Chemical Speciation and Ecological Risk of Heavy Metals in Municipal Sewage Sludge from Bangkok, Thailand

Abstract

Municipal sewage sludge is a potential soil amendment rich in organic matter and nutrients, yet its reuse is often constrained by heavy metal contamination. This study evaluated six heavy metals (Cd, Cr, Cu, Ni, Pb, and Zn) in sludge collected from seven centralized wastewater treatment plants in Bangkok, Thailand, by analyzing physicochemical properties, total metal concentrations, and chemical speciation. Three ecological risk indices, the geo-accumulation index (I_geo), risk assessment code (RAC), and potential ecological risk index (PERI), were applied to assess contamination status, mobility, and ecological threat. The sludge exhibited high levels of organic matter and essential nutrients, indicating potential for agricultural reuse; however, elevated electrical conductivity at some sites may pose salinity risks if unmanaged. Speciation analysis revealed that Cd and Zn were largely present in mobile and redox-sensitive fractions, Cr and Pb were primarily in stable residual forms, and Cu and Ni occurred in moderately mobile forms influenced by environmental conditions. Across all indices, Cd consistently posed the highest ecological risk, followed by Zn, in a site-dependent manner, while Cr and Pb represented low risk. These findings provide a clearer understanding of metal behavior in sewage sludge and underscore the importance of integrating chemical speciation with multi-index risk assessment in sludge management. Incorporating such approaches into national guidelines, particularly in countries lacking established heavy metal limits, can strengthen monitoring frameworks, guide safe and sustainable reuse, and support regulatory development in contexts with limited monitoring data.

1. Introduction

Over the past two decades, sewage sludge generation has increased globally, largely due to rapid urbanization and advances in centralized wastewater treatment technologies. This growing volume has drawn attention to its composition, which is typically rich in organic matter and plant-essential nutrients such as nitrogen, phosphorus, and potassium. These properties have made sewage sludge a potential resource for land-based applications, particularly in agriculture and land rehabilitation, where both its organic matter and nutrient content can directly contribute to the restoration of degraded soils. This approach aligns with circular economy principles by transforming waste into a valuable resource and supporting sustainable land management. In practice, when applied appropriately, sewage sludge can enhance soil fertility, improve water retention, strengthen soil structure, and promote microbial activity, all of which are key to long-term soil health and productivity [1,2].

Despite these benefits, the reuse of sewage sludge is frequently restricted by the presence of heavy metals. Heavy metals are elements with high atomic weights and densities that are toxic, even at low concentrations. They are persistent in the environment, non-biodegradable, and capable of accumulating in biological systems. While a wide range of metals can be found in sewage sludge, including highly toxic elements such as arsenic and mercury, six metals are routinely monitored due to their frequent occurrence at elevated levels. These include cadmium (Cd), chromium (Cr), copper (Cu), nickel (Ni), lead (Pb), and zinc (Zn). Among these, Cd and Pb are particularly hazardous, with well-documented health effects even at trace concentrations [3]. Others, such as Cr, Cu, and Ni, pose concern due to their environmental persistence and potential to bioaccumulate. Regulatory frameworks often rely on total metal concentrations to limit exposure, yet this metric alone fails to accurately predict environmental behaviors such as mobility, bioavailability, or ecological impact [4,5,6].

Heavy metal contamination is not confined to sewage sludge but extends across multiple environmental matrices, including soil, water, air, and food, illustrating its widespread and persistent nature. Globally, metal accumulation patterns vary considerably across regions, shaped by factors such as industrial development, governance capacity, and socioeconomic inequality. For example, Jones et al. [7] documented racial disparities in heavy metal concentrations in urban soils in the southeastern United States, linking contamination patterns to socioeconomic status and land-use legacies. In a broader context, Oloruntoba et al. [8] conducted a systematic review of Nigerian data spanning from 2000–2019, revealing widespread metal contamination across soils, surface waters, and food systems, largely driven by weak environmental regulation and unmonitored industrial discharges. These and other international studies underscore that heavy metal pollution is a global environmental and public health challenge. However, comparable speciation-based ecological risk assessments in tropical and Southeast Asian sewage sludge remain scarce, despite the region’s rapid urbanization and growing wastewater treatment capacity.

Recognizing this limitation, researchers have increasingly focused on chemical speciation analysis, since the environmental fate of heavy metals is determined not merely by their total amounts, but by the specific forms in which they occur. For instance, metals that exist in exchangeable or carbonate fractions tend to be more mobile and bioavailable, whereas those in residual fractions are generally inert. Techniques like the Tessier sequential extraction method have been widely used to identify these fractions in environmental matrices [9,10]. Complementing this, risk assessment tools such as the geo-accumulation index (I_geo) [11], risk assessment code (RAC) [12], and potential ecological risk index (PERI) [13] offer structured ways to evaluate the environmental threats posed by individual metals. Together, speciation analysis and ecological indices provide a more comprehensive and realistic understanding of metal behavior and their potential environmental impacts, offering a more holistic approach than total concentration alone. Understanding metal bioavailability is essential for evaluating the actual risks of land-applied sludge, particularly in agricultural settings where uptake by crops can lead to food chain contamination and long-term health implications.

Thailand, like many rapidly urbanizing countries, faces challenges in managing its increasing volume of sewage sludge. In Bangkok, centralized wastewater treatment plants generate large quantities of sludge, yet its reuse remains limited. National guidelines exist for general waste management, but specific thresholds for heavy metal concentrations in land-applied sludge are lacking. Moreover, most domestic studies focus on total metal content, often overlooking the critical role of chemical speciation and ecological risk [5,14]. This highlights a notable gap in both research and the development of comprehensive regulatory frameworks for sustainable sludge management in Thailand.

