Characterization_utilization_and_disposa.pdf

April 2002 The Arabian Journal for Science and Engineering, Volume 27, Number 1B. 3
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‫ﺍﻟﻤﻌﺎﻟﺠﺔ‬ ‫ﻤﺭﺍﻓﻕ‬ ‫ﻭﺘﻁﻭﻴﺭ‬ ‫ﺍﻟﺼﺤﻲ‬ ‫ﺍﻟﺼﺭﻑ‬ ‫ﻤﻴﺎﻩ‬ ‫ﻤﻌﺎﻟﺠﺔ‬ ‫ﻤﺤﻁﺎﺕ‬ ‫ﻤﺠﺎل‬ ‫ﻓﻲ‬ ‫ﺍﻻﺴﺘﺜﻤﺎﺭ‬ ‫ﺯﻴﺎﺩﺓ‬ ‫ﺃﺩﺕ‬
‫ﺇﺩﺍﺭﺓ‬ ‫ﺒﺭﺍﻤﺞ‬ ‫ﺘﺒﻨﻲ‬ ‫ﻀﺭﻭﺭﺓ‬ ‫ﻤﻥ‬ ‫ﺯﺍﺩ‬ ‫ﻤﻤﺎ‬ ،‫ﺍﻟﻤﺭﺍﻓﻕ‬ ‫ﺘﻠﻙ‬ ‫ﻤﻥ‬ ‫ﺍﻟﻨﺎﺘﺠﺔ‬ ‫ﺍﻟﺤﻤﺄﺓ‬ ‫ﻜﻤﻴﺎﺕ‬ ‫ﺘﺯﺍﻴﺩ‬ ‫ﺇﻟﻰ‬ ‫ﺍﻟﺤﺎﻟﻴﺔ‬
‫ﺘﻜﻭﻥ‬ ‫ﻟﻠﺤﻤﺄﺓ‬
‫ﻟﻠﺸﺭﻭﻁ‬ ‫ﻤﺭﺍﻋﻴﺔ‬
‫ﻭﺍ‬ ‫ﺍﻟﺒﻴﺌﻴﺔ‬
‫ﻻ‬
،‫ﻗﺘﺼﺎﺩﻴﺔ‬
‫ﺤﻴﺙ‬
‫ﺍﻟﺭﻏ‬ ‫ﺍﻻﻋﺘﺒﺎﺭ‬ ‫ﺒﻌﻴﻥ‬ ‫ﺘﺄﺨﺫ‬
‫ﺍﻟﻤﺘﺯﺍﻴﺩﺓ‬ ‫ﺍﻟﻌﺎﻟﻤﻴﺔ‬ ‫ﺒﺔ‬
‫ﺍﻟﺤﻤﺄﺓ‬ ‫ﺘﻠﻙ‬ ‫ﺍﺴﺘﺨﺩﺍﻡ‬ ‫ﻹﻤﻜﺎﻨﻴﺔ‬
.
‫ﺍﻟﺤﻤﺄﺓ‬ ‫ﻭﻤﻌﺎﻟﺠﺔ‬ ‫ﺇﻨﺘﺎﺝ‬ ‫ﻤﺠﺎل‬ ‫ﻓﻲ‬ ‫ﺍﻟﻤﻨﺸﻭﺭﺓ‬ ‫ﻟﻸﺩﺒﻴﺎﺕ‬ ‫ﻤﺭﺍﺠﻌﺔ‬ ‫ﺍﻟﻭﺭﻗﺔ‬ ‫ﺘﻘﺩﻡ‬
‫ﻤﻨﻬﺎ‬ ‫ﻭﺍﻟﺘﺨﻠﺹ‬ ‫ﺍﻻﺴﺘﻔﺎﺩﺓ‬ ‫ﻭﺒﺩﺍﺌل‬
.
‫ﺇ‬
‫ﺍﻟﻤﺘﻌﻠﻘﺔ‬ ‫ﺍﻷﻨﻅﻤﺔ‬ ‫ﺤﻭل‬ ‫ﻤﻌﻠﻭﻤﺎﺕ‬ ‫ﺍﻟﻭﺭﻗﺔ‬ ‫ﺘﻘﺩﻡ‬ ،‫ﺫﻟﻙ‬ ‫ﺇﻟﻰ‬ ‫ﻀﺎﻓﺔ‬
‫ﺍﻟﺼﺭﻑ‬ ‫ﻤﻴﺎﻩ‬ ‫ﻤﻌﺎﻟﺠﺔ‬ ‫ﻤﺤﻁﺎﺕ‬ ‫ﻓﻲ‬ ‫ﺍﻟﻤﻨﺘﺠﺔ‬ ‫ﺍﻟﺤﻤﺄﺓ‬ ‫ﺒﺎﺴﺘﺨﺩﺍﻡ‬
‫ﻭﺍﻟﺘﻲ‬ ،‫ﻤﻨﻬﺎ‬ ‫ﺍﻟﺘﺨﻠﺹ‬ ‫ﻭﻭﺴﺎﺌل‬ ‫ﺍﻟﻤﻨﺯﻟﻴﺔ‬
‫ﺍﻷﻤﺭﻴﻜﻴﺔ‬ ‫ﺍﻟﺒﻴﺌﺔ‬ ‫ﺤﻤﺎﻴﺔ‬ ‫ﻭﻜﺎﻟﺔ‬ ‫ﻤﺜل‬ ‫ﺍﻟﻌﻼﻗﺔ‬ ‫ﺫﺍﺕ‬ ‫ﺍﻟﻤﻨﻅﻤﺎﺕ‬ ‫ﻗﺒل‬ ‫ﻤﻥ‬ ‫ﺃﻋﺩﺕ‬
.
‫ﻤﻥ‬ ‫ﺍﻷﻨﻅﻤﺔ‬ ‫ﺘﻠﻙ‬ ‫ﺃﻋﺩﺕ‬ ‫ﻭﻗﺩ‬
‫ﺍﻟﺤﻤﺄﺓ‬ ‫ﻤﻊ‬ ‫ﻴﺘﻌﺎﻤﻠﻭﻥ‬ ‫ﺍﻟﺫﻴﻥ‬ ‫ﺍﻟﻌﻤﺎل‬ ‫ﻭﺼﺤﺔ‬ ‫ﺍﻟﻌﺎﻤﺔ‬ ‫ﻭﺍﻟﺼﺤﺔ‬ ‫ﺍﻟﺒﻴﺌﺔ‬ ‫ﺤﻤﺎﻴﺔ‬ ‫ﺃﺠل‬
.
CHARACTERIZATION, UTILIZATION, AND DISPOSAL
OF MUNICIPAL SLUDGE: THE STATE OF-THE-ART
Muhammad H. Al-Malack*, Nabil S. Abuzaid, Alaadin A. Bukhari
and Mohammed H. Essa
King Fahd University of Petroleum & Minerals
Dhahran, Saudi Arabia
*Address for Correspondence:
KFUPM Box 1150
King Fahd University of Petroleum & Minerals
Dhahran 31261
Saudi Arabia
e-mail: mhmalack@kfupm.edu.sa

M.H. Al-Malack, N.S. Abuzaid, A.A. Bukhari, and M.H. Essa
4 The Arabian Journal for Science and Engineering, Volume 27, Number 1B. April 2002
ABSTRACT
The rapid increase in sludge production, following massive investment in new
wastewater treatment plant installations and upgrading of existing facilities, creates the
need for adoption of economically and environmentally acceptable sludge management
schemes, which take into account the worldwide growing interest in reuse and recovery
possibilities. This paper presents a literature review concerning sludge generation,
production, and treatment. Alternatives of sludge utilization and disposal are also
presented. In addition, the paper provides information on the regulations on municipal
sludge reuse and disposal, set by related agencies, mainly the United States
Environmental Protection Agency (U.S.EPA). The regulations were formulated to
protect the environment, the health of workers handling municipal sludge, and the
health of the public.
Keywords: Municipal Sludge Generation, Production, Characteristics, Treatment,
Utilization, Disposal, Regulations.

