Contaminated Water and Pressures Produced in the Landfill Site

1. Types of Contaminants

Leachate produced in a landfill site generally results from the interaction of infiltrating liquids with the landfilled material and from the liquids contained within the landfill material at the time of disposal. The properties of leachate change from community to community as well as from season to season. The parameters and their range of concentrations are shown on Table 2.

Time

Figure 2 Schematic diagram illustrating cumulative settlement of waste due to decomposition of organic matter versus time. Decomposition stages: (1) aerobic, (2) anaerobic (nonmethanogenic), (3) anaerobic (methanogenic), (4) anaerobic decline, (5) return to anaerobic growth. (After Wardwell et al. [9].)

Time

Figure 2 Schematic diagram illustrating cumulative settlement of waste due to decomposition of organic matter versus time. Decomposition stages: (1) aerobic, (2) anaerobic (nonmethanogenic), (3) anaerobic (methanogenic), (4) anaerobic decline, (5) return to anaerobic growth. (After Wardwell et al. [9].)

Table 2 Range of Composition of Leachate from Municipal Solid Waste

Component

Range

PH

3.7-8.5

Hardness, CaC03

200-7600

Alkalinity, CaC03

720-9500

Ca

240-2400

Mg

64-410

Na

85-3800

K

28-1700

Fe

0.15-1640

Ferrous ion

8-9

Chloride

50-2400

Sulfate

20-750

Phosphate

0.5-130

Organic N

3.0-490

NH4 nitrogen

0.3-480

BOD

22,000-30,000

COD

800-50,000

Zn

0.02-130

Ni

0.15-0.9

Suspended solids

13-27,000

BOD, biological oxygen demand; COD, chemical oxygen demand.

BOD, biological oxygen demand; COD, chemical oxygen demand.

For purposes of examining the effect of leachate on the landfill liner system components, leachate can be grouped into inorganic and organic pore fluids.

Aqueous inorganic fluids are those in which water is the solvent and the solute is mostly inorganic. Aqueous organic fluids are those in which water is the solvent and solutes are predominantly organic. The organic part covers organic wastes and organic fluids. Organic wastes are those in which an organic fluid is the solvent and the solutes are other organic chemicals dissolved in the organic solvent. Organic fluids can be classified as organic acids and organic bases. Organic acids include those organic fluids that react with bases and include proton donors; and organic bases include any organic fluid capable of accepting a proton to become an ionized cation [10].

2. Acidity at the Linear Interface

Pore fluids produced from landfills are generally acidic. The acidic leachate seeps through from the landfill site into the liner; regardless of the type of liner used, the polluted fluid will interact with soil at various stages. All soils contain H + ions in their aqueous phases since water itself is dissociated into H+ and OH- ions. Higher H+ concentrations are due to exchangeable H + ions on the soil particles and/or the presence of mineral or organic acids such as H2S04 from oxidation of pyrites and other organic acids from the decomposition of vegetation. Soil acidity varies with the season [11].

Soil acidity and acidic pore fluid affects many geostructural members. For examples, they lead to corrosion of metals and other construction materials. On the other hand, acidity may have a desirable impact such as the catalytic effect on certain reactions employed in soil stabilization [12].

Table 3 Hydraulic Conductivity and Mass Transport Phenomena in Fine-Grained Clay

Liner Material

Mechanical energy field

Hydraulic conductivity due to mechanical (hydrostatic) potential

Multimedia energy fields

Energy conductivity (environmental)

(a) Hydration energy due to the hydration energy of ions, related to the heat of wetting

(b) Osmotic energy due to the osmotic energy of ions either held in a kind of Donnan equilibrium on the solid particle surfaces or free in the aqueous solution

(c) Capillary potential due to the surface tension of water and the size and geometry of the soil pores

(d) Electric potential; electroosmosis; electrokinetic phenomena

(e) Thermal potential; thermoosmosis, thermal-electric effect

(f) Magnetic potential; electromagnetic force

(g) Vapor pressure potential

Source: Based on Fang [14].

3. Stresses from Landfills

The stresses or pressures developed with the soil-water system are caused mainly by changes in the overlying stress conditions. There are at least three types of pressures to be considered, including external loads such as surcharge weight from landfill, lateral earth pressures, and loads due to construction activities around the landfill area [13].

4. Conductivity Phenomena in Landfills and Liner Systems

Flow through fine-grained liner material is at low velocity, unsteady, nonuniform, and sometimes discontinuous and contains contaminants in solid or gas forms. There are at least eight possible causes (potentials) of flow from one place to another. These are summarized in Table 3. In most cases, the potentials (and thus the flow) are small in magnitude. However, the accumulation of these small amounts can be significant, affecting the performance of waste control systems. In addition to factors shown on Table 3, bacteria, chemical corrosion, and erosion may contribute to barrier leakage as previously indicated. Case studies indicate that liners or barrier walls may leak because of these factors, which are not commonly considered in transport calculations.

0 0

Post a comment