Environment Protection Engineering

Vol. 45 2019 No. 4

DOI: 10.5277/epe190405

KAROL KRÓL1, KRZYSZTOF ISKRA2, WIESŁAW FERENS1, JAN M. MIODOŃSKI2

TESTING PROPERTIES OF SEWAGE SLUDGE FOR ENERGY USE

The properties of sewage sludge in the context of their further energy use have been examined.
For this purpose, 34 samples of sewage sludge from municipal sewage treatment plants from the area
of Lower Silesia with a capacity higher than the 2000 population equivalent (PE) with separate sludge
management were tested. As part of the study, tests were made to determine the technological useful-
ness of fuels and their elemental composition, i.e., technical analyses and elemental analyses. The ob-
tained results show a large diversity of basic physicochemical properties of the tested sewage sludge.
The share of volatile components important for energetic use ranged from 38.4 to 59.8 wt. %. The
content of carbon (C) in the mass of tested sewage sludge ranged from 22.4 to 39.2 wt. %, which means
that they have a lower content of elemental carbon compared to solid fuels. The higher heating value
(HHV) of sewage sludge ranged from 9.3 to 17.4 MJ/kg dry mass.

1. INTRODUCTION

With ever-increasing water consumption, and therefore, the necessity to protect es-
sential for life water resources, wastewater treatment plants (WWTP) have been pro-
gressively established to eliminate negative impacts of industrial and municipal
wastewater (especially sewage from urban agglomerations) on the natural environment.
Consequently, water reusing is achievable as a result of a series of treatments and un-
complicated processes [1, 2].
Consistent with the requirements of the European Union, on 1 January 2016, Poland
introduced a legal standard [3] which imposed a ban on depositing on landfills for non-
hazardous waste the substances which contain more than 8% of dry matter (DM) of
organic materials, determined as 5% DM losses during calcination of total organic car-
bon as well as with the combustion heat above 6 MJ/kg DM. In practice, this excludes
_________________________
1Wrocław University of Science and Technology, Chair of Energy Technologies, Turbines and Modelling
of Thermal and Fluid Flow Processes, corresponding author K. Król, e-mail address: karol.krol@pwr.edu.pl
2The Institute of Environmental Protection, National Research Institute (IEP-NRI), Department of

Wastewater Technology in Wrocław.

62 K. KRÓL et al.

depositing on landfills all the wastes generated during wastewater treatment processes,
even if they have been put through washing or biologically stabilized.
Currently, numerous activities are carried out to use WWTPs generated waste again [4].
The methods of sludge reuse include, among others, recovery in composting or biogas
plants and waste thermal treatment. Taking into account restrictions in landfilling [3],
the most advantageous method of WWTP waste disposal is burning. Thermal drying is
a widely used method to evaporate water from wastewater residual solids with the use
of heat. The method involves an uncomplicated technological process and is economi-
cally justified [5]. Among 27 EU Member States, sludge thermal drying is the most
common method used in two countries, quite often used – in 12 countries, and hardly
ever – in one country [6].
In most cases, the risks due to wastewater processing have been well recognized.
The chemical factors include: heavy metals [7], volatile organic substances [8] and other
organic compounds such as: polychlorinated biphenyls (PCBs) [9], dioxins – polychlo-
rinated dibenzo-p-dioxins (PCDD) and polychlorinated dibenzofurans (PCDF) [10], as
well as polycyclic aromatic hydrocarbons (PAHs) [11]. As far as biological factors are
concerned, there have been distinguished: bacteria, fungi, viruses, protozoa [7] and sus-
pended in the air aerosols containing endotoxins [12]. In the case of thermal drying, i.e.,
removing water from a sample by drying at elevated temperature under normal or re-
duced pressure, there occurs a risk of forming explosive atmospheres (ATEX) by dust,
which may lead to explosions [13]. All industrial plants where dust is present are ex-
posed to this hazard, thus specific risk control activities should be undertaken to avoid
accidents [14]. Dust generated during thermal drying of sewage sludge is a fuel product
[15] and can cause an explosion as a result of a chemical reaction with oxygen [16].
Physical and chemical properties of sewage sludge differ depending on sampling
points. A number of factors affect sewage sludge composition, including the city’s ur-
banization level, the intensity and character of local industry and services, the pres-
ence/absence of agro-food processing or animal husbandry and the wastewater treat-
ment technology used. For the most part, sewage sludge consists of organic matter and
mineral substances [17]. About 75% of suspended solids and 40% of dissolved sub-
stances are of organic nature [18]. The characteristics of the examined sewage sludge
sample depends on sludge type, and some parameters are contingent on seasonal/tem-
poral variability and the collection site. Up to date studies [19, 20] have shown signifi-
cant physical and chemical differences between the sewage sludge types and resultant
effects on their combustion heat.
According to relevant guidelines [21], in order to fully determine sewage sludge
properties and suitability for various processes, about 150 tests should be carried out,
with different levels of validity and accuracy. However, not all chemical determinations
are needed for a particular treatment process. The regulation of the Minister of the En-
vironment [22] specifies a scope of research necessary to evaluate municipal sewage
sludge and defines reference test methods. However, the chemical determinations listed
 Testing properties of sewage sludge for energy use 63