CHARACTERIZATION, UTILIZATION, AND DISPOSAL OF MUNICIPAL SLUDGE:
THE STATE OF-THE-ART
SLUDGE GENERATION AND PRODUCTION
Solids generation in biological wastewater treatment plants is difficult to estimate, where all suspended solids in the
influent do not appear in the sludge. Some are biologically metabolized to soluble or gaseous end-products in the
biological treatment process or sludge digestion process. Some soluble wastewater components will be transformed into
biological solids that can be reduced during digestion. Solids generation in a biological treatment process is a function of
the type and the operation of the process.
The sources of sludge in a wastewater treatment plant vary according to the type of plant and its method of operation
[1]. The principal sources of sludge at municipal wastewater treatment plants are the primary sedimentation basin and
the secondary clarifiers. Additional sludge may also come from chemical precipitation, nitrification–denitrification
facilities, screening, grinder, and filtration devices. Table 1 shows the principal sources of solids and sludge and the
types generated [1].
Table 1. Sources of Solids and Sludge from a Conventional Wastewater Treatment Plant [1].
Unit Operation or Process Types of Solids or Sludge Remarks
Screening Coarse solids Coarse solids are removed by mechanical
and hand-cleaned bar screens. In small
plants, screenings are often comminuted
for removal in subsequent treatment units.
Grit removal Grit and scum Scum removal facilities are often omitted
in grit removal facilities.
Preaeration Grit and scum In some plants, scum removal facilities are
not provided in preaeration tanks. If the
preaeration tanks are not preceded by grit
removal facilities, grit deposition may
occur in preaeration tanks.
Primary sedimentation Primary sludge and scum Quantities of sludge and scum depend
upon the nature of the collection system
and whether industrial wastes are
discharged to the system.
Biological treatment Suspended solids Suspended solids are produced by the
biological conversion of BOD. Some form
of thickening may be required to
concentrate the waste sludge stream from
biological treatment.
Secondary sedimentation Secondary sludge and scum Provision for scum removal from
secondary settling tanks is a requirement
of the U.S. Environmental Protection
Agency.
Sludge-processing facilities Sludge, compost, and ashes The characteristics of the end products
depend on the characteristics of the sludge
being treated and the operations and
processes used. Regulations for the
disposal of residuals are becoming
increasingly stringent.

Steel [2] reported that the per capita production of suspended solids could be assumed as 91 g/capita/day. If there is a
moderate amount of industrial wastes, this may rise to 100 g/capita/day, and if it is a combined sewage with considerable
industrial waste, it may be 113 g/capita/day. If ground garbage is added to the sewage, then there will be an additional 32
to 50 g/capita/day. Schmidtke [3] reported that, without sludge digestion, the per capita sludge production rates in
Ontario are approximately 80 and 115 g/d for primary and secondary treatment, respectively. Adding iron or aluminum
salts for phosphorus removal will increase sludge production by about 40% for primary treatment or 25% for secondary
treatment. Koch et al. [4] evaluated procedures for estimating sludge production. They concluded that more detailed data
needed to be collected on wastewater characteristics, the treatment process, and the operating parameters to reliably
predict sludge production. Lishman et al. [5] investigated the effect of temperature on wastewater treatment under
aerobic and anoxic conditions. They concluded that the observed yields were 35 to 52 percent higher for the anoxic
reactors than they were for the aerobic ones. Jardin and Popel [6] investigated the effect of an enhanced biological
phosphorus removal process on waste activated-sludge production and the type of phosphorus storage in two continuous-
flow activated-sludge systems in pilot-scale experiments. They reported that the additional uptake of phosphorus resulted
in an increase in the inorganic sludge mass, but the organic sludge mass did not change. Davis and Cornwell [7] reported
that for primary treatment, the quantities of sludge produced may be 0.25 to 0.35 percent by volume of wastewater
treated. When treatment is upgraded to activated sludge, the quantities increase to 1.5 to 2.0 percent of the volume of
wastewater treated. Use of chemicals for phosphorus removal can add another 1.0 percent. Vesilind et al. [8] reported
figures of dry sludge production in Europe ranging from 30 to 124 g/capita/day. In Metropolitan Seattle, Machno [9]
reported that the amount of solids generated per household is 125 g/day (about 35 g/capita/day). In the United Kingdom,
raw primary sludge produces 52 g/capita/day, co-settled activated produces 74 g/capita/day and co-settled activated
Table 2. Sludge Production and Methods of Disposal in 1990 [10].
Disposal method (%)
Country
Population
(total)
(million)
Population
served (%)
Sludge
produced
(TDS×1000/yr) Agriculture Landfill Incineration Other
Austria
Belgium
Denmark
France
West Germany
Greece
Ireland
Italy
Luxembourg
Netherlands
Portugal
Spain
Switzerland
UK
Japan
USA
7.8
9.9
5.1
56
62
10
3.
57
0.4
15
10.3
39
6.4
57
123
249
48
33
100
64
90
-
44
30
92
90
47
47
80
84
42
-
320
75
130
700
2500
15
24
800
15
282 (871)a
200
280
215
1075
2440
800–1600b
13
31
37
50
25
3
28
34
81
44
80
10
60
51
24
16
56
56
33
50
63
97
18
55
18
53
13
15
30
16
41
43
31
9
28
0
12
0
0
11
0
3
0
10
20
5
22
21
0
4
2
0
0
0
54
0
1
0
7
30
0
28
13
21
a
The production of sludge in the Netherlands is expected to go from 282 000 TDS/yr in 1990 to 871 000 TDS/yr in 2000.
b
A range of 800 million to 1.6 billion wet tons in the United States.

tertiary sludge produces 76 g/capita/day [10]. Sakai et al. [11] reported that no excess wastewater sludge was produced
in a full-scale wastewater treatment plant if the return sludge was ozonated. Soluble solids concentration of 2 to 15 mg/l
in the effluent were higher than those in the treatment processes without ozonation. The annual sludge production in
1990 for a number of countries is shown in Table 2 [10]. It should be noted that while the population of each country is
given, the population served by wastewater facilities is typically about one half of the total. Table 3 shows data on the
quantities of sludge produced from various processes and operations [1]. It is worth mentioning that the quantity of
sludge produced varies widely. Corresponding data on the sludge solids concentration to be expected from various
processes are given in Table 4 [1].
SLUDGE CHARACTERISTICS
The characteristics of sludge to be measured are strongly related to its ultimate fate. For example, if the sludge is to be
thickened by gravity, its settling and compaction characteristics are important. On the other hand, if the sludge is to be
digested anaerobically, the concentrations of volatile solids and heavy metals are of importance. The first characteristic,
solids concentration, is perhaps the most important variable in defining the volume of sludge to be handled, and
determining whether the sludge behaves as a liquid or a solid. The specific gravity of inorganic solids is about 2–2.5 and
that of the organic fraction is 1.2–1.3. Table 5 shows solids concentrations in various municipal sludges [12].
Table 3. Typical Data for the Physical Characteristics and Quantities of Sludge Produced from Various Wastewater
Treatment Operations and Processes [1].
Dry solids, lb/103
gal
Treatment operation or Process
Specific gravity
of sludge solids
Specific gravity
of sludge
Range Typical
Primary sedimentation 1.4 1.02 0.9–1.4 1.25
Activated sludge (waste sludge) 1.25 1.005 0.6–0.8 0.7
Trickling Filtration (waste sludge) 1.45 1.025 0.5–0.8 0.6
Extended aeration (waste sludge) 1.30 1.015 0.7–1.0 0.8a
Aerated lagoons (waste sludge) 1.30 1.01 0.7–1.0 0.8a
Filtration 1.20 1.005 0.1–0.2 0.15
Algae removal 1.20 1.005 0.1–0.2 0.15
Chemical addition to primary
sedimentation tanks for phosphorus
removal
Low lime (350–500 mg/l) 1.9 1.04 2.0–3.3 2.5b
High lime (800–1600 mg/l) 2.2 1.05 5.0–11.0 6.6b
Suspended-growth nitrification -c
- - -
Suspended-growth denitrification 1.20 1.005 0.1–0.25 0.15
Roughing filter 1.28 1.02 - -d
a
Assuming no primary treatment
b
Sludge in addition to that normally removed by primary sedimentation
c
Negligible
d
Included in sludge production from biological secondary treatment processes