This study addresses that gap by examining the chemical speciation and ecological risks of six key heavy metals (Cd, Cr, Cu, Ni, Pb, and Zn) in sewage sludge collected from centralized treatment facilities in Bangkok. The Tessier method was employed to determine metal distribution across five geochemical fractions [9]. Simultaneously, three widely used ecological risk indices, I_geo, RAC, and PERI, were applied to assess potential environmental impacts [11,12,13]. The goal is to generate insights that better reflect the true risk profile of heavy metals in sludge and to support future policies for its responsible reuse while contributing to the development of science-based guidelines for sustainable sewage sludge utilization in Thailand.

2. Materials and Methods

2.1. Sampling Locations and Collection

Bangkok, the capital and most populous city of Thailand, is located in the central region of the country and relies on centralized municipal wastewater treatment to manage its domestic effluents. As of 2022, the city operated eight centralized municipal wastewater treatment plants (WWTPs). For this study, sewage sludge samples were collected from seven of these facilities, designated WWTP1 through WWTP7 (Figure 1). One plant was excluded due to a temporary malfunction of its sludge dewatering system. These seven WWTPs treat between 30,000 and 350,000 m³ of municipal wastewater per day, collectively handling over 1 million m³/day. The selected WWTPs also produced dewatered sludge in varying amounts, ranging from 0.59 to 10.18 tons per day, depending on treatment scale and influent composition. Technical specifications of the selected WWTPs, including their locations, treatment capacities, process types, and average sludge production, are summarized in

Table 1. Technical specifications of the selected WWTPs, including locations, treatment capacities, process types, and average sludge production.

WWTP ID	WWTP Locations	Treatment Capacity (m3/day)	Wastewater Treatment Technology	Average Sludge Production (ton/day)
WWTP1	Si Phraya	30,000	Contact-stabilization-activated sludge	0.59
WWTP2	Thung Khru	65,000	Vertical-loop-reactor-activated sludge	2.52
WWTP3	Bang Sue	120,000	Activated sludge process with nutrient removal	4.37
WWTP4	Chatuchak	150,000	Cyclic-activated sludge system	6.06
WWTP5	Nong Khaem	157,000	Vertical-loop-reactor-activated sludge	10.18
WWTP6	Chong Nonsi	200,000	Cyclic-activated sludge system	4.01
WWTP7	Din Daeng	350,000	Activated sludge process with nutrient removal	8.33

.

Samples were collected during a designated campaign between February and March 2022 using a grab sampling method from piles that had undergone mechanical dewatering. To obtain a representative sample, sludge was collected from multiple points within each pile using a plastic shovel. A composite sample of approximately 10 kg was prepared per site, sealed in polyethylene bags, and transported to the laboratory for preparation and analysis.

2.2. Sample Preparation

Following collection, a portion of each fresh sludge sample was separated for moisture determination. The remaining samples were oven-dried at 60 ± 2 °C on polypropylene trays for several days. The dried sludge was then ground using a mechanical grinder until the entire sample passed through a 0.5 mm sieve. After sieving, the samples were re-dried at 105 ± 2 °C for 24 h, cooled in a desiccator, and sealed in polyethylene bags for subsequent analysis.

2.3. Physicochemical Analysis of the Sewage Sludge

Moisture content (MC) was determined gravimetrically from fresh sludge samples [15], while dried and sieved samples were used for subsequent physicochemical and nutrient analyses. Volatile solids (VS) and fixed solids (FS) were measured by ignition at 550 ± 50 °C [15]. pH and electrical conductivity (EC) were assessed using a 1:5 sludge-to-water extract with pH and conductivity meters, respectively [15]. Organic matter (OM) was measured using the Walkley and Black method [16]. Cation exchange capacity (CEC) was assessed using 1N ammonium acetate (pH 7.0), followed by steam distillation for ammonium quantification [17]. Total Kjeldahl nitrogen (TKN) was determined using the standard Kjeldahl method [15]. Available phosphorus (Avail P) was extracted with the Bray I method (1:10 sludge-to-extractant ratio) and quantified via the molybdenum blue colorimetric method [18]. Exchangeable potassium (Exch K) was extracted with 1N ammonium acetate (1:5 sludge-to-extractant ratio) and determined by flame photometry [19].

2.4. Determination of Total and Fractionated Heavy Metals

To determine the total concentrations of heavy metals in sewage sludge, approximately 2 g of dried and powdered samples were digested using a ternary acid (9 mL of 65% HNO₃, 3 mL of 98% H₂SO₄, and 3 mL of 37% HClO₄), following a modified Twyman protocol [20]. The digestion was performed in a 250-mL digestion tube using an infrared-assisted unit (TURBOTHERM TT125, C. Gerhardt GmbH & Co. KG, Königswinter, Germany), with controlled temperature ramping to ensure complete mineralization. After cooling, the digests were filtered through Whatman No. 5 filter papers, transferred to 100-mL volumetric flasks, and diluted with deionized water. As hydrofluoric acid (HF) was not used, the digestion process may not have fully released silicate-bound metals. Therefore, the concentrations are operationally defined as pseudo-total rather than true total values (due to the absence of HF).

Chemical speciation was assessed using a five-step sequential extraction procedure adapted from Tessier et al. [9], which partitioned metals into exchangeable (EXCH), carbonate-bound (CARB), iron and manganese oxide-bound (Fe/Mn OX), organic matter-bound (OM), and residual (RESD) fractions. The extraction steps, reagents, and operational conditions are detailed in

Table 2. Sequential extraction procedure with reagents and operational conditions.

