Rate of Microelement Quantitative Changes during the Composting of Sewage Sludge with Various Bulking Agents

Abstract

The composting of sewage sludge (SS) with structure-forming additives is a popular and inexpensive method for the management of biodegradable waste. During this process, a number of transformations of organic matter and nutrients occur. This study presents the rates and directions of quantitative changes in Cu, Zn, and Ni during the composting of various mixtures. The following substances were used for preparing compost mixtures: SS, sawdust, straw, and bark. These substances were mixed together in appropriate proportions as follows: C1: 45% SS + 50% sawdust + 5% straw; C2: 45% SS + 50% bark+ 5% straw; and C3: 45% SS + 35% bark + 15% sawdust + 5% straw. Single (DTPA solution) and sequential extraction methods (BCR protocol) were used for microelement mobility assessment. Compost valorization was performed in each individual composting phase. The chain relative increment analysis was used for such assessment. Additionally, the potential metal pollution in the composts was evaluated by applying the following indices: the pollutant accumulation index (PAI), heavy metal enrichment index (HMEI), and heavy metal pollution load index (HMPLI). During composting, generally, the amounts of Ni, Cu, and Zn increased in the various extracted combinations, which was independent of their susceptibility to decomposition. Despite of this, the tested composts should be considered as a source of slowly available microelements for plants. The most intense quantitative changes in metals occurred in the thermophilic phase, and the weakest changes occurred in the cooling phase. At the same time, the calculated indices indicated a lack of contamination of the composts with the analyzed metals, emphasizing their environmental safety and lack of negative impact. The bulking agents used for composting with SS did not significantly influence the intensity of the quantitative changes in the tested metals.

1. Introduction

The generation of a large amount of waste is a characteristic of human civilization development. One of the most troublesome wastes systematically generated by humans is sewage sludge (SS). According to the Local Data Bank [1], in the last 12 years (2010–2022), the amount of SS increased by 10% in Poland. This may not be a dramatic pattern, but the proper management of this waste is needed, especially since the landfill of SS is prohibited in Poland, and its incineration is not the most popular method. The above data [1] indicate that the use of SS for agricultural and reclamation purposes is a popular method of utilization, in addition to its use for composting and the production of organic fertilizer, i.e., compost. The basis for this direction of SS management is its chemical properties related to its significant and attractive fertilization value [2,3,4,5,6,7]. On the other hand, the possibility of the presence of pollutants that have a toxic effect on the environment, such as heavy metals, PAHs, PCBs, and AOX, should be considered [8,9]. In particular, the last negative aspect resulted in the introduction of standards tightening the conditions for the use of SS in agriculture in some European countries (Germany, Norway, and Sweden), up to including a complete ban (e.g., in Switzerland) [10]. However, in many countries around the world (the USA, China, and South Korea), SS has been successfully used for agricultural purposes because the chemical composition of SS is rich in organic matter and nutrients; thus, it contributes significantly to the improvement in soil fertility [11]. However, when SS is applied repeatedly, there is a possibility of heavy metal accumulation in the soil and its subsequent incorporation into the food chain [12,13,14]. Considering the dual nature of SS, an alternative is composting. It should be emphasized that the composting process is an environmentally friendly technique that is relatively inexpensive and attractive for agriculture.

At the household scale, composting has been used since immemorial times and is likely the oldest form of recycling [15]. As a biological process, composting involves simultaneous biochemical and microbiological processes (mineralization and humification), which lead to the transformation of organic matter and physicochemical changes in the composted materials. Additionally, the composting of SS solves many environmental problems. The most frequently mentioned are the avoidance of waste disposal, elimination of bothersome odors, reduction in the mass of waste, degradation of toxic organic compounds, reduction in the bioavailability of heavy metals or transformation of organic matter, and maintenance of nutrient circulation [2]. Moreover, during the composting process of SS, the quality of organic matter improves, and the sludge is dewatered and sanitized [15]. The most important product of the entire process of composting SS is the production of organic fertilizer, compost, which is a valuable source of both macro- and micronutrients and organic matter [11,16,17,18]. However, the use of compost from organic waste is not only related to the assessment of its effectiveness as a fertilizer but also to concerns about whether it can be safely used in agriculture due to the potential content of heavy metals. The origin of the raw materials used for compost production is the most important factor when determining the level of heavy metals in the mature product. In general, composts made from plant waste contain small amounts of metals, and the highest content is found in composts with a high proportion of sludge sewage [19,20]. This raises concerns about their possible excessive accumulation in the soil when using composts prepared from sewage sludge. When assessing the value of compost, we consider, among other factors, the total amount of metals that cannot exceed the permissible standards for use as fertilizer. However, as was proven by Jakubus [12,21,22], the total amounts of metals in SS do not necessarily mean that their use both as fertilizers and co-substrate for composting is a threat to soils. Using only a total content, it is assumed that all forms of metals have the same impact on the environment, which is contrary to the actual situation. Therefore, research on the chemical forms of metals in SS-derived composts requires great attention, with particular emphasis on the degree of their bioavailability to plants. Many authors [12,14,21,22,23] also claim that, compared to the total metal content, the chemical forms of metals in the solid phase are more important in determining their bioavailability for plants and leaching into groundwater. This approach is of great practical importance because it provides the opportunity to predict the possible behavior of the introduced metal along with the compost in the environment. During a proper process, metals are strongly complexed with stabilized organic matter. As a result, metals from composts enter the soil in more chemically stable combinations, and therefore, they become less available to plants [15]. As reported by Barthod et al. [19], this is noticeable in the case of composting with various additives that significantly reduce the bioavailability of metals, resulting in a marketable, safe material. The use of various additives (organic and inorganic) in the process of composting sewage sludge is also highly important because of its high moisture content, small particle size, and thick texture [19,24]. According to Zhao et al. [24], bulking agents, especially those with a high level of recalcitrant carbon, reduce organic matter degradation, enhance the humification process, and consequently improve the quality of final compost products. Additionally, bulking agents are desirable because they significantly improve the structure of the mixture, ensuring proper gas exchange and reducing moisture. For this purpose, various types of straw, sawdust, and bark are most often recommended [15].

Considering that some metals, such as Cu, Zn, Mn, Ni, and Fe, are also micronutrients and necessary for plants, it is important to consider both their availability for plants and the undesirable nature of excess amounts. Therefore, the analysis of metal combinations becomes necessary for the soil application of compost, especially when the compost is made from SS with various additives as bulking agents. It should be noted that there are no specific methods for evaluating compost. The same techniques, as in the case of soils, are routinely used, and their usefulness and verifiability are satisfactory [12,21,22]. Considering the assessment of the mobility and bioavailability of metals in composts, two independent groups of analyses are proposed: sequential and single. Each of these methods has advantages and disadvantages. Sequential methods are time-consuming but provide precise and comprehensive information. The general idea of sequential extraction is based on subjecting the analyzed sample to subsequent sample “attacks” of solutions (reagents with different chemical properties: pH, redox potential, and complexing abilities) with increasing aggressiveness. In this way, the basic goal of sequential extraction is to achieve the successive washing of the component from various compounds and its various connections with the phase constant of the sample, with extraction taking place from the most mobile, easily soluble forms and ending with permanent, hard-to-dissolve forms. Such a goal can be achieved using the BCR sequential extraction method, which allows the separation of four fractions describing the combinations of elements with the solid phase of the tested sample with different degrees of solubility [25]. In turn, single extractions are quick and useful in routine monitoring, and although these methods can extract bioavailable amounts of metals, they allow only for general conclusions. There are many single extractants used in the discussed technique, and in this group, both strong mineral acids and neutral salts can be distinguished, but it is customary to use them to determine the bioavailability of buffered salts or complexing reagents, which is related to their ability to create stable complexes with a wide spectrum of elements. Due to the large number of extraction methods used, efforts are being made to standardize them, and common single extraction procedures involving 0.005 mol·L⁻¹ DTPA + 0.01 mol·L⁻¹ CaCl₂ + 0.1 mol·L⁻¹ TEA have been proposed [26].

