Reagent Deodorization and Detoxification of Sewage Sludge with the Production of Reclamation Material

Abstract

This article is devoted to the search for effective ways of neutralizing sewage sludge to obtain sludge-based reclamation material. It was necessary to solve the problems of deodorizing the sludge and fixing the mobile forms of heavy metals in its composition. The composition, effective for solving the assigned problems, was experimentally determined; it included quicklime, sodium hypochlorite and peat. In the presence of sodium hypochlorite in the sludge-based composition, oxidation processes occur with the chemical transformation of ammonia and hydrogen sulfide into non-toxic and odorless compounds. Lime combined with peat promotes the humification of sewage sludge and the binding of heavy metals. Based on the composition that showed the best results, a technology has been developed for the chemical treatment of sewage sludge in situ to produce reclamation material.

1. Introduction

Large- and medium-sized settlements (cities and towns) in most countries of the world have centralized facilities for treating domestic wastewater. Treatment results in the formation of sewage sludge (SS). SS is a mixture of excess activated sludge from aeration tanks and sludge from primary settling tanks. The characteristic properties of SS comprise high water content and a wide range of pollutants that it contains, including heavy metals, pathogenic microorganisms, organic compounds and inorganic compounds. Along with this, SS is characterized by a high content of macronutrients, namely nitrogen and phosphorus, which are necessary for plant growth [1,2].

SS is considered large-scale waste, the total volume of which is increasing both due to the growth of the total population of the Earth and as a result of urbanization processes [2,3,4], e.g., in China, sewage sludge production exceeded 60 million tons (12 million tons dry matter—MtDS) in 2019 and is expected to exceed 90 million tons (18 MtDS) in 2025 [5]. China’s population increased from 1.321 to 1.424 billion people between 2007 and 2020, respectively [6]; over the same period of time, the share of the urban population in the country increased from 17% to 61% [7]. In EU-27, 22.5 kgDS of SS is produced per person per year, resulting in a total mass of approx. 10 MtDS [8]. Cities, unlike rural settlements, in most cases (statistics), are equipped with wastewater treatment plants, which are the source of SS generation. Due to the large and constantly growing amount of SS, simple dumping/landfilling of sludge is not a sustainable practice, as it leads to the generation of greenhouse gas emissions and the loss of energy and material (in the form of nutrients) potential of the landfilled SS. In this regard, the key task of municipal administrations is the treatment of SS, which is aimed at reducing the volume and utilizing the energy and/or material potential of accumulated and newly formed sludge [9].

Methods for utilizing SS can be divided into two groups:

• Methods that make it possible to minimize the volume of waste (thermal methods) and utilize its energy potential.
• Methods that use waste as a material/nutrients resource.

In both cases, SS must be prepared for processing. This need for preliminary preparation is due to the specific properties of SS—high water content and the presence of a wide range of pollutants in the sludge. When using thermal methods (1st group above), drying is required; when using “resource” methods (2nd group above), disinfection, deodorization and detoxification are required.

From the standpoint of maximizing the reduction of the volume of this waste that comprises many tons, thermal methods of waste disposal (which include incineration, pyrolysis, gasification and hydrothermal carbonization) are most effective. The use of each of these methods is associated with high capital (due to a multi-stage processing scheme using technologically complex equipment) and operating costs (due to the required temperature and in some cases pressure) [10,11]. For this reason, wastewater treatment plant operators often focus only on the “resource” methods of SS disposal.

The possibilities of using SS as a material resource are limited due to the wide range of pollutants they contain, including persistent pollutants such as heavy metals and pharmaceuticals. As a result, for example, countries with developed economies are seeking to eliminate the practice of using sewage sludge as a fertilizer and soil conditioner in agriculture [12,13]. At the same time, SS is suitable in cases where large volumes of soil material are required, the quality requirements for which are not as high as in agriculture, for example, for the reclamation of disturbed lands. SS can be mixed with bentonite, cement, lime (CaO) or fly ash and can then be used as temporary landfill cover material or even as a recultivation layer on top of a landfill [14,15]. However, even when using SS as a basis for the production of technical soil, preliminary deodorization and detoxification of the sludge is mandatory. Let us clarify that deodorization of SS is understood as a reduction in emissions of foul-smelling compounds below threshold odor values, and detoxification is the binding of mobile forms of heavy metals into insoluble compounds that are not absorbed by plants to prevent the leaching of heavy metals in subsequent periods of time. There is a wide range of methods used to deodorize and/or detoxify sewage sludge. These include the following:

−

Sludge pasteurization and thermal conditioning at temperatures above 70 °C [16,17,18].

