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.
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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 applica-tion 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 hygi-enization 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 (<16) based on the potential ecological risk factor (Er), which includes not only total metals concentration,
but also their toxicity (Zhu et al., 2012; Kulikowska and Gusiatin, 2015).
To date, as compost has been used mainly as a fertilizer, most attention has focused on transforming of OM, obtaining ther-mophilic conditions, transforming and conserving nitrogen (Zhu et al., 2004; Cayuela et al., 2006; Kalamdhad and Kazmi, 2009) and establishing the content of heavy metals in the mature
compost (Wong and Selvam, 2006; Manungufala et al., 2008). In recent years, compost has also been used in remediation of soils that are contaminated with heavy metals. It is known that such soil poses a potential risk of groundwater contamination, which increases when the metals are in mobile and potentially mobile fractions. Thus, one of the main goals of soil remediation should be to decrease the concentration of metals in the bioavailable and mobile fractions (stabilization) (Castaldi et al., 2005; Liu et al., 2009). Although this process does not decrease the total metal concentration in soil, decreasing metal mobility does sub-stantially decrease environmental risk (Gusiatin and Kulikowska, 2015). An alternative strategy for soil remediation is metals mobi-lization and thereafter removal from soil by soil washing (Conte et al., 2005; Tsang and Yip, 2014).
In both remediation strategies, compost plays an important role. In the former strategy (stabilization), compost acts as an organic amendment which contributes to immobilization of metals in the soil. In the second strategy (washing), compost is a source of HS, which after extraction may be used as washing agents (Kulikowska et al., 2015). However, in both cases, HS are one of the crucial factors. Thus, research on the composting of waste should also include analyses of the humification of OM. Further-more, it is important to determine not only the concentration of HS and their fractions, i.e. HA and fulvic acids (FA), but also the kinetics of HS formation, which is extremely important in predict-ing the time durpredict-ing compostpredict-ing at which the humification process occurs most intensely.
http://dx.doi.org/10.1016/j.wasman.2016.01.005
0956-053X/Ó2016 Elsevier Ltd. All rights reserved. E-mail address:dorotak@uwm.edu.pl
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Waste Management
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In previous study on sewage sludge composting in a two-stage system with rape straw and grass as amendments, the kinetic con-stants of both HS and HA were determined (Kulikowska and Klimiuk, 2011; Kulikowska, 2012): in both cases, humification pro-ceeded according to 1. order kinetics. However, although other studies about composting have provided information about changes in HS content during composting of different wastes (Paredes et al., 2001; Domeizel et al., 2004; Goyal et al., 2005), the author of the present study is unaware of any other informa-tion on the kinetic constants of the process. So, in order to verify if the kinetic model proposed in the earlier research can be useful for calculation of the rate of both HS and HA formation for other types of waste or composting technologies, further research is needed. Therefore, the aim of this study was to analyze the trans-formation of OM during sewage sludge composting, including determination of both the kinetic parameters of OM removal and of HS and HA formation during sewage sludge composting in an aerated bioreactor with wheat straw and leaves as amendments.
2. Materials and methods
2.1. Bioreactor for composting
The study was conducted in an aerated bioreactor with a capac-ity of 1 m3. A detailed description of the bioreactor was presented
in the earlier work ofKulikowska and Klimiuk (2011). The bioreac-tor was aerated by air pumped from the fan to the aeration box, which was built into the bioreactor floor. The intensity of the aeration was controlled by a frequency converter coupled to the fan drive. The bioreactor was equipped with PT 100 temperature sensors, coupled with LED displays, which allowed precise deter-mination of temperature to within 0.1°C. The sensors were mounted at two levels, i.e. at depths of 7 cm and 63 cm.
The intensity of aeration was maintained 1–1.5 L/kg d.m.min in such a way as to prevent overheating of the compost.
2.2. Characterization of sewage sludge and feedstock preparation
Dewatered sewage sludge from a municipal wastewater treat-ment plant (operated as an SBR system) was mixed with lignocel-lulosic materials in the following proportions (w/w): sewage sludge 65%, wood chips 15%, wheat straw 10%, and leaves 10%. The sewage sludge did not contain bacteria ofSalmonellaspp. or parasite eggs ofAscarsis,TrichurisorToxocaraspp.
