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Article

Composting of Municipal Sewage Sludge and Lignocellulosic Waste: Nitrogen Transformations and Humic Substances Molecular Weight

by
Dorota Kulikowska
* and
Katarzyna Bernat
Department of Environmental Biotechnology, Faculty of Geoengineering, University of Warmia and Mazury in Olsztyn, 10-709 Olsztyn, Poland
*
Author to whom correspondence should be addressed.
Energies 2023, 16(1), 376; https://doi.org/10.3390/en16010376
Submission received: 7 December 2022 / Revised: 21 December 2022 / Accepted: 25 December 2022 / Published: 29 December 2022
(This article belongs to the Special Issue Environmental Evaluation and Energy Recovery in Waste Management)
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Abstract

:
As increasing soil organic matter is considered one of the main strategies for reducing desertification in Europe, the production and use of high-quality composts has gained importance during the last decade. It is undisputed that the quantity and quality of humic substances (HS) and their fractions, i.e. fulvic acids (FA) and humic acids (HA) in compost are considered important indicators of compost maturity and chemical stability. Other important aspects are concentrations of macro- and micronutrients and heavy metals (HMs) that are introduced to the soil with mature compost. In this light, this study assessed the detailed characteristic of compost from municipal sewage sludge. Moreover, kinetic of organic matter (OM), and nitrogen transformations, therein nitrogen loss, were analysed. OM degradation proceeded according to first-order kinetics. In the bioreactor, the rate constant of OM removal and the rate of OM removal were 0.134 d−1 and 12.6 mg/(g d.m.d), respectively. In the windrow, these constants were 5.2-fold and 16.7-fold lower, respectively. In mature compost, the concentration of HS equaled 240.3 mg C/g OM (1.65-fold higher than in the feedstock) and the concentrations of HA and FA were 120.7 mg C/g OM and 119.6 mg C/g OM, respectively. In FA predominated those with a molecular weight in the range of 10–30 kDa (47.2%), FA with a molecular weight >100 kDa accounted for only 14.4%. In HA, however, fraction with the highest molecular weight (>100 kDa) accounted for more than half (51.2%), while the share of HA with a molecular weight <10 kDa was only 6.8%. During composting, nitrogen loss was observed, which resulted from NH3 rather than N2O emission. In mature compost, organic nitrogen predominated (17.82 g/kg d.m.; ca. 92% of the overall nitrogen). The final concentrations of ammonia nitrogen and nitrate nitrogen were 0.23 and 1.12 g/kg d.m., respectively. The compost met the Polish requirements for the content of HMs (the HMs concentrations were as follows: Cd 1.85 mg/kg d.m., Pb 12.16 mg/kg d.m., Ni 11.05 mg/kg d.m., Cr 24.14 mg/kg d.m., Cu 104.24 mg/kg d.m., Zn 854 mg/kg d.m., Hg 0.12 g/kg d.m.).

