A technical and economic evaluation of the pyrolysis of sewage
sludge for the production of bio-oil
Y. Kim, W. Parker
*
Department of Civil Engineering, University of Waterloo, Waterloo, Canada
Received 27 June 2005; received in revised form 22 January 2007; accepted 22 January 2007
Available online 26 March 2007
Abstract
Pyrolysis to produce bio-oil from sewage sludge is a promising way, to not only improve the economical value, but also to reduce
pollutants associated with sludge. The aim of this study was to evaluate the production of oil from primary, waste activated and digested
sludges. The pyrolysis was performed in a laboratory-scale horizontal batch reactor. The operating temperature ranged from 250 Cto
500 C, while a gas phase residence time of 20 min was maintained with 50 ml/min of nitrogen gas as a purge flow. The maximum oil
yield was achieved with primary sludge at 500 C. Temperature and volatile solids were the most important factors affecting the yield
of oil and char, however, sludge type also affected both results. Pre-treatment of sludge with either acids, a base or a catalyst (zeolite)
did not improve the quantity of oil produced. The economic values of the oil produced from primary, TWAS, and digested sludges were
estimated as 9.9, 5.6, and 6.9 ¢/kg-ds when the value of oil is 32 ¢/kg-oil.
2007 Elsevier Ltd. All rights reserved.
Keywords: Pyrolysis; Sewage sludge; Bio-oil; Economic estimation
1. Introduction
The management of municipal wastewater treatment
sludges is a difficult and expensive problem to solve for
many utilities. The sludge resulting from domestic waste-
water treatment processes consists of a complex hetero-
geneous mixture of organic and inorganic materials
(Metcalf and Eddy, 2002). With aerobic treatment, gener-
ally, 0.5–1 kg of sludge are produced per kilogram of bio-
logical oxygen demand (BOD
5
) treated (Eckenfelder,
2000). The solids typically contain 60–80% organic matter.
The organic materials in primary sludge are comprised of
20–30% crude protein, 6–35% fats and 8–15% carbohy-
drates (Metcalf and Eddy, 2002). Although sewage sludge
contains various valuable materials, it is often disposed
of as an undesirable and invaluable substance. Over
7·10
6
tons of dried sewage sludge were produced in
1990 in the US (McGhee, 1991). Canadian municipalities
spend $12–15 billion annually for sewage sludge treatment
(Buberoglu and Duguay, 2004).
The common disposal processes for sewage sludge
include landfilling, land application and incineration. How-
ever, conventional disposal processes have certain limita-
tions. Disposal in landfills is still the most frequently
chosen alternative for sludge in Europe and the US (Hall
and Dalimier, 1994; McGhee, 1991). Landfilling is not
always desirable because of limitations in available landfill
volume. Land application or the use of sewage sludge as a
fertilizer can result in the accumulation of harmful compo-
nents, such as toxic metals, in the soil (Vasseur et al., 1999).
Incineration is an effective way to reduce the sludge volume
and provide stabilization of the organic material in the
sludge (Werther and Ogada, 1999). When combined with
cogeneration, incineration processes can effectively recover
energy from the sludges. However, the air emissions pro-
duced from incineration are undesirable and are restricted
by regulation.
Pyrolysis is the thermal decomposition of organic sub-
stances under oxygen-free circumstances. The process
0960-8524/$ - see front matter 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.biortech.2007.01.056
*
Corresponding author.
E-mail address: wjparker@uwaterloo.ca (W. Parker).
Available online at www.sciencedirect.com
Bioresource Technology 99 (2008) 1409–1416
involves a complex series of chemical reactions to decom-
pose organic materials (Mu
¨hlen et al., 1989). The products
of sludge pyrolysis include oils (organic liquids and tar),
gases, char and reaction water. The synthesized oil, char
and gas can be used as alternative fuels and temperature
has been shown to be an important factor in determining
the yields of the various products (Campbell and Bridle,
1989; Lu et al., 1995; Caballero et al., 1997). Generally,
lower temperature conversion processes in the temperature
range between 275 C and 500 C have been used to pro-
duce oil from sewage sludge. Pyrolysis is of interest due
to the recovery of oil with low emissions of NO
x
and
SO
x
. It also avoids the formation of toxic organic com-
pounds such as dioxins, with low operating costs, as com-
pared to incineration (Werther and Ogada, 1999).