Table 4. Expected Sludge Concentrations from Various Treatment Operations and Processes [1].
Sludge solids concentration, %
dry solids
Operation or process application
Range Typical
Primary settling tank
Primary sludge
Primary sludge to a cyclone
Primary sludge and trickling-filter humus
Primary sludge with iron addition for phosphorus removal
Primary sludge with low lime addition for phosphorus removal
Primary sludge with high lime addition for phosphorus removal
Scum
Secondary settling tank
Waste activated sludge
With primary settling
Without primary settling
High purity oxygen activated sludge
With primary settling
Without primary settling
Trickling-filter humus sludge
Rotating biological contractor waste sludge
Gravity thickener
Primary sludge only
Primary and waste activated sludge
Dissolved-air flotation thickener
Waste activated sludge only
With chemical addition
Without chemical addition
Centrifuge thickener
Gravity belt thickener
Waste activated sludge only with chemical addition
Anaerobic digester
Primary sludge only
Aerobic digester
Primary sludge only
4.0–10.0
0.5–3.0
3.0–8.0
4.0–10.0
2.0–8.0
4.0–16.0
3.0–10.0
0.5–1.5
0.8–2.5
1.3–3.0
1.4–4.0
1.0–3.0
1.0–3.0
5.0–10.0
2.0–8.0
4.0–9.0
4.0–6.0
3.0–5.0
4.0–8.0
3.0–6.0
5.0–10.0
2.5–7.0
3.0–8.0
2.5–7.0
1.5–4.0
0.8–2.5
5.0
1.5
4.0
5.0
4.0
10.0
5.0
0.8
1.3
2.0
2.5
1.5
1.5
8.0
4.0
5.0
5.0
4.0
5.0
5.0
7.0
3.5
4.0
3.5
2.5
1.3

The rheological characteristics of sludge are very important because they are one of only few truly basic parameters
describing the physical nature of sludge. Sludge varies from a Newtonian fluid, where shear is proportional to the
velocity gradient, to a plastic fluid, where a threshold shear must be reached before the sludge starts to move. Most
wastewater sludges are pseudoplastic. McCartney and Tingley [13] developed a rapid infrared moisture content
measurement technique. The method was as accurate as moisture contents determined by other standard methods. The
physical characteristics of sludge from different treatment processes are given in Table 6 [10].
The chemical characteristics of sludge are of great importance for several reasons. Characteristics of sludge that affect
its suitability for beneficial use include organic content (usually measured as volatile solids), nutrients, pathogens,
metals, and toxic organics. Henry et al. [14] developed a new technique for field determination of nitrogen
mineralization from sludge, which provides a simple and inexpensive test that yields accurate results. Table 7 shows the
mean concentrations of several elements in different sludge types, while typical nutrient values of sludge as compared to
commercial fertilizers are reported in Table 8 [1, 10]. Moreover, Table 9 gives a range of typical chemical compositions
of different sludges [10].
Table 5. Solids Concentration in Various Sludge Types [12].
Sludge type Solids concentration %
Primary sludge
Waste activated sludge
Fixed film waste sludge
Primary and fixed film sludge
Aerobically digested sludge (thickened)
Anaerobically digested sludge (thickened)
5–8%
0.5–2.0%
3–10%
2.5–4%
3–5%
1–2%
6–12%
Table 6. Physical Characteristics of Sludge [10].
Parameter Primary sludge Secondary sludge Dewatered sludge
Dry solids
Volatile solids
Sludge specific gravity
Solids specific gravity
Shear strength (kN/m2)
Energy content (MJ/kg VS)
Particle size (90%)
2–6%
60–80%
1.02
1.4
<5
12–22
<200 µm
0.5–2%
50–70%
1.05
1.25
<2
12–20
<100 µm
15–35%
30–60%
1.1
1.2–1.4
<20
25–30
<100 µm
Table 7. Concentrations of Several Elements in Different Sludge Types [10].
Mean Concentration (% of dry solids)
Parameter
Anaerobic Aerobic All
K
Na
Ca
Mg
Ba
Fe
Al
0.52
0.7
5.8
0.58
0.08
1.6
1.7
0.46
1.1
3.3
0.52
0.02
1.1
0.7
0.4
0.57
4.9
0.54
0.06
1.3
1.2

Trace elements in sludge are those inorganic chemical elements that, in very small quantities, can be essential or
detrimental to plants and animals. The term ‘heavy metals’ is used to denote some of the trace elements present in
sludge. Several investigators described analytical methods for measuring trace metals and organic compounds [15–20].
Table 8. Comparison of Nutrient Levels in Commercial Fertilizers and Wastewater Sludge [1].
Nutrients, %
Nitrogen Phosphorus Potassium
Fertilizers for typical agricultural use
Typical values for stabilized sludge
5
3.3
10
2.3
10
0.3
Table 9. Typical Chemical Composition of Sludges [10].
Parameter Primary sludge
Anaerobically
digested sludge
Aerobically digested
sludge
pH
Alkalinity (mg/l CaCO3)
Nitrogen (N% of TS)
Phosphorus (P2O5% of TS)
Fats, grease (% of TS)
Protein (% of TS)
Organic acids (mg/l as Hac)
5– 8
500– 1500
1.5– 4
0.8– 2.8
6– 30
20– 30
6800– 10 000
6.5– 7.5
2500– 3500
1.6– 6
1.5– 4
5– 20
15– 20
2700– 6800
0.5– 7.6
1.1– 5.5
Table 10. Concentrations of Heavy Metals in Sludge [1].
Dry sludge, mg/kg
Metal
Range Median
Arsenic
Cadmium
Chromium
Cobalt
Copper
Iron
Lead
Manganese
Mercury
Molybdenum
Nickel
Selenium
Tin
Zinc
1.1– 230
1– 3410
10– 99 000
11.3– 2490
84– 17 000
1000– 154 000
13– 26 000
32– 9870
0.6– 56
0.1– 214
2– 5300
1.7– 17.2
2.6– 329
101– 49 000
10
10
500
30
800
17000
500
260
6
4
80
5
14
1700

With respect to microbiological characteristics of municipal sludges, wastewaters generally contain four major types
of pathogens: bacteria, protozoa, viruses, and helminths. The concentrations of these pathogenic organisms in
wastewater depend on the health condition of the community. In times of epidemics, these concentrations would be high.
Since pathogens come from relatively large volumes of wastewater, when ending up in the sludge, they, in general,
become concentrated and very infectious. Gaspard et al. [21] performed a parasitological analysis of helminths on
89 sludge samples, three sediments, and seven composts. The average concentration of helminths was 130 eggs per
100 grams of dry matter. Sludge from all types of treatment (mesophilic anaerobic and aerobic digestion, composting,
and liming) contained 10 or more viable eggs per 100 grams of dry matter. Antibiotic resistance of Escherichia coli in
sludge and wastewater was affected by location but not the digestion process [22]. The E.coli strains in digested
municipal sludge from El Paso, Texas had more antibiotic resistance than those from any other site. Table 11 shows
some disease-producing protozoa and helminths in sludges, while Table 12 shows the levels of indicator pathogenic
organisms in municipal sludges [10, 23]. Table 13 shows survival times of various pathogens in soil and on plant
surfaces [23].
Table 11. Pathogenic Organisms in Wastewater and Sludge [23].
Organism Disease/symptoms
Protozoa
Cryptosporidium
Entamoeba histolytica
Giardia lamblia
Balantidium coli
Toxoplasma gondii
Helminths
Ascaris lumbricoides
Ascaris suum
Trichuris trichiura
Toxocara canis
Taenia saginata
Taenia solium
Necatur americanus
Hymenolepis nana
Gastroenteritis
Acute enteritis
Giardiasis
Diarrhea and dysentery
Toxoplasmosis
Digestive and nutritional disturbances; abdominal pain, vomiting
May produce symptoms such as coughing, chest pain, and fever
Abdominal pain, diarrhea, anemia, weight loss
Fever, abdominal discomfort, muscle aches, neurological symptoms
Nervousness, insomnia, anorexia, abdominal pain, digestive disturbances
Nervousness, insomnia, anorexia, abdominal pain, digestive disturbances
Hookworm disease
Taeniasis
Table 12. Levels of Indicator and Pathogenic Organisms in Different Sludges (no. per gram of dry weight) [10].
Sludge
(untreated)
Total
coliform
Faecal
coliform
Faecal
Streptococci
Salmonella
species
Pseudomonas
aeruginosa
Enteric
viruses
Primary
Secondary
Mixed
106
–108
107
–108
107
–109
106
–107
107
–109
105
–106
106
106
106
4×102
9×102
5×102
3×103
1×104
103
–105
0.002–0.004
0.015–0.026
--

