Page 213 - Wastewater Solids Incineration Systems

Ash Handling and Recycling 179

3.1.1.5 Mechanical Conveyance
Screw conveyors may be used for conveyance of wet ash in a dewatering operation
or ash conditioning application. Where used in wet ash conveyance, a ribbon screw
is an effective design, allowing some minimal roll back of ash. Wet ash can easily
form a hard and thickened layer at the bottom of the trough which can lead to stress
and potential breakage of the conveyor shaft if it builds up sufficiently. An ultra-high
molecular weight plastic liner to facilitate “slippage” along the trough can be a valu-
able addition.

3.1.2 Storage
3.1.2.1 Ash Lagoon
Ash lagoons provide a simple and basic means for storing ash. The ash lagoon
receives the ash slurry and provides an area where the ash can separate from the
water and settle to the bottom of the lagoon. Water is decanted from the lagoon and
recycled to the plant for treatment. The settled ash is then excavated and transported
for ultimate disposal or is recycled. These lagoons can be constructed to suit local
topography; depth can range from 1 to 3 m (3 to 10 ft), with length and width vari-
able. At least two cells should be constructed to allow clearing of one while the
second receives slurry.

3.1.2.2 Bins
Bins or hoppers are used for storage of ash that is dewatered mechanically. Alterna-
tively, dewatered ash is directly conveyed to a truck or dumpster.

3.1.2.3 Mechanical Thickening and Dewatering
If mechanical thickening and dewatering are used, then the ash slurry is typically
conveyed to a gravity style thickener or decanter. Thickened ash slurry is then
pumped to a dewatering device. Vacuum filters are predominantly used for mechan-
ical ash dewatering. Belt filter presses also have been used for dewatering thickened
ash. The dewatered ash is then transported to a disposal site or is recycled.

3.2 Dry Systems
Dry systems historically have been installed only on MHFs. More recently, dry ash
conveyance systems have been installed on fluid bed incinerators as well. Dry ash
handling systems may be mechanical or pneumatic. The pneumatic systems may be
either pressure or vacuum and dilute or dense phase. Ash grinding may be required
for bottom ash systems to allow effective ash transport and to protect downstream