Despite the differences between the sequential and single methods described above, in the opinion of the authors of this study, it is appropriate to use them simultaneously when assessing changes in the bioavailability of metals from composts. As a result of these various extractions, we can obtain a complete database with complementary information, which allows for accurate interpretation and conclusions. Unfortunately, this is not a popular approach to the issue in the literature, and studies of this type are few [27]. Researchers focus on either method, and the use of sequential extraction methods dominates. However, as reported by Zimmerman and Weindorf [28], the application of this technique is controversial due to the nonselectivity of used reagents in protocols. In the opinion of the cited authors, reliance on sequential extraction alone is not feasible and needs to be complemented with some other kind of analytical technique to positively identify the solid components involved. Therefore, this work was undertaken to determine the following: 1. the fractional distribution of microelements (Cu, Zn, and Ni) during the composting process of biodegradable wastes using sequential methods (BCR) as well as a single extractant (DTPA solution); 2. the rate of the microelements’ quantitative changes during the composting process depending on bulking agents and the process stage; and 3. the assessment of potential metal pollution in composts on the basis of various indices.

We assumed that the bulking agents used have a small impact on the quantitative changes and rate of microelement transformations during the composting process and that the main driving force of transformations is the naturally occurring processes during composting.

2. Materials and Methods

2.1. Experimental Design

The experiment was conducted under controlled conditions, where the temperature, humidity, and air flow were monitored. The individual waste mixtures were mixed according to the proportions presented in

Table 1. Composition of the investigated mixtures.

Component	C:N	Share in Mixture (%)
Compost 1 (C1)	17:1
Sewage sludge		45
Sawdust		50
Wheat straw		5
Compost 2 (C2)	16.6:1
Sewage sludge		45
Pine bark		50
Wheat straw		5
Compost 3 (C3)	16.6:1
Sewage sludge		45
Pine bark		35
Sawdust		15
Wheat straw		5

. The piles were created and placed in chambers with a volume of 125 dm³, isolated from external conditions, and aerated (air flow of 4 dm³·min⁻¹). The piles were adjusted by adding the amount of water required to obtain the values of 60–70% of dry matter (water was applied as needed to maintain the respective moisture level). The temperature was measured daily, and the mean values for the phase and mixture are presented in Figure 1.

The fundamental changes in the temperature (by 20 °C) related to microbiological activity and organic matter transformations are described as particular composting phases that determine the time at which compost samples are collected from the individual composting mixtures. Thus, samples representing the successive stages of the composting process, PI (mesophilic after day 14), PII (thermophilic after day 21), PIII (cooling, the end of the active period after day 45), and PIV (maturing after day 60), were selected for the analyses. Samples were collected at six random locations in each pile for each stage. These six samples were mixed to obtain one bulk sample and then divided into three subsamples (replications). The samples collected during the successive composting periods were dried at 105 °C for 12 h. The samples were ground into a fine powder, sieved (<1 mm), and stored in plastic bags at 4 °C. The sewage sludge for the experiment was obtained from a local wastewater treatment plant (Poznań district area) and was chemically stabilized with lime. The stabilization process is usually performed on dewatered SS using quickly reacting calcium compounds (calcium oxide or calcium hydroxide is preferably used). Sewage sludge and lime are mixed thoroughly and homogeneously. The addition of lime increases the SS dry matter content, decreases organic matter amounts and water, as well as killing pathogens and microorganisms. Wheat straw, sawdust, and pine bark were obtained from local producers (Poznań district area). The chemical compositions of the individual mixtures and their components are listed in

Table 2. Chosen properties of the composted wastes.

Waste	Dry Matter (%)	C:N	TOC	Ntot	Nitot	Cutot	Zntot
			g∙kg−1		mg∙kg−1
Sewage sludge	17	6:1	330	53	15.7	262.2	736.4
Sawdust	82	500:1	500	1	2.3	2.7	21.4
Wheat straw	86	147:1	440	3	1.3	4.4	19.4
Pine bark	57	165:1	496	3	6.5	2.9	24.8

.

2.2. Chemical Analysis

The total organic carbon (TOC) and total nitrogen (N_tot) contents in the wastes were assessed with a Vario Max CNS analyzer. The total contents of Ni, Cu, and Zn in the wastes were determined according to the ISO procedure [29]. One-step extractions using the DTPA solution (0.005 mol ∙L⁻¹ DTPA + 0.1 mol∙L⁻¹ TEA + 0.01 mol∙L⁻¹ CaCl₂, pH = 7.3; at a 1:2 ratio; 3 h of shaking at room temperature) [26] allowed us to assess the bioavailable forms of metals (bioava). The sequential extraction method used in this study was developed by the Community Bureau of Reference [25]. The details of the experimental protocol are shown in

Table 3. BCR sequential extraction scheme [25].

Fraction	Extracting Agent	Extracting Conditions
		Shaking Time	Temperature
Fr. I—Exchangeable and water- and acid-soluble	0.11 mol·L−1 CH3COOH (pH = 7)	16 h	20–25 °C
Fr. II—Reducible, e.g., bound to iron and manganese oxyhydroxides	0.5 mol·L−1 NH2OH-HCl (pH = 1.5)	16 h	20–25 °C
Fr. III—Oxidizable, e.g., bound to organic matter and sulfides	30% H2O2 (pH = 2.0) and then 1.0 mol·L−1 CH3COONH4 (pH =2.0)	1, 2, 16 h	20–25, 85, 20–25 °C
Fr. IV—Residual; nonsilicate-bound metals	Aqua regia	2.5 h	60–70 °C

. The concentrations of Cu, Zn, and Ni in the extracts and digests were determined by flame atomic absorption spectrometry (FAAS) using Varian Spectra AA 220 FS.

2.3. Statistical Analysis

Chain relative increments were used to compare the changes in the contents of the elements studied during the successive phases of composting. They are important tools in the analysis of data dynamics. These indices, owing to their variable basis, measure how the value of a phenomenon has changed in a given period relative to the previous period. In other words, they express the ratio of the value in a given period to the value in the previous period [30,31,32]. Therefore, observation $y^t$ equals

$$y^t={\displaystyle \frac{x_t-x_{t-1}}{x_{t-1}}},t=2,3,4.$$

(1)

Moreover, the obtained values are expressed as percentages, where $y^t$ is the relative growth during period $t$ and $x_a$ is the content of the studied parameter during period $t$. The relative growth, that is, the absolute growth divided by the size of the phenomenon from the base period, was analyzed. The values are expressed as percentages. The contents of the studied parameters were compared to the contents from the previous intake date. This concept is mainly used in the analysis of dynamics, i.e., the determination of the magnitude and direction of change over time, i.e., the determination of the level of increase or decrease in the phenomenon under study [33,34]. Relative increments report how much higher or lower the level of the phenomenon under study is in a given period relative to the immediately preceding period. Comparisons of the changes in the content of the elements under study using chain relative increments revealed changes greater than 5%.

