A red water occurrence in drinking water distribution systems caused ...

Front. Environ. Sci. Eng. 2014, 8(3): 417–426 DOI 10.1007/s11783-013-0558-4

RESEARCH ARTICLE

A red water occurrence in drinking water distribution systems caused by changes in water source in Beijing, China: mechanism analysis and control measures Xiaojian ZHANG1, Zilong MI1, Yang WANG1,2, Shuming LIU1, Zhangbin NIU1,3, Pinpin LU1, Jun WANG1, Junnong GU4, Chao CHEN (✉)1

1 School of Environment, Tsinghua University, Beijing 100084, China 2 Beijing General Municipal Engineering Design and Research Institute, Beijing 100082, China 3 Ministry of Housing and Urban-Rural Construction, Beijing 100037, China 4 Beijing Water Works Group, Beijing 100192, China

© Higher Education Press and Springer-Verlag Berlin Heidelberg 2013

Abstract A red water phenomenon occurred in several communities few days after the change of water source in Beijing, China in 2008. In this study, the origin of this problem, the mechanism of iron release and various control measures were investigated. The results indicated that a significant increase in sulphate concentration as a result of the new water source was the cause of the red water phenomenon. The mechanism of iron release was found that the high-concentration sulphate in the new water source disrupted the stable shell of scale on the inner pipe and led to the release of iron compounds. Experiments showed that the iron release rate in the new source water within pipe section was over 11-fold higher than that occurring within the local source water. The recovery of tap water quality lasted several months despite ameliorative measures being implemented, including adding phosphate, reducing the overall proportion of the new water source, elevating the pH and alkalinity, and utilizing free chlorine as a disinfectant instead of chloramine. Adding phosphate was more effective and more practical than the other measures. The iron release rate was decreased after the addition of 1.5 mg$L–1 orthophosphateP, tripolyphosphate-P and hexametaphosphate-P by 68%, 83% and 87%, respectively. Elevating the pH and alkalinity also reduced the iron release rate by 50%. However, the iron release rate did not decreased after replacing chloramine by 0.5–0.8 mg$L–1 of free chlorine as disinfectant. Keywords

system, sulphate, phosphate, red water control, water quality stability

1

Introduction

The occurrence of “red water” in drinking water distribution systems always lead to abundant complaints from the public [1]. In early October of 2008, a “red water” problem affected several communities, about 100000 residents in Beijing, China. The tap water of the first flush in the morning had a yellow coloration and high turbidity (Fig. 1). Iron concentration, turbidity and discoloration of the water increased sharply, and exceeded the drinking water standard for China by 36.0, 35.0 and 20.0 times,

iron release, drinking water distribution

Received December 31, 2011; accepted December 3, 2012 E-mail: [email protected]

Fig. 1 Discolored tap water sourced in one apartment during the first flush in the morning

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respectively. Through the duration of this particular problem, the water company received more than 300 customer complaining calls in the affected area in a few days regarding the poor tap water quality, which was dozens of times of normal level (Fig. 2). This red water problem occurred so severely and drastically that it could not be explained by the re-suspension of sediments within reticulation pipes after night stagnation under normal circumstances. Red tap water is caused by the release of corrosion products (primarily iron compounds) from iron pipes, which leads to the formation of ferric particles and causes red or yellow color [2]. Iron release occurs as the transportation of iron elements from the corrosion scale and pipe matrix into the bulk water supply [3]. This transportation occurs as a result of the direct corrosion, dissolution of scale and hydraulic flushing. Under normal conditions, direct corrosion does not lead to a sharp increase in the iron concentration of water in the pipes since these processes do not result in obvious loss of the iron matrix. In this reported red water occurrence, there was no change in the hydraulic situation before and during the red water occurrence. Therefore, red water as a result of iron release via scale dissolution or breaching in drinking water distribution systems was the focus of this study. In past decades, many studies have investigated the effect of water quality changes on iron corrosion and red

