Best Practice Guide on the Control of Iron and Manganese in Water Supply

by Adam Postawa, Colin Hayes, Alessandra Criscuoli, Francesca Macedonio, Andreas N. Angelakis

Overview

This Best Practice Guide on the Control of Iron and Manganese in Water Supply is one of a series produced by the International Water Association’s Specialist Group on Metals and Related Substances in Drinking Water. 


Iron and manganese are often found in soft upland water sources associated with natural organic matter and are also commonly found in the groundwater abstracted from confined and unconfined aquifers. The presence of iron and manganese in water is one of the most frequent reasons for customers’ complaint due to aesthetic issues (yellow, brown and black or stains on laundry and plumbing fixtures). These two metals can be removed fairly readily by physico—chemical treatment. The municipal treatment systems deployed derive benefit from their larger scale, particularly in relation to control, but the processes used are less suitable for the numerous small supplies that are the most common water supplies throughout Europe, especially in rural areas. One important source of iron in drinking water is from old corroded cast—iron water mains, historically the material used most commonly in supply networks. Replacement and refurbishment is very expensive and the major challenge is how best to prioritize available expenditure. 


The purpose for this Best Practice Guide on the Control of Iron and Manganese in Water Supply is to give readers the broad view of a problem based on state—of—the—art compilation of the range of scientific, engineering, regulatory and operational issues concerned with the control of iron and manganese in drinking water. The Guide is of interest to water utility practitioners, health agencies and policy makers, as well as students on civil engineering and environmental engineering courses. 


Authors: Dr Adam Postawa, AGH University of Science and Technology, Faculty of Geology, Geophysics and Environment Protection, Krakow, Poland and Dr Colin R Hayes, University of Swansea, UK, Chair of IWA Specialist Group on Metals and Related Substances in Drinking Water.

Product Details

ISBN-13:	9781780400044
Publisher:	IWA Publishing
Publication date:	08/14/2013
Series:	Best Practice Guides on Metals and Related Substances in Drinking Water
Pages:	146
Product dimensions:	6.12(w) x 9.25(h) x 0.75(d)

Read an Excerpt

CHAPTER 1

Introduction: Why there are problems with iron and manganese in water supply

NATURE OF IRON AND MANGANESE

Iron and manganese are among the 10 most abundant elements in the earth's crust. Iron is the most common element (by mass) forming the planet Earth as a whole.

Both metals may be present in environment in various oxidation states. In water iron exists mainly as Fe (II) or Fe(III) and manganese as Mn(II) and Mn(IV). In anoxic conditions, such as in ground waters or the hypolimnion of eutrophic reservoirs, iron and manganese occur in relatively soluble +2 oxidation state. When exposed to oxygen or disinfectant during water treatment and distribution, Fe (II) is oxidized to the insoluble Fe(III) form, which precipitates and causes the release of iron from distribution system.

The presence of manganese in raw water presents special problems during water treatment because, unlike iron, manganese is not oxidized by air at neutral pH and is not removed during water treatment processes unless a chemical oxidation step is included. The presence of even low levels of manganese in water supplied to distribution system may result in deposition of manganese oxides on pipes. This may cause sloughing of manganese oxide deposits and result in poor aesthetic quality of water. The water has a brown-black colour and undesirable taste and which causes staining of fixtures, equipment, swimming pools, and laundry.

Iron is vital for all living organisms because it is essential for multiple metabolic processes to include oxygen transport, DNA synthesis, and electron transport. Iron is accumulated in the liver and spleen. Haemoglobin and myoglobin contain about 70% of total iron content in a human organism.

Like iron, manganese is regarded as a nuisance rather than a toxic component of drinking water but presents a special problem for risk assessment because it is both an essential nutrient and a potent neurotoxicant. Its neurotoxic properties have emerged almost exclusively from inhalation exposures, although some epidemiological data suggest that high concentrations in drinking water may be associated with neurological impairment. Reported concentrations ranging from 160 to 1200 µg Mn/l are much higher than aesthetic limits – 50 µg Mn/l, suggested by WHO and implemented in drinking water directive.

