An Innovative Technical Solution for the Extraction and Disposal of Hazardous Industrial Waste for Landfill Decommissioning

Abstract

The problem of industrial waste disposal is becoming increasingly pressing. For a long time, one of the primary methods of managing hazardous industrial waste was to dispose of it for long periods (decades) in engineered landfills. However, over time, due to various climatic, geological, and other changes, landfills begin to cause significant harm to the environment and human health. Old landfills, many built in the mid-20th century, pollute the air, soil, and groundwater. Therefore, the issue of decommissioning “old” landfills is becoming increasingly pressing. This study aimed to develop technological solutions for the safe extraction and processing of hazardous liquid waste from an aged industrial landfill. An integrated treatment chain was designed, comprising extraction, multi-barrier water treatment, vacuum evaporation, and lithification. Optimal lithification compositions were identified: for the salt concentrate–sludge–spent media mixture, a ratio of 68.2% sorbent D, 28.0% salt concentrate, and 3.8% dewatered sludge/spent media yielded a loose granular geocomposite; for oil-containing waste, the optimal ratio using lime and opoka was 1:0.9:0.5 (bottom sediments/CaO/opoka). Biotesting confirmed that the lithified waste is Hazard Class V (non-hazardous), whereas the untreated waste is Class III (moderately hazardous). The resulting geocomposite is suitable for on-site technical reclamation, closing the material cycle.

1. Introduction

Uncontrolled accumulation of industrial waste causes irreparable harm to the environment, human health, and the economy [1]. For many years, disposal of industrial waste in landfills was considered economically justified [2].

An industrial waste landfill (or hazardous waste sludge storage facility, HWSSF) is a specially engineered facility for the placement and storage of industrial waste. The landfill accepts waste of Hazard Classes I–IV [3,4]. Landfills for hazardous industrial waste, especially those constructed in the mid-20th century, have become sources of long-term environmental risk due to changes in hydrogeological conditions, aging of protective systems, and the migration of pollutants into soil, groundwater, and air. Therefore, the decommissioning and remediation of such facilities require technologies that ensure safe treatment of the accumulated waste and minimize secondary environmental impacts [5,6]. In the Russian Federation, the “Ecology” national project has already addressed 191 such sites, emphasizing land restoration and the creation of recreational zones [7].

The decommissioning of such facilities is a complex multi-stage undertaking. It must combine the safe extraction of liquid waste, deep treatment of effluents, and the disposal of secondary waste streams—all under strict environmental regulations [8]. Existing remediation approaches can be grouped into four families. Containment-only solutions (multilayer capping, slurry walls) are well-established but do not eliminate the source and are often insufficient for Category I hazardous sites [9]. In situ stabilization technologies (cement and geopolymer binders, bioremediation), reviewed by Zhang et al. [10], Drouiche et al. [11], and Hoeffner et al. [12], have been validated mainly for homogeneous sludges. Complex ex situ treatment trains with thermal or chemical units are emerging but often face high costs and challenges with managing “secondary waste” [13,14].

While obtaining secondary products from industrial waste aligns with circular economy principles [15,16,17], a significant gap remains in managing the high-salinity concentrates and heterogeneous sludges generated by modern treatment plants. Most studies on lithification focus on municipal solid waste (MSW) leachate [18,19], leaving a deficit of integrated, modular technologies for hypersaline industrial concentrates.

The objective of the present work is to develop and experimentally validate a comprehensive technological solution for the decommissioning of a legacy hazardous landfill. The study aims to: (i) design an integrated treatment chain; (ii) formulate a specialized lithification recipe for multicomponent waste; and (iii) evaluate the environmental safety of the resulting geocomposite via biotesting [20].

The scientific novelty lies in the first-time integration of a full extraction-treatment-lithification chain for a legacy site, the formulation of a multicomponent sorbent (Composition D) tailored for high-salinity concentrate (>50 g/L), and the experimental proof that combined waste streams can be converted into Hazard Class V material. Our proposed technology promotes the principles of a circular economy by converting hazardous waste into a secondary material resource [21,22].

2. Materials and Methods

Three elements of the experimental approach are taken from the prior art: the general principle of lithification as encapsulation of toxicants in a mineral matrix [12,19,23,24,25,26]; the use of opoka as a natural sorbent for oil-containing waste [27,28,29,30]; and the BAT-based wastewater-treatment train [13,14,31,32,33].

2.1. Object of Study

One such landfill is located in the Russian Federation and is one of the most hazardous sites of accumulated environmental damage (Category I) [3]. Its prolonged operation (over 50 years) led to the accumulation of significant volumes of liquid and paste-like waste of Hazard Classes I–IV in open sludge storage cells. The contents of these cells constitute a highly aggressive medium characterized by high toxicity, chemical activity, and depth stratification. During the landfill’s operation, the risk of contaminant migration into groundwater and the basin of a major river increased significantly, creating a direct threat to the environmental safety of the entire region. The current study focuses on the remediation of this specific site, addressing four secondary waste streams: evaporator concentrate, dewatered sludge, spent filter media, and contaminated topsoil.

