Use of Sands from Wastewater Treatment Plants as a Substitute for Natural Aggregate in the Context of a Circular Economy

Abstract

In light of the global raw material crisis and the ongoing degradation of the natural environment, this study provides a significant contribution to the advancement of the circular economy in the construction sector. The authors conducted a comprehensive analysis of the feasibility of using waste sands originating from wastewater treatment plants as substitutes for natural fine aggregates in concrete mixtures. The investigation included the evaluation of the physicochemical, environmental, and mechanical properties of the analyzed waste sands. The results demonstrate a high application potential for sewer cleaning sand (SC), which, in its current form, can be used in non-structural applications. The key advantages of the sand that was examined include a high sand-equivalent value (98.2%), low contents of impurities (LOI < 1.5%), and a favorable chemical composition. Leaching tests for harmful substances, including heavy metals, for both the sand and the mortar samples, did not indicate any significant environmental risk. One principal conclusion of the study is the identification of the possibility of closing the waste life cycle at the wastewater treatment plant stage, which could significantly contribute to the reduction of landfilled waste volumes and operational costs.

1. Introduction

The construction industry is a sector characterized by dynamic growth. One of the fundamental materials used in construction is concrete. It is a versatile, durable, and highly resilient material, applied in the construction of various structures, ranging from foundations and load-bearing walls to bridges, tunnels, and other infrastructural facilities. It is important to emphasize that concrete production involves significant consumption of natural resources, such as sand, gravel, and cement, the extraction of which is costly and leads to the gradual depletion of natural resources. The growing demand for concrete, driven by intensive urbanization and infrastructure development, further exacerbates this issue, contributing to environmental degradation. In light of the needs of future generations, there is an urgent need to develop alternative solutions that will reduce the exploitation of primary raw materials and mitigate the negative impacts of construction on the natural environment [1,2]. In response to contemporary challenges, materials science and engineering are focusing on the search for innovative technologies and substitutes for traditional concrete components. One promising direction is the use of recycled materials, such as fly ash, blast furnace slags, construction waste, and waste sands. Additionally, technologies for low-carbon-footprint concrete, including geopolymers, are being developed. These materials eliminate the need for traditional portland clinker, which has a high carbon footprint, within their structure [3,4]. Efforts aimed at protecting natural resources and reducing the negative environmental impacts of construction are essential elements of sustainable development. The implementation of new technologies and the development of alternative construction materials can help minimize the effects of environmental degradation and ensure the long-term availability of raw materials for future generations.

One essential component of concrete is sand. Due to the diversity of its properties, sand is an indispensable raw material in construction. Its physical and chemical properties make it irreplaceable in various applications. It is most commonly used in concrete mixtures, masonry, and plastering mortars, as well as serving as a stabilizing component in foundations in road construction [5]. Additionally, it is utilized in various types of bedding materials for paving stones, backfilling to protect pipes and cables, and backfills used in earthworks and foundation construction [6]. Due to the depletion of sand resources, alternative materials are being sought that could replace this valuable raw material [7]. One promising direction is the use of waste sands from various industrial sectors [8,9], such as the mining [10,11], metallurgy [12], and foundry industries [13,14]. The use of waste sands as substitutes for natural aggregates in concrete production is an innovative and future-oriented solution that could significantly contribute to sustainable natural resource management and the reduction of waste deposited in landfills. The application of waste sands in concrete could not only limit the extraction of natural resources but also reduce the carbon footprint in the construction sector. Implementing waste sands in concrete technology requires detailed research to determine their impacts on the mechanical properties and durability of the resulting materials [15]. Regulatory and standardization aspects are also of key importance and must be adjusted to new technologies to ensure the safety and longevity of structures. Furthermore, it is essential to develop and implement modern methods of processing and modifying waste sands, enabling their effective use in the construction industry.

In the context of searching for alternative construction materials, one potential substitute for natural sand could be sand originating from wastewater treatment plants. Sand is generated in two main areas of the wastewater treatment process: in sand traps and as a result of the separation of contaminants during the cleaning of sewage systems [16]. In standard operational practices at wastewater treatment plants, sand is treated as a waste material of varying quality and requires further processing before potential reuse [17]. It is most commonly handed over to specialized external companies that undertake its cleaning, which involves the removal of excess organic substances, and further processing is possibly required to adjust its properties given the relevant environmental and industrial requirements [16,17]. Depending on the quality and degree of contamination, further handling is regulated by relevant legal acts [18]. If the sand meets the specified standards for chemical composition and as to its contents of harmful substances, it can be further utilized, for example, in road construction or land reclamation in degraded areas [6,7]. If the required criteria are not met, it usually ends its lifecycle in a landfill. The use of sand from wastewater treatment plants as a substitute for natural sand is preliminarily feasible and aligns with the principles of the circular economy. Further research is necessary on the technologies used for processing this sand and analyzing the impact of this sand on the mechanical properties of construction materials where it could be applied.