1. Introduction

Composting of organic wastes such as sewage sludge, with a view to recovering resources from them, has gained importance in the last decade. In municipal sewage sludge management, the main objective is to maximize the use of nutrients while meeting all sanitary, chemical, and environmental safety requirements, and it can be achieved using composting. The second reason for organic waste composting and compost production is the loss of organic matter in soils and the need to recycle stable organic matter to the soil to maintain long-term soil functions. Increasing soil organic matter is considered one of the main strategies for reducing desertification in Europe [1,2]. Therefore, it is essential to step up activities to reduce soil erosion and increase soil organic matter and fertility. Therefore, a model of organic waste management, based on the production and use of high-quality composts, is strongly recommended to contribute to the mitigation of carbon release into the atmosphere Some authors indicate that another promising solution for waste disposal is sewage sludge pyrolysis to obtain char [3].
To have favorable properties for soils, composts should contain a high content of humic substances (HS). HS application has been reported to increase microbial population and activities [4,5]. The addition of HS to sandy soils increases their compactness, while their addition to soils with a heavier granulometric composition loosens the soil [6]. HS have been reported to also increase soil water holding capacity [7,8], which is particularly important in the case of sandy soils, as their water capacity depends mainly on their content of humic substances. HS increase the buffer capacity of soils [9] and regulating and stabilizing its pH. They also affect the sorption capacity of soils: the sorption capacity of HS (300–1400 meq/100 g) exceeds the capacity of the mineral components of soil by 4–12 times, which means that HS are responsible for 20–70% of the total sorption capacity of soil. Moreover, organic matter with a high degree of humification, such as that found in compost, can protect soil and water environment from heavy metal contamination as HS serve as a natural barrier protecting the soil and water [10]. Contamination of soils with metals poses a potential risk of groundwater contamination (a particularly high risk of migration of heavy metals exists in light soils) and exclusion of soils from agricultural use due to metal uptake by plants. In such cases, it is necessary to introduce compounds with high sorption capacity, which enables immobilization of metals in the soil. Due to their high sorption capacity, HS may limit the phytoextraction of heavy metals by plants as well as the leaching and migration of the metals into groundwater.
In light of these considerations, it is undisputed that the quantity and quality of HS in composts are considered important indicators of their maturity and chemical stability. Moreover, the complex biological activity of HS depends on its concentration, chemical characteristics, and molecular weight [11,12]. HA with low molecular weights contain more phenolic and carboxylic functional groups than HA with high molecular weights [13,14]. The chelating ability of HA, which enables efficient alteration of the soil biochemical properties, has also been attributed to the possession of a low molecular weight, while high molecular weight enables efficient improvements of the physical characteristics of the soil [14,15]. The molecular size of HS affects their mode of action in plants, as low molecular weight HS can penetrate the root cells, while high molecular weight HS bind to external cell receptors [16]. It was found that the low molecular weight fraction of HS play a major role in the transformation of metals or pollutants in soil micropores [17].
Apart from HS, important indicators of compost quality are macro- and micronutrients and heavy metals (HMs) concentrations in mature product.
In course of composting, however, kinetic of OM removal, temperature profile and nitrogen transformation cannot be overlooked. During nitrogen transformation, nitrogen loss is observed caused by NH3 and N2O emissions: NH3 emissions cause undesirable and other odor nuisances and N2O emissions have a direct impact on the global warming [18]. This means that it is important to assess whether nitrogen loss during composting resulted from NH3 or N2O emissions. In mature compost that is introduced to soil, nitrogen content and its availability for plants is important [19,20].
This review indicates that during composting, the course of the process as well as the quality of the compost should be analyzed in detail. These all were done in the presented study: in addition to analyze the course of the composting process (kinetic of organic matter removal, temperature profiles, nitrogen transformations), detailed characteristics of mature compost (micro- and macronutrients, HMs and HS content) were performed. To understand the behavior and the role of HS and their fractions, i.e., humic acids (HA) and fulvic acids (FA) in natural processes, knowledge of the molecular weight of HA and FA is essential, which also done in this study. The novelty follows from the multi-faceted analysis of composting process and compost characteristics

2. Materials and Methods

2.1. Bioreactor and Windrow

Composting was carried out in a two-stage system: made of acid-resistant steel bioreactor (100 dm3) was the first stage and the periodically turned windrow was a second stage. The bioreactor was equipped with a fan (that forced air into the aeration box) and a PC THERM REM-84 m dual-channel temperature sensor. In the upper part of the bioreactor, there was a cover, which allowed batch loading and sampling of the composted material. The second stage of composting was carried out under natural conditions.

