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Article

Composting of Urban Sewage Sludge and Its Application in Quarry Soil Reclamation: A Field Case Study

by
Luyao Zhang
1,
Chong Li
1,
Zengbiao Tian
1,
Mengchao Zhang
1,
Xueyuan Feng
1,
Guannan Liu
2,*,
Zihan Zhu
3,4,
Liming Dong
3,4 and
Yuhao Wang
2,5
1
No. 519 Geological Party, North China Geological Exploration Bureau, Baoding 071000, China
2
MNR Key Laboratory of Metallogeny and Mineral Assessment, Institute of Mineral Resources, Chinese Academy of Geological Sciences, Beijing 100037, China
3
Key Laboratory of Cleaner Production and Integrated Resource Utilization of China National Light Industry, Beijing Technology and Business University, Beijing 100048, China
4
State Environmental Protection Key Lab of Food Chain Pollution Control, Beijing Technology and Business University, Beijing 100048, China
5
College of Chemical Engineering, Beijing University of Chemical Technology, Beijing 100029, China
*
Author to whom correspondence should be addressed.
Agronomy 2025, 15(9), 2179; https://doi.org/10.3390/agronomy15092179
Submission received: 17 August 2025 / Revised: 2 September 2025 / Accepted: 10 September 2025 / Published: 12 September 2025
(This article belongs to the Special Issue Innovative Agricultural Inputs and Soil–Plant–Microbe Interactions for Sustainable Agronomy)
Browse Figures
Versions Notes

Abstract

Mining activities often result in ecosystem degradation and landscape destruction. The restoration of abandoned mine lands is particularly challenging due to the poor physicochemical properties and low fertility of the soil, which necessitate the use of soil amendments. Sewage sludge, which contains abundant nutrients, has potential for use in mine soil restoration. Four separate piles of sewage sludge, each weighing 5 tons, were subjected to aerobic composting and then applied at different rates (0%, 2%, 5%, 10%, and 20%) to reclaim an abandoned mine land site (500 m2). During the composting process, the pH, moisture content, organic matter (OM), and dissolved organic matter (DOM) of the sewage sludge all decreased, while electrical conductivity (EC) and germination index (GI) increased. The sewage sludge compost reached maturity after 83 days. Soil pH and bulk density decreased with increasing application levels, whereas soil fertility, such as OM, alkali-hydrolyzable nitrogen, available phosphorus (AP), and available potassium (AK), significantly improved after application of sewage sludge compost. Vegetation coverage (ryegrass and alfalfa) reached 100% after 2 months at the 20% application level. Fresh biomass and plant height were significantly higher at all application levels compared to the control (p < 0.05). Results from Pearson’s correlation, redundancy analysis (RDA), and the random forest model indicated that soil fertility, particularly AP, OM, and alkali-hydrolyzable nitrogen, rather than soil physicochemical properties, was the key factor influencing the restoration success of the mine site. The use of sewage sludge compost as a soil amendment for reclaiming abandoned mine lands is feasible and can help reduce the ecological restoration costs of mining.