2.2. Project Execution Stages

To achieve this objective, the following tasks were addressed, corresponding to the six stages of the project:

• Analysis of the territory condition and planned activities.
• Development of a schematic diagram for the safe extraction of liquid waste, including Hazard Class I and II waste [34].
• Analysis of existing technologies for processing hazardous liquid waste, including stages of phase separation, physicochemical destruction of toxicants, and desalination. Development and pilot testing of the technology.
• Development of a solution for the disposal of generated secondary waste (SW—concentrates and paste-like waste) by lithification.
• Analysis of environmental risks of implementing the proposed technology.
• Assessment of the economic efficiency of the developed technical solutions.

The six stages listed above correspond to the project execution stages of landfill liquidation, not to Technology Readiness Levels. The technological solution presented in this article corresponds to TRL 4–5 (component validation in the laboratory and in a relevant environment, in accordance with ISO 16290:2013) [35]; subsequent on-site pilot testing is planned at TRL 6.

2.3. Wastewater Treatment Plant

In the third stage, after averaging, the liquid waste is directed to treatment facilities with a capacity of 60 m³/h. These include the following units: chemical and electrochemical treatment, flotation, multi-stage filtration using zeolites and activated carbon, reverse osmosis, and concentrate evaporation. The schematic diagram is presented in Appendix A.

The unified combined landfill waste processing scheme, developed based on the conducted research, includes a full set of processes for collecting and extracting waste from open cells, their deep processing to produce water meeting the requirements for discharge into fishery water bodies [8], as well as producing commercial products used in the remediation of accumulated environmental damage (AED). Improvement of the project economics is achieved through the use of modular facilities that can be transported to a new site after the reclamation process is completed, and the obtained commercial product—topsoil [21].

2.4. Regulatory Framework and Best Available Techniques

The technological solutions developed in this study comply with the following Russian Information and Technical Reference Books on Best Available Techniques (BAT or ITS), the national counterpart of the European BREF documents [31]: (a) ITS 8-2022 [36] “Wastewater Treatment in the Production of Goods, Performance of Works and Provision of Services at Large Enterprises”; (b) ITS 15-2021 [37] “Waste Recovery and Neutralization (Except Thermal Neutralization)”; (c) ITS 17-2024 [38] “Disposal of Production and Consumption Waste”; (d) ITS 52-2022 [34] “Waste Management of Hazard Classes I and II”. The Russian BAT reference documents are structurally similar to the EU BREFs, as both are based on the principles of the Industrial Emissions Directive (IED, 2010/75/EU). The EU BAT conclusions for Waste Treatment, established by Commission Implementing Decision (EU) 2018/1147, serve as the primary regulatory benchmark in Europe. The Russian Hazard Classes I–V are generally comparable to the European Waste Catalogue (EWC) and the Basel Convention classification, although the specific criteria for assigning hazard levels differ.

2.5. Materials and Reagents

During the operation of the liquid waste treatment plant designed in the third stage, the SW of various physical states was calculated to be generated:

• evaporated salt concentrate obtained from the evaporation unit at a rate of 2.7 t/h; the calculated composition of the salt concentrate is presented in

  Table 1. Calculated Composition of the Evaporated Salt Concentrate.

  Component Name	Concentration, mg/L
  Na+	86,614.5
  K+	12,570.2
  NH4+	9469.0
  Ca2+	1553.9
  Mg2+	2216.8
  Cl−	92,277.7
  SO42−	93,185.9
  NO3−	277.6
  F−	157.1
  Phenol	398.7
  Anionic Surfactants	88.6
  Petroleum Products	36.1
  BOD5	170,990.0
  COD	313,512.6
  Inhibitor IOMS-1	505.8

  ;
• dewatered sludge from the filter press, generated during the reagent treatment of liquid waste at a rate of 0.4 t/h; the calculated composition of the dewatered sludge is presented in

  Table 2. Calculated Composition of the Dewatered Sludge after Reagent Treatment.

  Component Name	Concentration, mg/L
  Na+	3141.4
  K+	453.6
  NH4+	342.3
  Ca2+	57.4
  Mg2+	79.8
  Al(OH)3	2955.5
  Mn(OH)2	2268.9
  Cd(OH)2	70.9
  Co(OH)2	65.5
  Cu(OH)2	4.9
  Fe(OH)3	169,633.5
  Ni(OH)2	1173.1
  Zn(OH)2	4673.7
  Pb(OH)2	16.0
  Cr(OH)3	698.2
  Cl−	3329.8
  SO42−	2528.0
  NO3−	10.4
  Anionic Surfactants	139.2
  Petroleum Products	792.6
  Suspended Solids	116,965.5
  BOD5	342,858.5
  COD	717,295.7

  ;
• spent filtering and sorption media used in the liquid waste treatment plant in a total amount of 63.9 t/year, the composition of which is presented in

  Table 3. Composition of Spent Filtering and Sorption Media.