One of the lesser-studied, yet consistently produced, by-products of sewer system maintenance is the sand collected during routine cleaning of sewer pipelines. Unlike the grit removed in primary treatment stages such as grit chambers, this material originates from non-technological processes and exhibits distinct physical and chemical characteristics. It is primarily composed of inorganic particles, including sand and fine gravel, along with the minor organic content typical of deposits accumulating within sewer conduits. Due to the necessity of regular maintenance operations carried out by wastewater utility services, the generation of this type of waste is predictable and its collection is generally not associated with logistical challenges. The material is typically gathered and stored at accessible sites, ensuring a relatively stable supply. The physicochemical properties of sewer cleaning sand are influenced by site-specific factors, including the design and condition of the sewer network, as well as the pre-treatment practices employed by wastewater treatment plants, such as washing, dewatering, or mechanical separation. Therefore, any assessment of its potential reuse, whether in construction applications, land reclamation, or environmental engineering, must be based on a detailed characterization tailored to its specific origin.

The aim of the conducted research was a comprehensive analysis of the potential use of waste sands from sewage treatment plants as a secondary raw material in construction. Specifically, the focus was on assessing their physicochemical, mechanical, and environmental properties. The analyses were carried out to determine the impacts of waste sands from sewage treatment plants on the strength and safety of building structures, as well as the compliance of the resulting materials with applicable standards and legal regulations. Additionally, ecological aspects were considered, emphasizing the importance of waste recycling in the context of sustainable development.

It is important to emphasize that this research is pioneering and represents a breakthrough in the field of waste management in wastewater treatment plants. The innovation of this study lies in the possibility of closing the “waste loop” within the treatment plant, marking a significant step towards the sustainable development of the sewage treatment sector. The reuse of sand, one of the by-products of the treatment process, can significantly contribute to reducing the amount of waste sent to landfills, while minimizing the environmental impact. Furthermore, implementing such solutions will positively affect efforts to reduce operational costs in treatment plants. A holistic approach to this issue enables synergy between technical, ecological, and economic aspects, making this research particularly valuable and potentially influential in shaping the future of waste management in the water and sewage sector.

2. Materials and Methods

2.1. Materials

The analyzed material consisted of waste with code 19 08 02, produced during the mechanical treatment of wastewater [19]. The sands under study came from two sources: the contents of sand from desanding (SD, Figure 1) and the cleaning of sewage systems (SC, Figure 2). The SD sample was dominated by quartz grains (Figure 3). Along with the quartz, organic fragments were clearly visible—stems and seeds, as well as a large fragment of a snail shell. Ceramic fragments and small pieces of aluminum foil and green-colored plastic were also observed. The SC sample had a milky color and a neutral odor (Figure 4). Organic matter in the form of seed or wood pieces was noticeable. Single plastic elements were also present.

2.2. Methods

2.2.1. Sample Preparation and Physicochemical Characterization of Raw Sand

In the initial phase of the study, the water content in the collected samples was determined in accordance with the PN-EN 15934 standard [20]. To obtain the research material, the sand was subjected to a drying process at a temperature of 105 °C, followed by cooling. The pre-treated material was then subjected to selected physicochemical analyses. Bulk density was determined following the PN-EN 1097-3 standard [21]. The total organic carbon (TOC) content in the analyzed sand samples was measured. Biodegradability was assessed using the dichromate method [22]. TOC content was calculated based on the amount of degradable organic substances present. The nitrogen content was determined in accordance with PN-Z-15011-3 [22]. This method involved the mineralization of the sample using concentrated sulfuric acid with a catalyst, followed by ammonia distillation and titration. The determinations of potassium (K), sodium (Na), calcium (Ca), lithium (Li) and barium (Ba) content were performed using flame photometry [23]. The procedure involved converting the elements present in the sample into a soluble form by ashing at a temperature of 550 ± 50 °C, followed by hot dissolution in hydrochloric acid with the addition of nitric acid. The resulting solution was introduced into an acetylene--air flame, in which excitation of potassium, sodium, calcium, lithium, and barium atoms occurred, with the intensity of emitted radiation being proportional to the concentration of these elements in the analyzed sample [23].

