Development and Substantiation of Approaches to the Management of Sewage Sludge of Different Storage Periods

Abstract

A widespread method of sewage sludge disposal is still simple storage in sludge lagoons. Subsequent thermal utilization is hardly possible because sludge properties change over time and energy content is reduced. Use as a soil conditioner in agriculture or landscaping is usually not possible due to high heavy metal contents. This paper describes a method in which a 10-year-old accumulated sewage sludge can be utilized as technical soil by mixing it with pyrolized fresh sewage sludge. For this purpose, physicochemical and toxicological characteristics of sewage sludge of different storage periods were identified, processes of thermal destruction of sewage sludge analyzed, toxicological characteristics of solid products of thermal sludge treatment determined, and the possibility of using the sewage sludge–pyrolysate mixture as technical soil was assessed. Results show that the gross calorific value of fresh and one-year stored sewage sludge is with approx. 15,000 kJ/kg dry basis sufficient to produce pyrolysate autothermally. It is also shown that when the pyrolysis residue is mixed with fresh or 1-year old sewage sludge, heavy metals can be immobilized and thus the leaching of heavy metals significantly reduced by up to 75%. The method described can thus be a possible option for recycling accumulated sewage sludge.

1. Introduction

In the field of waste management, a non-trivial task often arises of disposing of waste that is not newly formed, but placed on a temporary or permanent storage site. The period of such storage can be several years, and during this time, the waste changes its properties [1,2]. As a rule, there is no standard solution for such a problem; in each specific case, it is necessary to conduct a study in order to select a technology for the treatment of accumulated waste.

One example of such waste is sewage sludge (a mixture of excess activated sludge from aeration tanks and sediments from preliminary sedimentation tanks) of different storage periods, which is the object of this study. The problem of disposal of accumulated sewage sludge (SS) is typical for many countries, including Russia and a number of EU countries [3].

In Russia, the most widely used method of disposal of SS is its long-term accumulation on special sites (sludge lagoons and/or sludge reservoirs/tanks), which depending on local geophysical and hydrological conditions, can be a simple earth basin or a concrete reservoir with lining to prevent groundwater contamination [4,5], as well as joint storage (disposal) at landfills with municipal solid waste. The widespread use of this sludge management method is explained by its low capital and operating costs [6,7]. However, the possibility of using this method of disposal of SS is limited by the capacity of the sites for accumulation, and even properly designed, sludge lagoons and landfills present higher risk of negative environmental impact than any other solutions [8]. Russian companies controlling urban wastewater treatment plants report that in many cities, the situation with the available capacity of sludge ponds/sludge reservoirs is critical or close to critical [9].

Such an approach to the treatment of SS not only leads to the occupation of significant land resources [7], but also does not meet modern requirements for ensuring environmental safety, leads to an irretrievable loss of the energy [10] and resource potential of sludge [11], as well as pollution of environmental objects [12,13,14].

In 2015, around 23.1% of SS was disposed on landfills without any utilization in the EU countries [15]. Within 3 years, the share for landfilling of SS in the EU dropped to 5.6% [16]. Long-term storage of SS has been widely used in 9 out of 27 EU countries, including Spain, Ireland, Bulgaria, Hungary, Poland, and Lithuania, where it was the most common practice [7]. Actual data on the quantities of SS stored long-term in the European Union are not published. Eurostat only quantifies “sludge disposal—other”. Especially in the case of the more recent EU-member states, it is unclear what this share represents [17]. Even if fresh sewage sludge is no longer disposed of in sludge lagoons in the EU, it can be assumed that due to the lack of disposal routes for these long-term stored sewage sludges, they are still present in large quantities.

An alternative disposal technology is the treatment of SS in reed beds, which is an economical option [18] for small as well as large centralized treatment plants [19,20]. Sludge drying/reed beds increase the dry matter content of the sludge, reducing the overall sludge volume while producing a safe, high quality end product that can be suitable for spreading on green areas or arable land [21]. A sludge drainage reed bed system requires little maintenance, uses little to no energy, and can be operated for 8 to 10 years before the sludge needs to be removed [22]. However, the accumulation of organic and inorganic pollutants (i.e., heavy metals (HMs), fecal microbial indicators, etc.) in the sludge may impede such applications [22]. In fact, the sites for long-term sludge storage represent an object of accumulated damage to the environment. It is difficult to further treat such waste, because the characteristics of sludge strongly differ depending on the storage period, climate [23], wastewater composition [22], steps of wastewater treatment [22], and other factors, and today there is no single reasonable approach for the disposal of sludge of different “ages”.

