Heavy Metal Control and Dry Matter Assessment in Digested Sewage Sludge for Biogas Production

Abstract

The expansion of sewage networks and treatment facilities results in considerable amounts of municipal sludge, which is essential for biogas production as part of energy diversification efforts. Principal Component Analysis (PCA) demonstrated a strong correlation between biogas production and its utilization in power generation units. Modernization efforts led to an increase in biogas utilization in power units but a decrease in boiler utilization, independent of the overall biogas production levels. The general linear model (GLM) further confirmed that biogas production was positively influenced by the amount of waste digested, while utilization in power units increased post modernization. A repeated measures ANOVA (Analysis of Variance) indicated significant increases in both dry matter and mineral content in digested sludge compared to raw sludge. SIMPER (Similarity Percentage) analysis revealed that the addition of glycerin water significantly reduced the nitrogen, ammonium nitrogen, and calcium content, while modernization increased these elements and slightly decreased the magnesium concentration. Multivariate dispersion analysis showed that samples treated with glycerin water exhibited less variability in metal content. Regression models explored the factors influencing mineral elements and dry mass in fermented sludge. The zinc content was positively associated with mineral content, while copper showed a negative correlation. The addition of glycerin water increased the mineral content, whereas modernization had the opposite effect. The nitrogen content was negatively correlated with dry mass. These findings provide valuable insights into optimizing sewage sludge treatment and biogas production processes by underlining the approaches for enhancing sludge properties to support efficient biogas production.

1. Introduction

In a traditional linear economy, goods are produced, consumed, and then disposed of as waste [1]. However, the circular economy aims to keep products, materials, and resources in circulation for as long as possible, minimizing waste [2]. In December 2015, the European Commission introduced the communication “Closing the Loop—an EU Action Plan for the Circular Economy”. This document, addressed to the European Parliament, Council of Europe, the European Union Economic and Social Committee, and the Committee of the Regions, outlines new strategies for transforming the European economy [3]. It emphasizes the shift from a linear to a circular economy, promoting the use of technologies to recover resources and energy. The use of sewage sludge as a source of energy and resource recovery aligns with the principles of the circular economy [4].

In 2022, Poland produced 580.7 thousand tons of total solids in sewage treatment plants [5]. The development of the water supply and sewage systems has led to an increase in the capacity of municipal sewage treatment plants and the application of advanced nutrient removal processes [6].

The construction and modernization of sewage treatment plants, such as the implementation of supercritical water oxidation for industrial and sewage sludge [7], are important for integrating these facilities into the circular economy [8]. Innovative technologies for processing sewage sludge can improve the energy balance of treatment plants and create opportunities to use sludge as a valuable raw material in other industries [9,10]. The circular economy will drive future activities in sewage sludge management, focusing on maximizing energy and material recovery [11]. Technologies enabling biogas production from sludge through methane fermentation, biofuel production, and energy generation in microbial fuel cells are becoming increasingly important [12]. Additionally, methods for producing construction materials and recovering nutrients and other reusable substances from sludge are being explored. Municipal wastewater treatment plants (WWTPs) are forecasting an increase in the amount of sewage sludge requiring management. Sewage sludge is generated at different stages of wastewater treatment, and its composition can vary significantly [13]. The primary challenge is that almost all pollutants entering the treatment plant are converted into biomass, making sludge management a complex task [14,15]. The quantity and composition of wastewater and sludge are subject to significant daily, weekly, monthly, and annual fluctuations [16]. There is no uniform composition for municipal wastewater, complicating treatment processes [17,18]. In WWTPs, the growth of microorganisms in biological reactors produces large amounts of sewage sludge. The overgrowth of microorganisms and the accumulation of non-biodegradable organic matter can clog and collapse natural treatment systems. Thus, alternative sludge treatment and reduction methods are increasingly sought after [19]. Considering sludge as a resource rather than waste has led to the recovery of valuable components like organic carbon and nutrients [20]. Implementing sewage sludge treatment technologies in developing countries involves assessing local needs and resources, adopting low-cost and low-maintenance technologies, and providing training and capacity building [21]. Engaging local communities, securing partnerships and funding, and establishing robust monitoring and evaluation frameworks are essential for success [22]. This approach offers a balanced solution to meet the current and future energy requirements while addressing environmental concerns. Advances in sludge reduction strategies, such as identifying peak periods for heavy metals and biological pollutants, can improve WWTP-generated sludge management and technology implementation [23] and deep leering advancement [24]. The levels of heavy metals such as Zn, Mn, Cu, Cr, Ni, Pb, As, and Co in sewage sludge can be potentially hazardous to health and should be monitored [25].

For effective sludge management, several critical research gaps must be addressed to enhance dry matter utilization and maximize biogas production. Scalability assessments are necessary to evaluate the feasibility and cost-effectiveness of various methods while ensuring that treated sludge consistently maintains low heavy metal concentrations. Increasing biogas production calls for comprehensive studies on best practices in co-digestion, particularly the integration of sewage sludge with various organic wastes, heating boilers, and power-generating systems. A critical gap in this study is the integration of glycerin water into the fermentation process for biogas production, which remains unexplored, particularly in the context of its effects on waste disposal and wastewater treatment efficiency. This study also identifies the need for robust monitoring systems to track operational parameters, ensuring consistent biogas output and optimizing production processes. These aspects, especially the use of glycerin water, represent a novel approach within the Polish context, addressing the lack of prior research in this area. In this research, we aim to carry out the following:

(i)

Evaluate the relationship between dry matter in sewage sludge before and after digestion;

(ii)

Determine the heavy metal and nutrient content after the digestion process;

(iii)

Verify whether the addition of glycerin water to sewage sludge increases biogas production in the cogeneration system (boilers and power generation units).