An analysis of variance was performed separately for each element, each fraction, and each compost. The analysis was aimed at comparing the content of the studied elements [35]. To assess at which phase the greatest changes in absolute value in the content of the element under study occurred, the coefficient ${\gamma }_i$ was introduced:

$${\gamma }_i={\sum }_s{\left(y_{si}^t\right)}^2,$$

(2)

where $y_{si}^t$ denotes the content of the studied parameter at time point $t$ in the $s$ fraction in the $i^{th}$ compost, $t=2,3,4,s\in \left\{\mathrm{F}\mathrm{r}.\mathrm{I},\mathrm{F}\mathrm{r}.\mathrm{I}\mathrm{I},\mathrm{F}\mathrm{r}.\mathrm{I}\mathrm{I}\mathrm{I},\mathrm{F}\mathrm{r}.\mathrm{I}\mathrm{V},\mathrm{F}\mathrm{r}.\mathrm{b}\mathrm{i}\mathrm{o}\mathrm{a}\mathrm{v}\mathrm{a}\right\}$, and $i=1,2,3$. The minimal and maximal values were then determined for each element and for each compost.

To assess the risk of contamination to the soil and other environmental parameters, the impact of human activity on the grade of each metal in the compost was rated by using the pollutant accumulation index (PAI) [36]:

$${\mathrm{P}\mathrm{G}\mathrm{I}}_{sj}^k={log}_2{\displaystyle \frac{{conc}_{sj}^k\left(n\right)}{x{B}^k\left(n\right)}},$$

(3)

where ${conc}_{sj}^k\left(n\right)$ is the mean concentration of $k^{th}$ elements in the $s^{th}$ fraction and $j^{th}$ phase, $k\in \left\{\mathrm{C}\mathrm{u},\mathrm{Z}\mathrm{n},\mathrm{N}\mathrm{i}\right\},$ $s\in \left\{\mathrm{F}\mathrm{r}.\mathrm{I},\mathrm{F}\mathrm{r}.\mathrm{I}\mathrm{I},\mathrm{F}\mathrm{r}.\mathrm{I}\mathrm{I}\mathrm{I},\mathrm{F}\mathrm{r}.\mathrm{I}\mathrm{V},\mathrm{F}\mathrm{r}.\mathrm{b}\mathrm{i}\mathrm{o}\mathrm{a}\mathrm{v}\mathrm{a}\right\}$, and $j=1,2,3,4$; $x=1.5$ is the correction index; $B^k\left(n\right)$ is the maximum permissible concentration of the metals in the compost; and $B^{\mathrm{Cu}}\left(n\right)=100,B^{\mathrm{Zn}}\left(n\right)=300,\mathrm{a}\mathrm{n}\mathrm{d}{B}^{\mathrm{Ni}}\left(n\right)=50$. The thresholds were established on the basis of references provided by the Commission Decision (EU) 2015/2099 [37]. Based on Muller [36], the extent of pollution was categorized as follows: ${\mathrm{P}\mathrm{G}\mathrm{I}}_{sj}^k<0$ denotes no pollution, ${1<\mathrm{P}\mathrm{G}\mathrm{I}}_{sj}^k<2$ denotes low pollution, $2\le {\mathrm{P}\mathrm{G}\mathrm{I}}_{sj}^k<3$ denotes moderate pollution, $3\le {\mathrm{P}\mathrm{G}\mathrm{I}}_{sj}^k<4$ denotes high pollution, and ${\mathrm{P}\mathrm{G}\mathrm{I}}_{sj}^k\ge 4$ denotes very high pollution.

The heavy metal enrichment index (HMEI) was applied for the determination of the increase in the pollutant in the compost relative to the amount of the element naturally occurring in the environment due to human activity:

$${\mathrm{H}\mathrm{M}\mathrm{E}\mathrm{I}}_{sj}^k={\displaystyle \frac{{conc}_{sj}^k\left(n\right)/conc\left(X\right)}{B^k\left(n\right)/conc\left(X\right)}}={\displaystyle \frac{{conc}_{sj}^k\left(n\right)}{B^k\left(n\right)}},$$

(4)

where $conc\left(X\right)$ is the total concentration of a reference/baseline element for normalization, and $t$ is the number of parameters under study. According to Gyamfi et al. [38], ${\mathrm{H}\mathrm{M}\mathrm{E}\mathrm{I}}_{sj}^k<1$ denotes deficiency or no enrichment, ${1<\mathrm{H}\mathrm{M}\mathrm{E}\mathrm{I}}_{sj}^k<3$ denotes moderate enrichment, ${3\le \mathrm{H}\mathrm{M}\mathrm{E}\mathrm{I}}_{sj}^k<6$ denotes considerable enrichment, and ${\mathrm{H}\mathrm{M}\mathrm{E}\mathrm{I}}_{sj}^k\ge 6$ denotes very high enrichment.

The heavy metal pollution load index (HMPLI) was calculated according to the formula:

$${\mathrm{H}\mathrm{M}\mathrm{P}\mathrm{L}\mathrm{I}}_{sj}=\sqrt[t]{{\prod }_k{\mathrm{H}\mathrm{M}\mathrm{E}\mathrm{I}}_{sj}^k},$$

(5)

where $t$ is the total number of heavy metals assessed in the study. According to the methodology of Gupta et al. [39], ${\mathrm{H}\mathrm{M}\mathrm{P}\mathrm{L}\mathrm{I}}_{sj}\le 0$ denotes the control, ${0<\mathrm{H}\mathrm{M}\mathrm{P}\mathrm{L}\mathrm{I}}_{sj}\le 1$ denotes the baseline level of pollutants, and ${\mathrm{H}\mathrm{M}\mathrm{P}\mathrm{L}\mathrm{I}}_{sj}>1$ denotes the continuous degradation of the environment due to increased heavy metal contamination.

The calculations were made using Microsoft^® Excel^® 2021 MSO and R version 4.4.0.

3. Results

3.1. The Pattern of Copper Quantity Changes

Regardless of the type of compost tested, the copper content of all sequentially separated fractions significantly differed among the composting phases (

Table 4. ANOVA performed separately for each element, each fraction, and each compost. The letters of the alphabet denote the division into homogeneous groups, where “a” denotes the group with the highest content of the element under study. “Fr” denotes fraction and “C” denotes compost.