Priority approaches to the treatment of SS today are methods aimed at utilizing the material or energy potential of sludge [11]. The purpose of this study was to substantiate the directions of treatment of SS of different storage periods for further efficient utilization. To achieve this goal, the researchers:

• Conducted laboratory studies to determine the physicochemical and toxicological characteristics (in terms of HMs content) of SS of different storage periods,
• Studied the processes of thermal destruction of SS during incineration and pyrolysis;
• Studied the toxicological characteristics of solid products of sludge thermal treatment by means of incineration and pyrolysis;
• Developed and substantiated recommendations on the preferred directions for the treatment of fresh sludge and accumulated sludge based on the data obtained.

2. Materials and Methods

The object of the study was the sludge from domestic wastewater from municipal wastewater treatment plants in a Russian city with a population of about 1 million people. The studied SS is formed in the process of urban wastewater treatment by mechanical and biological methods. The sludge is a mechanically dewatered mixture of compacted excess activated sludge from aeration tanks with a water content of 97–98.5% and raw sludge pumped out from preliminary sedimentation tanks with a water content of 95%. Dewatering is carried out on decanter centrifuges; the final water content of the SS is 71–73%.

Sludge is placed for approx. 1 year in 2 m deep sludge lagoons before the material is transported for permanent storage in approx. 17 m deep sludge reservoirs. The sludge lagoons and reservoirs are equipped with a drainage system. The sludge reservoir, put into operation in 1982, has an area of 20 ha.

The subject of the study was the characteristics of three different types of sludge: fresh SS (mechanically dewatered sludge after decanters), sludge with a storage period of 1 year (from a sludge lagoon), and sludge with a storage period of 10 years (sludge from the sludge reservoir). We assumed that fresh sludge has sufficient calorific value for its treatment by thermal methods, while for accumulated sludge more proper treatment methods are necessary to immobilize the HMs in the sludge and use the treated sludge as “technical soil” for disturbed lands reclamation or the recultivation of landfill sites. Sweden, for example, allows the use of sewage sludge as landfill cover [17], which means that about a quarter of Swedish SS is disposed of this way [24]. By technical soil, we mean here a technically produced material with positive soil properties such as good soil structure, increased water capacity, and increased ion exchange capacity (nutrient content). Such a material can be used for technical remediation of disturbed lands, e.g., spent pits, flats developed during open pit mining, extraction of useful minerals, development of sand, clay, gravel, for backfilling trenches in construction and repair of linear facilities, etc. [GOST 54534-2011].

To substantiate this hypothesis, a number of studies of the properties of sludge and solid products of its thermal treatment (bottom ash from incineration and pyrolysate from the pyrolysis) were carried out.

For the three SSs (fresh, 1 year old, and 10 years old) a representative mixed sample of approx. 10 kg (dry matter) was prepared and all considered parameters were determined in triplicate. The 10 kg mixed samples were compiled from a large number of individual samples.

The water content of SS samples was determined according to the standard method [DIN EN 12880; GOST 28268-89] by drying the samples in an oven to constant weight at a temperature of 105 °C in porcelain bowls, previously dried to constant weight and weighed.

The ash content of the samples was determined according to the standard method [DIN EN 15935; GOST 27784-88] by calcining the samples in a muffle furnace to constant weight at a temperature of 525 ± 25 °C in porcelain crucibles, previously calcined to constant weight and weighed.

The pH of the aqueous extract was determined according to the standard method [DIN EN 15933; GOST 26423-85] with a pH meter adjusted to buffer solutions.

Total phosphorus was determined according to the standard method [GOST 26205-84] in soil extracts in the form of a blue phosphorus–molybdenum complex on a photoelectron calorimeter.

An aqueous extract from SS to determine the concentrations of phosphate, chloride, and sulfate ions was prepared at a ratio of sludge to water of 1:10.

The concentration of chloride ions in the aqueous extract was determined according to the standard argentometric method according to Mohr [ISO 9297; GOST 26425-85] by titrating the chloride ion in the aqueous extract with a solution of silver nitrate, which forms with the chloride ion insoluble compound.

The concentration of sulfate ions in the aqueous extract of SS was determined by capillary electrophoresis according to the standard method [PND F 14.1:2:4.157-99] based on the migration of inorganic anions and separation under the action of an electric field due to their different electrophoretic mobility.

The content of HMs was determined by the standard method by atomic absorption spectrometry [DIN EN ISO 11885; M-MVI 80-2008]. Decomposition of samples when determining the gross content heavy metals were carried out by the method of microwave decomposition [DIN EN 13656].

To determine the mobile forms of heavy metals according to the standard method [RD 52.18.289-90], the samples were treated with an ammonium acetate buffer solution with a pH of 4.8, the flask with the resulting suspension was kept for 24 h at room temperature, stirred occasionally (5–7 times in total). The suspension was filtered and the given elements were determined by atomic absorption spectroscopy.