Step	Fraction	Reagent(s)	Extraction Procedure
1	EXCH	1.0 M MgCl2, pH 7.0	Add 16 mL MgCl2 to 2 g of dried and finely powdered sample. Shake at 25 °C for 16 h.
2	CARB	1.0 M NaOAc, pH 5.0	Add 16 mL NaOAc to residue from Step 1. Shake at 25 °C for 5 h.
3	Fe/Mn OX	0.04 M NH2OH·HCl, 25% HOAc	Add 40 mL NH2OH·HCl in HOAc to residue from Step 2. Heat at 96 °C for 6 h.
4	OM	0.02 M HNO3 + 30% H2O2, pH 2.0 30% H2O2, pH 2.0 3.2 M NH4OAc, 20% HNO3	Add 6 mL HNO3 and 10 mL H2O2 to residue from Step 3. Heat at 85 °C for 2 h. Add another 10 mL H2O2 and heat at 85 °C for 3 h. Then, add 10 mL NH4OAc and shake at 25 °C for 30 min.
5	RESD	HNO3:H2SO4:HClO4 (3:1:1, v/v/v)	Digest the residue from Step 4 with 9 mL HNO3, 3 mL H2SO4, and 3 mL HClO4.

. After each extraction step (Steps 1–4), the suspension was centrifuged at 4000 rpm for 10 min, filtered through Whatman No. 5 filter paper, acidified with 1 mL of 65% HNO₃, and then diluted to 50 mL.

The concentrations of Cd, Cr, Cu, Ni, Pb, and Zn in both total digests and sequentially extracted fractions were determined using flame atomic absorption spectrophotometry (AAS, model novAA 800, Analytic Jena, Jena, Germany). Calibration was performed using certified single-element standard solutions.

2.5. Quality Control

All digestion and extraction procedures were performed in triplicate with reagent blanks included to detect contamination. For total metal analysis, spike recovery tests with known metal concentrations yielded 82–102% recovery, confirming digestion accuracy. Sequential extraction reliability was evaluated by comparing the sum of all fractions with the corresponding total metal concentration.

2.6. Contamination Indices and Ecological Risk Assessment Methods

2.6.1. Geo-Accumulation Index (_geo)

The geo-accumulation index (I_geo), originally developed by Müller [11], was used to evaluate the extent of heavy metal contamination in sewage sludge samples. It was calculated using the following equation:

$${\mathrm{I}}_{\mathrm{geo}}={\log }_2\left({\displaystyle \frac{{\mathrm{C}}_{\mathrm{i}}}{1.5\times {\mathrm{B}}_{\mathrm{i}}}}\right)$$

(1)

where ${\mathrm{C}}_{\mathrm{i}}$ is the total concentration of heavy metal $\mathrm{i}$ (mg/kg) and ${\mathrm{B}}_{\mathrm{i}}$ is the background reference concentration in Thai soils. The values used, 0.15 for Cd, 80 for Cr, 45 for Cu, 45 for Ni, 55 for Pb, and 70 mg/kg for Zn, were derived from the 95th percentile concentrations reported by Zarcinas et al. [14]. Although these values cover multiple soil types, including agricultural soils, they remain the most widely recognized dataset for Thailand and are used here as a pragmatic baseline. A correction factor of 1.5 was applied to account for natural geochemical variation and minor anthropogenic influences. Contamination severity was classified according to the criteria in

Table 3. Contamination levels based on the geo-accumulation index (I_geo).

Class	Igeo Value Range	Contamination Level
0	Igeo ≤ 0	Uncontaminated (UC)
1	0 < Igeo ≤ 1	Uncontaminated to moderately contaminated (UMC)
2	1 < Igeo ≤ 2	Moderately contaminated (MC)
3	2 < Igeo ≤ 3	Moderately to heavily contaminated (MHC)
4	3 < Igeo ≤ 4	Heavily contaminated (HC)
5	4 < Igeo ≤ 5	Heavily to extremely contaminated (HEC)
6	Igeo > 5	Extremely contaminated (EC)

.

2.6.2. Risk Assessment Code (RAC)

The risk assessment code (RAC), introduced by Perin et al. [12], evaluates the environmental risk of heavy metals based on their chemical speciation. It focuses on the EXCH and CARB fractions, which are the most bioavailable and environmentally labile, making them critical in sewage sludge risk assessments [21,22,23,24].

In this study, the RAC index was applied to quantify the proportion of each metal present in the EXCH and CARB fractions relative to the total of all five sequentially extracted fractions, as shown in the following Equation:

$$\mathrm{RAC}=\left({\displaystyle \frac{{\mathrm{C}}_1+{\mathrm{C}}_2}{{\mathrm{C}}_{\mathrm{t}}}}\right)\times 100$$

(2)

where ${\mathrm{C}}_1$ and ${\mathrm{C}}_2$ represent the concentrations (mg/kg) of the metal in the EXCH and CARB fractions, respectively, and ${\mathrm{C}}_{\mathrm{t}}$ is the sum of the metal concentrations (mg/kg) in all five fractions. Based on the calculated RAC values, metals were classified into five risk levels, as summarized in

Table 4. Classification criteria for the risk assessment code (RAC).

Category	RAC Range (%)	Risk Level
I	RAC ≤ 1	No risk (NR)
II	1 < RAC ≤ 10	Low risk (LR)
III	11 < RAC ≤ 30	Medium risk (MR)
IV	31 < RAC ≤ 50	High risk (HR)
V	RAC > 50	Very high risk (VHR)

.

