Environmental Engineering Reference
In-Depth Information
10
ChaPTer
solids handling
10.1 inTroduCTion
The wastewater treatment unit pro-
cesses described to this point remove solids
and biochemical oxygen demand (BOD) from
the wastestream before the liquid effluent
is discharged to its receiving waters. What
remains to be disposed of is a mixture of solids and wastes called
pro-
cess residuals
, more commonly referred to as
sludge
or
biosolids
. The
most costly and complex aspect of wastewater treatment can be the
collection, processing, and disposal of sludge because the quantity of
sludge produced may be as high as 2% of the original volume of wastewa-
ter, depending somewhat on the treatment process being used. Because
sludge can be as much as 97% water content and because the cost of dis-
posal will be related to the volume of sludge being processed, one of the
primary purposes or goals of sludge treatment (along with stabilizing it
so it is no longer objectionable or environmentally damaging) is to sepa-
rate as much of the water from the solids as possible. Sludge treatment
methods may be designed to accomplish both of these purposes.
Key Point:
Sludge
is the commonly accepted
name for wastewater solids; however, if
wastewater sludge is used for beneficial
reuse (e.g., as a soil amendment or fertilizer),
it is commonly referred to as
biosolids
.
Note:
Sludge treatment methods are generally divided into three major
categories:
thickening
,
stabilization
, and
dewatering
. Many of these
processes include complex sludge treatment methods (heat treatment,
vacuum filtration, and incineration, among others).
10.1.1 sludge vs. biosolids
When we speak of
sludge
or
biosolids
, we are speaking of the same
material; each is defined as the suspended solids removed from waste-
water during sedimentation and then concentrated for further treatment