The gross calorific value of analytical samples was determined by the calorimetric method according to the standard method [DIN 51900-1].

Synchronous thermal analysis (STA) is the combined use of thermogravimetry and differential scanning calorimetry of the same sample. Thermogravimetry (TG) records the change in sample mass as a function of temperature; differential scanning calorimetry (DSC) records heat flow curves as a function of temperature or time. The DSC curve can be used to calculate the enthalpies of phase transitions. Studies of thermal degradation of sewage sludge samples were carried out on a synchronous thermal analysis device NETZSCH STA 449 C Jupiter in air (imitation of incineration) and in argon (imitation of pyrolysis). Al₂O₃ was used as a reference material for calibration.

• Analysis conditions:
• Initial temperature: 20–25 °C
• Dynamic heating segment: 800–1000 °C
• Heating rate: 10 °C/min
• Furnace gas flow rate: 40 mL/min (air)
• Protective gas flow rate: 20 mL/min (argon)
• Crucible material: platinum
• Mass of the sample: 17–30 mg

To study the properties of ash and pyrolysate of fresh sewage sludge, a batch of ash and pyrolysate was produced under optimal temperature conditions determined using STA. Ashing was carried out in porcelain cups, previously calcined to constant weight and weighed. The sample weight was 78–92 g. The cups were calcined in a muffle furnace at a temperature of 550 °C, the retention time was 4 h. Pyrolysis was carried out in porcelain crucibles, preliminarily calcined to constant weight and weighed; to create an inert atmosphere, the crucibles were covered tightly with a cap of blue clay on top. The sample weight was 29–37 g. The crucibles were calcined in a muffle furnace at a temperature of 450 °C, the retention time was 2 h.

The adsorption of HMs from 10-years SS was carried out using pyrolysate of fresh SS, as well as with the combined use of pyrolysate and humic–mineral preparation “Gumikom”. The humic–mineral complex “Gumikom” is produced in accordance with the requirements of Technical Conditions No. 2186-002-13787869-2009 from brown coal of the Irkutsk deposits, characterized by a high content of humic compounds. The active component of the humic–mineral complex “Gumikom” is salts of humic acids (alkali metal humates). The preparation technology consists of the treatment of brown coals with aqueous solutions of alkalis, thus forming soluble salts of humic acids. Humic acids (HAs) are specific high-molecular, polyfunctional, nitrogen-containing compounds of a cyclic structure and acid nature, which are products of the condensation of phenol-type aromatic compounds with amino acids and proteins [25]. The intensification of the process of bioremediation of contaminated soils in the presence of the preparation “Gumikom” is due to the interaction of alkali metal humates, which are part of the preparation, with the mineral component of the soil. The interaction of HAs with soil minerals includes adsorption, cation exchange, protonation, ligand exchange, binding via hydrogen bonds, cationic bridges, and Van Der Waals forces. Detoxification of contaminated soils in the presence of the preparation “Gumikom” is due to the binding of HM ions into poorly soluble complex compounds of HMs with humic acids as a result of ion and ligand exchange [26,27]. In the interaction of HAs with metal ions, the functional groups of humic acids, which differ in acid strength and can form compounds of varying degrees of stability with metal ions, are of great importance. For the adsorption tests, 10 years old SS was mixed with 5 and 7.5% of crushed pyrolysate and in 2 tests additionally with 0.1% “Gumikom” (all amounts/shares are in ds and related to the ds of the 10 years SS). The mixtures were kept at room temperature for 5 days, stirred occasionally.

When studying the ability of fresh sludge pyrolysate to adsorb HMs in 10-years SS, the efficiency of the detoxification process was monitored by the content of HMs in the aqueous extract (1 part SS ds/10 parts distilled water), since when using the standard method for determining mobile forms of metals with an ammonium acetate buffer solution desorption of heavy metal ions from the surface of the pyrolysate itself can occur. After the samples were treated/leached with distilled water, the flasks with the resulting suspension were kept for 24 h at room temperature, stirred occasionally (5–7 times in total); the suspension was then filtered, and the given elements were determined in the filtrate by atomic absorption spectroscopy.

3. Results and Discussion

3.1. Characteristics of Sewage Sludge of Different Storage Periods

At the first stage of laboratory studies, the physicochemical and toxicological characteristics of SS of different storage periods were determined. The results of the study are presented in

Table 1. Physical and chemical properties of SS.