2. Materials and Methods

2.1. Research Object

The sewage sludge used in this study was derived from the Komorowice wastewater treatment plant, located in Bielsko-Biała, Silesian Voivodeship, southern Poland. A detailed description of the technological parameters of this wastewater treatment plant can be found in the paper by Michalski et al., 2024 [4]. This study analyzed the biodegradability rates of the Biochemical Oxygen Demand (BOD5) and Chemical Oxygen Demand (COD) and their relationships to TP and TN in raw and treated sewage. The calorific value, methane content, and electricity consumption and production were also taken into account.

2.2. Stages of Sewage Sludge Processing

2.2.1. Process Control, Automation, and Real-Time Monitoring

The methodology employs advanced wastewater treatment systems incorporating automation and real-time monitoring. Overflow systems and control gates manage the flow to prevent equipment overload, with modernization completed in 2018. SCADA, a Supervisory Control and Data Acquisition technology, ensures real-time process oversight. Grit traps, screens, and separators remove heavy and mechanical impurities, while sedimentation units, washing systems, and pumps optimize impurity reduction. The sludge is processed in fermentation chambers and reception areas using shredders, mixers, macerators, and gravity thickeners, enhancing preparation and treatment efficiency.

2.2.2. Biological Treatment, Precision Aeration, and Chemical Intervention

In this system, wastewater undergoes advanced biological treatment across three bioreactors featuring labyrinthine chambers that facilitate smooth piston flow for uninterrupted processing. Efficient aeration, driven by robust air blowers and fine bubble tubular diffusers, maintains a steady oxygen supply to support microbial activity. The chambers are segmented into zones with integrated submersible mixers that prevent sedimentation by keeping suspended solids uniformly distributed in anoxic and anaerobic areas. Internal recirculation systems reduce the nitrate nitrogen, promoting effective natural denitrification. The oxygen levels are precisely managed by a blower station utilizing sensors, inverters, and soft starters to ensure responsive and stable aeration. Phosphorus removal is addressed via a specialized PIX (iron-based coagulants) dosing station, functioning both as an emergency measure for rainwater tanks during peak flows and for pre-treating fermented sludge before dewatering. PIX doses are calibrated manually based on phosphate analyzer readings or laboratory-tested data. The station’s chemically resistant cylindrical tank, mounted on an acid-resistant tray, ensures safe and environmentally friendly operations.

2.2.3. Excess Wastewater and Robust Emergency Response Mechanisms

The methodology employed in this study focuses on optimizing secondary treatment, rainwater management, and emergency storage systems within wastewater treatment plants. Secondary clarifiers perform the final clarification of wastewater after biological treatment. Radial clarifiers are equipped with devices to scrape sludge and floating debris. Settled sludge is collected in a central hopper and returned to Pumping Station P1 for recirculation, while excess sludge is pumped to gravity thickeners for further processing. Rainwater clarifiers store excess wastewater during heavy rainfall and remove pollutants through mechanical and chemical processes. Rectangular clarifiers with hydraulic bottom scrapers direct sludge to a front chamber. During periods of high inflow, coagulants such as ferric sulfate and powdered polyelectrolytes are added to incoming wastewater to improve coagulation and reduce the contaminant levels before discharge. A retention tank stores and homogenizes daily deliveries of wastewater from non-drainable tanks before treatment. It is equipped with a pump to transfer wastewater to the treatment system and a mixer to ensure even distribution. Emergency storage tanks accommodate excess wastewater during intense rainfall or when the inflow parameters exceed the norms. The overflow chamber between tanks features both manual and automated gates, allowing excess wastewater to be diverted to emergency tanks. These systems are integrated into the plant’s automated PLC (Programmable Logic Controller) and SCADA systems for efficient control and monitoring. This comprehensive methodology ensures effective treatment, the reliable handling of excess wastewater, and robust emergency response mechanisms, enhancing the overall sustainability of wastewater management.

2.2.4. Precise Pumping and Advanced Biological Treatment

The methodology employed in this study integrates advanced pumping systems, buffer storage, biological treatment, and aeration technologies to optimize wastewater management. The pumping stations manage the flow of wastewater and sludge across different treatment stages. Pumps are utilized for draining emergency tanks, rainwater clarifiers, and transferring effluents to subsequent treatment processes. Specialized pumps administer polymers to enhance sludge settling during treatment and optimize coagulant dosing for efficiency. Buffer tanks provide temporary storage for effluents, enabling regulated flow to reactors. Equipped with pumps and mixers, they ensure uniform retention and controlled delivery to deammonification units or biological treatment systems. Deammonification tanks remove nitrogen from sludge dewatering effluents through partial nitrification and denitrification under strict pH control. Submersible mixers, aeration systems with blowers, and sieves support the segregation of Anammox bacteria for efficient nitrogen removal. Biological reactors facilitate advanced wastewater treatment via Dynamic Step Feed systems. These reactors feature multi-chamber layouts with aeration grids and submersible mixers, maintaining suspended solids and enabling denitrification and dephosphatation. Automated valves regulate the oxygen supply, optimizing the treatment conditions in individual reactor zones. The centrifuge, thickener, and blower station handle the sludge management and aeration needs. The thickening processes, aided by mechanical belt thickeners and polymer additives, concentrate excess sludge before fermentation. Centrifuges dewater digested sludge, with liquid effluents directed to buffer tanks for further processing. The SCADA system monitors and controls the equipment, ensuring efficient operation and resource management. This multi-stage approach, combining precise pumping, advanced biological treatment, and robust sludge handling systems, enables sustainable and efficient wastewater treatment operations.