1. Introduction

Mineral resources serve as a fundamental basis for social and economic development. However, the exploitation of these resources has resulted in numerous abandoned mine lands, which give rise to a variety of ecological and environmental problems, such as ecosystem disturbances, biodiversity loss, soil erosion, and heavy metal pollution [1,2,3,4,5]. Especially in the case of opencast mining activities, landscapes and ecosystems are often profoundly damaged. In China, surface mining areas were estimated to cover 4746 km2 by 2015, showing a significant expansion from 1990 to 2015, with an increase of 2.7 times after 2000 [6]. These mining activities have had a profound impact on the sustainable development of ecosystems in China. Abandoned mining lands are particularly difficult to revegetate due to limited soil depth or soil degradation, such as acidification, low fertility, poor physical structure, salinization, alkalization, and contamination [7]. Therefore, to effectively implement mine ecological restoration, it is essential to improve the soil quality of abandoned mine lands through appropriate ecological restoration technologies and to restore both vegetation and landscape features.
Organic waste, such as paper machine sludge, crushed maize straw residues, cereal straw, livestock manure, and sewage sludge, has been utilized to reclaim mine soils, aiming to improve soil physicochemical properties and enhance fertility [8,9,10,11,12]. Urban sewage sludge is a by-product of the urban sewage treatment process and contains significant amounts of organic matter (OM) and essential nutrients such as nitrogen, phosphorus, and potassium [13]. In addition, sewage sludge can serve as a source of microorganisms and enhance soil respiration, owing to its high water-holding capacity and OM content [14]. Therefore, it holds potential for use in the reclamation of abandoned mine lands. However, urban sewage sludge may contain pathogens, heavy metals, and antibiotics, which could pose potential ecological risks to soil and groundwater if applied directly to the soils of abandoned mine lands [10,15]. During aerobic composting of urban sewage sludge, pathogens can be effectively eliminated, and other contaminants may be transformed into more stable forms or degraded [16,17]. Following aerobic composting, sewage sludge compost can be applied to mine soils as a soil amendment, significantly enhancing soil fertility, improving soil physical structure, and promoting vegetation recovery [10,18].
Although sewage sludge compost has been used to improve soil quality, its application in field reclamation of abandoned mine lands remains limited. The study focused on a system involving the composting of different sludge components and its subsequent application in quarry soil reclamation. The objectives were (1) to monitor changes in various parameters during the composting process of different sludge components in order to produce soil amendment and (2) to evaluate the effectiveness of the composted sludge in reclaiming abandoned mining soil under field conditions. Reclamation of mine soil with application of sewage sludge compost facilitates the reduction of ecological restoration costs for abandoned mining lands, while simultaneously accomplishing the dual goals of solid waste utilization and mine site rehabilitation.

2. Materials and Methods

2.1. Materials

Urban sewage sludge was collected from the sewage treatment plant in Baoding City, China. Corn straw and cobs are collected from farmland near the studied area, air-dried, and cut into small pieces. The physicochemical properties, including moisture content, total carbon (TC), and total nitrogen (TN), are presented in Table 1.
The quarry site is located in Baoding City, China, and the native soil physicochemical properties are shown in Table S1. The soil is alkaline and sandy with low contents of organic matter (OM), TN, total phosphorus (TP), total potassium (TK), available P (AP), and available K (AK).

2.2. Aerobic Composting of Urban Sewage Sludge

In March 2023, four separate aerobic composting piles were established: PJ (sewage sludge + corn straw + microbial inoculants), PY (sewage sludge + corncob), PH (sewage sludge + corn straw + corncob), and PX (sewage sludge + corn straw + corncob + microbial inoculants) (Figure S1). Each pile’s weight is about 5 t, and the composting periods were conducted for 83 days. The C/N ratio of the mixture from the four piles was adjusted to approximately 15 by separately adding sewage sludge, corn straw, or corncob in two different applications. Generally, initial C/N ratios ranging from 25 to 30 are regarded as optimal for the composting process. However, composting can also be effectively conducted at lower initial C/N ratios [19]. Operating at reduced C/N ratios may enhance the capacity of manure treatment [20]. In this study, an excessive amount of sewage sludge was prepared, while insufficient quantities of corn straw and corncobs were available. Consequently, the C/N ratio was adjusted to 15. For the PJ and PX piles, 10 kg of microbial inoculants was added at 38 days and 32 days, respectively. Temperature probes were buried at a depth of 50 cm within each pile to continuously monitor temperature at 10-min intervals and transmit the collected data to a computer through a 4G network. On days 1, 16, 26, 37, 42, 47, 58, 64, 73, 78, and 83, the piles were turned to maintain aerobic conditions. At the same time, subsamples were collected from various locations within the piles at a depth of approximately 50 cm for further analysis.

2.3. Reclamation of Quarry Soil by Sewage Sludge Compost

The abandoned mine land at a quarry site located in Baoding City, China, covers an area of approximately 500 m2. Within the study area, vegetation growth is severely restricted due to the poor soil structure and low fertility. The field area was divided into four plots for the purpose of reclaiming surface soil (0–20 cm) by applying four different piles of sewage sludge compost (PJ, PY, PH, and PX) at rates of 0% (CK), 2%, 5%, 10%, and 20%. After 1 week of applying sewage sludge compost and irrigation, the seeds of ryegrass and alfalfa (1:1 dry weight ratio, 10 g/m2) were sown uniformly. Four quadrats (0.5 m × 0.5 m) were collected in each plot treated with varying application levels of sewage sludge compost to assess the plant density of ryegrass and alfalfa after 15 days. After 2 months of reclamation, the vegetation coverage, plant height, and above-ground fresh biomass of ryegrass and alfalfa were measured. Five surface soil samples were collected from each plot with varying application rates of sewage sludge compost and then combined into one sample to assess soil physicochemical properties and fertility.