Name of the Indicator, Unit of Measurement	Value
	Sludge, Fresh Output	Sludge Accumulated, Storage Period 1 Year	Sludge Accumulated, Storage Period 10 Years
Water content, % as received	69.27 ± 7	69.04 ± 7	66.02 ± 6.5
Mass fraction of organic substances, % dry basis	49.88 ± 1.5	46.31 ± 1.4	9.42 ± 0.3
Mass fraction of ash, % dry basis	50.12 ± 1.5	53.69 ± 1.6	90.58 ± 2.7
Gross calorific value, MJ/kg dry basis	15.286 ± 22	14.237 ± 35	5.831 ± 38
pH of water extract	8 ± 0.1	7.8 ± 0.1	7 ± 0.1
Total phosphorus (P2O5), % dry basis	5.5 ± 1.1	5.1 ± 1	2.5 ± 0.5
Chloride ion Cl−, mg/kg dry basis	886.25 ± 186	354.5 ± 74.3	267.4 ± 56
Sulphate ion SO42−, mg/kg dry basis	343.7 ± 34.3	253.8 ± 25.3	183.4 ± 36.6

.

An analysis of the physicochemical properties of sludge from different storage periods showed that the water content of sludge in the process of 10-year storage decreases slightly—by about 3%. This is due to the climate of the area, where the average monthly temperature is negative for five months a year, and in spring and autumn, it often changes from positive to negative. Thus, the level of precipitation is more or less equal to the level of evaporation.

Organic substances in the sludge decompose almost completely over a 10-year period of being kept in the sludge reservoir, their content decreases by 5.3 times to 9.42%. The gross calorific value of the dry matter of freshly generated sludge and sludge of short storage periods is comparable to the calorific values of traditional low-calorie fuels such as brown coal [28,29], peat [30,31,32], and wood [33]. This suggests the expediency of treating such sludge by thermal methods; to select a specific thermal treatment method, additional studies are required, which will be presented below. At the same time, the studied sewage sludge has an extremely high ash content, which will complicate thermal disposal by incineration, since a large amount of ash residue will be formed in the reactor, which will need to be removed and safely disposed of. The calorific value of sludge from long storage periods (10 years or more) is insufficient for its effective treatment by thermal methods.

During the storage of SS, the pH indicator changes slightly from light alkaline to neutral. The concentration of chloride ions is reduced by 2.5 times, sulfate ions by 1.87 times. With the slow biodegradation of sewage sludge under natural conditions in the sludge tank, the process of mineralization of phosphorus compounds occurs with the formation of both water-soluble forms, for example, calcium hydrogen phosphate (CaHPO₄), and sparingly soluble compounds [34]. As a result, part of the phosphorus is leached out. Thus, the content of total phosphorus decreased by a factor of 2 but remained over 2% on a dry basis, which is considered sufficient for a source for manufacturing phosphate fertilizers [35]; therefore, it suggests the possibility of using sludge with a 10-year storage period as a technical soil. However, before making such a recommendation, it is necessary to determine the degree of toxicity of the sludge.

The toxicity of SS was evaluated by the content of seven HMs: Cd, Cu, Mn, Ni, Pb, Zn, and Hg. As a Russian limiting standard, we referred to the sanitary norms of MPC/APC for the content of HMs in the soil. GOST (Federal standard) on the application of SS as a fertilizer was not used as a reference, since it involves the introduction of a certain dose of SS per unit area. In our case, we are considering the possibility of applying huge volumes of accumulated SS as a technical soil, i.e., material for technical reclamation of disturbed lands. The results of the study of the total content of HMs in SS are presented in

Table 2. The total content of HMs in SS.

HM, Unit of Measure- Ment	Sludge Fresh Output	Sludge Accumu- Lated, Storage Period 1 Year	Sludge Accumu- Lated, Storage Period 10 Years	MPC/APC of Total Content of HMs in Soil [SanPiN 1.2.3685-21]	Limit Values for Content of HMs in Soil [Council Directive 86/278/EEC]	Limit Values for Content of HMs in SS for Use in Agriculture [Council Directive 86/278/EEC]
Cd, mg/kg dry basis	24 ± 7	25 ± 7	10 ± 3	/0.5 *; 1.0 **; 2.0 ***	1–3	20–40
Cu, mg/kg dry basis	309 ± 93	324 ± 97	223 ± 67	/33.0 *; 66 **; 132 ***	50–140	1000–1750
Mn, mg/kg dry basis	760 ± 228	1660 ± 498	1750 ± 525	1500/	-	-
Ni, mg/kg dry basis	193 ± 58	208 ± 62	198 ± 59	/20.0 *; 40.0 **; 80.0 ***	30–75	300–400
Pb, mg/kg dry basis	42 ± 13	52 ± 16	34 ± 10	/32.0 *; 65 **; 130 ***	50–300	750–1200
Zn, mg/kg dry basis	1957 ± 587	2062 ± 619	909 ± 273	/55.0 *; 110 **; 220 ***	150–300	2500–4000
Hg, mg/kg dry basis	0.063 ± 0.019	0.022 ± 0.006	0.016 ± 0.005	2.1/	1–1.5	16–25

MPC (Maximum Permissible Concentrations); APC (Approximately Permissible Concentrations); * sandy and sabulous soils; ** acidic (loamy and clayey) soils, pH KCl < 5.5; *** close to neutral or neutral (loamy and clayey) soils, pH KCl > 5.5. Values written in bold exceed threshold limits for SanPiN 1.2.3685-21.