−

Heat treatment during anaerobic digestion at temperatures of above 55 °C [16,18,19].

−

Aerobic stabilization in stacks or reactors with the addition of various structural materials [16,18].

−

High-temperature drying with air, water or thermal oil [16,18].

−

Irradiation with gamma rays and electron accelerators [20].

−

Stabilization on sludge maps/sites or plant beds [18].

−

Treatment with chemical reagents [16,18,21].

This study focuses on the disposal of SS using chemical treatment. The goal of this study was to develop a method for recycling sewage sludge to obtain so called technical soil, i.e., a material suitable for the reclamation of disturbed lands. To achieve this goal, the following objectives were reached:

• The choice of reagents for deodorization and detoxification of SS was experimentally substantiated, and the optimal composition was determined.
• A technology was developed for recycling SS with the production of reclamation material based on sludge.

2. Materials and Methods

The object of this study was SS formed at a municipal wastewater treatment plant of a Russian city, with a population of a million, as a result of sewage treatment using mechanical and biological methods. SS is formed from the mixing of excess activated sludge (EAS) from aeration tanks and raw sludge from primary settling tanks (PSTS). The water content of EAS is 88.4%; the water content of PSTS is 71.1%. In accordance with the technological regulations of the wastewater treatment plant operator, the ratio of sludge from primary settling tanks to excess activated sludge during their joint dewatering can be 60:40 or 50:50 in terms of dry matter. After dehydration in decanter centrifuges, the water content of sewage sludge is about 72%.

The subject of this study consisted of the characteristics of SS that limit the possibility of its use as reclamation material (odor, content of heavy metals) before and after treatment with different additives. The hypothesis underlying this study was that the use of certain combinations of preparations for chemical treatment of SS is more effective than the use of these preparations separately, which in turn will allow for complex treatment of sludge with the simultaneous solution of the tasks of deodorization and detoxification. The rationale for the choice of reagents is provided in Section 3.1 of this paper. The characteristics of the reagents/additives used in the main series of experiments are presented in

Table 1. Characteristics of reagents/additives used in the main series of experiments.

Indicator	Unit	Standard Value
1. Peat (according to [GOST R 51661.3])
Moisture content W, no more	%	60
Ash content Ad, no more	%	25
Acidity of the salt suspension (pHKCl), not less	-	4.6
Clogging (pieces of peat, tow, stumps, wood chips over 60 mm in size), no more	%	8
2. Quicklime (according to [GOST 9179-77])
Content of active CaO + MgO, not less	%	85
Content of active MgO, no more	%	1.6
Degree of dispersion: residue on a sieve with mesh No. 0.2, no more	%	0.01
Degree of dispersion: residue on a sieve with mesh No. 0.08, no more	%	0.1
Specific surface area	cm2/g	5800
3. Sodium hypochlorite (according to [GOST 11086-76])
Appearance		Greenish-yellow liquid
Light transmission coefficient, not less	%	20
Mass concentration, not less	g/dm3	190
Mass concentration of alkali	%	10–20
Mass concentration of Fe, no more	g/dm3	0.02

.

From PSTS and EAS, representative mixed samples weighing about 10 kg were formed with a dry matter component ratio of 60:40 and 50:50. In the main experiment, the compositions were obtained by sequentially adding a solution of NaClO grade A (diluted 1:10) to a sample of SS with constant stirring, followed by CaO and peat. The total duration of sample processing was 6 days. Sampling of gas emissions to monitor the deodorization process was carried out 1 h after mixing, and then after 3 and 6 days. Before collecting gas samples, the containers with compositions were hermetically sealed; after 15 min, gas samples were obtained with a syringe. All studied characteristics were determined at least three times to ensure statistical reliability of the measurement results.

To assess the effectiveness of deodorization, the following indicators were chosen:

-

Organoleptical (intensity of odor).