The characteristics of the composted waste and feedstock are shown inTable 1. Sewage sludge was characterized by the highest moisture and nitrogen content, wheat straw and wood chips had high OM content, and wheat straw had the lowest moisture content. Lignocellulosic materials were added to increase the C/N ratio, lower moisture content and improve the structure of the composted feedstock.
2.3. Analytical methods
Samples of composted waste were collected in accordance with the guidelines contained in the Polish standard PN-Z-15011-1.
The number of samples was adjusted to the volume of the bioreac-tor. Primary samples (3 samples, about 1 kg each, taken from the top, middle and bottom of the bioreactor) were piled up on a paved surface, and then thoroughly mixed to create a representative sam-ple. In order to reduce volume, the representative sample was piled up in the shape of a truncated pyramid with a square base and a height not exceeding 30 cm. It was then divided with diagonals into 4 parts. Two of the opposing parts were discarded and the remaining two parts were mixed. The procedure was repeated in the same manner until a representative laboratory sample with a mass of 0.5 kg was obtained. The samples were mixed, dried at 105°C and ground to a diameter of 0.5 mm using a Retsch SM 100 mill.
OM was determined by ignition of the samples at 550°C (PN-Z-15011-3:2001); TOC in compost, HS and HA was determined using a Shimadzu Liquid TOC-VCSN analyzer; total N by the Kjeldahl method (PN-Z-15011-3:2001).
The dewatered sewage sludge was tested forSalmonellaspp., and eggs of ofAscarsis,TrichurisandToxocaraspp. in a specialized
Laboratory of Microbiology and Parasitology which meets the requirements of the Polish standard PN-EN ISO/IEC 17025:2005.
2.3.1. Humic substances (HS) extraction
Before extraction of the HS, samples of compost were washed three times with distilled water to eliminate soluble non-humic substances (e.g. sugars and proteins). 50 ml of water were added to 2.5 g of compost, and the resulting mixture was shaken for approximately 1 h at 150 rpm. Then, the sample was centrifuged for 10 min at 9000 rpm. Next, the supernatant was discarded, and the above procedure was repeated three times.
In the second step, the samples were defatted with a chloro-form:methanol (2:1) mixture (Jouraiphy et al., 2005) in a MarsX-press microwave oven. 1.5 g of the compost was transferred to a Teflon vessel and 20 ml of chloroform were added, as well as 10 ml of methanol. The extractions were carried out at 60°C for 10 min. After extraction, the supernatant was discarded and a chlo-roform:methanol (2:1) mixture was again added to the sample. The extraction was repeated until a colorless supernatant was obtained. After that, the defatted samples were evaporated to elim-inate the solvents.
The study used a two-stage extraction process (Boratyn´ski and Wilk, 1965), which allows two kinds of HA to be obtained, i.e. labile humic acids (L-HA) (extraction with 0.1 M Na4P2O7at pH 7) and
stable humic acids (S-HA) (extraction with 0.1 M NaOH at pH 12, after separation of L-HA). Sequential extraction with Na4P2O7and
then NaOH enables the contents of L-HA and S-HA to be deter-mined separately, and thus their contribution to total HA content can also be determined.
0.3 g of compost were shaken with 6 ml of 0.1 M Na4P2O7for
1 h, and then the mixture was centrifuged at 15000 rpm for 15 min. After the supernatant was transferred to a 50 ml volumet-ric flask, the procedure was repeated until a colorless supernatant was obtained. The extract in the 50 ml flask was filtered through a 0.45
l
m filter, and then 0.1 M Na4P2O7was added for a finalvol-ume of 50 ml.
Next, this extract was acidified to pH 1 with H2SO4, after which
the precipitated L-HA were left to coagulate at 4°C for 24 h. After
Table 1
Characteristic of the components used for composting and composted feedstock.
Characteristic Components Feedstock
Sewage sludge Wood chips Wheat straw Leaves
Moisture (%) 87.0 ± 3.2 34.0 ± 1.5 14.0 ± 0.8 58.0 ± 2.1 70.4 ± 1.4
Organic matter (%) 73.2 ± 2.6 97.2 ± 2.3 97.8 ± 1.4 63.0 ± 4.1 72.4 ± 1.7
TOC (% d.m.) 38.6 ± 1.8 48.1 ± 2.2 51.3 ± 2.4 19.04 ± 1.1 38.4 ± 0.8
N (% d.m.) 5.34 ± 0.6 0.88 ± 0.03 0.97 ± 0.03 0.42 ± 0.02 3.32 ± 0.08
24 h, L-HA were separated from the fulvic fraction (FF) by centrifu-gation at 15000 rpm for 15 min. Then, L-HA was dissolved in 0.1 M Na4P2O7 and the content of TOC in the solution was determined
using a Shimadzu Liquid TOC-VCSN analyzer. The FF content was calculated by subtracting the content of HA from that of HS.
