Solid Wastes
I. Introduction
Solid waste, sometimes called "the third pollution" (after air and water) has undergone rapid changes in recent years. In order to place these changes in perspective, it is important to begin by considering several dimensions of the problem. The first dimension is to consider the sheer amount of solid waste. A very rough estimate is about 4 billion tons of solid waste/ year in the United States. Several issues become apparent in considering this number. First, this computes to about 16 tons/person/year, or more than one ton per month! Clearly, most of the solid waste we generate is not seen by the average resident. However, the goods we enjoy in our society are not possible without this volume of waste. For example, we discard roughly 8 million motor vehicles/year. Second, most of the solid waste generated in this country is agricultural wastes returned to the soil or mining wastes returned to the mine. As we shall see, the EPA definition of municipal solid wastes excludes these wastes. This usually accounts for the differences in waste estimates in this country. Using the EPA definition of municipal solid waste, the actual figures are about 180 million tons per year, which computes to roughly 4 pounds/person/day. Third, the U.S. has one of the highest per capita rates of the developed countries. The rates have doubled in the last 20 years and appear to be growing still.
A second dimension of the problem is the technology, which lags behind air or water pollution control. Through the 1960s, the predominanat method of waste disposal was through the use of open dumps. On the one hand, the technology may be more challenging because the public health concerns are a combination of air pollution, water pollution, vector controls, and other public health concerns. On the other hand, open dumps are a far cry from the technologies in other areas of environmental engineering.
A third dimension of the problem is the lack of data in analyzing solid waste problems. While we have improved dramatically in the last several decades, there are still fundamental question about solid waste management that are difficult to answer: how much waste is generate on-site; how much illegal dumping really occurs; what is the true potential for source reduction; most of all, what are the true health effects from solid wastes? We are improving in our ability to answer these questions, but much remains to be done in minimizing the technical uncertainties of solid waste management.
II. Definitions
This text will start by considering three types of waste. The first is nuclear waste. These wastes will be discussed in a later chapter, because the laws that govern nuclear wastes are separate from other solid wastes. The second category is hazardous waste. Similarly, federal law defines hazardous waste separate from nuclear waste -- both are hazardous, but radiation hazards are considered separately. The overlap of the two is called "mixed waste." The third category is non - hazardous waste, referred to as "municipal solid waste."
Within this third category are several terms that describe the characteristics of municipal solid waste. The most common of these are garbage and rubbish. Garbage is relatively decomposable wastes such as kitchen and other food wastes. Rubbish is the reltively non-decomposable wastes, which include combustibles such as paper, wood, and cloth, as well as the non-combustibles such as metal, glass, and stone.
Other terms include refuse, which refers to community wastes. Ash is the residue from incineration,which may present problems of disposal. Trash generally refers to larger items.
III. Major sources
Non-household sources of solid waste include the following:
The above discussion raises a fundamental problem to solid waste management -- how do we estimate the quantities of solid wastes? Generally, there are two different methods that have been use; input analysis and output analysis. Input analysis is based on products people use. For example, total sales from the soft drink industry can be used to estimate the total number of bottles and cans that have been purchased by consumers. The problem is that this technique can only be applied in isolated communities. In our mobile society, the purchases in a given local area can be easily distributed to other regions. Likewise, waste can be entering from outside the region. Thus, planning for solid waste management would rarely if ever rely solely on input analysis. By contrast, output analysis is characterized by weighing trucks as they arrive at a solid waste facility. While the same issues apply, it nevertheless describes the capacity needs for a given facility. The biggest adjustment to be made is the moisture content than can affect this estimate.
Ideally, professionals should be able to reconcile the differences between input and output analysis in order to bring about a better understanding of just how wastes move in and out of a region. It should also provide insights into the type of technologies most needed.
IV. Risk Management
In order to place techniques for the management of solid wastes, we rely on a life cycle analysis that follows the waste from the moment of consumer disposal to it final disposal. Later in this chapter, we will consider interventions earlier in the generation process.
A. collection
The traditional starting point in following disposal of solid wastes in the collection of wastes. This point is surprisingly the most important point in the whole cycle, acounting for roughly 80% of the total cost of solid waste management.
A quick review of methods for collection shows that we have not advanced much beyond the traditional garbage truck as a means of collection. Longer considered a a study of inefficiency, some advancements have been put forth by EPA in the form of micro routing and macro routing. Micro-routing refers to the patterns of trucks in the actual collection areas, and macro routing refers to the drive to and from the disposal site. Districting refers to the delinaeation of borders for collection of wastes.
The high cost of collection has lead to a search over the years for alternatives. The search has often lead to creative albeit impractical solutions such as pneumatic pipes, where refuse is ground and sucked through pipes. While such a technique would reduce the need for trucks, it would be very costly.
The use of garbage disposals has certainly increased over the years and played a major role in disposal of kitcken wastes. However, rather than reduce the total solid waste stream, we have simply seen an increase in BOD at wastewater treatment sites.
Trash compacters would also reduce collection costs, but only if everyone had one. In more recent years, the push for transfer stations has played a role in reducing costs. The waste can be compacted into larger trucks, which reduces the total miles travelled per truck. It can also be an advantage for managing hazardous wastes by lowering the risk through simple processing.