2.2. Sewage Sludge, Lignocellulosic Materials and Feedstock Characteristics

The sewage sludge came from a municipal wastewater treatment plant located in northeastern Poland. Sewage sludge was characterized by high moisture and high contents of OM and nutrients (Table 1). Concentrations of HMs were lower than limit values reported in [21], which are as follows: Cd 20 mg/kg; Pb 750 mg/kg; Ni 300 mg/kg; Cr 500 mg/kg; Cu 1000 mg/kg; Zn 2500 mg/kg and Hg 16 mg/kg.
Additionally, no live intestinal parasite eggs were found in the sewage sludge samples; however, bacteria of the genus Salmonella were isolated (in 100 g).
High moisture (87%) and low C/N ratio (6.67) in sewage sludge caused that. To prepare an appropriate feedstock, the sludge was mixed with coniferous bark and grass characterized by low moisture, a low nitrogen content, and a high organic carbon content. The characteristics of the lignocellulosic materials used were as follows: moisture 47.9%, OM 97.6%, TOC 54.6%, N 0.41% (coniferous bark) and moisture 32.4%, OM 62.1%, TOC 34.8%, N 0.98% (grass). Taking into account the characteristics of all components, the feedstock was prepared and the share of sewage sludge, coniferous bark and grass in the feedstock were: 75, 15, and 10% in wet mass and 40.1, 32.1, and 27.8% in dry mass. In the feedstock, the moisture and C/N ratio were ca. 76% and 16, respectively.

2.3. Analytical Methods

In the fresh samples taken from the bioreactor and the windrow, the pH, moisture, and dry matter (d.m.) were determined [22]. In water extracts, the ammonium nitrogen [23], nitrite nitrogen [24], and nitrate nitrogen [25] were analyzed. Water extracts were prepared by shaking compost samples with distilled water in a 1:10 (w/w) ratio for 2 h. The mixture was centrifuged at 9000 rpm (15 min) and then filtered through a membrane filter [26]. All analyses were done according to procedures given by the Polish Committee for Standardization.
In dried samples of organic matter (OM) [27], macronutrients [28] and HMs [29] were analyzed. The OM, by ignition of the samples at 550 °C, and macronutrients were completed with the procedures given by the Polish Committee for Standardization. HMs content were analyzed with an atomic absorption spectrometer (Varian AA280FS, Agilent, Mulgrave, Australia). Additionally, the contents of HS, the fulvic acids (FA), and the humic acids (HA) were determined with the use of a 0.1 M NaOH as extractant. However, before the HS extraction, the samples were extracted three times with distilled water in order to eliminate soluble non-humic substances (e.g., sugars and proteins). After that, the samples were defatted with a mixture of chloroform and methanol (2:1) in MARSXpress (CEM, Matthews, NC, USA). Then, the defatted samples were evaporated. The detailed procedure was presented in [26,30]. The content of FA was determined by taking the difference between the content of HS and that of HA (organic C in HS and HA were analyzed using Shimadzu Liquid TOC–VCSN analyzer).
For size-fractionation of dissolved organic matter (DOM), FA and HA in mature compost Amicon Ultra centrifugal membrane filters (Millipore) were used. The pore sizes of Amicon Ultra filters corresponded to nominal molecular weights of 10, 30, and 100 kDa.
In raw sewage sludge and mature compost, the content of live eggs of intestinal parasites (Ascaris sp., Trichuris sp., Toxocara sp.) (PB-102/LM) and the presence of Salmonella [31] were analyzed.