After extraction of L-HA, S-HA was extracted from the samples by repeating the above procedure with 0.1 M NaOH in place of 0.1 M Na4P2O7. After S-HA extraction, TOC was again determined
in the extract as above.
3. Results and discussion
3.1. Organic matter (OM) degradation and temperature and moisture evolution
3.1.1. OM degradation
OM content in the feedstock equaled 72.4 ± 1.7%. During com-posting, OM was lost, due to the intensive mineralization. Loss of OM during composting (OM removal efficiency,EOM,loss) was
calcu-lated according toParedes et al. (2000):
EOM;loss¼100 100
½X1ð100 X2Þ
½X2ð100 X1Þ ð1Þ
where:
EOM,lossOM removal efficiency (%) X1ash concentration in the feedstock (%) X2ash concentration during composting (%)
In this studyEOM,lossequaled 61.2%. It is worth emphasizing that
OM mineralization proceeded most intensively within the first 15 days; after 15 days efficiency of OM removal was 57%, which accounted for 93% of overall efficiency (EOM,loss) (Fig. 1a). The
effi-ciency of composting can vary over a wide range, depending on the kind of composted waste and its characteristics. It is known that the one of the crucial factors for effective composting is the C/N ratio in the feedstock (an initial C/N ratio of 20–30:1 is recommended).
When the C/N ratio is too low, N may be lost from the system as ammonia. If the C/N ratio is too high, the synthesis of biomass is lim-ited. However, it should be emphasized that the degree and rate of OM mineralization also depend on the chemical composition of the feedstock, and in particular on the concentration of fibers, especially lignin.Eklind and Kirchmann (2000)composted household waste alone (C/N 13) and in various mixtures: with straw (C/N 28), leaves (C/N 22), soft wood (C/N 32), hardwood (C/N 34), paper (C/N 30) or peat (C/N 28). Composting of household waste (C/N 13; lignin con-tent of 0.9%) was most efficient in terms of OM removal (approxi-mately 80%). With household waste mixed with peat (C/N 28), the concentration of lignin was 20.8%, and the efficiency of OM removal was about 40%. With the other mixtures, irrespective of the C/N ratio, the process efficiency averaged 70%, except for the mixture with leaves, with which the efficiency was 50%.
In this study, although the C/N ratio was low (11.5; OM/N ratio 21.8), efficiency of OM removal was relatively high. This was nected with the high biodegradable OM content and the low con-tent of lignin in the sewage sludge (2.3 ± 0.04% d.m.) as sewage sludge accounted 65% of the feedstock. In contrast to our findings, OM removal is much lower when anaerobically stabilized OM is composted.Ponsa et al. (2009), during composting of this kind of sludge with wood chips in periodically turned windrows, showed that OM removal ranged from 11% to 30%, depending on the ratio of sludge to wood chips (OM removal was 10.55% when the ratio of sewage sludge:wood chips was 1:2, 29.5% at 1:2.5, and 15.5% at 1:3). Similarly, OM removal was 8.45% and 15.3% after 90 days of composting anaerobically stabilized sewage sludge with sawdust at ratios of 1:1 and 1:3, respectively (Hernandez et al., 2006). In both studies mentioned, the low process efficiency was due to the low concentration of readily biodegradable organic compounds in the sludge after stabilization and the high share of lignocellu-losic materials.
The kinetics of OM degradation (OMloss) was calculated with the
1. order kinetic equation:
OMloss¼Ae kOMt ð2Þ
where:
Athe maximum degradation of OM (g/kg d.m.)