.

The SS from fresh output contains concentrations of HMs that exceed the MPC of the Russian SanPiN 1.2.3685-21 for all the studied metals except for Mn and Hg. Two standards exist in the European Union, one that defines the maximum heavy metal content in soils to which sewage sludge can be applied, and one that defines the maximum heavy metal content in sewage sludge [Council Directive 86/278/EEC].

In the process of storage in an open sludge reservoir for 10 years, an increase in the concentration of Mn occurs; Ni concentration remains practically unchanged, while the concentrations of other studied metals decrease, which is explained by the leaching out of mobile forms of metals [36]. The accumulation of Mn in the sludge is due to the low mobility of this metal. Most likely, over time, Mn is oxidized to MnO₂, which has low solubility [37]. The concentrations of Cd, Cu, Mn, Ni, Pb, and Zn in the sludge of a 10-year storage period exceed the MPCs.

Sewage sludge, regardless of its storage period, contains concentrations of HMs that exceed the regulatory limits (Table 2). We assume that some of these metals are in a mobile form. That is why, in order to utilize the material potential of sludge by using it as a technical soil, preliminary detoxification with immobilization of HMs is required. Because of the low calorific value of 10 years sludge, thermal methods can only be used for the disposal of SS of short storage periods—this was shown by an analysis of the results of a study of the physicochemical properties of sludge (Table 1). To select a method with maximum energy efficiency and economic feasibility, studies were carried out on the processes of thermal destruction of SS, as well as the toxicological properties of solid products of incineration and pyrolysis of sludge.

3.2. Study of the Processes of Thermal Destruction of Sewage Sludge during Incineration and Pyrolysis

The processes of thermal destruction of SS during incineration and pyrolysis were studied using synchronous thermal analysis (STA) that combines thermogravimetry (TG) and differential scanning calorimetry (DSC) in air and inert gas (argon); the results of these studies made it possible to make a reasonable choice of the method and modes of sludge thermal treatment. The tasks that needed to be solved using STA included:

• Determination of the optimal conditions for the incineration and pyrolysis of SS;
• Determination of heat flows during dewatering and destruction of SS;
• Determination of the amount of thermal energy released during the incineration and pyrolysis of sludge in order to establish the possibility of carrying out the process in an autothermal mode.

The analysis made it possible to determine the changes in the mass of the samples and the amount of heat released/absorbed as the ambient temperature increased from 20 °C to 800 °C. Two types of samples were analyzed: wet (mechanically dewatered fresh SS after a decanter or sludge samples from a sludge lagoon/sludge tank), and samples dried to constant weight at a temperature of 105 °C. The nomenclature of the samples is presented in

Table 3. Nomenclature of samples studied by the method of synchronous thermal analysis.

Water Content	Sample No.
	Sludge Fresh Output	Sludge Accumulated, Storage Period 1 Year	Sludge Accumulated, Storage Period 10 Years
Approx. 70% (mechanically dewatered sludge after the decanter (fresh), or sludge with its own water (from a sludge lagoon or sludge tank)	1.1	2.1	3.1
1% (sludge dried to constant weight at 105 °C)	1.2	2.2	3.2

. The results of the analysis are presented in Figure 1 and Figure 2.

Analysis of the results of STA in air environment showed that the process of thermal destruction of SS is characterized by several exo- and endo-effects. The decomposition of dewatered samples occurs in three stages: at the first stage, in the temperature range of 25–175 °C, water is removed; at the second stage, up to 630 °C, the processes of oxidation and combustion of organic substances occur. The total mass loss of sludge samples with a storage period of 0, 1, and 10 years is 89.45%, 85.07%, and 75.29%, respectively. As a result of complete incineration of dewatered samples with a storage period of 0, 1, and 10 years, 10.55%, 14.93%, and 24.71% ash is formed, respectively. With the complete incineration of dried samples with storage period of 0, 1, and 10 years, 45.50%, 42.52%, and 80.81% ash is formed, respectively, i.e., the high ash content of sludge with a long storage period makes its incineration irrational. Analysis of the results of the study indicates the expediency of preliminary drying of sludge. It has been established (

Table 4. Thermal characteristics of SS samples in air.