  Component Name	Component Content, %
  Gravel (fraction 2–5 mm)	14 ± 1.4
  Quartz Sand (fraction 0.7–1.2 mm)	14 ± 1.4
  Hydroanthracite Grade A	8 ± 0.8
  Activated Carbon Silcarbon k 835 spezial	30 ± 3.0
  Activated Carbon Silcarbon k 0.3–0.8	28 ± 2.8
  Ion Exchange Resin Tokem 150 in Na-form	7 ± 0.7

  .

Characteristics of Oil-Containing Waste

Oil-containing waste, including bottom sediments and sludge from Cells No. 1 and No. 2, was sampled separately at a rate of 0.5 t/h. The physicochemical characteristics of the samples are presented in

Table 4. Composition of Oil-Contaminated Soil in Cells No. 1 and No. 2.

No.	Parameter Name, Units of Measurement	Results of Quantitative Chemical Analysis of Cell
		No. 1	No. 2
1	Mass Fraction of Petroleum Products, mg/kg	372	9341
2	Mass Fraction of Organic Matter, %	3.38	8.70
3	Mass Fraction of Ash, %	96.62	91.30
4	Mass Fraction of Moisture, %	28.47	46.65

and

Table 5. Composition of Bottom Sediments (Cell No. 2).

No.	Parameter Name, Units of Measurement	Value	Measurement Method Code *
1	pH Value	8.21	PND F 14.1:2.253-09
2	Total Hardness, meq/dm3	2.43	PND F 14.1:2:4.167-2000
3	Calcium (Ca), mg/dm3	37.54	PND F 14.1:2:4.154-99
4	Magnesium (Mg), mg/dm3	6.68	PND F 14.1:2:4.254-2009
5	Total Alkalinity, mg/dm3	1.74	PND F 14.1:2.253-09
6	Bicarbonates (HCO3−), g/dm3	106.14	PND F 14.1:2.253-09
7	Sodium (Na), mg/dm3	12.72	PND F 14.1:2.253-09
8	Potassium (K), mg/dm3	2.42	PND F 14.1:2:3:4.282-18
9	Aluminum (Al), mg/dm3	10.03	PND F 14.1:2:4.168-2000
10	Total Iron (Fe), mg/dm3	118	PND F 14.1:2.253-09
11	Iron II (Fe2+), mg/dm3	<0.05	PND F 14.1:2:3:4.282-18
12	Copper (Cu), mg/dm3	0.108	PND F 14.1:2:3:4.282-18
13	Zinc (Zn), mg/dm3	0.583	PND F 14.1:2:4.187-02
14	Ammonium (NH4+), mg/dm3	3.89	PND F 14.1:2.253-09
15	Nitrate Ions (NO3−), mg/dm3	0.85	PND F 14.1:2.253-09
16	Nitrite Ions (NO2−), mg/dm3	<0.2	PND F 14.1:2:4.167-2000
17	Sulfate Ions (SO42−), mg/dm3	5.3	PND F 14.1:2:4.154-99
18	Phosphate Ions (PO43−), mg/dm3	<0.25	PND F 14.1:2:4.254-2009
19	Fluoride Ions (F−), mg/dm3	<0.1	PND F 14.1:2.253-09
20	Chloride Ions (Cl−), mg/dm3	15.41	PND F 14.1:2.253-09
21	Sulfides (S2−), mg/dm3	<0.05	PND F 14.1:2.253-09
22	Anionic Surfactants, mg/dm3	2.14	PND F 14.1:2:3:4.282-18
23	Petroleum Products, mg/dm3	4190	PND F 14.1:2:4.168-2000
24	Phenols, mg/dm3	3.69	PND F 14.1:2.253-09
25	Formaldehyde, mg/dm3	0.234	PND F 14.1:2:3:4.282-18
26	COD, mgO2/dm3	3870	PND F 14.1:2:3:4.282-18
27	Permanganate Oxidizability, mgO2/dm3	368	PND F 14.1:2:4.187-02
28	Turbidity, mg/dm3	4900	PND F 14.1:2.253-09
29	Suspended Solids, mg/dm3	21,620	PND F 14.1:2.253-09
30	Salinity, mg/dm3	139	PND F 14.1:2:4.167-2000
31	Dry Residue, mg/dm3	–	PND F 14.1:2:4.154-99

* PND F—standardized methods of quantitative chemical analysis (https://metod.fcao.ru accessed on 30 April 2026).

.

2.6. Lithification Compositions and Screening Procedure

For the processing of the indicated waste, lithification technology was employed [11,19]. Lithification involves the principle of pollutant immobilization, combined with the production of material with the required physicomechanical characteristics. Ideally, this should be a secondary material resource (SMR) suitable for reuse. The application of lithification technology allows the use of local raw materials for its implementation, including natural sorbents [39,40].

Four compositions (multicomponent mixtures) were used for SW processing. The compositions were formulated from commercially available ingredients.

Composition “A” is a mixture consisting of bentonite flour (12% bentonite and less than 1% polyacrylamide) and sand (up to 88%).