kOMthe rate constant of OM degradation (d 1) ebase of natural logarithm
tcomposting time (d)
Degradation of OM proceeded according 1. order kinetics (Fig. 1b) and the rate constant of OM degradation kOMequaled
0.196 d 1. The maximum degradation of OM (
A) was 201 g/kg d.m.,
whereasrOM(rate of OM degradation) equaled 39.4 g/kg d.m.d. In
the literature, there is little data on the kinetics of OM removal during sewage sludge composting. Instead, most studies on the kinetics of organics removal during composting used municipal waste or waste from food processing, with a variety of composting technologies. However, most of these studies showed that OM degradation proceeded according 1. order kinetics, e.g. Hamoda et al. (1998) (municipal solid waste); Eklind and Kirchmann (2000) (organic household waste with different litter amend-ments); Paredes et al. (2001) (olive mill wastewater);
Bustamante et al. (2008) (distillery waste with animal manure),
Alburquerque et al. (2009)(by-product from olive oil industry). An exception was the research of Doublet et al. (2010). They showed that the kinetics of OM mineralization during aerobically-digested sewage sludge composting were linear (0. order kinetics). So, on this basis it can be supposed that OM degra-dation, in most cases, irrespective of the kind of waste being com-posted, proceeds according to 1. order kinetics. Thus, it may be
(a)
(b)
400 500 600 700 800
composting time [d]
OM
los
s
[g/kg d.m.]
OM 1. order kinetic
kOM = 0.196 d
-1
A = 201 g/kg d.m.
rOM = 39.4 g/kg d.m.·d 0
10 20 30 40 50 60 70 80
0 5 0 5
10 15 20 25 30 35 40 45 50 55 60 10 15 20 25 30 35 40 45 50 55 60
composting time [d]
EOM
,l
o
ss
[%]
Fig. 1.OM removal efficiency (a) and OM removal kinetics (b) during sewage sludge composting.
stayed that this model is useful for calculation of the rate of OM degradation during composting of most types of waste and with different composting technologies.
However, it should be emphasized that in the above studies, the values ofkdiffered, depending on the kind of waste being com-posted, its chemical composition, the conditions during compost-ing and the compostcompost-ing technology. For example, Eklind and Kirchmann (2000)established kinetic constants for organics min-eralization during household waste composting with different additives. The authors showed that kOM values ranged from
0.024 d 1 for kitchen waste to 0.233 d 1 for kitchen waste with
leaves. Much lower kOMvalues (0.048–0.085 d 1) were obtained
byBustamante et al. (2008)during composting of distillery waste with animal manure. These differences between the values ofkOM
may result from the kind of waste composted and its composition (excess sewage sludge in this study vs. kitchen waste or waste with high fibre content, e.g. distillery waste). In general, the lowest values of thekOMwere recorded during composting of waste with
significant amounts of fibers. It is known that lignocellulosic materials are characterized by low biodegradability, which signifi-cantly reduces the rate of mineralization. This was confirmed also by He et al. (2014). They found that during municipal waste composting, OM was degraded in the following order: aliphatic substances > proteinaceous compounds > polysaccharide-like mat-ter and lignin.
In the present study, OM removal efficiency (EOM,loss) and the
values ofkOMandrOMwere 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 con-stituted 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 pro-portion of sewage sludge and influence the economy of the com-posting 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).
3.1.2. Temperature and moisture profiles
During composting, there is an increase in temperature, which depends on the initial temperature and the rate of transformation of readily available substrates, which releases heat. On the basis of these changes in temperature, the composting process often is divided into different phases, i.e. mesophilic, thermophilic and cooling.
In the present study, thermophilic conditions were reached after 1 day. The maximum temperature in the upper layer of com-posted feedstock was 72°C, but in the middle layer it equaled 66.3°C (Fig. 2). This increase in temperature indicates intense min-eralization of easily degradable organic compounds, which is accompanied by the release of a large amount of heat and is char-acteristic of the initial phase of composting.
Thermophilic conditions sanitize the compost because the high temperatures kill pathogens. In this study, thermophilic conditions
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 temper-ature was 69°C. At a lower C/N ratio (C/N 15), they observed that thermophilic conditions were reached after 7 days, the ther-mophilic phase was shorter and the maximum temperature was lower (60°C). According to the authors, the reduction of the max-imum 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 tem-perature 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 tem-perature did not exceed 46°C during 90 days of composting. The main reason for the low temperature of the compost was the pres-ence 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 dur-ing compostdur-ing. 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 penetra-tion 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
(a)
(b)
0 20 40 60 800
composting time [d]
temperature [
oC]
7 cm from the upper edge of bioreactor
63 cm from the upper edge of
0 20 40 60 80
0
20 40 60 80 100 120 140 160 180
20 40 60 80 100 120 140 160 180
composting time [d]
moisture content [%]
Fig. 2.Temperature (a) and moisture (b) profiles during sewage sludge composting.
