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Article

Effect of Water Treatment Plant Sludge Addition on the Composting Efficiency, Quality, and Environmental Sustainability of Sewage Sludge, Food Waste, and Agro-Industrial Waste

by
Daví Matos Lopes
1,
Monica Luci Oliveira de Brito
2,
Josiel Isaac Domingues de Almeida
2,
Danilo Corado de Melo
2,
Jhon Adno de Almeida Santana
2,
Manoel Ferreira Lima Neto
2 and
Maico Chiarelotto
1,2,*
1
Graduate Program in Environmental Sciences, Center for Biological and Health Sciences, Federal University of Western Bahia (UFOB), Barreiras 47808-021, Brazil
2
Center for Exact Sciences and Technologies, Federal University of Western Bahia (UFOB), Barreiras 47808-021, Brazil
*
Author to whom correspondence should be addressed.
Recycling 2026, 11(4), 74; https://doi.org/10.3390/recycling11040074
Submission received: 11 September 2025 / Revised: 24 October 2025 / Accepted: 26 October 2025 / Published: 7 April 2026
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Abstract

This study aimed to evaluate the effects of adding sludge generated in water treatment plants on the composting of sewage sludge, urban organic waste, and agroindustrial waste. Four treatments were conducted with different proportions of water treatment plant sludge (WTS). Four treatments were conducted with 0%, 10%, 20%, and 30% proportions of WTS. The different proportions allowed for the evaluation of the effects of WTS addition on composting. The study was carried out in composting reactors. Kinetic models were applied to study the degradation of organic matter. Physicochemical and microbiological parameters were analyzed. During the process, temperature variation and basal respiration exhibited similar patterns. Principal component analysis showed that the 30WTS (32.2% water treatment sludge) treatment presented higher values of cation exchange capacity (CEC)/total organic carbon (TOC) ratio (3.83), and germination index (94.35%), and lower values of TOC (23.67%) and C/N (carbon/nitrogen) ratio (14.45). The composts produced in all treatments complied with Brazilian regulations for the environmental and agronomic quality of organic composts. It was concluded that the inclusion of up to 30% of WTS in composting did not negatively affect the composting process and did not compromise the environmental or agronomic quality of the final organic composts.

Graphical Abstract

1. Introduction

Given the inevitable generation of waste in various economic sectors, waste management systems must find new ways to promote its valorization. Organic solid waste may contain valuable substances with reuse potential [1]. In addition to waste reduction, it is necessary to manage organic waste productively, as its generation is unavoidable [2].
Composting stands out as a technology capable of treating and valorizing organic waste from different sectors. During the composting process, microorganisms transform organic matter into a stabilized and hygienically safe product [1]. This process plays a prominent role in the circular economy, as it enables the return of nutrients and organic matter to the soil [3]. Characterizing and assessing the potential use of sanitation, urban, and rural waste as sources of nutrients and organic matter in agriculture is an important strategy for recycling these materials [4]. The nutrients present in organic composts produced through composting can be directly reintroduced into the agricultural production cycle, demonstrating technical, economic, and environmental feasibility [5].
Composting allows for the integration of various animal and plant-based wastes, considering their physical, chemical, and microbiological characteristics to control the process. The combination of organic waste from agroindustrial, sanitation, and urban sectors can serve as a strategy to mitigate the negative effects of uncontrolled degradation processes while enhancing the joint valorization of different types of waste. One of the main types of waste generated in urban environments is food waste (FW), which contains high concentrations of degradable substances such as sugars, proteins, and lipids [6]. Gavilanes-Teran et al. [5] indicated that composting horticultural plant waste, with the addition of animal residues and sawdust, is effective for recycling these materials and producing high-quality composts suitable for farm production cycles.
In the agroindustrial textile sector, cotton harvesting and processing wastes are particularly noteworthy. According to Díaz et al. [7], cotton wastes (CW) can be recycled for biosolid production, as they contain organic matter and nutrients. A study by Costa et al. [8], showed that using cotton waste as a carbon source in the composting of broiler industry waste was a viable alternative due to the reduced composting time and nutrient retention in the final product. Sewage sludge (SS) is one of the main wastes generated by the sanitation sector. Sewage sludge (SS) can contain concentrations of heavy metals, emerging pollutants such as pharmaceuticals and microplastics, as well as pathogenic microorganisms [9].
The SS contains high concentrations of nutrients and organic matter and can be recycled for agricultural purposes and the restoration of degraded areas [10]. Chiarelotto et al. [11] concluded that the composting of sewage sludge and cotton waste was a viable strategy for waste treatment and valorization when using C/N ratios between 24.9 and 35.2. SS is widely used as biosolids for application in the restoration of degraded areas and in agriculture, highlighting the need for monitoring and control of treatment technologies [12].
In the sanitation sector, in addition to sewage sludge generation, the production of water treatment sludge (WTS) is also significant. In the process of making natural water potable, solids and other compounds must be removed. One commonly used treatment technology is the complete-cycle method, which includes coagulation, flocculation, sedimentation and filtration steps. Among these, the sedimentation stage is primarily responsible for sludge generation within a water treatment plant [13]. The composition of WTS may include pollutants, sand, silt, clay, and humic substances present in water bodies [14]. In Brazil, WTS is generally discharged into rivers and lakes, constituting the main pollution issue associated with water treatment plants [15]. One alternative for treatment and final disposal of WTS is dewatering followed by landfilling. However, scientific literature contains few studies exploring alternative treatments and the valorization of WTS through composting. There are no known records of composting-based WTS valorization in combination with other residues such as SS, FW or CW. WTS is characterized by low concentrations of carbon and nitrogen [16]. The addition of WTS does not provide significant direct benefits to the composting process, but it may represent a new alternative for final disposal with reduced environmental impact. Therefore, this study operates under the hypothesis that the addition of up to 30% WTS, relative to the total mass under degradation, does not impair the composting process of CW, SS, and FW.
Given the challenges involved in treating, valorizing, and disposing of WTS, studies assessing the effects of WTS addition to composting systems with other organic wastes are essential. The objective of the study was to evaluate how the addition of WTS influences the composting of sewage sludge, urban, and agro-industrial waste, considering process monitoring parameters, stability, and the agronomic and environmental quality of the final compost.