Composition “B” is a homogeneous, opaque, flame-retardant liquid without mechanical impurities, with a viscosity of no more than 2000 cP at 25 °C. It is a product of the interaction of a polyol mixture with an excess of polyisocyanate and contains 19–20% free isocyanate groups. Composition “B” is typically used for waterproofing coatings, anti-corrosion protection of metals, as well as an adhesive or binder in the production of construction materials, and possesses high strength and special characteristics.

Composition “C” is a dry modified multicomponent bio-mixture (powder) with high absorbing properties, intended for use in animal and poultry housing facilities. The use of the mixture eliminates unpleasant odors, prevents active bacterial growth, and transforms liquid waste into a final solid substance. This composition is characterized by ecological purity and has no negative impact on the environment, animals, or human health.

Composition “D” is a multicomponent mixture containing the following reagents and mineral components, presented in

Table 6. Composition of Composition “D”.

Component Name	Component Content, %
Sodium Polyacrylate	3.5 ± 0.5
Construction Gypsum	24.5 ± 2.5
Soil	30.0 ± 15.0
Chalk	35.0 ± 15.0
Sorbent MIU-S (activated carbon)	7.0 ± 0.7

.

Natural sorbents (opoka from the Kamennoyarsk deposit, Russian Federation) and construction lime were used to process oil-containing waste [22]. Opokas are often used for lithification due to their binding and sorption properties [30]. Opoka from the Kamennoyarsk deposit was used in the 0.7–1.8 mm fraction with the following chemical composition (

Table 7. Average Chemical Composition of the Studied Opoka for the 0.7–1.8 mm Fraction (wt.%).

Oxide	$\overline{\mathit{x}}$	±Δ	s	Min	Max	CV, %
SiO2	82.75	1.18	1.05	78.88	85.65	1.44
Al2O3	6.04	0.31	0.20	5.46	6.63	5.07
Fe2O3	3.88	0.66	0.57	2.99	5.28	14.40
TiO2	0.36	0.07	0.06	0.27	0.62	19.02
CaO	1.47	0.46	0.39	0.77	2.39	31.47
MgO	1.11	0.07	0.05	0.95	1.26	6.60
Na2O	0.57	0.06	0.06	0.44	0.63	11.05
K2O	1.68	0.15	0.15	1.33	1.87	8.65
LOI	2.31	0.82	-	-	-	-

where $\overline{x}$—arithmetic mean, ±Δ—mean linear deviation, s—standard deviation, C_v—coefficient of variation.

).

For waste disposal using lithification technology, a technological complex in a block-modular layout is envisaged, delivered to the production site in full factory configuration. The lithification technological unit consists of the following blocks:

• dry reagent metered dosing block;
• waste feed block for processing;
• mixing block for component blending;
• packaging unit with finished product discharge into bulk bags.

The technology does not require the use of significant amounts of water for mixing.

The operating principle of the technological complex consists of mixing SW with specially selected reagents and mineral components supplied in a specific proportion.

Research on the selection of compositions for lithification was conducted using various combinations of commercially available mineral and organic materials. The physicomechanical characteristics of the thickened product were evaluated. All lithification experiments were performed at ambient temperature (22 ± 2 °C) using a mechanical mixer operating at 60 rpm for 15 min. Each formulation was prepared in triplicate (n = 3), and the reported values represent the arithmetic mean ± standard deviation. The consistency and setting behavior of the solidified products were quantified using a fall-cone penetrometer (penetration resistance, kPa). Phase separation was determined as the supernatant volume fraction after 2 h of settling. Biotesting was carried out in duplicate according to the standard method FR.1.39.2007.03222 [41]. To obtain the most optimal soil samples, a preliminary analytical assessment of the aqueous extract was performed for pH, COD, and salinity indicators, and further tests were conducted to determine the toxicity of the aqueous extract using biotesting, similar to the studies [20]. The optimal mixing ratio for each composition was selected against the following acceptance criteria: (i) final physical state of the lithified product—free-flowing, granular, non-sticky solid (paste-like or two-phase mixtures rejected); (ii) absence of free-liquid exudation (syneresis) within 24 h; (iii) hazard class of the cured product determined by biotesting on Daphnia magna and Chlorella vulgaris [20]—target Hazard Class V (practically non-hazardous) for the salt-concentrate/sludge stream and not lower than Hazard Class IV for the oil-containing stream, in accordance with Order No. 536 of the Ministry of Natural Resources and Ecology of the Russian Federation; (iv) aqueous extract of the cured product—pH 6.0–9.0, salinity below 1 g/L, COD below 30 mg O₂/L; (v) minimum reagent consumption per tonne of waste, subject to all of the above being satisfied. In the text below, the ratio that satisfies all five criteria is referred to as the optimal mixing ratio.

3. Results and Discussion

3.1. Lithification of Evaporated Salt Concentrate Using Composition “A”

During testing of the composition, it was found that when the salt concentrate and Composition “A” were mixed, a two-phase system formed, with phase separation observed. Therefore, the introduction of a solid phase (bentonite, sand) was found to be necessary for the lithification of the liquid salt concentrate.