Cu		C	p-Value	Phase
				1	2	3	4
	Fr. I	1	2.39 × 10−9 ***	d	c	ba	a
		2	0.000115 ***	c	a	b	ab
		3	8.72 × 10−8 ***	c	b	b	a
	Fr. II	1	3.99 × 10−7 ***	a	b	b	c
		2	9.55 × 10−8 ***	a	a	b	b
		3	6.5 × 10−8 ***	a	b	b	b
	Fr. III	1	3.18 × 10−9 ***	d	c	b	a
		2	2.16 × 10−5 ***	bc	b	a	a
		3	2.19 × 10−7 ***	c	b	b	a
	Fr. IV	1	1.4 × 10−8 ***	a	ab	ab	c
		2	0.000478 ***	c	bc	ba	a
		3	7.54 × 10−5 ***	b	a	a	a
	bioava	1	7.68 × 10−11 ***	d	c	b	a
		2	3.47 × 10−9 ***	d	c	b	a
		3	1.22 × 10−10 ***	d	c	b	a
Zn	Fr. I	1	8.48 × 10−10 ***	c	c	b	a
		2	3.83 × 10−5 ***	a	b	b	b
		3	6.1 × 10−8 ***	a	b	c	d
	Fr. II	1	2.25 × 10−7 ***	c	a	b	a
		2	2.82 × 10−10 ***	d	c	b	a
		3	3.24 × 10−7 ***	c	b	ab	a
	Fr. III	1	4.64 × 10−6 ***	c	d	b	a
		2	5.47 × 10−9 ***	c	b	a	a
		3	1.28 × 10−11 ***	c	c	b	a
	Fr. IV	1	0.00054 ***	cb	b	b	a
		2	0.365
		3	0.291
	bioava	1	1.14 × 10−7 ***	c	c	a	b
		2	1.64 × 10−12 ***	c	b	a	a
		3	8.34 × 10−11 ***	c	a	a	b
Ni	Fr. I	1	1.63 × 10−6 ***	a	b	c	c
		2	5.74 × 10−6 ***	a	b	bc	c
		3	7.23 × 10−7 ***	a	b	b	b
	Fr. II	1	0.0578
		2	0.0179 *	b	a	ab	a
		3	3.3 × 10−6 ***	b	b	b	a
	Fr. III	1	1.21 × 10−5 ***	c	b	b	a
		2	0.00162 **	b	a	a	a
		3	0.00348 **	b	a	a	a
	Fr. IV	1	2.54 × 10−8 ***	d	c	b	a
		2	0.0266 *	a	ab	b	ab
		3	4.65 × 10−5 ***	c	b	ab	a
	bioava	1	1.57 × 10−6 ***	a	ab	b	c
		2	6 × 10−4 ***	a	a	c	ba
		3	0.144

Signif. codes: 0 “***” 0.001 “**” 0.01 “*” 0.05 “.” 0.1 “ ” 1.

). Additionally, the amounts of bioavailable Cu (Cubioava) determined in the different composting phases of the mixtures were significantly different. A comparison of the chain relative increments in the Cu content among the composts revealed that, in all the compost mixtures, the Cu content in Fr. I observed at the end of the mesophilic phase was significantly lower than that observed in the subsequent composting stage (

Table 5. Chain relative increment for Cu, Zn, and Ni. In the table, $y^t=(x_t-x_{t-1})/x_{t-1}$; $t=2,3,4$; $x_t$ is the content of the studied parameter during time point $t$ (see (1)); “Fr” denotes fraction; and “C” denotes compost.

Parameter			Cu			Zn			Ni
		${\mathit{y}}^{\mathbf{2}}$	${\mathit{y}}^{\mathbf{3}}$	${\mathit{y}}^{\mathbf{4}}$	${\mathit{y}}^{\mathbf{2}}$	${\mathit{y}}^{\mathbf{3}}$	${\mathit{y}}^{\mathbf{4}}$	${\mathit{y}}^{\mathbf{2}}$	${\mathit{y}}^{\mathbf{3}}$	${\mathit{y}}^{\mathbf{4}}$
	C
Fr. I	1	191.76	21.13	14.20	4.35	7.93	31.37	−26.22	−34.77	6.71
	2	173.87	−32.57	14.72	−20.10	−2.80	−8.22	−31.57	−16.85	−14.18
	3	58.03	1.62	29.23	−11.57	−11.95	−18.72	−53.77	−29.06	2.55
Fr. II	1	−29.42	−13.48	−30.22	36.15	−9.90	18.88	−10.84	−3.62	33.21
	2	−0.77	−40.98	−0.47	73.41	31.26	22.11	9.05	32.50	−5.35
	3	−43.53	−11.22	−12.23	43.72	8.37	8.06	−16.75	3.74	85.38
Fr. III	1	18.79	21.33	10.73	1.32	8.27	11.04	56.85	−5.84	34.30
	2	5.71	8.96	3.37	30.09	13.64	3.04	35.64	9.59	8.00
	3	36.03	1.76	19.56	5.26	21.75	31.68	93.87	5.02	−15.75
Fr. IV	1	−5.36	−2.64	−25.01	−8.10	9.89	24.98	35.73	24.87	40.13
	2	25.41	16.04	5.40	−4.29	−3.62	11.29	6.77	23.45	−7.74
	3	53.89	−4.47	11.50	8.59	4.24	−0.71	50.56	28.86	7.62
bioava	1	109.01	22.97	19.45	3.03	31.98	−8.66	−9.40	−7.65	−36.11
	2	55.02	29.55	28.38	27.54	5.37	0.47	−8.61	−24.98	29.05
	3	97.01	11.43	23.00	75.13	5.72	−24.83	−7.17	19.47	−5.13

, Figure 2). For C1 and C3, the Cu content in fraction I increased throughout the composting period, with the greatest relative increase in content occurring in the thermophilic phase, at 191.76% for C1 and 58.03% for C3. Between composting phases, the increase in the bioavailable Cu content did not exceed 20.13%. During the observation of the changes that occurred during the composting of C2, an increase of 173.87% in the amount of Cu Fr. I in the thermophilic phase was observed, with a decrease of 32.57% in the next cooling phase and an increase of 14.72% in the maturation phase.

In all composts, the amount of Cu Fr. II observed at the end of the mesophilic phase was significantly greater than that observed in the subsequent composting stage (Table 4). The rate of change in the copper content of Fr II was significantly influenced by the composition of the compost, with reducible Cu levels decreasing throughout the process (Table 5, Figure 2). In C1, the greatest reduction of 30.22% in metal content was recorded in the mature compost. In C2, the difference in the decrease in the reducible amount of Cu during the cooling phase was −40.98%. Regardless of the type of compost and the period of comparison, this was the greatest decrease in the Cu content of fraction II. In C3, the greatest reduction in the Cu fraction II content was recorded at the beginning of the composting period in the thermophilic phase, at 43.53% (Table 5, Figure 2).

In all composts, the Cu Fr. III content observed at the end of the mesophilic phase was significantly lower than that observed in the subsequent composting stage (Table 4). A comparison of the changes in the metal amounts between the phases revealed that the rates of the quantitative changes in the oxidizable Cu bond content were similar for C1 and C2, with the highest values occurring in the cooling phase (Table 5, Figure 2). In C1, greater increases in Cu content were recorded in Fr. III (21.33%) than in C2 (8.96%). In C3, the smallest increase in the amount of Cu Fr. III was found in the cooling phase (1.76%), while the largest increase was at the beginning of the thermophilic phase (36.03%).

The content of Cu Fr. IV at the end of the mesophilic phase in C1 was comparable to the values observed up to the end of the cooling phase and was significantly greater than the values observed in the mature compost (Table 4). In C2, the Cu residual amounts at the end of the mesophilic phase were comparable to the values observed up to the end of the thermophilic phase and were significantly lower than the values observed in the mature compost. In C3, the Cu content of fraction IV increased significantly after the end of the mesophilic phase. The rate of change in the Cu content in Fr. IV was significantly influenced by the tested composts (Table 5, Figure 2). In C1, the residual Cu content decreased, and the rate of reduction was the highest in the mature compost, reaching a value of 25.01%. In C2, the Cu Fr. IV content successively increased, but the greatest increase (25.41%) was observed in the initial part of the process in the thermophilic phase. At C3, there was a 53.89% increase in the residual copper content in the thermophilic phase, followed by a 4.47% decrease and a slow increase in the mature compost, reaching 11.50%.