2.4. Chemical Analysis

2.4.1. Sewage Sludge Compost

Moisture content of fresh sewage sludge compost was determined using gravimetric method. Sewage sludge compost was air-dried and ground into 2 mm. The pH and electrical conductivity (EC) were measured with a pH meter and conductivity meter, respectively, at a soil-to-water ratio of 1:10. OM was determined by measuring the loss of dry-solid mass after ignition at 550 °C in a muffle furnace for 24 h. A weight of 2 g sewage sludge compost was added to 20 mL of deionized water and shaken for 12 h. The resulting extract was centrifuged, filtered through a 0.45 μm membrane filter, and analyzed for dissolved organic matter (DOM) using a total organic carbon analyzer (TOC-V, Shimadzu, Japan). The germination index (GI) was determined by pakchoi seeds and water extracts. In total, 20 pakchoi seeds were placed on filter paper in a sterile dish (9 cm diameter), followed by adding 5 mL of the compost water extract. The dish was then incubated at 20 °C in the dark for 3 days [21,22]. GI was calculated as follows:
GI % = [ Seed germination of treatmen % ] [ Rootlength of treatment ] [ Seed Rootlength of control % ] [ Rootlength of control ] ( % )

2.4.2. Soil and Vegetation

Bulk density was determined by means of the clod method. Soil pH, EC, and OM were determined followed the methods of sewage sludge compost. Alkali-hydrolyzale nitrogen was measured using alkaline hydrolysis diffusion [23]. Available phosphorus (AP) in soils was extracted by a NaHCO3 solution and measured using molybdenum–antimony–scandium colorimetry [24]. Available potassium (AK) was determined after extraction of ammonium acetate using flame photometry [25]. Vegetation coverage for each plot was determined by calculating the number of green pixels in aerial photographs using Adobe Photoshop CS 5.1 [26].

2.5. Multivariate Statistical Analysis

Statistical analyses and Spearman’s correlation were performed with the statistical software package SPSS version 20.0 for Windows. A one-way ANOVA test was used to statistically analyze the differences between datasets. Redundancy analysis (RDA) was performed by Canoco 5. To account for the influence of soil physicochemical properties and fertility on the plant growth, random forest model was performed with the “randomForest” in R 4.3.1 (R Core Team, Vienna, Austria). Variable contribution degrees were quantified by permutation importance (%IncMSE), and p-values were derived through permutation tests.

3. Results and Discussion

3.1. Aerobic Composting of Urban Sewage Sludge

3.1.1. Temperature and Moisture Content

Temperature is considered one of the key parameters for evaluating the composting process [27,28]. The changes in composting temperature are shown in Figure 1, Table S2. The temperatures exhibited notable fluctuations throughout the composting process. At the beginning, the temperature of all piles increased, with a sharp rise observed in PJ, PY, and PH. The temperatures of PJ and PX remained largely below 50 °C during the initial process of composting because of their improper C/N. After adjusting the C/N ratio by adding corn straw or corncob and microbial inoculation for PJ and PX, the temperature rose sharply. The durations with high temperature (>50 °C) were 20, 32, 24, and 33 days for PJ, PY, PH, and PX, respectively. High temperatures can effectively inactivate pathogens during the composting process of sewage sludge [28,29]. The prolonged presence of high temperatures indicates the maturity of compost. Notably, temperature fluctuations during the composting process vary across different piles due to differences in their composition.
An appropriate moisture content is essential for microbial activities and nutrient transport during the aerobic composting process, as it supports the growth and metabolism of microorganisms [30]. Changes in moisture content are shown in Figure 2, Table S2. The moisture content of all piles decreased during the aeration composing. At the end of composting process, the moisture content of PJ, PY, PH, and PX reduced to 39.18%, 43.3%, 43.1%, and 43.3%, respectively. The elevated temperature during composting facilitated water evaporation from the composting materials [31].