2. Results and Discussion

2.1. Thermal Profile

Temperature is widely recognized as an essential parameter for monitoring the composting process and is directly related to microbial activity [17], serving as an indicator of OM degradation. The bio-oxidative phase of composting is when microbial activity peaks, leading to the most significant degradation of organic matter. During the thermophilic phase, high temperatures prevail, maximizing OM degradation while eliminating pathogens and weed seeds [3]. All treatments exhibited both thermophilic and mesophilic phases (Figure 1).
It was observed that by the second day of the process, all treatments reached temperatures above 40 °C. Gavilanes-Terán et al. [5] reported similar behavior in composting piles composed of poultry manure, sawdust, and vegetable waste, in which temperatures exceeded 40 °C within the first few days. The treatments 20WTS and 30WTS, which included higher amounts of WTS, showed the longest thermophilic phases, lasting 20 days (Table 1). These results suggest that the addition of up to 30% WTS did not hinder microbial activity or the stabilization of residues via composting. To maintain the same C/N ratio (25) for all treatments at the start of composting, the addition of 20% and 30% WTS to the decomposing mass particularly reduced CW. The TOC and TKN concentrations in WTS are low compared to CW (Table 6). This behavior suggests that the TOC and TKN present in the WTS were degraded by microorganisms during the 61 days of composting. The ease of degradation of these compounds may enhance microbial activity during composting and influence higher temperature values and basal respiration [18]. The temperature behavior during composting directly affects compost stabilization and may influence TOC concentrations and the C/N ratio [19]. Therefore, in composting involving a mixture of two or more residues, defining the ideal proportions is necessary for process efficiency.
A way to assess the degradation efficiency in relation to temperature rise is through the exothermic accumulation index (Table 1), calculated by the quadratic sum of the daily differences between the average temperature of the composting mass and the ambient temperature [20]. Higher values of this index indicate a more intense thermophilic phase and more vigorous degradation [21]. Other temperature-related control parameters were also determined (Table 1), including the duration of the bio-oxidative phase (BP), the duration of the thermophilic phase (TV), and the EXI2 index. Lower BP/TV ratios observed in the 20WTS and 30WTS treatments confirm their prolonged thermophilic phase. On the other hand, the higher EXI2/BP ratios found in the 20WTS and 0WTS treatments indicate that these systems operated at higher temperatures more frequently during the process, reflecting greater microbial activity and degradation intensity. In a study by Oliveira et al. [21] evaluating composting treatments of food waste with various carbon sources, the most efficient treatment, which included cotton waste, showed an EXI2 value lower than those recorded for all four treatments in the present study.