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.
3.2. Organic matter humification
During the humification process, there are changes in the con-centrations 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 wastew-ater treatment, humification occurs simultaneously with the min-eralization of OM.Réveillé et al. (2003)showed that the content of HS in sewage sludge increases as stabilization proceeds. In unsta-ble 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 sew-age sludge and waste cotton. Increases in HA during composting were also reported byAmir et al. (2006)(from 30 mg/g to approx-imately 55 mg/g OM) andDomeizel 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
considered to be precursors of HS), the different composting tech-nologies and the differences in the conditions during composting, e.g. the maximum temperature and the duration of the ther-mophilic 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 ther-mophilic 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 effi-ciency of lignin degradation was 50%, and in windrows with forced aeration it did not exceed 30%. The authors explained the low effi-ciency of lignin degradation in the aerated windrow as being due to the short thermophilic phase in those conditions. A longer dura-tion of the thermophilic phase favors the formadura-tion of precursors from which humus is created in the cooling phase. In the study byCayuela et al. (2006), there were different proportions of HA and FA in the mature compost: in the windrow with a forced aer-ation, 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 nat-ure 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 con-centrations were 7.3–15.1 mg C/g OM).
Although research on the content of HS and HA in mature com-post is relatively common, data on the kinetics of the humification process are lacking. In this study, the kinetics of HS and HA forma-tion proceeded according to the 1. order kinetic equaforma-tion:
C¼Cmax ð1 e ktÞ þCi; ð3Þ
1. order kinetic S-HA+ L-HA L-HA
kHA = 0.045 d-1
Cmax,HA = 72.1 mg C/g OM
rHA= 3.24 mg C/g OM·d
Fig. 3.Changes in HS (a) and HA (b) concentrations and kinetic constants of humification during sewage sludge composting.
where
Cconcentration of HS or HA in time (mg C/g OM)
Cmaxthe maximum increase in concentration of HS (Cmax,HS) or
HA (Cmax,HA) (mg C/g OM)
kthe rate constant of HS (kHS) or HA (kHA) formation (d 1) ebase of natural logarithm
tcomposting time (d)
Cithe 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,rHSandrHA, equaled
3.46 mg C/g OM d and 3.24 mg C/g OM d, respectively. The values ofkHSandkHAobtained 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 two-stage 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 concentra-tions 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 pro-cess can only be made on the basis of own research. In these stud-ies on sludge composting with different amendments and different technological conditions, humification did indeed proceed accord-ing 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 con-stants for humification and predicting the time at which humifica-tion 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 (kHSandkHA)
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 humi-fication 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 humi-fication 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 compost-ing (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 for-mation 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 relation-ships between OM degradation and humification during compost-ing 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, gener-ated during partial degradation of lignin, inversely correlgener-ated with the humification indices throughout the composting process, which suggests they acted as precursors in the humification pro-cess. According to the authors, this means that HS were synthe-sized from precursors that originated from lignin (the lignin theory). However,Veeken et al. (2000)showed that the condensa-tion route (polyphenol theory) may also have an important contri-bution 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 humifica-tion process during composting of olive mill waste. They found a statistically significant correlation between the lignin/holocellu-lose ratio and humification indices in all composting mixtures which, according to the authors, reflected the involvement of holo-cellulose degradation products (simple carbohydrates) in the for-mation 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
mark-edly 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 frac-tions of HA and FF in HS). TOC decrease is related to the amount of CO2released, 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 matura-tion time has a greater impact on changes within HS (i.e. polymer-ization 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
(per-centage of HA, PHA= (CHA/CHS)100) to evaluate humification
dur-ing compostdur-ing 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 humifica-tion 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 PHAfrom 16.4 to 47.4. Similar results were obtained byParedes 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 indi-cated by increases in DP and PHA. However, they showed that HR
and HI generally did not show a clear tendency during the com-posting process. In contrast,Jouraiphy et al. (2005)noted signifi-cant changes in all analyzed indices (HR, HI, PHAand DP) during
composting of sewage sludge and green waste mixtures. After
135 days of composting there was an increase in HR from 18.1 to 27.9; HI from 7.4 to 19.2; PHAfrom 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 polymer-ization 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 sup-ported by the author’s previous study, which shows that it is pos-sible 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 mobil-ity and stabilmobil-ity 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 humifi-cation 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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