3. Results and Discussion

3.1. Temperature Profiles and Organics Removal

During composting, one of the important indicators is the temperature, which must be high enough for hygienization to take place. During the first 2 days of the experiment, the temperature increased intensively. A temperature above 40 °C was reached after one day, and thermophilic conditions above 55 °C were obtained on the second day of the process (with a maximum temperature of 62 °C). A temperature above 55 °C was maintained until the 9th day. Thermophilic conditions are conducive to hygienization: under these conditions, pathogenic bacteria of the genus Salmonella, eggs of intestinal parasites (Ascarsis sp., Trichuris sp., Toxocara sp.), and weed seeds die, which is important in the sewage sludge composting. After the 9th day, the temperature gradually began to decrease, dropping below 40 °C after about 15 days, and reached 25 °C after 3 weeks. The temperature remained at 20–24 °C during maturation in the turned windrow (Figure 1a). Simultaneously, with the temperature decrease, a decrease in the moisture was observed (Figure 1b).
The changes in temperature are the result of mineralization of organic matter in the feedstock. In the first phase of composting, i.e., mineralization, easily biodegradable simple organic compounds, such as sugars, amino acids, and some easily degradable polymers and lipids, are oxidized. Polysaccharides, such as starch, pectins, proteins, and lipids, are hydrolyzed to sugars, amino sugars, uronic acids, amino acids, alcohols, and fatty acids. Ammonia is formed during the decomposition of proteins. In the initial phase of mineralization, organic acids may accumulate, but they are quickly degraded to carbon dioxide and water. As mineralization proceeds at a high rate, the temperature rises, which initiates the next (thermophilic) phase of the process, when the temperature often exceeds 60–65 °C.
Depletion of easily biodegradable organics and the low biodegradability of the remaining substrates in the composting mass reduce the activity of the microorganisms and leads to a gradual decrease in the temperature. In the cooling phase, intensive decomposition of cellulose and lignin begins.
In this study, the kinetic constants of OM removal were established. It was found that OM removal proceeded according to first-order kinetics. During 21 days of composting in the bioreactor, the rate constant of OM removal, kb, equaled 0.134 d−1 and the rate of OM removal, rb, was 12.6 g/(kg d.m.·d) (Figure 2). The lower values of kinetic constants of OM removal (0.102–0.104 d−1 and 7.8–10.1 g/(kg d.m.·d), respectively) was noted by Kulikowska et al. [32] during composting of sewage sludge, but with different amendments.
The high OM removal rate resulted from the presence of easily biodegradable organic compounds in the sewage sludge. As a result of the aerobic degradation of organic substances, heat is produced, which is retained within the compost, thus raising the temperature. However, it should be emphasized that the most intense OM removal took place during the first 9 days in the bioreactor, which was reflected in an intensive increase in temperature and the maintenance of thermophilic conditions until the 9th day (Figure 1a). After the 9th day, the temperature in the bioreactor began to decrease gradually due to the exhaustion of easily biodegradable organics, reaching the ambient temperature on the 21st day. During composting in the windrow, despite the low temperature, a slight decrease in organic matter content was observed (mainly during the first 20 days of composting in windrow) and the values of the rate constant of OM removal, kw, and the rate of OM removal, rw, were 5.2-fold and 16.7-fold lower than during composting in the bioreactor. However, it must be emphasized that the rate calculated from the 1st order kinetic model is the so-called initial rate, which means that the value of rw (0.754 g/(kg d.m.·d)) resulted from the removal of OM during the first 20 days of composting in the windrow.
The loss of organic matter (OMloss) was calculated with equation given by Paredes et al. [33]. In the present study, the greatest OMloss, up to 49%, was noted during composting in bioreactor, as expected. The overall OMloss (as the sum of OMloss in the bioreactor and windrow) was 56% (Figure 2). Although this value was higher than the results obtained by other authors who composted digested sewage sludge [34,35], some authors have reported even higher OMloss, e.g., Kulcu, Yaldiz [36], who composted goat manure with wheat straw (72%).
As mentioned above, thermophilic conditions in this study lasted ca. 7 days, which was shorter than recommended for technical scale (according to EU regulations, a temperature of 55 °C or more should be maintained for at least 14 days; a temperature of 60 °C or above, for at least 7 days; one of 65 °C or above, for at least 5 days; and 70 °C or above, for at least 3 days). Thermophilic conditions shorter than in technical scale is typical for experiments in laboratory conditions. For example, Bai et al. [37] observed that during composting of cattle manure with fermentative residue, the thermophilic phases lasted only 4–8 days, and depended on the share of substrates in the feedstock; ca. 6–7 days of thermophilic phase was reported also by Yuan et al. [38] when composting kitchen waste with dry cornstalks. The brevity of the thermophilic phase was due to the fact that the volume of the laboratory reactor was limited to ca. 100 L (in this study), and thus the mass of feedstock and, in turn, organic matter used for conversion by microorganisms was limited. In other words, mineralization increased temperature, leading to thermophilic conditions, but after depletion of the relatively small amount of highly biodegradable organics in laboratory scale, the temperature dropped after short time.
Moisture determines the development of microorganisms in compost, as the transport of nutrients necessary for the metabolic and physiological activity of microorganisms takes place in aqueous conditions. Too low moisture inhibits biological processes, while excess moisture fills the compost pores with water, reducing air flow, thus creating anaerobic conditions and inhibit composting. At the beginning of the process, the moisture in the feedstocks and the composting mass ranged from 76 to 74%. When thermophilic conditions were reached, the moisture started to gradually decrease. During maturation in the windrow the moisture did not drop below 50% (Figure 1b).

