Economic and environmental evaluation of municipal sewage sludge
treatment methods
RIGAS F., KORDOUTIS K.
School of Chemical Engineering
National Technical University of Athens
Iroon Polytechniou 9, Zografou, 15700 Athens, Greece
http://www.chemeng.ntua.gr
Abstract: -. This work aims at designing and comparing alternative methods of sewage sludge treatment and
finding the best solution based on economic and environmental criteria. The SuperPro Designer software was
used to work out data found in the literature. Five alternative projects were designed for the sludge treatment
in a continuous operating mode. The influent of each project varies in quantity, corresponding to the flow rate
of sludge from the wastewater treatment plants. All plants are assumed to treat sewage sludge derived from
populations of between 10,000 and 5,000,000 equivalent population (EP). The first project includes anaerobic
digestion for the stabilization of the sludge, dewatering via a belt-filter press and finally incineration in a
multiple hearth incinerator. The second project involves aerobic digestion for sludge stabilization, followed by
dewatering and incineration. The third project deals with wet-air sludge oxidation, thickening and dewatering.
The fourth project introduces alkaline thermal sludge hydrolysis, while the fifth one uses oxidation with
hydrogen peroxide for sludge stabilization. In the last two projects, sludge is dewatered and incinerated for
further mass reduction. The treatment of data from literature with SuperPro Designer provided results useful
for economic and environmental evaluation of the studied projects.
Key-Words: sewage sludge treatment, economic and environmental evaluation, SuperPro Designer
1 Introduction
Sewage sludge is produced worldwide in
increasingly great quantities in municipal
wastewater treatment plants, during primary and
secondary treatments. The increase in sludge
production is caused by the increase of households
connected to wastewater treatment plants, the fact
that laws governing effluent discharges are
becoming stricter and the availability of advanced
technological methods, able to achieve more
efficient wastewater purification. The impact of
increased environmental awareness globally should
also not be ignored. On the other hand, due to
raising transportation and disposal costs, great effort
is made to reduce the quantity of sludge for final
disposal. Therefore, the final quantity of sludge to
be disposed of depends on the environmental,
financial and technological limitations of a city or
country [1].
Given that sludge is the mixture of primary
sedimentation of raw wastewater and secondary
sedimentation of biologically treated wastewater, it
varies in quantity and quality depending on many
factors, such as treatment parameters, sewage
sources, climate and regional conditions. Therefore,
it forms the main by-product of wastewater
treatment plants and it concentrates most of sewage
polluting, infectious, and undesirable components.
These can be inorganic, organic and toxic
substances and pathogenic or other microorganisms.
Sewage sludge treatment aims at reducing the
quantity of organic solids, eliminating pathogenic
bacteria and improving dewatering ability, so that
less environmental and economic drawbacks will
occur at the final disposal. These can be achieved
through
sludge
stabilization,
conditioning,
dewatering and thermal processing.
Concentrated sludge poses severe threats to the
environment and therefore, needs to be deactivated
through stabilization before disposal. Worldwide,
the most commonly applied method is digestion.
Digestion achieves sludge volume reduction and
stabilization at the same time, either with aerobic or
anaerobic conditions.Recent research focuses on
improving the dewatering ability of sludge through
stabilization processes and achieving lower water
content after dewatering process. Laboratory
experiments indicated that alkaline thermal
hydrolysis [2] and oxidation with hydrogen peroxide
[3] are two alternative promising methods, able to
achieve a high percentage of solids in sludge cake
after dewatering.
Sludge conditioning also improves dewatering
ability. Thermal processes or chemical additives,
either organic (polyelectrolytes) or inorganic
chemicals, are used to achieve conditioning. The
main advantage of polymers is that they do not
increase considerably the mass of final sludge,
while every kilogram of inorganic chemicals added
remains to the dry solids.
Stricter regulations for sludge disposal and
restriction in available disposal areas limit the
disposal options and raise the disposal cost.
Therefore, thermal treatment methods have become
more suitable to recent economic and environmental
requirements.
Incineration is the thermal process that
implements high temperature in excess air
conditions to partially or totally oxidize sludge
organic content mainly to carbon dioxide and water
[4]. The main objective is to reduce the total solid
quantity for disposal. Efficient dewatering of sludge
affects the amount of supplementary fuel needed for
combustion [5].
2 Conceptual process design
Five projects were designed for sewage sludge
treatment in continuous operating mode, using
SuperPro Designer software. Stabilization and
conditioning methods are of paramount importance
in sludge treatment. Utilizing the particular software
capabilities, all methods were chosen either for their
application worldwide, such as anaerobic and
aerobic digestion, or for the improved results and the
demand for minimization of the final quantity.