-

Content of ammonia in gas emissions.

-

Content of hydrogen sulfide in gas emissions.

-

The pH value of the water extract.

The type and intensity of the odor were assessed with the organoleptic method according to the standard method [GOST 3351-74]. Closed flasks with samples were heated in a water bath to 50–60 °C. Having opened the flask, the nature and intensity of the odor was quickly determined using a five-point system, where 1 represents that the odor is not felt, 2 represents that the odor is noticeable if you pay attention to it, 3 represents that the odor is easily noticed, 4 represents that the odor attracts attention, and 5 represents the most intense odor.

Ammonia (NH₃) and hydrogen sulfide (H₂S) were chosen as indicators of deodorization efficiency, since both of them are included in the group of key indicators of the danger of gas emissions of SS, as well as in the group of indicators of pollutants’ high mass concentrations [22]. Key indicators are characterized by a high dilution factor (the ratio of the mass concentration of a chemical to its odor threshold value), which explains the actual impact of odors on the environment. Indicators of high mass concentration include compounds contained in gas emissions with a mass concentration over 100 μg/m³ [22]. The mass concentration of NH₃ in gas emissions was determined according to a standard method (MUK 4.1.3181-14) using ion chromatography. This method is based on the capture of NH₃ from an air sample with deionized water in an absorption unit consisting of two Richter devices and the quantitative determination of ammonium ions in the resulting aqueous solutions using ion chromatography. The mass concentration of H₂S in gas emissions was determined using the qualitative analysis method, and a solution of lead acetate was used as an indicator. In the presence of H₂S, the solution becomes turbid as a result of the formation of slightly soluble lead sulfide. The release of H₂S was assessed by the degree of turbidity of the solution. The turbidity of the solution was assessed photometrically according to a standard method (GOST 3351-74) on a calibration scale from 0 to 5, where 5 is a turbid solution and 0 is transparent.

The pH of the aqueous extract was chosen as an indicator of the effectiveness of deodorization, since when using an alkaline reagent (CaO), its value will change towards an alkaline environment. At the same time, the pH value is regulated by the requirements of regulatory documents for the quality of reclamation materials and must remain within the medium alkaline environment, not exceeding 8.5 (GOST R 54534-2011).

The efficiency of detoxification of sewage sludge was determined by the content of mobile forms of heavy metals using a standard method (RD 52.18.289-90). Samples with compositions based on SS were treated with an ammonium acetate buffer solution with pH 4.8, the flask with the resulting suspension was kept for 24 h at room temperature and was stirred periodically (a total of 5–7 times). The suspension was filtered, and the specified elements were determined in the resulting filtrate using atomic absorption spectroscopy.

The hazard class of the obtained soil materials based on sewage sludge was determined via biotesting according to a standard method (FR.1.39.2007.03222), in which the lower crustaceans Daphnia magna Straus are used as test objects. The results of the experiment on the effects on aquatic organisms were correlated with the standard criteria for classifying waste into hazard classes (S0P 2.1.7.1386-03).

The overall layout of this study is presented in Figure 1.

3. Results and Discussion

3.1. Selection of Reagents for Deodorization and Detoxification of Sewage Sludge

The principal step to solving the problem of SS deodorization involves cleaning exhaust gases from foul-smelling emissions of H₂S and NH₃; for this, strong oxidizing agents are used, such as hypochlorous acid (HClO), CaO [23], calcium hypochlorite (Ca(ClO)₂), hydrogenperoxide (H₂O₂), sodium nitrite (NaNO₂) [24], manganese hydroxide (Mn(OH)₂) [25] and potassium permanganate (KMnO₄) [26,27]. However, it is most economically feasible to use cheaper [28] and more effective [29] reagents for these purposes, for example, NaClO, which has a similar deodorizing effect on the mentioned compounds’ emissions. In advanced oxidation processes, the structure of organic materials can be destroyed, and organic pollutants can be directly undergo the process mineralization [27].