The humus content in the aggregate was determined using a visual method, as described in PN-EN 1744-1+A1:2013-05 [24]. This procedure involves evaluating the color change of a 3% sodium hydroxide (NaOH) solution after interaction with the sample. Samples were placed in glass beakers filled to a predefined level with the NaOH solution and stirred for one minute. Subsequently, the mixtures were allowed to rest undisturbed for 24 h. After this incubation period, the color of the solution was visually compared against a reference solution. The degree of coloration served as an indicator of humus presence, where a lightly colored solution suggested minimal humus content, whereas a more intense coloration indicated a higher humus concentration.

Additionally, the loss on ignition (LOI) of the dry sample mass was determined in accordance with PN-EN 15935 [25]. This analysis involved combusting the material at 600 °C to quantify the organic matter content.

2.2.2. Leaching Procedure and Analysis of Aqueous Extracts

Water extracts were prepared from the sand samples, following the PN-EN 12457-2 standard [26]. The aqueous extract was obtained using a liquid-to-solid (L/S) ratio of 10 L/kg, with distilled water (pH 6.6) serving as the leaching agent [26]. The mixture was agitated for 24 h on a laboratory shaker, after which it was filtered through a soft filter. The analysis of the leachates included determinations of pH, chloride, sulfate, sodium, potassium, calcium, lithium, barium, phosphorus, fluorides, and selected heavy metals (Zn, Pb, Cu, Cd, Cr, Co, Fe, Mn, and Ni).

The pH values of the extracts were measured potentiometrically using an Elmetron CPC-501 m, in accordance with the PN-Z-15011-3 standard [22]. Chloride content was determined by the Mohr titration method, employing silver nitrate and potassium chromate as an indicator, following the PN-ISO 9297 standard [27]. Sulfates (SO₄²⁻) were quantified gravimetrically using barium chloride, and in accord with PN-ISO 9280 [28]. Concentrations of sodium, calcium, potassium, lithium, and barium were measured via flame emission spectrometry, in compliance with PN-ISO 9964-3 [23]. Phosphorus was analyzed spectrophotometrically using ammonium molybdate and tin(II) chloride as a reducing agent, as specified in PN-EN ISO 6878:2006 [29]. Fluoride content was determined by a photometric method using the SPADNS reagent (1,8-dihydroxy-2-(4-sulfophenylazo)-naphthalene-3,6-disulfonic acid) [30].

Total inorganic carbon (IC) and total organic carbon (TOC) in the water samples were quantified with a Shimadzu TOC-L analyzer (Kyoto, Japan). This device utilizes catalytic oxidation by combustion at 680 °C, followed by detection of the produced CO₂ using nondispersive infrared (NDIR) technology. Heavy metal concentrations were determined by atomic absorption spectrometry using the GBC AVANTA PM spectrometer, applying the flame atomization technique.

2.2.3. Granulometric Analysis of Waste Sand

At the next stage of the research, waste sand samples were dried at 105 °C and then cooled to room temperature. Each sample, having a mass of 3 kg, was subjected to granulometric analysis in compliance with the PN-EN 933-1:2012 standard [31]. The analysis employed a series of sieves with mesh sizes of 63.0, 31.5, 16.0, 8.0, 6.3, 2.0, 1.0, 0.65, 0.200, 0.125, and 0.063 mm. The material retained on each sieve was weighed, allowing for the calculation of the proportion of each grain size fraction relative to the total sample mass. This method enabled the determination of the fines content as well as the sand point. Afterwards, the samples were combined and divided into two granulometric fractions: particles larger than 2 mm and those smaller than or equal to 2 mm. The fine fraction (≤2 mm) underwent further physicochemical analyses, using the same procedures previously applied to the untreated material, as described earlier in the text.

2.2.4. Preparation and Environmental Assessment of Concrete Mortars

Concrete mortars were prepared using the previously characterized waste sand fractions, following the procedure outlined in the PN-EN 197-1:2012 standard [32]. The mixing process was conducted using an automatic mortar mixer to ensure homogeneity and reproducibility of the specimens. The prepared mortars were molded into blocks for further testing.

To assess the potential environmental impacts of the incorporation of waste materials into cementitious composites, a leachability study examining harmful substances, particularly heavy metals, was conducted. The leaching behavior of concrete is known to depend on its physical form. In monolithic forms, the release of contaminants typically results from surface leaching and diffusion, whereas in fragmented materials, percolation dominates the leaching process.