in the regulation have no practical meaning for the preparation of thermal utilization of
given sewage sludge and do not ensure trouble-free and high-efficiency operation of the
power unit.
This paper presents the results of the study carried out with the aim to: (1) identify
physical and chemical as well as fuel properties of sewage sludge materials, (2) deter-
mine correlations between sewage sludge various parameters (the content of elements,
ash, organic matter and moisture), (3) examine the effects of individual components on
the heat of combustion, and to (4) specify the possibility of the use of sewage sludge for
energy purposes.

2. MATERIALS AND METHODS

The study was carried out on selected and then collected diversified research material,
which comprised 34 sewage sludge samples. The material was collected from municipal
WWTPs which adopted the solutions most commonly used in sewage sludge management
in Poland. These include the sequence of sludge treatment processes: thickening + bio-
logical stabilization (aerobic or anaerobic) + mechanical dewatering. The present study
focused on sludge material just from municipal WWTPs, like that from industrial facili-
ties is subject to different legal requirements and usually utilized in company heat plants
or incinerators.
Sewage sludge is a non-homogeneous material, showing differences in properties
between individual samples taken from different locations. For the purpose of this study,
the mean (representative) sample was prepared according to the Polish Standard PN-EN
ISO 5667-15:2009 from wastewater treatment plants of Lower Silesia area. Once under
laboratory conditions, the content of the containers with the samples collected were
spread over a large area so as to obtain a thin layer of sewage sludge material for further
analyses, which was afterwards dried out in a well-ventilated and well-lit room. After
reaching the humidity equilibrium with the environment, the materials tested were
ground using a Retsch laboratory mill intended for grinding fuels of different origin, to the
size of grains below 200 μm. Then the tested material was stored in tightly closed containers.
The technical analysis comprised the determination of the content of moisture, ash,
volatile matter and solid combustible parts. The elemental analysis (using an analizator
LECO TruSpec Micro) was carried out to assess the content of the following elements:
C, H, O, S, N. Additionally, the heat of combustion was determined. The technical anal-
yses were carried out in accordance with the Polish Standards for solid fuels: PN-EN
ISO 18134-3:2015-11 (determination of moisture content by gravimetric method),
PN-EN ISO 18122:2016-01 (determination of ash content by gravimetric method),
PN-ISO 1928:2002 (higher heating value and calculation of calorific value by calori-
metric methods) and PN-EN ISO 18123:2016-01 (volatile matter content by the gravi-
64 K. KRÓL et al.