2.2.5. Precise Dosing, Effective Clarification, and Robust Monitoring Systems

The study’s methodology integrates PIX dosing stations, secondary clarifiers, and advanced pumping systems to enhance wastewater treatment and monitoring processes. The PIX dosing station, featuring an acid-resistant tank and automated dosing based on phosphorus analyzer readings, removes excess phosphorus effectively. Secondary clarifiers evenly distribute wastewater and sludge, employing rotary scrapers, floating sludge collectors, and overflow cleaning systems with heated tracks for efficient clarification. Floating sludge is transferred back to Bioreactor II for further processing via specialized pumps. The process water pumping station ensures stable water pressure for cleaning equipment with a hydrophore system and three efficient pumps. Finally, the containerized treated wastewater measurement station monitors effluent quality and compliance with environmental standards using automated samplers, flow meters, and analyzers before discharge into the Biała River. This integrated approach emphasizes precision, efficiency, and sustainability in wastewater management.

2.2.6. Effective Resource Recovery and Sustainable Energy Use

This stage incorporates advanced processes for efficient sludge management, biogas handling, and wastewater treatment system sustainability. Sludge storage and handling facilities include sedimentation fields and a warehouse to temporarily store dewatered sludge, protecting it from rehydration prior to disposal. A pumping station manages the transfer of recirculated and excess sludge for further processing, while the excess sludge tank ensures stabilization and even mixing before thickening. Anaerobic digestion plays a pivotal role in treating sludge, utilizing fermentation chambers equipped with spiral heat exchangers to generate biogas. This biogas is purified in a desulfurization unit and stored in a membrane tank for future use. Any surplus biogas is safely burned in a flaring system to prevent overproduction. Biogas is effectively utilized in cogeneration units to produce both electricity and thermal energy, supporting operational heating requirements. Additionally, gas boilers serve as a backup heat source, capable of utilizing either biogas or natural gas depending on availability. Administrative and operational infrastructure includes SCADA-controlled systems for real-time monitoring and adjustment, with facilities for laboratory analysis, staff amenities, and heat recovery systems. The treatment plant features an advanced leachate reception station for logging and processing delivered wastewater. The biogas drying station ensures optimal humidity levels before directing biogas to cogeneration units and boilers. This integrated methodology ensures effective resource recovery, sustainable energy use, and robust wastewater treatment operations.

2.3. Laboratory Analysis

Measurements of biogas production, the utilization of biogas in power generation units and in boilers, as well as variables describing sewage sludge—dry mass, mineral content, heavy metal content, and nutrients—covered the years 2014–2023 and were conducted once in the following months: January, March, May, July, September, and November.

Dewatered sludge samples were collected in accordance with the standard PN-EN ISO 5667-13:2011, Section 6.3.6 in [26]. Meanwhile, centrifuged sludge was collected as composite samples from waste piles (30 samples for sludge volumes exceeding 100 m³).

Glycerin water was delivered to the wastewater treatment plant usually once a week. Dosing was performed by a separate pump from the glycerin water tank at a maximum rate of 800 L/day. It was applied to the pipeline supplying sludges and immediately mixed before introducing it into the anaerobic digestion chambers. The glycerin water has the following parameters: oxidation-reduction potential (77 mV), conductivity (13.97 mS/cm), dry mass concentration (55.9 g/dm³), organic carbon (48.38%), organic nitrogen (0.17%), ammonium (254 mg/dm³), and a pH value of 5.6. Glycerin water was added to one part of the sewage sludge. However, for comparison purposes, no glycerin water was added to the few control samples.

Digested sludge from Shared Fermentation Chambers (SFCs) was collected as single samples directly from the installation. The mineralization of samples for metal content was performed in accordance with PN-EN 13346:2002 [27]. A wet sludge sample was mineralized with aqua regia in a microwave mineralizer. The dry mass content was also determined in accordance with PN-EN 12880:2004 [28]. The sludge was neither dried nor ground prior to analysis. For heavy metal determination, aqua regia was used as the reagent (extractant). The analytical equipment used at this stage of the analysis included a Microwave Mineralizer (Anton Paar, Graz, Austria). The content of heavy metals was measured using the ICP-OES method in compliance with the standard PN-EN ISO 11885:2009 [29] and using the Agilent 5800 VDV spectrometer (Agilent, Santa Clara, CA, USA). Reference materials were also applied, specifically CRM Trace Metals—Sewage Sludge 3 (Sigma-Aldrich, St. Louis, MO, USA, catalog no. CRM031-40G). Parameters characterizing the method (working range, repeatability, intermediate precision, accuracy, recovery, and uncertainty) were established during the method’s validation. The threshold values for the working range were defined based on the identified analytical needs and the scope specified in the standard. The samples of sludge analyzed had elemental contents within the predefined working range. The analysis adhered to the standards PN-EN ISO 11885:2009 [29] and PN-EN 13346:2002 [27], and was performed in compliance with the Minister of Climate and Environment’s Regulation of 31 December 2021, amending the regulation on municipal sewage sludge [30].