In all compost mixtures, the bioavailable amounts of Cu observed at the end of the mesophilic phase were significantly lower than the values observed in the subsequent composting stage (Table 4). An analysis of the chain relative increments in the bioavailable copper content revealed an increase in the copper content in all phases. The greatest increases in its content were recorded in the thermophilic phase, with values of 109.21% (C1), 55.02% (C2), and 97.01% (C3). Regardless of the type of compost, the amount of bioavailable copper increased further in the composting process, not exceeding 29.55% for C2 and the cooling period (Table 5, Figure 2).

3.2. The Pattern of Zinc Quantity Changes

The zinc content of fractions I–III and Znbioava differed significantly in the subsequent composting phases of all the composts tested (Table 4). In C1, significant differences were observed in the zinc content of fraction IV. In the other composts, no significant differences were observed in the amount of the element in the residual combinations.

The Zn content fraction I zinc content at the end of the mesophilic phase in C1 was significantly lower than the values observed in the cooling phase. In C2 and C3, the Zn Fr. I content at the end of the mesophilic phase was significantly greater than that in the subsequent composting phases (Table 4). Throughout the composting process, the zinc content of Fr. I in C1 fluctuated, increasing by 4.35% in the thermophilic phase and then decreasing by 7.93% in the cooling phase (Table 5, Figure 3). Analyzing the relative changes, it was noted that the highest rate of increase in the content of this element was in the mature compost, as evidenced by a 31.37% increase in its amount compared to that in the previous period, the cooling phase. The exchangeable Zn content at C2 and C3 decreased throughout the process, with the greatest reduction in Zn Fr. I observed at C2 in the thermophilic phase by 20.10% and at C3 in the mature material by 18.72%. (Table 5, Figure 3)

An analysis of the average rate of change during the successive stages of composting revealed that, irrespective of the compost studied, the Zn content in the reducible fraction underwent a systematic increase during the composting process of the mixtures studied (Table 5, Figure 3). The greatest increase in the amount of metal was observed between the thermophilic phase (36.15%, 73.41%, and 43.72% in C1, C2, and C3, respectively). At all time points studied, the greatest increases in the Zn content of fraction II were recorded in C2 compared to the other composts. It should be mentioned that only in C1 was there a decrease in the Zn fraction II content of 9.90% during the cooling phase.

The zinc content of fraction III at the end of the mesophilic phase, regardless of the type of compost, was significantly lower than the values observed in the subsequent stages of the composting process (Table 4). Considering the chain relative increments, the trend of the changes in the amount of Zn Fr. III was similar to that observed for fraction II. In each successive composting phase, the Zn Fr. III content increased (Table 5, Figure 3). Notably, for C1 and C3, the greatest increase in the amount of Zn Fr. III was detected in the mature compost (11.04% for C1 and 31.68% for C2) and in C2 in the thermophilic phase (30.09%). The largest relative increase in the fraction III zinc content occurred in mature C3.

The zinc content of fraction IV in C1 increased significantly only in the mature compost and reached comparable values in the other phases. No significant differences were observed between C2 and C3 (Table 4). On the basis of the results obtained for the comparisons of the increments in the element content, it was observed that, in the thermophilic phase, the content of residual zinc decreased by 8.10% in C1 and by 4.29% in C2 (Table 5, Figure 3). During further composting, the content of Zn Fr. IV increased, reaching 24.98% in the mature C1 group. In C2, the decreasing trend in the residual zinc content also continued in the cooling phase, while an increase of 11.29% in the amount of Zn Fr. IV occurred in the mature compost. In C3, the residual zinc content increased up to the cooling phase, reaching the highest value in the thermophilic phase (8.59%), followed by a decrease of 0.71% in the mature compost phase.

The amount of bioavailable zinc at the end of the mesophilic phase, regardless of the type of compost, was significantly lower than the values observed during other phases of the composting process (Table 4). Based on the relative increases (Table 5, Figure 3), the bioavailable zinc content in C1 fluctuated during the composting process, resulting in increases of 3.03% in the thermophilic phase and 31.98% in the subsequent phase, followed by a decrease of 8.66% in the mature material. In C2 and C3, the changes in bioavailable zinc content followed a similar pattern, and during composting, the bioavailable amounts of Zn increased until the end of the cooling phase, reaching the highest values in the thermophilic phase (27.54% for C2 and 75.13% for C3). In the mature compost, there was a slight increase in the bioavailable Zn content of 0.47% in C2 and a decrease of 24.83% in C3. It should be emphasized that the greatest increase in the content of the aforementioned metal was observed in the thermophilic phase in C3.

3.3. The Pattern of Nickel Quantity Changes

The nickel content of fractions I-IV and the bioavailable amounts in all the composts differed significantly among all the composting phases (Table 4). The exceptions are the lack of differences in the contents of Ni fraction II for C1 and bioavailable amounts of nickel for C3. The Ni Fr. I content at the end of the mesophilic phase, irrespective of the type of compost, was significantly greater than the values observed at the following time points (Table 4). Evaluating the changes in nickel content on the basis of the mean increases, it should be noted that the nickel content in fraction I decreased in all composts up to the maturing phase, with the greatest decreases of 34.77%, 31.57%, and 53.77% in composts 1, 2, and 3, respectively (Table 5, Figure 4). In the maturing phase, increases of 6.71% in C1 and 2.55% in C3 were observed for this metal. The opposite trend occurred in C2, where a decrease in the Ni content of Fr. I was observed throughout the composting process, reaching 14.18% in the maturing compost stage. The greatest decrease in the content of this element of 53.77% was recorded in C3 in the thermophilic phase.

The nickel content of fraction II in C1 at the end of the mesophilic phase was not significantly different from the values observed in the other phases of the composting process. The nickel content of the abovementioned fraction was significantly lower at the end of the mesophilic phase than at the end of the thermophilic phase. In C3, no significant differences were observed between the nickel content of this fraction in the mesophilic and thermophilic phases (Table 4). On the basis of the evaluation of the change in nickel content performed by means of average increments, it can be concluded that the amount of Ni Fr. II decreased in the thermophilic phase by 10.84% in C1 and by 16.75% in C3 (Table 5, Figure 4). The decreasing trend of the Ni content in Fr. II Ni in C1 continued until the end of the cooling phase. After this time, there was a 33.21% increase in the amount of Ni in the maturation phase. In C3, after the end of the thermophilic phase, the content of Ni Fr. II increased, reaching the highest value (85.38%) in the mature compost. In C2, the nickel content of fraction II increased until the end of the cooling phase, reaching the highest value of 32.50%, while in the mature compost, the content of this element decreased by 5.35%.

The nickel content of fraction III at the end of the mesophilic phase, regardless of the type of compost, was significantly lower than the values observed at the next time points (Table 4). A comparison of the chain relative increments revealed that, irrespective of the content of the compost, the content of NI Fr. III increased from the end of the thermophilic phase, reaching 56.85% (in C1), 35.64% (in C2), and 93.87% (in C3) (Table 5, Figure 4). It is worth noting that the increase in the content of this metal was maintained only in C2 and did not exceed 9.59% in the subsequent phases. In C1, a decrease of 5.84% in the amount of Ni Fr. III was observed in the cooling phase. A decrease of 15.75% in the content of this metal was also observed in C3 in the maturing phase.

The nickel content of fraction IV at the end of the mesophilic phase was significantly lower than the values observed at other time points in C1 and C3. The residual Ni content in C2 at the end of the mesophilic and thermophilic phases was not significantly different (Table 4). On the basis of the chain relative increments, it can be concluded that the amount of Ni in this fraction increased in all the composts throughout the composting process (Table 5, Figure 4). The only exception to this rule was the mature C2, where a decrease in residual Ni of 7.74% was found. The greatest increase of 40.13% in the amount of Ni Fr. IV was observed in the mature C1 phase, and an increase of 50.56% was observed in the thermophilic phase in the C3.