This study focused on fragmented mortar specimens. Water extracts were prepared in accordance with the applicable standard [33], ensuring consistency with the procedure used for raw sand extracts. The prepared samples underwent 24-h of agitation, followed by filtration. The resulting eluates were analyzed for pH, major ions (e.g., Na⁺, K⁺, Ca²⁺, SO₄²⁻, and Cl⁻), and heavy metals (Zn, Pb, Cu, Cd, Cr, Co, Fe, Mn, and Ni), using the analytical methods described in Section 2.2.3, including flame atomic absorption spectrometry (AAS), potentiometric pH measurement, and gravimetric or photometric determinations where appropriate [22,23,27,28,29,30].

The results of this analysis allowed for an initial evaluation of the environmental safety and durability of the developed concrete composites, as well as their suitability for potential use in construction applications.

3. Results and Discussion

Table 1. Basic physicochemical properties of the materials tested.

Parameter	Unit	SD	SC
Total humidity (Htot.)	%	13.50	2.98
Analytical humidity (Ha)	%	0.74	0.45
Bulk density in a dry state (pn)	kg/m3	1330	1350
Degradable organic substances (DOS)	%	8.85	3.79
Total organic carbon (TOC)	%	2.75	1.78
Total nitrogen (Ntot.)	%	0.17	0.09
Sodium (Na)	%	0.03	0.02
Calcium (Ca)	%	0.34	0.10
Potassium (K)	%	0.05	0.03
Lithium (Li)	%	blq 1	blq 1
Bar (Ba)	%	0.10	0.01

¹ blq—values below the limit of quantification.

presents the results of the physicochemical characterization of the materials investigated. The SD sample exhibited higher total moisture content than the SC sample. On the other hand, the SC sample had slightly higher bulk density than SD, which may suggest that SC is denser or has fewer void spaces in its structure. The content of degradable organic matter (DOS) was higher in the SD sample (8.85%), compared to the SC sample (3.79%), indicating that SD contains more organic substances that are more easily biodegradable. The SD sample also contained more organic carbon (2.75%), compared to SC (1.78%), which may suggest a higher content of organic materials in SD. The contents of calcium (Ca), potassium (K), and sodium (Na) were marginally higher in the SD sample compared to SC. No detectable amounts of lithium (Li) were found in either material.

Figure 5 illustrates the results of the visual analysis of humic substance content in the examined sand samples. The humus content in individual samples was determined based on the color assessment of the solution. The intensity of coloration was correlated with the amount of humic substances—the dark brown color of sample SD indicated a high humus content in the aggregate. In contrast, the solution obtained for sample SC exhibited a less intense coloration, suggesting a lower humus content. Both analyzed samples (SD and SC) were compared with a solution prepared using standardized sand, which served as the reference sample (RS).

Figure 6 illustrates the results of the organic matter content (LOI) determination for each individual sample. The examined sands originating from the wastewater treatment plant exhibited variation within a range of organic matter content. The organic matter content in SD was determined to be 7.80% based on dry mass, whereas in SC, it was 1.11%. The reported organic matter contents may be overestimated due to the presence of synthetic plastics and other anthropogenic materials in the samples.

Table 2. Leachability of harmful substances and heavy metals from tested waste samples.

Substance Name	SD	SC	Permissible Value According to [34]
pH, -	7.0	8.6	6.5–9.0
Total suspended solids (TSS), mg/L	24	18	35
Chlorides (Cl−), mg/L	50.69	9.22	1000
Sulphates (SO42−), mg/L	98.05	41.41	500
Total nitrogen (Ntot.), mg/L	0.56	3.68	10
Phosphorus (P), mg/L	0.09	0.05	1
Potassium (K), mg/L	4.50	2.69	80
Calcium (Ca), mg/L	27.12	9.95	ng 1
Sodium (Na), mg/L	4.19	1.82	800
Fluoride (F−), mg/L	1.26	<0.05	25
Barium (Ba), mg/L	1.91	1.36	2
Total organic carbon (TOC), mg/L	27.76	4.21	30
Total carbon (TC), mg/L	31.16	9.14	ng 1
Inorganic carbon (IC), mg/L	3.40	4.93	ng 1
Zinc (Zn), mg/L	0.08	0.23	2
Copper (Cu), mg/L	0.02	0.03	0.5
Lead (Pb), mg/L	<0.05	<0.05	0.5
Cadmium (Cd), mg/L	0.03	<0.005	ng 1
Chromium (Cr), mg/L	<0.05	<0.05	0.1
Cobalt (Co), mg/L	<0.05	0.06	1
Manganese (Mn), mg/L	0.06	0.03	ng 1
Iron (Fe), mg/L	0.18	0.15	10
Nickel (Ni), mg/L	<0.04	<0.04	0.5

¹ ng—no guidelines.