2.2. Basal Respiration

BR is among the key indicators of microbiological activity during composting and reflects the overall metabolic activity of all microbial processes involved in the degradation of organic matter [22]. It corresponds to the CO2 production by microorganisms in the composting medium. The progression of BR throughout the composting process exhibited a pattern similar to that of temperature (Figure 2). This behavior aligns with the microbial metabolic activities responsible for CO2 production, which are directly influenced by the temperature of the composting process.
During the thermophilic phase, the 10WTS treatment exhibited the highest BR peak, closely followed by 0WTS. As the process transitioned into the mesophilic phase and readily available nutrients were likely depleted, microbial activity began to decline, with 0WTS maintaining higher BR values for a longer period. Toward the end of the process, BR values across treatments converged, except for 30WTS, which showed a more pronounced decrease in CO2 production. The addition of WTS to the treatments did not significantly alter microbial activity, as CO2 production followed a similar pattern across all treatments. This suggests that WTS addition did not impair microbial function during the composting process.
Furthermore, the addition of WTS did not enhance microbial metabolic activity. This outcome can be attributed to the low organic matter content of WTS. This behavior is evidenced by the consistent BR levels observed in the 0WTS treatment throughout the degradation period.

2.3. Organic Matter Losses

Composting is a mineralization process in which various aerobic and facultative organisms decompose organic matter [23]. Through the microbial degradation of organic matter, nutrients such as nitrogen, phosphorus, potassium, calcium, and magnesium are released and transformed into mineral forms [21]. The degradation profile of OM over time for the four treatments (Figure 3) followed a first-order kinetic equation. Curve fitting yielded the parameters necessary to describe the degradation behavior for each treatment (Table 2).
Parameter A reflects the maximum mineralization of OM (%). The higher the value of A, the greater the capacity for OM degradation during the process. Treatments containing WTS exhibited higher maximum mineralization values, significantly greater than those observed in the control treatment (0WTS). This suggests that the addition of WTS did not act as a limiting factor in the degradation of waste materials. In a composting study using sewage sludge (SS) and construction waste (CW) in different combinations, Chiarelotto et al. [11] reported A values ranging from 61.2% to 67.5% in their most efficient treatments lower than those obtained in this study for treatments containing WTS.
The parameter k represents the OM degradation rate; higher values indicate that degradation was concentrated within a specific time frame, whereas lower values suggest a more evenly distributed degradation process throughout the composting period. The 10WTS treatment stood out with a higher k value compared to the other treatments. This can be attributed to the composition of the initial waste mixture, which included more balanced proportions of easily degradable materials such as FW, and CW with slower degradation. This combination may have resulted in a short-term peak in OM degradation and increasing k values. The 0WTS treatment had a higher proportion of FW but also a higher proportion of CW, which may have reduced the degradation rate (k). The 20WTS and 30WTS treatments showed lower degradation rates compared to 10WTS. This behavior can be explained by the higher proportion of WTS, which contains TOC with slower degradation by microorganisms.

2.4. Evolution of Parameters During the Composting Process

Assessing composting efficiency parameters from the beginning to the end of the process is essential (Table 3). During the bio-oxidative phase, pH values increased in all treatments, reaching alkaline levels by the end of the process. The rise in pH during the bio-oxidative phase may be attributed to ammonia production associated with protein degradation and the volatilization of organic acids during periods of high temperature [24].
TOC levels decreased across all treatments. This reduction is directly related to intense microbial activity, particularly during the thermophilic phase. The decline results from the release of heat, water vapor, and CO2 associated with the degradation of organic matter [5]. Greater TOC reductions were observed in treatments with the addition of WTS (10WTS, 20WTS, and 30WTS). On the other hand, TKN concentrations increased in all treatments. This increase may be due to mass loss and TOC reduction, along with minimal nitrogen losses during the process.
One of the most critical parameters for monitoring composting is the C/N ratio. This ratio directly influences waste degradation, as microorganisms use carbon as an energy source and nitrogen for reproduction [25,26]. An optimal C/N ratio at the start of composting enables greater degradation in a shorter processing time [4]. All treatments in this study showed a decrease in the C/N ratio, with final values below 20, as recommended by both the literature and regulatory standards [25,27]. Therefore, it can be inferred that the addition of up to 30% WTS to the waste mixture undergoing composting did not hinder the stabilization process.
EC exhibited a similar increasing trend when comparing initial and final values. This behavior can be explained by the mineralization of materials and the accumulation of soluble salts in the medium [28]. Similar patterns were also observed by Jalili et al. [29] in composting pistachio waste, cattle manure, and sewage sludge and by Chiarelotto et al. [11] in treatments with higher proportions of cotton waste (CW) in composting food waste (FW) and sewage sludge (SS). Even though the 30WTS treatment showed the greatest TOC reduction, the main factor influencing EC was the proportion of CW. The 30WTS treatment had the lowest proportion of CW in its initial composition and, consequently, the lowest final EC. In contrast, the 0WTS treatment had the highest proportion of CW and, consequently, the highest final EC. CW presented the highest EC value among the composted wastes (Table 6). This behavior affects the quality of the final compost, as EC is an important parameter for agronomic quality [30].