water occurrences [4–7]. Indeed, a transition of water sources can influence the chemical stability of drinking water distribution systems, thereby accelerating scale dissolution and breaching and resulting in enhanced iron release and the occurrence of “red water” at the tap. This phenomenon had been reported in several cases, such as Tucson, Arizona in the United States [8,9]. This phenomenon had also occurred in several cities in China including Tianjin, which utilized multiple water sources that change irregularly [10]. In recent years, the red water problem has been given much attention since more and more cities will use multiple and alternating water sources in the future. Accordingly, there is an urgent and high demand for technologies that prevent or ameliorate the red water problem. Corrosion scale can be divided into three layers, namely, a top surface layer, a shell-like layer and a porous core. The thin and loose surface layer is composed of sediment particles that settle upon the pipe wall. The shell-like layer is very dense, compact and hard, which prevents the water from interacting with the inner porous core of scale [3,11,12]. The shell-like layer is predominantly composed of Fe3O4 and α-FeOOH [13,14]. The inner layer is porous, loose and composed of Fe(II) compounds, such as FeCl2 and FeCO3, as well as some ferric compounds [15,16]. If the dense shell-like layer is compromised, the soluble Fe (II) compounds inside are released into the bulk water, then

Fig. 2 Overview of the customer complaints received in Beijing City regarding the red water occurrence

Xiaojian ZHANG et al. Mechanism of red water occurrence and control measures

oxidized into the non-soluble Fe(III) compounds by oxygen or chlorine in the bulk water and form suspended iron particles with yellow or red color, which results in the discoloration of tap water. Factors resulting in the dissolution or penetration of the shell-like layer of scale and the release of iron include the depletion of dissolved oxygen and residual chlorine [17], the increase in sulphate and chloride [18,19], and low pH and alkalinity [20]. The Larson Ratio (LR) value is an index that is used to quantify the corrosiveness of water to metals [21]. The LR considers the sulphate, chloride and alkalinity as very important parameters influencing metal corrosion (Eq. (1)). 2½SO24 –  þ ½Cl –  LR ¼ , ½HCO3– 

(1)

where concentrations are expressed in mol$L–1. Under normal conditions, the LR is a sufficient indication of the corrosive ability of source water. A higher LR value is believed to indicate a more aggressive water to iron, when the LR value is greater than 1, the water is considered to be corrosive seriously. Besides, the Langlier Saturation Index (LSI) and Ryznar Stability Index (RSI) were used to predict the precipitation potential of carbonate in pipe water [22]. A trend of carbonate precipitation in pipe water was regarded to be able to prevent the solution of corrosion scale. However, these two indices are more suitable for the assessment of carbonate stability. Around China, the water resources are not distributed evenly, with water abundance in the southern regions while water shortages occur in the north. Meanwhile, water pollution has further restricted social and economic development in the northern cities of the country. In recent years, the South-to-North Water Transfer Project was implemented to transfer water from the water-rich Yangtze River to the drier regions of northern China, including Beijing and Tianjin. The total capacity of transferring is about 9.5 billion m3 of water annually from the Danjiangkou Reservoir, which is located on one main branch of Yangtze River and 1267 km away from Beijing. However, the water quality of water stored in the Danjiangkou Reservoir is very different from that of the local water sources in Beijing. Therefore, the drastic change in water quality chemical components caused by the switch of water source would have a large influence on the iron release in drinking water distribution systems and bring much concern to the water companies and local governments, and probably lead to the red water problem. In this respect, the effect of sudden water quality change on iron release in the pipelines is of great concern. Accordingly, a study was conducted in order to evaluate the situation, discover the cause and find the solutions for the occurrence of the red water problem in Beijing. The water company accepted the suggestions base on the outcomes of this investigation. They applied the sugges-

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tion of blending water sources so as to reduce the corrosive potential of the source water. This remedial action attenuated the problem within three months. This paper summarizes the background to the red water problem, the mechanism of iron release, and the control measures, which are presented to help guide the response to similar problems in the future.