presents the parameters of the selected pollutants washed from the tested sample sands and subsequently determined to constitute a potential environmental nuisance. The results obtained were then compared with the permissible pollution values specified in the regulation of the Minister of Maritime Economy and Inland Navigation referencing substances particularly harmful to the aquatic environment, and the conditions to be met when sewage is introduced into water or on land. The pH values of the aqueous extracts from the studied sands (SD-7.0 and SC-8.6) were neutral or slightly alkaline. This pH range is typically desirable for wastewater treatment plant by-products, as alkaline aqueous extracts promote chemical stability and minimize corrosion. Additionally, this is beneficial for the further processing of sands and their potential applications. The aqueous extracts from the waste sands derived from wastewater treatment plants, for most substances, fell within the acceptable limits of the standards, indicating the appropriate quality of these materials in terms of their environmental impact. However, substances such as boron and total organic carbon require monitoring, particularly with regard to their potential applications, such as in construction for concrete production. These values may also provide insights into optimizing purification processes and further treatment of sands in wastewater treatment facilities.

Figure 7 presents the results of the grain size distribution analysis of the examined sands. The analyses conducted allowed for a detailed characterization of the proportions of individual fractions. One important parameter characterizing the gradation of aggregates used in concrete is the sand equivalent. The sand equivalent for average sand is 100%. For the SD sample, it was 95.0%, and for the SC sample, it was 98.2%. The sand equivalent for the reference sand was 99.8%. Based on the analysis conducted, it can be concluded that the sands from the wastewater treatment plant meet most of the requirements for second-class refined sand, in accordance with the PN-EN 12620 standard [35].

Figure 8 presents a simplified granulometric classification of the analyzed samples. The samples were divided into two main fractions: particles with a diameter greater than 2 mm and particles with a diameter smaller than 2 mm. Granulometric composition analysis revealed that in both examined samples, the finer fraction (≤2 mm) was predominant, accounting for approximately 90% of the total material. In contrast, the coarser fraction (>2 mm) constituted only about 10% of the analyzed material. The obtained results indicate that the grain size distribution may significantly influence the physical and mechanical properties of the material, including its porosity and water retention capacity, which could determine its potential applications in various sectors of the economy.

After rejecting the fraction larger than 2 mm, which contained significant amounts of organic matter and contaminants, further investigations were conducted on the properties of sands derived from wastewater treatment plants. The study focused on assessing the physical and chemical characteristics of these materials, including organic matter content, density, and the impacts of these substances on the strength of cement mortars. The findings are crucial for evaluating the potential utilization of this raw material in construction, which could contribute to reducing the exploitation of natural aggregate deposits and promote a circular economy.

A detailed physicochemical analysis of the examined sands was conducted as the first step (

Table 3. Basic physicochemical properties of the tested materials with fractions below 2 mm.

Parameter	Unit	SD	SC	RS
Analytical humidity (Ha)	%	0.66	0.46	0.33
Bulk density in a dry state (pb)	kg/m3	1333	1350	1329
Degradable organic substances (DOS)	%	3.79	blq 1	blq 1
Total organic carbon (TOC)	%	1.78	blq 1	blq 1
Total nitrogen (Ntot.)	%	0.14	0.04	blq 1
Phosphorus (P)	%	0.06	0.01	blq 1
Sodium (Na)	%	0.02	0.02	0.01
Calcium (Ca)	%	0.03	0.05	blq 1
Potassium (K)	%	0.03	0.02	0.01
Lithium (Li)	%	blq 1	blq 1	blq 1
Bar (Ba)	%	blq 1	0.01	blq 1

¹ blq—values below the limit of quantification.

). The bulk density in the dry state showed minimal variation between the samples, ranging from 1329 to 1350 kg/m³, indicating similar structures. However, significant differences in the chemical composition of the individual samples were observed during the chemical analysis. The presence of degradable organic substances (DOS) was detected only in sample SD, reaching a value of 3.79%, while in the other samples, the levels of this substance were below the detection limit. Organic carbon content was found exclusively in sample SD (1.78%); this correlates with the presence of organic substances. In summary, sample SD exhibited a higher content of organic components and nutrients, such as nitrogen, phosphorus, and organic carbon. In contrast, sample SC contained higher amounts of minerals, including calcium and barium. The obtained results confirm differences in the origin of the sands and suggest potential applications based on their unique chemical properties.

The results of the analysis of humic substance content in the studied sand samples with a fraction below 2 mm, conducted based on a visual assessment of solution color, are presented in Figure 9. The solution obtained for the <2 mm fraction of the SD sample still exhibited an intensely dark brown coloration, indicating a persistently high humic substance content. In contrast, the solution obtained for the <2 mm fraction of the SC sample displayed significantly less intense coloration compared to the raw sample, suggesting a reduction in humic content. Furthermore, the color of the SC sample solution was similar to that of the reference sample (RS), which may indicate an advanced degree of dehumification in this fraction.