The bioavailable nickel amounts at the end of the mesophilic phase, regardless of the type of compost, were comparable to the values observed at the end of the thermophilic phase (Table 4). The values obtained from the comparison of the average increments indicate that the bioavailable Ni content decreased in C1 during the whole composting process, reaching the highest value of −36.11% in the maturing phase (Table 5, Figure 4). The aforementioned downwards trend in the bioavailable Ni content continued in C2 until the end of the cooling phase, with a 29.05% increase in the Ni content of this fraction in the mature compost. In C3, there were fluctuations in the bioavailable Ni content, which were determined by the timing of sampling. In the thermophilic phase, the Nibioava content decreased by 7.17%. During the cooling period, the metal content of the aforementioned fraction increased by 19.47%, and in the mature compost, it decreased again by 5.13%.

Comparing all the fractions of the metals studied, the largest changes in copper, nickel, and zinc contents were recorded in the thermophilic phase, except for Zn in C1 and nickel in C2 (

Table 6. The composting phase for which the minimal and maximal values of the coefficient ${\gamma }_i$ were determined according to (2). The coefficient ${\gamma }_i$ determined separately for each test element and each compost denotes the sum of the squares of the changes in the content of the test elements in all fractions; “C” denotes compost.

Parameter	C	Maximal Value	Minimal Value
Cu	1	thermophilic phase	cooling phase
	2	thermophilic phase	maturing phase
	3	thermophilic phase	cooling phase
Zn	1	maturing phase	cooling phase
	2	thermophilic phase	maturing phase
	3	thermophilic phase	cooling phase
Ni	1	thermophilic phase	cooling phase
	2	cooling phase	maturing phase
	3	thermophilic phase	cooling phase

). The smallest changes in the content of all the elements discussed were recorded in C1 and C3 in the cooling phase. In C2, the smallest changes occurred in the mature phase.

3.4. Assessment of Metal Pollution in Composts

The microelements examined in this research are essential ingredients for plants, but it must not be forgotten that they also belong to the group of heavy metals and are characterized by a certain mobility in the environment. Therefore, it is justified and necessary to assess the possible contamination of composts, especially when we consider their fertilizing nature. Therefore, the selected climatic indices help in the valorization of composts and indicate their possible impact on the soil environment after their application. On the basis of the ${\mathrm{P}\mathrm{G}\mathrm{I}}_{sj}^k$ values determined according to (3), regardless of the sampling date and the fraction of metals tested, the contamination of each heavy metal studied in each compost was recorded at a level indicating no contamination (

Table 7. Indices ${\mathrm{P}\mathrm{G}\mathrm{I}}_{sj}^k,{\mathrm{H}\mathrm{M}\mathrm{E}\mathrm{I}}_{sj}^k,\mathrm{a}\mathrm{n}\mathrm{d}{\mathrm{H}\mathrm{M}\mathrm{P}\mathrm{L}\mathrm{I}}_{sj}$ of the $k^{th}$ element in the $s^{th}$ fraction and the $j^{th}$ phase, where $k\in \left\{\mathrm{C}\mathrm{u},\mathrm{Z}\mathrm{n},\mathrm{N}\mathrm{i}\right\},$ $s\in \left\{\mathrm{F}\mathrm{r}.\mathrm{I},\mathrm{F}\mathrm{r}.\mathrm{I}\mathrm{I},\mathrm{F}\mathrm{r}.\mathrm{I}\mathrm{I}\mathrm{I},\mathrm{F}\mathrm{r}.\mathrm{I}\mathrm{V},\mathrm{b}\mathrm{i}\mathrm{o}\mathrm{a}\mathrm{v}\mathrm{a}\right\}$, and $j=1,2,3,4$; “Fr” denotes fraction; and “C” denotes compost; see (3–5).

			Parameters
			Cu		Zn		Ni
	C	Term	PGI	HMEI	PGI	HMEI	PGI	HMEI	HMPLI
Fr. I	1	1	−6.749	0.014	−2.379	0.288	−4.434	0.069	0.065
		2	−5.204	0.041	−2.317	0.301	−4.873	0.051	0.086
		3	−4.927	0.049	−2.207	0.325	−5.489	0.033	0.081
		4	−4.736	0.056	−1.814	0.427	−5.395	0.036	0.095
	2	1	−6.708	0.014	−2.228	0.320	−4.600	0.062	0.066
		2	−5.254	0.039	−2.552	0.256	−5.147	0.042	0.075
		3	−5.823	0.027	−2.593	0.249	−5.414	0.035	0.061
		4	−5.625	0.030	−2.717	0.228	−5.634	0.030	0.059
	3	1	−6.266	0.019	−2.262	0.313	−4.246	0.079	0.078
		2	−5.606	0.031	−2.439	0.277	−5.359	0.037	0.068
		3	−5.583	0.031	−2.622	0.244	−5.855	0.026	0.058
		4	−5.213	0.040	−2.921	0.198	−5.818	0.027	0.060
Fr. II	1	1	−1.652	0.477	−1.600	0.495	−4.310	0.076	0.261
		2	−2.155	0.337	−1.155	0.674	−4.476	0.067	0.248
		3	−2.364	0.291	−1.305	0.607	−4.529	0.065	0.226
		4	−2.883	0.203	−1.056	0.722	−4.115	0.087	0.233
	2	1	−2.574	0.252	−1.933	0.393	−4.671	0.059	0.180
		2	−2.585	0.250	−1.139	0.681	−4.546	0.064	0.222
		3	−3.346	0.148	−0.746	0.894	−4.140	0.085	0.224
		4	−3.353	0.147	−0.458	1.092	−4.219	0.081	0.235
	3	1	−2.345	0.295	−2.373	0.290	−4.609	0.061	0.174
		2	−3.170	0.167	−1.849	0.416	−4.873	0.051	0.153
		3	−3.341	0.148	−1.733	0.451	−4.820	0.053	0.152
		4	−3.529	0.130	−1.622	0.487	−3.930	0.098	0.184
Fr. III	1	1	−1.020	0.740	−1.992	0.377	−5.022	0.046	0.234
		2	−0.772	0.879	−1.973	0.382	−4.373	0.072	0.290
		3	−0.493	1.066	−1.858	0.414	−4.460	0.068	0.311
		4	−0.346	1.180	−1.707	0.459	−4.034	0.092	0.368
	2	1	−0.372	1.159	−2.108	0.348	−4.701	0.058	0.285
		2	−0.291	1.226	−1.728	0.453	−4.261	0.078	0.351
		3	−0.168	1.336	−1.544	0.514	−4.129	0.086	0.389
		4	−0.120	1.381	−1.501	0.530	−4.018	0.093	0.408
	3	1	−0.838	0.839	−1.874	0.409	−4.866	0.051	0.260
		2	−0.395	1.141	−1.800	0.431	−3.911	0.100	0.366
		3	−0.369	1.161	−1.516	0.524	−3.840	0.105	0.400
		4	−0.112	1.388	−1.119	0.691	−4.087	0.088	0.439
Fr. IV	1	1	−3.994	0.094	−3.639	0.120	−4.935	0.049	0.082
		2	−4.073	0.089	−3.761	0.111	−4.494	0.067	0.087
		3	−4.112	0.087	−3.625	0.122	−4.174	0.083	0.096
		4	−4.527	0.065	−3.303	0.152	−3.687	0.116	0.105
	2	1	−4.974	0.048	−2.697	0.231	−4.249	0.079	0.095
		2	−4.647	0.060	−2.760	0.221	−4.155	0.084	0.104
		3	−4.433	0.069	−2.814	0.213	−3.851	0.104	0.115
		4	−4.357	0.073	−2.659	0.237	−3.967	0.096	0.119
	3	1	−4.968	0.048	−2.987	0.189	−4.419	0.070	0.086
		2	−4.346	0.074	−2.868	0.205	−3.828	0.106	0.117
		3	−4.412	0.070	−2.808	0.214	−3.463	0.136	0.127
		4	−4.255	0.079	−2.818	0.213	−3.357	0.146	0.135
bioava	1	1	−2.662	0.237	−1.583	0.501	−4.070	0.089	0.220
		2	−1.598	0.495	−1.540	0.516	−4.212	0.081	0.275
		3	−1.300	0.609	−1.139	0.681	−4.327	0.075	0.314
		4	−1.043	0.728	−1.270	0.622	−4.974	0.048	0.279
	2	1	−2.445	0.275	−1.806	0.429	−4.442	0.069	0.201
		2	−1.812	0.427	−1.455	0.547	−4.572	0.063	0.245
		3	−1.439	0.553	−1.379	0.577	−4.986	0.047	0.247
		4	−1.079	0.710	−1.372	0.579	−4.619	0.061	0.293
	3	1	−2.408	0.283	−1.871	0.410	−5.013	0.046	0.175
		2	−1.430	0.557	−1.062	0.718	−5.121	0.043	0.258
		3	−1.273	0.621	−0.982	0.759	−4.864	0.052	0.290
		4	−0.975	0.763	−1.394	0.571	−4.940	0.049	0.277