2.1 Influent Quality and Quantity
The quality and quantity of sewage sludge are
assumed to be the same for every alternative project
as shown in Τable 1.
According to Kiely [6], typical rate for
municipal wastewater is 225L/capita/day and
800mg/L or 180gr/capita/day total solids. Suspended
solids are about 40% of total solids or 320mg/L.
From these, about 200mg/L usually separate through
physical methods. The other 60% of total solids,
about 450mg/L, remain dissolved in wastewater as
part of BOD5 and COD. Through chemical,
biochemical and biological wastewater treatment in
municipal plants a ratio of about 2/3 ends up in
sludge. Therefore, sludge produced after municipal
wastewater treatment is estimated at 120gr
DS/capita/day. Based on this estimation, wastewater
treatment in a plant of a city of 100,000 EP produces
12,000 Kg of dry solids, daily.
Table 1. Sludge main components.
Component
Mass
Flow rate
Concentration
Content
(kg/h)
(gr/L)
(%)
Alkalinity
5.760
(CaCO3)
0.005
Cd
101.775
Cellulose
0.250
Cr
0.400
Cu
152.662
Fats
8.500
Fe
0.250
Pb
0.040
Ni
2.500
K
183.304
Proteins
43.750
Si
9,500.000
Water
0.850
Zn
Total(kg/h) 10,000.045
Total(L/h) 10,044.500
0.058
0.573
0.000
1.018
0.002
0.004
1.527
0.085
0.002
0.000
0.025
1.833
0.437
95.000
0.008
0.001
10.132
0.025
0.039
15.198
0.846
0.025
0.004
0.249
18.249
4.356
945.791
0.085
2.2 Alternative Sludge Treatment Projects
2.2.1 Project No. 1 – Anaerobic Digestion
Anaerobic digestion of sewage sludge is a controlled
partial degradation of volatile solids and other
organic compounds by anaerobic bacteria,
producing a gas mixture of mainly methane, carbon
dioxide and hydrogen traces as well as stabilized
innocuous sludge. Optimum conditions for an
efficient methane production include generally good
mixing, sludge temperature between 30 and 350C
and residence time for the high-rate digestion of
about fifteen days.
During anaerobic digestion (Fig. 1) of sewage
sludge, volatile solids get hydrolyzed by
microorganisms to simpler dissolved organic
compounds, such as amino acids, fatty acids and
glucose with hydrolysis constant of 0.04-0.13/day
for cellulose, 0.08-1.7/day for fats and 0.02-0.03/day
for proteins according to Malina and Pohland [7].
Acidogenic bacteria convert these simpler organic
compounds to volatile acids, carbon dioxide and
traces of gas hydrogen. Anaerobic methanogenic
bacteria produce methane using volatile acids.
Acidogenesis and methanogenesis follow the Monod
model.
Volatile solids turn to methane, carbon dioxide
and hydrogen traces at a percentage of about 54%.
The gas product contains 65% CH4 and 35% CO2
and outflow for further utilization.
Digested sludge gets conditioning by use of
polymer additives. Subsequently, sludge gets
dewatered in a belt filter press and the sludge cake
(24%) ends up to a multiple hearth incinerator.
2.2.2 Project No. 2 – Aerobic Digestion
During aerobic digestion, biological mechanisms
destroy partially the degradable organic components
of
sludge
and
reduce
the
pathogenic
microorganisms. According to WEF [5], cell-tissues
get oxidized aerobically to carbon dioxide, water
and nitric salts or ammonia during digestion. The
remaining sludge after digestion is in such a low
energy level that is actually biologically stabilized.
Consequently, it is suitable for a variety of
alternative disposal methods. Oxygen requirements
of aerobic digestion depend on the operating
temperature, the existence of primary sludge and the
residence time of active sludge systems, where
secondary sludge derives from.
The aerobic digester operates in ambient
conditions with a hydraulic residence time of fifteen
days, in continuous operating mode (Fig. 2).
Volatile solids and biomass get oxidized at about
42% by microorganisms to carbon dioxide, water,
ammonia and partially nitrates, following first
degree kinetic with constant kd<0,08 days according
to WEF [8]. Conditioning and dewatering follow
digestion. Dewatered sludge (18%cake) ends up to a
multiple hearth incinerator.