2

Materials and methods

2.1

Water quality of the water sources

This red water phenomenon occurred in several communities just after a new water source more than 300 km away was added to Beijing’s original water supply, resulting in a dramatic change to the finished water quality. The water quality of the original as well as the new water source is shown in Table 1. The greatest evident difference in water quality between the two water sources was that the new source water had higher sulphate and chloride levels, as well as a greater LR value. The values of LSI and RSI did not differ much between the two sources. Therefore, LR value was found to be more useful for predicting the degree of iron release in this study. Specifically, the new source water had an LR value that was four times higher than that of the local source water, which indicated that it was relatively corrosive. A third water source, one of ground water sources in Beijing, which was reserved as backup water source in the case of emergency, with even lower concentrations of sulphate and chloride and higher alkalinity than the local source, was blended with the new water source, so as to decrease the LR value of effluent and tap water. All three water sources supply raw water to the same water treatment plants. The conventional water treatment process did not facilitate the removal of soluble sulphate and chloride. 2.2 Description of the distribution systems of the affected area

The affected area covered dozens of square kilometres, and contains about 500 buildings and a population of about 100000 people. The pipes (DN75/100) under the streets of the community are made of unlined cast iron. The pipes (DN15/20/40/50) within houses are made of galvanized steel. These pipes are about 20 years old. The hydraulic situation and flow pattern within the network were not changed since treated water originates from the same water treatment plant at the same flux, although the source water to the water treatment plant changed. 2.3

Water quality analysis

First flush samples of approximately 500 mL were obtained from the taps of kitchens and washrooms of

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Table 1 Basic water quality of the three water sources water quality parameters pH –1

conductivity /(μS$cm ) –1

sulphate /(mg$L ) –1

chloride /(mg$L ) –1

local source water

new source water

third source water

7.8

7.9

7.8

350

750

400

47

180–220

26

27

50

22

alkalinity (CaCO3) /(mg$L )

150

120

190

calcium (CaCO3) /(mg$L–1)

120

116

132

Larson Ratio

0.51

1.70–1.95

0.27

Langlier Saturation Index

0.01

– 0.07

0.14

Rynzar Stability Index

7.79

8.05

7.52

Note: water samples were taken at the influent well of the water treatment plant

several affected apartments at about 6:00 a.m. every morning before the majority of residents wake up. According to information obtained from a survey of residents, the first flush tap water prior to the red water incident was clear and colorless. Therefore, it stands to reason that the red water incident occurred as a result the iron release in pipes overnight, and not from the flushing of sediments. The water quality of the first flush tap water showed the undisturbed effect of iron contamination. Conductivity, pH, dissolved oxygen and turbidity were determined onsite. The analysis of iron, color, sulphate, chloride, alkalinity, hardness and calcium were further measured in the laboratory. The analytical techniques used to measure these water quality parameters are showed in Table 2. Table 2 Analytical methods used to analyze water quality parameters water quality parameters

analytical methods

pH

Thermo Orion model 5 STAR benchtop pH meter

conductivity/(μS$cm–1)

Thermo Orion model 5 STAR benchtop conductivity meter

dissolved oxygen/(mg$L–1)

Thermo Orion model 5 STAR benchtop DO meter

turbidity/(NTU)

HACH 2011P Turbidimeter

sulphate/(mg$L–1)

barium sulphate turbidimetry

chloride /(mg$L–1)

silver nitrate titration –1

alkalinity (CaCO3)/(mg$L )

acid-base titration

hardness (CaCO3)/(mg$L–1)

EDTA titration

–1

calcium (CaCO3)/(mg$L ) color iron/(mg$L–1)

