Kinetics of organic matter removal and humification progress during sewage sludge composting
Waste Management xxx (2016) xxx–xxx
Contents lists available at ScienceDirect
Waste Management
journal homepage: www.elsevier.com/locate/wasman
Kinetics of organic matter removal and humification progress during
sewage sludge composting
Dorota Kulikowska
University of Warmia and Mazury in Olsztyn, Department of Environmental Biotechnology, Słoneczna St. 45G, 10-709 Olsztyn, Poland
a r t i c l e
i n f o
Article history:
Received 25 August 2015
Revised 31 December 2015
Accepted 4 January 2016
Available online xxxx
Keywords:
Sewage sludge
Composting
Kinetic constants
Organics
Humic substances
Humic acids
a b s t r a c t
This study investigated the kinetics of organic matter (OM) removal and humification during composting
of sewage sludge and lignocellulosic waste (wood chips, wheat straw, leaves) in an aerated bioreactor.
Both OM degradation and humification (humic substances, HS, and humic acids, HA formation)
proceeded according to 1. order kinetics. The rate constant of OM degradation was 0.196 d 1, and the
rate of OM degradation was 39.4 mg/g OM d. The kinetic constants of HS and HA formation were
0.044 d 1 and 0.045 d 1, whereas the rates of HS and HA formation were 3.46 mg C/g OM d and
3.24 mg C/g OM d, respectively. The concentration profiles of HS and HA indicated that humification
occurred most intensively during the first 3 months of composting. The high content of HS
(182 mg C/g OM) in the final product indicated that the compost could be used in soil remediation as a
source of HS for treating soils highly contaminated with heavy metals.
Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction
Municipal sewage sludge can be a source of valuable fertilizer,
due to its high content of organics, nitrogen, phosphorus and trace
elements. However, the presence of pathogenic organisms may
pose health risks, limiting the direct application of sludge to soil
fertilization. Therefore, sludge should be treated prior to application by methods such as composting.
The composting process lowers sewage sludge mass and
moisture content. In addition, thermophilic conditions destroy
pathogenic organisms present in waste and ensure complete hygienization of the compost. Additionally, compost contains relatively
low concentrations of heavy metals and fulfills the requirements
for soil amendments in most cases. For example, earlier study
showed that the metal concentration in sewage sludge compost
poses low ecological risk ( proteinaceous compounds > polysaccharide-like matter and lignin.
In the present study, OM removal efficiency (EOM,loss) and the
values of kOM and rOM were relatively high, which indicates that,
despite a C/N ratio that was lower than optimal, the composting
process proceeded efficiently. This could be due to the high content
of readily biodegradable organics in the sewage sludge, which constituted 65% of the feedstock. Efficient composting at a relatively
low C/N ratio is important from a practical point of view, as it
means that when composting sewage sludge, which is usually
characterized by a low C/N ratio, there is no need to add very large
amounts of amendments. Although the process may proceed more
efficiently in optimal conditions, this would require large amounts
of amendments (e.g. straw or leaves), which would reduce the proportion of sewage sludge and influence the economy of the composting process. Although nitrogen is lost from the system as
ammonia during composting at a low C/N ratio, and conservation
of nitrogen in the composted biomass is one of the objectives of
composting, the high N content of sewage sludge means that the
mature compost will still fulfill the requirements for fertilizers
even if it is composted at a low C/N ratio. In the present study, N
content in the final compost was 2.11%. Relatively high N content
in compost produced from feedstock characterized by a low C/N
ratio has also been confirmed by earlier research on sewage sludge
composting in a two-stage system with rape straw and grass as
amendments (Kulikowska and Gusiatin, 2015).
80
60
40
20
0
0
20
40
60
80
100
120
140
160
180
composting time [d]
7 cm from the upper edge of bioreactor
63 cm from the upper edge of
(b)
moisture content [%]
4
80
60
40
20
0
0
20
40
60
80
100
120
140
160
180
composting time [d]
Fig. 2. Temperature (a) and moisture (b) profiles during sewage sludge composting.
lasted 19 and 24 days in the upper and middle layers, respectively.
The maximum temperature during composting and the length of
the thermophilic phase depend not only on the content of readily
biodegradable organic compounds but also on the C/N ratio. In a
study by Huang et al. (2004), thermophilic conditions were
reached after 3 days and lasted for 40 days during composting of
manure in a windrow at a C/N ratio of 30; the maximum temperature was 69 °C. At a lower C/N ratio (C/N 15), they observed that
thermophilic conditions were reached after 7 days, the thermophilic phase was shorter and the maximum temperature was
lower (60 °C). According to the authors, the reduction of the maximum temperature and the shortening of the thermophilic phase
were associated with insufficient carbon content in the composted
mass (low C/N). However, Goyal et al. (2005) showed that the temperature profiles of compost depend more on the biodegradability
of the OM present in the waste than the C/N ratio. The authors
composted sugar cane waste with cattle manure at ratios of 4:1
and 1:1, sugar cane waste alone and water hyacinth biomass.
The C/N ratios in these wastes were as follows: 51.1, 32.0, 14.5
and 18.1. They found that, regardless of the type of waste, the temperature did not exceed 46 °C during 90 days of composting. The
main reason for the low temperature of the compost was the presence of high amounts of fibres, mainly lignin, as heat generation is
associated with intensely occurring decomposition of readily
biodegradable organic compounds.