3.2. Nitrogen Transformations

During composting the concentration of ammonia nitrogen increases mainly in thermophilic phase, when the decomposition of nitrogenous organic substances is most intense. In this study, the concentration of ammonia nitrogen increased (up to 6.5 g/kg d.m.) during the first 9 days of the process. In this period, the greatest increase in cumulative nitrogen loss, up to 40%, was observed (Nloss was calculated based on the equation given by Cayuela et al. [39]) (Figure 3).
In the next days, increase in nitrogen loss was lower and total cumulative Nloss was ca. 50–52%. Initial increase in Nloss is caused by the high temperature and high pH, which shifted the equilibrium towards the formation of the free form of ammonia (NH3). Due to its low vapor pressure, NH3 is easily released from the composting material. However, relatively high total cumulative Nloss in this study may be due to low C/N ratio in the feedstock.
It should be emphasized that in our study there was no increase in Nloss after temperature decrease. This suggests that nitrogen loss resulted from NH3, rather than nitrous oxide (N2O), emissions. Similar results were provided by Yang et al. [40] who showed that during composting of dairy manure, accumulated NH3 emissions were the main contributor to nitrogen loss, whereas accumulated N2O emissions were insignificant. Since the pH is a key factor for NH3 emissions, to limit this process an excessive increases in pH should be avoided. Thus, to minimize NH3 emissions, Cao et al. [41] recommended acidification of the feedstock. Authors reported, however, that although manure acidification significantly reduced NH3 emissions, excessive acidification (to pH 6 or pH 5) increased N2O emissions (to 17.6 and 18.6%, respectively). An additional drawback of acidification was a delay in OM degradation (ca. 7–10 days).
As already mentioned, nitrogen loss may also result from N2O emissions. It is known that N2O is a byproduct of both nitrification and denitrification. He et al. [42] found a positive correlation between surface N2O emissions and the nitrite nitrogen concentration in the composted material: a significant increase in N2O emissions after the addition of nitrite (NO2−) indicated that N2O was produced mainly from NO2−. According to the authors, more than 95% of N2O was produced when readily available carbon sources were depleted. An intermittent addition of fresh food waste reduced the mass-based N2O emissions by 20%. In contrast, a weak correlation was found between the nitrate nitrogen concentration and the N2O emissions: in the latter period of composting, N2O emissions decreased in spite of an increase in the nitrate nitrogen, up to 900 mg/kg. Wu et al. [43] stated that during composting of sewage sludge with chicken manure and rice hulls, the main source of N2O emission was denitrification.
Changes in ammonia concentrations are related to the release of ammonia from the composting material (pH > 8), assimilation of nitrogen by the microorganisms (an increase in the concentration of organic nitrogen), and nitrification, which is indicated by the increase in the concentration of nitrate nitrogen. In the past, the formation of nitrate nitrogen was considered an indicator of maturation; however, nowadays, nitrate nitrogen is also considered a way of nitrogen conservation in compost [44].
Nitrification occurs during the maturation phase [45,46] and this was also the case here. In this study, the predominant oxidized form of nitrogen was nitrate nitrogen. Nitrite nitrogen concentrations were low (<0.001 g/kg d.m.), which is a positive aspect because nitrite nitrogen is considered one of the most harmful forms of nitrogen. However, Chen et al. [47] found that when dairy manure was composted, the nitrite nitrogen concentration was much higher than that of nitrate nitrogen. One possible reason for this is the high level of free ammonia, which could inhibit the oxidation of nitrite nitrogen to nitrate nitrogen. Fukumoto et al. [48] suggested that in manure composting, accumulation of nitrite nitrogen resulted from incomplete nitrification. Thus, improving the composition of the nitrifying communities would be a useful solution for decreasing nitrite concentration in manure composting.
An important factor affecting N2O emission from the composting process is that nitrite nitrogen can be easily accumulated [49], and thus, any solutions that limit nitrite nitrogen concentration would mitigate N2O emissions from composted manure. For example, it was found that biochar amendments significantly reduced the nitrite nitrogen concentration during composting, thereby reducing the amount of nitrogen available for further conversion and N2O production [50].
The nitrogen forms introduced into the soil with compost are of great importance for the quality of mature compost and nitrogen availability for plants. In the present study, organic nitrogen constituted ca. 92% of the overall nitrogen concentration in mature compost (17.82 g/kg d.m.), the rest was in the forms of ammonia nitrogen or nitrate nitrogen (V) (0.23 and 1.12 g/kg d.m., respectively).
As organic nitrogen is not directly available for plants, nitrogen mineralization in soil is essiential and it is known that different factors can affect this process, e.g. temperature, water content and soil properties. However, N availability can depend also on C/N ratio. A C/N ratio lower than 25/1 generally results in N mineralization and C/N ratio higher than 25/1 can lead to N immobilization [51,52]. In the present study the C/N ratio was below 22/1, which indicates possible N mineralization.