The study of the applicability of Composition “A” for the lithification of the concentrate generated after evaporation was conducted at various concentrate: “A”: bentonite ratios, with the physicomechanical characteristics of the solidified product being evaluated. The results are presented in

Table 8. Characteristics of the Product Obtained by Treatment of Salt Concentrate with Composition “A” at Various Ratios.

Mass Ratio Salt Concentrate: Composition “A”: Bentonite	Characteristics of Lithified Waste
2:1:1	After solidification, the waste is a sticky mass with a resistance of only 12 ± 5 kPa.
1:1:1	Waste in solid form
1:1:0.3	Waste in solid form represents a solidified foamed material with a penetration resistance of 180 ± 25 kPa after 6 h

Note. Mass ratios are expressed in parts by mass. The order of components is liquid waste (salt concentrate)–solid filler–additional solid (bentonite, where applicable). For example, a ratio of “1:1:1” corresponds to 1 g of salt concentrate, 1 g of dry Composition “A”, and 1 g of bentonite (or any equivalent mass units). In the present experiments, the total mass of each test mixture was 100 g, and the individual component masses were calculated according to the given ratios.

.

It was noted that when Composition “A” is used in a moist environment, a foaming effect is observed; solidification and transformation into a monolith occur 6 h after treatment. The foaming observed with Composition “A” is caused by the presence of anionic surfactants (88.6 mg/L) in the salt concentrate, which reduces the surface tension of the liquid. During mixing, the polyacrylamide component of Composition “A” stabilizes the entrapped air bubbles, forming a persistent foam.

Thus, as a result of the work performed, it was established that for the lithification of the salt concentrate after evaporation with Composition “A”, their mixing in a ratio of 1 part liquid waste to 1 part Composition “A” to 0.3 parts bentonite was required.

3.2. Lithification of Evaporated Salt Concentrate Using Composition “B”

The study of the applicability of composition (sorbent) “B” for the lithification of the concentrate generated after evaporation was conducted at various liquid-to-sorbent ratios, with the physicomechanical characteristics of the thickened product evaluated. The results are presented in

Table 9. Characteristics of the Product Obtained by Treatment of Salt Concentrate with Sorbent “B” at Various Ratios.

Mass Ratio Salt Concentrate: Composition “B”	Characteristics of Lithified Waste
6:1	The treated waste is a two-phase system; phase separation is observed
4:1	Waste in the form of a mobile suspension
2:1	Waste of thick paste-like consistency
1:1	Waste in the form of lumps

Note. Mass ratios are expressed in parts by mass. The order of components is liquid waste (salt concentrate)–solid filler–additional solid (bentonite, where applicable). In the present experiments, the total mass of each test mixture was 100 g, and the individual component masses were calculated according to the given ratios.

.

The optimal ratio for lithification with Composition “B” was found to be 2 parts liquid waste to 1 part sorbent. At this ratio, a thick paste with a penetration resistance of 45 ± 8 kPa was obtained, while the 1:1 mixture formed lumps with a resistance >200 kPa but was not workable.

During the studies, it was found that, for the lithification of salt concentrates (initial salinity above 50 g/L), a greater amount of sorbent is required for thickening compared to analogous experiments with water.

Thus, as a result of the work performed, it was established that for the lithification of the salt concentrate after evaporation with Composition “B”, their mixing in a ratio of 2 parts liquid waste to 1 part sorbent was required.

3.3. Lithification of Evaporated Salt Concentrate Using Composition “C”

As a result of the work performed, it was established that, for the lithification of the salt concentrate with composition (sorbent) “C”, mixing in a ratio of 10 parts liquid waste to 1 part sorbent was required, and the lithified concentrate after treatment was a dense, gel-like product.

The hazard class of the lithified sample obtained by treating the salt concentrate with sorbent “C” was determined by biotesting. According to the biotesting results, the studied lithified waste sample was classified as Hazard Class V. Biotesting was performed on Daphnia using a previously described method [20]. An extract from the biotesting results protocol is presented in Appendix B.

3.4. Lithification of Evaporated Salt Concentrate, Dewatered Sludge After Reagent Treatment, and Spent Filtering and Sorption Media Using Composition “D”

As a result of the work performed, the optimal proportions for mixing the plant’s secondary waste with composition (sorbent) “D” were determined, necessary for obtaining an environmentally safe mineral geocomposite, which was subsequently named “adaptive mineral soil.”

Thus, the adaptive soil consisted of evaporated salt concentrate, dewatered sludge after reagent treatment, spent filtering and sorption media, and a lithifying composition “D” in the specific ratio presented in

Table 10. Composition of Adaptive Mineral Soil.

Component Name	Component Content, %
Evaporated Salt Concentrate	28.0 ± 2.8
Dewatered Sludge after Filter Press	3.7 ± 0.4
Spent Filtering and Sorption Media	0.1 ± 0.01
Composition “D”	68.2 ± 0.7

.

The obtained adaptive mineral soil sample was a loose granular material.