Sample No.	I Effect			II Effect			III Effect			Total Weight Loss at 800 °C ∆mtotal, %	Total Thermal Effect ∆Htotal, J/g
	Temperature Range, °C Direction of the Effect	Mass Loss Δm1, %	Thermal Effect ΔH1, J/g	Temperature Range, °C Direction of the Effect	Mass Loss Δm2, %	Thermal Effect ΔH2, J/g	Temperature Range, °C Direction of the Effect	Mass Loss Δm3, %	Thermal Effect ΔH3, J/g
Wet samples
1.1	25–165 endopeak	76.56	+1944	203–509 exopeaks	11.77	−2254	665–725 endopeak	1.12	+15	89.45	−295.00
2.1	25–175 endopeak	69.41	+1768	195–630 exopeaks	13.86	−2622	671–750 endopeak	1.79	+27.81	85.07	−826.19
3.1	25–150 endopeak	63.89	+1626	212–620 exopeaks	9.32	−1723	659–730 endopeak	2.09	+26.8	75.29	−70.20
Dried samples
1.2	25–156	3.16		156–364 exopeak 364–647 exopeak	48.52	−9625	647–730 endopeak	2.82	+76.32	54.50	−9548.68
2.2	25–132	3.01		132–365 exopeak 365–642 exopeak	50.86	−9824	652–740 endopeak	3.61	+89.78	57.48	−9734.22
3.2	25–219	2.37		219–650 exopeak	13.26	−2634	650–740 endopeak	3.56	+99.31	19.19	−2534.69

) that the incineration process is energy efficient: even when incinerating wet fresh sewage sludge, pre-drying requires less heat (1944 J/g) than is released at the incineration stage (2254 J/g).

Analysis of the results of STA in argon environment showed that the process of thermal destruction of SS is characterized by several exo- and endo-effects, the decomposition of dewatered samples occurs in three stages: at the first stage, in the temperature range of 25–175 °C water is evaporated; at the second stage, in the temperature range of 190–400 °C, an exo-peak is observed, associated with the destruction of part of the sludge organic matter; finally, in the temperature range of 650–750 °C, a small endo-peak is observed, apparently associated with the decomposition of part of the inorganic substances of the sludge (e.g., aluminosilicates, phosphates, carbonates such as CaCO₃ in the presence of clay substances). The main process of dewatered sludge degradation proceeds in the temperature range of 200–450 °C, the total heat flux (consumed heat) ranges from +1605 J/g for fresh sludge to +1359 J/g for sludge of 10 years storage period. The main part of heat consumption is associated with water desorption. It should be noted that in an inert environment, the gaseous products of the destruction of fresh SS and sludge of short storage periods are pyrolysis gases—hydrocarbons C1–C4, hydrogen, CO—which make up about 30% of the mass of the sample [38,39]. These gases have high calorific value [40,41] and can be used as a fuel for maintaining the temperature in the pyrolysis reactor or for drying dewatered sludge. For dried sludge, the main degradation process occurs in the temperature range of 220–400 °C; the weight loss in this case is 29–31% for sludge with a storage period of up to 1 year, 19% for sludge with a storage period of 10 years. It has been established (

Table 5. Thermal characteristics of SS samples in argon.

Sample No.	I Effect			II Effect			III Effect			Total Weight Loss at 800 °C ∆mtotal, %	Total Thermal Effect ∆Htotal, J/g
	Temperature Range, °C Direction of the Effect	Mass Loss Δm1, %	Thermal Effect ΔH1, J/g	Temperature Range, °C Direction of the Effect	Mass Loss Δm2, %	Thermal Effect ΔH2, J/g	Temperature Range, °C Direction of the Effect	Mass Loss Δm3, %	Thermal Effect ΔH3, J/g
wet samples
1.1	25–175 endopeak	73.20	+1747	198–391 exopeak	7.74	−166.4	650–740 endopeak	1.60	+24.47	85.98	+1605.07
2.1	25–160 endopeak	70.84	+1682	192–419 exopeak	8.48	−354.7	660–740 endopeak	2.10	+39.35	84.72	+1366.65
3.1	25–175 endopeak	64.03	+1522	197–396 exopeak	5.10	−210.3	651–755 endopeak	2.39	+47.39	74.29	+1359.09
Dried samples
1.2	25–155	6.09		155–390 exopeak	22.65	−1126	639–800 endopeak	6.67	+156	47.23	−970
2.2	25–152	6.68		152–390 exopeak	25.07	−1319	644–800 endopeak	7.01	+183.9	49.85	−1135.1
3.2	25–195	7.22		200–500 exopeak	10.80	−489.4	700–760 endopeak	4.98		23.00	−489.4

) that during the pyrolysis of wet sewage sludge from a fresh output, pre-drying requires 1747 J/g of thermal energy, while 166 J/g of heat is released at the pyrolysis stage. Therefore, carrying out pyrolysis in an autothermal mode is possible only if energy-saturated pyrolysis gases are utilized as fuel for drying the dehydrated sludge and partially for the pyrolysis reactor. Comparison of the weight loss of a dried sample of fresh sludge (sample 1.2) during its thermal destruction in the temperature range of 25–550 °C in air and in an inert atmosphere (50% and 38%, respectively) suggests that during the pyrolysis of the sludge, pyrocarbon is formed on the surface of the inorganic component [42], constituting 12% of the mass of the resulting solid pyrolysis residue (pyrolysate). The presence of pyrocarbon in the pyrolysate suggests that it has sorption properties and can be used as a sorption material, i.e., when restoring disturbed areas.