2.5. Final Compost Quality and Principal Component Analysis

Composting results in an organic product that can be used as biosolids for land reclamation [31] and agricultural production [32]. Assessing the environmental and agronomic quality of the final product is essential. One of the most important parameters for evaluating compost quality is the CEC, which tends to increase throughout the composting process due to the humification of organic matter. According to Bernal et al. [25], recommended CEC values should exceed 60 meq/100g. The CEC values obtained in all four treatments surpassed this minimum threshold, indicating stabilization of the composted material. This finding is supported by the CEC/TOC ratio, which serves as an index of organic matter humification [11]. Literature reports minimum CEC/TOC values of 1.7 [33] and 1.9 [34]. All treatments in this study showed CEC/TOC ratios above these reference values (Table 4). The final composts analyzed were not subjected to a maturation process.
The GI can indicate the degree of stabilization and maturation of a compost and is an effective method for evaluating phytotoxicity [25]. According to the classification proposed by Belo [35], a compost is considered non-phytotoxic and stabilized when it exhibits a GI between 80% and 100%. Among the four composts analyzed, the 0WTS treatment showed a GI of 74.29%, suggesting potential phytotoxicity (Table 4). This behavior may be associated with the high electrical conductivity values observed for 0WTS, which had the highest EC among treatments. The high salt concentration indicated by EC for 0WTS may have caused the phytotoxicity of the final compost. The high EC in the final compost can be associated with the higher proportion of CW in the initial composition of 0WTS. A maturation phase for 0WTS could reduce the salt concentration and, consequently, the phytotoxicity of the compost [36]. Elevated salt concentrations, as indicated by EC, can inhibit plant development, particularly during germination [37,38].
The environmental and agronomic quality of composts produced through composting must comply with each country’s regulations [4]. The environmental and agronomic parameter values of the composts produced in this study were compared to Brazilian standards (Table 4). All treatments met Brazilian regulatory requirements for the environmental and agronomic quality of organic composts. The quality of the organic composts was compared with general regulations of the European Union [39] and Italy [40]. All composts met the minimum requirement of 15% organic matter [39]. The final composts also showed pH values (between 5.5 and 11), total nitrogen (>1.5%), total organic carbon (>20%), and total phosphorus (>0.4%) in accordance with the ranges established by Italian standards for biosolids produced from sewage sludge [40].
To assist in interpreting the results of the evaluated parameters in the final composts, principal component analysis (PCA) proved to be an effective tool [41]. Ten variables were analyzed through PCA, and two principal components (PCs) were selected, which together explained 86.88% of the data variance (Table 5). PC1 accounted for 73.83% of the total data variance. The variables EC, TOC, TKN, P, and CEC were positively correlated with PC1, while pH, K, CEC/TOC, and GI showed negative correlations. This group of variables was explained by PC1 due to the characteristics of the wastes and their proportions in the treatments, which influenced the quality of the final compost. For example, treatments that exhibited higher TOC concentrations also showed higher values for EC, TKN, P, and CEC. Conversely, treatments with higher GI values also exhibited higher pH, K, and CEC/TOC ratio. PC2 explained 13.05% of the total variance, with the C/N ratio negatively correlated with this component.
PCA allowed the identification of three distinct groups (Figure 4): group 1 comprised 30WTS; group 2 consisted of 0WTS; and group 3 was represented by 10WTS and 20WTS. Through PCA, it was evident that the 30WTS treatment exhibited higher CEC/TOC ratios and germination index (GI), along with lower TOC content and C/N ratios. This behavior suggests that 30WTS resulted in a slight improvement in the final quality of the organic compost compared to the other treatments. The group composed of 10WTS and 20WTS treatments showed higher CEC/TOC ratios and GI values than the 0WTS treatment. It can be inferred that the addition of up to 30% WTS in the composting process does not impair the environmental or agronomic quality of the organic composts. These results reinforce the potential for the treatment and valorization of WTS in combination with wastes such as SS, CW, and FW through composting.