2.2.3 Project No. 3 – Wet-air Oxidation
Wet-air oxidation is a thermal process that
makes use of high temperature and pressure
conditions to oxidize the organic content of sludge
in liquid phase with air oxygen (Fig. 3). All sludge
organic solids get oxidized under high temperature
(about 2320C) and pressure (about 38 bar)
conditions. The whole process takes place in liquid
state and the oxygen required is provided through an
air stream. According to Kiely [6] and WEF [5], one
hour residence time results in 90% oxidation of
volatile solids. The liquid effluent follows
concentration to increase the solid content and
finally ends up dewatered in a belt filter press.
Project No. 4 – Alkaline Thermal
Hydrolysis
Sludge and Ca(OH)2 enter the reactor at a ratio of
0.068gr Ca(OH)2/g DS. The reactor (Fig. 4) operates
at 1000C on continuous mode and pH=10 [2]. After
1 hour residence time all pathogens are eliminated
and then the sludge comes through a heat exchanger
to reduce temperature. In sequence, sludge is
conditioned with a polymer additive and dewatered
in a belt filter press. The dewatered sludge cake
(37.7%) ends up in the multiple hearth incinerator.
2.2.4
2.2.5 Project No. 5 – Oxidation with H2O2
In ambient conditions sludge is mixed in the reactor
with sulfuric acid in ratio 145gr H2SO4/kg DS
(stream concentration 1750 H2SO4 gr/L) and ferrous
sulfate in ratio 1.67gr FeSO4/kg DS adjusting pH=3
and hydrogen peroxide (stream concentration
390gr/L) in ratio 25gr H2O2/kg DS, in continuous
operating mode (Fig. 5). Organic compounds
convert to carbon dioxide and water following the
Fenton’s reagent chain [3]. Although, stabilizing is
based on the mechanism of the Fenton’s
peroxidation, which is well known, the mechanisms
responsible for enhanced properties of the treated
sludge are not fully understood. The oxidative
conditioning might be based on partial oxidation and
rearrangement of the surface components of the
sludge flocs. After a residence time of 75 minutes,
the sludge mixture is neutralized with Ca(OH)2 in
ratio of 55,6gr Ca(OH)2/Κg DS and polyelectrolyte
(PE) is added.
Finally, the sludge is dewatered in the belt filter
press and the sludge cake (43%) end up to the
incinerator.
3 Economic and Environmental
Analysis
The processing of data from literature using the
SuperPro Designer provided results useful for
economic and environmental evaluation of the
alternative projects.
3.1 Economic Analysis
Economic analysis of sludge treatment projects aims
at comparing alternative methods investment and
operating cost and indicating those of lower total
cost. Table 2 shows basic economic parameters of
the alternative projects, such as total investment
cost, operating cost, purchase equipment cost and
Net Present Value (NPV) of each investment for a
particular plant capacity, needed for 2,000,000
equivalent population. NPV was calculated for
fifteen years investment time with interest i m=7%.
Minus NPV means the investments are not
profitable. For a thorough comparison, economic
sizes were studied for a wide range of plant capacity
representing population between 10,000 and
5,000,000 EP. Figure 6 shows the total cost for each
project.
Table 2. Main economic parameters.
PROCESSING
TOTAL CAPITAL OPERATING
RATE (kg/year of
INVESTMENT ($) COST ($/year)
Influent)
Cost Item
Project No. 1 –
Anaerobic Digestion
Project No. 2 – Aerobic
Digestion
Project No. 3 – Wet-air
Oxidation
Project No. 4 – Alkaline
Thermal Hydrolysis
Project No. 5 –
Oxidation with H2O2
EQUIPMENT
PURCHASE
COST ($)
NPV
(at 7.0 %
interest)($)
38,704,000
7,635,000
79,200,357
6,594,000
-93,324,000
61,931,000
12,155,000
79,200,357
10,551,000
-148,745,000
43,054,000
8,694,000
79,200,357
6,585,000
-105,242,000
33,881,000
7,598,000
79,200,357
5,563,000
-88,709,000
32,701,000
7,630,000
79,200,357
5,366,000
-87,803,000
Dewatering results are of great importance
regarding economic and environmental issues, too.
Table 4 shows the solid mass content of dewatered
sludge for all projects.
3.2 Environmental Analysis
Stricter regulations for sludge disposal and
restriction in available disposal areas limit the
disposal options and raise the disposal cost.
Therefore, there is an increasing demand for
minimizing the final product for disposal. Due to
this fact, incineration was chosen for further sludge
reduction in the projects, apart from wet-air
oxidation.
Incineration produces mainly carbon dioxide,
vapor and ash. The final solid product (ash) for each
project is shown in Table 3. Comparing the results,
the projects processing aerobic and anaerobic
digestion produce less ash.
Table 4. Solid content after dewatering.