In the process of thermal treatment of SS by incineration and pyrolysis, secondary solid wastes are formed—bottom ash and pyrolysate, respectively. For further testing of their toxicological properties, ash and pyrolysate samples from the fresh SS were produced in a muffle furnace. The results of studies of the content of mobile HMs fraction in the ash residue from sludge incineration and in the pyrolysate are presented in

Table 6. Total content of HMs and HMs mobile fraction in the ash from the incineration of SS and the pyrolysate.

HM, Unit of Measurement	Total Content of HMs in SS Incineration Ash [mg/kg]	Total Content of HMs in SS Pyrolysate [mg/kg]	MPC/APC of Total Content of HMs in Soil [SanPiN 1.2.3685-21]	Mobile Fraction of HMs in SS Incineration Ash, mg/kg	Mobile Fraction of HMs in SS Pyrolysate, mg/kg	MPC/APC of Mobile Fraction of HMs in Soil [SanPiN 1.2.3685-21]
Cd, mg/kg	48 ± 14.4	36 ± 10.8	/0.5 *; 1.0 **; 2.0 ***	6.7 ± 2.3	2.4 ± 0.8	-
Cu, mg/kg	618 ± 185.4	468 ± 140.4	/33.0 *; 66 **; 132 ***	47 ± 7.6	2.3 ± 0.4	3.0/
Mn, mg/kg	1520 ± 456	1151 ± 345.3	1500/	135 ± 24.8	62 ± 11.4	60′; 80″; 100‴/
Ni, mg/kg	386 ± 115.8	292 ± 87.6	/20.0 *; 40.0 **; 80.0 ***	7.0 ± 1.6	2.5 ± 0.6	4.0/
Pb, mg/kg	84 ± 25.2	63 ± 18.9	/32.0 *; 65 **; 130 ***	<1	<1	6.0/
Hg, mg/kg	0.126 ± 0.038	0.095 ± 0.003	2.1/	<0.7	<0.7	-

* sandy and sabulous soils ** acidic (loamy and clayey) soils, pH KCl < 5.5 *** close to neutral or neutral (loamy and clayey) soils, pH KCl >5.5 ′ sod-podzolic soil, pH 4.0 ″ sod-podzolic soil, pH 5.1–6.0 ‴ sod-podzolic soil, pH 6.0 Values written in bold exceed threshold limits.

.

In the ash, after incineration of SS, HMs accumulate, which indicates the need for its detoxification by binding HMs into poorly soluble compounds. The content of mobile forms of Cu, Mn, and Ni in the ash exceeds the normatively permissible level for soils.

The conducted studies of the toxicological properties of the final solid products of the thermal treatment of SS by the methods of incineration and pyrolysis showed that pyrolysate is safer for their disposal in the environment which has been also investigated by [25]. At the same time, our synchronous thermal analysis showed that the process of pyrolysis of dried SS with storage period of up to one year is quite energy efficient, and the heat released is enough to pre-dry the sludge and operate the pyrolysis reactor in autothermal mode. The results of synchronous thermal analysis also show a 12% mass difference between ash and pyrolysate which results from the pyrolytic carbon in the composition of the pyrolysate.

3.3. Use of Solid Residue of Sludge Pyrolysis for the Detoxification of Accumulated Sludge

To determine the sorption capacity of the pyrolysate of fresh SS, studies were carried out on the use of pyrolysate as a sorbent for the immobilization of mobile forms of HMs contained in SS with a 10-year storage period. The investigated pyrolysate obtained by SS pyrolysis is a hydrophobic dispersed material containing, depending on the pyrolytic conditions and mass fraction of organic substances in sewage sludge, from 10 to 45% of pyrolytic carbon [43]. The content of HMs in the mobile form in the pyrolysate does not exceed the maximum permissible concentrations of for soils according to the Russian SanPiN 1.2.3685-21 (Table 6), which allows it to be used for various purposes. Comparable limit values in European standards do not exist.