While pyrolysis of sewage sludges for the production of
oils has been of interest for some time, full scale implemen-
tation of the technology has been limited (Bridle and
Skrypski-Mantele, 2004). Acceptance of the technology
has been limited by the low economic value of the pro-
duced oil as well as the relative complexity of the process-
ing equipment. The economic viability of pyrolysis may be
improved if the yield of oil were enhanced or if value-added
products such as adsorbents could be produced from the
pyrolysis chars.
There have been a limited number of studies that have
evaluated alternative strategies for enhancing the yield of
oils from pyrolysis of wastewater sludges. The use of zeolite
as a catalyst to assist in sludge decomposition was found to
increase the production of gas and oil versus tar due to
cracking of tar (Stammabach et al., 1989). More recently,
the use of acidic pre-treatment has been employed to
enhance the adsorptive properties of chars that were gener-
ated by sludge pyrolysis (Rio et al., 2005). However, the
impact of the acid treatment on the yield of oil was not
reported. Studies that assess the impact of acid and base
treatment on the generation of oils by pyrolysis are lacking.
It was hypothesized that pre-treatments may modify the
structure of sludge-based organic matter through hydro-
lytic mechanisms and that this may result in enhanced oil
production during pyrolysis.
The objective of this study was to examine the impact of
pyrolysis conditions and the use of pre-treatments, that
may be considered for adsorbent production, on the gener-
ation of oil from a cross-section of wastewater treatment
sludges. Pre-treatments that were considered included the
use of zeolites, acids and strong bases. An energy-based
economic analysis was conducted to identify the sludge
source and pyrolysis conditions that were most economi-
cally viable for oil generation.
2. Experimental methods
2.1. Sample preparation
Two types of sewage sludges and centrifuged anaerobi-
cally digested biosolids were collected from the municipal
wastewater treatment plant in Ottawa, Canada. Primary
sludge was collected from the primary settling tank while
thickened waste activated sludge (TWAS) was generated
by Alfa Laval (Toronto, Ont.) model 76000 thickening cen-
trifuges. Digested sludge was collected as cake after anaer-
obic digestion and subsequent centrifugal dewatering by
Alfa Laval model 76000DS dewatering centrifuges.
In the laboratory, the primary sludge and TWAS were
further dewatered using a Thermo Electron model 2349
laboratory centrifuge (Waltham, MA) that was operated
at 6000 rpm for 10 min. Each sample was then dried for
24 h at 105 C in a Fisher Scientific Model 506G laboratory
air convection oven (Pittsburgh, PA) and subsequently
stored in an airtight container. The primary sludge was pul-
verized by hand as it tended to agglomerate during treat-
ment. TWAS and digested sludge did not require size
reduction.
2.2. Catalyst and pre-treatment of dried sludge samples
The use of zeolite as a potential catalyst to enhance pro-
duction of bio-oil was investigated. A zeolite (SiO
2
) with-
out alumina was blended with 5 g of dried sludge prior to
pyrolysis. Mixing ratios of 0.15, 0.2, 1 and 1.5 g-zeolite/g-
ds were evaluated to find the optimum amount of catalyst
for the dried sludge.
The agents employed for pre-treatment of the dried
sludges included 3 M HCl (pH 0.8), 98% and 0.1 M acetic
acid (pH 1.0 and 3.5) and 3 M NaOH (pH 13.5) with the
pH values measured with a Corning model 320 pH meter
(Woburn, MA) before contacting with the sludges. Pre-
cisely weighed dried sludge samples (50 g each) were
soaked in 200 ml of the agents in a 500 ml flask. The flasks
were then placed on a Lab-line Instruments model 3545
orbital shaker (Melrose Park, IL) for 20 min at 150 rpm.
After shaking, the sludges were washed several times on fil-
ter paper with deionized water to minimize solids loss and
subsequently dried at 105 C for 24 h in an air convection
oven. The mass and volatile fraction of each sludge sample
was then measured as per standard methods (APHA, 1998)
to determine the mass reduction associated with the pre-
treatment.
2.3. Experimental apparatus
A schematic of the laboratory-scale cylindrical batch
pyrolysis reactor is shown in Fig. 1. The body of the appa-
ratus consisted of a horizontal stainless steel reactor that
was 160 mm long and 40 mm in diameter. The reactor
was mounted on a 5slope inside a temperature-controlled
chamber for easy oil flow into the separator. The reactor
was wrapped with a temperature-controlled heating tape
to provide the additional heat input that was required to
achieve the pyrolytic temperatures.
