the corrosion rate was observed when the buffer intensity was increased at a constant pH.
Phosphate based inhibitors although used to control copper uptake, (Edwards et al. 2002) however this may be adopted in the prevention of iron corrosion as well as red water problems.
This preventive measure works on the theory/principle that its addition to finished waters will result in the formation of lower solubility lead phosphate complexes on the inner walls of the pipe thus inhibiting iron release by increasing scale impermeability.
Application of phosphate inhibitor products applied to drinking water treatment could be categorized into 3 which include:
Orthophosphate / polyphosphate blend
Polyphosphate: these are the first phosphorus compounds to be used in corrosion control in which the most common and original one used was Calgon (Na20P20O61). These inhibitors are however used for iron sequestering. They stabilize iron particles and have been observed to cause decrease in red coloration of water.
Orthophosphate: the simplest form of this category of inhibitor is is the phosphoric acid. Unlike the polyphosphate which works as an iron sequestering, the addition of orthophosphate to water enhances scale formation thus making it denser and reducing its chances of dissolution into the water.
Orthophosphate / polyphosphate blend: this are available in propriety blends and are available and applied when multiple treatment objectives need to be met. Examples of this include the Bi-metallic (Zinc) phosphate which is a blend of poly and orthophosphate with 5 - 25% Zinc to form bi-metallic phosphates in which some literatures conclude they are stronger and more effective in decreasing corrosion compared to regular poly or orthophosphate (Klebe, 1965 and Murray, 1970).
Generally phosphate inhibitors are useful in the prevention of Iron corrosion as well as red water problems via its iron sequestering and formation of protective layers which acts as barriers to corrosion and eliminate the release of iron into water.
It is however essential to note that the effectiveness of this phosphate inhibitor control measure is dependent on pH. In cases where pH rises above 7.8, metal phosphate precipitation could become more problematic as orthophosphate may react with other cation such as calcium or magnesium which may be present in the water). Thus a stable pH needs to be maintained throughout the distribution system (USE EPA, 2004).
This is also called Saturation Index (S.I) or the Langelier index (L.I) which characterizes the ability of water to precipitate Calcium carbonate. Reports confirms that there is no saturation correlation between the Langelier index and the corrosion rate of iron pipes and it might not be a generally accepted way for corrosion control (Pisigan et al. 1987 and Piron et al 1986) however, it is essential not to neglect the role in which calcium carbonate could play in the prevention of corrosion and iron release from old iron pipes.
The precipitation of calcium carbonate (CaCO3) from over saturated waters on the pipe surfaces would protect the pipe surface by forming a protective layer under proper chemical conditions which prevents more contact of the distributed water from the pipe wall thus reducing further corrosion of the iron pipes as well as release of iron.
To achieve this chemical condition and implement this control measure, CaCO3 must be adjusted to speed up the deposition of CaCO3 layer on pipe walls however, in cases where CaCO3 is present in the raw water / source water, increase of pH (by adding sodium hydroxide (NaOH)) during treatment could come in handy.