In addition to temperature, moisture is an important factor during composting. The correct moisture content is a compromise
between allowing oxygen flow to maintain aerobic conditions, on
the one hand, and having enough water for microorganisms to
move and transport nutrients, on the other. If this balance is not
achieved, the metabolic and physiological activities of the desired
microorganisms will be impaired. When the moisture content of
composting material is too low, it can slow or stop the biological
processes. With too much water, pores (holes) in the composting
materials can be filled with water, which reduces oxygen penetration and creates anaerobic processes.
Although sources differ slightly, there is general agreement that
moisture contents around 60–70% are best for process efficiency.
For example, Liang et al. (2003) studied, among other factors, the
Please cite this article in press as: Kulikowska, D. Kinetics of organic matter removal and humification progress during sewage sludge composting. Waste
Management (2016), http://dx.doi.org/10.1016/j.wasman.2016.01.005
5
D. Kulikowska / Waste Management xxx (2016) xxx–xxx
HS [mg C/g OM]
(a)
200
150
50
20
0
40
60
80
100
120
140 160 180
composting time [d]
(b)
HS
k HA = 0.045 d-1
C max,HA = 72.1 mg C/g OM
r HA = 3.24 mg C/g OM·d
200
150
80
60
100
40
50
20
0
20
0
40
60
80
100
120
140
160
L-HA [mg C/g OM]
1. order kinetic
0
180
composting time [d]
1. order kinetic
3.2. Organic matter humification
During the humification process, there are changes in the concentrations of HS and their fractions, i.e. HA and FA, in composting
waste. During this process, the concentration of FA decreases,
while that of HA increases, which has been confirmed by many
authors (Inbar et al., 1990; Veeken et al., 2000; Paredes et al.,
2002; Domeizel et al., 2004; Jouraiphy et al., 2005; Amir et al.,
2006). Studies on the HS content in compost are quite numerous,
but the author of the present study is unaware of any information
on the kinetics of the process. This information is important
because it provides information about the time at which HS are
most intensively formed.
In this study, the content of HS and HA was already high in the
feedstock (HS 118 mg C/g OM, HA 57 mg C/g OM), due to the large
share of sewage sludge (Fig. 3). It is believed that during wastewater treatment, humification occurs simultaneously with the mineralization of OM. Réveillé et al. (2003) showed that the content of
HS in sewage sludge increases as stabilization proceeds. In unstable sewage sludge, the concentration of HS, expressed as the sum
of HA and FA, was 56.8% of TOC content, while in sewage sludge
after anaerobic stabilization, it was 83.1% of TOC. According to
many authors, depending on the type of sludge, the content of
HS ranges from 2% to 7%, and the ratio of HA to FA ranges from
0.3 to 3.0 (Riffaldi et al., 1982; Aiwa and Tabatabai, 1994;
Iakimenko and Velichenko, 1997).
During composting in the present study, the concentrations of
HS and HA increased to 182 mg C/g OM and 128 mg C/g OM,
respectively. An increase in the concentration of HS and HA during
composting of different types of waste, including sewage sludge, is
a typical phenomenon. For example, Paredes et al. (2002) found
that HA increased from 4.89% to 6.29% during composting of sewage sludge and waste cotton. Increases in HA during composting
were also reported by Amir et al. (2006) (from 30 mg/g to approximately 55 mg/g OM) and Domeizel et al. (2004) (from 16.2 mg/g
OM to 24.2 mg/g OM). Although an increase in HA concentrations
was observed in all these cases, it should be emphasized that the
HA concentrations in the final compost varied. This may not only
be due to differences in HA concentration in the feedstock, but also
due to the various amendments used (especially lignocellulosic
materials, whose degradation leads to formation of compounds
k HS = 0.044 d-1
C max,HS = 78.6 mg C/g OM
r HS = 3.46 mg C/g OM·d
100
0
HA [mg C/g OM]
impact of different moisture contents (30%; 40%; 50%; 60% and
70%) on microbial activity during sewage sludge composting, and
proved that it was the dominant factor impacting aerobic microbial
activity. The authors showed that 50% moisture content was the
minimum necessary for a rapid increase in microbial activity, while
moisture in the range of 60–70% provided maximum activity.
Makan et al. (2013) had similar but slightly higher results. They
evaluated the effect of initial moisture content (55%, 65%, 70%,
75% and 85%) on in-vessel composting of the organic fraction of
municipal solid waste. They showed that the least amount of OM
degradation took place at the lowest initial moisture content
(65% and 55%). The highest initial water content (85%) inhibited
the composting process, and thus the smallest amount of OM
was degraded with this moisture content. The greatest amount of
OM biodegradation took place at a moisture content of 70–75%.
In this study, high moisture content (70.4%) was noted in the
feedstock and during the first days of composting (70.6–72.6%);
this is when mineralization takes place, and water is one of the
products of the decomposition of OM. Then, the moisture gradually
decreased (Fig. 2b), which is a typical phenomenon. For example,
Larré-Larrouy and Thuriès (2006) also showed that during
composting of sheep manure with grape and coffee by-products,
moisture gradually decreased from 59% in the feedstock to 40%
after 130 days, and to 22.6% after 305 days of composting.
S-HA+ L-HA
L-HA
Fig. 3. Changes in HS (a) and HA (b) concentrations and kinetic constants of
humification during sewage sludge composting.
considered to be precursors of HS), the different composting technologies and the differences in the conditions during composting,
e.g. the maximum temperature and the duration of the thermophilic phase.