2.4

EDTA titration Pt-Co colorimetry bathophenathroline spectrophotometry

Pipe scale analysis

The pipes within the affected communities are constructed of cast iron and had been in service for 20 years at the time

of the red water problem. Sections of pipe approximately 1 m long (DN 100) without any coating, were dug out from one affected community for analysis and pipe section reactor investigation. The interior corrosion scales within the pipe sections were dried with nitrogen, after which the microstructure and elemental composition of the scales were analyzed by Scanning Electron Microscopy (SEM) (Scanning Electron Microscope, JSM-6460LV, Joel, Japan), Energy Dispersive Spectroscopy (EDS) (Energy Dispersive Spectroscope, JSM-6460LV, Detectable Element from Be to U) and X Ray Diffraction (XRD) (X-ray Single Crystal Diffractometer, BRUKER2P4, Bruker, Germany). 2.5 Influence of different water quality on iron corrosion rate

The corrosion rates of the two types of source water were evaluated based on the weight loss of coupons using annular reactors [23]. Virgin cast iron coupons of the same material as the pipes in the affected area were washed with acetone and ethanol, dried with filter paper and then stored in a vacuum desiccator for more than 4 h. The initial weights of the coupons were then determined, after which they were placed in the annular reactors that were filled with pre-treated source water. The pre-treated source water was prepared by coagulation and sedimentation in a jar tester and filtration through paper to simulate the drinking water treatment process. The samples were then incubated in the annular reactors while rotating at 120 r$min–1 for 117 h. Subsequently, the coupons were removed, gently brushed and washed with an acidic solution (8 g of hexamethylenetetramine dissolved into 1000 mL 37% HCl) for 3 min, washed briefly with clear tap water, washed with 60 g$L–1 sodium hydroxide for 30 s and then soaked in ethanol for 2 min. The samples were then dried using filter paper and placed in a vacuum desiccator for over 4 h. Finally, the coupons were weighed and the corrosion rates were calculated.

Xiaojian ZHANG et al. Mechanism of red water occurrence and control measures

2.6

Influence of different water quality on iron release

The influence of different source water on scale breaching and iron release was evaluated using the pipe section reactor which was designed to be made of pipe sections dug from the affected area (Fig. 3). Pipe sections were cut into 10 cm-long sections using a lathe, after which they were covered entirely with ethoxylate, except for the interior surface that was covered with scale. The sections were then immersed into the different water samples for the duration of the experimental period. Since the outer surface of the pipe section was covered with ethoxylate, this treatment enabled the reactions between the scale and the water to be observed and monitored without interference from the outer surface. The water quality before the experiment and after 8 h of reaction was then determined. The iron concentration, turbidity and color of the water samples were determined to reflect the influence of different water quality on iron release. The pipe section was immersed in the test water throughout the experimental period and the water was renewed every day.

Fig. 3

2.7

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phosphate, elevating the pH and alkalinity, and utilizing free chlorine as the disinfectant instead of chloramines. The conditions of these control measure experiments are showed in Table 3. The measure finally selected by the local water treatment plant was to greatly reduce the ratio of the new water source to the original one. The efficiency of this measure was monitored by determining the water quality of the tap water in several apartments in the affected area.

3

Results and discussion

3.1 Influence of water quality on iron corrosion and iron release

Results of iron corrosion and iron release experiments revealed that iron release from the scale was the main cause of the red water phenomenon. More specifically, the iron release rate of the used pipe section in the new source water was over 11 times of that in the local source water (Fig. 4). Water from the new source increased the corrosion rate on the virgin cast-iron coupons by only 22%, when being compared with the local water after the continuous corrosion test for 117 h. The average corrosion rates of the local and new source water were 0.49 and 0.60 mm$a–1,

Schematic diagram of the pipe section reactor

Control measure experiments

The pipe section reactors were also used to research measures for ameliorating and controlling the red water phenomenon. These measures included adjusting the ratio of the new water source to the original one, adding

Fig. 4 Relationship between the Larson Ratio and the iron release rate. The percentage is the ratio of the new water source