). The values of the enrichment factor index ${\mathrm{H}\mathrm{M}\mathrm{E}\mathrm{I}}_{sj}^k$, determined according to (4), indicate the deficiency or lack of enrichment of the pollutant in the composts relative to the amount of the element naturally occurring in the environment due to human activity in all composts and all metal fractions. The exceptions to this rule were the amount of Cu Fr. III in all composts throughout the study period and the amount of Zn Fr II in C2 in the maturation phase. In these cases, a moderate enrichment was observed. The value of the index of the total contamination in the soil, ${\mathrm{H}\mathrm{M}\mathrm{P}\mathrm{L}\mathrm{I}}_{sj}$, indicates the baseline levels of pollution under all conditions (Table 7); see (5).

4. Discussion

Understanding changes in metal bioavailability during the composting of SS with various bulking agents largely depends on the changes in composting organic matter that occur during the process. Composting can be divided into four basic phases: mesophilic, thermophilic, cooling, and maturation [40]. They differ in temperature and pH, which have a direct impact on the microorganisms dominating in particular stages. In the mesophilic phase, at a temperature close to the ambient temperature, mesophilic bacteria predominate and use easily degradable organic compounds, such as simple sugars, proteins, starch, and fats. Thanks to thermophilic bacteria, the second phase of the process is the most active stage of the composting, during which most readily available organic compounds are decomposed into carbon dioxide and converted into humus. The material composted in the cooling phase consists of, among others, substances that are difficult to decompose: cellulose, chitin, lignin, and hemicellulose. Due to their structure and low susceptibility to degradation, the listed compounds can only be broken down by extracellular enzymes secreted by fungi and actinomycetes [41]. Therefore, the population of these organisms begins to dominate during the maturation phase. This stage of the process is the longest, and during this stage, the stabilization of the composted mixture occurs, expressed, among other things, by an increased share of humus and a lack of phytotoxic substances [42]. As can be seen from this general description of the individual composting phases, a number of microbiological and biochemical transformations occur during the process, which are reflected in the transformation of organic matter, pH, and temperature. These factors also have a direct and indirect impact on the mobility of metals and, consequently, their potential bioavailability expressed by their individual distribution in specific fractions and combinations. In general, the largest changes in the amounts of microelements were in the thermophilic phase, and the smallest changes were in the cooling phase. Changes in the organic matter of composts are one of the main drivers of the variable distribution of metals in their fractions. Therefore, special attention, especially in waste composting, should be paid to reducing the mobility and availability of metals to plants, even when their total content increases as a result of reducing the composted mass. During the composting process, a decrease in the amount of organic matter can cause either dilution or an increase in the concentration of nutrients, depending on the combinations in which metals occur [43]. The heterogeneity and dynamics of the transformations of individual metals were demonstrated in this research.

The amounts of Cu and Zn in the separated fractions generally increased during the composting process of the individual mixtures. Additionally, the bioavailable amounts of Cu and Zn were greater in mature composts than at the beginning of the composting process, which the chain relative increment data show. The pattern of quantitative changes in Ni was slightly different because the content of Ni in Fr. I decreased while that in the remaining connections increased, which was particularly visible in the organic and residual bonds. The amounts of bioavailable Ni, apart from those in C1, were at a constant level. The bioavailable amounts of Ni in C1 decreased significantly during composting. An experiment conducted with composted sewage sludge by Jakubus [27] confirmed a significant reduced mobility of this metal during the composting process. Zorpas and Loizidou [44] also noted a significant share (75%) of nickel in organic and residual connections of sewage sludge composted with sawdust and zeolites, although here, the decisive role in binding metals may have been that of zeolites. Rehana et al. [45] and Pecorini et al. [46] also found the largest amounts of Ni in Fr. IV and observed a significant reduction in the mobile fraction of Ni. According to the authors of the cited studies, the additives used for composting SS, such as sawdust, zeolites, and fly ash, strongly stabilized the amount of this metal, reducing the mobility of Ni and its probable uptake by the plant. This statement is acceptable and can be interpreted in relation to the changes in the distribution of Ni in the sequentially separated fractions that were found for the mixtures examined in this paper in relation to those determined for SS alone, so that each compost had the same percentage of SS (45%) in its composition. Jakubus [21] found that, in SS from the Poznan district, the amount of Ni in the fractions increased in the opposite direction, i.e., Fr. IV < Fr. III < Fr. II < Fr. I. In this study, regardless of the compost, the Ni content in the fractions of the mature composts increased as follows: Fr. I < Fr. II < Fr. III < Fr. IV (based on absolute values not directly presented here). This allows us to assume that the additives used in the form of sawdust, straw, or bark were effective structural agents that reduced the mobility of nickel in the exchangeable, water-soluble, and acid-soluble bonds (Fr. I). The bulking agents used for composting with SS are characterized by different chemical compositions and, therefore, susceptibility to microbiological decomposition. This will also determine their greater or lesser ability to absorb metals. Straw was used in the same proportion (5%), and its significant impact on the sorption process was rather negligible because, compared to sawdust or bark, straw contains less lignin and more water-soluble sugars, proteins, and starch [47], i.e., compounds that are easily accessible sources of carbon for microorganisms carrying out the mineralization process. Therefore, it can be assumed that, when sawdust and bark are used as wood materials, they do not decompose as dynamically as straw; thus, they are likely to play a more important role in the sorption process. It can be assumed that the decomposition of organic matter in SS contributed to the release of certain amounts of Ni, which in the initial phases of composting was sorbed on both sawdust and bark. Such a possibility is indicated by Kovacova et al. [48], although their studies were conducted for Cu and Zn. The cited authors demonstrated that sawdust is an effective adsorbent for the removal of these metal ions from model solutions. Xiong et al. [49] indicated that the addition of swelling agents decreases the heavy metal content and reduces the mobility of heavy metals. Unfortunately, such a possible sorption with respect to Cu and Zn cannot be clearly stated in these studies because, during composting, both the bioavailable amounts and contents in the separated fractions increased, which was a phenomenon independent of the composition of the mixture. The only exceptions were the amounts of Zn Fr. I for C2 and C3, which decreased during composting. Moreover, the distributions of Zn in the separated fractions of SS [12] or mature composts were very comparable, which raises doubts as to the effectiveness of the additives used, such as stabilizing agents. As cited by the above author, in SS, the following sequence of increasing Zn amounts was detected: Fr. IV < Fr. I < Fr. III < Fr. II, and in the mature composts in this study, increasing amounts of this metal were detected in the following series of fractions extracted: C1, Fr. IV < Fr. I < Fr. III < Fr. II; C2, Fr. I < Fr. IV < Fr. III < Fr. II; and C3, Fr. I < Fr. IV < Fr. II < Fr. III (based on absolute values not directly presented here). Gao et al. [43] also confirmed such a distribution of Zn because they determined most of the Zn in fractions III and II. On the other hand, Pecorini et al. [46] reported the highest percentage of Zn in the mobile fractions of compost.