In the present study, the concentration of HS and HA increased
(in comparison to the feedstock) 1.5 times (HS) and 2.2 times (HA),
which could be related to the fairly long thermophilic phase, and to
the presence of waste that contained lignocellulosic materials,
which is degraded to precursors of HS. The beneficial effect of thermophilic conditions on lignin decomposition has been confirmed
by other authors (Tomati et al., 1995; Solano et al., 2001).
Cayuela et al. (2006) showed that in a turned windrow, the efficiency of lignin degradation was 50%, and in windrows with forced
aeration it did not exceed 30%. The authors explained the low efficiency of lignin degradation in the aerated windrow as being due
to the short thermophilic phase in those conditions. A longer duration of the thermophilic phase favors the formation of precursors
from which humus is created in the cooling phase. In the study
by Cayuela et al. (2006), there were different proportions of HA
and FA in the mature compost: in the windrow with a forced aeration, the HA/FA ratio was 2.7–3.3, while in the turned windrows it
was 5.0–6.2.
Among the HA, L-HA (extracted with Na4P2O7) and S-HA
(extracted with NaOH) are distinguished. It is believed that L-HA
are weakly bound to mineral surfaces via cation bridges. Their nature is more similar to that of FA, whereas S-HA is more stable and
bond strongly to the mineral soil fraction. L-HA is characterized by
low and medium molecular weight and aromatic character,
whereas S-HA is larger, with a more aliphatic nature. In this study,
the share of L-HA was not high, not exceeding 14% of HA (their concentrations were 7.3–15.1 mg C/g OM).
Although research on the content of HS and HA in mature compost is relatively common, data on the kinetics of the humification
process are lacking. In this study, the kinetics of HS and HA formation proceeded according to the 1. order kinetic equation:
C ¼ C max ð1
e
kt
Þ þ Ci;
ð3Þ
Please cite this article in press as: Kulikowska, D. Kinetics of organic matter removal and humification progress during sewage sludge composting. Waste
Management (2016), http://dx.doi.org/10.1016/j.wasman.2016.01.005
6
D. Kulikowska / Waste Management xxx (2016) xxx–xxx
where
C concentration of HS or HA in time (mg C/g OM)
Cmax the maximum increase in concentration of HS (Cmax,HS) or
HA (Cmax,HA) (mg C/g OM)
k the rate constant of HS (kHS) or HA (kHA) formation (d 1)
e base of natural logarithm
t composting time (d)
Ci the initial concentration of HS or HA (mg C/g OM)
The kinetic constants of HS and HA formation obtained from Eq.
(3) were 0.044 d 1 (kHS) (Fig. 3a) and 0.045 d 1 (kHA) (Fig. 3b),
respectively. The rates of HS and HA formation, rHS and rHA, equaled
3.46 mg C/g OM d and 3.24 mg C/g OM d, respectively. The values
of kHS and kHA obtained in this study are similar to those obtained
in the author’s earlier study that also examined the kinetics of HS
and HA formation during sewage sludge composting, but in a twostage system and with rape straw and grass as amendments (in
that study, kHS was 0.014–0.058 d 1; kHA was 0.037–0.053 d 1)
(Kulikowska, 2012).
In the literature, data about humification kinetics are lacking. In
most papers, authors only give information about HS concentrations in the feedstock and mature compost, and sometimes show
the changes in HS and their fractions during composting, but do
not determine the kinetic constants. Thus, conclusions about the
suitability the 1. order kinetics to describe the humification process can only be made on the basis of own research. In these studies on sludge composting with different amendments and different
technological conditions, humification did indeed proceed according to 1. order kinetics, and the process proceeded most intensively
during the first 3–4 months of composting (Kulikowska and
Klimiuk, 2011; Kulikowska, 2012; unpublished data). Thus, a 1.
order kinetic model seems to be useful for determining kinetic constants for humification and predicting the time at which humification will proceed most intensely, but this needs to be confirmed by
more experiments using different kinds of waste.
In this study, the kinetic constants of humification (kHS and kHA)
were an order of magnitude lower than the kinetic constants of OM
mineralization (kOM) which means that humification proceeded
slower than mineralization, as expected. Mineralization occurred
most intensively in the first 15 days of composting, whereas humification was most intensive when the temperature decreased
below 60 °C (ca. 2 weeks after the beginning of the process) and
lasted for the next 3 months of the process (in this study the humification process was analyzed through 6 months). Similarly, Vieyra
et al. (2009) showed that the most intensive OM degradation
occurred during the first 30 days of composting of domestic solid
waste; however, the highest increase in humification took place
between 75 and 120 days of the process.
The decrease in OM is greatest during the first days of composting (mineralization phase), due to the degradation of readily
biodegradable compounds, i.e. proteins, simple carbohydrates
and lipids. As a result of this process, heat is produced and retained
within the compost, which raises the temperature. The range of
heat generated during decomposition of proteins, carbohydrates
and lipids is 9–40 kJ/g, with lipids yielding about twice as much
heat per unit weight as the other two (Mathur, 1998).
In thermophilic conditions, lignin degradation leads to the formation of phenolic and quinonic moieties, which could serve as
precursors for humification (Sánchez-Monedero et al., 1999).
Therefore, an increase in HS and HA is observed in the latter phase
of composting. Sánchez-Monedero et al. (1999) analyzed relationships between OM degradation and humification during composting of six different organic-waste mixtures, in which the main
components were sewage sludge, sorghum bagasse and municipal
solid waste. They showed that the concentration of phenols, generated during partial degradation of lignin, inversely correlated with
the humification indices throughout the composting process,
which suggests they acted as precursors in the humification process. According to the authors, this means that HS were synthesized from precursors that originated from lignin (the lignin
theory). However, Veeken et al. (2000) showed that the condensation route (polyphenol theory) may also have an important contribution to the formation of HA during biowaste composting, which
was also shown by He et al. (2014), who demonstrated that
polysaccharide-like and proteinaceous compounds can also be
integrated into HS during composting.