metric method). The results refer to the analytical state, i.e., the state of fuel with mois-
ture and ash content equivalent to that in a sample brought to the level of equilibrium
with the surrounding atmosphere. Fixed carbon (FC) is the solid combustible residue
that remains in fuel after volatile materials and ash are expelled. There were tested
200 μm fraction, i.e., polydispersed dust from sewage sludge processed in agreement
with generally applied principles used for hard and brown coals, according to the norm
PN-G-04502: 2014-11. FC contents was calculated with the use of the following the
formula:

 FC   100    A    M    VM   (1)

where: [FC] – fixed carbon content, wt. %, [A] – ash content, wt. %, [M] – moisture
content, wt. %, [VM] – volatile matter content, wt. %.
The quotient of [FC] and [VM] is called the fuel ratio (FR). For the purpose of the
present study, the FR value was calculated with the use of the following the formula:

 FC 
FR  (2)
 VM 

FR is used to classify coal varieties: anthracite – FR at least 10, semi-anthracite – FR 6–10,

semi-bituminous – FR 3–6, bituminous – FR 3, and alternative fuels – FR 1 or less.

3. RESULTS AND DISCUSSION

The results of the technical and elemental analyses along with data on the higher
heating value (HHV) of the studied 34 sludge samples (SS) are presented in Table 1.
For comparison, Table 2 shows parameters for solid fuels. As shown in the tables, sew-
age sludge materials differ from other solid fuels in terms of the content of moisture
[M], volatiles [VM] and ash [A]. They also have a different elementary composition
when compared with solid fuels.
The results presented in Table 1 show large diversification in the basic physical and
chemical properties of the examined materials. Analytical moisture content ranges from
7.2 wt. % (SS19) to 18.2 wt. % (SS22). The water content in sludge accumulated in
the wastewater treatment process can considerably vary and reach up to 95%, which is
important as the level of moisture can considerably affect combustion. Increased mois-
ture content results in an additional amount of water vapor in the exhaust. This leads to
an increase in the volume of exhaust gas per energy unit, which in turn, in the case of
high water content, is associated with the need to build devices with appropriately larger
volumes. The results obtained show that the higher heating value of sewage sludge de-
creases with increasing moisture content (Table 1). Too high moisture content in sewage
sludge can, among others, impede ignition and decrease combustion temperature due to
 Testing properties of sewage sludge for energy use 65

