Ch2o o2 co2 h2o

Some organic matter is partially oxidized to aldehydes, ketones, and alcohols, which give fresh waste its characteristic sweet smell.

• In the second, anaerobic acid phase, the process of acidic fermentation occurs, generating ammonia, hydrogen, and carbon dioxide gases and large quantities of partially degraded organic compounds, especially organic acids. The pH of the leachate in this phase, 5.5-6.5, is chemically aggressive. Other organic and inorganic substances dissolve in this leachate due to its acidity. Again, carbon dioxide is released. This phase of the reaction can be approximated by the reaction

2 CH2O-* ch3cooh although longer-chain fatty acids, which subsequently decompose into acetic acid, are formed initially, as is hydrogen gas.

In this phase, the leachate has a high oxygen demand (see BOD and COD, Chapter 13), as well as relatively high heavy-metal concentrations. Anaerobic decomposition produces volatile carboxylic acids and esters, which dissolve in the water present. The sickly sweet smell that emanates from landfills during this phase is due to these esters and to thioesters.

• The third, anaerobic—or methanogenic—stage starts about six months to a year after coverage and can continue for very long periods of time. Anaerobic bacteria work slowly to decompose the organic acids and hydrogen that were produced in the second stage. Since the organic acids are consumed in the process, the pH rises to about 7 or 8, and the leachate becomes less reactive. The main products of this stage are carbon dioxide and methane, CH4. To a first approximation, the overall reaction is ch3cooh —» ch4 + co2

Methane generation usually continues for a decade or two and then drops off relatively quickly. Some methane is also formed when hydrogen gas combines with carbon dioxide. Much lower BOD values and a smaller volume are associated with landfills in this phase. Because the leachate is not acidic in this phase, the heavy-metal concentrations drop since these substances are not as soluble at higher pH.

Often the methane gas produced by a landfill is vented to the atmosphere by being directed into wells or gravel-packed seams in the landfill. In some municipalities, the methane gas is burned as it is released through vents (see Figure 16-3) rather than being released into the air. This treatment of the methane is especially desirable, given that the greenhouse gas potential per molecule of CH4 is much greater than that of the C02 produced by its combustion (Chapter 6). The heat produced from the combustion of this gas can be used for practical purposes. Indeed, the second and third stages of decomposition in landfills are identical to those used in the deliberate production of biogas (biomethane) for energy in reactors using municipal solid waste sludge, food processing waste, livestock waste, and other biodegradable materials.

PROBLEM 16-1

Calculate the volume of methane gas, at 15°C and 1.0 atm pressure, that is released annually by the anaerobic decomposition of 1 kg of garbage, assuming the latter is 20% biodegradable organic in nature and that decomposition occurs evenly over a 20-year period. [Hint: Add together the equations for the two anaerobic stages of decomposition,]

Leachate from a Landfill

Engineering is needed to control the leachate from a landfill. Otherwise, the liquid can flow out at the bottom of the landfill and percolate through porous soil to contaminate the groundwater below it. Alternatively, if the soil under the landfill is nonporous, the leachate can build up and gradually overflow the site (the overflowing bathtub effect), possibly contaminating nearby surface waters.

The typical components used to control the leachate consist of:

• A leachate collection and removal system, followed by treatment of the liquid. Often the potential effect of leachate on groundwater is monitored by digging and testing several wells in the vicinity.

• A liner placed around the walls and bottom of the landfill. The liner material is either synthetic (e.g., a plastic such as 2-mm-thick high-density polyethylene) or natural (e.g., compacted clay). The material chosen is impervious to water and will largely prevent the leakage of the contaminated leachate into the groundwater, especially if and when the collection system fails due to clogging, etc. Since 1991 new landfills in the United States must have at least six layers of protection between the garbage and the underlying groundwater! Liners have been developed that consist of bentonite clay—an excellent sealant that efficiently binds heavy metals, preventing their migration out of the landfill—sandwiched between two layers of a plastic such as polypropylene.