Nitrogen gas was employed to maintain an oxygen-free
environment. The pressure from the gas cylinder was fixed
at 10 psig with a regulator and the flow was controlled by a
1410 Y. Kim, W. Parker / Bioresource Technology 99 (2008) 1409–1416
rotameter at a value of 50 ml/min. The gas that was ini-
tially at room temperature was preheated through a stain-
less steel tube using the latent heat in the chamber. The
preheating of the nitrogen gas minimized the temperature
drop when the gas stream entered the pyrolysis reactor.
The pyrolysis time was measured from the time that the
temperature controller indicated that the target tempera-
ture was achieved. The oil and gases formed as products
from sewage sludge pyrolysis and nitrogen gas from the
reactor were transferred to a separator through a connect-
ing tube (50 mm in length and 0.64 mm OD). The separator
(0.3 cm OD Swagelok
Tee connector) split the oil and gas
flows. The condenser located at the top of the separator
allowed the condensable gases to drip back down to the
oil collector at ambient temperatures. The mass of sludge
that could be processed in a batch varied with the differing
sludge sources but was in the range of 25–50.
2.4. Pyrolysis
In this study, 5 g of dried sewage sludge was used for
each pyrolysis run, even though some of the pretreated
sludges had lost mass during treatment. Therefore, the oil
and char yields of the pretreated sludge samples required
additional consideration in the analysis. After sample prep-
aration, 5 g of dried sample was placed in the reactor and it
was tightly sealed using caps at both ends. At least dupli-
cate runs were conducted for each combination of sludge
source, sludge pre-treatment and pyrolysis temperature.
Oil and reaction water derived during the pyrolysis was
collected in weighed and labeled 2.5 ml vials that were
located on the bottom of the separator. The reaction water
that was mixed in the collected oil was difficult to separate
mechanically in a separatory funnel as the amount of oil
was insufficient to apply this method. Hence it was
removed using an evaporation method. The oil collector
(vial) was connected to the separator with a flexible vinyl
tube. Non-condensable gases (NCG) were vented through
the condenser. After collecting the oil and reaction water,
the vials were placed in a weighed 25 ml glass test tube.
The oil residual (mainly tar) was washed from the stainless
steel tube between the separator and condenser with ace-
tone and collected. The acetone solution was placed in a
50 C water-bath for 24 h to evaporate the acetone, and
the retrieved oil mass was measured. The residual char in
the reactor was collected and weighed after cooling and
stored in airtight containers.
The masses of volatile (VS) and total (TS) solids of the
dried feed samples and chars were measured as per stan-
dard methods (APHA, 1998). The elemental composition
(C, H, N, and S) of all sludge, oil and char samples was
measured with an Elementar Americas Vario EL III ele-
mental analyzer (Mount Laurel, NJ). A Parr Instrument
series 1108 bomb calorimeter (Moline, IL) was employed
to measure the calorific values of the three types of dried
sludge samples, oils, and chars. Prior to the calorimetry
tests, the samples were mixed with diesel fuel because they
did not completely combust in the calorimeter. The mea-
sured enthalpy of the diesel fuel was 42.5 ± 2 kJ/g. The
procedure and calculation employed in the calorimetry
tests followed the method provided in the manual from
the manufacturer.
3. Results and discussions
3.1. Summary of analysis results
All subsequent data are expressed as the averages of val-
ues that were obtained from replicate measurements that
were collected in the replicate pyrolysis tests. At least dupli-
cate runs were conducted for each experimental condition
and at least duplicate measurements were taken for each
of the responses reported in this paper. All error bars that
are presented in the plots represent the 95% confidence
intervals (C.I.) that were estimated on the basis of the rep-
licate tests and replicate analytical measurements. In gen-
eral, the 95% C.I. of the oil and char yields were
estimated to be within ±2.5% of the mean values. The
Vent
1
2
3
4
5
6
7
8
9
10
11
12
16cm
4cm
13
13
T
Fig. 1. Laboratory-scale pyrolysis apparatus. 1. Nitrogen gas cylinder; 2. flowmeter; 3. preheating coil; 4. chamber; 5. reactor; 6. heating tape; 7.
thermocouple; 8. temperature controller; 9. separator; 10. vial; 11. condensing coil; 12. connector; 13. lid.
Y. Kim, W. Parker / Bioresource Technology 99 (2008) 1409–1416 1411
95% C.I. of the calorific values were found to be within
±10% of the mean values. In the elemental analysis, the
95% C.I. of the oil samples was estimated to be within
±1%, while the char and dried sludge values were within
±5% of the mean values.