Reagent methods for detoxification of sewage sludge are based on the replacement of heavy metals with ions of alkali and alkaline earth metals or on the binding of mobile ions of heavy metals into poorly soluble complex compounds. It is known that heavy metal ions are capable of forming durable complex compounds with humic acids, humates and carbonates [30,31] that are not assimilated by plants. The source of humic substances during the processing of sewage sludge can be peat, black soil, as well as industrial humic preparations, for example, “Gumikom” (Certificate of state registration of a pesticide or agrochemical No. 2423 dated 16 October 2012).

To increase the efficiency of SS detoxification, it is necessary to intensify the process of isolating humic substances from peat or other humic-containing material. Since the purpose of this study was to obtain reclamation material, it was necessary to choose a reagent that would not interfere with plant growth for binding heavy metals. Quicklime (CaO) was used as this reagent. It is known that Ca [32] and Na humates are highly soluble in water, i.e., interact more effectively with heavy metal ions. Studies [33,34] have established that when SS is treated with calcium-containing reagents, for example, chalk, gypsum, lime or calcium phosphate, microorganisms are immobilized on the surface of the reagent, and this process is accompanied by the replacement of heavy metal ions with calcium. The introduction of calcium-containing preparations into sewage sludge also intensifies the processes of water separation, which is associated with the adsorption of fine sludge particles on the surface of the preparations and their enlargement.

3.2. Preliminary Experiment—Study of the Influence of Different Reagent Compositions on Sewage Sludge Deodorization

At the preliminary experiment stage, the prospects for using various compositions for deodorizing SS were studied. The goal of this stage was to identify ineffective/interchangeable preparations and form the composition for the reagent treatment of SS.

The following preparations were considered as possible reagents for deodorization (reducing emissions of foul-smelling substances) when processing sewage sludge in preliminary and main experiments:

-

Black soil was selected as a source of humic substances and indigenous microflora.

-

Peat was considered as an alternative to the black soil. Lowland peat is an additional source of organic substances and bacterial microflora, which promote the humification of sewage sludge as well as the formation of humic compounds that can interact with heavy metal ions to form poorly soluble complex compounds.

-

Quicklime helps to disinfect wastewater and increase water yield. In the presence of this alkaline reagent, humic compounds, which are involved in the processes of sewage sludge humification, detoxification and binding of heavy metals [34], are leached from peat. Therefore, it can be expected that in the presence of calcium oxide, the activity and efficiency of rich black soil/peat in detoxifying SS will be increased.

-

Humic preparation. Humic preparation “Gumikom” (Certificate of state registration of pesticide or agrochemical No. 2423 dated 16 October 2012) is produced at the industrial site of Emulsion Technologies LLC in the city of Kinel, Samara Region, from lignite coal from the Irkutsk deposits, characterized by a high content of humic compounds. The preparation meets the requirements of TU 2186-002-13787869-2009 “Humic-mineral complex “Gumikom”.

-

Sodium hypochlorite was chosen for processing SS due to its ability to easily oxidize ammonium ions and H₂S. It was assumed that NaClO would maximize the reduction of emissions of foul-smelling substances from SS. In the experiments, hypochlorite solutions with an active chlorine concentration of 10–15 g/L were used.

During the preliminary experiment, ten compositions based on SS (half 60:40 and 50:50 sludge from primary settling tanks to excess activated sludge) were prepared with five of them using peat as the main source of humic substances and the other five using black soil (Table S1). Gumikom was used each in one of the two sewage sludge compositions. Prepared CaO and “Gumikom” samples were also used as additives. NaClO was not used in the compositions of the preliminary experiment.

The effectiveness of SS deodorization was determined by organoleptic characteristics. Based on the results of the organoleptic evaluation, the intensity of odor decreased, but none of the compositions provided a sufficient deodorization of SS. Rich black soil, compared to peat, exhibited a lower deodorizing ability; therefore, in further experiments, peat was used as a cheaper alternative. The introduction of the “Gumikom” sample did not have a significant effect on the deodorization of sewage sludge, so we decided to exclude it from the composition.

To reduce the intensity of the odor when processing SS, it was necessary to introduce an additional strong deodorant, which was used in the main experiment as a sodium hypochlorite solution. As already noted, NaClO is capable of oxidizing H₂S and NH₃ as well as being a strong bactericidal agent, which helps to reduce the decay of SS. Reagent treatment of SS is effective, provided that the tasks of deodorization and detoxification of sludge are simultaneously solved. In this regard, in the main series of experiments, SS was treated with different ratios of selected reagents, namely CaO, NaClO and peat.