3.3. Humic Substances Concentration and Molecular Weight of HA, FA and DOM

An important process during composting is humification, which efficiently converts organic matter into stable HS. The HS content in mature compost gives an information of its ability to sequestrate carbon.
In the mature compost in this study, the concentration of HS was 240.3 mg C/g OM, which was 1.65-fold higher than that in the feedstock (146.5 mg C/g OM). This indicates that during composting, humification proceeded intensively. In the HS in the feedstock, FA predominated: the content of FA was 104.7 mg C/g OM (71.5% of HS), and the rest was HA, with a content of 41.8 mg C/g OM (28.5% of HS). In mature compost, in HS the share of HA and FA was the same: 50% HA and 50% FA (120.7 mg C/g OM and 119.6 mg C/g OM, respectively) (Figure 4a,b). However, it should be noted that the concentration of HA was almost three times higher compared to the feedstock. Although most authors have reported that HA predominates in the HS in mature compost, this is not always the case. In our previous study, it was found that FA predominated in mature compost from sewage sludge with bark (HA 90.8 mg/g OM, FA 221.9 mg/g OM) [53].
Although the importance of HS (including HA and FA) in the soil is undeniable, to understand their role in natural soil processes, knowledge of molecular weight is useful [14,15,16,54].
In this study, therefore, the HA and FA in mature compost were divided into different molecular weight fractions, i.e., <10 kDa, 10–30 kDa, 30–100 kDa, and >100 kDa. The same fractionation was also completed for DOM. In DOM, compounds with a molecular weight below 30 kDa predominated (75.7%) (this fraction was subdivided into 40.2% compounds with a molecular weight of <10 kDa and 35.5% of ones with a molecular weight of 10–30 kDa). The percentage share of compounds with the highest molecular weight (>100 kDa) was only 8.9% (Figure 5a).
In the FA extracted from compost predominated those with molecular weights in the range of 10–30 kDa (47.2%), and FA with a molecular weight >100 kDa accounted for only 14.4% of the total FA (Figure 5b). In contrast, in HA, the largest share (51.2%) consisted that with the highest molecular weight (>100 kDa), and the share of HA with a molecular weight < 10 kDa was only about 6.8% (Figure 5c). It was shown that HA in sewage sludge compost have a similar molecular weight to that of HA from weathered coal. Zhang et al. [55] divided HA derived from Chinese weathered coal into three fractions with molecular weights >100 kDa, 10–100 kDa, and <10 kDa. These authors found that more than 60% of HA had a molecular weight above 100 kDa, while only 3.25% had one lower than 10 kDa. Li et al. [56] characterized FA and HA from sewage sludge. They found that HA were the major component of HS from sludge and that the amount of FA was only one-eighth that of HA. However, 37.7% of the FA had a molecular weight in the range of 30–50 kDa, and 87.8 % of HA had a molecular weight greater than 100 kDa.