2.6.3. Potential Ecological Risk Index (PERI)

The potential ecological risk index (PERI), originally proposed by Hakanson [13], was selected in this study for its ability to integrate both contamination levels and metal-specific toxicities into a single ecological risk index. This approach enables a comprehensive assessment of individual and cumulative ecological risks posed by heavy metals in sewage sludge. The ecological risk factor for each metal (${ER}_r^i$) and the overall PERI were calculated using the following equations:

$${\mathrm{ER}}_{\mathrm{r}}^{\mathrm{i}}= {\mathrm{T}}_{\mathrm{i}}\hspace{0.17em}\times {\displaystyle \frac{{\mathrm{C}}_{\mathrm{i}}}{{\mathrm{B}}_{\mathrm{i}}}}$$

(3)

$$\mathrm{PERI}=\sum {\mathrm{ER}}_{\mathrm{r}}^{\mathrm{i}}$$

(4)

where ${\mathrm{ER}}_{\mathrm{r}}^{\mathrm{i}}$ is the potential ecological risk factor for metal $\mathrm{i}$, ${\mathrm{T}}_{\mathrm{i}}$ is the toxic-response factor, ${\mathrm{C}}_{\mathrm{i}}$ is the total concentration of the metal (mg/kg) measured in sewage sludge, and ${\mathrm{B}}_{\mathrm{i}}$ is the corresponding background concentration (mg/kg) in Thai soils [14]. The toxic-response factors ${\mathrm{T}}_{\mathrm{i}}$ used in this study were 30 for Cd, 5 for Cu, Pb, and Ni, 2 for Cr, and 1 for Zn, based on Hakanson [13]. These toxic-response factors, though widely used, may not fully reflect current toxicological knowledge or regional conditions; however, they are retained here to allow comparability with previous studies. Together, these criteria provide a standardized framework for ecological risk interpretation, as summarized in

Table 5. Classification criteria for ecological risk assessment.

Class	ER Range	PERI Range	Risk Level
I	ER ≤ 40	PERI ≤ 150	Low risk (LR)
II	40 < ER ≤ 80	150 < PERI ≤ 300	Moderate risk (MR)
III	80 < ER ≤ 160	300 < PERI ≤ 600	Considerable risk (CR)
IV	160 < ER ≤ 320	PERI > 600	High risk (HR)
V	ER > 320		Very high risk (VHR)

.

2.7. Statistical Analysis

All statistical analyses were conducted using IBM SPSS Statistics version 27.0.1. Descriptive statistics were reported as means and standard deviations for both physicochemical parameters and total heavy metal concentrations. The Shapiro–Wilk test (p > 0.05) confirmed data normality. Accordingly, one-way analysis of variance (ANOVA) was used to test for significant differences among WWTPs (p < 0.05), with Tukey’s Honestly Significant Difference (HSD) test applied for post-hoc pairwise comparisons. For non-detected values, one-half of the method detection limit (MDL) for the corresponding metal was substituted to minimize analytical bias.

3. Results and Discussion

3.1. Physicochemical Characteristics of the Sewage Sludge

The physicochemical properties of sewage sludge samples collected from seven municipal WWTPs in Bangkok are summarized in

Table 6. Physicochemical properties of sewage sludge from seven municipal WWTPs in Bangkok.

WWTP ID	Fresh Sludge	Dry Sludge
	MC (%)	VS (%)	FS (%)	pH (1:5)	EC (1:5) (dS/m)	OC (%)	OM (%)	TKN (%)	Avail P (g/kg)	Exch K (g/kg)	CEC (cmol(+)/kg)	C:N Ratio
WWTP1	85.76 ± 0.04 e	52.04 ± 0.09 f	47.96 ± 0.09 a	6.53 ± 0.04 ab	3.19 ± 0.00 c	27.90 ± 0.08 e	46.87 ± 0.13 e	2.61 ± 0.04 b	0.46 ± 0.00 c	1.25 ± 0.03 bc	55.54 ± 0.12 c	10.69
WWTP2	75.84 ± 0.05 b	37.45 ± 0.06 b	62.55 ± 0.06 e	6.66 ± 0.09 c	2.43 ± 0.04 a	22.06 ± 0.40 c	37.07 ± 0.66 c	4.17 ± 0.03 g	0.56 ± 0.02 d	1.30 ± 0.04 c	45.81 ± 0.22 a	5.29
WWTP3	75.89 ± 0.79 b	38.28 ± 0.04 c	61.72 ± 0.04 d	6.58 ± 0.01 bc	3.37 ± 0.05 d	20.61 ± 0.16 b	34.62 ± 0.27 b	2.91 ± 0.02 c	0.39 ± 0.01 b	1.17 ± 0.01 b	52.44 ± 0.43 b	7.07
WWTP4	85.27 ± 0.08 e	45.35 ± 0.50 e	54.65 ± 0.50 b	6.51 ± 0.02 bc	2.74 ± 0.06 b	24.43 ± 0.38 d	41.04 ± 0.64 d	3.47 ± 0.07 e	0.75 ± 0.02 e	1.46 ± 0.04 d	52.77 ± 0.11 b	7.05
WWTP5	77.56 ± 0.08 c	41.26 ± 0.05 d	58.74 ± 0.05 c	6.49 ± 0.02 b	4.34 ± 0.08 e	22.67 ± 0.11 c	38.09 ± 0.20 c	3.15 ± 0.04 d	0.49 ± 0.01 c	0.91 ± 0.04 a	51.98 ± 0.06 b	7.19
WWTP6	71.78 ± 0.48 a	27.55 ± 0.02 a	72.45 ± 0.02 f	7.24 ± 0.12 d	2.62 ± 0.05 b	15.30 ± 0.10 a	25.70 ± 0.18 a	1.86 ± 0.01 a	0.34 ± 0.01 a	0.96 ± 0.02 a	46.44 ± 0.50 a	8.23
WWTP7	81.12 ± 0.25 d	52.13 ± 0.06 f	47.87 ± 0.06 a	6.02 ± 0.04 a	3.49 ± 0.01 d	28.21 ± 0.24 e	47.39 ± 0.40 e	3.99 ± 0.03 f	2.23 ± 0.02 f	3.60 ± 0.04 e	57.20 ± 0.78 d	7.07
Min	71.87	27.55	47.87	6.02	2.43	15.30	25.70	1.86	0.34	0.91	46.44	5.29
Max	85.76	52.13	72.45	7.24	4.34	28.21	47.39	4.17	2.23	3.60	57.20	10.69
Mean	79.03	42.01	57.99	6.58	3.17	23.03	38.68	3.17	0.74	1.52	51.74	7.51
SD	5.22	8.74	8.74	0.36	0.65	4.46	7.49	0.80	0.63	0.89	4.26	1.65