3. Materials and Methods

3.1. Characterization of Organic Wastes

The organic waste materials used were water treatment sludge (WTS), sewage sludge (SS), cotton waste from processing (CW), and food waste (FW) from urban sources. The physicochemical parameters analyzed (Table 6) for each waste type included pH, electrical conductivity (EC), organic matter (OM), total organic carbon (TOC), total Kjeldahl nitrogen (TKN), and the C/N ratio.
Table 6. Characterization of the physicochemical parameters of organic waste.
Table 6. Characterization of the physicochemical parameters of organic waste.
Parameter WTSSSCWFW
pH5.17 ± 0.154.93 ± 0.156.4 ± 0.005.5 ± 0.00
EC (mS/cm)0.30 ± 0.013.94 ± 0.0812.40 ± 0.267.12 ± 0.03
OM (%)15.36 ± 1.1051.71 ± 1.9888.86 ± 0.5279.61 ± 0.72
TOC (%)8.54 ± 0.6128.73 ± 1.1049.37 ± 0.2944.23 ± 0.40
TKN (%)0.26 ± 0.002.44 ± 0.021.54 ± 0.041.61 ± 0.15
C/N ratio31.95 ± 0.3311.59 ± 0.3532.09 ± 0.8527.73 ± 2.82
EC: electrical conductivity; OM: organic matter; TOC: total organic carbon; TKN: total Kjeldahl nitrogen.
SS was collected from a drying bed at a wastewater treatment plant. This sludge resulted from the discharge of the sludge blanket from an upflow anaerobic sludge blanket (UASB) reactor treating domestic wastewater in the city of Barreiras, Bahia, Brazil. CW was generated during the cleaning and post-harvest processing of cotton and was collected from agroindustrial facilities in Luís Eduardo Magalhães, Bahia, Brazil. FW was collected from a residential area in the city of Barreiras, Brazil, and included all types of discarded food, except meat.
The WTS, the central focus of this study, was collected from a conventional full cycle drinking water treatment plant in the city of Barreiras, Bahia, Brazil. Coagulation and flocculation processes occur in hydraulic tanks with aluminum sulfate as the coagulant. Sedimentation is carried out in high-rate settlers with bottom discharge, where sludge samples were collected for this study. Following sedimentation, water passes through rapid filters, disinfection, and chemical stabilization. The plant has a treatment capacity of 487 L/s, operates 23 h per day, and produces an average flow of 358.97 L/s [42].

3.2. Experimental Design

Four treatments were conducted with varying combinations of SS, FW, and CW, primarily differing in the proportion of WTS. Waste proportions in each treatment were calculated to achieve an initial C/N ratio close to 25 (Table 3):
  • 10WTS: 50% CW + 10.3% WTS + 19.9% SS + 19.9% FW
  • 20WTS: 43.6% CW + 21.7% WTS + 17.3% SS + 17.3% FW
  • 30WTS: 37.8% CW + 32.2% WTS + 15% SS + 15% FW
  • 0WTS: 55.7% CW + 0% WTS + 22.1% SS + 22.1% FW
The mixtures were placed in cylindrical composting reactors with a capacity of 20 L, equipped with drainage systems for leachate collection (Figure 5). Each treatment was conducted in a composting reactor. Monitoring of the degrading mass was carried out during the bio-oxidative phase, characterized by sustained temperatures above ambient levels. Internal and ambient temperatures were recorded daily using a digital thermometer, allowing for the evaluation of the thermophilic phase duration and peak temperature. The exothermic accumulation index (EXI2) was calculated as the quadratic sum of the daily difference between the average compost temperature and ambient temperature throughout the stabilization period [20].
Aeration and homogenization of the composting waste mass were controlled through manual turning on a weekly basis. Moisture was monitored by collecting samples during turning and determining the water content using gravimetry and an oven. After determination, if necessary, water was added to the reactors. During each turning, samples were collected for physicochemical and microbiological analysis. The composting process was considered complete when the internal temperature of the compost mass remained within ±3 °C of the ambient temperature for at least five consecutive days. The composting process lasted 61 days. Final samples were collected for evaluation of compost quality.