2.4. Statistical Analysis

To identify the key factors affecting biogas production and its utilization in a wastewater treatment plant, a GLM (Generalized Linear Model) analysis and a PCA (Principal Component Analysis) were conducted [31]. The first GLM included biogas production in m³ as the dependent variable and the following independent variables: the percentage of dry mass in digested sewage sludge, the amount of waste accepted for the fermentation process (Mg·year⁻¹) as a continuous variable, and the technological process modernization factor as a categorical variable. We use one dataset with multiple variables, each measured across 60 samples.

All predictors had VIF values below the commonly used threshold of 5, indicating acceptable levels of multicollinearity. The second model included biogas utilization in power generators as the dependent variable, with the percentage of dry mass and the modernization factor as independent variables. For the variables included in the model, the VIF values were below 2. This indicates that multicollinearity was not a concern in the analysis. The PCA aimed to illustrate the relationship between biogas production and its utilization in power generation units and boilers in the context of technological process modernization in the wastewater treatment plant.

The second analysis aimed to compare the dry mass % and mineral content % in raw and fermented sewage sludge, using repeated measures analysis of variance (ANOVA). To test the impact of the application of glycerin water and technological process modernization in wastewater treatment on the content of heavy metals and biogenic elements, a PERMANOVA (Permutational Multivariate Analysis of Variance) [32,33,34] with 999 permutations was performed, supplemented by a post hoc SIMPER (Similarity Percentage) analysis [35,36]. In the next step, a multivariate dispersion analysis of the samples was applied. The first analytical approach examines how the content of metals and biogenic elements differs between the studied groups. The second analytical approach assesses whether the measurements of the metal and biogenic contents within a given study group are more homogeneous, meaning more similar to each other compared to measurements obtained in the other group. This will help determine whether changes in the wastewater treatment process or the application of glycerin water stabilize the content of metals and biogenic elements in the sludge and whether the treatment process itself is resistant to random fluctuations and changes. In summary, the first approach identifies differences in the metal and biogenic contents, while the second analytical approach determines whether the obtained measurements are consistent with each other, indicating a more efficient and reliable technological process.

The vegan package in R was used to compute multivariate dispersion, which is based on the average distances of points from their group centroid in the space defined by the distance matrix. The formula for this dispersion is as follows [37,38]:

$$D_i=\sqrt{{\displaystyle \frac{1}{n_i}}\sum _{j=1}^{n_i}{d}_{ij}^2}$$

where:

• D_i is the dispersion for group i;
• n_i is the number of samples in group i;
• d_ij is the distance of point j from the centroid of group i.

The analysis works by performing PCoA (Principal Coordinates Analysis) on the input distance matrix. After projecting points into a multidimensional space, it calculates the variance of distances to the group centroid, which allows for the comparison of dispersion between groups using permutation F-tests with 999 permutations.

The choice of dissimilarity measures aligns with the objectives of the analyses performed. For PERMANOVA and SIMPER analysis, the Bray–Curtis distance is appropriate as it focuses on compositional differences rather than absolute values, making it suitable for assessing variations in the content of heavy metals and biogenic elements in sewage sludge. This measure accounts for relative differences between samples and is robust to variations in concentration scales, which is critical for analyzing environmental data. On the other hand, the Euclidean distance [39] is well suited for the multivariate dispersion analysis, as it directly measures absolute differences and provides an effective way to assess group dispersion in multivariate space. Since the analysis evaluates the homogeneity of group variances, the Euclidean distance allows for a straightforward interpretation of the dispersion of samples within groups. In order to assess the effects of the selected factors, including the modernization of the wastewater treatment process and the addition of glycerin water, on the dry mass content and mineral content of fermented sludge, we employed model selection and model averaging procedures based on information theory. In the first global model, the mineral content was the dependent variable, while the independent variables (predictors) included process modernization and glycerin water addition as categorical variables, and heavy metals—zinc, copper, cadmium, lead, and chromium—as continuous variables. For the variables included in the global model, the variance inflation factor (VIF) values were close to 2, typically below or around 2, and did not exceed 2.5. This indicates that multicollinearity was not a concern in the analysis. Nickel was excluded from the analysis due to its strong correlation with zinc (r = 0.71) and its relatively high VIF = 2.74, the highest among the considered variables.

In the second global model, the dry mass content was the dependent variable, with process modernization as a categorical predictor and nitrogen, calcium, and phosphorus as continuous predictors. For the variables included in the global model, the VIF values were below 2, and did not exceed this number. To identify the set of best-supported models among all possible models, we applied the Akaike information criterion corrected for small sample size (AICc). We ranked all models based on their ΔAICc values, where ΔAICc represents the difference between a given model and the one with the lowest AICc. Models with ΔAICc < 2 were considered well supported. Model results were then averaged across all supported models using their Akaike weights, which indicate the probability of a given model being the best among those tested [40]. The parameters included in the models were at most moderately correlated, with Pearson’s r below 0.5, ensuring that they remained within the acceptable range for treating variables as independent in multivariate analyses [36]. ANOVA for repeated measurements and GLMs were calculated using STATISTICA 13.3 [41]. Multivariate analysis was performed using R software R-4.5.0 [42], the vegan package [43], whereas model selection and parameter averaging procedures were run in R software using the MuMIn package [44]. The PCA was calculated using Canoco for Windows 4.5 [31].

