In all types of incinerators, the gases from combustion must be brought to and kept at a temperature of 1250° F to 1400° F. until they are completely burned. This is essential to prevent odor nuisance from stack discharge. It is also necessary to maintain effective removal of dust, fly ash and soot from the stack discharge. This may be done by a settling chamber, by a centrifugal separator, or by a Cottrell electrical precipitator. The selection depends on the degree of removal efficiency required for the plant location. All types of sludge, primary, secondary, raw or digested sludge, may be dried and burned. Raw primary sludge with about 70 percent volatile solids contains about 7800 Btu per pound of dry solids and when combustion is once started will burn without supplementary fuel, in fact an excess of heat is usually available. Digested sludge may or may not require supplementary fuel, depending on the moisture content of the cake and percent volatile solids or degree of digestion. Raw activated sludge generally requires supplementary fuel for drying and burning. In all cases, supplementary fuel is necessary to start operation and until combustion of the solids has been established.
Incineration of sludge has gained popularity throughout the world, especially at large plants. It has the advantages of economy, freedom of odor, independence of weather and the great reduction in the volume and weight of end product to be disposed of. There is a minimum size of sewage treatment plant below which incineration is not economical. There must be enough sludge to necessitate reasonable use of costly equipment. One of the difficulties in operating an incinerator is variations in tonnage and moisture of sludge handled.
There are two major incinerator technologies used in this process. They are (1) the multiple hearth incinerator, and (2) the fluidized bed incinerator. An incinerator is usually part of a sludge treatment system which includes sludge thickening, macerations, dewatering (such as vacuum filter, centrifuge, or filter press), an incinerator feed system, air pollution control devices, ash handling facilities and the related automatic controls. The operation of the incinerator cannot be isolated from these other system components. Of particular importance is the operation of the thickening and dewatering processes because the moisture content of the sludge is the primary variable affecting the incinerator fuel consumption.
Incineration may be thought of as the complete destruction of materials by heat to their inert constituents. This material that is being destroyed is the waste product (i.e., the sludge). Sewer sludge as sludge cake normally contains from 55 to 85% moisture. It cannot burn until the moisture content has been reduced to no more than 30%. The purpose of incineration is to reduce the sludge cake to its minimum volume, as sterile ash. There are three objectives incineration must accomplish:
3. dry the sludge cake,
4. destroy the volatile content by burning, and
There are four basic types of incinerators used in wastewater treatment plants. They are the multiple hearth incinerator, the fluid bed incinerator , the electric furnace , and the cyclonic furnace. Each system has it's own distinct method of incineration and while one may be more cost efficient, another may have more of an environmental impact.
The basic configuration and features of the multiple hearth incinerator are illustrated in Figure 23. This incinerator is the most prevalent incinerator technology for the disposal of sewage sludge in the U.S. due to it's low ash discharge. Sludge cake enters the furnace at the top. The interior of the furnace is composed of a series of circular refractory hearths, which are stacked one on top of the other. There are typically five to nine hearths in a furnace. A vertical shaft, positioned in the center of the furnace has rabble arms with teeth attached to them in order to move the sludge through the mechanism. Each arm is above a layer of hearth. Teeth on each hearth agitate the sludge, exposing new surfaces of the sludge to the gas flow within the furnace. As sludge falls from one hearth to another, it again has new surfaces exposed to the hot gas. At the top of the incinerator there is an exit for flue gas, an end product of sludge incineration. At the bottom of the furnace there is an exit for the ashes.
The basic configuration and features of the fluid bed incinerator are illustrated in Figure 24. This technology has been around since the early 1960s. In this system, air is introduced at the fluidizing air inlet at pressures of 3.5 to 5 psig. The air passes through openings in the grid supporting the sand and creates fluidization of the sand bed. Sludge cake is introduced into the bed. The fluidizing air flow must be carefully controlled to prevent the sludge from floating on top of the bed. Fluidization provides maximum contact of air with sludge surface for optimum burning. The drying process is practically instantaneous. Moisture flashes into steam upon entering the hot bed. Some advantages of this system are that the sand bed acts as a heat sink so that after shutdown there is minimal heat loss. With this heat containment, the system will allow startup after a weekend shutdown with need for only one or two hours of heating. The sand bed should be at least 1200° F when operating.