3.3. Physicochemical and Microbiological Analyses

For basal respiration (BR) analysis, 8 g of sample were placed in sealed plastic containers alongside smaller vials containing 10 mL of 1M NaOH. The containers were incubated at 25 °C for seven days. CO2 produced during incubation was absorbed by NaOH, and CO2 evolution was quantified by titration with 0.5 M HCl with the aid of a glass burette [43]. For pH and EC, 5 g of sample were mixed with 50 mL of distilled water, shaken for 30 min at 160 rpm, and then allowed to rest for 30 min before measurements were taken using a benchtop pH (Hanna HI2221) and EC meter (Alfakit AT255).
Samples were dried at 105 °C and then incinerated in a muffle furnace (Jung LF7012) at 550 °C for two hours. The mass loss was used to calculate volatile solids (VS). TOC was estimated by dividing the VS concentration by a constant 1.8 [44]. For TKN, samples were dried at 50 °C, ground, and digested at 350 °C with sulfuric acid and 0.7 g of digestion solution in a digestion block (Solab SL25/40R). The digested samples were distilled using a Kjeldahl nitrogen distiller (Solab SL74) with 40% NaOH and titrated with 0.025 mol/L sulfuric acid [43]. The C/N ratio was calculated as the ratio of TOC to TKN.
For phosphorus (P) and potassium (K), dried and ground samples were pre-digested with a nitric-perchloric acid solution and further digested at 220 °C in a digestion block. P concentration was determined via spectrophotometry (Kasvi K37) and K via flame photometry (Tecnow 7000) [45].
For cation exchange capacity (CEC), 2.0 g of sample was mixed with 1.0 g of activated carbon and 100 mL of 0.5 M HCl, then shaken for 30 min. The samples were filtered and rinsed with distilled water until a volume of 400 mL was reached. The residue was washed with 10 mL of calcium carbonate solution and further rinsed to 350 mL. The filtrate was titrated with 0.1 M NaOH using phenolphthalein as an indicator to quantify CEC [46]. The CEC/TOC ratio was determined from the values obtained.
For the germination index (GI), 1:10 (dry matter/water) extracts were prepared and agitated for 24 h at 160 rpm. The solutions were centrifuged at 3000× g rpm for 30 min and filtered through a 0.45 µm membrane. Three mL of the extract were placed in Petri dishes lined with double filter paper and seeded with ten Allium cepa seeds. The dishes were incubated (Limatec LT320TFP-I) for 72 h at 25 °C [47], and root length measurements were used to calculate phytotoxicity.

3.4. Statistical Methods

To model OM degradation over time using a first-order kinetic model [48,49], the function OMloss = A(1 − e−kt) was employed, where A represents the maximum OM degradation (%C), k the degradation rate constant (d−1), and t the composting time (days). The model was selected based on random distribution of residuals, the lowest residual mean square (RMS), and significant F-values. Principal component analysis (PCA) was applied to the measured parameters to assess the final compost quality. To eliminate the influence of variable units, principal components were extracted from the correlation matrix. Components were selected based on eigenvalues greater than 1.0 and a cumulative variance proportion of at least 70% [50].

4. Conclusions

Multivariate statistical analysis of principal components highlighted the differences between the 0WTS, 30WTS, and 10WTS/20WTS treatments. The treatments with WTS addition showed the better stabilization of the wastes. The addition of WTS up to 30% in the composition of the waste mass did not negatively affect the composting process, which showed intense microbial activity through basal respiration values and exothermic indexes. The addition of up to 30% WTS did not negatively impact the agronomic quality of the organic composts produced through the composting process.
The 10WTS, 20WTS, and 30WTS treatments, with 10%, 20%, and 30% WTS of the degrading mass, showed lower EC values and produced a non-phytotoxic final compost. The 0WTS treatment, without WTS, presented an EC of 14.95 mS/cm in the final compost and potential phytotoxicity (GI = 74.29%). The addition of WTS in composting promotes a balance among the proportions of wastes in the degrading mass, enhancing agricultural use while reducing the potential phytotoxicity of the stabilized organic compost.
The results emphasize composting as a promising alternative for the treatment and valorization of WTS in combination with sewage sludge, food, and agroindustrial waste. The results of this study position composting as a strategy for the co-treatment of WTS and SS, offering sanitation companies a potential solution for the proper management of these wastes and the valorization of by-products within a circular economy framework.
Further studies are needed to explore different proportions of WTS in composting, the removal of pathogenic microorganisms, greenhouse gas emissions during the process, the assessment of heavy metal accumulation in the final compost, and the maturation of stabilized composts. Large-scale composting studies combining WTS, SS, FW, and CW should be conducted.

Author Contributions

Conceptualization, D.M.L. and M.C.; methodology, D.M.L. and D.C.d.M.; formal analysis, D.M.L., M.L.O.d.B.; investigation, D.M.L., J.I.D.d.A., J.A.d.A.S. and M.F.L.N.; resources, M.C.; writing—original draft preparation, D.M.L.; writing—review and editing, M.C.; visualization, all authors; supervision, M.C.; project administration, D.M.L.; funding acquisition, M.C. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by Conselho Nacional de Desenvolvimento Científico e Tecnológico (grant number PVA998-2021) and Bahia State Research Support Foundation (FAPESB) through FAPESB Notice No. 0020/2025.