Values are presented as mean ± standard deviation (n = 3). Different superscript letters (^a–f) within the same column indicate significant differences among WWTPs at p < 0.05 based on Tukey’s HSD test.

. Considerable variability was observed among the samples in terms of moisture content (MC), volatile solids (VS), fixed solids (FS), pH, electrical conductivity (EC), organic carbon (OC), organic matter (OM), total Kjeldahl nitrogen (TKN), cation exchange capacity (CEC), available phosphorus (Avail P), exchangeable potassium (Exch K), and the carbon-to-nitrogen ratio (C/N). These parameters are critical for evaluating the potential reuse of sludge in agricultural applications.

Moisture content ranged from 71.87% to 85.76%, with WWTP1 and WWTP4 showing the highest values, while WWTP6 exhibited the lowest (p < 0.05). This variation reflects differences in sludge dewatering processes. Similarly, the volatile solids content was significantly higher in WWTP1 and WWTP7 (>52%), indicating a higher proportion of biodegradable organic matter, whereas WWTP6 had the lowest VS (27.55%) and the highest fixed solids (FS, 72.45%), suggesting greater inorganic content. These differences are likely due to operational conditions and influent wastewater composition at each site.

Sludge pH varied between 6.02 and 7.24, ranging from slightly acidic to neutral, which is within the optimal range for microbial activity and nutrient stability in soils WWTP6 had the most alkaline pH, significantly higher than that of WWTP7 (p < 0.05), which may reduce heavy metal solubility and improve sludge stability. Electrical conductivity ranged from 2.43 to 4.34 dS/m, with WWTP5 showing the highest value. Elevated EC levels may indicate salt accumulation risks if applied to poorly drained soils and require site-specific application strategies.

Organic matter content ranged from 25.70% to 47.39%, with WWTP7 showing the highest value. This aligns with its high CEC (57.20 cmol(+)/kg), reinforcing the well-established correlation between organic matter and cation exchange capacity. Organic carbon (OC) ranged from 15.30% in WWTP6 to 28.21% in WWTP7, mirroring the trends observed in OM. The C/N ratio ranged from 5.29 to 10.69 across all sites, which falls within the optimal range for nitrogen mineralization and organic matter decomposition. These values suggest that the sludge is generally stable and biologically active, with a low risk of nitrogen immobilization when applied to soil.

TKN values varied significantly among sites, from 1.86% to 4.17%, with WWTP2 exhibiting the highest nitrogen content, further supporting its agronomic potential. Available phosphorus (Avail P) and exchangeable potassium (Exch K) also differed significantly across WWTPs. WWTP7 recorded substantially higher concentrations of both (2.23 g/kg and 3.60 g/kg, respectively), suggesting enhanced fertilizer potential. The high nutrient content observed is consistent with findings from previous studies on municipal sludge reuse [25], which highlight the benefits of sewage sludge as an alternative to synthetic fertilizers. This is further supported by broader reviews indicating that sewage sludge is rich in organic matter and essential nutrients, though elevated EC levels may pose a salinity risk if unmanaged [26].

However, it is important to recognize that elevated EC values and the potential accumulation of salts or contaminants must be managed carefully to avoid soil degradation over time. Balanced application, proper treatment, and periodic monitoring are essential to optimize the benefits of sludge reuse while minimizing environmental risks. These findings support the viability of sewage sludge in circular nutrient management systems, particularly when heavy metals and other pollutants are adequately controlled.

3.2. Total Concentrations of Heavy Metals in the Sewage Sludge

Sewage sludge samples from seven WWTPs in Bangkok were analyzed for total concentrations of Cd, Cr, Cu, Ni, Pb, and Zn, revealing statistically significant differences among treatment plants (

Table 7. Total concentrations of heavy metals in sewage sludge from seven WWTPs in Bangkok.

WWTP ID	Concentration (mg/kg DM)
	Cd	Cr	Cu	Ni	Pb	Zn
WWTP1	1.70 ± 0.13 a	158.32 ± 3.84 c	369.73 ± 4.69 d	84.86 ± 0.74 b	19.80 ± 1.87 a	911.58 ± 14.93 c
WWTP2	4.09 ± 0.02 c	184.65 ± 0.40 e	432.55 ± 23.74 e	161.21 ± 1.21 e	23.88 ± 0.50 ab	1948.47 ± 34.56 e
WWTP3	3.51 ± 0.05 b	130.84 ± 1.66 b	125.36 ± 8.33 a	73.84 ± 0.28 a	28.69 ± 0.83 bc	575.79 ± 3.99 a
WWTP4	4.10 ± 0.08 c	202.12 ± 0.98 f	181.83 ± 1.94 b	182.80 ± 0.59 f	33.10 ± 3.75 cd	851.61 ± 21.62 b
WWTP5	4.87 ± 0.06 d	169.88 ± 3.52 d	1140.97 ± 14.43 f	153.30 ± 0.40 d	45.24 ± 5.09 e	1296.26 ± 10.66 d
WWTP6	4.70 ± 0.07 d	165.50 ± 3.27 cd	248.59 ± 2.60 c	88.86 ± 0.07 c	38.61 ± 2.92 de	878.68 ± 13.07 bc
WWTP7	4.18 ± 0.01 c	122.52 ± 0.49 a	1124.00 ± 8.21 f	75.78 ± 0.59 a	25.00 ± 0.94 ab	891.25 ± 15.55 bc
Min	1.70	122.52	181.83	73.84	19.80	575.79
Max	4.87	202.12	1,140.97	182.80	45.24	1948.47
Mean	3.88	161.98	517.58	117.24	30.62	1050.52
SD	1.06	28.11	432.89	46.53	8.96	448.30

