The Pollution And Purification Of Water

In this chapter, the following introductory chemistry topics are used:

■ Acid-base and equilibrium concepts and calculations; pH

■ Basic structural organic chemistry (as in the Appendix)

■ Oxidation numbers; redox half-reactions

■ Catalysis

■ Distillation

Background from previous chapters used in this chapter:

■ Maximum contaminant levels (Chapter 10)

■ Adsorption (Chapter 4)

■ Photochemical reactions; UV light (Chapters 1-5)

■ Free radicals (Chapter I)

■ BTX hydrocarbons (Chapter 7)

■ ppm concentration scale in water (Chapter 10)

■ No effects level, NOEL (Chapter 11)

Introduction

The pollution of natural waters by both biological and chemical contaminants is a worldwide problem. There are few populated areas, whether in developed or undeveloped countries, that do not suffer from one form of water pollution or another. In this chapter, we shall survey the various methods—both traditional and innovative—-by which water can be purified. We begin by discussing techniques that are used to purify drinking water from relatively uncontaminated sources, and then consider the pollution and remediation of groundwater and of sewage and wastewater. Lastly, we investigate modern advanced techniques whereby polluted air and water can be cleansed.

Water Disinfection

The quality of "raw" (untreated) water, whether drawn from surface water or groundwater, that is intended eventually for drinking varies widely, from almost pristine to highly polluted . Because both the type and quantity of pollutants in raw water vary, the processes used in purification also vary from place to place. The most commonly used procedures are shown in schematic form in Figure 14-1. Before discussing the major topic of disinfection, we shall discuss the various nondisinfection steps that are often taken in the overall purification process.

Aeration of Water

Aeration is commonly used in the improvement of water quality. Municipalities aerate drinking water that is drawn from underground aquifers in order to remove dissolved gases such as the foul-smelling hydrogen sulfide, H2S, and organosulfur compounds, as well as volatile organic compounds, some of which may have a detectable odor. Aeration of drinking water also results in reactions that produce C02 from the most easily oxidized organic material. If necessary for reasons of odor, taste, or health, most of the remaining organics can be removed by subsequently passing the water over activated carbon, although this process is relatively expensive, so rather few communities use it (see Box 14-1). Another advantage to aeration is that the increased oxygen content of water oxidizes water-soluble Fe2+ to Fe3+, which then forms insoluble hydroxides (and related species) that can be removed as solids.

(Recall that ions listed in equations without a state specified are assumed to be in aqueous solution.)

After aeration, colloidal particles in the water are removed. If the water is excessively hard, calcium and magnesium are removed from it before the final stages of disinfection and the addition of fluoride. All these procedures are described below (see Figure 14-1).

Removal of Calcium and Magnesium

If the water comes from wells in areas having limestone bedrock, it will con-

tain significant levels of Ca and Mg ions, which are usually removed during processing since these ions can interfere with soaps and detergents used by consumers for washing. Calcium can be removed from water by addition of phosphate ion in a process analogous to that discussed later for phosphate removal; here, however, phosphate is added in order to precipitate the

BOX 14-1

Activated Carbon

Activated carbon (activated charcoal) is a very useful solid for purifying water of small organic molecules present in low concentrations. The ability of this material to remove contaminants from water and to improve its taste, color, and odor has been known for a long time; indeed, the ancient Egyptians used charcoal-lined vessels to store water for drinking purposes.

Activated carbon is produced by anaerobi-cally charring a high-carbon-content material such as peat, wood, or lignite (a soft brown coal) at temperatures below 600°C, followed by a partial oxidation process using carbon dioxide or steam at a slightly higher temperature.

The removal of contaminants by activated carbon is a physical adsorption process and therefore is reversible if sufficient energy is applied. The characteristic that makes activated carbon such an excellent adsorber is its huge surface area, about 1400 m2/g- This surface is internal to the individual carbon particles, so that crushing the material neither increases nor decreases the area. The internal structure of the solid involves series of channels (pores) of progressively decreasing size that are produced by the charring and partial oxidation processes. The internal sites where adsorption occurs are large enough only for small molecules, including chlorinated solvents. At the typical ppm concentrations found for organic contaminants in water, each gram of activated carbon can adsorb a few percent of its mass in contaminants such as chloroform and the dichloroethenes as well as much higher masses of TCE, PCE, and pesticides such as dieldrin, heptachlor, and DDT

Once a sample of activated carbon has reached near-saturation in terms of adsorbed organics, three alternatives are available. It can be simply disposed of in a landfill, it can be incinerated to destroy it and the adsorbed contaminants, or it can be heated to rejuvenate the surface by driving off the organic pollutants, which can then be incinerated or catalytically oxidized.