Iron and manganese are commonly found in groundwater due to the weathering and leaching of manganese bearing minerals and rocks into the aquifers; concentrations can vary by several orders of magnitude and very often exceed parametric values set for drinking waters. Figure 1.1 present an example from Poland. Only 30% of samples present concentrations of iron below 0.2 mg/l and 25% of samples show concentrations of manganese below than 0.05 mg/l. This means that about % ¾ of groundwater require iron and manganese removal before it will meet aesthetic limits and can be used for human consumption.

In surface waters iron is generally present as salts containing Fe(III) when the pH is above 4.5. Most of those salts are insoluble and settle out or are adsorbed onto surfaces; therefore, the concentrations of iron in well aerated waters are seldom high.

Manganese is generally present in natural surface waters as dissolved or suspended matter at concentrations below 0.05 mg/l.

However, in some surface waters, particularly in upland catchments and reservoirs increased concentrations of iron and manganese may present serious problems. The UK uplands provide headwaters of many major British rivers and they are a major source of potable, industrial and agricultural water supplies. The main focus on upland water quality has been on those components directly linked to acidification and forestry rotation cycles (pH, aluminium, chloride, sulphate and nitrate). Recent studies from UK have shown that concentrations of iron and manganese can be high in upland waters (Abesser et al. 2006).

An additional problem associated with lakes and ponds is stratification and seasonal turnover. During summer stratification, temperature and dissolved oxygen decrease with increasing water depth and correlate with the production of reduced soluble chemical species of iron, manganese and ammonia and hydrogen sulphide in the hypolimnion or anoxic bottom waters of the lake, as a result of the decomposition of organic matter. In late autumn when the temperature of the epilimnion decreases the density difference and wind forces acting on the surface are powerful enough to bring about a reversal of the layers allowing the water from hypolimnion to rise to the surface. Seasonal temperature variations additionally modify the vertical structure and red-ox conditions. Iron, manganese and other compounds (i.e. sulphur as hydrogen sulphide) may become increasingly mobile and when released from anoxic bottom sediments may cause taste and odour problems. Fluctuations in concentrations of iron and manganese may affect treatment process and as a result of insufficient treatment lead to customers complains.

1.2 CONSUMERS' PERCEPTION

A large proportion of the customer contacts that drinking water supply companies receive stem from the occurrence of discoloured water. "Coloured water" describes the appearance of drinking water that contains suspended particulate iron or manganese where the actual suspension colour may range from light yellow to red to brown and brown-black due to water chemistry and particle properties. The coloured water stains household appliances and porcelain ware. Clothes laundered in such water are also stained.

Discoloured water incidents greatly affect customer's confidence in tap water quality and the quality of service provided by water companies. Currently, consumer complaints are dealt with in a reactive manner. However, water companies are being driven to implement planned activities to control discolouration prior to contacts occurring. Water discolouration was, in early 1990's, the single largest cause of complains in the Yorkshire water region (Pattison et al. 1995). The proportions of complaints have changed significantly in many English companies since 1995, with discolouration no longer the largest component, although this has taken extensive investment in the rehabilitation of iron water mains over a number of years. According to Vreeburg & Boxall (2007) water discolouration is the reason for 34% of customer contacts with Water Company in UK.

Water with unacceptable colour will undermine the confidence of consumers which could lead to the use of water from sources that are aesthetically more acceptable, but potentially less safe. The consumers however, have no means of judging the safety of their drinking water themselves. Their attitude towards their drinking water supply and their drinking water suppliers will depend on aspects that they will be able to perceive with their own senses.

In general, number of complaints increase with increasing distance from the treatment plant. This is in part due to the fact that the velocity of flow decreases with increase in distance from the plant. The iron dissolved or the decrease in dissolved oxygen per unit distance travelled by a unit volume of water near the plant is far less than that at the outer ends of the distribution system. The time of contact with the pipe per unit volume of water is relatively short for the larger mains and longer for the smaller pipes where the demand is low or negligible for significant periods of time. Furthermore, as corrosion takes place and rust particles develop, they settle to the bottom of the pipe at periods of low or negligible demand, and are picked up and redistributed at periods of high or instant demand.