Serramia et al. (2010) analyzed the relationship between the
degradation of the lignocellulosic fraction of OM and the humification process during composting of olive mill waste. They found a
statistically significant correlation between the lignin/holocellulose ratio and humification indices in all composting mixtures
which, according to the authors, reflected the involvement of holocellulose degradation products (simple carbohydrates) in the formation of humic-like molecules.
In this study, 2 humification indexes were determined: the
humification ratio HR (HR = (CHS/CTOC) 100) and the degree of
polymerization DP (DP = CHA/CFA). The value of HR changed markedly during the first 2 months of composting, after which it
increased only slightly. This is due to the fact that, during the later
stages of humification, TOC and the total amount of HS did not
change much (although there were marked changes in the fractions of HA and FF in HS). TOC decrease is related to the amount
of CO2 released, and it does not change substantially if the degree
of mineralization is small, as it was during the latter part of
humification.
The DP is a measure of the creation of complex molecules of HA
from simpler FA molecules. In this study, the DP value more than
doubled from 0.93 to 2.58 during the course of humification. It is
worth emphasizing that the DP value increased throughout the
entire time of composting, even when the HS concentration did
not change. This indicates that the elongation of compost maturation time has a greater impact on changes within HS (i.e. polymerization of FA to HA) than on creation of more HS.
Other authors also analyzed the suitability of humification
indices to evaluate progress in humification. For example
Sánchez-Monedero et al. (1999) analyzed 4 different indices, i.e.
HR, HI (humification index, HI = (CHA/CTOC) 100), DP and PHA (percentage of HA, PHA = (CHA/CHS) 100) to evaluate humification during composting of six kinds of waste (primary aerobic sewage
sludge, cotton waste, sorghum bagasse, pine bark, brewery sludge
and the organic fraction of selectively collected municipal solid
waste) in different mixtures. They showed that, for all mixtures,
DP was the most sensitive indicator for evaluation of the humification processes. The values of DP in mature compost were from 1.58
to 3.07-times higher than in the feedstock. Moreover, changes in
the value of other indicators were less evident. Similarly, Hsu
and Lo (1999) showed a considerable increase in the values of DP
during composting of pig manure, from 0.60 in the feedstock to
3.33 after 122 days of composting. Moreover, they noted increases
in PHA from 16.4 to 47.4. Similar results were obtained by Paredes
et al. (2001), although they composted different waste (sewage
sludge, industrial waste from orange juice extraction, and cotton
gin waste). They found that humification progress was best indicated by increases in DP and PHA. However, they showed that HR
and HI generally did not show a clear tendency during the composting process. In contrast, Jouraiphy et al. (2005) noted significant changes in all analyzed indices (HR, HI, PHA and DP) during
composting of sewage sludge and green waste mixtures. After
Please cite this article in press as: Kulikowska, D. Kinetics of organic matter removal and humification progress during sewage sludge composting. Waste
Management (2016), http://dx.doi.org/10.1016/j.wasman.2016.01.005
D. Kulikowska / Waste Management xxx (2016) xxx–xxx
135 days of composting there was an increase in HR from 18.1 to
27.9; HI from 7.4 to 19.2; PHA from 40.9 to 68.9 and DP from
0.69 to 2.21.
It is worth emphasizing that, in the present study, a continuous
increase in the values of DP occurred during the entire composting
process, whereas the most intense increase in the concentration of
HS occurred during the first 3 months of the process (after this
time the rate of HS formation was low). As a result, the amount
of HS in compost matured for a longer period of time differed only
slightly from that matured for a shorter period of time. This means
that lengthening the maturation time affects mainly the polymerization of FA to HA, i.e. transformation from one kind of HS to
another does not considerably increase the total concentration of
humic substances. This small change in total HS concentration
has practical significance because it means that it is possible to
use compost after 3 months of maturation. This conclusion is supported by the author’s previous study, which shows that it is possible to use compost after 3 months of maturation for stabilization
of metals in contaminated soil (Gusiatin and Kulikowska, 2015).
That study examined how the redistribution pattern, metal mobility and stability of Cu and Zn were affected by the maturation time
(3, 6 and 12 months) of sewage-sludge compost that was added to
the contaminated soil. Although Cu redistribution, bioavailability
and stability were favorably affected by compost addition, these
results were not affected by lengthening the maturation time of
the compost and thus increasing the share of HA in the compost.
4. Conclusions
In the present study, the kinetic constants of OM removal were
an order of magnitude higher than the kinetic constants of humification during sewage sludge composting. Organics removal
occurred mainly during the first 15 days of composting, whereas
humification occurred most intensively during the first 3 months
of composting, as indicated by the concentration profiles of OM,
HS and HA. Lengthening compost maturation time over 3 months
increased HS concentration only slightly but the polymerization
of FF to HA took place, as shown by the DP values. The high content
of HS (182 mg C/g OM) indicated that the compost could also be
used in soil remediation, both as an amendment in stabilization
or as a source of HS in soil washing.
Acknowledgement
The study was supported by the Ministry of Science and Higher
Education in Poland (Statutory Research, 528-0809-0801).
