Domestic Waste

It is estimated that approximately 4 lb of domestic refuse is generated every day for every person in the United States, of which about 3 lb (1.4 kg) per day goes to domestic land-fills and 11 percent is recycled. It is estimated that about 185 million tons (168 X 109 kg) of domestic waste is generated per year in the United States. Several of these wastes have a potential for reuse in highways.

Refuse. Landfill refuse is not sought for reuse in highway construction because there is little homogeneity among landfill refuse, and so a great deal of analysis and separation would be required at individual landfills to determine the potential for use. However, there have been occasions when a highway right-of-way traverses a landfill. In such cases, analysis to find appropriate on-site placement of the refuse instead of costly relocation and disposal has been found to be cost-effective. The refuse was spread in thin layers and compacted into embankment material or used for raised medians.

Paper and Paperboard. Approximately 40 percent of the domestic waste generated in the United States is paper or cardboard. Approximately 25 percent of the wastepaper products are recycled each year and used primarily in making more paper, cardboard, and related materials. A highway use of wastepaper, particularly slick paper such as magazine paper, is in the production of mulch material.

Yard Waste and Compost. There are over 1400 yard waste composting stations in the country. Yard waste is banned completely from landfills in many states. Compost material must meet pathogen control, pH, metal concentration, nitrogen ratio, water-bearing capacity, maturity, particle size, and nutrient content control standards set by the EPA. Compost mate­rials are used for mulching, soil amendment, fertilizers, and erosion control. Concerns related to leaching potential, odors, worker health and safety, long-term exposure, and public accep­tance have limited use in highways to the experimental stage, except in landscape use.

Plastics. The amount of plastic waste generated each year is growing. Recycling plastics is complicated in that plastics are developed from at least six different resin bases, which must be sorted for the most-effective recycling. About 30 percent of the plastics made from polyethylene terephthalate (PET), the resin base of soda bottles, is recycled. One use of PET is as a geotextile. Low-density polyethylene (LDPE) resin from film and trash bags can be recycled into pellets for use as an asphalt modifier in paving mixes. High-density polyethylene (HOPE) from milk jugs has been used in manufacturing plastic posts. Reuse of commingled plastics is more difficult but has been applied in fencing and posts. Such plastics have also been used as traffic delineators.

Glass. The amount of glass containers produced each year is declining, but about

12.5 million tons (11.3 X 109 kg) of glass is disposed of as domestic waste each year. To be reused in glass manufacturing, glass must be sorted according to color. Uses in high­ways include as fine aggregate in unbound base courses, as pipe bedding, as aggregate in asphalt mixes, and as glass beads in traffic paint.

Ceramics. Ceramic waste consists of factory rejects and discarded housewares and plumbing fixtures. Only in infrequent instances are large quantities of waste ceramics avail­able for reuse in large applications, such as highway projects. In California, crushed porce­lain has been used as an unbound base course aggregate. Crushed porcelain has been found to meet or exceed quality requirements for concrete aggregate.

Incinerator Ash. Incinerator ash results from the burning of municipal waste. About 26 million tons (24 X 109 kg) of incinerator ash is produced each year, of which 90 percent is bottom ash and the remainder is fly ash. Fly ash often exceeds regulatory limits for con­centrations of lead and cadmium. Fly ash is most often mixed with bottom ash, and this mixture generally does not contain sufficient concentrations of metals to render it haz­ardous. Incinerator ash has been used successfully as a partial replacement of coarse aggre­gate in asphalt mixtures, as roadway fill, and in base course construction when stabilized with Portland cement. Concerns on the part of the EPA about the leaching of heavy metals have initiated several studies.

Sewer Sludge Ash. More than 15,000 municipal wastewater treatment plants in the country produce over 8 million tons (7 X 109 kg) of dry solids of sewage sludge. Following dewatering, sludge cake contains between 18 and 24 percent solids consist­ing mostly of nitrogen and phosphorus, but may be contaminated from various waste­water streams. Much of this sludge cake is incinerated, producing about 1 million tons (0.9 X 109 kg) of ash a year. Sludge ash has the potential for use as an asphalt filler and use in brick manufacturing. Studies indicate that with heat treatment, the ash can pro­duce lightweight pellets that can increase concrete compressive strength by 15 percent when replacing aggregate. Sewage sludge ash has been used as a mineral filler in asphalt paving in Iowa, Minnesota, and other states. Sewage sludge can be composted for agricultural uses such as soil amendments, compost, or fertilizer. Recycled munic­ipal sewage sludge can be a health and safety concern for highway workers using it in landscaping.

Scrap Tires. In 1994, NCHRP published findings of a 5-year review and synthesis of all of the states’ highway practices involving the use of waste tires. This document, entitled Uses of Recycled Rubber Tires in Highways, is the result of a compilation of over 500 sources of information on the topic. The discussion in this section is a synopsis of the infor­mation provided in that document. A copy of the document can be obtained through the Transportation Research Board of the National Research Council 2101 Constitution Avenue NW, Washington, DC 20418.

It is estimated that 2 to 3 billion waste tires have accumulated in the United States, about 70 percent of which are dumped illegally throughout the countryside or disposed of in unauthorized, uncontrolled stockpiles. Also, scrap fires account for about 2 percent of the solid waste that is disposed in regulated landfills. Each year an additional 242 million more scrap tires add to the nation’s solid waste dilemma. Scrap tires are regulated under RCRA Subtitle D as a nonhazardous waste. However, if they are burned, the resulting residue, which may consist of oils, carbon black, and metal-concentrated ash, may be hazardous. In addition, leachate from tire-based products may also be a hazardous or toxic concern. Potential uses of scrap tires in highways and related facilities are numerous.

Table 1.17 identifies the uses of tires in transportation facilities in several states. The environmental implications of the use of scrapped tires in pavement are issues of emis­sions from the manufacture and placement of rubber asphalt. Leachate is also a major concern, particularly of metals (arsenic, barium, cadmium, chromium, lead, selenium, and zinc) and PAHs (polyaromatic hydrocarbons). A Minnesota study conducted in wet­land areas concluded that the use of waste tires in asphalt-rubber pavements may affect groundwater quality. The study’s results were comparable to two other studies with regard to metal leachates, but PAH leachate concentrations were not confirmed by the other studies. Mitigation measures suggested in the Minnesota study would be to place tire materials only in unsaturated zones of the subgrade or fill areas and not below the water table or within surface water boundaries. A Wisconsin study that scrap, shredded, and crumbed tires were not hazardous, nor did they release significant amounts of priority pollutants. Several studies have indicated that the emissions in asphalt-rubber operations are not significantly higher than with conventional asphalt concrete. The one exception to this may be the release of methyl isobutyl ketone, which appears to be consistently slightly higher than with the conventional mixture. The results of these studies should be used with caution, in that the tires from which asphalt rubber is made are not of the same chemical composition, and are continuing to change. The rubber-asphalt formulation process also varies significantly, changing the emissions and leachable properties of the asphalt rubber. Comparison difficulties are compounded in that the composition and

formulation processes for the conventional concrete asphalt that is being used for a stan­dard vary tremendously also. Common and innovative uses of scrap tires are summarized in Table 1.18.