3. Results

3.1. Heavy Metal and Nutrient Contents

The data present a comparison of sewage sludge characteristics before and after digestion. The dry matter content significantly increases post digestion, rising from 5.3% to 25.18%, indicating a substantial reduction in the water content. Likewise, mineral substances grow from 23.46% to 37.99%, suggesting concentration due to the decomposition process. The heavy metal concentrations of metals such as Cd, Ni, Pb, Cu, Zn, Cr, and Hg varied across samples, with Pb showing the widest range (13.9 to 617 mg·kg⁻¹ DM). Nutrients such as nitrogen and phosphorus remained relatively stable, while calcium and magnesium demonstrated modest variation (

Table 1. Variables measured in sewage sludge in the years 2014–2023.

Variable	N	Mean	Confidence −	Confidence +	Minimum	Maximum
Dry matter of sewage sludge before digestion (%)	60	5.30	5.123	5.474	3.26	6.44
Mineral substances in sewage sludge before digestion (%)	60	23.46	22.589	24.336	18.6	33.77
Dry matter in sludge after digestion (%)	60	25.18	24.755	25.607	20.82	28.36
Mineral substances in sewage sludge after digestion (%)	60	37.99	36.997	39	30.78	47.03
Content after digestion
Cd (mg·kg−1 DM *)	60	2.56	2.323	2.788	1.21	5.42
Ni (mg·kg−1 DM)	60	65.25	57.7	72.792	35.3	196
Pb (mg·kg−1 DM)	60	108.08	80.102	136.062	13.9	617
Cu (mg·kg−1 DM)	60	305.89	294.26	317.52	147	388
Zn (mg·kg−1 DM)	60	1308.56	1236.54	1380.587	582	2480
Cr (mg·kg−1 DM)	60	103.99	96.578	111.398	43.8	242
Hg (mg·kg−1 DM)	60	0.73	0.609	0.854	0.00	2.56
Ammonium N (% DM)	60	0.85	0.801	0.889	0.56	1.24
N (% DM)	60	5.38	5.236	5.53	4.11	6.47
P (% DM)	60	2.88	2.767	2.997	2.18	5.38
Ca (% DM)	60	1.98	1.887	2.08	0.48	2.57
Mg (% DM)	60	0.71	0.677	0.734	0.21	0.94

* DM—dry mass.

).

3.2. Biogas Production

Biogas production and its utilization in power generation units were closely related, meaning that the more biogas produced, the more was used in the power generation units. These variables were strongly associated with the first ordination axis (scores of 0.981 and 0.993, respectively). Biogas utilization in power generation units increased after modernization, whereas biogas utilization in boilers decreased. Additionally, it was not dependent on the amount of biogas produced. This variable was associated with the second ordination axis (score of 0.874), which explained 4.6% of the data variability. The first ordination axis of the PCA explained 95.3% of the variability in the data describing biogas production and utilization (Figure 1).

The addition of glycerin water and process modernization of the inclusion of statistically significant effects were introduced to the nutrient content in the fermented sludge (F = 29.119, p = 0.001; F = 2.876, p = 0.039, respectively). For the content of heavy metals in the sludge, process modernization was statistically insignificant (F = 1.912, p = 0.114), and the controlled glycerin water was on the border of significance, which is understandable (F = 2.053, p = 0.094). The SIMPER analysis maintains that glycerin water results from the content of total nitrogen and ammonium nitrogen and the content of the sludge (

Table 2. Factors affecting biogas production and utilization in wastewater treatment plant (N = 60).

Biogas production (m3·month−1)
	df	F	p
Intercept	1	16.363	<0.001
Dry mass	1	1.924	0.171
Waste approved	1	8.766	0.004
Modernization	1	0.189	0.666
Error	56
Utilization of biogas in power generation units (m3·month−1)
	df	F	p
Intercept	1	18.765	<0.001
Dry mass	1	2.904	0.094
Modernization	1	11.608	0.001
Error	57

). On the other hand, in process modernization, which was determined by the content of total nitrogen, ammonium nitrogen, and sea nitrogen, as well as a slight discharge of chemical substances (Table 2), the addition of glycerin water and the modernization of the treatment process had a statistically significant effect on the content of biogenic elements in the digested sludge (F = 29.119, p = 0.001; F = 2.876, p = 0.039, respectively). As for the content of heavy metals in the sludge, process modernization was statistically insignificant (F = 1.912, p = 0.114), while the addition of glycerin water was on the border of significance, which suggests a small effect (F = 2.053, p = 0.094).

The GLM analysis revealed that biogas production increased when a larger amount of waste was accepted for the digestion process (Table 2). Biogas utilization in power generation units was higher after technological process modernization (Table 2, Figure 2). Both the dry mass content and the mineral fraction content were higher in fermented sewage sludge compared to raw sludge (

Table 3. Results of repeated measures ANOVA for the comparison of raw sludge and digested sludge (N = 60).

Dry mass content %
	df	F	p
Intercept	1	16,789.542	<0.001
Repeat measure	1	7803.729	<0.001
Error	59
Mineral elements content %
Intercept	1	5173.307	<0.001
Repeat measure	1	1389.108	<0.001
Error	59

, Figure 3).

SIMPER analysis revealed that the addition of glycerin water led to a decrease in the total nitrogen, ammonium nitrogen, and calcium contents in the sludge. On the other hand, the modernization of the treatment process resulted in an increase in the total nitrogen, ammonium nitrogen, and calcium contents, as well as a slight decrease in the magnesium content (

Table 4. Results of SIMPER analysis for biogenic element contents in digested sewage sludge.