Values are presented as mean ± standard deviation (n = 3). Different superscript letters (^a–f) within the same column indicate significant differences among WWTPs at p < 0.05 based on Tukey’s HSD test.

).

Among the analyzed elements, Zn exhibited the highest mean concentration (1050.52 ± 448.30 mg/kg), followed by Cu (517.58 ± 432.89 mg/kg), Cr (161.98 ± 28.11 mg/kg), and Ni (117.24 ± 46.53 mg/kg). In contrast, Pb and Cd were detected at comparatively lower levels, with mean values of 30.62 ± 8.96 mg/kg and 3.88 ± 1.06 mg/kg, respectively. This ordering (Zn > Cu > Cr > Ni > Pb > Cd) is consistent with previous reports that identify Zn and Cu as the most abundant metals in municipal sludge, due to their widespread use in domestic and industrial products [27,28], and agrees with findings from an industrialized city in China [29].

When comparing across individual WWTPs, concentrations varied markedly. High-concentration sites included WWTP5, which recorded the highest Cu concentration (1140.97 mg/kg), and WWTP2, which had the highest Zn level (1948.47 mg/kg). Intermediate concentrations were observed in samples from WWTP7 and WWTP6, whose service areas contain a substantial number of manufacturing facilities (e.g., chemical, plastic, metal, printing, and electrical sectors) that may be associated with their intermediate concentrations of heavy metals. Low-concentration sites included WWTP3, which serves mainly light industries such as food processing, woodworking, and assembly.

WWTP5 also lies within a service area containing numerous factories engaged in metalworking, plastics, and chemical production, which could be potential sources if their wastewater is discharged into the municipal system. Nonetheless, these associations are indicative rather than definitive, as no direct industrial effluent composition data were incorporated into the analysis. In addition to potential industrial contributions, non-point sources such as urban runoff and diffuse household discharges, including detergents, cosmetics, and cleaning agents, could also influence heavy metal accumulation in municipal sludge [30]. The total heavy metal concentrations (sum of all six metals) ranged from 938.04 to 2811.00 mg/kg, with the highest totals in WWTP5 and WWTP2. This variability likely reflects differences in wastewater composition, catchment characteristics, and treatment processes. Studies from Hanoi and Kuala Lumpur have reported similar patterns, with Zn and Cu commonly dominant [31,32]. Although Zn and Cu were consistently dominant, elevated concentrations, especially in WWTP2 and WWTP6, may present environmental risks if sludge is applied to land. Nevertheless, all observed values remained below EU Directive 86/278/EEC limits for agricultural use (Zn: 2500–4000 mg/kg; Cu: 1000–1750 mg/kg) [33]. In Thailand, while there are no regulatory limits for heavy metals in sewage sludge, the National Environment Board [34] specifies soil quality standards for residential and agricultural land use (e.g., Cd ≤ 1 mg/kg, Ni ≤ 10 mg/kg, Zn ≤ 300 mg/kg, Cu ≤ 50 mg/kg, Cr ≤ 50 mg/kg, and Pb ≤ 75 mg/kg). Notably, these thresholds are considerably lower than the concentrations detected in most sludge samples, indicating that uncontrolled land application could drive exceedances in soil over time. Collectively, these findings indicate that the heavy metal composition in Bangkok’s municipal sewage sludge reflects trends observed in other urban regions worldwide, with Zn and Cu dominating the profiles. The variation among WWTPs emphasizes the influence of local factors and the need for site-specific monitoring in risk management and resource recovery planning.

3.3. Speciation of Heavy Metals in the Sewage Sludge

The concentrations of Cd, Cr, Cu, Ni, Pb, and Zn in each chemical fraction of the sewage sludge from seven municipal WWTPs in Bangkok are provided in the Supplementary Materials (Table S1), and their percentage distributions are illustrated in Figure 2. These results reflect the average chemical speciation profiles across the sampled sludge. The recovery rates (R%) observed in this study ranged from 81.0% to 120.2%, which aligns with the generally accepted range of 80–120% for sequential extraction procedures such as the Tessier and modified BCR protocols. Comparable values were reported by Tytła [31], where most recoveries fell between 85% and 115%, with occasional exceedances due to matrix effects. These findings confirm the reliability and consistency of the applied method, indicating no substantial losses or overestimations of metal content.

Overall, Pb and Cr were predominantly found in the stable RESD fractions, while Zn, Cd, and Ni exhibited higher proportions in more mobile fractions (EXCH, CARB, and Fe/Mn OX). Differences among the seven WWTPs, as shown in Figure 2, may reflect variations in influent composition, wastewater sources, and operational conditions. For example, Zn and Cu tended to be more enriched in mobile fractions in sludge from WWTP2 and WWTP7, which are located in service areas that include diverse manufacturing activities; such patterns may relate to differences in influent quality, although no direct industrial effluent data were obtained in this study. In contrast, sludge from WWTP3 and WWTP6 showed higher proportions of Cr and Pb in RESD forms, indicating greater chemical stability and lower mobility.