Projects
Project No. 1 – Anaerobic Digestion
Project No. 2 – Aerobic Digestion
Project No. 3 – Wet-air Oxidation
Project No. 4 – Alkaline Thermal
Hydrolysis
Project No. 5 – Oxidation with H2O2
Table 3. Ash product.
Projects
Project No. 1 – Anaerobic Digestion
Project No. 2 – Aerobic Digestion
Project No. 3 – Wet-air Oxidation
Project No. 4 – Alkaline Thermal
Hydrolysis
Project No. 5 – Oxidation with H2O2
DS – sludge
cake (mass
%)
24
18
22
38
43
Outflow environmental parameters of each
stabilizing method are presented in Table 5. Wet-air
oxidation and anaerobic digestion reduce
significantly the sludge organic content and the main
environmental parameters, while aerobic digestion is
less efficient in stabilizing sludge.
Ash (kg/h)
55.906
55.907
76.983
84.813
Table 5. Basic environmental parameters for stabilization outflow.
STREAM
PROPERTIES
Influent
Anaerobic
Digestion
Effluent
Aerobic
Digestion
Effluent
Wet-air
Oxidation
Effluent
Alkaline Thermal
Hydrolysis
Effluent
Oxidation with
Η2Ο2 Effluent
TOC (mg C/L)
24,804
9,661
14,528
2,508
22,090
20,618
COD (mg O/L)
87,768
27,896
55,663
8,874
77,750
76,353
BODu (mg O/L)
71,144
23,937
45,944
7,193
59,174
58,289
BOD5 (mg O/L)
55,904
18,658
32,959
5,652
46,864
43,623
TKN (mg N/L)
2,080
412
2,152
210
1,978
2,480
365
72
215
37
347
358
TS (mg Slds/L)
43,729
3,836
28,655
4,557
26,787
34,836
TSS (mg Slds/L)
22,798
155
13,408
2,305
0.0
12,728
VSS (mg Slds/L)
17,098
20,932
132
3,681
10,071
15,247
1,729
2,252
0.0
26,787
9,546
22,108
TP (mg P/L)
TDS (mg Slds/L)
Fig. 1. Project No. 1 – Anaerobic Digestion.
Fig. 2. Project No. 2 – Aerobic Digestion.
Fig. 3. Project No. 3 – Wet-air Oxidation.
Fig. 4. Project No. 4 – Alkaline Thermal Hydrolysis.
Fig. 5. Project No. 5 – Oxidation with H2O2.
Anaerobic
Digestion
Millions
9000
Aerobic
Digestion
Total Cost ($)
6000
Wet-air
Oxidation
Alkaline
Thermal
Hydrolysis
3000
operating cost for each alternative scenario, known
that this process requires high cost equipment and
high operating cost. The more efficient stabilization
and dewatering processes are the less expensive
incineration and final disposal become.

Wet-air oxidation is very efficient in sludge
volume and pollutant reduction. However, making
use of high temperature and pressure conditions, the
mechanical equipment becomes very expensive and
the method presents high operating cost.
Oxidation
with H2O2
0
0
100000
200000
300000
400000
500000
Capacity (Sludge Influent - Kg/h)
Fig. 6. Total cost for variable capacity.
4 Conclusion

The economic evaluation of the proposed
projects indicates that the projects No. 4 and 5 that
include alkaline thermal hydrolysis and stabilization
with hydrogen peroxide have lower total costs, for
sewage sludge treatment plants suitable for
equivalent populations between 10,000 and
5,000,000. For populations greater than 2,000,000
EP, the first project that includes anaerobic digestion
for sludge stabilization was found to be the most
economical investment. Generally, the projects that
include wet-air oxidation and aerobic digestion have
comparatively higher total costs.

The environmental evaluation of the alternative
projects showed that the use of alkaline thermal
hydrolysis and oxidation with hydrogen peroxide for
stabilization causes overloading of the effluent with
inorganic additives. Therefore, solid residue (ash)
for final disposal is increased.

Anaerobic digestion decreases considerably
sludge pollutants and volume by changing partially
the volatile solids to methane, which can also be
utilized for energy production for the needs of the
plant. Meanwhile, the whole scenario proves to be
the most inexpensive for great scale plants.

The methods of alkaline thermal hydrolysis and
oxidation with hydrogen peroxide do not reduce
sufficiently sludge environmental parameters. They
mainly improve dewatering ability and they reduce
the sludge volume. Moreover, these methods require
cheap equipment and low operating cost.

Implementing incineration for the further
reduction of sludge for final disposal, increases
considerably the cost of investment and the
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