Table 3 Conditions of the control measure experiments control measures

conditions

adjust ratio of new water source

the ratio of water from the new source to original water was adjusted to 0%, 20%, 40%, and 100%

add phosphate

orthophosphate-P, tripolyphosphate-P and hexametaphosphate-P (chain length, n = 12) were added at a dose of 1.5mg$L–1

elevate pH and alkalinity

2 mol$L–1 NaOH was added to increase the pH from 7.6 to 8.20.1; NaHCO3 was added to adjust the alkalinity from 135 to 260 mg$L–1 (calculated as CaCO3)

utilizing free chlorine as the disinfectant instead of chloramine

free chlorine: 0.5–0.8 mg$L–1

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respectively. Additionally, the LR value increased from about 0.5 to 0.7 while the iron release rate increased, which indicated the dissolution or breaching of scale. As the LR value increased further, the curve of the iron release turned to level off. When the LR value was greater than 1.2, the iron release rate reached the maximum value. 3.2

Characteristics of pipe scale

The characteristics of the pipe scale revealed the source of the iron release. Figure 5 showed the section plane of scale and the SEM pictures of the shell-like layer and porous core. It was very clear that a compact but thin shell-like layer covered the bulky porous core of the scale. The results of XRD and EDS indicated that the dominant components of the shell-like layer were α-FeOOH (Goethite) and Fe3O4 (Magnetite), while the porous core

was mainly composed of FeCO3 (Siderite), γ-FeOOH (Lepidocrocite) and SiO2 (Quartz). The compact and hard shell-like layer of scale acted as the passive layer that prevented the penetration of oxidants such as chlorine, monochloramine and oxygen into the core. In addition, this layer prevented the release of loose compounds into the bulk water. Once this layer was disrupted, the abundant ferrous compounds inside the scale could be released into the bulk water and resulted in the contamination of tap water with high turbidity and discoloration. Accordingly, it is safe to assume that the iron release from the scale was the direct source of the red water problem. 3.3 Mechanism of red water phenomenon following water source switch and blending

Disruption of the passive layer was likely induced by the

Fig. 5 Different layers of scale on the cast iron pipe. (a) Section plane of iron pipe scale; (b) SEM of shell-like layer of scale; (c) SEM of porous core of scale

Xiaojian ZHANG et al. Mechanism of red water occurrence and control measures

conditions such as low dissolved oxygen, low residual chlorine, high sulphate, high chloride, low alkalinity and low pH. During the stagnation period, chlorine and oxygen would be depleted slowly and completely, which occurred commonly at the end of the distribution system. The absence of oxidants would then enable the passive layer to be reduced or dissolved slowly (Eq. (2)). This explains the slightly yellow color of tap water that is often observed in the first flush in the morning. This amount of iron release depends on the time of stagnation. With a longer stagnation time, a greater amount of iron is released into the bulk water. Under normal conditions, the ferric matter formed during one night was fairly limited and could be flushed clear in seconds. FeOOH þ H2 O↕ ↓Fe3þ þ 3OH –

(2)

High concentrations of sulphate or chloride can greatly accelerate the dissolution of the passive layer and the release of iron. Once released, these ions can bond with ferric hydroxides such as α-FeOOH, forming unstable components with higher solubility that lead to the release of ferric iron (Eqs. (3)–(4)) [24]. Moreover, anions such as chloride and sulphate play an important role in maintaining electroneutrality within the core of the scale, and thus affect the rate of iron release [25]. The high ionic strength could accelerate the speed of the iron reaction by increasing the rate of the chemical reaction via increased conductivity and migration of electrons. FeOOH þ SO24 – þ H2 O↕ ↓Fe3þ þ SO24 – þ 3OH –

(3)

FeOOH þ Cl – þ H2 O↕ ↓Fe3þ þ Cl – þ 3OH –

(4)

Once the passive layer of scale was dissolved or penetrated, the inner ferrous iron in the porous core would be leached out. Any released ferrous compounds would be oxidized into ferric compounds by dissolved oxygen and residual chlorine in the bulk water. These compounds could then hydrolyze into the non-soluble ferric hydroxide. A high level of small ferric hydroxide particles could then bind together, forming a particle suspension that can lead to increased turbidity [19]. In addition, released ferrous iron would also be oxidized into ferric iron by oxidants, forming soluble complexes with an inorganic ligand such as chloride in the bulk water, which lead to increased discoloration. If the iron concentration in the pipe water was very high, the color of the water would be red, whereas a mediate iron concentration would lead to the formation of yellow tap water. 3.4