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PN-Z-15011-1. Polish standards. Municipal
Contents lists available at ScienceDirect
Waste Management
journal homepage: www.elsevier.com/locate/wasman
Kinetics of organic matter removal and humification progress during
sewage sludge composting
Dorota Kulikowska
University of Warmia and Mazury in Olsztyn, Department of Environmental Biotechnology, Słoneczna St. 45G, 10-709 Olsztyn, Poland
a r t i c l e
i n f o
Article history:
Received 25 August 2015
Revised 31 December 2015
Accepted 4 January 2016
Available online xxxx
Keywords:
Sewage sludge
Composting
Kinetic constants
Organics
Humic substances
Humic acids
a b s t r a c t
This study investigated the kinetics of organic matter (OM) removal and humification during composting
of sewage sludge and lignocellulosic waste (wood chips, wheat straw, leaves) in an aerated bioreactor.
Both OM degradation and humification (humic substances, HS, and humic acids, HA formation)
proceeded according to 1. order kinetics. The rate constant of OM degradation was 0.196 d 1, and the
rate of OM degradation was 39.4 mg/g OM d. The kinetic constants of HS and HA formation were
0.044 d 1 and 0.045 d 1, whereas the rates of HS and HA formation were 3.46 mg C/g OM d and
3.24 mg C/g OM d, respectively. The concentration profiles of HS and HA indicated that humification
occurred most intensively during the first 3 months of composting. The high content of HS
(182 mg C/g OM) in the final product indicated that the compost could be used in soil remediation as a
source of HS for treating soils highly contaminated with heavy metals.
Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction
Municipal sewage sludge can be a source of valuable fertilizer,
due to its high content of organics, nitrogen, phosphorus and trace
elements. However, the presence of pathogenic organisms may
pose health risks, limiting the direct application of sludge to soil
fertilization. Therefore, sludge should be treated prior to application by methods such as composting.
The composting process lowers sewage sludge mass and
moisture content. In addition, thermophilic conditions destroy
pathogenic organisms present in waste and ensure complete hygienization of the compost. Additionally, compost contains relatively
low concentrations of heavy metals and fulfills the requirements
for soil amendments in most cases. For example, earlier study
showed that the metal concentration in sewage sludge compost
poses low ecological risk ( proteinaceous compounds > polysaccharide-like matter and lignin.
In the present study, OM removal efficiency (EOM,loss) and the
values of kOM and rOM were relatively high, which indicates that,
despite a C/N ratio that was lower than optimal, the composting
process proceeded efficiently. This could be due to the high content
of readily biodegradable organics in the sewage sludge, which constituted 65% of the feedstock. Efficient composting at a relatively
low C/N ratio is important from a practical point of view, as it
means that when composting sewage sludge, which is usually
characterized by a low C/N ratio, there is no need to add very large
amounts of amendments. Although the process may proceed more
efficiently in optimal conditions, this would require large amounts
of amendments (e.g. straw or leaves), which would reduce the proportion of sewage sludge and influence the economy of the composting process. Although nitrogen is lost from the system as
ammonia during composting at a low C/N ratio, and conservation
of nitrogen in the composted biomass is one of the objectives of
composting, the high N content of sewage sludge means that the
mature compost will still fulfill the requirements for fertilizers
even if it is composted at a low C/N ratio. In the present study, N
content in the final compost was 2.11%. Relatively high N content
in compost produced from feedstock characterized by a low C/N
ratio has also been confirmed by earlier research on sewage sludge
composting in a two-stage system with rape straw and grass as
amendments (Kulikowska and Gusiatin, 2015).
80
60
40
20
0
0
20
40
60
80
100
120
140
160
180
composting time [d]
7 cm from the upper edge of bioreactor
63 cm from the upper edge of
(b)
moisture content [%]
4
80
60
40
20
0
0
20
40
60
80
100
120
140
160
180
composting time [d]
Fig. 2. Temperature (a) and moisture (b) profiles during sewage sludge composting.
lasted 19 and 24 days in the upper and middle layers, respectively.
The maximum temperature during composting and the length of
the thermophilic phase depend not only on the content of readily
biodegradable organic compounds but also on the C/N ratio. In a
study by Huang et al. (2004), thermophilic conditions were
reached after 3 days and lasted for 40 days during composting of
manure in a windrow at a C/N ratio of 30; the maximum temperature was 69 °C. At a lower C/N ratio (C/N 15), they observed that
thermophilic conditions were reached after 7 days, the thermophilic phase was shorter and the maximum temperature was
lower (60 °C). According to the authors, the reduction of the maximum temperature and the shortening of the thermophilic phase
were associated with insufficient carbon content in the composted
mass (low C/N). However, Goyal et al. (2005) showed that the temperature profiles of compost depend more on the biodegradability
of the OM present in the waste than the C/N ratio. The authors
composted sugar cane waste with cattle manure at ratios of 4:1
and 1:1, sugar cane waste alone and water hyacinth biomass.
The C/N ratios in these wastes were as follows: 51.1, 32.0, 14.5
and 18.1. They found that, regardless of the type of waste, the temperature did not exceed 46 °C during 90 days of composting. The
main reason for the low temperature of the compost was the presence of high amounts of fibres, mainly lignin, as heat generation is
associated with intensely occurring decomposition of readily
biodegradable organic compounds.
In addition to temperature, moisture is an important factor during composting. The correct moisture content is a compromise
between allowing oxygen flow to maintain aerobic conditions, on
the one hand, and having enough water for microorganisms to
move and transport nutrients, on the other. If this balance is not
achieved, the metabolic and physiological activities of the desired
microorganisms will be impaired. When the moisture content of
composting material is too low, it can slow or stop the biological
processes. With too much water, pores (holes) in the composting
materials can be filled with water, which reduces oxygen penetration and creates anaerobic processes.
