Environmental Engineering Reference
In-Depth Information
taBle 32.17
typical data for the Physical characteristics and quantities of sludge Produced from
various Wastewater treatment operations and Processes
dry solids (kg 10
-3
/m
3
)
treatment operation
or Process
specific Gravity
of solids
specific Gravity
of sludge
range
typical
Primary sedimentation
1.4
1.02
110-170
150
Activated sludge (waste sludge)
1.25
1.005
70-100
80
Trickling filter (waste sludge)
1.45
1.025
60-100
70
Extended aeration (waste sludge)
1.30
1.015
80-120
100
a
Aerated lagoon (waste sludge)
1.30
1.01
80-120
100
a
Filtration
1.20
1.005
12-24
20
Algae removal
1.20
1.005
12-24
20
Chemical addition to primary
tanks for phosphorous removal
Low lime (350-500 mg/L)
1.9
1.04
240-400
300
b
High lime (800-1600 mg/L)
2.2
1.05
600-1300
800
b
Suspended growth nitrification
Negligible
Suspended growth denitrification
1.20
1.005
12-30
18
Roughing filters
1.28
1.02
-
-
c
a
Assuming no primary treatment.
b
Solids in addition to that normally removed by primary sedimentation.
c
Included in solids production from secondary treatment processes.
32.5.2.3 mass-volume-concentration relationships
The volume of sludge depends mainly on its water content and only slightly on the character of
the solid matter. A conventional 4% sludge, for example, contains 96% water by weight, has a
concentration of 40.2901 g/L and a density of 1007.25 kg/m
3
. Examples for the sludge relationships
between concentration, density and percent of solids are gathered in Table 32.19.
32.5.2.4 anaerobic sludge digestion, Biogas Production, and use
Waste municipal sludge can be easily digested. Anaerobic digestion is the worldwide most common
treatment for municipal sludge before disposal or further treatment. The properties and principles
of anaerobic digestion have been thoroughly discussed in the previous section; in this section we
will focus on specifics that concern municipal sludge itself. Generally municipal sludge does not
cause any problems in anaerobic digestion if no inhibitors and toxic substances are present in the
wastewater. Primary sludge (or any organic sludge that comes from sedimentation) requires no
special pretreatment for anaerobic digestion. Secondary sludge (biological sludge from wastewater
treatment processes) like activated sludge or trickling filter sludge are in fact cellular bacterial matter
and take usually more time to be digested. The cells of microorganisms need to be ruptured for
anaerobic microorganisms to process the material. Conventionally no special pretreatment methods
are applied for primary sludge, except homogenization if large chunks are present. For secondary
sludge, being cellular material some pretreatment methods can enhance the digestion and therefore
lower the retention time in the digester (Pham et al. 2008; Salsabil et al. 2008). There are several
methods present and commercially available; some of the most common are ultrasonic pretreatment,
pressure drop pretreatment, and different kinds of thermal pretreatment. Ultrasonic pretreatment
uses vibrating forks of 0.5 m in size at the frequency of 40,000-50,000 kHz, to generate ultrasound
and rupture the cell walls. Pressure drop pretreatment uses pressures of 200 bar and subsequently a
quick release to normal conditions. In this case, rapid expansion causes the cell to rupture. Thermal
