15-24 WATER AND WASTEWATER ENGINEERING
and the remaining water evaporating. Sometimes the amount of free water available to drain is
enhanced by natural freeze-thaw cycles. In mechanical dewatering, some type of device is used
to force the water out of the sludge. Table 15-4 provides a method for screening the selection of
an alternative from those that are discussed in the following paragraphs.
In the following discussion the desirability of conducting pilot tests to develop design data
is m entioned in several instances. Cornwell (2006) provides some general, as well as specific,
guidance on conducting pilot tests.
Nonmechanical Dewatering
Lagoons. Lagoons can be constructed as either permanent storage lagoons or dewatering lagoons.
Permanent storage lagoons are designed to act as a final disposal site. They will not be discussed.
Some authors (for example, Cornwell, 1999) consider a lagoon to be a dewatering lagoon only
if it has a sand u nderdrain bottom. Others (for exa
mple, Kawamura, 2000, and MWH, 2005)
consider the underdrain an optional feature. The difference in concept is critical in selection of
the method for determining the design dimensions. If an underdrain system is provided, then the
design methodology is the same as that u
sed for a sand drying bed. This method is discussed in
the next section. In this discussion the term dewatering lagoon is used with the understanding that
an underdrain will not be provided.
Lagoons are generally operated in a cyclic fashion: fill, settle, decant. This cycle is repeated
until the lagoon is full or the decant can no longer meet discharge limitations. The solids are then
removed for final disposal. The standing water is removed by decanting or pumping to facilitate
drying. To recover the water, the decant is often returned to the head end of the plant. As noted
for thickening, this may be a problem because it inc
reases the concentration of Cryptosporidium
and Giardia.
Coagulant sludges can only be expected to reach a 7 to 10 percent solids concentration in
dewatering lagoons. The remaining solids must be cleaned out wet. Evaporation to dryness
is generally not practical. Depending upon the depth of the wet solids, evaporation
can take
years. The top layers will often form a crust, preventing evaporation of the bottom layers of
sludge.
Lime-soda softening sludges dewater more readily than coagulant sludges. Typical values
vary from 30 to more than 50 percent. For these sludges it is important to de
sign the lagoon so
that the sludge does not remain submerged after initial filling because it does not c ompact well
under water.
Dewatering lagoons , which are generally earthen basins, have no size limitations but have
been designed with surface areas from 2,000 to 60,000 m
2
, and depths ranging from 2 to 10 m.
Typically, several lagoon cells are provided to allow for a drying period after the lagoon is
full. Dewatering lagoons should be equipped with inlet structu res designed to dissipate the
velocity of the incoming sludge. This minimizes turbulence in the lagoons and help
s prevent
carryover of solids in the decant. The lagoon outlet structure is designed to skim the settled
supernatant.
When land is readily available, lagoons may serve as both thickeners with continuous decanting
and drying beds. A common approach for coagulant sludges i
s to provide sufficient volume for
three months of filling and three months of dewatering. For lime softening sludges, a three-year
cycle for filling and concurrent dewatering has been used.