Although sources differ slightly, there is general agreement that
moisture contents around 60–70% are best for process efficiency.
For example, Liang et al. (2003) studied, among other factors, the
Please cite this article in press as: Kulikowska, D. Kinetics of organic matter removal and humification progress during sewage sludge composting. Waste
Management (2016), http://dx.doi.org/10.1016/j.wasman.2016.01.005
5
D. Kulikowska / Waste Management xxx (2016) xxx–xxx
HS [mg C/g OM]
(a)
200
150
50
20
0
40
60
80
100
120
140 160 180
composting time [d]
(b)
HS
k HA = 0.045 d-1
C max,HA = 72.1 mg C/g OM
r HA = 3.24 mg C/g OM·d
200
150
80
60
100
40
50
20
0
20
0
40
60
80
100
120
140
160
L-HA [mg C/g OM]
1. order kinetic
0
180
composting time [d]
1. order kinetic
3.2. Organic matter humification
During the humification process, there are changes in the concentrations of HS and their fractions, i.e. HA and FA, in composting
waste. During this process, the concentration of FA decreases,
while that of HA increases, which has been confirmed by many
authors (Inbar et al., 1990; Veeken et al., 2000; Paredes et al.,
2002; Domeizel et al., 2004; Jouraiphy et al., 2005; Amir et al.,
2006). Studies on the HS content in compost are quite numerous,
but the author of the present study is unaware of any information
on the kinetics of the process. This information is important
because it provides information about the time at which HS are
most intensively formed.
In this study, the content of HS and HA was already high in the
feedstock (HS 118 mg C/g OM, HA 57 mg C/g OM), due to the large
share of sewage sludge (Fig. 3). It is believed that during wastewater treatment, humification occurs simultaneously with the mineralization of OM. Réveillé et al. (2003) showed that the content of
HS in sewage sludge increases as stabilization proceeds. In unstable sewage sludge, the concentration of HS, expressed as the sum
of HA and FA, was 56.8% of TOC content, while in sewage sludge
after anaerobic stabilization, it was 83.1% of TOC. According to
many authors, depending on the type of sludge, the content of
HS ranges from 2% to 7%, and the ratio of HA to FA ranges from
0.3 to 3.0 (Riffaldi et al., 1982; Aiwa and Tabatabai, 1994;
Iakimenko and Velichenko, 1997).
During composting in the present study, the concentrations of
HS and HA increased to 182 mg C/g OM and 128 mg C/g OM,
respectively. An increase in the concentration of HS and HA during
composting of different types of waste, including sewage sludge, is
a typical phenomenon. For example, Paredes et al. (2002) found
that HA increased from 4.89% to 6.29% during composting of sewage sludge and waste cotton. Increases in HA during composting
were also reported by Amir et al. (2006) (from 30 mg/g to approximately 55 mg/g OM) and Domeizel et al. (2004) (from 16.2 mg/g
OM to 24.2 mg/g OM). Although an increase in HA concentrations
was observed in all these cases, it should be emphasized that the
HA concentrations in the final compost varied. This may not only
be due to differences in HA concentration in the feedstock, but also
due to the various amendments used (especially lignocellulosic
materials, whose degradation leads to formation of compounds
k HS = 0.044 d-1
C max,HS = 78.6 mg C/g OM
r HS = 3.46 mg C/g OM·d
100
0
HA [mg C/g OM]
impact of different moisture contents (30%; 40%; 50%; 60% and
70%) on microbial activity during sewage sludge composting, and
proved that it was the dominant factor impacting aerobic microbial
activity. The authors showed that 50% moisture content was the
minimum necessary for a rapid increase in microbial activity, while
moisture in the range of 60–70% provided maximum activity.
Makan et al. (2013) had similar but slightly higher results. They
evaluated the effect of initial moisture content (55%, 65%, 70%,
75% and 85%) on in-vessel composting of the organic fraction of
municipal solid waste. They showed that the least amount of OM
degradation took place at the lowest initial moisture content
(65% and 55%). The highest initial water content (85%) inhibited
the composting process, and thus the smallest amount of OM
was degraded with this moisture content. The greatest amount of
OM biodegradation took place at a moisture content of 70–75%.
In this study, high moisture content (70.4%) was noted in the
feedstock and during the first days of composting (70.6–72.6%);
this is when mineralization takes place, and water is one of the
products of the decomposition of OM. Then, the moisture gradually
decreased (Fig. 2b), which is a typical phenomenon. For example,
Larré-Larrouy and Thuriès (2006) also showed that during
composting of sheep manure with grape and coffee by-products,
moisture gradually decreased from 59% in the feedstock to 40%
after 130 days, and to 22.6% after 305 days of composting.
S-HA+ L-HA
L-HA
Fig. 3. Changes in HS (a) and HA (b) concentrations and kinetic constants of
humification during sewage sludge composting.
considered to be precursors of HS), the different composting technologies and the differences in the conditions during composting,
e.g. the maximum temperature and the duration of the thermophilic phase.
In the present study, the concentration of HS and HA increased
(in comparison to the feedstock) 1.5 times (HS) and 2.2 times (HA),
which could be related to the fairly long thermophilic phase, and to
the presence of waste that contained lignocellulosic materials,
which is degraded to precursors of HS. The beneficial effect of thermophilic conditions on lignin decomposition has been confirmed
by other authors (Tomati et al., 1995; Solano et al., 2001).
