Civil Engineering Reference
In-Depth Information
coagulation plants.
71
Spent-filter backwash waters were discharged to streams and lakes
by 83 percent of the plants responding in 1953 and 49 percent in 1969.
Results on water treatment plant waste disposal methods from a 1979 survey of 75
alum coagulation plants and a 1981 survey of 100 softening plants are shown in Table
23-21.
12,66
The percentage of softening plants discharging sludge to rivers or lakes
had decreased to 13 percent by 1981, while 20 percent of alum coagulation plants still
practiced this method of sludge disposal in 1979. Substantial numbers of water treat-
ment plants continue to discharge sludge and spent-filter backwash water to surface
waters; however, it is obvious that this practice is steadily being restricted.
Disposal Options
There are eight basic sludge disposal options that can be used by water treatment
plants:
1. Discharge to waterway
2. Discharge to sanitary sewers
3. Codisposal with wastewater biosolids
4. Lagooning with ultimate disposal of the residue
5. Mechanical dewatering with landfilling of residue
6. Coagulant recovery
7. Land application
8. Use for building or fill materials
Discharge to Waterway
The oldest disposal method is to discharge sludges to the
nearest available waterway—stream, pond, lake, or ocean—but its use is decreasing
under the pressure of state regulatory agencies and federal laws.
TABLE 23-21. Methods for Disposal of Water Treatment
Plant Waste
Percent of Plants Using Indicated
Disposal Method
Softening*
Sludge
Coagulation†
Sludge
Sludge lagoon
34‡
43
Sanitary sewer
8
27
River of lake
13
20
Recalcination
5
—
Direct land application
5
—
Other
—
10
* Reference 12.
† Reference 66.
‡ Fifty-six percent of softening plants surveyed had sludge lagoons, 60 percent of
which were considered ''permanent lagoons''; thus, 34 percent of plants used sludge
lagoons for disposal.