3.1.2. pH and EC

At the beginning of composting, the pH of PJ, PY, and PH all rose dramatically within the first 16 days (Figure 3, Table S2) due to generation of NH4+. As the temperature of the compost piles rose, most of the NH4+ was converted into NH3 and subsequently volatilized, leading to a gradual decrease in the pH values of PJ, PY, and PH [32]. For PX, the pH initially slightly increased firstly and subsequently decreased gradually, which might be attributed to the relatively low temperature at the beginning of composting process and slightly less NH3 volatilization than other composting piles. Generally, EC is gradually increased during composting and reached 1830, 2220.5, 2207.5, and 1777 μS/cm, which are comparable with Temel’s study [33]. As the temperature of the piles increases, the metabolism of microorganisms accelerates, leading to enhanced decomposition of organic matter, the production of low-molecular-weight organic acids and water-soluble minerals, and a higher EC [32].

3.1.3. OM and DOM

OM of all piles was reduced gradually due to degradation, and at the end of aeration composting, OM was 43.0%, 34.6%, 36.3%, and 32.9% for PY, PH, PJ, and PX, respectively (Figure 4, Table S2). DOM, which constitutes a heterogeneous mixture of various organic compounds that differ in molecular size, structure, and functional properties, remains an active fraction throughout the composting process [34,35]. Notably, the DOM of sewage sludge compost also decreased during the composting process, which could be divided into two distinct stages: a stage of dramatic reduction (days 1–26) and a stage of gradual reduction (days 27–83) (Figure 4, Table S2). The results indicated that in the initial stage of composting, bioavailable DOM components, such as protein-like compounds, were rapidly degraded by microorganisms. In contrast, stable humic substances within the DOM increased during the later stage of composting [35].

3.1.4. GI

GI serves as a crucial biological indicator for assessing compost maturity and phytotoxicity [22]. A GI exceeding 50% indicates a phytotoxic-free medium, while a GI above 80% indicates mature compost [36]. On the first day of composting, the GIs of all compost samples are very low, ranging from 10–20%, indicating high phytoxicity (Figure 5, Table S2). The GI increased throughout the composting process; almost all compost samples achieved GIs above 80%, indicating the compost had reached maturity. During the composting of sewage sludge, ammonium and organic acids in the materials decreased gradually, thereby reducing phytotoxicity and promoting an increase in GI.
After composting, sewage sludge matures, and the levels of heavy metals and hygienic indicators are presented in Table S3. The results indicated that these parameters meet the requirements specified in NY/T 525-2021 [37], except for Pb. However, the Pb concentration was below the risk screening value for agricultural soil (GB 15618-2018) [38] and the limit for planting soil for greening (CJ/T 340-2016) [39]. Therefore, the findings suggest that the utilization of compost as a soil amendment for soil restoration is safe.

3.2. Effect of Sewage Sludge Compost on Soil Properties and Fertility

3.2.1. Soil Physicochemical Properties

The native soil is slightly alkaline with a pH of 8.47, and the pH of sewage sludge compost is lower than that of the native soil. However, the soil pH changes less after the application of sewage sludge compost at low addition levels (Figure 6). Application of sewage sludge compost at a 20% level resulted in a slightly reduced soil pH. The results were inconsistent with the raising effect of sewage sludge compost on acidic soil pH [40]. The pH values of the reclaimed soils ranged from 7.77 to 8.49, which fall within the range suitable for plant growth. Bulk density of soils generally decreases with increasing application rates of sewage sludge compost across most reclaimed soils. A decrease in soil bulk density indicates an increase in soil porosity, which improves drainage capacity, enhances oxygen availability, and provides more space for root growth [14].
EC reflects the salt content of the material during the composting process and serves as an important indicator for assessing compost quality. High salinity in sewage sludge compost can inhibit plant growth and microbial activity [33]. Notably, EC increased with the addition of sewage sludge compost, showing a general upward trend with increasing application rates (except for PX 20%), which can be attributed to the higher EC of the sewage sludge compost itself. No obvious increase in EC in PX 20% may be attributed to an uneven application during the field study. The highest EC values in the reclaimed soils were 0.841, 0.274, 0.601, and 0.516 mS/cm for PJ, PY, PH, and PX, respectively. Following the application of sewage sludge compost, the EC levels of the soils were found to align more closely with the technical standards for greening planting soil (0.15–0.9 mS/cm), as specified in CJ/T 340–2016 [39].

