Municipal Solid Wastes

Introduction

The amount of waste generated each year is staggering. No one really knows how much waste humans generate, but much of it originates from the developed countries. Just as the United States is a leader in energy consumption and pollution, this country also produces the most waste per capita. Estimates of the amount of solid waste generated by the United States range from 6 to 10 billion tons a year. This amounts to about 24 tons of waste a year for every American or slightly more than 130 pounds of waste a day. Now the average person does not literally throw away 130 pounds of trash each day. Most waste in the United States comes from mining, agricultural, and industrial operations; but ultimately these operations exists to feed and provide for the necessities and desires of the consuming public. The trash that individuals put out for the garbage collector, the municipal solid waste (old newspapers, packaging materials, empty bottles, and so forth), makes up only a bit more than 3% of America's waste, but this still amounts to nearly 200 million tons per year. The average American directly disposes of a little over four pounds of trash and garbage each day, significantly more than just a few decades ago and also more than is generated per person in various other countries.

Municipal solid wastes,

What's in Our Trash?
Paper and Paperboard	38.1 %
Plastics	9.4 %
Yard wastes	13.4 %
Metals	7.7 %
Wood	5.2 %
Food wastes	10.4 %
Glass	5.9 %
Other	9.9 %
Source: EPA, 1998

although proportionally far smaller in amount than agricultural and mining wastes, arguably represent our greatest waste management challenge (with the possible exception of industrial hazardous wastes). This is because urban wastes are generated where people live and must be quickly removed and properly disposed of in order to prevent serious environmental health problems. Municipal wastes are also much more heterogeneous than are the wastes produced by agriculture, mining, or specific industries. Paper and paper products constitute the single largest portion of household rejects, but an examination of a typical garbage container would also reveal glass, metal, plastic containers; rubber, leather, and cloth items; food wastes; grass clipping, tree trimmings, discarded appliances, and numerous other items.

Municipal Waste Collection and Disposal

From the public health and environmental quality standpoint proper disposal of urban refuse is just as important as regular collection. Historically, because the public has been far more concerned that refuse be regularly removed that with what happens to it once the garbage truck rounds the nearest corner (the "out of sight, out of mind" philosophy), municipal solid waste budgets have traditionally allocated a significant greater proportion of their resources to refuse collection than to disposal.

Until the 1970s,

the most common method of urban refuse disposal was open dumping, a practice that simply involved hauling collected garbage to a location at the edge of town and dumping it on the ground. Regarded today as environmentally unacceptable, open dumping represents the problems that can arise when solid wastes are mismanaged. Open dumps support large populations of rats, flies, and cockroaches that frequently invade nearby dwellings. They contaminate adjacent surface or groundwater supplies when leachates (liquids resulting from the interaction of water with wastes) containing dissolved pollutants run off or seep downward through the soil from the dumpsite. Open dumping as a method for disposing of municipal refuse was outlaws by the federal government in 1976.

Current Waste Disposal Alternatives

• Sanitary Landfill - A sanitary landfill differs from open dump in that collected refuse is spread in thin layers and compacted by bulldozers. When the compacted layers are 8-10 feet deep, they are covered with about 6 inches of dirt, which is again compacted. At the end of each working day another thin layer of soil is placed over the fill to prevent litter from blowing about, to keep away insect and rodent pests, and to minimize odor problems. When the landfill has reached its ultimate capacity, a final earth cover two feet deep is place over the entire area and the land can be used for a park, golf course, or other kinds of recreational facilities.

Diagram of a Sanitary Landfill. Please note that a series of wells are placed to check for possible underground water contamination. Things do not biodegrade in a landfill. Anaerobic processes are the only ones that take place after the landfill is sealed, and this makes organic matter decay very slow, as there no oxygen or moisture to support the decomposition process.

When properly sited, well designed, and efficiently operated, a sanitary landfill can be a perfectly adequate means of urban refuse disposal, free from offensive odors, vermin, or pollution problems.

According to the EPA in 1990, some municipal solid waste sanitary landfill are:

Design without:

• Liners - 85%
• Leachate collection systems - 95%
• Methane gas collection systems - 83%

Operating without environmental monitoring systems for:

• Groundwater - 75%
• Surface water - 88%
• Air pollution - 96%
• Explosive gases - 95%

Leachate is the liquid waste that can accumulate between the landfill cells and the liner. Liners are plastic or clay layers installed to keep liquid wastes from contaminating soil or water. Methane gas is the normal end result of biological decomposition. Methane is flammable and explosive if not vented.

• Source Reduction - The best and cheapest way of managing wastes is not to produce them in the first place. Policies of reducing wastes at their source are top-ranked in every priority listing of waste management options. The present sense of urgency to relieve pressures on existing disposal facilities has prompted a more serious look at the potential for source reduction strategies. Most advocates estimate could cut present urban waste streams by about 5%. Approaches to source reduction target consumers, manufactures, or government and can include either voluntary or mandatory components.
  The following are all methods of initiating source reduction:
• Do not purchase as much, or reduce use.
• Purchase products with reduced toxics.
• Purchase environmentally preferred products.
• Purchase products with less packaging.
• Purchase concentrated products.
• Purchase products in bulk or larger sizes.
• Buy multiple use products.
• Do not replace for style.
• Purchase more durable products.
• Maintain properly and repair instead of replace.
• Purchase reusable products, and then reuse or donate to charity.
• Borrow, share, or rent product.
• Recycling - Also referred to as resource recovery. Recycling is perceived by many citizens as "the right thing to do" and by local officials as a way of extending the remaining lifespan of existing land disposal facilities. The increase emphasis on recycling has been promoted primarily for its very real ecological benefits such as:
• resource conservation,
• energy conservation, and
• pollution abatement.

We can summarized by using the three Rs: Reuse, Reduce, Recycle.

• Waste-to-Energy - Also know as incineration. These facilities not only burn refuse, thereby reducing its volume by 80-90%, but also capture the heat of combustion in the form of electricity or process steam.

• The major concern about the used of these facilities is the release of possible toxic air emissions, especially dioxins, furans, and heavy metals (e.g. lead, cadmium, and mercury). Smokestacks are now being fitted with acid gas scrubbers, baghouse filters for trapping metal particulates, and activated carbon injection systems for capturing mercury vapors. The combustion temperature has to be high enough to prevent the creation of dioxin from plastic, and low enough so that not to many nitrogen oxides are produced. When talking about incineration we have to consider that incineration:
• reduces the volume of solid waste - only the ash needs landfilling (80 to 90 % volume reduced),
• metals may be recovered from the incineration process, and are usually recycled by the incineration plant,
• converts waste to energy, by using the heat given off by incinerating waste to fuel a turbine, which is used to generate electricity,
• may result in air pollution, and
• ash may be considered a hazardous waste.