The pyrolysis conditions and the corresponding charac-
teristics of the sewage sludges and products are summa-
rized in Table 1. Primary sludge had the highest VS
content with an average value of 84%, while digested sludge
had the lowest VS content with an average value of 59%.
The calorific value of the dried sludges corresponded well
with the VS and did not differ significantly between sludge
types when expressed on a VS basis with values ranging
from 27 to 30 MJ/kg-VS. The calorific value of the pro-
duced oils ranged from 36 to 39 MJ/kg-oil and did not
seem to be related to the operating temperatures and sludge
types. The calorific value of the pyrolyzed chars ranged
from 10 to 21 MJ/kg-TS and decreased, as the pyrolysis
temperature increased. This agreed with the reduced VS
content of the char that was generated at the higher tem-
peratures. The calorific values of the oils and chars were
similar to that reported by Campbell and Bridle (1986)
where values of 32–42 MJ/kg-oil and 7–23 MJ/kg-TS,
respectively were observed.
The elemental composition of the oil did not vary with
operating condition or sludge type. The values of carbon,
oxygen, nitrogen, hydrogen, and sulfur concentrations ran-
ged between 62 and 74, 8 and 22, 2.7 and 8.5, 9.5 and 9.9
and less than 1%, respectively. Oxygen composition was
determined by difference since the elemental analyzer only
measured C, H, N and S. Hence, the oxygen values
included any uncertainties in the measurements of the
other elements. The composition of the char with respect
to C, H and O content was a function of the pyrolysis tem-
perature and decreased with increasing temperature.
The energy loss values presented in Table 1 were deter-
mined from an energy balance that considered the dried
sludges and the pyrolysis products (oil and char) as shown
in Eq. (1).
Eloss ¼MdsEds ðMoilEoil þMcharEcharÞð1Þ
where, Mand Erefer to the mass and calorific values and
the subscripts ds, oil and char refer to the dried sludge, oil
and char, respectively. The calculation was based on the
energy per unit mass of dried sludge and the energy that
was present in the oil and in the char. It was assumed that
the energy loss (E
loss
) values quantified energy associated
with the gas phase. The energy of the vented gases (non-
condensable gases, NCG) was difficult to measure and they
were not considered as major products in this study.
The energy lost to the NCG as determined from the
energy balance was 3–10% (0.5–2.3 MJ/kg-ds) with the
exception of TWAS at 300 C that presented a negative
value (energy generation). In the latter case, the calculated
energy loss was less than 1% of the calorific value of the
dried sludge and hence, was within the variability that
was associated with the measurements of the solids and
energy content of the samples. The most energy loss was
observed with digested sludge at 500 C with a value of
2.3 MJ/kg-ds. For all cases, the energy loss increased with
increasing pyrolysis temperature and was less than 10% of
the initial energy content of the dried sludges. In this work,
it was assumed that all mass loss was due to NCG. The
measured total yields that the dried sludges transferred to
the oils, chars and reaction water during pyrolysis ranged
from 80% to 91%, hence 9–20% of the mass was estimated
to be lost as NCG.
Campbell and Bridle (1989) reported slightly different
results with NCG yields for digested sludge of 4–12%
and included 4–6 MJ/kg-TS at 450 C in a bench-scale
continuous system. They found the NCG yields were pri-
marily a function of operating temperature. Caballero
et al. (1997) reported the NCG was composed mainly of
CO and CO
2
.
3.2. Effect of temperature on oil and char yields
Oil yields versus pyrolysis temperature on the basis of
both TS and VS are illustrated Fig. 2. The oil yields based
on the TS of primary, TWAS and digested sludge ranged
between 8 and 42, 12 and 33 and 4 and 26 wt.%, respec-
tively. On the basis of TS, the primary and digested sludge
oil yields continuously increased with temperature, while
TWAS had an optimum yield of oil at around 400 C.
The results from primary sludge were 10–20 wt.% higher
than the other sludge types over most of the temperature
range with the exception of 250 C. The oil yields of pri-
mary and digested sludge increased more steeply over the
temperature range between 250 C and 350 C, while
TWAS increased steadily up to 400 C. These results indi-
cate that most of the oil from sludge came at temperatures
under 400 C for all sludges.