Cayuela et al. (2006) showed that in a turned windrow, the efficiency of lignin degradation was 50%, and in windrows with forced
aeration it did not exceed 30%. The authors explained the low efficiency of lignin degradation in the aerated windrow as being due
to the short thermophilic phase in those conditions. A longer duration of the thermophilic phase favors the formation of precursors
from which humus is created in the cooling phase. In the study
by Cayuela et al. (2006), there were different proportions of HA
and FA in the mature compost: in the windrow with a forced aeration, the HA/FA ratio was 2.7–3.3, while in the turned windrows it
was 5.0–6.2.
Among the HA, L-HA (extracted with Na4P2O7) and S-HA
(extracted with NaOH) are distinguished. It is believed that L-HA
are weakly bound to mineral surfaces via cation bridges. Their nature is more similar to that of FA, whereas S-HA is more stable and
bond strongly to the mineral soil fraction. L-HA is characterized by
low and medium molecular weight and aromatic character,
whereas S-HA is larger, with a more aliphatic nature. In this study,
the share of L-HA was not high, not exceeding 14% of HA (their concentrations were 7.3–15.1 mg C/g OM).
Although research on the content of HS and HA in mature compost is relatively common, data on the kinetics of the humification
process are lacking. In this study, the kinetics of HS and HA formation proceeded according to the 1. order kinetic equation:
C ¼ C max ð1
e
kt
Þ þ Ci;
ð3Þ
Please cite this article in press as: Kulikowska, D. Kinetics of organic matter removal and humification progress during sewage sludge composting. Waste
Management (2016), http://dx.doi.org/10.1016/j.wasman.2016.01.005
6
D. Kulikowska / Waste Management xxx (2016) xxx–xxx
where
C concentration of HS or HA in time (mg C/g OM)
Cmax the maximum increase in concentration of HS (Cmax,HS) or
HA (Cmax,HA) (mg C/g OM)
k the rate constant of HS (kHS) or HA (kHA) formation (d 1)
e base of natural logarithm
t composting time (d)
Ci the initial concentration of HS or HA (mg C/g OM)
The kinetic constants of HS and HA formation obtained from Eq.
(3) were 0.044 d 1 (kHS) (Fig. 3a) and 0.045 d 1 (kHA) (Fig. 3b),
respectively. The rates of HS and HA formation, rHS and rHA, equaled
3.46 mg C/g OM d and 3.24 mg C/g OM d, respectively. The values
of kHS and kHA obtained in this study are similar to those obtained
in the author’s earlier study that also examined the kinetics of HS
and HA formation during sewage sludge composting, but in a twostage system and with rape straw and grass as amendments (in
that study, kHS was 0.014–0.058 d 1; kHA was 0.037–0.053 d 1)
(Kulikowska, 2012).
In the literature, data about humification kinetics are lacking. In
most papers, authors only give information about HS concentrations in the feedstock and mature compost, and sometimes show
the changes in HS and their fractions during composting, but do
not determine the kinetic constants. Thus, conclusions about the
suitability the 1. order kinetics to describe the humification process can only be made on the basis of own research. In these studies on sludge composting with different amendments and different
technological conditions, humification did indeed proceed according to 1. order kinetics, and the process proceeded most intensively
during the first 3–4 months of composting (Kulikowska and
Klimiuk, 2011; Kulikowska, 2012; unpublished data). Thus, a 1.
order kinetic model seems to be useful for determining kinetic constants for humification and predicting the time at which humification will proceed most intensely, but this needs to be confirmed by
more experiments using different kinds of waste.
In this study, the kinetic constants of humification (kHS and kHA)
were an order of magnitude lower than the kinetic constants of OM
mineralization (kOM) which means that humification proceeded
slower than mineralization, as expected. Mineralization occurred
most intensively in the first 15 days of composting, whereas humification was most intensive when the temperature decreased
below 60 °C (ca. 2 weeks after the beginning of the process) and
lasted for the next 3 months of the process (in this study the humification process was analyzed through 6 months). Similarly, Vieyra
et al. (2009) showed that the most intensive OM degradation
occurred during the first 30 days of composting of domestic solid
waste; however, the highest increase in humification took place
between 75 and 120 days of the process.
The decrease in OM is greatest during the first days of composting (mineralization phase), due to the degradation of readily
biodegradable compounds, i.e. proteins, simple carbohydrates
and lipids. As a result of this process, heat is produced and retained
within the compost, which raises the temperature. The range of
heat generated during decomposition of proteins, carbohydrates
and lipids is 9–40 kJ/g, with lipids yielding about twice as much
heat per unit weight as the other two (Mathur, 1998).
In thermophilic conditions, lignin degradation leads to the formation of phenolic and quinonic moieties, which could serve as
precursors for humification (Sánchez-Monedero et al., 1999).
Therefore, an increase in HS and HA is observed in the latter phase
of composting. Sánchez-Monedero et al. (1999) analyzed relationships between OM degradation and humification during composting of six different organic-waste mixtures, in which the main
components were sewage sludge, sorghum bagasse and municipal
solid waste. They showed that the concentration of phenols, generated during partial degradation of lignin, inversely correlated with
the humification indices throughout the composting process,
which suggests they acted as precursors in the humification process. According to the authors, this means that HS were synthesized from precursors that originated from lignin (the lignin
theory). However, Veeken et al. (2000) showed that the condensation route (polyphenol theory) may also have an important contribution to the formation of HA during biowaste composting, which
was also shown by He et al. (2014), who demonstrated that
polysaccharide-like and proteinaceous compounds can also be
integrated into HS during composting.