The basic features of the electric furnace are illustrated in Figure 25. The electric furnace is basically a conveyor belt system passing through a long rectangular refractory lined chamber. Heat is provided by electric infrared heating elements within the furnace. Cooling air prevents local hot spots in the immediate vicinity of the heaters and is used as secondary combustion air within the furnace. The conveyer belt is made of continuous woven wire mesh chosen of steel alloy that will withstand the 1300 to 1500° F temperatures. The sludge on the belt is immediately leveled to one inch. The belt speed is designed to provide burnout of the sludge without agitation.
The basic features of the cyclone furnace are illustrated in Figure 26. The cyclonic furnace is a single hearth unit where the hearth moves and the rabble teeth are stationary. Sludge is moved towards the center of the hearth where it's discharged as ash. The furnace is a refractory lined cylindrical shell with a domed top. The air, heated with the immediate introduction of supplemental fuel creates a violent swirling pattern which provides good mixing of air and sludge feed. The air, which later turns into flue gas, swirls up vertically in cyclonic flow through the discharge flue in the center of the doomed roof. One advantage of these furnaces is that they are relatively small and can be placed in operation, at operating temperature within an hour.
A good question for us to ask at this stage is what does a sludge treatment plant do with the ash that is discharged out of the furnace? As ash falls into a wet sump, turbulence is created by the entrance of water. This turbulence is necessary so that the ash doesn't collect and cake up. This water containing the ash is pumped into a holding pond or lagoon, with a residence time of at least 6 hours. During this time, 95% of the ash will have settled to the bottom and the overflow is taken back to the treatment plant. There has to be a minimum of two lagoons with one being used to hold the ash-water discharge and the other for drying. When dry, the ash is hauled to a landfill or used for concrete. Mixing one part of ash to four parts cement will produce a slow-setting concrete with no loss in strength.
A serious environmental impact that incineration has is on the air. An incinerator's smoke discharge or flue gas should be colorless. Flue gas is an emission mainly made up of nitrogen, carbon dioxide and oxygen. There are traces of chloride and sulfides in the gas and if these levels become to high, they could cause the possibility of corrosion. With respect to the color of the discharge again, if there is a significant amount of particulate matter in the emission, it will be detected by color. The stream can range from a black to white appearance and will have a pale yellow to dark brown trail. The discharge should also have no discernable odor and there should be no detectable noise due to incinerator operation at the property line. Unfortunately colored emissions and odor problems do occur and treatment plants take the proper actions to correct it. Air pollution controls are critical factors that add significant costs onto these technologies. A discussion of these technology options and requirements are quite extensive and beyond the scope of this volume. There are some good references cited at the end of this chapter where you can gain valuable information from.
When dealing with incinerators, fuel is generally the most expensive part of the process from an operational standpoint. There should be a ratio calculated before hand that represents the amount of fuel used for the amount of sludge inputted. If there is a significant change to the amount of fuel consumed, it could mean that there is a problem in the fuel supply system, air flow to the incinerator, or that an extensive furnace cleaning is in order. Minimal cost of operation and equipment maintenance is another economic parameter for sludge incineration. Preventive maintenance is the single most important factor in reduction of operating costs. Semiannual or quarterly appointments must be scheduled to allow time for complete furnace check-out and cleaning (referred to as "turnarounds"). Not having your furnace inspected at least semiannually is a federal violation in the U.S.. The following (Table 2) is a break down of the costs of each incinerator. Essentially costs can be related to one basic parameter, namely - the lower the moisture content is in the sludge, the less expensive the incinerator will be to operate. Also incinerators are bought based on what moisture level of sludge they are going to be effective with. Some incinerators can burn out sludge with 20% moisture levels and some cannot. Table 4 provides some costs for the four basic incinerators plus installation:
Sludge Moisture Content, %
Installed Cost (U.S. $)
Multiple Hearth Furnace
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