Settling and precipitation

A1 or Fe salt to precipitate colloids o I

Hardness removal

Phosphate

Disinfection o

o O

o O

Ammonia and fluoride o 0y o o o o ° o o I

Ca2+ pptd. as phosphate Cl2 or ozone or CJ02

Consumer o Water

• Suspended particles

FIGURE 14-1 The common stages of purification of drinking water.

calcium ion. More commonly, calcium ion is removed by precipitation and filtering of the insoluble salt calcium carbonate, CaC03. The carbonate ion is either added as sodium carbonate, Na2C03, or if sufficient HC03" is naturally present in the water, hydroxide ion, OH , is added in order to convert dissolved b ic ci rbonat e ion to carbonate:

OH" + HtXy->C032- + Hp

Ca2+ + C032" v CaC03(s)

Magnesium ion precipitates as magnesium hydroxide, Mg(OH)2, when the water is made sufficiently alkaline, i.e., when the OH" ion content is increased. After removal by filtration of the solid CaC03 and Mg(OH)2, the pH of the water is readjusted to near-neutrality by bubbling carbon dioxide into it.

PROBLEM 14-1

Ironically, calcium ion is often removed from water by adding hydroxide ion in the form of Ca(OH) ?. Deduce a balanced chemical equation for the reaction of calcium hydroxide with dissolved calcium bicarbonate, Ca(HC03)2, to produce insoluble calcium carbonate. What molar ratio of Ca(OH)2 to dissolved calcium should be added to ensure that almost all the calcium is precipitated?

Disinfection to Prevent Illness

In terms of causing immediate sickness and even death, biological contaminants of water are almost always much more important than chemical ones. For that reason, we begin our discussion of the purification of water by extensively discussing its disinfection, i.e., the elimination of microorganisms that can cause illness.

Many of the microorganisms in raw water are present as a result of contamination by human and animal feces. The microorganisms are principally

• bacteria, including those of the Salmonella genus, one species of which causes typhoid. In this category is also Escherichia coli 0157:H7, whose transmission in water has caused a number of deaths in recent years, including those from an outbreak in Walkerton, Ontario, in 2000;

• viruses, including polio viruses, the hepatitus-A virus, and the Norwalk virus; and

• protozoans (single-celled animals), including Cryptosporidium and Giardia lamblia.

Because many microorganisms of these three types are pathogenic, causing mild to serious and sometimes fatal illnesses, they must be largely removed from water before it is suitable for drinking.

Notwithstanding well-known techniques for water disinfection, many of which have been used extensively for more than a century in developed countries, there are still about 1 billion people in the world who do not yet have access to safe drinking water. According to the World Health Organization, about 4500 children die daily from the consequences of polluted water and inadequate sanitation.

Filtering of Water

In addition to dissolved chemicals, the raw water that is obtained from rivers, lakes, or streams contains a multitude of tiny particles, some of which consist of or contain microorganisms. Many of the small, suspended particles consist of clay, resulting from the erosion of soil and rock, whether by natural forces or due to plowing of land for agriculture, mining, or commercial or housing development. The suspended particles increase water's turbidity and thereby reduce the ability of light to penetrate deeply enough to support photosynthesis.

The larger of the particles suspended in water are often removed by simply filtering it. Indeed, the filtration of water by passing it through a bed of sand is the oldest form of water purification known, dating back to ancient times. The sand retains suspended solids of all types, including microorganisms, down to about 10 /am in size.

Recently it was realized that forcing raw water through filters having especially small openings can be used instea d of chemicals or light to disinfect water of some viruses and bacteria, and even some dissolved chemicals, by just removing them. ;

Removal of Colloidal Particles by Precipitation

Most municipalities allow raw water to settle, since this permits large particles to settle out or to be readily separated. However, much of the insoluble matter originating from rocks and soil, and from the disintegration and decomposition of water-based plants and animals, will not precipitate spontaneously since it is suspended in water in the form of colloidal particles. These are particles that have diameters ranging from 0.001 to 1 /xm and consist of groups of molecules or ions that are weakly bound together. These groups dissolve as a unit, rather than breaking up and dissolving as individual ions or molecules. In many cases, the individual units within a colloidal particle are spatially organized such that the surface of the particles contains ionic groups. The ionic charges on the surface of one particle repel those of like charge on neighboring particles, preventing their aggregation and subsequent precipitation.