Sodium polyacrylate, a component of the multicomponent sorbent, can absorb extremely large amounts of liquid relative to its own mass and is designed to retain the liquid phase within the volume of the lithified material through hydrogen bonding with water molecules.

The mechanism of lithification proceeds through several interrelated stages [23,24,25], as supported by our experimental observations. First, sodium polyacrylate rapidly absorbs the liquid phase (salt concentrate) via hydrogen bonding, forming a dense gel that eliminates free water and prevents phase separation—consistent with the gel-like consistency observed in the optimal formulation. Subsequently, in the alkaline environment created by the mineral components, construction gypsum and chalk react to form ettringite-type phases and calcium carbonates. These newly formed phases encapsulate heavy metals and organic pollutants, as evidenced by the biotesting results. After lithification with Composition D, the aqueous extract showed no acute toxicity (Hazard Class V), demonstrating effective immobilization. In the opoka/lime system, CaO provides the necessary alkalinity and reacts with the silica-rich opoka to generate C-S-H-like binding phases. This reaction physically encapsulates petroleum products and is corroborated by the rapid transformation from a paste to a dry, loose product within 20 min (Experiments 1–3,

Table 11. Results of Lithification of Oil-Containing Waste, Experiment No. 1.

Cell No. 2, Sampling Point No. 5, Bottom Sediments Ratios: Initial Solution/CaO/Opoka Ratios: Initial solution/CaO/Opoka
1:1.8:0.4	1:1.4:0.4	1:1.2:0.4	1:1.1:0.2
1:1:0.2	1:0.9:0.5	1:0.9:0.2

,

Table 12. Results of Lithification of Oil-Containing Waste, Experiment No. 2.

Waste Sludge from Cell No. 1 Ratios: Sludge/CaO/Opoka Ratios: Sludge/CaO/Opoka
1:0.4:0.1	1:0.2:0.1	1:0.2:0.2

and

Table 13. Results of Lithification of Oil-Containing Waste, Experiment No. 3.

Waste Sludge from Cell No. 2 Ratios: Sludge/CaO/Opoka Ratios: Sludge/CaO/Opoka
1:0.9:0.2	1:0.6:0.2	1:0.6:0.1
1:0.5:0.3	1:0.4:0.2	1:0.3:0.3

). The absence of free liquid and the non-hazardous classification of the product confirm the formation of a stable, inert matrix [26,39].

The lithification technology for secondary waste enables the formation of an environmentally safe mineral geocomposite—adaptive mineral soil—suitable as a construction material in the form of reclamation mixtures for creating soil masses during backfilling of cell excavations.

The obtained soil sample, consisting of evaporated salt concentrate, dewatered sludge after reagent treatment, spent filtering and sorption media, and mixed with lithifying sorbent—Composition “D”—was sent to a certified laboratory for hazard class determination by biotesting. The waste hazard class is established based on the degree of possible harmful impact on the environment through direct or indirect exposure of the hazardous waste to it. During biotesting, the hazard class of samples is established by the dilution factor of the aqueous extract at which no impact on hydrobionts was detected.

According to the biotesting results, the studied adaptive mineral soil sample belonged to Hazard Class V. An extract from the biotesting results protocol is presented in Appendix C. To evaluate the long-term stability of the adaptive mineral soil, we performed leaching tests under various pH conditions. The results showed that the concentration of heavy metals, petroleum products, and phenols in the leachate remained below the detection limits or the maximum permissible concentrations for Hazard Class V waste, even at extreme pH values (4.0 and 9.0) (

Table A2. Extract from the Protocol of Biotesting Results of the Evaporated Reverse Osmosis Salt Concentrate Sample Lithified with Sorbent “D”.

Test Object	Observation Duration, hours	Parameter	Result
Dafnia magna Stratus	96	toxicity	absent
Chlorella vulgaris Beijer	72	toxicity	absent

Conclusion. The aqueous extract of the sample (treated with sorbent) has a harmful effect on hydrobionts. In accordance with Order No. 536 of the Ministry of Natural Resources and Ecology of the Russian Federation, the tested sample can be classified as Hazard Class V (non-hazardous) based on the degree of environmental impact.

, Appendix C). This confirms the high immobilization capacity of the mineral matrix.

3.5. Lithification with Natural Materials (CaO + Opoka)

For the lithification of oil-containing waste (oil-contaminated soils and bottom sediments from cells No. 1 and No. 2), the use of construction lime (CaO) in combination with natural opoka from the Kamennoyarsk deposit (Russian Federation) was justified, with optimal mass ratios determined experimentally (Table 11, Table 12 and Table 13); the resulting product corresponded to Hazard Class IV by biotesting and exhibited a loose, free-flowing consistency suitable for further reclamation use [21,22].

Experiment No. 1. The optimal mixing ratios were obtained at the following proportions:

-

bottom sediments/CaO/Opoka

-

1:0.9:0.5

-

1:0.9:0.2.

Observations: when the smallest amount of fillers was added, the mixture turned into a paste, heated up, and dried within 20 min.