3.4. Compost Quality

Mature compost had a high content of organic matter (ca. 75 g/kg d.m.) and macronutrients: 1.78% N, P2O5 20.9 g/kg d.m., K2O 13.42 g/kg d.m., MgO 6.02 g/kg d.m., CaO 18.2 g/kg d.m. HMs content met the requirements of the Regulation of the Minister of Agriculture and Rural Development [57] and were as follows: Cd 1.85 mg/kg d.m., Pb 12.16 mg/kg d.m., Ni 11.05 mg/kg d.m., Cr 24.14 mg/kg d.m., Cu 104.24 mg/kg d.m., Zn 854 mg/kg d.m., Hg 0.12 g/kg d.m. Low concentrations of HMs in compost from municipal solid waste were noted also by Borisova et al. [58]. In mature compost, not only the contents of HMs are important but also their bioavailability. In this study, total HMs concentrations were relatively low; however, low HMs content in compost is not a rule. Therefore, to decrease the HMs bioavailability, different additives are used. For example, Yan et al. [59] showed that the addition of biochar of different quality to composted biogas residue could change the bioavailability of HMs by decreasing the content of exchangeable HMs in the compost, increasing the content of residual HMs, and decreasing the bioavailability of HMs.
No bacteria of the genus Salmonella were found in the compost, which means that both the temperature in the thermophilic phase and duration of this phase were sufficient for hygienization.

4. Conclusions

During composting, the high rate of OM removal provides a sufficiently long thermophilic phase (with a temperature above 55 °C) to obtain higienization. Nitrogen loss was observed mainly in the bioreactor (at high temperature and pH) implying that the loss resulted from NH3 rather than N2O emissions. The total cumulative Nloss was ca. 50–52%.
In the mature compost, the concentration of the most stable organic fraction, i.e., HS, was 240.3 mg C/g OM. Although the concentration of HA increased almost 3-fold during composting (from 41.8 mg C/g OM to 119.6 mg C/g OM), in the HS extracted from the mature compost, the share of HA and FA was almost 50% each. In FA predominated those with a molecular weight in the range of 10–30 kDa (47.2%), but in HA, those with a molecular weight >100 kDa had the largest share (51.2%), and the share of HA with a molecular weight <10 kDa was only 6.8%. In the mature compost, organic nitrogen predominated (92% of total nitrogen; the concentration of organic nitrogen reached 17.82 g/kg d.m.), and the ammonium nitrogen and nitrate nitrogen (V) concentrations were 0.23 and 1.12 g/kg d.m., respectively. The HMs concentrations were as follows: Cd 1.85 mg/kg d.m., Pb 12.16 mg/kg d.m., Ni 11.05 mg/kg d.m., Cr 24.14 mg/kg d.m., Cu 104.24 mg/kg d.m., Zn 854 mg/kg d.m., Hg 0.12 g/kg d.m.). The obtained results provide useful information on the compounds that are introduced into the soil with sewage sludge composts.