Table 13. Survival Times Of Various Pathogens In Soil And On Plant Surfaces [23].
SOIL PLANTS
Absolute Common Absolute Common
Pathogen
Maximum Maximum Maximum Maximum
Bacteria
Viruses
Protozoan cysts
Helminth ova
1 year
6 months
10 days
7 years
2 months
3 months
2 days
2 years
6 months
2 months
5 days
5 months
1 month
1 month
2 days
1 month
SLUDGE THICKENING, DEWATERING, CONDITIONING, AND STABILIZATION
Thickening is the pre-processing of sludge prior to dewatering. Thickeners are used in wastewater treatment plants
where solids must be concentrated to increase the efficiency of further treatment. Thickening is economically attractive
because considerable volume reduction is achieved with even modest increases in sludge solids concentrations. Such
reduction can provide significant savings in the cost of dewatering, digestion, or other downstream activities. The proper
location of the thickener in a wastewater treatment plant is important. If the sludge is to be digested, thickening a blend
of raw, primary, and waste activated sludge would be very efficient [24]. On the other hand, if raw primary and
secondary sludges are to be dewatered and incinerated, the sludge should be thickened separately and blended
immediately before dewatering. The three commonly used methods for sludge thickening are gravity, flotation, and
centrifugation methods.
Thickening by gravity is the most common concentration process in use at wastewater treatment plants due to its
simple and inexpensive operation. Gravity thickening is a sedimentation process similar to that which occurs in all
settling tanks. The degree to which waste sludge can be thickened depends on many factors. The most important factor is
the type of sludge being thickened and its volatile-solids concentration. Bulky biological sludge, particularly from an
activated sludge process, will not concentrate to the same degree as raw primary sludge. The degree of biological
treatment and the ratio of primary to secondary sludge will affect the ultimate solids concentration obtained by gravity
thickening [24]. The design parameters for gravity thickeners include the floor loading and the supernatant overflow
rates. Another design factor is the sludge volume ratio (SVR), which is defined as the volume of the sludge blanket
divided by the daily volume of sludge pumped from the thickener. Values for SVR are normally maintained between 0.5
and 2 days, with the lower values being used during warmer weather.
Flotation thickening units are useful in sewage treatment plants for handling waste-activated sludge. They have the
advantage over gravity thickening tanks of offering higher solids concentrations and lower initial equipment cost due to
higher hydraulic and solids loadings which can be applied. Flotation thickeners in wastewater treatment are often used to
thicken activated sludge before digestion. According to Butler et al. [25] dissolved air flotation can be used to thicken a
mixture of primary and waste activated sludges. The advantages of co-thickening these sludges are a significant
reduction in the organic loading to the secondary treatment process and reduction of the amount of grit transferred to the
sludge digesters.
Centrifugation was shown to be an effective method to thicken a portion of the wastewater sludge before digestion
[26]. The thickened portion was then diluted with wastewater sludge to achieve the desired six percent solids content in
the digester. Although centrifuges have been used widely for dewatering, they have had limited use for thickening
because of their relatively high cost. They have been used for thickening waste activated sludge where space limitations
or sludge characteristics make other methods unsuitable.
Sludge dewatering is a physical unit process used to remove as much water as possible from sludge to produce a
highly concentrated cake. Dewatering differs from thickening, as the sludge should behave as a solid after it has been
dewatered. Metcalf and Eddy [1] reported some reasons for performing dewatering.

A sludge suspension contains bulk water, which is not bound to the sludge particles, and bound water. Kopp and
Dichtl [27] reported that in a sewage sludge suspension, four different types of water can be distinguished according to
their physical bonding to the sludge particles. These are free water, interstitial water, surface water, and intracellular
water.
The free water content represents the largest part in sewage sludge, which can be separated mechanically. The
interstitial water is kept between the interstices of the sludge particles and microorganisms in the sludge floc, which is
bound physically by active capillary forces. The surface water covers the entire surface of sludge particles in several
layers of water molecules and is bound by adsorptive and adhesive forces. The surface water is physically bound to the
particles and cannot move freely. The intracellular water contains the water in cells as well as water of hydration. Smith
and Vesilind [28] reported that intracellular water can only be determined together with the surface water and is often
called bound water content. Bound water, which contributes the smallest proportion of water content, has the strongest
physical–chemical bonding to the particles and can only be removed thermally. Dewatering devices primarily remove
free water; some interstitial water can be removed as well, but it is likely that the major fraction of bound water is vicinal
water that cannot be removed mechanically. Robinson and Knocke [29] reported that freeze–thaw conditioning
physically disrupts the floc and cell structure and produces the greatest degree of bound water release for dewatering.
Lajoie et al. [30] reported that the abundance of zoogleal clusters and centrifuged solids content were negatively
correlated, as determined by linear regression. Wu and Huang [31] concluded that recycling-sludge operation can
decrease the average bound water content by nearly 13–24 percent and increase the effective floc density, thereby
forming a compact structure with a high fractal dimension and low floc porosity. Chin et al. [32] investigated the use of
capillary suction time (CST) as a measure of sludge dewaterability and reported that CST was a good index for sludge
filterability. Besides the use of chemical coagulants and anaerobic digestion to improve the dewaterability of sludge,
Cantet et al. [33] reported that the addition of talqueous powders improved sludge dewaterability.
A number of sludge dewatering techniques are currently in use. The selection of a sludge-dewatering system depends
on: (1) the characteristics of the sludge to be dewatered; (2) the available space; and (3) the moisture content
requirements of the sludge cake for ultimate disposal. When land is available and the sludge quantity is small, natural
dewatering systems are most attractive. These include drying beds and drying lagoons. Mechanical dewatering systems
are generally selected where land is not available. Common mechanical sludge-dewatering systems include vacuum
filter, centrifuge, filter press, and belt filter press.
Ockier et al. [34] reported that mechanical dewatering of the sludge directly extracted from municipal wastewater
treatment plants is often applied. Dorical et al. [35] reported that sludge dewatering efficiency varied substantially at the
mills. On the other hand, Harvey and Boulanger [36] concluded that 65 percent of the reduction of chemical cost could
be achieved by focusing on areas such as blend ratio and consistence, equipment modification, and optimization of the
chemical addition points.
Drying Beds
Sludge drying beds are the oldest method of sludge dewatering and are still used extensively in small-to-medium sized
plants to dewater sludge. They are relatively inexpensive and provide dry sludge cake. In recent years, many advances
have been made to conventional drying bed technology, and new systems are used on medium- and large-sized plants.
These variations of the drying beds are: (1) conventional sand; (2) paved; (3) wire-wedge; and (4) vacuum assisted.
Conventional Sand Beds
Typical sand beds consist of a layer of coarse sand 15–25 cm in depth and supported on a gravel bed (0.3–2.5 cm) that
incorporates selected tiles or perforated pipe under-drain. Sludge is placed on the bed in 20–30 cm layers and allowed to
dry. Sludge cake removal is manual by shoveling into wheelbarrows, trucks, scraper, or front-end loader. The under-
drained liquid is returned to the plant. The drying period is 10–15 days, and the moisture content of the cake is
60–70 percent. Sludge loading rate is 100–300 kg dry solids per m2
per year for uncovered beds. Several investigators
such as Hossam and Saad [37], Marklund [38], Marklund [39], and Nishimura et al. [40] studied the use of conventional
sand beds.