1.3 ECONOMIC AND TECHNICAL PROBLEMS Treatment

Traditional technology of water treatment is based on transformation of Fe(II) and Mn(II) compounds to the form of insoluble oxides of Fe(III) and Mn(IV) that are easy to be removed by filtration.

Manganese removal requires oxidation of Mn (II) by two degrees of valence and higher redox potentials which makes this process technologically more demanding than iron removal. In order to intensify the process of manganese oxidation stronger oxidation conditions have to be used than oxygen at neutral pH. This may be achieved by the use of stronger oxidising agents such as chlorine, chlorine dioxide, potassium permanganate and ozone, or the use of autocatalytic or biological processes.

Corrosion in distribution network

Corrosion in water distribution systems is one of the most important problems in water supply. In many countries worldwide cast iron and other iron-based materials are the most commonly used materials for mains and service pipes.

In the United States approximately 15% of all pipelines supplying drinking water are unlined cast iron pipes (AWWA 2003). The American Water Works Association estimates that in order to upgrade water distribution systems in United States, $ 325 billion should be spent over the 20 years period (AWWARF 1996). This AWWA value is built on the United States Environmental Protection Agency (US EPA) estimate of U$ 77.2 billion for service and replacement of transmission and distribution system lines over the 20 years period (Davies et al. 1997). Latest estimations are even higher reaching $526 billion during period 2011-2035 (AWWA 2011).

The majority of distribution system pipes are composed of iron material: cast iron (41%), ductile iron (25%), and steel (3%) (AWWA 2011). According to American Water Works Association Research Foundation corrosion of cast iron pipes is the most common distribution system problem (AWWARF 1996). Corrosion of iron pipes in a distribution system can cause many problems such as:

(1) Pipe mass is lost through oxidization to soluble iron species or iron-bearing scale.

(2) The scale can accumulate as large tubercles that increase head loss and decrease water capacity (see Figure 1.2a).

(3) Localised corrosion leading to pipes breaks (see Figure 1.2b).

(4) The build-up of corrosion products into tubercles inside the pipes may provide sites for microbial regrowth.

(5) Corrosion reactions can also directly consume disinfectants, thereby lowering residuals and allowing increased biological activity. Harmful disinfection by-products such as chloroform may be formed as chlorine disinfectants react with tubercles in corroded iron pipes.

(6) The release of soluble or particulate iron and manganese corrosion by-products to the water decreases its aesthetic quality and often leads to consumer complaints of "red water" or "black water" at the tap.

Corrosion of iron pipe is a complicated process that is influenced by many different factors. The most important are:

• Water quality and composition (DO, pH, redox potential, chlorides, sulphates).

• Flow conditions.

• Biological activity.

• Pipe age.

• Temperature.

• Corrosion inhibitors usage.

Concentrations of chemical constituents affecting the rate of corrosion can change during passage through the system. A certain water may be only slightly corrosive in one part of a system and strongly corrosive in another due to the difference in time of contact for corrosion to take place per unit volume of water.

Corrosion scales and deposits formed within drinking water distribution systems have the potential to retain inorganic contaminants.

Dissolved oxygen in water system gives rise to conditions which may prevent or enhance "red water". Inhibition is experienced by the formation of a protective ferric oxide calcium carbonate coating in the mains which can prevent penetration of dissolved oxygen to the metal itself as well as stop corrosion products from sloughing off.

The presence of ammonia and oxygen as a source of energy for bacteria gives rise to bacterial growths in the mains. Corrosion is enhanced by the bacterial transformation of basic ammonia and oxygen to acidic and oxidizing nitrites and nitrates.

Depletion of dissolved oxygen at the "dead" ends and service lines of the system gives rise to a condition where ferrous iron is not oxidized to insoluble ferric oxide and solution of iron to the ferrous state takes place.

CHAPTER 2

Natural sources of iron and manganese in water

2.1 HYDROGEOCHEMISTRY OF IRON AND MANGANESE

Iron

It commonly occurs in the Earth's crust (4.65%) and the primary sources of iron in groundwater and surface water are the magmatic rock minerals such as: pyroxenes, amphiboles, magnetite, olivines and pyrite. In these minerals iron mainly occurs in the bivalent form. As a result of the process of magma rock decay, iron gets to the environments of sediment rocks, where it occurs mainly in the form of oxides and hydroxides (hematite, limonite, goethite), as well as pyrites and siderites.

