Wastes Soils And Sediments

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

■ Thermochemistry

■ Concept of oxidation and reduction as electron loss or gain; oxidation number; basic electrochemistry

■ Background organic chemistry (see Appendix)

■ Concepts of acids and bases; pH S Phase diagrams

Background from previous chapters used in this chapter:

■ Adsorption; NOx; particulates (Chapters 3 and 4)

■ Aerobic and anaerobic decompositions; methane (Chapter 6)

■ PCBs, dioxins, and furans (Chapter 11)

■ PAHs; phthalates; BTEX, chlorinated solvents (Chapter 12)

■ BOD, COD, and water carbonate chemistry (Chapter 13) m Heavy-metal chemistry (Chapter 15)


In this chapter, we turn our attention to the environmental aspects of the solid state—particularly of soil and of the sediments of natural water systems—and of ways that polluted soils and sediments can be remediated.

A closely related issue is the nature and disposal of concentrated wastes of all kinds, including both domestic garbage and hazardous waste, and their possible recycling.

The material in this chapter has been arranged in the order of generally increasing toxicity and hazard. Thus we begin with the least toxic substances—domestic and commercial garbage—and consider its disposal by landfilling, incineration, or recycling. We then consider soils and sediments and their contamination by chemicals. Finally, we look at hazardous wastes and some of the high-technology methods that are being developed to dispose of them.

Domestic and Commercial Garbage: Its Disposal and Minimization

The great majority of the material that we discard and that must be disposed of is not hazardous but is simply garbage or refuse. The greatest single constituent of this solid waste (defined as waste that is collected and transported by a means other than water), is construction and demolition debris, almost all of which is either reused or eventually buried in the ground. The second largest volume of waste is that generated by the commercial and industrial sectors, followed by the domestic waste generated by residences. Typically, a North American generates about 2 kg of domestic and commercial waste a day, twice as much as the average European. In these discussions, we will not consider the much larger amounts of waste generated by the petroleum industries, by agriculture, as ashes from power plants, or as sewage, which was discussed in Chapter 14.

A breakdown by the type of solid waste typically generated in countries at various levels of economic development is shown in Figure 16-1. Notice that the fraction of the waste that is vegetable matter declines as the level of economic development rises. The opposite is true of paper, which in industrialized countries is the largest single component of waste and dominates commercial-sector waste. Historically, the largest component of paper waste was newspapers; now the volume of paper packaging is similar. The amount of packaging has grown, in part, because so many goods are now produced far from their ultimate destination and must be transported safely over long distances.

A tractor moving shredded paper in the warehouse of a paper-recycling plant. The paper can be reused or recycled in several different ways. (Digital Vision)

Industrialized countries

Middle-income countries

Industrialized countries

Paper I I Vegetable

Middle-income countries

Low-income countries

Low-income countries

Paper I I Vegetable

FT! Other □ Plastics □ Textiles, rubber, leather & wood



FIGURE 16-1 Typical composition of solid waste for countries at different levels of economic development. [Source: "Wjste and the Environment," The Economist (29 May 1993): 5 Environment Survey section,.]

Plastics, glass, and metals each account for about one-tenth of the volume of solid waste in developed countries, whereas organic matter (food waste) accounts for about twice this value. These proportions would differ significantly in areas that collect materials for recycling or composting: The glass and metals components would be much smaller.

Burying Garbage in Landills

The main method used for disposal of municipal solid waste, MSW, is to place it in a landfill (also variously called a garbage dump or a rubbish tip), which is a large hole in the ground that is usually covered with soil and/or clay after it is filled. For example, 85-90% of domestic and commercial waste is currently landfilled in the United Kingdom, about 6% is incinerated, and the same fraction is recycled or reused; similar figures apply to many municipalities in North America. Landfilling dominates the disposal methods because its direct costs are substantially lower than disposal by any other means.

In the past, landfills were often simply large holes in the ground that had been created by mineral extraction—especially old sand or gravel pits. In many instances, they leaked and contaminated the aquifers that lay beneath them; this was especially true for landfills that used former sand pits, since water easily percolates through sand. These landfills were not designed, controlled, or supervised, and they accepted many types of wastes, including hazardous materials.

Soil cover Clay cap


Soil cover Clay cap


\ Cell

y \\


Plastic' liner

FIGURE 16-2 Components of a modern landfill (in the process of being filled).

^ Leachate collection system


Modern municipal landfills are much more elaborately designed and engineered, often accept no hazardous waste, and have their sites selected to minimize impact on the environment. The components of a typical modern landfill are illustrated in Figure 16-2.

In a sanitary landfill, the MSW is compacted in layers (to reduce its volume) and is covered with about 20 cm (8 in.) of soil at the conclusion of each day's operations. Thus the landfill consists of many adjacent cells, each corresponding to a day's waste (Figure 16-2). After one layer of cells is completed, another is begun, and the process is continued until the hole is filled. Usually, the landfill is eventually capped by a meter or so of soil, or preferably clay, a material that is fairly impervious to rain. A geomembrane made of plastic may be added on top as a liner instead of the clay, or over it. The system recommended by the U.S. EPA is illustrated in Figure 16-3.

During the time that municipal wastes in a landfill are decomposing— aerobically at first, then anaerobically after a few months or a year—water

^ Leachate collection system

FIGURE 16-3 Landfill design with cover system recommended by the U.S. EPA.

Vegetation/soil ■ top layer

Filter layer-

Drainage layer ■ Geomembrane ■

Clay layer-


<-Methane gas collection

* a « - V « c t " .

; '/jk. y ^ j yj !» flfei V v V * " - f- r 1 < £ j¡vW-i C , ' t

%1 è '^^cy^ Vp,. 5,'

^ ^ * ^ .o

15 cm

60 cm

60 cm

15 cm

60 cm


from precipitation, liquid from the waste itself, and groundwater that seeps into the landfill all percolate through the garbage, producing a liquid called leachate. This liquid contains dissolved, suspended, and microbial contaminants extracted from the solid waste, Leachate volume is relatively high for the first few years after a site is covered. Typically, leachate contains

volatile organic acids such as acetic acid and various longer-chain fatty acids;

• heavy metals, usually in low concentration (those of most concern in leachate are lead and cadmium); and

• salts of common inorganic ions such as Caz".

The micropollutants present even in MSW leachate include common volatile organic compounds such as toluene and dichloromethane.

Continue reading here: Stages in the Decomposition of Garbage in a Landfill

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