3.2.2. Soil Fertility

Soil fertility is presented in Figure 7. After application of sewage sludge compost, soil OM also increased with the application rate. The highest contents of OM are 81.3, 47.5, 35.8, and 22.6 g/kg for PJ, PY, PH, and PX, respectively. High soil OM can significantly improve the water retention capacity, porosity, and soil microbial activities [11]. Sewage sludge is rich in nitrogen, phosphorus, and potassium; therefore, the application of sewage sludge compost can enhance their concentrations in soils [41]. Moreover, during the composting process, microorganisms convert these nutrients into bioavailable forms [42]. Soil fertility, including alkali-hydrolyzable nitrogen, AP, and AK, increased after the application of sewage sludge compost and generally rose with higher application rates in most plots. The maximum contents of alkali-hydrolyzable nitrogen, AP, and AK reached 245, 112, and 592 mg/kg, respectively. Notably, the contents of alkali-hydrolyzable nitrogen, AP, and AK were similar under the 5%, 10%, and 20% application rates of PX, which may be attributed to uneven application of sewage sludge compost during the field study. A significant improvement in soil fertility following the application of sewage sludge compost has also been reported in other studies [40,43,44,45].

3.3. Effect of Sewage Sludge Compost on Revegetation

3.3.1. Germination and Vegetation Coverage

Sewage sludge compost, when used as a soil conditioner, can significantly improve soil physicochemical properties and nutrient content. Consequently, plots treated with sewage sludge compost tend to exhibit higher germination rates and enhanced vegetation growth. Raghunathan et al. [9] found application of organic amendments boosted the survival of plants from 80% to 100%. However, no significant improvement in plant density of ryegrass and alfalfa was observed 15 days after reclamation (Table 2). During the first 15 days, there was no significant difference in moisture content among the various groups due to frequent irrigation. Seed germination may depend more on soil moisture content. Notably, a greater amount of alfalfa was observed in each quadrat compared to ryegrass. Although no significant differences in plant density were found across various application levels, higher vegetation coverages were observed in the reclaimed soils 15 days and 2 months after reclamation (Table 3, Figures S2–S4).
Higher vegetation coverages were observed after reclamation (Figures S3 and S4). For most groups, vegetation coverage increased with the application level of sewage sludge compost. Fifteen days after application, the highest vegetation coverages were 42.3%, 38.0%, 42.3%, and 16.0% for PJ, PY, PH, and PX, respectively. The vegetation coverage at the 20% application level almost reached 100%, and high vegetation coverages were also observed in groups treated with lower application levels of sewage sludge compost. These results indicate that the application of sewage sludge compost can significantly enhance the restoration of abandoned mine lands. Notably, vegetation coverage increased with higher application rates of sewage sludge compost in the present study. However, excessive application of sewage sludge compost may lead to increased plant mortality, likely due to elevated levels of DOM and ammonium [46].

3.3.2. Plant Height and Fresh Biomass

The plant heights of ryegrass and alfalfa are shown in Figure 8. The application of compost significantly enhanced the plant heights of both ryegrass and alfalfa, with PJ, PY, and PH showing significant differences for ryegrass and PJ and PY for alfalfa (p < 0.05). In general, increasing the application rate of sewage sludge compost promoted the growth of ryegrass and alfalfa. At the end of the experiment, the average plant heights under the 20% compost addition level were 30.36, 35.18, 48.92, and 34.25 cm for ryegrass and 10.07, 11.90, 12.59, and 9.82 cm for alfalfa, respectively.
Ryegrass and alfalfa were collected and mixed to determine the fresh biomass of the above-ground parts (Figure 9). The fresh biomass of the above-ground parts from the four piles at the highest addition level was significantly greater than that of the control (CK) (p < 0.05). The improvement effect of sewage sludge compost on plant height and biomass has also been observed in other studies [14,40,47].