Addition of glycerin water
	YES (mean amount)	NO (mean amount)	p
N	5.036	5.730	0.001
Ca	1.755	2.211	0.001
P	2.915	2.850	0.669
Ammonium N	0.754	0.936	0.001
Mg	0.697	0.714	0.278
Modernization process
	BEFORE (mean amount)	AFTER (mean amount)	p
N	5.240	5.956	0.001
Ca	1.938	2.163	0.005
P	2.897	2.823	0.990
Ammonium N	0.816	0.961	0.017
Mg	0.707	0.700	0.041

).

In the case of multidimensional dispersion, only the addition of glycerin water significantly enabled the use of dispersion (F = 5.713, p = 0.012), showing that the sending from the glycerin organization was more similar to itself and less alternative in terms of the range of the group from the group to which glycerin water was not available (Figure 2). In the case of multivariate dispersion, only the addition of glycerin water significantly modified the dispersion of the samples (F = 5.713, p = 0.012), making the samples with glycerin water more similar to each other and less diverse in terms of metal content compared to the group where glycerin water was not used (Figure 4).

The weighted-average (

Table 5. Outcome of analysis of factors affecting mineral elements and dry mass in fermented sewage sludge (N = 60).

Mineral elements %
	Estimate	Adjusted SE	z value	p
Intercept	32.454	3.204	10.13	<0.001
Zn	0.005	0.002	3.295	<0.001
Addition of glycerin water (YES)	2.440	1.097	2.223	0.026
Cu	−0.021	0.010	2.153	0.032
Modernization (BEFORE)	4.193	1.194	3.513	<0.001
Cd	1.002	0.638	1.572	0.116
Pb	0.005	0.004	1.183	0.237
Cr	−0.016	0.014	1.148	0.251
Dry mass %
	Estimate	Adjusted SE	z value	p
Intercept	34.021	2.0586	16.527	<0.001
N	−1.838	0.3495	5.258	<0.001
Ca	0.687	0.4837	1.420	0.155
Modernization (BEFORE)	0.638	0.4885	1.307	0.191
P	0.351	0.3842	0.913	0.361

) results of the models explaining the patterns of the mineral element content in fermented sewage sludge showed that a higher zinc content had a significant positive effect on the mineral fraction of the sludge. In contrast, a higher copper content in the sludge was associated with a lower proportion of mineral matter in the sewage sludge. The addition of glycerin water increased the mineral fraction in the sludge, while process modernization led to a reduction in the mineral content. On the other hand, a higher total nitrogen content contributed to a decrease in the dry mass of the fermented sewage sludge.

4. Discussion

4.1. Managing Sewage Sludge in a Circular Economy: Challenges, Digestion Treatment Effects, and Resource Utilization Benefits

Digestion significantly altered the sewage sludge composition, increasing the dry matter from 5.30% to 25.18% and mineral content from 23.46% to 37.99%. Heavy metal concentrations varied, with Pb ranging from 13.9 to 617 mg·kg⁻¹ DM and Zn averaging 1308.56 mg·kg⁻¹ DM. The nutrient content remained notable, with total nitrogen at 5.38% and phosphorus at 2.88%, highlighting the impact of digestion on the sludge properties (Table 1). Cu and Zn are often used together in alloy production and galvanization processes, while Ni and Cr are used in alloy production and metal plating due to their corrosion resistance. The high correlations indicate that these elements may be interdependent or share common sources. Significant differences were found in the distribution of nutrient concentrations among the sampled WWTPs. Higher total concentrations of nutrients were observed in WWTPs, with Fe, Ca, P, and N being the most abundant [45].

The circular economy is a growing global trend aimed at optimizing resource use and minimizing waste. In this model, resources are utilized for as long as possible, and waste is repurposed as raw materials [46]. This approach is increasingly influencing water and wastewater management practices in the EU [47]. Specifically, sewage sludge is now viewed as a resource for nutrients and energy, rather than as mere waste. Existing legal frameworks adequately address sewage sludge management, encompassing its use in land recovery and thermal processing [48]. However, challenges persist due to the absence of specific guidelines on sediment stabilization, resulting in varying sludge quality [49]. Strict criteria for landfilling municipal sewage sludge often render this option impractical [50]. Issues arise when nearby incineration facilities are unavailable or when sludge fails to meet the incineration quality standards [51]. Moreover, the sludge may not find suitable applications above ground. The effective management of sludge at both regional and central levels is hindered by inadequate data on sludge quality from wastewater treatment plants [52]. Currently, there is no legal obligation to test the sludge not intended for above-ground use. It is critical to ascertain whether challenges in using municipal sewage sludge above ground stem from quality deficiencies or other barriers [53].

Managing sewage sludge in the context of a circular economy presents several challenges. Technical and regulatory hurdles are significant [54]. The diverse composition of sewage sludge, containing organic materials, heavy metals, and pathogens, complicates its treatment and safe reuse [55,56]. Varied regulatory frameworks across regions necessitate advanced treatment technologies and substantial investment. Public perception and the acceptance of using treated sludge in agriculture or land reclamation applications can also be a major barrier. An analysis of sewage sludge composition revealed significant variations in heavy metal concentrations, both before and after digestion. Samples exhibited elevated levels of Zn, Cu, and Pb, with maximum values reaching 2480 mg·kg⁻¹ DM, 388 mg·kg⁻¹ DM, and 617 mg·kg⁻¹ DM, respectively. These levels, along with fluctuations observed in Cd, Ni, Cr, and Hg, underscore the necessity for rigorous monitoring and treatment strategies to mitigate the potential environmental risks associated with sludge disposal (Table 1). Concerns about health risks and environmental impacts require thorough public education and transparent risk communication [57]. However, these risks include ash-containing heavy metals, which must be carefully managed. The quality of the end product depends on the treatment parameters and initial sludge composition [58]. These approaches not only mitigate the environmental impacts of waste disposal, but also extract economic value from previously underutilized resources [59]. While hazardous waste requires testing before incineration, there is no universal requirement to test all sludge. Nevertheless, testing is critical to prevent exceeding air emission limits when sludge enters incineration facilities [60].