Data Availability Statement

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

Acknowledgments

The authors would like to thank the National Council for Scientific and Technological Development (CNPq) for granting research scholarships. The authors would also like to thank the Research Support Foundation of the State of Bahia (FAPESB) for providing financial support for the publication of this research. The authors would like to thank the Bahia Water and Sanitation Company (Embasa) and the Bahia Association of Cotton Producers (Abapa) for their support in developing the research.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Temperature profile during the composting process.
Figure 1. Temperature profile during the composting process.
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Figure 2. Graph showing the evolution of basal respiration values during the bio-oxidative phase.
Figure 2. Graph showing the evolution of basal respiration values during the bio-oxidative phase.
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Figure 3. Organic matter losses during composting for the treatments.
Figure 3. Organic matter losses during composting for the treatments.
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Figure 4. Biplot graph extracted from principal component analysis (PCA).
Figure 4. Biplot graph extracted from principal component analysis (PCA).
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Figure 5. Schematic diagram of composting reactors.
Figure 5. Schematic diagram of composting reactors.
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Table 1. Temperature control parameters during the bio-oxidative phase.
Table 1. Temperature control parameters during the bio-oxidative phase.
Parameter10WTS20WTS30WTS0WTS
Thermophilic phase duration (days)17202019
Maximum temperature (°C)45.649.147.547.8
EXI2 Index (°C2)8009.179426.528323.998610.10
BP/TV ratio3.593.053.053.21
EXI2/BP ratio131.30154.53136.46141.15
Duration of the bio-oxidative phase: 61 days. EXI2: quadratic exothermic index (quadratic sum of the daily difference between the average temperature of the degrading mass and the ambient temperature). BP/TV ratio: days of duration of the bio-oxidative phase/days of duration of the thermophilic phase (temperature > 40 °C). EXI2/BP ratio: EXI2 index values/days of duration of the bio-oxidative phase.
Table 2. Parameter values of the first-order equation describing the degradation of OM in the treatments.
Table 2. Parameter values of the first-order equation describing the degradation of OM in the treatments.
AkFR2aRMSSEEA × k
10WTS75.90.100282.50.9818.414.297.59
20WTS72.60.081673.30.997.302.705.88
30WTS69.30.06267.80.9273.828.594.30
0WTS52.30.068298.80.988.062.843.56
A—maximum MO mineralization (%); k—degradation rate (d−1); R2a—adjusted coefficient of determination; RMS—residual mean square; SEE—standard error of estimate. For parameters A and k, p < 0.05.
Table 3. Physicochemical parameters for stabilizing the process.
Table 3. Physicochemical parameters for stabilizing the process.
Process StagepHEC (mS/cm)TOC (%)TKN (%)C/N Ratio
10WTS: 50% CW + 10.3% WTS + 19.9% SS + 19.9% FW
Initial5.47 ± 0.158.55 ± 0.3540.06 ± 0.001.60 ± 0.0025.03 ± 0.00
Final8.65 ± 0.2111.60 ± 0.1428.89 ± 1.501.74 ± 0.1316.60 ± 0.61
20WTS: 43.6% CW + 21.7% WTS + 17.3% SS + 17.3% FW
Initial5.07 ± 0.154.90 ± 0.1236.05 ± 0.001.43 ± 0.0025.22 ± 0.00
Final8.55 ± 0.0711.35 ± 0.2127.93 ± 0.351.69 ± 0.1616.52 ± 0.68
30WTS: 37.8% CW + 32.2% WTS + 15% SS + 15% FW
Initial5.47 ± 0.066.77 ± 0.1932.37 ± 0.001.27 ± 0.0025.40 ± 0.00
Final8.60 ± 0.147.68 ± 0.0523.67 ± 0.701.58 ± 0.0514.98 ± 0.21
0WTS: 55.7% CW + 0% WTS + 22.1% SS + 22.1% FW
Initial5.30 ± 0.007.41 ± 0.1643.66 ± 0.001.75 ± 0.0024.88 ± 0.00
Final8.35 ± 0.0714.05 ± 0.2136.42 ± 0.272.50 ± 0.0414.57 ± 0.03
EC: electrical conductivity; TOC: total organic carbon; TKN: total Kjeldahl nitrogen.
Table 4. Final quality parameters of the organic compost at the end of the bio-oxidative phase.
Table 4. Final quality parameters of the organic compost at the end of the bio-oxidative phase.
Parameter10WTS20WTS30WTS0WTSBrazilian Regulations
pH8.65 ± 0.218.55 ± 0.078.60 ± 0.148.35 ± 0.07ad 1
EC (mS/cm)11.60 ± 0.1411.35 ± 0.217.68 ± 0.0514.05 ± 0.21-
TOC (%)28.89 ± 1.5027.93 ± 0.3523.67 ± 0.7036.42 ± 0.27≥15 1
TKN (%)1.74 ± 0.131.69 ± 0.161.58 ± 0.052.50 ± 0.04≥0.5 1
C/N ratio16.43 ± 0.6117.33 ± 0.6814.45 ± 0.2114.75 ± 0.03≤20 1. 2
P (g/kg)4.48 ± 0.074.04 ± 0.433.25 ± 0.286.01 ± 0.09-
K (g/kg)4.60 ± 0.194.24 ± 0.444.46 ± 0.433.78 ± 0.18-
CEC (meq/100g)97.27 ± 1.4393.77 ± 0.9690.67 ± 0.45103.3 ± 0.46ad 1
CEC/TOC ratio3.37 ± 0.203.36 ± 0.063.83 ± 0.092.84 ± 0.02ad 1
GI (%)88.97 ± 0.0990.82 ± 0.1294.35 ± 0.0874.29 ± 0.05-
EC: electrical conductivity; TOC: total organic carbon; TKN: total Kjeldahl nitrogen; CEC: cation exchange capacity; GI: germination index; as: as declared. 1 Ministry of Agriculture, Livestock and Supply Normative Instruction No. 61/2020. 2 National Environmental Council Resolution 481/2017.
Table 5. Matrix of factor loadings for principal component analysis (PCA).
Table 5. Matrix of factor loadings for principal component analysis (PCA).
ParameterPC1PC2
pH−0.7347 *−03153
EC0.9082 *−0.3949
TOC0.9806 *−0.1391
TKN0.9594 *0.1834
C/N ratio−0.3574−0.8949 *
P0.9764 *−0.1391
K−0.7326 *−0.2388
CEC0.9369 *−0.1936
CEC/TOC−0.9453 *0.2397
GI−0.8644 *−0.1580
Eigenvalues7.38291.3055
Variance (%)73.8313.05
Cumulative variance (%)73.8386.88
* High load factor. EC: electrical conductivity; TOC: total organic carbon; TKN: total Kjeldahl nitrogen; CEC: cation exchange capacity; GI: germination index.
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Matos Lopes, D.; Brito, M.L.O.d.; Almeida, J.I.D.d.; Melo, D.C.d.; Santana, J.A.d.A.; Neto, M.F.L.; Chiarelotto, M. Effect of Water Treatment Plant Sludge Addition on the Composting Efficiency, Quality, and Environmental Sustainability of Sewage Sludge, Food Waste, and Agro-Industrial Waste. Recycling 2026, 11, 74. https://doi.org/10.3390/recycling11040074