heat demand for water evaporation. Reduced combustion temperature affects the com-
position of combustion products and, as a result, hazardous emission levels [24].
Table 1
The results of analyses of sewage sludge samples
Technical analysis Elemental analysis
HHV
Sample [M] [A] [VM] [FC] [C] [H] [N] [S] [O]
FR [MJ/kg]
[wt. %] [wt. %]
SS1 9.8 28.4 51.0 10.8 0.21 32.7 4.0 4.6 1.0 19.5 14.1
SS2 10.7 32.7 46.9 9.8 0.21 30.7 3.8 3.8 1.0 17.4 13.2
SS3 10.0 27.2 52.4 10.4 0.20 32.2 4.2 4.4 1.4 20.7 14.0
SS4 10.3 30.8 49.8 9.1 0.18 30.4 3.9 4.1 1.8 18.7 13.1
SS5 10.2 25.1 54.1 10.6 0.20 34.4 4.1 4.5 1.2 20.4 15.3
SS6 11.7 29.6 48.4 10.3 0.21 30.5 3.8 3.8 1.1 19.5 12.8
SS7 9.2 31.4 50.9 8.6 0.17 30.1 3.8 4.2 1.2 20.1 12.9
SS8 8.9 46.5 38.4 6.2 0.16 22.4 3.0 2.8 2.0 14.4 9.3
SS9 9.4 25.3 54.1 11.1 0.21 35.1 4.2 5.0 0.9 20.1 15.1
SS10 10.6 25.4 53.5 10.6 0.20 35.5 4.3 4.6 1.1 18.6 15.0
SS11 9.1 32.5 50.5 7.9 0.16 32.0 3.9 2.9 1.1 18.6 13.5
SS12 9.8 35.5 46.1 8.5 0.19 30.2 3.6 3.2 1.5 16.3 12.7
SS13 9.9 24.7 55.5 10.0 0.18 36.4 4.5 4.6 1.4 18.6 15.4
SS14 9.6 38.4 45.5 6.5 0.14 27.6 3.6 3.4 1.8 15.8 11.6
SS15 10.1 26.7 52.1 11.2 0.21 34.4 4.2 4.6 0.9 19.2 14.4
SS16 11.3 28.3 49.6 10.8 0.22 31.4 3.7 4.0 0.8 20.5 13.5
SS17 9.0 31.1 49.3 10.5 0.21 31.0 4.0 4.4 0.9 19.5 13.2
SS18 9.1 31.2 49.4 10.3 0.21 30.4 3.8 4.4 1.0 20.2 13.4
SS19 7.2 31.1 53.2 8.5 0.16 36.2 4.5 3.1 1.1 16.8 16.0
SS20 10.9 31.9 47.4 9.8 0.21 29.7 3.7 3.6 1.4 18.9 12.9
SS21 12.1 23.6 53.2 11.0 0.21 33.5 4.0 4.8 1.4 20.5 14.6
SS22 18.2 25.2 46.4 10.3 0.22 28.7 3.4 3.6 1.5 19.4 12.4
SS23 9.8 19.7 59.8 10.6 0.18 39.2 4.7 4.0 1.8 20.8 17.4
SS24 10.0 34.4 46.7 9.0 0.19 27.2 3.5 3.7 2.7 18.5 11.8
SS25 11.9 29.4 48.8 9.9 0.20 31.5 3.9 4.0 1.6 17.7 13.1
SS26 8.5 35.0 46.2 10.3 0.22 29.1 3.6 3.4 1.7 18.7 12.8
SS27 10.7 24.1 53.6 11.6 0.22 32.7 4.0 4.2 1.4 23.0 14.9
SS28 10.1 34.9 45.1 10.0 0.22 27.2 3.5 3.6 2.1 18.6 12.0
SS29 12.0 27.6 49.1 11.3 0.23 29.9 3.7 4.0 1.2 21.6 13.2
SS30 8.5 31.9 50.9 8.8 0.17 32.8 4.2 3.1 1.3 18.4 14.7
SS31 12.7 21.8 52.6 12.9 0.24 34.5 4.0 4.6 0.9 21.5 15.0
SS32 13.3 23.6 50.0 13.1 0.26 33.0 3.6 4.2 1.0 21.4 14.2
SS33 12.2 23.3 53.0 11.6 0.22 33.1 3.9 4.4 0.9 22.2 14.7
SS34 12.6 25.5 52.0 9.9 0.19 31.8 3.7 4.3 1.7 20.4 13.8
Min 7.2 19.7 38.4 6.2 0.14 22.4 3.0 2.8 0.8 14.4 9.3
Avg 10.6 29.2 50.2 10.1 0.20 31.7 3.9 4.0 1.3 19.3 13.7
Max 18.2 46.5 59.8 13.1 0.26 39.2 4.7 5.0 2.7 23 17.4
66 K. KRÓL et al.

Table 2
The parameters of solid fuels [25, 31]
Technical analysis Elemental analysis
HHV
Fuel [M] [A] [VM] [FC] [C] [H] [N] [S] [O]
FR [MJ/kg]
[wt. %] [wt. %]
Bituminous coal 2.0 17.3 26.9 53.8 2.00 65.4 3.7 1.3 1.2 9.1 27.8
Lignite 10.7 19.5 41.6 28.2 0.68 43.5 4.9 0.7 2.6 19.4 16.6
Biomass 2.8 2.9 76.2 18.0 0.24 47.7 5.7 0.4 0.0 40.4 18.9

The sewage sludge materials tested showed a high share of volatile matter (Table 1),
characterized by large variation: from 38.4 wt. % (SS8) to 59.8 wt. % (SS23). The gases
formed as a result of sewage sludge degassing are, likewise in the case of other fuels,
light hydrocarbons, carbon monoxide and dioxide, hydrogen and water vapor. The pro-
portions of individual components depend on the temperature at which the reaction takes
place and on the rate of heating of the combusted material. Due to the high content of
volatiles, sewage sludge can ignite at relatively low temperatures, i.e., at 470–600 °C
[25]. Therefore, sewage sludge combustion proceeds quickly, and consequently, the
process should be adequately supervised [24]. Figure 1 shows the dependence of the
fixed carbon (FC) on the volatile matter (VM) content. The FR value is between 0.26
and 0.14. A linear correlation between [FC] and [VM] was detected.