Paved Drying Beds
Paved drying beds may be of a drainage type, decanting type, or a combination. The drainage-type paved drying beds
are rectangular and are similar to the conventional drying beds with vehicular track for cake removal. The paving is
made of concrete, asphalt, or soil cement. The drainage pipe is placed below the unpaved area of the drying bed. In the
decanting type, the sludge feed pipe is vertical and is at the center of the bed. The settled sludge is agitated periodically
by a tractor-mounted horizontal auger or other device to regularly mix and aerate the sludge to promote evaporation and
percolation. Solids concentration may range between 40 to 50 percent for 30–40 days of drying period in an arid climate
for a 30 cm sludge layer [1]. Lienard et al. [41] studied dewatering of activated sludge in experimental reed-planted and
unplanted sludge concrete drying beds. They concluded that reeds were found to contribute to maintaining a high and
regular liquid conductivity in the sludge, which allows easier and higher dosing of planted beds.
Wedge-Wire Drying Beds
In wedge-wire drying beds, artificial media made of stainless steel wire wedge and high-density polyurethane formed
into panels have been successfully utilized. Drainage and evaporation are the two mechanisms utilized to form a sludge
cake. The U.S. EPA reported the following advantages for the system: (1) no clogging of the media; (2) constant and
rapid drainage; (3) higher throughput rate than sand beds; (4) easier removal of sludge cake; (5) ability of drying
difficult-to-dewater sludge; and (6) ease of maintenance [42].
Vacuum-Assisted Drying Beds
Dewatering of sludge can be accelerated by applying vacuum to the drying bed. The operation of a vacuum-assisted
sludge-drying bed involves the application of preconditioned sludge to a depth of 30–75 cm, while solids levels of
14–23 percent are reached in the sludge cake. The solids loading per application ranges from 5 to 20 kg/m2
.
Drying Lagoons
Drying lagoons may be used as a substitute for drying beds for the dewatering of digested sludge. However, lagoons
are not suitable for dewatering of untreated sludges, limed sludges, or sludges with a high-strength supernatant because
of their odor and nuisance potential. Metcalf and Eddy [1] reported that climate, precipitation, and low temperatures
affect the performance of drying lagoons, like that of drying beds.
Lagoons are most applicable in areas with high evaporation rates. Dewatering by subsurface drainage and percolation
is limited by increasingly stringent environmental and groundwater regulations. It may be necessary to line the lagoon, if
a groundwater aquifer used for a potable water supply underlies the lagoon site. Evaporation is the main dewatering
mechanism utilized in drying lagoons. Sludge is removed mechanically, usually at a solids content of 25 to 30 percent.
The cycle time for lagoons varies from several months to several years. Solids loading ranges from 36 to 39 kg/m3
.yr of
lagoon capacity. Schweizer et al. [43] and Tarven et al. [44] conducted studies to improve the performance of sludge
lagoons by using Roller Compacted Concrete (RCC) and covers.
Vacuum Filter
A vacuum filter consists of a rotating cylindrical drum covered with a filtering material or fabric, partially submerged
in a vat of conditioned sludge. A vacuum is applied inside the drum to extract water, leaving the solids, or filter cake, on
the filter medium. As the drum completes its rotational cycle, a blade scrapes the filter cake from the filter and the cycle
begins again. There is a wide variety of filter fabrics, ranging from Dacron to stainless steel coils, each with its own
advantages. Vacuum filters produce a sludge cake with solids content ranging between 15 and 30 percent. Gingerich
et al. [45] investigated the use of applied direct pressure and constant voltage direct current to dewater anaerobically and
aerobically digested municipal sludge. They reported that final cake solids were increased to 50 percent with an applied
voltage of 60 V. Increasing pressure, voltage, or time may enhance final cake solids, and additional water may be
removed from a conventionally dewatered cake by further application of dc voltage.

Centrifuge
Centrifuges are analogous to sedimentation tanks except that the separation of suspended particles is accelerated by a
centrifugal force that is higher than the gravity force. In a typical unit, sludge is pumped into a horizontal, cylindrical
bowl, rotating at 800 to 2000 rpm. Polymers used for sludge conditioning are also injected into the centrifuge. The solids
are spun to the outside of the bowl where they are scraped out by a screw conveyor. The liquid, or concentrate, is
returned to the treatment plant, ahead of the primary sedimentation tank. Davis and Cornwell [7] reported that
centrifuges are sensitive to changes in the concentration or composition of sludge, as well as to the amount of polymer
applied. The sludge cake from the centrifuge contains 20 to 35 percent solids.
Filter Press
Filter presses are also called plate and frame presses or recessed pressure filters. Filter presses consist of round or
rectangular recessed plates that, when pressed together, form hollow chambers. A filter cloth is mounted on the face of
each individual plate. The sludge is pumped under high pressure (350–1575 kN/m2
) into the chamber. The water passes
through the cloth while the solids are retained and form a cake on the surface of the cloth. The sludge filling continues
until the press is effectively full of cake. Filter presses were reported to attain up to 40 percent solids. The filter is then
mechanically opened, and the dewatered cake drops from the chamber onto a conveyor belt for removal. Table 14 shows
typical dewatering performance of filter presses [46]. Krieger [47] discussed new developments in the field of reject and
sludge dewatering. He noted that the Kufferath Company has developed a new screw press to be used specifically for
dewatering of sludges. Rehmat et al. [48] described a laboratory scale sludge press. The effects of applied pressure and
press time on filtrate flow rate and sludge cake solids were discussed, as well as the effects of various combinations of
primary and secondary sludges.
Table 14. Typical Dewatering Performance of Filter Presses [46].
Chemical Dosage
(percent dry solids)
Type of Sludge
Feed Solids
(percent)
FeCl3 CaO
Filter Yield
(kg/m2
.h)
Cake Solids
(percent)
Primary and secondary 4 5 15 5 40
Anaerobically digested,
Primary and secondary
4 6 16 5 40
Thermally conditioned,
Primary and secondary
14 0 0 12 60
Belt Filter Press
The belt filter press operates by bending a sludge cake contained between two filter belts around a roll, which
introduces shear and compressive forces in the cake. This allows water to find its way to the surface and out of the cake,
thereby reducing the cake moisture content. The press consists of two converging belts mounted on rollers. The lower
belt is made of fine wire mesh and is very porous. As conditioned sludge moves onto the belt, some of the entrained
water drains through the belt in the gravity drain zone. The sludge passes through the press zone where the pressure
provided by the converging belts and rollers causes dewatering. Table 15 shows typical data for various types of sludges
dewatered on belt filter press [46]. Bullard et al. [49] investigated the factors that influence belt filter press performance.
They reported that characterizing the inherent dewatering potential of an activated sludge would help in refining the
treatment processes leading to enhancement of dewatering potential. Such an action can result in reduced water content
of dewatered cake solids, increased belt filter press production capacity, and reduced conditioning chemicals usage.
It was reported that the use of belt filter presses for dewatering of municipal sludges provides cost-effective solutions for
sludge dewatering [50].

Chemical conditioning is performed using either inorganic chemicals or organic polyelectrolytes. Inorganic chemicals
used include ferric chloride, lime, and a combination of ferrous sulfate and lime. For conditioning of raw or digested
sludges, prior to dewatering in presses or vacuum filters, a combination of lime and lime salts has traditionally been
used. Ferrous sulfate, ferric sulfate, or ferric chloride, may also be used. Lime is used for pH adjustment and to provide
some degree of odor control and disinfection. Calcium carbonate and other calcium compounds formed by reaction with
constituents of sludge no doubt assist in providing a granular structure for dewatering. Iron salts, which should be added
before lime, are thought to form positively-charged complexes which react with negatively-charged sludge solids and
also form hydroxides which act as flocculating agents. The introduction of polyelectrolytes in sludge conditioning has
undoubtedly been the most significant development in the past years. Essentially, both anionic and cationic
polyelectrolytes may be effective in sludge conditioning. Many polyelectrolytes are of high molecular weight and of high
charge density. The required dose of polyelectrolyte is critical to the performance of the dewatering equipment and to the
cost of operation. Several investigators used organic and inorganic chemicals, and heat as a mean of sludge conditioning
[51–58].
Sludge stabilization processes are used to convert raw wastewater sludge to inoffensive forms by decreasing the
organic content in the sludge or by otherwise rendering them inert. This is especially essential when sludge is to be
disposed of on land. This process renders the sludge or sludge end-products pathogen-free. Much recent legislative
emphasis is placed on successfully stabilizing sludge or compost products. The four major stabilization processes are
anaerobic digestion, aerobic digestion, chemical (lime, chlorine, and oxygen) stabilization, and heat treatment. Reimers
Table 15. Typical Data for Various Types of Sludge Dewatered on Belt Filter Presses [46].
Type of Sludge
Feed Solids
(percent)
Solids Loading Rate
(kg/m.belt width.h)
Polymer Dose
(g/kg)
Cake Solids
(percent)
Raw
P
WAS
P+WAS
P+TF
Anaerobically Digested
P
WAS
P+WAS
Aerobically Digested
P+WAS
P+TF
Oxygen Activated
WAS
Thermally Conditioned
P+WAS
3– 10
0.5– 4
3– 6
3– 6
3– 10
3– 4
3– 9
1– 3
4– 8
90– 180
290– 910
360– 680
45– 230
180– 590
180– 590
360– 590
40– 135
180– 680
90– 230
135– 230
90– 180
290– 910
1– 5
1– 10
1– 10
2– 8
1– 5
2– 10
2– 8
2– 8
2– 8
4– 10
0
28– 44
20– 35
20– 35
20– 40
25– 36
12– 22
18– 44
12– 20
12– 30
15– 23
25– 50
P = primary sludge; WAS = waste activated sludge; TF = trickling filter sludge.