Cd exhibited a relatively mobile profile. It was primarily distributed in the EXCH and CARB fractions, with average proportions of 36% and 21%, respectively. Cd in the RESD fractions accounted for only 24% on average. Notably, sludge from WWTP1 exhibited 57% of Cd in the EXCH fraction, the highest among all samples, followed by WWTP2 (37%), WWTP3 (36%), and WWTP5 (35%). This pattern suggests that Cd in the sewage sludge is highly available and poses significant ecological risk, particularly under acidic conditions or ionic shifts. Similar trends have been reported by Kowalik et al. [29] and You et al. [25], who found high proportions of Cd in mobile fractions in urban sewage sludge.

Among all examined metals, Cu showed the strongest association with the OM fraction, with an average of 68% across all samples. This was followed by the RESD fraction at 20%, while Fe/Mn OX, CARB, and EXCH forms accounted for only minor portions (5%, 4%, and 3%, respectively). Cu in OM fractions were highest in WWTP7 (71%), WWTP5 (70%), and WWTP6 (54%), indicating Cu’s strong affinity for organic ligands and humic substances. This strong affinity may reduce Cu’s immediate mobility; however, the potential for release remains under oxidative conditions, particularly in the sewage sludge enriched with biodegradable organic inputs. Such trends are consistent with findings by Kowalik et al. [29], who also reported high proportions of Cu in OM fractions in aerobic sludge systems.

Ni exhibited a moderately mobile distribution pattern, with the highest average proportion found in the Fe/Mn OX fraction (35%), followed by the RESD (24%) and OM (22%) fractions. Smaller proportions were present in the EXCH (13%) and CARB (6%) fractions. Notably, Ni in Fe/Mn OX fractions was predominant in WWTP6 (45%) and WWTP3 (33%), indicating potential redox-sensitive mobilization. Elevated Ni in OM fractions was also observed in WWTP4 (40%) and WWTP5 (38%), suggesting contributions from organic-rich influents. This distribution reflects the transitional behavior of Ni between mobile and stable forms, which is consistent with the findings of [32].

Pb exhibited the highest chemical stability among all studied metals. Nearly 100% of Pb in all sludge samples was found in the RESD fraction, indicating negligible mobility and bioavailability under typical environmental conditions. This uniform distribution pattern reflects Pb’s strong tendency to form stable mineral compounds, rendering it relatively inert in the sewage sludge. Similar findings have been reported by Liu and Sun [35], who observed Pb primarily in RESD forms in municipal sewage sludge across various regions in China.

Zn demonstrated relatively high environmental lability, primarily associated with Zn in the Fe/Mn OX (52%) and OM (13%) fractions. Only minor proportions were found in the CARB (6%), EXCH (5%), and RESD (15%) fractions. The predominance of Zn in redox-sensitive fractions suggests a strong potential for mobilization under reducing conditions. Notably, sludge from WWTP2 and WWTP4 contained the highest proportions of Zn in the Fe/Mn OX fractions (65% and 51%, respectively), indicating site-specific susceptibility to metal release. These findings align with observations by Feng et al. [30] and Tytła [36], who identified Zn as one of the most mobile and redox-responsive metals in municipal sewage sludge.

Considering the cumulative proportions in the mobile fractions, namely EXCH, CARB, and Fe/Mn OX, the relative mobility of the studied metals can be ranked as follows: Cd > Zn > Cu > Ni > Cr > Pb. This trend reflects the chemical characteristics and binding preferences of each metal within the sewage sludge matrix. Cd and Zn, which were predominantly present in exchangeable and redox-sensitive fractions, pose the highest mobility and potential environmental risk. In contrast, Pb and Cr showed a strong affinity for the RESD fractions, indicating high chemical stability and limited bioavailability under normal environmental conditions. Cu and Ni demonstrated intermediate behavior, with substantial associations with organic matter and Fe/Mn oxides, making them sensitive to oxidative or reductive changes. These findings highlight the need for site-specific management strategies based on metal speciation, particularly in WWTPs with high levels of mobile Cd and Zn. A summary of the dominant chemical fractions of each metal, along with the representative WWTPs and their corresponding environmental implications, is presented in

Table 8. Dominant chemical fractions of heavy metals detected in the sewage sludge across seven WWTPs in Bangkok, including site-specific highlights and environmental implications based on mobility.

Metal	Dominant Fraction(s)	Representative WWTP(s)	Environmental Implication
Cd	EXCH > CARB	1, 3	High mobility; pH-sensitive; elevated potential risk
Cr	RESD	3, 4	Low mobility; stable under normal pH
Cu	OM	5–7	Bound to organic matter; redox-sensitive
Ni	Fe/Mn OX ≈ OM ≈ RESD	3–6	Moderately mobile; redox- and OM-sensitive
Pb	RESD	All sites	Inert; negligible mobility and bioavailability
Zn	Fe/Mn OX > OM	2, 4	Mobile; redox-sensitive; possibly influenced by local wastewater sources

.

3.4. Ecological Risk Assessment of Heavy Metals in the Sewage Sludge

I_geo, RAC, and PERI values for all six heavy metals across the seven WWTPs are summarized in the Supplementary Materials (Table S2), which compiles contamination levels, mobility classifications, and ecological risk categories for each metal at each site.