3.3.3. Relationships Between Soil Physicochemical Properties and Plant Growth

The relationships between soil physicochemical properties and plant growth indices are illustrated in Figure 10. Notably, both soil pH and bulk density exhibited negative correlations with vegetation coverage, plant height, and fresh biomass, with all correlations involving pH being statistically significant (p < 0.05). In alkaline soils, a slight decrease in pH promotes plant growth. Meanwhile, lower bulk density indicates higher soil porosity, which enhances oxygen availability and provides more space for root development [14]. Soil OM, which plays a crucial role in soil restoration, contributes to the formation of soil structure and aggregates, improves soil water-holding capacity, and supplies energy for soil microflora [48,49]. Significant positive correlations were observed between plant growth indices and OM content (p < 0.05). Alkali-hydrolyzable nitrogen, AP, AK, and EC were also significantly correlated with plant growth (p < 0.05). These findings indicate that the application of sewage sludge can significantly enhance soil fertility and improve soil physicochemical properties, thereby promoting plant growth.
RDA results are presented in Figure 11. Soil physicochemical properties explained 70.1% of the total variation, with the first and second axes accounting for 61.39% and 3.80%, respectively. Based on the direction of the arrows, plant growth indices were positively correlated with soil OM, EC, alkali-hydrolyzable nitrogen, AP, and AK, while they were negatively correlated with soil pH and bulk density. These findings are consistent with the results of the correlation analysis. Notably, AP was identified as the primary factor influencing plant growth (p < 0.05), explaining 57.0% of the total variation. Phosphorus plays a crucial role in numerous biological processes, including energy production, nucleic acid synthesis, photosynthesis, glycolysis, respiration, membrane synthesis and stability, enzyme activation and inactivation, redox reactions, cellular signaling, carbohydrate metabolism, and nitrogen fixation [50]. Therefore, the results suggest that phosphorus is a key nutrient for vegetation restoration in this study. CK and most treatments with low application rates of sewage sludge compost were located on the right side of Figure 11, indicating that these groups, characterized by higher pH and bulk density, were associated with reduced plant growth.
The results of random forest model are shown in Figure 12. Vegetation coverage was primarily influenced by OM and AP, followed by alkali-hydrolyzable nitrogen and EC (p < 0.01). Similar results were observed for fresh biomass, where the influence followed the order of AP, OM, and alkali-hydrolyzable nitrogen (p < 0.01), along with AK (p < 0.05). The highest contribution of AP to plant growth was also identified through RDA analysis. Except for AK’s effect on ryegrass height, none of the other soil physicochemical properties or soil fertility indicators showed a significant impact on plant height. These findings suggest that soil fertility, particularly AP, OM, and alkali-hydrolyzable nitrogen, rather than soil physicochemical properties, plays a critical role in mine site restoration in this study.

4. Conclusions

Four separate aerobic composting piles with various urban sewage sludge and crop straw and corncob were conducted to obtain sewage sludge compost. With the composting, moister content, pH OM, and DOM of the sewage sludge decrease, while EC and GI are increasing. Prolonged high temperature and high GI (>80%) indicated sewage sludge is mature after 83 days of aerobic composting. After application of sewage sludge, soil physicochemical properties and soil fertility are apparently improved. Soil pH and bulk density decrease, while EC, alkali-hydrolyzable nitrogen, AP, AK, and OM increase with application level of sewage sludge. Vegetation coverage, plant height, and fresh biomass were apparently enhanced after application sewage sludge compost. The vegetation coverage at the 20% application level almost reached 100%. At the end of the experiment, the average plant heights under the 20% compost addition level were 30.36, 35.18, 48.92, and 34.25 cm for ryegrass and 10.07, 11.90, 12.59, and 9.82 cm for alfalfa, respectively. The application of sewage sludge compost promotes plant growth, primarily by improving soil properties and enhancing soil fertility. Soil fertility, particularly AP, OM, and alkali-hydrolyzable nitrogen, rather than soil physicochemical properties, plays a critical role in mine site restoration in this study. In the future, long-term observations are necessary to further evaluate the effects of sewage sludge compost on soil reclamation. Additionally, indicators related to enzyme activity and microbial communities should be incorporated into such studies. The potential risk of heavy metal release also warrants continued attention and investigation.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/agronomy15092179/s1, Figure S1: Aerobic composting of urban sewage sludge; Figure S2: Photos of plants after reclamation 15 days; Figure S3: Revegetation after application of sewage sludge compost two months later; Figure S4: Revegetation after application of sewage sludge compost (PY); Table S1: Physicochemical properties of native soil; Table S2: Characteristics of composting piles before and after the composting process; Table S3: Heavy metals and hygienic indicators of mature compost; Table S4: Heavy metal concentration in reclaimed soils.