et al. [59] reviewed current and future advances of disinfection technologies such as heat drying, biological processes,
and alkaline treatment. Switzenbaum et al. [60] evaluated different stability criteria for different disinfection processes.
The authors concluded that no single method can assess stability for all types of biosolids and recommended tests for all
stabilization methods evaluated. Vesilind and Hsu [61] noted that specific criteria based on the use of the biosolids are
needed. Leboucher et al. [62] suggested that the measurement of the potential of biosolids to generate hydrogen sulfide
be used as a stability criterion.
Anaerobic digestion is one of the most commonly employed processes for sludge stabilization. The process, which
biologically reduces the amount of volatile suspended solids that must be handled by subsequent dewatering and ultimate
disposal operations, renders the organic material nonputrescible, and destroys a large number of pathogenic organisms.
A major gaseous end-product of anaerobic digestion is methane, which is often used as a source of fuel in wastewater
treatment plants [1]. Digested sludge is an excellent soil conditioner and has found some utility for this purpose. A major
advantage associated with anaerobic digestion is its low energy requirement. Power consumption is much less than that
required for aerobic stabilization or heat treatment. The methane produced is normally more than sufficient to generate
the heat required to maintain optimum digester temperatures and thus provides a surplus energy source that can be used
elsewhere. The attractions of anaerobic digestion of sludge are methane production, 30 to 50 percent reduction in sludge
volume, odor-free sludge end product, and pathogen-free sludge.
Several investigators reviewed, described, and presented results related at anaerobic digestion [63–68].
Aerobic digestion is somewhat similar to the activated sludge process. Sludge is fed to a tank where it is mixed
aerobically. The main objective of the process is to reduce the solids content for ultimate disposal. The volatile solids are
reduced as in anaerobic digestion and, thus, a stabilized highly fertilizable humus is produced. Advantages of this
process are low capital cost, easy operation, low odor, non-explosive gas, and more purified supernatant than in
anaerobic digestion. The disadvantages are high operation costs for power and oxygen, reduced performance in cold
weather, and difficult to dewater sludge [1]. Aerobic digestion has the advantage of a complete food chain and a mixed
and varied ecology including some anaerobic and facultative organisms. The sludge produced from the aerobic process
during the first days of aeration often shows a marked increase in the sludge volume index, while drainability is severely
hampered. After about ten days of aeration, however, the situation improves.
Chemical stabilization includes chlorine and lime stabilization. Lime has long been used for sludge stabilization,
where high concentrations cause an increase in pH resulting in destruction of most of the biological life. Quicklime has
also been used for stabilization of raw and digested sewage sludge. It has been determined that a pH of 11–11.5, at 15 °C
and 4-hr detention time destroys all E. coli and Salmonella typhosa. Some spore-forming bacteria may survive, but few
pathogens form spores. Higher organisms such as hookworm and amoebic cysts may survive at least 24 hr at high pH.
Lime also tends to eliminate odors and improve sludge dewaterability. It was found that the addition of lime to raw
primary sludge produced a significant drop in specific resistance to filtration, a measure of how well a sludge dewaters.
Lime addition to raw sludge does not render it permanently stable because the pH eventually drops and surviving
organisms, or organisms that recontaminate the sludge, can create nuisance conditions.
Oxygen can also be used to stabilize sludge in either of two ways: biologically or chemically at elevated pressure and
temperature. Biological stabilization with oxygen resembles aerobic digestion except that pure oxygen is used instead of
air. Chemically, oxygen can be used to stabilize sludge at a sufficiently high temperature, pressure, and reaction time.
Sludges produced thus are sterile and normally are easily dewatered.
The heat treatment process involves heating sludge for short periods of time under pressure. It is essentially a
conditioning process, which prepares sludge for dewatering without the use of chemicals. In addition, the sludge is
sterilized and generally rendered inoffensive.
Composting is also utilized to stabilize municipal sludge for reuse purposes. Compost stability/maturity has become a
critical issue for land application of compost because immature compost can be detrimental to plant growth and the soil
environment. Wu et al. [69] compared several methods of evaluating the stability/maturity of sludge compost. They
concluded that pH, electrical conductivity (EC), CO2 evolution rate, seed germination rate, and Dissolved Organic
Carbon (DOC) could be used to monitor the stabilization and maturation processes. Vermicomposting is the use of

earthworms as a waste treatment technique, and is gaining popularity. Ndegwa et al. [70] investigated the effect of
stocking density and feeding rate on vermicomposting of sludge. They reported that vermicomposting produces a
product that is homogeneous, has desirable aesthetics, contains lower levels of contaminants, tends to hold more
nutrients over a longer period, and has no impact on the environment. Certain parameters need to be established for the
design of an efficient and economic vermicomposting system. Aggelides and Londra [71] investigated the effect of
compost produced from town wastes and sewage sludge on the physical properties of a loamy and clay soil. The organic
fertilizer was produced by composting 62 percent town wastes, 21 percent sewage sludge, and 17 percent sawdust by
volume. The product was applied to loamy and clay soils in areas characterized by a semi-arid climate. They reported
that the chemical properties of the soils were affected directly by the amendment compost. Moreover, the physical
properties of the amended soils were improved in terms of the saturated and unsaturated hydraulic conductivity, water
retention capacity, bulk density, total porosity, pore size distribution, soil resistance to penetration, aggregation, and
aggregate stability.
SLUDGE REUSE AND DISPOSAL
Sewage sludge production is a continuous process and requires a flexible and secure range of outlets for its disposal to
be economically and environmentally acceptable. The majority of wastewater sludge is disposed of on land, with
approximately three-quarters being used as a soil conditioner and the remainder buried in landfills. The predominant
treatment and disposal options available include municipal landfill, incineration (ash is landfilled), sludge farming or
composting, fertilizer, and others such as lagoons and ocean disposal.
Four options are known for utilizing sewage sludge on land as a resource, other than in agriculture. These are its use as
a forest fertilizer, as a soil conditioner for the restoration of disturbed soils, as a soil forming material for reclaiming
derelict land, and for producing soil for use on green areas in the urban environment. With the exception of ocean
dumping, all sludge treatment technologies are land based, which present formidable problems, including environmental
acceptability, air and water pollution problems, disposal of hazardous wastes on land, and groundwater and soil
contamination potentials. Several investigators have reported different means by which sludge can be disposed or
utilized [16, 72–79].
Environmental concerns regarding land disposal are surface water and groundwater pollution, contamination of the
soil and crops with toxic substances, and transmission of human and animal diseases. Problems of sludge on land may be
environmental, agronomic, and operational. Environmental problems are largely associated with emissions to the
atmosphere of odor, ammonia, and pathogens. The agronomic and operational aspects are very much interrelated and
primary concerns are soil conditions and crop response in relation to accuracy and timing of applications. The effect of
wastewater sludge in co-disposal landfills of municipal solid waste and wastewater sludge on the degradation rate and
leachate quality was investigated in field and laboratory lysimeter studies by Rohrs et al. [80]. In treatment with sludge
addition, chemical oxygen demand (COD) and nickel concentrations in the leachate were reduced, while ammonium and
phosphorus concentrations increased.
Coastal cities have discharged digested sludge into the ocean for decades. In recent years, this practice has been
questioned by regulatory agencies, particularly in the United States. The principal environmental concerns are
degradation of recreational waters, buildup of solids on the sea bottom, and toxicity to marine life. The contaminants
involved are the same as those related to disposal on land: heavy metals, pathogens, and organic pollutants. The Helsinki
agreement called for the banning of ocean dumping of sludges by 1987. By 1995 most developed countries had put this
ban into practice [10].
Another principal method of disposal is incineration. Important considerations in evaluating incineration methods
include the composition of the sludge feed and the amount of auxiliary fuel required. Air pollution constraints and
resultant equipment and treatment requirements, as well as ash disposal are also important [24]. Advantages,
disadvantages, and economical and environmental viability of various engineering designs for disposal of solid wastes,
such as wastewater sludge and municipal incinerator fly ash, were assessed and compared [81]. The effect of incineration
of municipal sludge on air quality has been studied by several researchers [82–84].