In the subsequent stage of the study, the organic matter content in waste sands with a fraction size of ≤2 mm was determined, using the loss on ignition (LOI) method, at three temperature points: 440 °C, 600 °C, and 950 °C. The results of this analysis are presented in Figure 10. The highest LOI value at 440 °C was recorded for sample SD, amounting to 1.79%. In contrast, the LOI for sample SC was lower and did not exceed 1%. A similar trend was observed at 600 °C, at which the LOI value for sample SD increased to 3.14%, while for sample SC, the ignition losses exceeded 1%. In the next phase of the study, LOI was determined at 950 °C with a heating duration of 1 h. The assessment of LOI at this temperature determines a crucial parameter in the quality control of aggregates intended for applications such as concrete production. Excessive contents of organic matter and other impurities may adversely affect the mechanical properties and durability of construction materials. The acceptable LOI level for waste sands designated for concrete production depends on their origin and intended use. However, technical literature suggests that it should not exceed 3%. The obtained results indicate that the LOI value for sample SC was 1.35%, which falls within the acceptable limits. In contrast, sample SD exhibited an LOI of 4.18%, exceeding the established threshold. It is noteworthy that waste sands with elevated LOI values may be utilized in subbase layers but are not recommended for structural concrete production. The LOI analysis conducted highlights the significance of quality control for raw materials, particularly waste materials intended for construction applications. Furthermore, it underscores the necessity for further investigations to optimize the parameters of waste sands so that they can serve as a potential substitute for natural aggregates.

The analysis of aqueous extracts for the SD and SC samples revealed very low levels of chemical contaminants, which were in compliance with the permissible standards outlined in the applicable legal regulations (

Table 4. Results of leachability tests for harmful substances, including heavy metals, in sands with a grain size fraction below 2 mm.

Substance Name	SD	SC	RS	Permissible Value According to [34]
pH, -	7.3 ± 0.1	6.9 ± 0.1	8.9 ± 0.1	6.5–9.0
Chlorides (Cl−), mg/L	69.12 ± 0.01	blq 1	blq 1	1000
Sulphates (SO42−), mg/L	44.29 ± 5.21	47.86 ± 5.21	2.88 ± 0.41	500
Total phosphorus (Ptot.), mg/L	0.17 ± 0.01	0.16 ± 0.01	0.44 ± 0.01	1
Potassium (K), mg/L	2.59 ± 0.01	1.03 ± 0.01	0.72 ± 0.01	80
Calcium (Ca), mg/L	15.85 ± 0.02	12.19 ± 0.02	5.15 ± 0.02	ng 2
Sodium (Na), mg/L	3.08 ± 0.01	1.27 ± 0.01	0.51 ± 0.01	800
Fluorides (F−), mg/L	0.40 ± 0.04	0.37 ± 0.04	0.13 ± 0.04	25
Barium (Ba), mg/L	blq 1	0.12 ± 0.01	0.15 ± 0.01	2
Total organic carbon (TOC), mg/L	18.63	2.17	1.92	30
Total carbon (TC), mg/L	25.30	7.78	9.29	ng 2
Inorganic carbon (IC), mg/L	6.67	5.61	7.37	ng 2
Zinc (Zn), mg/L	0.11	0.23	<0.01	2
Copper (Cu), mg/L	0.08	<0.02	<0.02	0.5
Lead (Pb), mg/L	<0.05	<0.05	<0.05	0.5
Cadmium (Cd), mg/L	0.02	<0.005	<0.005	ng 2
Chromium (Cr), mg/L	<0.05	<0.05	<0.05	0.1
Cobalt (Co), mg/L	<0.05	0.06	<0.05	1
Manganese (Mn), mg/L	<0.02	0.03	<0.02	ng 2
Iron (Fe), mg/L	<0.04	0.15	<0.04	10
Nickel (Ni), mg/L	0.11	<0.04	<0.04	0.5

¹ blq—values below the limit of quantification. ² ng—no guidelines.

). The pH values, chloride, sulfate, phosphorus, potassium, calcium, sodium, and fluoride concentrations in both analyzed samples were below the allowable limits. Furthermore, the concentrations of heavy metals such as zinc, copper, lead, cadmium, chromium, cobalt, manganese, iron, and nickel were below the detection limit. In conclusion, the results of the leaching study considering harmful substances from sands originating from wastewater treatment plants and with a fraction of ≤2 mm indicate that these substances do not pose a risk to human health or the natural environment.