A significant portion of municipal sewage sludge management involves unspecified methods (“Intended for other purposes”) and on-site storage at sewage treatment plants, indicating legal barriers to effective management. Phosphorus recovery from sewage sludge, integral to sewage itself, or from incineration ashes, underscores its potential resource value [61]. The cost implications of advanced sewage sludge treatment technologies can be substantial, but are often justified by long-term benefits and savings. Initial capital investments in technologies like anaerobic digesters, thermal treatment units, and advanced oxidation systems can be high [62]. Operating and maintenance costs vary, with advanced systems requiring skilled personnel and regular upkeep. Despite higher initial costs, advanced treatment technologies offer economic benefits through resource recovery, regulatory compliance, and a reduced environmental impact [63]. Advanced oxidation and chemical treatments break down pollutants, and nutrient recovery technologies like struvite precipitation extract valuable nutrients for fertilizers, driving more sustainable sludge management [64]. Sewage sludge showed significant nutrient variability: nitrogen 4.11–6.47% DM, phosphorus 2.18–5.38% DM, and ammonium up to 1.24% DM (Table 1). To achieve optimal nitrogen and phosphorus abatement in wastewater, a concerted focus on process modernization is indispensable. The Nowy Sącz facility, operating under stringent accreditation and adhering to esteemed standards, serves as a model for such optimization. Data analysis confirmed that progressive treatment stages yield marked reductions in the total nitrogen and total phosphorus, culminating in a highly purified effluent, thereby validating the strategic application of advanced treatment paradigms. Specifically, nutrient analysis revealed significant reductions across treatment stages. Raw sewage presented average levels of 63.17 mg/L TN and 7.68 mg/L TP. Mechanically treated sewage exhibited minor variations, whereas totally treated sewage demonstrated drastically reduced levels of 7.84 mg/L TN and 0.34 mg/L TP, conclusively indicating effective nutrient removal [65].

4.2. Contribution of Sewage Sludge Treatment to Renewable Energy Production

Sewage sludge treatment plays a crucial role in renewable energy production, primarily through anaerobic digestion, which generates biogas. This biogas can be used for electricity, heat, or upgraded to biomethane [66]. Biogas production was significantly predicted by “waste approved” (p = 0.004), indicating its influence, while dry mass and modernization were not significant (p > 0.05). Biogas utilization was strongly predicted by modernization (p = 0.001), but not dry mass (p = 0.094). Therefore, “waste approved” impacts production, and modernization impacts utilization in power generation units (Table 2). Additionally, thermal methods such as incineration, gasification, and pyrolysis convert sludge into energy-rich gases and char, which can be used as fuel [67]. These processes reduce reliance on fossil fuels and help wastewater treatment plants achieve energy self-sufficiency, thereby promoting a circular economy [68,69]. For both the dry mass content and mineral element content, the “Intercept” and “Repeat measure” (likely representing the difference between raw and digested sludge) demonstrated highly significant results (p < 0.001), indicating substantial differences between the two sludge types. This signifies that the digestion process significantly alters both the dry mass and mineral element composition of the sludge. The PCA plot showed how biogas production is influenced by operational factors: biogas production and power generation units strongly separated the two sample groups, while boilers had an opposing effect. The first axis captured 95.3% of the variance, indicating that it is the dominant factor in distinguishing the samples (Figure 1). Thermal treatment methods, including incineration, pyrolysis, and gasification, significantly affect the quality and usability of sewage sludge [70]. These processes reduce the volume of sludge, eliminate pathogens, and recover energy in the form of heat or syngas [71]. We also displayed the impact of modernization on biogas production and utilization, showing an increase in both metrics after the modernization process. Specifically, biogas production slightly decreased while utilization significantly increased post modernization, suggesting improved efficiency in the utilization process (Figure 2).

Recent advancements in sludge treatment focus on increasing efficiency, reducing environmental impact, and enhancing resource recovery [72]. Anaerobic digestion stabilizes sludge and produces biogas, while advanced thermal treatments convert sludge into biochar [73]. Addressing these limitations significantly enhanced the efficiency and sustainability of WWTPs, ensuring that they met the growing demands of modern society. Digested sludge showed a significantly higher dry mass content compared to raw sludge. Similarly, the mineral element content was substantially elevated in digested sewage sludge (Figure 3). These results indicated that the digestion process leads to a concentration of both dry mass and mineral elements in the sludge. Furthermore, robust monitoring systems for operational parameters such as temperature, pH, and retention time are important, but costly and technologically demanding.

The SIMPER analysis revealed significant differences in the biogenic element content within digested sewage sludge based on two factors: glycerin water addition and process modernization. When glycerin water was added, the mean amounts of nitrogen (N), calcium (Ca), and ammonium nitrogen were significantly lower (p = 0.001) compared to when it was not added. Similarly, process modernization resulted in significantly higher mean amounts of N (p = 0.001), Ca (p = 0.005), ammonium nitrogen (p = 0.017), and a slightly lower mean amount of magnesium (Mg) (p = 0.041) after modernization. Phosphorus (P) levels, however, did not show significant changes in relation to either glycerin water addition or modernization. These findings indicated that both the addition of glycerin water and modernization had distinct impacts on the biogenic element composition of digested sewage sludge (Table 4).