Control measures

3.4.1 Reducing the ratio of the new water source by blending the original one

As shown in Fig. 6, the area affected by the red water

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phenomenon expanded gradually within the first 16 days. Reducing the ratio of the new water source was applied on the 16th day to alleviate the red water phenomenon. More specifically, the plant reduced the ratio of the new water source from 100% to 20%, which resulted in a reduction in the LR value of the water from about 1.8 to approximately 0.7. However, the broken scale recovered very slowly and the tap water quality parameters such as iron concentration, turbidity and color did not improve in the next 30 days. The tap water quality was then adjusted again on the 41st day by adding high-alkalinity water from another local backup source. By this means, the LR value of the influent water was further reduced to 0.45. The high alkalinity apparently helped restore the shell-like layer and the tap water quality became nearly normal after 90 days. 3.4.2

Adding the phosphorus-containing corrosion inhibitor

Adding different types of phosphate decreased the release of iron from the pipe section. Tripolyphosphate and hexametaphosphate were more effective than orthophosphate at reducing the iron release rate. More specifically, after the addition of 1.5 mg$L–1 tripolyphosphate for 15 days, the iron release rate was decreased by 83%, from 47.80 to 7.65 mg$m–2$h–1. Hexametaphosphate displayed slightly better efficiency than tripolyphosphate, with the iron release rate decreasing by 87%, from 46.18 to 5.88 mg$m–2$h–1 (Fig. 7). It was suggested that a phosphorus-containing corrosion inhibitor be added to the networks of the affected area for 1–3 months. The recommended dosage of tripolyphosphate-P or hexametaphosphate-P is 0.15–0.35 mg$L–1 in large networks and 0.7–1.7 mg$L–1 in small networks. Orthophosphate controlled the release of iron probably via decreasing the porosity of the scale, which making it more difficult for Fe(II) species to diffuse out and for anions to diffuse into the core of the scale [25]. Orthophosphate also likely plays a secondary role as an anodic inhibitor by inducing the formation of iron phosphate precipitates (e.g., FePO4$2H2O) that plugged the breaks in the iron oxide film, thereby decreasing corrosion. Tripolyphosphate and hexametaphosphate reduced the release of iron from the scale more efficiently than orthophosphate. This was likely attributed that tripolyphosphate and hexametaphosphate can combine with ferric or ferrous iron to form compounds that subsequently precipitate upon the surface of the scale and alleviate the iron release problem. However, overuse of polyphosphates has the potential capability to disperse small particles into the water, which might exasperate the iron release problem. Therefore, it is important to select an appropriate chain length and dose of polyphosphate to control the red water phenomenon. The addition of a phosphorus-containing corrosion inhibitor poses no risk to the health of the public. Although

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Fig. 6 Tap water quality during the water source switch Note: The entire red water problem could be divided into the six stages listed below The dash line in the figures indicates the criteria for total iron, turbidity and color according to the Drinking Water Standard of China. Stage I: 90% new water source and 10% local water source; from Sep 27, 2008 to Oct 12, 2008 Stage II: 30% new water source and 70% local water source; from Oct 12, 2008 to Oct 18, 2008 Stage III: 20% new water source and 80% local water source; from Oct 18, 2008 to Nov 6, 2008 Stage IV: 20% new water source and 80% third water source; from Nov 6, 2008 to Nov 24, 2008 Stage V: 100% third water source; from Nov 24, 2008 to Dec 5, 2008 Stage VI: 20% new water source and 80% third water source; from Dec 5, 2008 to Jan 4, 2009

phosphate addition does increase the phosphorus load on wastewater treatment and the receiving water body, this side effect is very limited since the duration and area of application are very limited. 3.4.3