3.3. Main Experiments—Determination of the Composition for Optimum Sewage Sludge Deodorization and Detoxification

The results of the preliminary experiment made it possible to exclude rich black soil and “Gumikom” from the compositions. For more effective deodorization of SS, we decided to add NaClO to the compositions. In the second series of experiments, the optimum mixture of sludge and additives/reagents for the deodorization was determined. In the third series, the 3 best compositions of the deodorization tests were taken to evaluate the effectiveness of sludge detoxification.

At the second stage, two consequent series of experiments were carried out with the compositions based on 100 g DS sewage sludge:

• With a ratio of sludge of primary settling tanks to excess activated sludge = 60:40 DS.
• With a ratio of sludge of primary settling tanks to excess activated sludge = 50:50 DS.

The compositions of the first series, which showed the least effectiveness in sludge deodorization, were excluded. As a result, the number of compositions in the second series was halved.

The compositions are presented in

Table 2. Mass fractions of additives in the compositions based on 100 g DS of SS during the main experiment (for sewage sludge deodorization).

Composition No.	Weight of Additive, g OS *
	CaO	NaClO (g for Active Cl2)	Peat
Series 1
1	30.08	0.14	56.40
2	45.12	0.08	75.20
3	22.56	0.24	75.20
4	7.52	0.39	75.20
5	15.04	0.28	75.20
6	30.08	0.19	56.40
Series 2
7	6.33	0.15	63.31
8	12.66	0.15	47.48
9	6.33	0.23	47.48

* OS—original substance.

.

The results of experiments to determine emissions of NH₃ and H₂S, organoleptic assessment of deodorization efficiency and determination of the pH value of compositions 1 h, 3 and 6 days after treatment, converted into points on a scale from 0 to 10 and colored with shades of grey (0 colored with dark grey is the worst value, 10 colored with white is the best—see Table S2), are presented in

Table 3. Changes in sewage sludge deodorization indicators after 1 h, 3 and 6 days.

No.	1 h				3 Days				6 Days
	NH3	H2S	Odor	pH	NH3	H2S	Odor	pH	NH3	H2S	Odor	pH
1	0	6	0	6	1	6	0	7	2	4	2	6
2	7	10	6	4	8	10	8	7	8	8	8	6
3	5	2	0	4	5	2	0	6	4	4	0	6
4	8	6	8	9	8	10	8	9	8	10	10	7
5	5	10	2	6	7	10	2	6	8	10	2	6
6	7	4	2	6	7	4	0	4	7	4	0	3
7	8	4	6	9	8	10	8	9	9	10	10	9
8	2	6	0	4	5	6	0	6	8	10	6	7
9	7	10	6	6	8	10	6	7	9	10	10	9

.

It was found that 1 h after treatment, the maximum reduction in NH₃ emissions, odor intensity and the pH of the sample approaching a neutral level was observed in samples No. 4 and 7. In sample No. 9, H₂S emissions were below the detection threshold, and the odor also decreased significantly. Three days after treatment, the maximum reduction in the emissions of NH₃ and H₂S as well as odor intensity was observed in sample No. 2. In samples No. 4 and 7, all four indicators approached the optimal level. For sample No. 9, NH₃ emissions decreased to a minimum (9 points out of 10), H₂S emissions remained below the detection threshold and the odor increased slightly. Six days after treatment, the maximum improvement in all four indicators of sewage sludge deodorization was observed in samples No. 4, 7 and 9.

The results of the experiment showed that the reduction of odor emissions from compositions based on SS is positively influenced by an increase in the content of excess activated sludge relative to the proportion of sludge from primary settling tanks. The most effective of the reagents used in sludge deodorization is NaClO, while CaO and peat contribute to the humification and detoxification of sludge. It is known that NaClO is not only a bactericidal agent, but also a strong oxidizing agent that can undergo a reaction with reducing agents, such as NH₃ and H₂S. As a result of adding NaClO to SS-based compositions, oxidation processes occur with the formation of non-toxic compounds. The final chemical reactions of these processes involving NH₃ and H₂S can be described by the following equations:

4 NH₃ + 3 NaClO = 2 N₂ + 3 NaCl + 3 H₂O

H₂S + NaClO = Na₂SO₄ + NaCl

However, with a high dose of CaO and a corresponding increase in pH, anaerobic processes of organic compounds’ destruction can begin with the release of H₂S and/or NH₃ emissions [27]. The result of this is an increase in the concentration of H₂S/NH₃ in the emissions of compositions based on sludge, which is observed after several days from the moment the sludge is chemically treated. For example, in compositions No. 1 and 2 (see Table 3) with high doses of CaO, a deterioration in the H₂S indicator was observed 6 days after treatment; in composition No. 3, a deterioration in the NH₃ indicator was observed after 6 days. In composition No. 6 with a high dose of CaO, a deterioration in pH towards a highly alkaline environment was observed after 6 days.

To assess the effectiveness of sludge detoxification, compositions No. 4, 7 and 9, which showed the best results in sludge deodorization, were analyzed to determine the content of heavy metals in mobile form. Long-term studies of the content of heavy metals in SS show that the highest content of metals in mobile form is typical for Mn, Cu, Ni, Pb and Zn [9,35]. Therefore, in this experiment, the analysis was carried out for these five metals. The contents of Cd and Hg ions in mobile form in SS, which were also measured, were below the detection limits, which may be due to the formation of sparingly soluble Cd and Hg sulfides in the sludge in the presence of H₂S. The results of the analysis are presented in

Table 4. Mobile forms of heavy metals content in compositions based on sewage sludge.

Composition No.	HM, mg/kg DS
	Mn	Cu	Ni	Pb	Zn
4	53.00 ± 9.75	0.52 ± 0.08	0.90 ± 0.21	0.54 ± 0.13	65.00 ± 23.27
Reference SS *	69.87 ± 12.86	0.89 ± 0.14	2.97 ± 0.68	0.97 ± 0.24	129.74 ± 46.45
7	52.00 ± 9.57	0.58 ± 0.09	0.90 ± 0.21	0.60 ± 0.15	76.00 ± 27.21
9	23.50 ± 4.32	0.53 ± 0.09	0.55 ± 0.13	0.58 ± 0.14	71.00 ± 32.11
Reference SS **	84.21 ± 15.49	0.87 ± 0.14	2.88 ± 0.66	0.93 ± 0.23	119.30 ± 42.71
Maximum permissible concentrations (MPC) of mobile fraction of HMs in soil (SanPiN 1.2.3685-21)	60’; 80’’; 100’’’	3.0	4.0	6.0	23.0

* SS with the ratio PSTS/EAS = 60:40 DS. ** SS with the ratio PSTS/EAS = 50:50 DS. ’ 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. The maximum permissible concentration (MPC) of heavy metals, which is similar to a threshold value, is understood as their concentration, which, with prolonged action on the soil, does not cause any pathological changes or anomalies during biological processes and does not lead to the accumulation of toxic elements in plants; therefore, it cannot disrupt the optimum biological processes of animals or humans. MPC is usually considered the content of heavy metals, which is the limit for phytotoxicity [36].

.

In the initial sewage sludge, the content of mobile forms of heavy metals exceeds the established maximum permissible levels for Mn and Zn. At the same time, the contents of Cu, Ni and Pb are significant, but within the permissible range (MPC), amounting to 29.7 and 29% for Cu; 74.25 and 72% for Ni; and 16.17 and 15.5% for Pb, respectively.

The results of the impacts of the treatment on the content of all analyzed heavy metals are presented in Figure 2.

A reduction in the content of Mn to levels below the maximum regulatory permissible values is observed when treating SS with all three considered variants of reagent compositions. The content of Mn is reduced when performing treatment with compositions, as follows:

−

No. 4—by 24%.

−

No. 7—by 38%.

−

No. 9—by 72%.

Also, for Ni, high reductions between 69% and 81% of the mobile form after treatment can be achieved. For Cu, the reduction is between 33% and 42%, and it is between 35% and 45% for Pb.

The Zn content in the compositions decreased by 40–50% but remained above the MPC. Therefore, an experiment was conducted to determine the levels of toxicological hazards of the resulting soil-like materials using biotesting. The results of the experiment showed that in terms of their effect on hydrobionts, all compositions (Nos. 4, 7 and 9) belong to class IV—low hazard—which confirms the possibility of their use as reclamation materials.