3.4.1. Risk Assessment by the Geo-Accumulation Index

Based on the I_geo values (Figure 3A), Cd consistently recorded the highest enrichment levels across the seven WWTPs, ranging from moderately to heavily contaminated (MHC) to heavily to extremely contaminated (HEC) categories. Zn also exhibited elevated values, especially at WWTP2 and WWTP5, while Cu showed pronounced peaks at WWTP5 and WWTP7. In contrast, Ni and Cr generally fell within uncontaminated to moderately contaminated (UMC) or moderately contaminated (MC) classes, and Pb remained in the uncontaminated (UC) class across all sites.

When viewed spatially (Figure 3B), WWTP5 stood out with high I_geo values for Cd, Cu, and Zn, and WWTP2 for Cd and Zn. These patterns are consistent with findings from other urban sludge studies [25,29,30], where Cd and Zn frequently rank among the most enriched metals due to widespread anthropogenic inputs such as industrial effluents, traffic emissions, and urban runoff. Such consistency suggests these elements warrant priority monitoring in facilities handling sludge with multiple metals in higher contamination classes.

3.4.2. Risk Assessment by the Risk Assessment Code

Evaluation of RAC results (Figure 4A) indicated that Cd had the highest proportion of labile fractions, placing every site in the high risk (HR) or very high risk (VHR) categories. Zn and Ni showed moderate percentages in these fractions, with Zn mobility greater at WWTP2 and WWTP3 and Ni more pronounced at WWTP2. In contrast, Cu, Cr, and Pb were generally below 10%, with Pb consistently at 0%.

The spatial distribution (Figure 4B) highlighted WWTP2 and WWTP3 as notable for higher Zn mobility, while Cd risk remained high at all sites. Similar patterns for Cd and Zn mobility have been observed in municipal sludge from industrialized regions [25,37], underscoring their potential for environmental release, particularly under acidic or reducing conditions. These results highlight the value of integrating mobility-based metrics into ecological risk evaluations to address not only total concentrations but also the most labile fractions.

3.4.3. Risk Assessment by the Potential Ecological Risk Index

ER results by metal (Figure 5A) integrate contamination levels with metal-specific toxic-response factors, providing an overall measure of ecological threat. Cd dominated the risk profile at every site, with ER values placing all WWTPs in the VHR category. Zn and Cu followed, contributing to moderate to considerable risk depending on location; for example, Cu risks were most notable at WWTP5 and WWTP7, while Zn risks were elevated at WWTP2 and WWTP5. Ni, Pb, and Cr generally fell within the low to moderate risk categories.

The stacked bar chart (Figure 5B) shows that Cd accounted for the majority of the overall risk, often exceeding 80% of the PERI score. Sites with the highest total PERI, WWTP5 (1242) and WWTP2 (918), also showed notable contributions from Zn and Cu. Similar PERI patterns have been reported in urban sludge studies from China, Poland, and global reviews [25,29,30], and in long-term sludge application trials in tropical soils [37], confirming Cd and Zn as among the most mobile and environmentally relevant metals in sludge management.

3.4.4. Comparison of Assessment Based on I_geo, RAC, and PERI

Cross-comparison of the three indices, I_geo, RAC, and PERI, highlights their complementary perspectives on heavy metal risks in Bangkok’s municipal sewage sludge. I_geo emphasizes anthropogenic enrichment, identifying Cd, Zn, and Cu as the most enriched elements. RAC focuses on chemical lability, again ranking Cd as the most mobile element, followed by Zn and Ni, whereas Cu, Cr, and Pb were generally stable under current environmental conditions. PERI integrates contamination levels with toxicity, reinforcing Cd’s dominance in overall ecological risk, with Zn and Cu contributing to site-specific risks.

Although all three indices consistently identify Cd as the primary concern, variations in the relative positions of Zn, Cu, and Ni demonstrate the value of a multi-metric approach. For example, Cu’s elevated I_geo and PERI scores at WWTP5 and WWTP7 are not reflected in the RAC due to low mobility, while Ni appears minor in I_geo and PERI but registers higher mobility in the RAC at certain sites. This combined approach is consistent with recommendations from international sludge risk assessments [25,29,30,37] and supports the development of management strategies that address both widespread and site-specific risks.

4. Conclusions

This study presents a comprehensive assessment of heavy metal speciation and associated ecological risks in sewage sludge from centralized municipal WWTPs in Thailand, representative of tropical Southeast Asian conditions where such evaluations remain limited. By combining chemical fractionation (Tessier method) with three ecological risk indices (I_geo, RAC, and PERI), the study demonstrates the value of a multi-metric approach for identifying priority metals, understanding their environmental behavior beyond total concentrations, and informing targeted mitigation strategies. Cd consistently emerged as the highest-risk element, with Zn and Cu also contributing under site-specific conditions.

From a management perspective, the findings highlight the urgent need for science-based national guidelines on heavy metal limits in sewage sludge for land application. Such guidelines should consider site-specific variability, prioritize Cd control, particularly in WWTPs located in industrial catchments, and promote application plans that balance nutrient benefits with contaminant risk reduction. Routine monitoring that incorporates chemical speciation and multi-index risk assessment would provide a stronger basis for safe reuse decisions.

Although the results provide valuable baseline data, certain limitations remain. The study did not include quantitative industrial effluent data, preventing definitive source apportionment of metals, and was based on a single-season sampling campaign. Future work should expand to multi-season and multi-year datasets, incorporate direct measurements of industrial discharges, and evaluate the long-term impacts of repeated sludge applications on soil health, crop uptake, and ecosystem functions.

Overall, this work establishes a scientific foundation for integrating chemical speciation into sludge management frameworks in tropical urban settings. The approach demonstrated here can guide regulatory development, strengthen monitoring programs, and support safe and sustainable resource recovery, particularly in countries where formal guidelines for heavy metal limits in sludge are not yet in place.