AMA Style

Matos Lopes D, Brito MLOd, Almeida JIDd, Melo DCd, Santana JAdA, Neto MFL, Chiarelotto M. Effect of Water Treatment Plant Sludge Addition on the Composting Efficiency, Quality, and Environmental Sustainability of Sewage Sludge, Food Waste, and Agro-Industrial Waste. Recycling. 2026; 11(4):74. https://doi.org/10.3390/recycling11040074

Chicago/Turabian Style

Matos Lopes, Daví, Monica Luci Oliveira de Brito, Josiel Isaac Domingues de Almeida, Danilo Corado de Melo, Jhon Adno de Almeida Santana, Manoel Ferreira Lima Neto, and Maico Chiarelotto. 2026. "Effect of Water Treatment Plant Sludge Addition on the Composting Efficiency, Quality, and Environmental Sustainability of Sewage Sludge, Food Waste, and Agro-Industrial Waste" Recycling 11, no. 4: 74. https://doi.org/10.3390/recycling11040074

APA Style

Matos Lopes, D., Brito, M. L. O. d., Almeida, J. I. D. d., Melo, D. C. d., Santana, J. A. d. A., Neto, M. F. L., & Chiarelotto, M. (2026). Effect of Water Treatment Plant Sludge Addition on the Composting Efficiency, Quality, and Environmental Sustainability of Sewage Sludge, Food Waste, and Agro-Industrial Waste. Recycling, 11(4), 74. https://doi.org/10.3390/recycling11040074

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