Leachate treatment systems must address all the liquid's major components. The treatment of leachate, usually at a sewage treatment plant, is accomplished by aerobic degradation to rapidly decrease the BOD, sometimes using advanced oxidation methods that employ ozone (Chapter 14). In the past, collected leachate was often simply returned to the top of the landfill, since, during its second percolation through the waste, much of its organic content would be biologically degraded; however, this practice is now discouraged in the United States.

Incineration of Garbage

Besides landfilling, the most common way to dispose of wastes, particularly organic and biological ones, is by incineration—the oxidation by controlled burning of materials to simple, mineralized products such as carbon dioxide and water. The primary incentive in the incineration of municipal solid waste is to substantially reduce the volume of material that must be landfilled. In the case of toxic or hazardous substances, an even more important goal is to eliminate the toxic threat from the material. Incineration of hospital wastes is done to sterilize them as well as to reduce their volume.

Many municipalities throughout the world bum domestic garbage in incinerators. For example, Japan and Denmark burn more than half their domestic waste, but the practice is banned in some countries. The combustible components of the garbage, such as paper, plastics, and wood, provide the fuel for the fire. The most common domestic MSW incinerators are one-stage mass burn units; the two-stage modular type is more modem. In the latter, wastes are placed in the primary chamber and bum at a temperature of about 760°C. The gases and airborne particles that result from the first stage are then burned more completely, at temperatures in excess of 870°C, in the secondary combustion chamber. The quantity of waste gases that must later be controlled is greatly reduced in the two-stage units compared to the one-stage unit, although the gases are further heated as they exit the one-stage unit to produce more complete combustion. In some incinerators, an attempt is made to recover some of the heat of the combustion processes and to convert it to steam, hot water, or even electricity.

The output from municipal incinerators includes not only the final gases but also solid residues that amount to about one-third of the initial weight of the garbage. Bottom ash is the noncombustible material that collects at the bottom of the incinerator. Fly ash is the finely divided solid matter that is usually trapped by environmental pollution controls in the stack to prevent it from being released into the outside air. Much of the ash consists of the inorganic constituents of the waste, which form solids rather than gases even when fully oxidized. Although fly ash accounts for only 10-25% of the total ash mass, it is generally the more toxic component, since heavy metals, dioxins, and furans readily condense onto its small particles. The low density and small-particle character of the ash make inadvertent dispersal into the environment a significant risk. Of particular concern are heavy metals in the ash, which could potentially be leached from it and pollute nearby surface water and groundwater. For many years, it was common for incinerator ash to be taken to a hazardous waste landfill. Techniques such as the addition of an adhesive or melting and vitrification have now been developed to solidify ash into a leach-resistant material that need not be classified as hazardous waste. In some countries such as Denmark and the Netherlands, the ash is mainly recycled into asphalt.

The main environmental concern about incineration is the air pollution that it generates, consisting of both gases and particulates. The emission controls on MSW incinerators can control a large fraction, but not all, of the toxic substances emitted into the air from the combustion process. About half the capital costs of new incinerators is spent on air pollution control equipment. Typically, the controls include a baghouse filter, which is made of woven fabric and is used to filter particulates, especially those with diameters over 0.5 /mm, from the flow of output gas. Periodically, the bags are shaken or the air flow is reversed to collect the fly ash. Also typical is a gas scrubber, which is a stream of liquid or solid that passes through the gas stream, removing some particles and gases. If the liquid stream consists of lime, CaO, and water, which collectively form Ca(OH)2, or if the solid stream consists of lime, acid gases such as HC1 and S02 are efficiently removed since they are neutralized to salts by the lime. Heavy metals are also captured by the alkaline environment, since they form insoluble hydroxides. In some modem installations, nitrogen oxides are removed by spraying ammonia or urea into the hot exhaust gases (recall the chemistry explained in Chapter 3). In another new technology used in garbage incinerators, activated charcoal or lignite coke powder is blown into the exhaust gases, which are subsequently filtered by baghouse; much of the dioxin, furan, and met' cury content of the exhaust gases is removed, since these components adsorb onto the charcoal or coke surface.