Table 1
Summary of results for sludges and pyrolysis products
Results Sludge type
Primary TWAS Digested
Dried sludge
VS fraction (wt.%) 84 69 59
Calorific value (MJ/kg-TS)
TS-based 23 19 17
VS-based 27 27 30
Oil
Yield (wt.%) 8–42 12–33 4–26
Calorific value (MJ/kg-oil) 36–38 37 38–39
Elemental composition
C 62–74 63–65 69–74
H 2.7–8.5 9.4–9.7 9.7–9.9
O 8–22 17–20 8.4–15
N 9.5–9.9 6.8–8.5 5.6–6.3
Char
Yield (wt.%) 33–85 43–77 53–87
Calorific value (MJ/kg-TS)
TS-based 17–21 13–20 10–16
VS-based 32 32–35 34–36
Energy loss (MJ/kg-ds) 0–1.6 0.16–1.9 0.5–2.3
1412 Y. Kim, W. Parker / Bioresource Technology 99 (2008) 1409–1416
The oil generated in pyrolysis would be expected to orig-
inate with the material described by the VS (Bridle, 1982).
Hence, the data were also normalized on a VS basis
(Fig. 2). The trend of yields with temperatures did not
change compared to the TS basis. However, the VS-based
yields from primary sludge and TWAS were generally sim-
ilar, while those from digested sludge were slightly lower
than the other sludges. Work by Campbell and Bridle
(1989) indicated that the VS in primary sludge and TWAS
contained more oil precursors and these were destroyed
during the sludge digestion process.
Char yields for the different sludges versus pyrolysis
temperature are presented Fig. 3. The char yields decreased
with increasing temperature as increased quantities of VS
were converted to oil and NCG. Digested sludge produced
higher char yields than the other sludges over all tempera-
tures as the digested sludge contained less VS than the
other sludges. The char yields from primary, TWAS, and
digested sludges ranged between 33 and 85, 43 and 77
and 53 and 87 wt.%, respectively. The use of the chars as
adsorbent materials has been reported by other researchers
(Lu et al., 1995; Tay et al., 2001; Chen et al., 2002). For the
purpose of this study, however, the value of the chars was
not considered.
3.3. Effect of catalyst and chemical pre-treatment
The impact of incorporating zeolite into the sludge
matrices was evaluated to determine if the yield of oil might
be enhanced in the pyrolysis process. The yields of oil and
char, based on TS, from primary sludge are presented ver-
sus the ratio of catalyst to sludge solids masses in Fig. 4
when the samples were pyrolyzed at 500 C. It can be seen
that above a ratio of 0.2 g/g the addition of catalyst
resulted in lower char yields but did not substantially
impact the yield of oil. These results suggest that addition
of the catalyst resulted in increased conversion of VS to
gas. Hence, it was concluded that while catalysts may be
useful for generating pyrolysis gases, the production of oils
was not enhanced.
Acid pre-treatment of wastewater sludges has been eval-
uated in previous studies to enhance the adsorptive proper-
ties of chars that are generated by pyrolysis. Adsorbents
may represent an alternative value-added product of pyro-
lysis that would improve the economic viability of pyro-
lysis, if both oil and adsorbents could be generated
simultaneously. It was hypothesized that either strong
acids or strong bases may hydrolyze complex organics
and hence enhance their conversion to oil in the subsequent
pyrolysis. The impacts of acid and base addition on the
mass of solids during pre-treatment and the yield of oil dur-
ing pyrolysis are presented in Figs. 5a–5c.
Pre-treatment of the sludges was observed to modify the
total mass and the VS fraction of the sludges. Total mass
reductions of primary, TWAS and digested sludge with
acid treatment ranged from 15 to 38, 9 to 26 and 8 to 26
Temperature, oC
200 250 300 350 400 450 500 550
%.tw ,liO fo dleiY
0
10
20
30
40
50
60
Open Markers: Based on VS
Closed Markers: Based on TS
Primary Sludge
TWAS
Digested Sludge
Fig. 2. Effect of temperature on oil yield for three sludge sypes.
Temperature, oC
200 250 300 350 400 450 500 550
%.tw ,rahC fo dleiY
30
40
50
60
70
80
90
100
Primary Sludge
TWAS
Digested Sludge
Fig. 3. Effect of temperature on char yield for three sludge types.
Ratio (Zeolite/ds), g/g
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6
%tw,rahC&liOfodleiY
20
25
30
35
40
45
50
Oil
Char
Fig. 4. Effect of catalyst on yields of oil and char.
Y. Kim, W. Parker / Bioresource Technology 99 (2008) 1409–1416 1413