Pyrolysates contain heteroatoms, in particular oxygen, which is part of the surface oxygen-containing groups: carboxyl, phenolic, hydroxyl, capable of interacting with HM ions. In this case, mobile HM ions are presumably bound into sparingly soluble complex compounds due to the irreversible process of chemisorption occurring on the surface of the pyrolysate simultaneously with the process of physical adsorption. Pyrolysates have a porous structure and are capable of adsorbing organic compounds, hardly biodegradable components as well as foul-smelling compounds (hydrogen sulfide and other sludge degradation products) [25].

To confirm the possibility of using the pyrolysate of the fresh sludge for the adsorption of HMs from accumulated sludge of a 10-year storage period, laboratory studies of the content of mobile forms of HMs after adsorption were carried out. Pyrolysate was used as a sorbing component, with or without the preparation “Gumikom”.

The reference content of HMs in mobile form in 10-years SS and the MPC levels are presented in

Table 7. Content of HMs mobile fraction in 10-years SS and the respective MPC levels.

HM, Unit of Measurement	Ni, mg/kg Dry Basis	Cu, mg/kg Dry Basis	Pb, mg/kg Dry Basis	Cd, mg/kg Dry Basis
Reference *	14.0	9.0	7.5	1.5
MPC/APC of mobile fraction of HMs in soil [SanPiN 1.2.3685-21]	4.0/	3.0/	6.0/	-

* HMs mobile fraction determined in SS (storage period 10 years) according to the standard method [RD 52.18.289-90], i.e., by treating the samples with an ammonium acetate buffer solution with a pH of 4.8. Values written in bold exceed threshold limits.

. The results of studies of the content of HMs in a mobile form in aqueous extracts of samples of SS with a 10-year storage period, with the addition of pyrolysate of fresh SS with/without the preparation “Gumikom” are presented in Figure 3.

Analysis of the results of adsorption of HMs in 10-year-old SS presented in Figure 3 relative to the control sample without the addition of pyrolysate and the “Gumikom” preparation showed that the sorbent, which is a pyrolysate of fresh SS, works both separately and in combination with the “Gumikom” humic–mineral preparation. The content of Pb, Ni, and Cu in the water extract of 10-year SS is reduced:

-

with the addition of 5% pyrolysate by 1.4, 10.3, and 50.1% and

-

with the addition of 7.5% pyrolysate by 2.5, 59.1, and 40.6%, respectively.

With the addition of 1 g/kg of the Gumikom preparation, the Pb content in the aqueous extract of 10-year-old SS after sorption turned out to be below the detection limit. The results for the content of Ni and Cu improved and amounted to:

-

with the addition of 5% pyrolysate by 76.1 and 62.8% and

-

with the addition of 7.5% pyrolysate by 68.1 and 69.2%, respectively.

The results obtained make it possible to recommend the use of fresh SS pyrolysate as a sorbent for the immobilization of HMs contained in SS of 10-year storage period. SS of long storage periods treated in this way can be used as a source for the production of technical soil. Further stabilization of SS will be needed to ensure additional drying and mineralization of organic substances [GOST 54534-2011]. The resulting technical soil can then be used in the reclamation of disturbed lands or as part of the upper insulating layer at the recultivation of municipal solid waste landfills.

4. Conclusions

(1)

SS of short storage periods has a calorific value of 14–15 MJ/kg, which is comparable to the calorific value of traditional low-calorie fuels. The lower calorific value of SS of long storage periods (about 5.8 MJ/kg) makes its utilization by thermal methods inexpedient.

(2)

According to the content of HMs, SS of different storage periods is toxic to the environment, which indicates the need for immobilization of HMs before utilization of the material potential of SS.

(3)

The total thermal effect during the incineration of wet SS of fresh output is −295 J/g, which indicates the possibility of carrying out the incineration process in an autothermal mode, i.e., without the use of fuel after the reactor reaches operating temperature. The total thermal effect during the pyrolysis of wet SS of fresh output is 1605 J/g; therefore, the autothermal mode of pyrolysis is only possible using high-calorie pyrolysis gases as a fuel for drying SS and maintaining the temperature in the pyrolysis reactor. Thermal disposal of 10-years SS is inexpedient.

(4)

Ash from SS incineration contains HMs in concentrations exceeding the normative limits. The content of HMs in the pyrolysate does not exceed the normative limits.

(5)

Adsorption with the addition of 7.5 wt.% pyrolysate, generated from fresh SS, together with 1 g/kg of humic preparation allows to reduce the content of mobile forms of individual HMs (Ni and Cu) in 10-year SS by 60–70%.

In conclusion, we can say that the results of this study confirm the need for differentiation of approaches to the treatment of SS of different storage periods. The data obtained can be used in choosing approaches to the disposal of SS, especially the sludge of long storage periods that does not have the attractive properties of an alternative fuel.