Author Contributions

Conceptualization, L.Z., C.L., Z.T., M.Z., G.L. and L.D.; Methodology, L.Z., M.Z. and X.F.; Software, G.L.; Validation, G.L.; Formal analysis, Y.W.; Investigation, L.Z., C.L., Z.T., M.Z., X.F., G.L., Z.Z. and Y.W.; Data curation, X.F.; Writing—original draft, L.Z.; Writing—review and editing, G.L.; Visualization, Z.Z.; Supervision, G.L. and L.D.; Project administration, L.Z. All authors have read and agreed to the published version of the manuscript.

Funding

This study was financially supported by the Scientific Research Project (Category B) of North China Geological Exploration Bureau (HK2023-B18) and Ministry-Province Cooperative Project under the Ministry of Nature Resources (2024ZRBSHZ092).

Data Availability Statement

The original contributions presented in this study are included in the article/Supplementary Materials. Further inquiries can be directed to the corresponding author.

Conflicts of Interest

The authors declare that they have no conflicts of interest.

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Figure 1. Temperature variations in piles during composting process.
Figure 1. Temperature variations in piles during composting process.
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Figure 2. Moisture content variations in piles during composting process.
Figure 2. Moisture content variations in piles during composting process.
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Figure 3. Variations of pH and EC in piles during composting process.
Figure 3. Variations of pH and EC in piles during composting process.
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Figure 4. Variations of OM and DOM in piles during composting process.
Figure 4. Variations of OM and DOM in piles during composting process.
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Figure 5. Changes in GIs of piles during composting process.
Figure 5. Changes in GIs of piles during composting process.
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Figure 6. Physicochemical properties of reclaimed soils.
Figure 6. Physicochemical properties of reclaimed soils.
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Figure 7. Effect of sewage sludge compost on soil fertility.
Figure 7. Effect of sewage sludge compost on soil fertility.
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Figure 8. Plant height of ryegrass and alfalfa after soil reclamation by sewage sludge compost. Different letters indicate significant differences (p < 0.05, one-way ANOVA).
Figure 8. Plant height of ryegrass and alfalfa after soil reclamation by sewage sludge compost. Different letters indicate significant differences (p < 0.05, one-way ANOVA).
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Figure 9. Above-ground fresh biomass of vegetation after soil reclamation by sewage sludge compost. Different letters indicate significant differences (p < 0.05, one-way ANOVA).
Figure 9. Above-ground fresh biomass of vegetation after soil reclamation by sewage sludge compost. Different letters indicate significant differences (p < 0.05, one-way ANOVA).
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Figure 10. Spearman’s correlation between soil physicochemical properties and plant growth indices. Plant height-r. and plant height-a. are plant heights of ryegrass and alfalfa, respectively.
Figure 10. Spearman’s correlation between soil physicochemical properties and plant growth indices. Plant height-r. and plant height-a. are plant heights of ryegrass and alfalfa, respectively.
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Figure 11. Redundancy analysis demonstrating the relationship between soil physicochemical properties (red arrows) and plant growth indices (blue arrows). Plant height-r. and plant height-a. are plant heights of ryegrass and alfalfa, respectively.
Figure 11. Redundancy analysis demonstrating the relationship between soil physicochemical properties (red arrows) and plant growth indices (blue arrows). Plant height-r. and plant height-a. are plant heights of ryegrass and alfalfa, respectively.
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Figure 12. Random forest importance ranking for plant growth. ** p < 0.01, * p < 0.05. Plant height-r. and plant height-a. are plant heights of ryegrass and alfalfa, respectively.
Figure 12. Random forest importance ranking for plant growth. ** p < 0.01, * p < 0.05. Plant height-r. and plant height-a. are plant heights of ryegrass and alfalfa, respectively.
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Table 1. Physicochemical properties of urban sewage sludge, crop straw, and crop cob.
Table 1. Physicochemical properties of urban sewage sludge, crop straw, and crop cob.
Physicochemical PropertiesUrban Sewage SludgeCrop StrawCrop Cob
Moisture content (%)89.7411.047.98
TC (%)23.4839.1843.97
TN (%)3.881.180.70
C/N6.0533.2062.81
Table 2. Plant density of ryegrass and alfalfa 15 days after reclamation (plants/m2). Different letters indicate significant differences (p < 0.05, one-way ANOVA).
Table 2. Plant density of ryegrass and alfalfa 15 days after reclamation (plants/m2). Different letters indicate significant differences (p < 0.05, one-way ANOVA).
Addition LevelPJPYPHPX
RyegrassAlfalfaRyegrassAlfalfaRyegrassAlfalfaRyegrassAlfalfa
CK6.5 ± 3.1 bc35.5 ± 4.4 ab6.5 ± 4.6 c58.8 ± 16.9 a9.0 ± 1.4 b43.0 ± 16.4 a4.0 ± 2.8 bc37.0 ± 15.6 bc
2%10.0 ± 2.8 a46.5 ± 9.2 a8.0 ± 2.1 bc44.3 ± 6.7 b8.5 ± 1.7 b34.8 ± 10.0 a11.3 ± 3.8 a62.0 ± 9.8 a
5%7.5 ± 2.1 a48.0 ± 1.4 a15.3 ± 3.6 a56.5 ± 11.3 ab13.5 ± 2.1 a45.5 ± 20.5 a10.5 ± 3.5 a29.0 ± 1.4 c
10%10.0 ± 2.8 ab22.0 ± 2.8 b11.8 ± 3.5 ab42.8 ± 10.9 b13.0 ± 0.0 a44.5 ± 14.8 a8.5 ± 0.7 abc54.0 ± 0.0 ab
20%11.5 ± 9.2 a35.5 ± 13.4 ab16.3 ± 5.3 a57.5 ± 4.2 ab16.5 ± 2.1 a34.0 ± 5.7 a3.0 ± 1.4 b43.0 ± 14.1 ab
Table 3. Vegetation coverages of reclaimed soil by application of sewage sludge compost.
Table 3. Vegetation coverages of reclaimed soil by application of sewage sludge compost.
Addition LevelPJPYPHPX
15 Days60 Days15 Days60 Days15 Days60 Days15 Days60 Days
CK0.2%15.3%6.5%28.5%8.2%63.8%30.3%53.4%
2%28.2%71.8%34.7%89.4%28.2%97.3%35.4%50.4%
5%29.9%74.4%18.6%92.4%29.9%100.0%10.2%67.4%
10%26.4%90.3%29.3%100.0%26.4%96.9%14.7%89.4%
20%42.3%97.5%38.0%100.0%42.3%100.0%16.0%95.4%
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MDPI and ACS Style