Capping of municipal sludge is a cost-effective remediation method for soft contaminated sludge and the U.S. EPA
agreed to permit capping as a remediation method. Aydilek et al. [85] investigated the consolidation characteristics of
wastewater sludge. They concluded that the finite-strain model predicted the behavior observed in both the field and
laboratory more accurately than the consolidation theory.
To eliminate the possibility of transmitting contaminants to humans, the preferred vegetation is a non-food-chain crop
such as cotton. Grass is considered acceptable, provided cattle are restricted from grazing for a specified period after
sludge application. The variety and numbers of bacteria, viruses, and parasitic organisms pathogenic to humans and
animals found in wastewater relate to the state of health of the contributing community. Although treatment processes
reduce their numbers, often considerably, the effluent and sludge would still contain some of the species.
REGULATIONS FOR THE REUSE AND DISPOSAL OF SLUDGE
The volume of municipal sludges is expected to increase significantly as a result of a number of recent environmental
developments world-wide including:
1. The Helsinki Agreement called for banning ocean dumping of sludges by 1987. By 1995 most developed countries
had put this ban into practice, e.g. New York and most of the United States by 1992; Australia and New Zealand by
1993 [10].
2. EU Environmental Directive on Urban Wastewater, requiring better than secondary treatment and in sensitive areas
nutrient removal as well [10].
3. In the United States, ‘Part 503’, the USEPA Standards for the use and disposal of sewage sludge [86].
4. In New Zealand, the public health guidelines for the safe use of sewage effluent and sewage sludge on land [87].
Application of sludge to land, where feasible, is the most desirable alternative because it uses sludge in a natural cycle.
Ideally, sludges produced from wastewater treatment processes contain all foreign matter introduced into water through
domestic use that has not been removed or transformed into neutral substances by the wastewater treatment process.
Except for chemical additions, the residues accumulated in wastewater treatment processes largely consist of organics
and minerals ultimately derived from soil. Home use of synthetic agents may introduce metals above ambient levels and
toxic compounds into sludge. Pathogens or parasites occur in sludge as a result of sewage being a carriage vehicle for
human excrement.
The U.S. Environmental Protection Agency’s Part 503 Sludge Regulations impose significant monitoring, reporting,
and recordkeeping requirements on publicly owned treatment works (POTWs). These requirements will help ensure
environmental quality with long-term application of sludge or biosolids on land. Sludge is considered clean if the
concentration of certain pollutants is not exceeded. Additionally, certain pathogens and vector reduction requirements
must also be met for land application.
Some of the common pathogens of concern that appear in municipal wastewater and sludge include ascarids (Ascaris
lumbricoides and Toxocara), whipworms (Trichuris sp.), tapeworms (Hymenolepis sp. and Taenia sp.), amoeba
(Entamoeba coli), and giardia (Giardia lamblia). The requirements for pathogens vary, depending on the classification
of the sludge, either Class A or Class B. Class A sludge has either fecal coliform densities under 1000 MPN per gram of
dry solids or Salmonella sp. densities under 3 MPN per 4 grams of dry solids. For Class B sludge, the fecal coliform
density requirement is relaxed to under 2 × 106
MPN per gram of dry solids. Class A pathogens requirements shall be
met when bulk sewage is applied to a lawn or home garden or sold or given away in a bag or other container for
application to the land. The Class B pathogen requirements and site restrictions shall be met when bulk sewage sludge is
applied to agricultural land, forest, a public contact site, or a reclamation site. Recently, and besides the U.S. EPA
standards for use or disposal of sewage sludge, the National Institute for Occupational Safety and Health released a
report which recommended that workers who handle Class B sludge should follow standard personal hygiene practices
and use appropriate personal protective equipment where needed to prevent potential health problems [88].
The pathogens in sewage sludge pose a disease risk only if there are routes by which the pathogens are brought into
contact with humans or animals. A principal route for transport of pathogens is vector transmission. Part 503 of the U.S.
EPA regulations contain 12 options, summarized in Table 16, for demonstrating reduced vector attraction of sewage

Table 16. Summary of Requirements for Vector Attraction Reduction Under Part 503 [23].
Requirement What is Required Most Appropriate for
Option 1
Option 2
Option 3
Option 4
Option 5
Option 6
Option 7
Option 8
Option 9
Option 10
Option 11
Option 12
At least 38% reduction in volatile solids during sewage sludge
treatment
Less than 17% additional volatile solid loss during bench-scale
anaerobic batch digestion of the sewage sludge for 40 additional
days at 30°C to 37°C
Less than 15% additional volatile solids reduction during bench-
scale aerobic batch digestion for 30 additional days at 20°C
Specific oxygen uptake rate at 20°C is ≤1.5 mg oxygen/hr/g total
sewage sludge solids
Aerobic treatment of the sewage sludge for at least 14 days at over
40°C with an average temperature of over 45°C
Addition of sufficient alkali to raise the pH to at least 12 at 25°C
and maintain a pH≥12 for 2 hours and a pH≥11.5 for 22 more hours
Percent solids ≥75% prior to mixing with other materials
Percent solids ≥90% prior to mixing with other materials
Sewage sludge is injected into soil so that no significant amount of
sewage sludge is present on the land surface 1 hour after injection,
except Class A sewage sludge which must be injected within 8
hours after the pathogen reduction process
Sewage sludge is incorporated into the soil within 6 hours after
application to land or placement on a surface disposal site, except
Class A sewage sludge which must be applied to or placed on the
land surface within 8 hours after the pathogen reduction process
Sewage sludge placed on a surface disposal site must be covered
with soil or other material at the end of each operating day
PH of domestic septage must be raised to ≥12 at 25°C by alkali
addition and maintained at ≥12 for 30 minutes without adding more
alkali
Sewage sludge processed by:
- Aerobic biological treatment
- Anaerobic biological treatment
- Chemical oxidation
Only for anaerobically digested sewage
sludge that cannot meet the requirement of
Option 1
Only for aerobically digested sewage sludge
with 2% or less solids that cannot meet the
requirements of Option 1 (e.g. sewage
sludges treated in extended aeration plants)
Sewage sludges from aerobic processes
(should not be used for composted sludges)
Composted sewage sludge (Options 3 and 4
are likely to be easier to meet for sludges
from other aerobic processes
Alkali-treated sewage sludge (alkalies include
lime, fly ash, kiln dust, and wood ash)
Sewage sludges treated by an aerobic or
anaerobic process (i.e. sewage sludges that do
not contain unstabilized solids generated in
primary wastewater treatment)
Sewage sludges that contain unstabilized
solids generated in primary wastewater
treatment (e.g. any heat-dried sewage sludge)
Sewage sludge applied to the land or placed
on a surface disposal site. Domestic septage
applied to agricultural land, a forest, or a
reclamation site, or placed on a surface
disposal site.
Sewage sludge applied to the land or placed
on a surface disposal site. Domestic septage
applied to agricultural land, a forest, or a
reclamation site, or placed on a surface
disposal site
Sewage sludge or domestic septage placed on
a surface disposal site
Domestic septage applied to agricultural land,
a forest, or a reclamation site or placed on a
surface disposal site

