High-temperature processes have been used for the incineration or combustion of municipal wastewater solids since the early 1900s. Popularity of these processes has fluctuated greatly since their adoption from industrial combustion. Thirty years ago, combustion of wastewater solids was both practical and inexpensive. Solids were easily dewatered and the fuel required for combustion was cheap and plentiful. In addition, air-emission standards were virtually nonexistent. Today, wastewater solids are more complex and include sludge from secondary and advanced waste treatment processes. These sludge are more difficult to dewater and thereby increase fuel requirements for combustion. Due to environmental concerns with air quality and the energy crisis, the use of high-temperature processes for combustion of municipal solids is being scrutinized. More efficient solids dewatering processes and advances in combustion technology have renewed an interest in the use of high-temperature processes for specific applications.
High-temperature processes should be considered where available land is scarce, stringent requirements for land disposal exist, destruction of toxic materials is required, or the potential exists for recovery of energy, either with wastewater solids alone or combined with municipal refuse. High-temperature processes have potential advantages over other methods which include:
• Maximum volume reduction. Reduces volume and weight of wet sludge cake by approximately 95 percent, thereby reducing disposal requirements.
• Detoxification. Destroys or reduces toxics that may otherwise create
• Energy recovery. Potentially recovers energy through the combustion of waste products, thereby reducing the overall expenditure of energy.
Disadvantages of high-temperature processes include:
• Cost. Both capital and operation and maintenance costs, including costs for supplemental fuel, are generally higher than for other disposal alternatives.
• Operating problems. High-temperature operations create high maintenance requirements and can reduce equipment reliability.
• Staffings. Highly skilled and experienced operators are required for high-temperature processes. Municipal salaries and operator status may have to be raised in many locations to attract the proper personnel.
• Environmental impacts. Discharges to atmosphere (particulates and other toxic or noxious emissions), surface waters (scrubbing water), and land (furnace residues) may require extensive treatment to assure protection of
Combustion is the rapid exothermic oxidation of combustible elements in fuel. Incineration is complete combustion. Classical pyrolysis is the destructive distillation, reduction, or thermal cracking and condensation of organic matter under heat and/or pressure in the absence of oxygen. Partial pyrolysis, or starved-air combustion, is incomplete combustion and occurs when insufficient oxygen is provided to satisfy the combustion requirements. The basic elements of each process are shown on Figure 27. Combustion of wastewater solids, a two-step
A value commonly used in sludge incineration calculations is 10,000 Btu per pound of combustibles. It is important to clearly understand the meaning of combustibles. For combustion processes, solid fuels are analyzed for volatile solids and total combustibles. The difference between the two measurements is the fixed carbon. Volatile solids are determined by heating the fuel in the absence of air. Total combustibles are determined by ignition at 1,336° F (725° C). The difference in weight loss is the fixed carbon. In the volatile-solids determination used in sanitary engineering, sludge is heated in the presence of air at 1,021° F (550° C). This measurement is higher than the volatile-solids measurement for fuels and includes the fixed carbon. Numerically, it is nearly the same as the combustible measurement. If volatile solids are used in the sense of the fuels engineer, it will be followed parenthetically by the designation/wefe usage. If the term volatile solids or volatiles is used without designation, it will indicate sanitary engineering usage and will be used synonymously with combustibles. The amount of heat released from a given sludge is a function of the amounts and types of combustible elements present.
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