Zhang, L.; Li, C.; Tian, Z.; Zhang, M.; Feng, X.; Liu, G.; Zhu, Z.; Dong, L.; Wang, Y. Composting of Urban Sewage Sludge and Its Application in Quarry Soil Reclamation: A Field Case Study. Agronomy 2025, 15, 2179. https://doi.org/10.3390/agronomy15092179

AMA Style

Zhang L, Li C, Tian Z, Zhang M, Feng X, Liu G, Zhu Z, Dong L, Wang Y. Composting of Urban Sewage Sludge and Its Application in Quarry Soil Reclamation: A Field Case Study. Agronomy. 2025; 15(9):2179. https://doi.org/10.3390/agronomy15092179

Chicago/Turabian Style

Zhang, Luyao, Chong Li, Zengbiao Tian, Mengchao Zhang, Xueyuan Feng, Guannan Liu, Zihan Zhu, Liming Dong, and Yuhao Wang. 2025. "Composting of Urban Sewage Sludge and Its Application in Quarry Soil Reclamation: A Field Case Study" Agronomy 15, no. 9: 2179. https://doi.org/10.3390/agronomy15092179

APA Style

Zhang, L., Li, C., Tian, Z., Zhang, M., Feng, X., Liu, G., Zhu, Z., Dong, L., & Wang, Y. (2025). Composting of Urban Sewage Sludge and Its Application in Quarry Soil Reclamation: A Field Case Study. Agronomy, 15(9), 2179. https://doi.org/10.3390/agronomy15092179

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