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Publication Number: FHWA-RD-97-148
User Guidelines for Waste and Byproduct Materials in Pavement Construction
Sewage sludge ash is the by-product produced during the combustion of dewatered sewage sludge in an incinerator. Sewage sludge ash is primarily a silty material with some sand-size particles. The specific size range and properties of the sludge ash depend to a great extent on the type of incineration system and the chemical additives introduced in the wastewater treatment process.
At present, two major incineration systems, multiple hearth and fluidized bed, are employed in the United States. Approximately 80 percent of the incinerators used in the United States are multiple hearth incinerators.
The multiple hearth incinerator is a circular steel furnace that contains a number of solid refractory hearths and a central rotating shaft. Rabble arms that are designed to slowly rake the sludge on the hearth are attached to the rotating shaft. Dewatered sludge (approximately 20 percent solids) enters at the top and proceeds downward through the furnace from hearth to hearth, pushed along by the rabble arms. Cooling air is blown through the central column and hollow rabble arms to prevent overheating. The spent cooling air with its elevated temperature is usually recirculated and used as combustion air to save energy. Flue gases are typically routed to a wet scrubber for air pollution control. The particulates collected in the wet scrubber are usually diverted back into the sewage plant.(1)
Fluidized bed incinerators consist of a vertical cylindrical vessel with a grid in the lower sections to support a bed of sand. Dewatered sludge is injected into the lower section of the vessel above the sand bed and combustion air flows upward and fluidizes the mixture of hot sand and sludge. Supplemental fuel can be supplied by burning above and below the grid if the heating value of the sludge and its moisture content are insufficient to support combustion.(1)
Figure 17-1 shows a simplified flow diagram of a sludge incinerator. The complete system includes sludge pretreatment operations such as sludge thickening (sedimentation) and sludge dewatering (vacuum filter, centrifuge, or filter press), followed by incineration, air pollution control, and ash handling. Sludge dewatering may involve the addition of ferrous chloride, lime, or organic polymers to enhance the dewatering process. Auxiliary fuel is normally needed to maintain the combustion process.(2) The quantity of auxiliary fuel required depends on the heating value of the sludge solids and, primarily, on the moisture content of the incoming feed sludge.
Operating temperatures can vary, depending on the type of furnace, but can be expected to range from approximately 650°C (1200°F) to 980°C (1800°F) in the incinerator combustion zone. High operating temperatures above 900°C (1650°F) can result in partial fusion of ash particles and the formation of clinkers, which end up in the ash stream. Lime may also be added to reduce the slagging of sludge during incineration.(3)
Figure 17-1: Simplified sludge incinerator flow diagram.
Incineration of sewage sludge (dewatered to approximately 20 percent solids) reduces the weight of feed sludge requiring disposal by approximately 85 percent. There are approximately 170 municipal sewage treatment plant incinerators in the United States, processing approximately 20 percent of the nation's sludge, and producing between 0.45 million and 0.9 million metric tons (0.5 and 1.0 million tons) of sludge ash on an annual basis.(4)
CURRENT MANAGEMENT OPTIONS
Sludge ash has been previously used as a raw material in Portland cement concrete production, as aggregate in flowable fill, as mineral filler in asphalt paving mixes, and as a soil conditioner mixed with lime and sewage sludge.(5,6) Some California sludge ash with high copper content has reportedly been sent to an Arizona smelter for copper recovery.(7) Sludge ash has also been proposed as a substitute lightweight aggregate product, produced by firing sludge ash or a mixture of sludge ash and clay at elevated or sintering temperatures. Other potential uses that have been reported include the use of ash in brick manufacturing(8) and as a sludge dewatering aid in wastewater treatment systems.(9)
Applications that could potentially make use of sewage sludge ash in highway construction include the use of ash as part of a flowable fill for backfilling trenches or as a substitute aggregate material or mineral filler additive in hot mix asphalt.(6,10,11)
Most of the sludge ash generated in the United States is presently landfilled.
Sludge ash can be obtained directly from municipal wastewater treatment facilities with sludge incinerators or from private companies responsible for the disposal of the sludge ash. Due to the relatively small quantities of sludge ash generated, provisions for ash storage will be required to accumulate sufficient amounts for most applications.
Sludge ash properties (chemical) depend on the nature of the wastewater and the chemicals used in the treatment and sludge handling and incineration process. Since sludge is almost always dewatered prior to combustion, pretreatment of the sludge to enhance the dewatering process may include the addition of ferrous salts, lime, organics, and polymers. Ash produced at treatment plants that introduce ferrous salts or lime for sludge conditioning and dewatering contain significantly higher quantities of ferrous and calcium, respectively, than plants that do not. The pH of sludge ash can vary between values 6 and 12, but sludge ash is generally alkaline.
Sludge ash from multiple hearth incinerators will usually consist primarily of silty material mixed with some larger sand-sized particles. The formation of larger particles is normally the result of higher operating temperatures and the formation of clinkers. Fluidized bed furnaces produce only a very fine (silt-sized) ash.
HIGHWAY USES AND PROCESSING REQUIREMENTS
Asphalt Concrete Aggregate and Mineral Filler
Sludge ash has been used in asphalt paving mixes (6,10,11) to replace both fine aggregate and mineral filler size fractions in the mix. A number of test pavements have been successfully placed in Minnesota.(6)
Sludge ash can also be vitrified to produce a frit for use as an aggregate substitute material. A plant operated in New York State for approximately 3 years, but closed in 1995. It produced a vitrified frit-like product that was approved by the New York State Department of Transportation for use as fine aggregate substitute in paving mixes.(12)
Flowable Fill Aggregate
Sludge ash has reportedly been used as a fine aggregate substitute in flowable fill applications, although there is no documented use of sludge ash in this application.(6)
Table 17-1 presents physical property characterization data for sludge ash from several sources. Sludge ash is a silty-sandy material. A relatively large fraction of the particles (up to 90 percent in some ashes) are less than 0.075 mm (No. 200 sieve) in size. Sludge ash has a relatively low organic and moisture content. Permeability and bulk specific gravity properties are not unlike those of a natural inorganic silt. Sludge ash is a nonplastic material.
Sludge ash consists primarily of silica, iron and calcium. The composition of the ash can vary significantly as previously noted, and depends in great part on the additives introduced in the sludge conditioning operation. There are no specific data available relative to the pozzolanic or cementitious properties of sludge ash, but sludge ash is not expected to exhibit any measurable pozzolanic or cementitious activity. Table 17-2 lists the range of major elemental concentrations present in sludge ash reported from two sources.
Trace metal concentrations (e.g., lead, cadmium, zinc, copper) found in sludge ash are typically higher than concentrations found in natural fillers or aggregate. This has resulted in some reluctance to use this material; however, recent investigations (leaching tests) suggest that these trace metal concentrations are not excessive and do not pose any measurable leaching problem. (See references 6,11,13,14.)
Table 17-1. Typical physical properties of sewage sludge ash.
Table 17-2. Typical range of elemental concentrations in sewage sludge ash.