As part of the next stage of the research, cement mortars were designed and produced. The composition of the mortars is presented in

Table 5. Composition of mortars, expressed in grams.

Type of Waste	CEM I	Water	Sand acc. PN-EN 196-1 [36]	Sand from Cleaning the Sewage System
Reference sand mortar	450	225	1350	0
Mortar with waste sand	450	225	0	1350

. For the reference mortar, portland cement CEM I, 42.5R and standardized sand were used-PN EN 196-1 [36]. To investigate the effect of waste sand on the properties of the mortar, the same portland cement CEM I 42.5R was used, but instead of standardized sand, sand from sewage cleaning was applied. The mortars were prepared according to the procedure described in the PN-EN 197-1:2012 standard [32], using an automatic centrifugal mixer.

Figure 11 shows cement mortars in which 100% of the reference aggregate has been replaced with sand derived from the sewer cleaning process. Mortars containing waste sand exhibit no swelling and maintain their original dimensions. No abnormalities were observed in terms of performance or material structure.

Figure 12 and Figure 13 present the results of compressive strength and flexural strength tests of cement mortars after 28 days of curing. The analysis of the obtained data demonstrates that the use of sand derived from sewer cleaning processes, as a complete replacement for standardized aggregate, leads to significant reductions in the mechanical properties of the mortar. After 28 days of maturation, the compressive strength of the mortar containing 100% waste sand was 35% lower, compared to the reference mortar made with standardized sand. Similarly, the flexural strength decreased by 38% relative to the reference values. The negative impact of sewer-cleaning-derived sand on the mechanical properties of cement mortars is consistent with findings reported in the literature [37,38,39,40,41]. The observed reduction in strength can be attributed to the presence of contaminants and the differing physical properties of the waste sand compared to standard aggregates. Factors such as increased porosity, reduced adhesion to the cement matrix, and the potential presence of organic particles and fine clay fractions may adversely affect the development of the cementitious composite’s microstructure, consequently reducing its mechanical resistance. The obtained results highlight the necessity for further research focused on the cleaning and modification of sand originating from sewer processes in order to improve its properties and enhance its potential for use as an aggregate replacement in construction materials.

The leaching potential of hazardous substances, particularly heavy metals, relative to crushed cement mortars was assessed by preparing aqueous extracts at a weight ratio of 1:10 (dry mass of crushed mortar to water mass).

Table 6. Results of tests on the leachability of pollutants from the tested mixtures.

Substance Name	Mortar with Waste Sand	Reference Sand Mortar	Permissible Value According to [34]
pH, -	10.4 ± 0.1	10.3 ± 0.1	6.5–9.0
Chlorides (Cl−), mg/L	23.03 ± 0.01	34.56 ± 0.01	1000
Sulphates (SO42−), mg/L	1.44 ± 0.29	13.78 ± 0.87	500
Total phosphorus (Ptot.). mg/L	0.44 ± 0.01	0.93 ± 0.01	1
Potassium (K), mg/L	14.01 ± 0.12	1.89 ± 0.01	80
Calcium (Ca), mg/L	7.31 ± 0.10	8.72 ± 0.02	ng 1
Sodium (Na), mg/L	3.36 ± 0.01	1.89 ± 0.01	800
Fluorides (F−), mg/L	0.51 ± 0.02	0.61 ± 0.03	25
Barium (Ba), mg/L	4.43 ± 0.06	0.36 ± 0.06	2
Total organic carbon (TOC), mg/L	2.29	2.85	30
Total carbon (TC), mg/L	7.42	6.29	ng 1
Inorganic carbon (IC), mg/L	5.12	3.44	ng 1
Zinc (Zn), mg/L	<0.01	<0.01	2
Copper (Cu), mg/L	0.03	0.03	0.5
Lead (Pb), mg/L	0.05	<0.05	0.5
Cadmium (Cd), mg/L	<0.005	<0.005	ng 1
Chromium (Cr), mg/L	<0.05	<0.05	0.1
Cobalt (Co), mg/L	<0.05	<0.05	1
Manganese (Mn), mg/L	<0.02	<0.02	ng 1
Iron (Fe), mg/L	<0.04	<0.04	10
Nickel (Ni), mg/L	<0.04	<0.04	0.5

¹ ng—no guidelines.

presents the concentrations of selected contaminants in aqueous extracts obtained from fragmented cement mortars after 28 days of curing. The results were compared with permissible limits for the discharge of wastewater into water bodies or soil [34]. Notably, the concentrations of hazardous substances, including heavy metals, generally remained within the acceptable regulatory limits, often significantly below the threshold values. However, in one case—specifically for the mortar containing waste sand—the concentration of barium (Ba) exceeded the permissible value by a factor of two. This finding raises concerns regarding its long-term environmental impact, particularly in the context of bioaccumulation within ecosystems. The results suggest that mortars incorporating sand recovered from sewer cleaning processes may be relatively safe for aquatic environments, provided that appropriate monitoring and control measures are implemented throughout production and application.