Although public concern has centered on emissions from hazardous waste incinerators (to be discussed later in this chapter), several U.S. surveys in the 1990s indicated that many more dioxin and furan emissions emanate from medical waste and municipal waste incinerators than from hazardous waste ones, although cement kiln units used for hazardous wastes (discussed in a later section) also make a significant contribution. There are more than 1000 medical waste incinerators in the United States. Emissions to the air from incinerators are most likely to happen during start-up and when equipment fails.' Because medical waste and backyard barrel garbage incinerators operate in a start-and-stop mode, they tend to produce more airborne pollutants per unit mass of incinerated waste than do larger incinerators. Overall, municipal, backyard, and medical incinerators are believed to be a major anthropogenic source of both mercury and dioxins/furans in the U.S. environment and a moderately important source of cadmium and lead.

Green Chemistry: Polyaspartate—A Biodegradable Antiscalant and Dispersing Agent

In pipes, boilers, water cooling systems, and other devices that handle water, scale buildup (Figure 16-4) tends to reduce water flow and heat transfer, thereby lowering efficiency. In addition, scale may lead to corrosion and damage of these devices. Scale is generally the result of the precipitation of insoluble compounds such as calcium carbonate, calcium sulfate, and barium sulfate. Compounds called antiscalants or dispersants are employed to prevent the buildup of scale. Whereas antiscalants prevent the formation of scale, dispersants allow its formation but maintain the scale in a state of suspension so that it can simply be washed away.

One of the most commonly used antiscalants and dispersants is the poiyanion polyacrylate or PAC:

FIGURE 16-4 Scale buildup in a water pipe. vWard Lopes)

Short chains of this polymer act as antiscalants while longer chains act as dispersants. The anionic carboxylate groups, —COO", of PAC are able to form complexes with the cations (such as calcium and barium) normally found in scale, thus preventing the formation of scale or dispersing it. Globally, several hundred million kilograms of PAC are produced each year, a significant portion of which is used as a dispersant or antiscalant. Although PAC is nontoxic, it is nonvolatile and does not degrade in the environment. When used for water treatment, it builds up in lakes and streams, or at best it must be removed in wastewater treatment plants as a sludge and then landfilled.

To prevent this environmental burden, biodegradable antiscalants and dispersants such as polyasparate have been developed, Polyaspartate can be used to replace PAC, but because it undergoes biodégradation to innocuous products (such as carbon dioxide and water), it eliminates the need for removal in wastewater treatment plants and disposal in landfills.

Although the performance of polyaspartate is comparable to that of PAC, its price was formerly prohibitive. The Donlar Corporation developed a new synthesis of polyaspartate that lowered the cost of the polymer so that it was competitive with PAC, For this accomplishment, Donlar won a Presidential Green Chemistry Challenge Award in 1996. Donlar's synthesis (Figure 16-5) begins by heating aspartic acid (a naturally occurring amino acid) to produce polysuccinimide, followed by basic hydrolysis to produce polyaspartate. This straightforward synthesis is not only economically desirable but also environmentally sound. The first step simply requires heat and yields only water as a by-product, while the second step uses water under basic conditions to produce the desired product. The product of this synthesis is generally called thermal polyaspartate (TPA) because of the heat used

HzC^

aspartic acid

heat polysuccinimide

30% b cleavage 70% a cleavage polyaspartate

FIGURE 16-5 Don lar Corporation synthesis of polyaspartate.

in the synthesis. Polyaspartate can also be used in fertilizers (to enhance uptake of nutrients) and detergents (as builders).

Healthy Chemistry For Optimal Health

Healthy Chemistry For Optimal Health

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