sludge [23]. These requirements are designed to either reduce the attractiveness of sewage sludge to vectors (options 1
through 8 and 12) or prevent the vectors from coming in contact with the sewage sludge (options 9 through 11).
Metals, which are toxic and bio-accumulative as well as nutrients, are constituents of interest in sludge. The
composition of sludge varies widely depending on industrial activity and treatment applied. Some typical values for
heavy metals are given in Table 10 [1]. If the sludge contains high concentrations of toxic organics, it will be a
hazardous waste subject to more restrictive disposal measures. With respect to heavy metals in sewage sludge, Section
503.13 of the U.S. EPA standards for the use or disposal of sewage sludge states that:
1. Bulk sewage sludge or sewage sludge sold or given away in a bag or other container shall not be applied to the land
if the concentration of any pollutant in the sewage sludge exceeds the ceiling concentration for the pollutant (shown
herein in Table 17) [23].
2. If bulk sewage sludge is applied to agricultural land, forest, a public contact site, or a reclamation site, either:
(i) The cumulative loading rate for each pollutant shall not exceed the cumulative pollutant loading rate for the
pollutant (Table 17), or
(ii) The concentration of each pollutant in the sewage sludge shall not exceed the concentration for the pollutant
(Table 17).
3. If bulk sewage sludge is applied to a lawn or a home garden, the concentration of each pollutant in the sewage
sludge shall not exceed the concentration for the pollutant (Table 17).
4. If sewage sludge is sold or given away in a bag or other container for application to the land, either
(i) The concentration of each pollutant in the sewage sludge shall not exceed the concentration for the pollutant
(Table 17), or
(ii) The product of the concentration of each pollutant in the sewage sludge and the annual whole sludge application
rate for the sewage sludge shall not cause the annual pollutant loading rate for the pollutant in Table 17 to be
exceeded.
In comparison, Table 18 shows the Canadian standards and guidelines on heavy metals in sewage sludges for land
application [12].
Table 17. Concentration Limits on Sludge Quality and Application [23].
Pollutant
Ceiling
Concentration
(mg/kg)
Cumulative
Pollutant Loading
Rate (kg/ha)
Monthly Average
Concentration
(mg/kg)
Annual Pollutant
Loading Rate
(kg/yr/ha)
Arsenic
Cadmium
Chromium
Copper
Lead
Mercury
Molybdenum
Nickel
Selenium
Zinc
75
85
3000
4300
840
57
75
420
100
7500
41
39
3000
1500
300
17
-
420
100
2800
41
39
1200
1500
300
17
-
420
36
2800
2.0
1.9
150
75
15
0.85
-
21
5.0
140

Table 18. Canadian Regulations on Heavy Metals in Sewage Sludge for Land Application [12].
Pollutant
Maximum Acceptable
Concentration (mg/kg)
Cumulative Acceptable Loading
(kg/ha)
Arsenic
Cadmium
Chromium
Cobalt
Copper
Lead
Mercury
Molybdenum
Nickel
Selenium
Zinc
75
20
--
150
--
500
5
20
180
14
1850
15
4
--
30
--
100
1
4
36
2.8
370
Table 19. Pollutant Concentration- Active Sewage Sludge Unit
without a Liner and Leachate Collection [23].
Pollutant Concentration (mg/kg)
Arsenic
Chromium
Nickel
73
600
420
The U.S. EPA has also regulated disposal of sewage sludge. Subpart C of part 503-Standards for the Use or Disposal
of Sewage Sludge states that, for active sewage sludge unit without liner and leachate collection system, the
concentration of pollutants shall not exceed those concentrations listed in Table 19 [23].
In Europe, the problem of sewage sludge treatment and disposal has regularly taxed the minds of legislators. Whether
at EC level or in Member States, legislation has governed these processes for many years. Over the last decade, annual
sludge production has risen as more sewage is treated to a higher standard. As an example, sludge production is currently
running at over 960 000 tonnes of dry solids (tds) per year in the U.K. About 18% of sludge was disposed of at sea until
1998 when this option was banned by the EC. Recycling sludge to agricultural land has absorbed some of the excess
(about 60% is recycled to land now), but a certain proportion is inevitably landfilled and incinerated. There is still a need
for alternative disposal routes. A recent development in this field has been the implementation of the Safe Sludge Matrix
agreed between the water companies and the British Retail Consortium. This provides agreed procedures and quality
standards to the use of sewage sludge in agriculture. The Government’s proposed amendments to the long standing
Statutory Controls for the Agricultural Use of Sewage Sludge incorporate the main tenets of this agreement. These
controls effectively give the Safe Sludge Matrix a statutory basis and introduce the concepts of treated and enhanced
treated sludge [89].
The Safe Sludge Matrix sets minimum acceptable levels of treatment and use for any sewage sludge based product
used in agriculture. This goes beyond the currently regulated cropping and grazing restrictions. From the end of 2001,
water companies will cease the application of raw sludge to farmland. Where grassland is used for grazing, treated
sludge must be injected. Land used for growing vegetables and in horticulture will receive only Advanced Treated

sludge. This is an excellent start, but many commentators in the water industry are concerned that the work put in trying
to safeguard recycling of sewage sludge to agricultural land will be undermined if the European Council of Ministers
accepts the European Commission’s revisions to the sludge regulations next year. The likely implementation date is
2004/5. There is concern that some aspects may not be possible to meet in any case.
Under the previous EU Directive it is legal to apply sludge with levels of lead up to 1 200 mg/kg on agricultural land.
The new regulations propose to reduce this level initially to 750 mg/kg in the short term, probably by 2004. In the
medium and long term, that is 2015 and 2025, it is proposed that these levels will be further reduced to 500 mg/kg and
200 mg/kg, respectively. Permissible levels for zinc, copper, cadmium, mercury, and nickel would also be reduced.
If any of these levels are exceeded, then the application of that sludge to land will not be allowed. In addition to metals
and pathogens, such as E. Coli and Salmonella, sludge treatment (or source controls) must also reduce the levels of
certain organic compounds. The organic compounds listed in the draft document include nonylphenols, polycyclic
aromatic hydrocarbons (PAHs), polychlorinated biphenols (PCBs), halogenated organics, and dioxins.
Under the new regulations, only sludge that has undergone advanced treatment, over and above the usual digestion
process that is used to treat the majority of sewage sludge, can be applied to certain categories of agricultural land.
For instance, advanced treated sludge only should be used on land growing salad or soft fruit crops. Advanced treated
sludge can also be used on most land with no or lesser restrictions on cropping methods.
Levels of heavy metals in soils, under the new proposals, will also be limited. Water companies must test the soil
before applying sewage sludge. If tests show that the limit values for heavy metals have already been reached then
sludge must not be applied.
HEALTH PROBLEMS ASSOCIATED WITH SLUDGE
Wastewater sludge typically contains organics (protein, carbohydrates, fats, oils, greases, chemicals, etc.), pathogens
(bacteria, viruses, and parasites), heavy and toxic metals, and toxins (pesticides, and household and industrial chemicals).
All of these present a risk to humans and the environment, and because of this, sludge presents not only a sizable
disposal problem, but obvious risks and hazards. Untreated sludge contains a number of harmful pollutants such as
viruses, bacteria, parasites, and fungi. Many of the viruses that cause disease in man enter the sewers with feces or other
discharges and have been identified in sludge. Wastewater treatment, particularly chemical coagulation or biological
processes followed by sedimentation, concentrates viruses in sludge. Raw primary and waste activated sludges contain
10 000 to 100 000 PFU (plaque forming units) per 100 ml.
Man may be exposed to pathogens in wastewater sludge in a variety of ways and at greatly varying concentrations.
There is no firm scientific evidence to document a single confirmed case where human disease is directly linked to
exposure to pathogens from wastewater sludge. Viable pathogens, however, have been isolated from intermediate points
in the sludge management system, such as from surface runoff from sludge treated fields. Gaspard et al. [90] performed a
parasitological analysis of helminths on 89 biosolids samples. The average concentration of helminths was 130 eggs per
100 g of dry matter.
Studies of fecal coliform and Salmonella sp. regrowth in sandy soils amended with biosolids demonstrated that, in
most cases, concentrations of these organisms decreased to below detection after a long hot, dry summer in the field [91].
CONCLUSIONS
It is clear that sludge is an unavoidable part of wastewater treatment. Measures have to be implemented in order to
deal with sludge complications. Nowadays, the area of sludge disposal is a growing concern, due to the substantial
increase in sludge production. This increase is due to the increasing awareness concerning environmental pollution and
the consequent increasingly stringent effluent discharge standards. Hence, the use of secondary and, in some locations,
tertiary treatment of wastewater was found to be a necessary practice in order to comply with those standards. As the
quantity of sludge increases, so does the number of regulations attempting to control this material, as well as other
potentially toxic or hazardous wastes. This paper has reviewed the state of the art development in sludge related research

and has covered sludge production, characteristics, treatment, utilization, related health problems, and regulations
governing reuse and disposal of sludge.
Sludge could pose health hazards if not handled properly, due to its harmful characteristics. On the other hand, sludge
can be usefully utilized as a fertilizer and soil conditioner, if properly treated.
ACKNOWLEDGEMENT
The authors would like to express their thanks to King Abdulaziz City for Science and Technology (KACST) for
providing financial support to this work, which is part of project number AR-18-28. The Research Institute of King Fahd
University of Petroleum & Minerals (KFUPM) is also to be thanked for its support.
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Paper Received 18 March 2001; Revised 21 November 2001; Accepted 12 February 2002.

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