4. Conclusions

The concept of a circular economy applies to all economic sectors within EU member states, including the water and wastewater sectors. Wastewater treatment plants can be regarded not only as infrastructure elements but also as potential sources of secondary raw materials. In this context, the reuse of waste generated during the treatment process contributes to reducing both natural resource consumption and the volume of waste sent to landfills. Research has demonstrated that sand recovered from sewer cleaning processes shows potential for use in the construction industry. Despite its lower levels of mechanical performance, its incorporation into cementitious mortars supports sustainable development and the circular economy by providing a substitute for natural aggregates in non-structural applications. However, for more demanding uses, further enhancement of its material properties is required.

The following eight key scientific conclusions were drawn based on the comprehensive physicochemical, environmental, and mechanical assessments of sand materials derived from wastewater treatment processes:

• The loss on ignition (LOI) analysis at 950 °C revealed that sample SD exceeds the recommended threshold for organic matter content in construction applications (4.18%), which may significantly limit its suitability for structural concrete production. In contrast, sample SC meets the required quality criteria (1.35%), indicating its potential applicability in building materials.
• Visual evaluation of the humic substance content showed a significantly higher concentration of humic compounds in the SD sample compared to the SC sample. A high content of humic substances may affect the physicochemical properties and long--term stability of the building material under its operating conditions. Additionally, humic substances can form water-repellent films on sand-grain surfaces, which may disrupt the interaction between the cementitious matrix and the aggregate, thereby reducing the quality of the interfacial transition zone.
• The waste sands examined that originated from wastewater treatment plants (SD and SC) comply with the current legal regulations establishing permissible levels of harmful substances, including heavy metals. This confirms their potential suitability as secondary raw materials from an environmental protection perspective.
• Both analyzed samples exhibited a high content of fine fractions (<2 mm). However, the SC sample is characterized by a noticeably greater proportion of very fine (<0.063 mm) and fine (<2 mm) particles, which may indicate a higher degree of washing and a generally finer material composition. In contrast, the SD sample contains a higher percentage of particles within the 0.125–0.25 mm range, which may significantly influence its filtration properties. The observed differences in grain size distribution between the samples suggest distinct functional characteristics that are relevant in their potential engineering applications.
• The substitution for natural sand with waste sand derived from sewage cleaning processes in cement mortar formulations results in a significant reduction in mechanical performance. Specifically, declines of approximately 35% in compressive strength and 38% in flexural strength have been observed. This deterioration can be attributed to several factors, including the presence of organic and inorganic contaminants commonly found in sewage-derived sand. Moreover, the irregular morphology and suboptimal granulometric distribution of the particles may adversely affect the compaction behavior of the mix and hinder the bond development between the cement paste and the aggregate. To mitigate these negative effects and enhance the suitability of this secondary raw material, appropriate pretreatment measures should be considered prior to its incorporation into mortar mixtures.
• Despite its lower levels of mechanical performance, waste sand may still be considered for use in sub-base layers and other non-structural applications. Such an approach aligns with the principles of the circular economy and contributes to the reduction of natural aggregate exploitation.

The prospects and challenges for research in the coming years may encompass the following areas of activity:

• Modification of recycled concrete composition: Further research is required on the application of specialized chemical additives to enhance the mechanical properties of concrete in which natural aggregate has been entirely replaced with waste materials. Particular attention should be given to improving compressive strength and durability, while considering the long-term operational effects.
• Optimization of wastewater treatment plant sand content in recycled aggregate: Another potential research direction involves determining the optimal proportion of sand derived from wastewater treatment plants in concrete mixtures, with particular consideration of the compositional variability resulting from the diverse sources. Analyses should focus on the impacts of chemical and granulometric composition on the mechanical properties and durability of concrete.
• Innovative uses of waste sands in next-generation concrete: It would be worthwhile to simultaneously conduct research on the possibility of using sands from sewage treatment plants in innovative building materials, such as geopolymers or self-healing concrete. In the context of sustainable development, it is particularly important to determine the impacts of these materials on the reduction of CO₂ emissions and with reference to improvements in durability and resistance to environmental factors.

Conducting multi-faceted research in the areas mentioned above may contribute to the development of eco-friendly building materials in line with the concept of a circular economy.