Elevating pH and alkalinity

It has been shown that high levels of pH and alkalinity would also improve iron stability [25]. In this study, increased pH and alkalinity was found to effectively control the red water problem caused by changing the water source. More specifically, the iron release rate was reduced by 50% after adjusting the pH and alkalinity over

15 days of application (Fig. 8). Elevating the pH or alkalinity likely reduced the occurrence of red water via the formation of calcium carbonate precipitate [6]. The formation of such a precipitate would protect the inner pipe surface from subsequent contact with the water, thereby reducing corrosion. 3.4.4 Applying free chlorine as the disinfectant instead of chloramine

Residual disinfectant was also effective in controlling the red water phenomenon. The tap water sourced in kitchens

Xiaojian ZHANG et al. Mechanism of red water occurrence and control measures

Fig. 7 Effect of the addition of phosphate on iron release. The dose of orthophosphate, tripolyphosphate and hexametaphosphate (chain length, n = 12) was adjusted to provide 1.5 mg$L–1 (calculated as P)

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Fig. 9 Effect of utilizing free chlorine as the disinfectant instead of chloramine on iron release. The concentration of free chlorine was maintained at 0.5–0.8 mg$L–1

containing 0.5–0.8 mg$L–1 free chlorine for 10 days. These findings indicated that formation of the compact and passive layer of scale with free chlorine was a long-term process. Therefore, free chlorine could not be used for the rapid stabilization of scale in the pipes in response to the occurrence of red water.

4

Fig. 8 Effect of elevating pH and alkalinity on iron release. Elevation of the pH from 7.6 to 8.20.1 was accomplished by adding 2 mol$L–1 NaOH; the alkalinity was elevated from 135 to 260 mg$L–1 (calculated as CaCO3) by adding NaHCO3

was worse than that sourced in washrooms (Fig. 6), because the taps in kitchen are seldom used at night and the stagnation time was much longer than that of water in washrooms. Under the stagnant conditions, the level of dissolved oxygen and residual chlorine decreased to about zero, which facilitated the release of iron from the breached scale. Free chlorine has been found to benefit the formation of denser and thicker scale than chloramine because of the difference in their oxidation ability [19]. However, there was no difference in the re-stabilization capability of iron release by these two disinfectants observed in this investigation (Fig. 9). Indeed, the iron release rate changed only slightly when the pipe section that had previously been soaked in chloramine was soaked again in water

Conclusions

Based on the results presented here, the following conclusions could be drawn. 1) The significant increase in sulphate concentration brought by the introduction of the new water source was regarded as the origin of the red water problem. The iron compounds that were formed in the inner scale layer were released as the stable shell of the scale on the pipe was dissolved and corrupted by the high concentration of sulphate. 2) The effect of sulphate on iron release from scale was greater than its enhancement on iron corrosion. More specifically, the iron release rate of the used pipe section in the new source water increased by 11 times when being compared with that in the local source water. In addition, the corrosion rate of virgin cast-iron coupon in the new source water increased only 22% when being compared with that in the local one. 3) Reducing the ratio of the new water source by blending different water sources, labeled by a reduction in the Larson Ratio, led to the alleviation of the red water problem. However, it required approximately three months for full recovery to be achieved. Therefore, the gradual and monitored switching of source water is highly recommended to avoid and control sudden and dramatic changes in tap water quality. 4) Bench scale experiments revealed that adding

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polyphosphate was a more rapid, effective and practical measure for controlling the red water problem. Additionally, elevating the pH value and alkalinity were also effective in addressing the occurrence of red water. However, replacing the chloramines as disinfectant by free chlorine could not bring obvious improvement, which was not regarded as an effective emergency response to solve the red water problem.

12.

13.

14. Acknowledgements This research work was supported by the National Water Special Program of China (No. 2009ZX07424-003), the National High Technology Research and Development Program of China (No. 2009AA06Z308), and International Science & Technology Cooperation Program of China (No. 2010DFA91830).

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