It was established that composition No. 9 showed the best results in terms of the totality of analyzed indicators of deodorization and detoxification of sewage sludge, including the emissions of NH₃ and H₂S, organoleptic odor index, pH and the content of mobile forms of heavy metals in the aqueous extract.

3.4. Development of Technology for Reagent Treatment of Sewage Sludge with the Production of Reclamation Material

The results of the preliminary experiment made it possible to exclude rich black soil.

Moreover, it is proposed to process sewage sludge in two stages at two technological sites:

• Treatment for deodorization in the facility for SS mechanical dewatering.
• Treatment for detoxification in an open site (for example, in sludge lagoons).

In a mechanical dewatering facility equipped with a ventilation system, sewage sludge (a mixture of sludge from primary settling tanks and excess activated sludge from aeration tanks) is treated with a solution of NaClO and CaO for deodorization. Then, the deodorized SS is transported to sludge lagoons, where peat is added to the sludge. This two-stage treatment allows us to minimize odor emissions from sludge lagoons.

We calculated the material balance of the technological process. The consumption rates of reagents per 1 ton of dewatered sewage sludge with a water content of 78% was calculated to be as follows:

CaO	16.4 kg;
NaClO (190 g/dm3 active chlorine)	2.7 L;
Peat	123 kg.

The main component that increases the cost of the proposed chemical treatment is peat. Therefore, it is advisable to use this technology in settlements/areas where there are accessible reserves of natural peat.

Sewage sludge treated with NaClO and CaO is laid out on a strip 5–6 m wide. A uniform layer of 2/3 of a given portion of peat is added to the treated sewage sludge; then, the sludge is mixed with, e.g., a wheel loader, and a pile is formed. The height of the pile should not exceed 1.2 m to ensure aeration of the soil material. The remaining part (1/3) of the peat is evenly distributed over the surface of the pile and used as a sorption material that absorbs released gases. The layout of the piles is shown in Figure 3.

To maintain the necessary aeration and porosity of the mixture, it is necessary to aerate it after 2, 3 and 5 days by replacing/re-mixing the piles.

Each time the pile is turned over, samples must be taken for an analytical control to be performed. Samples are taken according to a standard quartering method (GOST R 58586-2019) to form an average sample.

4. Conclusions

This article examines the possibility of recycling sewage sludge as a reclamation material obtained by treating the sludge with reagents for its deodorization and detoxification.

Sewage sludge is a mixture of solid sludge from primary settling tanks and excess activated sludge. This study found that a larger proportion of excess activated sludge in sewage sludge has a positive effect on reducing odor emissions from sludge.

An experiment on the selection of reagents for deodorization of sewage sludge showed that NaClO is an effective reagent for this purpose. This is due to the fact that in the presence of NaClO in sludge-based compositions, oxidation reactions occur with the formation of non-toxic compounds: NaCl, H₂O, N₂ and Na₂SO₄.

It is a known practice to treat sewage sludge with calcium-containing reagents, such as chalk, gypsum, quicklime and calcium phosphate. In this work, it was found that high dosages of CaO added to sewage sludge lead to an increase in pH in a highly alkaline environment (12 and above) and can provoke processes of anaerobic destruction of organic compounds with an additional release of H₂S and NH₃ emissions, which can be observed after several days after treating the sludge with reagents.

Based on the results of the studies, it is recommended to treat sewage sludge for 10–15 days using reagents with a low content of CaO and using a significant dose of NaClO and peat. This combination of reagents, in addition to deodorization, also has a positive effect on the binding of mobile forms of heavy metals, the content of which is reduced by 15–81% in the sludge. Very good deodorization was achieved in the tests carried out, but satisfactory results could not yet be achieved in the immobilization of heavy metals, especially in the case of zinc. In order to advance this immobilization, further tests with other additive compositions and with substitutes for lime as a reagent must be carried out. Technologies such as photocatalytic treatment, which starts at the wastewater treatment stage, have also been proven to remove various organic wastewater components and immobilize heavy metal ions [37,38]; therefore, this technology can also be a promising alternative.