Author Contributions

Conceptualization and design of the study by D.K. and K.B.; investigation by D.K. and K.B.; result interpretation and writing of the paper by D.K. and K.B. All authors have read and agreed to the published version of the manuscript.

Funding

Research was financed from the Statutory Project No. 29.610.024-110 supported by the Minister of Education and Science, Poland. Linguistic verification of the paper was financed from the Project supported by the Minister of Education and Science under the program entitled “Regional Initiative of Excellence” for the years 2019–2023, Project No. 010/RID/2018/19, amount of funding 12.000.000 PLN.

Data Availability Statement

Not applicable.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Temperature profile and moisture changes in the bioreactor (a), moisture changes during the whole period of composting (b).
Figure 1. Temperature profile and moisture changes in the bioreactor (a), moisture changes during the whole period of composting (b).
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Figure 2. Changes in organic matter content during composting; in the table, kinetic parameters are given (k—the rate constant of OM removal; r—the rate of OM removal, A—maximal OM removal; subscript b refers to bioreactor, subscript w refers to windrow).
Figure 2. Changes in organic matter content during composting; in the table, kinetic parameters are given (k—the rate constant of OM removal; r—the rate of OM removal, A—maximal OM removal; subscript b refers to bioreactor, subscript w refers to windrow).
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Figure 3. Changes in ammonia nitrogen concentration and nitrogen loss (Nloss) versus pH and temperature conditions.
Figure 3. Changes in ammonia nitrogen concentration and nitrogen loss (Nloss) versus pH and temperature conditions.
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Figure 4. Humic substances (HS), fulvic acids (FA), and humic acids (HA) content in the feedstock and in mature compost (a), percentage share of fulvic acids (FA/HS) and humic acids (HA/HS) in the humic substances extracted from feedstock and mature compost (b).
Figure 4. Humic substances (HS), fulvic acids (FA), and humic acids (HA) content in the feedstock and in mature compost (a), percentage share of fulvic acids (FA/HS) and humic acids (HA/HS) in the humic substances extracted from feedstock and mature compost (b).
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Figure 5. Molecular weight fractions in DOM (a), FA (b), and HA (c) in mature compost.
Figure 5. Molecular weight fractions in DOM (a), FA (b), and HA (c) in mature compost.
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Table
Table 1. Municipal sewage sludge characteristics.
Table 1. Municipal sewage sludge characteristics.
CharacteristicsUnitConcentrations
Moisture%87.0
Organic matter (OM)% d.m.83.8
Organic carbon (TOC)% d.m.46.8
ReactionpH8.01
N% d.m.7.02
P2O5g/kg d.m.27.6
K2Og/kg d.m.9.26
MgOg/kg d.m.4.60
CaOg/kg d.m.7.13
Cd mg/kg d.m.1.89
Pb8.3
Ni9.4
Cr20.9
Cu 87.0
Zn599.0
Hg0.021
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Kulikowska, D.; Bernat, K. Composting of Municipal Sewage Sludge and Lignocellulosic Waste: Nitrogen Transformations and Humic Substances Molecular Weight. Energies 2023, 16, 376. https://doi.org/10.3390/en16010376

AMA Style

Kulikowska D, Bernat K. Composting of Municipal Sewage Sludge and Lignocellulosic Waste: Nitrogen Transformations and Humic Substances Molecular Weight. Energies. 2023; 16(1):376. https://doi.org/10.3390/en16010376

Chicago/Turabian Style

Kulikowska, Dorota, and Katarzyna Bernat. 2023. "Composting of Municipal Sewage Sludge and Lignocellulosic Waste: Nitrogen Transformations and Humic Substances Molecular Weight" Energies 16, no. 1: 376. https://doi.org/10.3390/en16010376

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