Instantaneous formation of a loose product is possible only with an increased consumption of fillers (total filler mass exceeding 1.4, where lime must be at least 0.9 parts of the bottom sediment mass).

Experiment No. 2. The optimal mixing ratios were obtained at the following proportions:

-

sludge from Cell No. 1/CaO/Opoka

-

1:0.2:0.2.

Experiment No. 3. The optimal mixing ratios were obtained at the following proportions:

-

initial sludge from Cell No. 2/CaO/Opoka

-

1:0.4:0.2

-

1:0.6:0.1

Observations: when the smallest amount of fillers was added, the mixture turned into a paste, heated up, and dried within 20 min. Instantaneous formation of a loose product is possible only with an increased consumption of fillers (total filler mass exceeding 0.8, where lime must be at least 0.6 parts of the sludge mass).

The best-performing samples were sent to an accredited laboratory for hazard class determination. Hazard Class IV was confirmed. The performance of the selected binders is comparable to or exceeds that reported in the recent literature. For instance, Yazev et al. (2020) [19] achieved successful lithification of landfill leachate using a similar mineral-based sorbent system. However, they required a higher binder-to-waste ratio than our Composition D. In the context of geopolymer solidification, Zhang et al. (2022) [10] reported effective immobilization of leachate concentrate contaminants using alkali-activated geopolymers; however, their approach required curing at elevated temperatures, whereas our process operates at ambient conditions. The combination of lime and opoka employed here for oil-containing waste yields results consistent with the work of Vurdova (2024) [22], confirming that this naturally derived binder system can reliably produce a solid, low-hazard product without thermal pre-treatment.

Thus, based on the conducted research to determine the optimal composition for the lithification of secondary waste in various physical states generated during operation of the liquid waste treatment plant at the studied landfill, the multicomponent sorbent D was selected. The feasibility was also studied, and it was experimentally proven that this sorbent can be applied to waste with high salt and organic matter content. It was established that the use of sorbent D renders the resulting waste less hazardous, rendering it non-hazardous to the environment. For the lithification of oil-containing waste (oil-contaminated soils and bottom sediments from cells No. 1 and No. 2), the use of construction lime (CaO) in combination with natural opoka from the Kamennoyarsk deposit (Russian Federation) was justified, with optimal mass ratios determined experimentally (Table 11, Table 12 and Table 13); the resulting product corresponded to Hazard Class IV by biotesting and exhibited a loose, free-flowing consistency suitable for further reclamation use.

The next stage involves obtaining regulatory documentation for the produced soil, which will allow its use as a reclamation material. The regulatory documentation includes the development of the following documents:

• Technical specifications for the product
• Certificate of conformity
• Safety data sheet
• Expert opinion (if necessary).

Quality control of the technical soil must be performed in accordance with the technical specifications. It must include the environmental and physicomechanical characteristics of the material, ensuring its suitability for backfilling vacated cells and for grading works.

3.6. Process Flow Diagram of the Lithification Unit for Evaporated Salt Concentrate, Dewatered Sludge After Reagent Treatment, and Spent Filtering and Sorption Media

For waste disposal using lithification technology, the use of a technological complex consisting of the following blocks is envisaged:

• metered dosing block for sodium polyacrylate (in dry form) Bd1–2;
• mixing block for blending evaporated salt concentrate with sodium polyacrylate, item SM1/1–2;
• metered dosing block for gel after treatment of evaporated salt concentrate with sodium polyacrylate to the mixing block, item SM3;
• metered dosing block for construction gypsum (in dry form);
• metered dosing block for natural chalk (in dry form);
• metered dosing block for spent filtering and sorption media;
• metered dosing block for sorbent D (in dry form);
• metered dosing block for topsoil combined with dewatered sludge;
• mixing block, item SM3;
• packaging unit with finished product discharge into bulk bags.

The process flow diagram of the lithification unit is presented in Appendix D.

The operating principle of the technological complex consists of mixing the plant’s secondary waste with specially selected reagents and mineral components supplied in a specific proportion.

Processing of the generated secondary waste occurs in two stages:

• Mixing of evaporated salt concentrate with sodium polyacrylate in mixer item SM1/1–2;
• Mixing of the gel after treatment of evaporated salt concentrate with sodium polyacrylate with the remaining components of sorbent D and other secondary waste from the plant in mixer item SM2.

Alternating operation of mixers item SM1/1–2 is provided: while one mixer receives evaporated salt concentrate and sodium polyacrylate and mixes them, the other mixer supplies the semi-product in gel form to the receiving hopper of mixer item SM2.

Dry reagents are delivered to the lithification site in bulk bags by forklift. Bag opening stations are provided for de-packaging. Using the existing lifting device in the facility, the bulk bag is raised and positioned over a special cutting system. In the lower part of the structure, there is a receiving hopper with a screw conveyor. To ensure uniform discharge from bulk bags, the opening station is equipped with a vibrating bottom. A shaking system is also provided. The screw conveyor is then activated and transfers sodium polyacrylate to the dosing hopper. Loading is regulated by a weight sensor installed in the dosing hopper.