Adding glycerin water to sewage sludge post digestion enhances biogas production and improves, stabilizes, and increases dewaterability [74]. Glycerol serves as an easily degradable carbon source that boosts microbial activity and methane yield [75]. It helps maintain the pH balance and prevents process inhibition. Additionally, glycerol reduces sludge viscosity, making it easier to handle and process. Utilizing waste glycerol from the biodiesel industry provides a cost-effective and sustainable method to improve sewage sludge management and energy recovery [76]. The boxplot depicted the dispersion of the metal content in digested sewage sludge, segregated by the addition of glycerin water. Samples without glycerin water showed greater variability in the metal content, while those with glycerin water had a more consistent, narrower distribution (Figure 4). Optimal dosage and continuous monitoring are essential to maximize these benefits. Addressing these limitations will significantly enhance the efficiency and sustainability of WWTPs, ensuring that they can meet the growing demands of modern society by maintaining low heavy metal concentrations, improving dry matter management, optimizing co-digestion processes, and increasing biogas production. For mineral elements, the model revealed that zinc (Zn), glycerin water addition (YES), copper (Cu), and pre-modernization (BEFORE) significantly influenced the mineral element content (p < 0.05). Specifically, Zn and glycerin water addition increased the mineral element content, while Cu decreased it, and pre-modernization samples had higher mineral element levels. For dry mass, only the N content showed a significant effect (p < 0.001), with higher N levels associated with a lower dry mass (Table 5).

4.3. Future Recommendations

Recent advancements in microbial and composting research emphasize optimizing bioenergy production and environmental sustainability. Studies have explored hydrogen production in anaerobic digestion [77], the role of soil in humus formation [78] and emission reduction during composting, and effective pretreatment methods to enhance biogas yield from lignocellulosic materials [79]. Effective wastewater treatment is crucial for addressing environmental challenges and ensuring public health. Sequencing Batch Reactor (SBR) technology integrated into wastewater treatment presents a modern solution by achieving significant advancements in pollutant removal, energy efficiency, and sustainable practices. This approach contributes to optimizing wastewater management in line with contemporary sustainability goals [80,81].

For effective sludge management and the modernization of wastewater treatment plants (WWTPs), several critical research limitations needed to be addressed to enhance dry matter utilization and maximize biogas production. Scalability assessments were essential to determine the feasibility and cost-effectiveness of various methods in real-world applications, yet they required substantial resources and faced challenges in translating laboratory successes to full-scale operations. Additionally, the lack of long-term monitoring data complicated the efforts to ensure that treated sludge maintained low heavy metal concentrations over time, as consistent and accurate long-term data collection was time-consuming and complex. In this study, we applied a minimum of 10 years of data to compare research technologies for heavy metals and nutrients. Technological limitations and the variable composition of sewage sludge hindered optimal dry matter management, while the scarcity of comprehensive studies on co-digestion processes limited the integration of sewage sludge with various organic wastes.

Strategic goals for managing municipal sewage sludge include reducing waste sludge generation, improving processing methods, and maximizing nutrient recovery while adhering to safety standards. This involves thermal transformation, land application, and the production of fertilizers or conditioners. Improving the public perception of treated sewage sludge requires comprehensive education about safety, benefits, and regulatory safeguards. Transparent communication, community involvement, and the certification of treated products can build trust and acceptance. This sludge, primarily composed of domestic and industrial wastewater, along with infiltration and rainwater, poses a management challenge. It should also be noted that the scope of the strategy does not include tasks related to investments in the construction or modernization of sludge lines in wastewater treatment plants. In each sewage treatment plant, the mass balance of pollutant loads should be closed, taking into account all inputs and outputs, and thus balance and design an appropriate sludge management. The quantitative and qualitative characteristics of municipal wastewater depend on the type and technical condition of the sewage system, the city’s industrialization, the amount of water used, and the standard of living of the inhabitants.

The functioning co-fermentation provides benefits in stabilizing the anaerobic fermentation process and some energy/economic benefits in the form of the increased utilization of organic waste due to the introduction of an easily biodegradable substrate. An important effect is the equalization and enrichment of the digested sludge into a product that can be used to produce biogas, which is useful for energy purposes.

5. Conclusions

This study evaluated the impact of anaerobic digestion, glycerin water addition, and wastewater treatment modernization on sewage sludge characteristics and biogas production. Digestion effectively increased the dry mass and mineral contents, demonstrating its efficiency in concentrating sludge solids. While post-digestion nutrient levels remained relatively stable, the heavy metal concentrations exhibited variability. The addition of glycerin water significantly reduced the nitrogen, ammonium nitrogen, and calcium levels, simultaneously enhancing the uniformity of heavy metal distribution within the sewage sludge. Conversely, modernization increased these N and Ca components, albeit slightly decreasing Mg. Furthermore, the research established a positive correlation between the Zn content and mineral fraction, while Cu displayed an inverse relationship. Based on these results, we recommend optimizing sewage sludge treatment to increase biogas production. For biogas production, prioritizing waste input and the technological modernization of waste treatment, along with using a cogeneration system with power generation units and boilers, are critical factors for increasing yield.