Fig. 1. Dependence of fixed carbon (FC) on the volatile matter (VM) content

The sewage sludge material tested was characterized by high and diversified ash
content ranging from 19.7 wt. % (SS23) to 46.5 wt. % (SS8). Due to considerable min-
 Testing properties of sewage sludge for energy use 67

eral content, a sewage sludge burning installation should be equipped with an appropri-
ately effective dust removal system. The high proportion of ash in fuel reduces its higher
heating value. Ash has negative effects on the heating surfaces, as it increases their ero-
sive wear and can cause the formation of slag and ash deposits [24, 26]. The analysis of
the results obtained showed a linear correlation between the amount of volatile matter
(VM) and ash (A) content. This correlation is shown in Fig. 2.

Fig. 2. Dependence of mineral substance (A) on volatile matter (VM) content

Fig. 3. Dependence of hydrogen (H) on elemental carbon (C) content

68 K. KRÓL et al.

The elemental carbon (C) content in the material tested in the present study does
not differ from the C content in sewage sludge reported in the literature [27]. In the
examined sewage sludge, C content ranged from 22.4 wt. % (SS8) up to 39.2 wt. % (SS23).
Figure 3 shows the linear correlation between hydrogen (H) and carbon (C) in content
in the examined sludge.
The content of nitrogen and sulfur in the fuels determine their potential to pose risks
related to emissions of nitrogen oxides and sulfur dioxide. The nitrogen content determined
in the sewage sludge tested in the present study is comparable with the values given by other
authors [28, 29]. The measured amount of nitrogen ranged from 2.8 wt. % (SS8) up to
5.0 wt. % (SS9). Sulfur content ranged from 0.8 wt. % (SS16) up to a maximum 2.8 wt. %
(SS24, Table 1). Similar values for this type of waste are given in the available literature
[28, 29].

Fig. 4. van Krevelen diagram of various fuels based on [30] and tested sewage sludge

The properties of alternative fuels are very different from those of coals. The tested
sewage sludge materials contained less elemental C when compared with anthracite and
hard coals, thus H/C ratios in sewage sludge materials showed higher values. In addi-
tion, increased oxygen (O) content observed in sewage sludge examined led to higher
O/C ratio and the lower heat of combustion when compared with carbon fuels (Fig. 4).
The atomic ratio is a measure of the ratio of atoms of one kind to another kind. A closely
related concept is the atomic per cent (or at.%), which gives the percentage of one kind
of atom relative to the total number of atoms. The results presented in Fig. 4 confirm
that the sewage sludge materials under investigation can be regarded biofuels, however,
due to the increased content of mineral matter and moisture, this does not mean that
sewage sludge has similar properties to those of biomass.
 Testing properties of sewage sludge for energy use 69

As part of the conducted study, there were compared sewage sludge fuel properties
(inside a relevant fuel group). There was also attempted to systematize sewage sludge
in search of the correlations (Figs. 1–3) between the properties characteristic of the sew-
age sludge materials tested. The content of elemental carbon constitutes a very im-
portant component of sewage sludge materials, decisive of their technological useful-
ness. Figure 5 shows the dependence of the higher heating value (HHV) on the content
of elemental carbon (C). With increasing C content, there increases energy from fuel
combustion caused by enlarged available chemical energy in fuel mass unit [27, 31].

Fig. 5. Dependence of the higher heating value (HHV) on the elemental carbon (C) content

Fig. 6. Dependence of the higher heating value (HHV) of the volatile matter (VM) content
70 K. KRÓL et al.