Sodium polyacrylate is loaded from the gravimetric dosing unit through a disc valve into the mixer vessel item SM1/1–2. The absorbent is then mixed with the evaporator concentrate. Sodium polyacrylate has the ability to absorb extremely large amounts of liquid relative to its own mass. It is designed to retain the liquid phase within the volume of the lithified material through hydrogen bonding with water molecules. The semi-product after treatment of the evaporated salt concentrate with sodium polyacrylate is a dense gel.

In mixer item SM2, the obtained semi-product is blended with the remaining sorbent D components and other secondary plant waste: dewatered filter press sludge and spent filtering media.

Dosing of dry materials (gypsum, chalk, spent media, and activated carbon MIU-S) and dewatered sludge after reagent treatment is performed in a manner similar to the process described above. Topsoil from the PFZ excavation is fed by an excavator into open hoppers, from which it is dosed into mixer item SM3.

Thus, the resulting adaptive mineral soil consists of the following components in the ratio presented in

Table 14. The sequence of introducing components when obtaining adaptive mineral soil.

Component Introduction Sequence	Component Name	Component Consumption, t/h
1.	Evaporated Salt Concentrate	2.7
2.	Sodium Polyacrylate	0.27
3.	Construction Gypsum	1.9
4.	Natural Chalk	1.3
5.	Spent Filtering and Sorption Media	0.007 (periodic supply)
6.	Activated Carbon MIU-S	0.5
7.	Dewatered Sludge after Filter Press	0.4
8.	Topsoil	3.7

.

After filling, the bulk bag enters a belt conveyor that moves it to the finished product shipping area.

After filling, the soil is immediately ready for further use—backfilling cell excavations—or can be placed at a storage site.

Thus, the studies justified selecting lithification technology as the optimal method for disposing of secondary waste (evaporator bottoms and dewatered sludge) generated during the treatment of landfill effluents.

The effectiveness of using a multicomponent sorbent based on mineral binders was experimentally confirmed. It was established that the optimal ratio for obtaining a strong geocomposite is: 68% sorbent, 28% salt concentrate, and 4% sludge.

The obtained product (adaptive mineral soil) belongs to Hazard Class V (practically non-hazardous waste) according to biotesting results. This allows its use at the work site for technical reclamation—backfilling vacated excavations (cells)—eliminating the costs of transporting waste to other disposal landfills.

The lithification technology solves a key environmental problem—the prevention of secondary environmental contamination by highly toxic brines, converting them into an insoluble, inert artificial stone matrix.

The use of block-modular equipment allows for a 25–30% reduction in capital costs for future projects. The energy consumption is 38.5 kWh per ton of processed waste, and the reagent costs for the lithification stage are 15.2 $/ton, which is significantly lower than the cost of conventional disposal methods (45–60 $/ton).

3.7. Discussion on Promising Methods

Alternative advanced wastewater treatment technologies, such as plasma gasification and supercritical water oxidation, offer high removal efficiencies and minimal secondary waste generation [42]. However, their industrial application remains limited by high capital and operating costs, as well as technological complexity.

In contrast, the developed multi-stage treatment and lithification scheme is more practical for the studied landfill because it is modular, scalable, and adapted to the specific composition of the waste streams [43,44].

4. Conclusions

Based on the experimental research and engineering analysis carried out, the following main results were obtained. (a) A multi-stage deep treatment scheme was developed and demonstrated to bring the highly toxic landfill filtrate to discharge standards for fishery water bodies (reagent treatment–pressure flotation–cascade mechanical filtration–three-stage reverse osmosis–vacuum evaporation). (b) The principal result of the study is the integrated lithification recipe—“adaptive mineral soil”—that converts the four secondary waste streams of the on-site liquid-waste treatment plant (evaporator concentrate, dewatered sludge, spent filter media, topsoil from cell excavation) into a single Hazard Class V geocomposite (Table 14, Section 3.6). (c) For the oil-containing fraction, treatment with construction lime and natural opoka was validated, yielding a Hazard Class IV product (Section 3.5). Given the unique composition of the landfill leachate, on-site pilot testing of at least 6 months is recommended prior to construction. The estimated capital cost is ~30 million USD; the block-modular (container-type) equipment ensures mobility between sites and reduces capital cost compared with permanent buildings. The results confirm the feasibility of using modular on-site treatment systems to reduce environmental risk and improve the economic efficiency of landfill remediation. The applicability envelope is bounded by the empirical composition of the studied object: the technology is directly applicable to liquid and paste-like industrial waste of Hazard Classes I–IV with composition similar to that of Table 1, Table 2, Table 3, Table 4 and Table 5; plausible transferability to chemical, petrochemical, metallurgical and old galvanic sludge accumulators is subject to short laboratory verification against the acceptance criteria of Section 2.6; the technology is not validated for radioactive waste, persistent organic pollutants, mercury-bearing waste, or strongly acidic/alkaline streams outside pH 3–12. The next step is on-site pilot testing of the block-modular lithification unit at TRL 6.