Colloidal particles must be removed from drinking water for both aesthetic and health and safety reasons. To capture the colloidal particles, a small amount of either iron(lll) sulfate, Fe2(S04)3, or aluminum sulfate,

A12(S04)3 ("alum"), is deliberately dissolved in the water. By then making the water neutral or alkaline in pH (7 and up), both the Fe3+ and Al3+ ions produced from the salts form gelatinous hydroxides that physically incorporate the colloidal particles and form a removable precipitate. The water is greatly clarified once this precipitate has been removed. Commonly, after the removal of the colloidal particles, the water is filtered through sand and/or some other granular material.

Although the idealized formulas of the precipitates are Fe(OH)3 and Al(OH)3, the actual situation is much more complex. For example, aluminum actually forms a polymeric cation, Al1304(0H)247+, which produces a loose network structure that is held together by hydrogen bonds. This network entraps the colloidal particles and forms the precipitate. Only if the pH rises to quite a high value does the aluminum in solution form the expected hydroxide Al(OH)3. Since the concentration of aluminum sulfate added to the water is only about 10 ¿¿m/L, very little residual aluminum ion is left in the treated water.

PROBLEM 14-2

Calculate the approximate number of atoms contained in colloidal particles of (a) 1'fim and (b) 0.01 -/im diameters, assuming that their densities are similar to that of water and that the atomic mass of the atoms averages 10 g/mol.

Disinfection of Water by Membrane Technology

Water can be purified of most contaminant ions, molecules, and small particles, including viruses and bacteria, by passing it through a membrane in which the individual holes, called pores, are of uniform and microscopic size. The range of sizes of the various contaminants in raw water are summarized in Figure 14-2. Clearly, for a technique to be effective in providing a barrier, the pore size of the membrane must be smaller than the contaminant size.

In the processes of microfiltration and ultrafiltration, a membrane or some other analogous barrier containing pores of 0.002- to 10-/im diameter (2-10,000 nm) is employed to remove larger constituents from water. The water can be forced through the barrier by pressure or can be drawn through it by suction, leaving behind the larger impurities. In one modern version of this technology, the barrier is composed of thousands of strands of plastic tubing having walls that are pierced with thousands of tiny pores of similar size.

Some bacteria and colloid particles are as small as 0.1 fira and so can pass through conventional filters and even some microfilters (Figure 14-2). Viruses can be as small as 0.01 fxm and therefore require at least the ultrafiltration level to eliminate them. However, filtration using membranes can be

FIGURE 14-2 Filtration of contaminants by various methods.

used to disinfect water if a sufficiently small pore size is used and if the water is later irradiated with ultraviolet light to eliminate any microbes that have passed through the filtration stage.

Neither microfiltering nor ultrafiltering removes dissolved ions or small organic molecules. Generally speaking, before water is treated using membranes with even smaller pores (see below), it must be pretreated to remove the larger particles—especially colloids—which would otherwise foul the finer membrane by leaving deposits.

Membrane systems have been developed recently that purify water of virtually all contaminants by nanofiltration. Water is pumped under pressure through fine membranes that have pores only about 1 nm wide, which therefore remove not only most bacteria and viruses, but also any larger organic molecules that would nourish the regrowth of bacteria. These nanofilters still allow water molecules to pass through the filter, since the molecules are only a few tenths of a nanometer in size. Unlike ultrafiltration, nanofiltration can be used to soften water, since hydrated divalent ions such as Ca2+ and Mg2+ are larger than the pores and so do not pass through. Hydrated monovalent ions such as sodium and chloride also pass through some nanofilters, but not through ones with subnanometer pore sizes. As a consequence, some nanofil-ter membrane systems can be used to desalinate seawater and to help purify wastewater, as discussed later in this chapter.

Reverse Osmosis

The ultimate in membrane filtration occurs in the widely used technique called reverse osmosis, sometimes called hyperfiltration. Here, water is forced under high pressure to pass through the pores in a semipermeable membrane, composed of an organic polymeric material such as cellulose acetate or triacetate or a polyamide. Since only water (and other molecules of its small size) can pass efficiently through the pores, the liquid on the other side of the membrane is purified water. The solution on the impact side of the membrane becomes more and more concentrated in contaminants as time goes on and is discarded. The procedure is called reverse osmosis because, by use of pressure, the natural phenomenon of osmosis—by which pure water would spontaneously migrate through the membrane into solution, thereby diluting it—is reversed.