Figures 3 and 5 show that with increasing carbon (C) and hydrogen (H) content,
there increases the higher heating value of the sewage sludge investigated. Due to the
fact that the heat of combustion of fuels depends on the flammable components H and C,
the increased content of these elements cause an increase in the energy released during
combustion [31].
Figure 6 shows the correlation between the content of volatiles (VM) and the higher
heating values (HHV). Sewage sludge HHVs increase proportionally with the increase
of VM content. Consistent with Fig. 2, [VM] is inversely proportional to those of ash
(A). At the same time, VM amounts are proportional to those of the solid combustible
flammable residue (FC) (Fig. 1), therefore an increase in volatile content entails an in-
crease in flammable residue amounts, which increases the heat of combustion [24–26].

Fig. 7. Dependence of the higher heating value (HHV)

on the moisture content with mineral substance content [M] +[A])

Figure 7 shows the correlation between the moisture and mineral content and the higher
heating value. The increase in the amount of ballast in fuel (moisture M + ash A) reduces
the share of combustible fraction, so the observed dependence is in line with expectations.
Figures 6 and 7 show a strong dependence of higher heating value (HHV) on the available
quantity of combustible parts. This indicates a similar composition (HHV) of the combus-
tible substance of the tested sewage sludge samples. This is in line with the correlation
shown in Figs. 2 and 6 because the increase in ballast in the form of M + A will result in
a reduction of the available energy released during combustion [24, 26].
To sum up, Figures 1, 3, 5 and 6 show that with increasing combustible residue
(FC), carbon (C), hydrogen (H) and volatile matter (VM) contents, there still occurs an
increase in the higher heating value, while with the increase of moisture (M) and mineral
substance (A) contents (Figs. 7 and 2), the higher heating value decreases.
 Testing properties of sewage sludge for energy use 71

Sewage sludge tests showed similar content and a strong correlation between hy-
drogen and carbon content. To determine higher heating value, a formula was used that
estimates the HHV value based on combustible elements such as C, H, S and fuel ballast
in the form of elementary oxygen (O). For the purpose of the present study, Mendeelev
formula was used (for solid fuels in the analytical state):

HHV  339.15  C   1255.9  H   108.9  S   108.9  O  (3)

The above formula is mainly used for fossil fuels, and especially for hard coal. Fig-
ure 8 shows the HHV of samples, calculated on the basis of Mendeleev formula using
data from Table 1 compared to the HHV determined in accordance with the Polish
Standard PN-ISO 1928:2002.

Fig. 8. HHV calculated with the use of Mendeleev formula

vs. HHV determined in accordance with the Standard PN-ISO 1928:2002

As a result of the simulation, it was found that in the case of the tested sewage sludge
materials, the Mendeleev formula is applicable to calculate the HHV. It is, therefore,
possible to determine the HHV just on the basis of the elemental analysis. The maxi-
mum uncertainty of determination is below 8% of the measurement value (Fig. 8).

4. CONCLUSIONS

The results of the analyses of municipal sewage sludge samples refer only to alter-
native fuels tested in the present study. Although the results of the simulation show high
72 K. KRÓL et al.

conformity, further research should be carried out to unambiguously answer the ques-
tion on applicability of the Mendeleev formula to calculate the higher heating values of
alternative fuels based on elemental analysis.
As shown in the present study, sewage sludge materials differ from other solid fuels
in terms of the content of moisture, volatiles and ash. They also have a different ele-
mentary composition when compared with solid fuels, especially – with bituminous
coal. However, sewage sludge can be a valuable complementary fuel that can be suc-
cessfully co-combusted in the power industry. In the case of large and medium-sized
cities, where the quality of sewage sludge is affected by industrial pollution, thus it may
be insufficient when evaluated in line with agricultural or natural criteria, combustion
or co-burning with energy recovery seems to be the best alternative among existing and
allowed methods of sewage sludge utilization.

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