Residuals Management - Public Water Systems - page 768

TABLE 23-17. Survey of Thickening Methods at Water
Treatment Plants in the United States
Treatment Methods

Existing Number of Facilities

Lagoons 180

Gravity thickening 48

Dissolved air flotation 0

Gravity belt thickeners 0

Source: Reference 43. (Adapted from Proceedings of 1991 AWWA /
WEF Joint Residuals Management Conference, by permission. Copy-
right 1991, American Water Works Association.)

WTP residuals. The 1991 WIDB survey also included dewatering methods, as shown
in Table 23-18. The 180 dewatering lagoons are the same 180 lagoons reported for
thickening in the previous table. The survey also did not differentiate between belt
filter presses and filter presses in dewatering.

Lagoons One of the oldest and also most common methods currently used for han-
dling of water treatment plant residuals is lagooning. Lagoons can be used for storage,
thickening, dewatering, or drying. At some locations, lagoons have also been used for
final disposal of residuals. The operating costs of this technique are low, but the land
requirements are high. Because of the space requirement, lagooning may be most
attractive for small, isolated plants.
Lagoons are generally built by enclosure of a land area with dikes or berms, or by
excavation, with no attempt to maximize drainage with underdrains or by a sand layer.
However, it is usually desirable for lagoons to have good drainage. This is best ac-
complished by constructing them with the bottom of the lagoon at natural ground
level; an exception occurs where the existing ground surface is a tight clay soil with
poor drainage initially. Preferably, sludge lagoons should not be built in excavated pits,
and never with a depth below groundwater level. To allow some dewatering, residuals
should have drainage into the subsoil, and the surface should be open to evaporation. 44

TABLE 23-18. Survey of Dewatering Methods at Water Treatment
Plants in the United States
Treatment Methods Existing Number of Facilities
Lagoons 180
Sandy drying beds 26
Freeze-assisted drying beds 33
Solar drying beds Number included with sand drying beds
Belt filter press Number included with filter presses
Centrifuges 10
Filter presses 20
Vacuum filters 4
Screw press 0
Source: Reference 43. (Adapted from Proceedings of 1991 AWWA / WEF Joint Residuals
Management Conference, by permission. Copyright

1991, American Water Works As-

sociation.)