Serramia et al. (2010) analyzed the relationship between the
degradation of the lignocellulosic fraction of OM and the humification process during composting of olive mill waste. They found a
statistically significant correlation between the lignin/holocellulose ratio and humification indices in all composting mixtures
which, according to the authors, reflected the involvement of holocellulose degradation products (simple carbohydrates) in the formation of humic-like molecules.
In this study, 2 humification indexes were determined: the
humification ratio HR (HR = (CHS/CTOC) 100) and the degree of
polymerization DP (DP = CHA/CFA). The value of HR changed markedly during the first 2 months of composting, after which it
increased only slightly. This is due to the fact that, during the later
stages of humification, TOC and the total amount of HS did not
change much (although there were marked changes in the fractions of HA and FF in HS). TOC decrease is related to the amount
of CO2 released, and it does not change substantially if the degree
of mineralization is small, as it was during the latter part of
humification.
The DP is a measure of the creation of complex molecules of HA
from simpler FA molecules. In this study, the DP value more than
doubled from 0.93 to 2.58 during the course of humification. It is
worth emphasizing that the DP value increased throughout the
entire time of composting, even when the HS concentration did
not change. This indicates that the elongation of compost maturation time has a greater impact on changes within HS (i.e. polymerization of FA to HA) than on creation of more HS.
Other authors also analyzed the suitability of humification
indices to evaluate progress in humification. For example
Sánchez-Monedero et al. (1999) analyzed 4 different indices, i.e.
HR, HI (humification index, HI = (CHA/CTOC) 100), DP and PHA (percentage of HA, PHA = (CHA/CHS) 100) to evaluate humification during composting of six kinds of waste (primary aerobic sewage
sludge, cotton waste, sorghum bagasse, pine bark, brewery sludge
and the organic fraction of selectively collected municipal solid
waste) in different mixtures. They showed that, for all mixtures,
DP was the most sensitive indicator for evaluation of the humification processes. The values of DP in mature compost were from 1.58
to 3.07-times higher than in the feedstock. Moreover, changes in
the value of other indicators were less evident. Similarly, Hsu
and Lo (1999) showed a considerable increase in the values of DP
during composting of pig manure, from 0.60 in the feedstock to
3.33 after 122 days of composting. Moreover, they noted increases
in PHA from 16.4 to 47.4. Similar results were obtained by Paredes
et al. (2001), although they composted different waste (sewage
sludge, industrial waste from orange juice extraction, and cotton
gin waste). They found that humification progress was best indicated by increases in DP and PHA. However, they showed that HR
and HI generally did not show a clear tendency during the composting process. In contrast, Jouraiphy et al. (2005) noted significant changes in all analyzed indices (HR, HI, PHA and DP) during
composting of sewage sludge and green waste mixtures. After
Please cite this article in press as: Kulikowska, D. Kinetics of organic matter removal and humification progress during sewage sludge composting. Waste
Management (2016), http://dx.doi.org/10.1016/j.wasman.2016.01.005
D. Kulikowska / Waste Management xxx (2016) xxx–xxx
135 days of composting there was an increase in HR from 18.1 to
27.9; HI from 7.4 to 19.2; PHA from 40.9 to 68.9 and DP from
0.69 to 2.21.
It is worth emphasizing that, in the present study, a continuous
increase in the values of DP occurred during the entire composting
process, whereas the most intense increase in the concentration of
HS occurred during the first 3 months of the process (after this
time the rate of HS formation was low). As a result, the amount
of HS in compost matured for a longer period of time differed only
slightly from that matured for a shorter period of time. This means
that lengthening the maturation time affects mainly the polymerization of FA to HA, i.e. transformation from one kind of HS to
another does not considerably increase the total concentration of
humic substances. This small change in total HS concentration
has practical significance because it means that it is possible to
use compost after 3 months of maturation. This conclusion is supported by the author’s previous study, which shows that it is possible to use compost after 3 months of maturation for stabilization
of metals in contaminated soil (Gusiatin and Kulikowska, 2015).
That study examined how the redistribution pattern, metal mobility and stability of Cu and Zn were affected by the maturation time
(3, 6 and 12 months) of sewage-sludge compost that was added to
the contaminated soil. Although Cu redistribution, bioavailability
and stability were favorably affected by compost addition, these
results were not affected by lengthening the maturation time of
the compost and thus increasing the share of HA in the compost.
4. Conclusions
In the present study, the kinetic constants of OM removal were
an order of magnitude higher than the kinetic constants of humification during sewage sludge composting. Organics removal
occurred mainly during the first 15 days of composting, whereas
humification occurred most intensively during the first 3 months
of composting, as indicated by the concentration profiles of OM,
HS and HA. Lengthening compost maturation time over 3 months
increased HS concentration only slightly but the polymerization
of FF to HA took place, as shown by the DP values. The high content
of HS (182 mg C/g OM) indicated that the compost could also be
used in soil remediation, both as an amendment in stabilization
or as a source of HS in soil washing.
Acknowledgement
The study was supported by the Ministry of Science and Higher
Education in Poland (Statutory Research, 528-0809-0801).
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Please cite this article in press as: Kulikowska, D. Kinetics of organic matter removal and humification progress during sewage sludge composting. Waste
Management (2016), http://dx.doi.org/10.1016/j.wasman.2016.01.005
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PN-Z-15011-1. Polish standards. Municipal