Incinerating sewage sludge has been practiced for the last sixty years. Early designs were either flash-drying or multiple-hearth types, while in recent years fluidized-bed incinerators have also been used. The flash-drying process has a low capital cost and is flexible in that it can produce the amount of dried sludge that markets need; the remainder can be incinerated. Its limitations are the added cost of pay fuel and the associated odor and pollution problems. The multiple-hearth design, the most widely used for sludge incineration, reduces odor and pollution but provides less operating flexibility because it cannot dry the sludge without incinerating it. The most recent and advanced design is the fluidized-bed sludge incinerator, which can operate in either the combustion or pyrolysis mode. The exhaust temperature from a fluidized-bed incinerator is higher than from a multiple-hearth furnace so afterburners are less likely to be needed to control odor.
The auxiliary fuel cost of sludge incineration is higher with fluidized-bed incinerators than with multiple hearths. The cost varies according to the moisture content of the sludge and the degree of heat recovery (Sebastian 1974a). Eliminating the need for auxiliary fuel requires that the dry-solid content exceed 25% for multiple-hearth and 32% for fluidized-bed incinerators (Sebastian 1974a). In some fluidized-bed installations in Japan, operating costs have been cut in half through heat recovery (Henmi, Okazawa, and Sota 1986).
In a multiple-hearth incinerator with a feed containing 10% solids, the ash is about 10% of the feed. Table 10.10.1 gives the composition of incinerator ashes. The ash is either landfilled or marketed as a soil conditioner. Table 10.10.2 gives the composition of Vitalin, the ash from Tokyo's Odai plant. (The Japanese word lin means
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