Particles, molecules (including small organic molecules), and ions down to less than 1 nm (0.001 ¿urn) in size, or about 150 g/mol in mass, are removed by reverse osmosis. It is particularly useful for removing alkali and alkaline earth metal ions, as well as salts of heavy metals. Thus it is employed in hospitals and renal units to produce water that is particularly free of ions. Reverse osmosis is used on large scale for the desalination, i.e., the removal of salts, from seawater and brackish water, a topic considered in Box 14-2.

BOX 14-2

The Desalination of Salty Water

Desalination is the production of fresh water from salty water, often seawater, by the removal of its ions. There are more than 15,000 large-scale desalination plants in operation, located in more than 125 countries. Reverse osmosis is widely used in some areas of the world, such as the Middle East, to generate drinking water from salt water.

The other main commercial desalination process is the thermal distillation—evaporation— of seawater or brackish water. Desalination of seawater by evaporation is a technique that goes back to ancient times; it is especially suited even today for seawater that contains particularly high levels of dissolved salts and suspended solids in areas such as the Persian

Gulf. The evaporation method is even more energy-intensive than is reverse osmosis. Modem, large-scale thermal distillation plants use energy to raise salty water to the boiling point, then reduce the air pressure above the liquid to create a partial vacuum into which the liquid readily "flash" evaporates, leaving the salt behind in the remaining liquid. The vapor is removed and condensed as desalted water. Thermal distillation plants are often incorporated within electricity-generating plants to use the low-grade waste steam from the latter as their energy source.

Desalination of water is also sometimes accomplished using the technique of electro-dialysis, which is described later in this chapter.

Water destined for drinking purposes is commonly pretreated, e.g., by filtering it through sand and gravel, and passing it over activated carbon to remove the larger particles such as bacteria, etc., and treating it with chlorine, before subjecting it to reverse osmosis in order to minimize fouling and degradation of the membrane.

Because of the high pressures needed to force water through the small pores in the membrane, reverse osmosis is an energy-intensive process. A pressure of about 2 atm is sufficient for portable and domestic units, but a greater force must be applied to brackish or salty water. However, advances in the engineering of large-scale desalination plants have markedly reduced energy consumption by redirecting pressure from waste brine to low-pressure incoming water.

Reverse osmosis tends to be wasteful of water, since so much of it—a third to a half—is discarded. Also, the accumulated discharges of brine— sometimes called concentrate—from desalination processes of any kind can cause cumulative environmental problems, such as harming fish populations, in the immediate area of the seacoast into which it is deposited if it is not first treated. In some locations, the brine is injected into an underground saltwater aquifer. In others, the brine is left to evaporate in large outdoor pools, and the salts disposed of later.

Some domestic consumers of drinking water have installed small under-the-sink reverse osmosis units to further purify their water by removing unwanted contaminants si jch as heavy metal cations (e.g., lead), hard water cations (calcium and magnesium), anions (e.g., nitrate and fluoride), and organic molecules from water obtained from domestic supplies. Small reverse osmosis units are also used 'in medical facilities for producing water that is particularly ion-free.

Some bottled water has been purified and deionized by reverse osmosis, but small amounts of salts are reintroduced into it before it is sold to consumers. Drinking large amounts of deionized water is not healthy, since the ion balance in the body can be upset as a consequence.

Disinfection by Ultraviolet Irradiation

Ultraviolet light can also be used to disinfect and purify water. Powerful lamps containing mercury vapor whose excited atoms emit UV-C light (see Chapter 1) centered at a wavelength of 254 nm are immersed in the water flow. About 10 seconds of irradiation are usually sufficient to eliminate the toxic microorganisms, including Cryptosporidium, which is resistant to treatment by some other methods. The germicidal action of the light disrupts the DNA in microorganisms, preventing their subsequent replication and thereby inactivating the cells. At the molecular level, absorption of UV-C light results in the formation of new covalent bonds between nearby thymine units on the same strand of DNA. If sufficient such thymine dimers are formed, the DNA molecule becomes so distorted that subsequent replication of the organism is prevented.

The use of ultraviolet light to purify water is complicated by the presence of dissolved iron and humic substances, both of which absorb the UV light and thus reduce the amount available for disinfection. Small solid particles suspended in the water also inhibit the action of the UV light since they can shade or absorb bacteria and also scatter or absorb the light. An advantage of UV disinfection technology is that small units can be employed to serve small population bases, whether in the developed or developing world, so the continuous monitoring activity of chemical systems is avoided. As discussed later, UV light can also be used to purify water of dissolved organic compounds, but by a different mechanism.

Continue reading here: Disinfection by Chemical Methods Ozone and Chlorine Dioxide

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