The migration of iron into natural water occurs as a result of decay with the participation of water and CO2, as well as oxidation and hydrolysis (Macioszczyk 1987).

In groundwater, iron usually occurs in the bivalent form, while in surface water it is in the form of a colloidal suspension of trivalent iron. The content of iron in groundwater is mainly determined by its form in sediments – Fe(II) or/and Fe(III), the pH of the environment and the reduction-oxidation potential (Eh). An example of such relationships, based on observations from western Poland, is presented in Figure 2.1 after Gorski (1981).

The trivalent iron is practically insoluble in water at the usual pH encountered. According to the nature of iron release into water, the environments characterized by the following conditions can be distinguished: oxidizing, transitional and reductive.

In the oxidizing environments with Eh > 400 mV (at pH > 7), iron transforms into the oxidized form Fe(III) and it does not occur in groundwater. It also does not occur in the water in highly reductive environments rich in organic matter. In these environments, as a result of anaerobic decomposition of organic matter, bivalent sulphur ions are released, which, when combined with iron, are precipitated in the form of insoluble sulphides. High concentrations of iron occur in the environments characterized by a relatively low redox potential (due to the content of organic matter) and acidification connected with the presence of carbonic acid and humic acids.

Moreover high concentrations of iron in water are observed in the oxidation zones of sulphide deposits, where sulphuric acid is formed as a result of the oxidation of sulphides.

Groundwaters usually contain the bivalent iron, which undergoes oxidation and turns into colloid suspension. Iron in the colloid form and the suspension form also get into rivers and reservoirs as a result of surface run-off.

The main source of iron in surface water is connected with groundwater drainage to rivers and lakes. It may also be generated by erosion and the dissolution of iron and/or manganese containing sediments and minerals at the sediment-water interface.

It was reported that in some rivers and stream waters in Sweden iron was present in concentrations up to 1.7 mg/l accompanied with manganese in concentrations up to 0.14 mg/l (Huser et al. 2011; Kritzberg & Ekstrom 2012). In the UK, uplands provide headwaters of main British rivers which are a major source of drinking water. Concentrations of iron occurring in stream and river waters have doubled over the past 20 years. In some small streams water contains up to 4.7 mg Fe/l and in main streams up to 0.8 mg Fe/l (Neal et al. 2008).

Manganese

Manganese occurs in the Earth's crust in amounts much smaller than iron (0.1%). In magmatic rocks, it occurs in the dispersed form as an admixture of dark minerals in olivines, pyroxenes, amphiboles and biotites. It occurs as Mn(II), Mn(III), Mn(IV). The trivalent form is transient and unstable. From magma rocks, it is mainly released as Mn(II) and it is more easily leached than iron. Oxidation to Mn(IV), which is very weakly soluble and is produced in water as MnO2, requires stronger oxidizing conditions than the oxidation from Fe(II) to Fe(III).

(Continues…)

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Table of Contents

About this Best Practice Guide, ix,
Authors, xi,
Acknowledgements, xiii,
Acronyms, xv,
Definitions, xvii,
Foreword, xix,
Executive Summary, xxi,
Chapter 1 Introduction: Why there are problems with iron and manganese in water supply, 1,
Chapter 2 Natural sources of iron and manganese in water, 7,
Chapter 3 Sources of iron and manganese in drinking water distribution network, 21,
Chapter 4 Regulatory background, 33,
Chapter 5 Health perspectives, 39,
Chapter 6 Aesthetic issues, 45,
Chapter 7 Sampling and monitoring, 51,
Chapter 8 Analytical methods, 61,
Chapter 9 Investigational methods, 69,
Chapter 10 Corrective water treatment and other engineering methods, 79,
Chapter 11 Economics of iron and manganese control, 97,
Chapter 12 Developing a strategy, 105,
Appendix 1 Case studies, 109,
References, 119,