B. Methods of disposal
Throughout the 1960s, open dumps were the predominant choice. With a fundamental lack of design, the problems are by now predictable: vectors, air pollution and odor, water pollution, and fire hazards immediately come to mind. But the lasting legacy of open dumps remains with us today -- lack of faith in other methods. We see this in the language of newspapers and T.V. shows: the term "open dump" is often used interchangeable with "sanitary landfill." As we shall see in the next section, a sanitary landfill is an engineered facility which, while it may not eliminate risks, reperesents deliberate planning to reduce risks.
Sanitary landfill:
The EPA defines a landfill in 3 fundamental ways. First, it is defined by
1. spreading the solid waste in thin layers,
2. compacting it, and
3. spreading cover material at the end of each operating day.This approach has several advantages: first, it minimizes vectors breeding on the site; second, air pollution is reduced, third, fire hazards are significantly reduced; and finally it has long been the cheapest of all methods
Nevertheless, there are disadvantages with the indiscriminate use of sanitary landfills. The fundamental problems is that they eventually leak, and leachatge eventuallymay find its way to surface and groundwater.
In order to place the risks within a landfill in perspective, it is important to understand two 2 processes occurring in a landfill. First, microbial action creates various gases, including CO2, NH3, H2S, and CH4. Of these, CH4 has the most economic importance interms of the potential for energy recovery. These gases produce settling in a landfill, making it illegal to site a residence on top of a closed site.
The second process in a landfill is the dissolution of metals, which is often enhanced by the presence of CO2 or NH3.
Recall that both constituents can lower pH, thus enhancing dissolution. Therefore proper venting of gases and draining of water from the site is a necessity.
A final disposal option is ocean dumping, permitted under the EPA ocean disposal program. The safety of such practices is controversial and restrictions continue to increase in this area.
IV. Treatment alternative
One of the longest known treatment alternative is incineration, dfined as the controlled combustion of solid wastes, leaving ash and non-combustibles. The control fall into three basic categories: temperature, turbulence, and time. Temperature controls are optimal at 1400-1800 deg. F, which minimizes smoke formation. Such temperatures can be assured throught the use of water cooling and heat exchangers. The second control, turbulence, refers to the delivery of adequate oxygen during combustion. This is generally achieved by the design of the combustion chamber. The third control is time -- of course, there must be adequate time must be allowed for thorough combustion, but time factors also reveal that continuous burn is preferred over batch. The advantage of incineration is that it can recover heat for energy use, which helps to defer the expenses of incineration. Also, incineration reduces municipal solid waste to about 1/4 its original weight (and can be better depending on efficiency).
The disadvantages include the need for landfill disposal of residue ash. Such ash may also be hazardous. Furthermore, air pollution control is also an issue during incineration, which typically must control for particulates (smoke) and various gases (including odors).
While the design of incinerators is fundamentally an engineering issue, there are three basic features to consider in such designs. First, an incinerator must have a storage area.
This area may be available to separate combustible from non-combustibles, although a mass burn (burning without any sorting) may also be applied. However, a large storage also is necessary to maintain a continuous burn.
The second design feature of incinerators is a system of grates for the purposes of loading, drying, and combustion. Over the years, there have been various types of grates, including rectangular (2 or more tiers), vertical circular (cone shaped), rotary kiln (like a cement truck), and many other propietary designs. The goal behind these designs is to deliver optimal amounts of oxygen during incineration.
The third design feature is the stack (flue), where air emissions are released. The controls discussed in air pollution (e.g., dust collectors and wet scrubbers) are relevant in this part of the design. B. Pyrolysis
Pyrolysis is defined as burning in an oxygen deficient environment, more likened to a roasting of wastes. It has been called destructive distillation due to the fact that the process does not fully oxidize the waste. The advantage of such a process is that it produces fuel (e.g., methanol, charcoal). It also reduces the total volume of waste. The disavantages are that it is expensive to build (although it may be cheaper than incinerators), the scrubbers are high in BOD, and the fuel has not found wide acceptance.
C. Biodegradation
Biodegradation techniques take advantage of the abilities of microbes to digest various nutrients. The advantages include methane production and recovery, and with the advances in genetic engineering, there are numerous possibilities for digestion of a broad array of wastes. The disadvantages are that the safety is questioned, and few people understand it adequately for full operation. Moreover, the variable content of waste makes it difficult to maintain the steady state nutrient conditions that are essential of stable systems.
However, a simple example of biodegradation that has broad applicability is composting. Composting is defined as controlled aerobic digestion of organic wastes into humus. The process typically takes about 2-12 months, although this depends on various conditions including nutrients and temperature. At the end of this process is created a material called humus, which is defined as decomposed animal and plant matter. Humus is considered a mulch or soil conditioner that improves soil texture -- this helps reduces erosion and increases the holding capacity of the soil for air and water. While humus does add nutrients to the soil, it is not considered a fertilizer, buecause the nutrient level is not high enough. Rough estimates are that 25% of refuse to landfills is compostable.
V. Resource Recovery
Resource Recovery is defined as any process where materials are recovered rather than discarded. This recovery can occur in any of three basic ways (from 1970 Res. Rec. Act): reuse, reclamation, and recycling. To illustration the differences among these three methods, consider the fate of glass bottles.
With reuse, the material is used again in exactly the same way. For example, the bottle can be cleaned and reused. With reclamation (also called utilization), the material is used in new ways. For example, glasphalt can be used to extend the use of asphault, and therefore represents a new use. With recycling, recycling, the raw material is recovered and used in either old or new uses. For example, glass bottles can be ground up and recast (wither into bottles or other glass products).