In light of the reports on the high mobility of nickel and zinc in composts [50,51], the low metal contents in fraction I of composts and the higher metal contents in stable chemical forms are interesting results. Such changes seem beneficial from the point of view of environmental protection but less so when we consider the fertilizer aspect of the composts. Despite their significant affinity for organic matter, nickel and zinc form organic ligands characterized by a weaker stability than those created through copper [52,53]. In this respect, Huang et al. [54] emphasized the greater affinity of zinc for fulvic acids. Mineralization occurring during composting could cause the breakdown of these organometallic complexes, and the released metal ions were subsequently adsorbed on iron and manganese oxides or created more durable organic compounds within the resulting compost humus. It is necessary to remember that the humification process occurs simultaneously with the mineralization process, as a result of which more stable structures of humus compounds are created with a high sorption capacity in relation to metals [54]. Amir et al. [50] particularly emphasized that, during the composting process, there was a reduction in both metals in water-soluble connections (and zinc additionally in the residual fraction) and increase in organic connections. An explanation for this phenomenon was also provided as the release of nickel and zinc ions from easily soluble connections that were the result of the decomposition of organic matter, which were then bound to stable organic connections. This finding is also confirmed by the results of the research by Liu et al. [55].

With respect to the fractional distribution of Cu, it should be emphasized that both the sewage sludge [12] and the tested composts had the same distribution. The amount of Cu, regardless of the composting phase, increased in the following order: Fr. I < Fr. IV < Fr. II < Fr. III (based on absolute values not directly presented here). The amounts of Cu increased everywhere except for Fr. II, where a decrease in the amount of metal was recorded. Leśniańska et al. [56] and Jakubus [27] reported the highest Cu amounts for Cu bound with organic matter of the tested composts. On the other hand, Pecorini et al. [46] reported the highest percentage of Cu in the mobile fractions of compost. The addition of bulking agents resulted in an increase in the amount of Cu labile connections and a decrease in the amount of the organic fraction [27,57]. The authors also noted an unclear trend in Cu mobility due to the transfer of the organic fraction into the changeable and carbonate fractions. The explanation may be the biogeochemistry of Cu, which is considered a metal that does not easily create water-soluble forms; therefore, it is not easily activated and is characterized by complexes with organic matter with high durability [53,58]. Moreover, as a result of the decomposition of organic matter during composting, copper is released from soluble organometallic ligands, and then, it may be subjected to precipitation in the form of carbonates, which are extracted via the BCR method with an acetic acid solution (a reagent in fraction I). The increased amount of copper in water- and acid-soluble connections can also be interpreted based on the opinion regarding the creation of stable copper complexes with fulvic acids [59]. Although the composting process stimulated an increase in the copper content in fraction I, the microelement amount in these combinations was lower than that in the other fractions. The quantitative dominance of copper in the organic fraction is in line with the common opinion about the durability and stability of the ligand copper with sulfur compounds and high-molecular-weight humus compounds of the nature of humic acids [54,60]. Additionally, Xiong et al. [60] stated that ligneous bulking agents may promote the formation of HA and reduce FA during the composting process. Ligneous bulking agents, especially sawdust, could improve the complexation ability of HA, but had little influence on that of FA. These findings indicate that organic materials, especially sawdust, may be used as bulking agents to reduce the mobility and bioavailability of metals in solid waste composts. In this context, the strong bonding of copper with carboxyl and carbonyl groups and phenolics is particularly important [59].

Although the amounts of Ni, Cu, and Zn in various combinations generally increased during composting, these metals mainly formed permanent bonds and were slowly activated in the environment. This finding indicates environmental safety, which is confirmed by metal pollution indices. The calculated values of the applied indices proved the lack of a negative impact of metals introduced into the soil from the composts in the form of potential contamination. Therefore, the process of composting SS with bulking agents should be considered beneficial, although considering that a low rate of quantitative transformation of micronutrients occurring in the forms most available to plants (bioavailable and isolated in fraction I) was detected. It should be borne in mind that composts of this type should be treated as fertilizers from which microelements are slowly released into the environment. On the one hand, this creates environmental safety, but on the other, it may lead to temporary shortages in nutrients for plants.

5. Conclusions

The direction of the quantitative changes in Cu, Zn, and Ni in the separated fractions and bioavailable contents was varied and depended on the experimental factors and the chemical nature of the metals. The used research methodology significantly presented the changes occurring during the successive stages of composting, and the indicators introduced had an important impact on the assessment of the environmental impact of the tested composts. Regardless of the composition of the mixtures, the amounts of Cu, Zn, and Ni generally increased for the majority of fractions during composting. The most substantial increment in copper content was evident in fraction I of C1 during the thermophilic phase (191.76%), as well as in C3 during the zinc bioavailability phase (75.13%) and in C3 within nickel fraction III (93.87%). Regardless of this, it should be emphasized that, throughout the composting process, the shifting levels of metal contents exhibited diverse trends, including both positive and negative values. Specifically, these variations were observed in C1 for Cu Fr I, Zn Fr IV, and bioavailable amounts, and Ni Fr I and II; C2 for Ni Fr IV and bioavailable amounts; and C3 for Znbioava, Ni Fr I, Fr II, Fr III, and bioavailable amounts. The obtained results confirm the initial assumptions about the significant influence of changes occurring during composting on the amount of microelements because the largest changes in copper, nickel, and zinc contents were observed during the thermophilic phase, except for zinc in C1 and nickel in C2. The smallest changes in the content of all discussed micronutrients were found in C1 and C3 during the cooling phase. For C2, the least changes occurred during the mature phase. According to the assumptions, the role of bulking agents in composting with SS was smaller in relation to the quantitative changes in the metals. Regardless of the bulking agent used, there was a reduction in the mobility of the quantity, and consequently, the availability of microelements, which was particularly visible in the case of Ni. Despite significant increases in the bioavailable amounts of Cu and Zn in all the tested mixtures, these composts did not have a potential negative impact on the environment, which was confirmed by the values of metal pollution indices. Valorizing composts as a source of available microelements, the most beneficial properties in this respect were observed in C3 and the least in C2. Regardless of this, it should be remembered and taken into account that composting SS with various bulking agents is justified and purposeful; however, the obtained composts are a slowly releasing source of Cu, Zn, and Ni, which indicates the need for supplementation with other fertilization.