Characterizing and Assessing Drinking Water Treatment Plant Sludges as a Soil Stabilizer in Trabzon, Türkiye

doi:10.21203/rs.3.rs-4774412/v1

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Characterizing and Assessing Drinking Water Treatment Plant Sludges as a Soil Stabilizer in Trabzon, Türkiye

https://doi.org/10.21203/rs.3.rs-4774412/v1

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Resources used with the increasing population and developing industry and related wastes are also increasing rapidly. The conscious use of the environment and natural resources for future generations, and the waste products resulting from the resources used should be evaluated with the most economical and environmentally friendly attitude without harming nature. Considering factors such as the country's economy, storage cost, efficient use of resources and most importantly environmental and public health, evaluating these wastes is quite important. Today, many studies have been carried out to assess these wastes with an economic and ecological attitude. Determining the characterization of waste is the most fundamental step to be taken for the selection of the waste evaluation method. Therefore, the most accurate evaluation method is decided by determining the characterization of the waste material to be used. In this study, it was aimed to determine the characterization of waste sludges from some of the drinking water treatment plants in Trabzon, Türkiye and to evaluate them in terms of ground stabilization. In this context, the waste sludge samples from the Esiroğlu, Akçaabat and Derecik Drinking Water Treatment Plants with traditional treatment systems were taken from the plants in slurry form and were first dewatered. Subsequently, a series of analyses and experimental studies on dewatered waste samples were carried out and characterizations of waste sludges were determined based on their mineralogical, chemical, physical and mechanical properties. As the material properties of the treatment sludges in the drinking water treatment plants were evaluated, the WS-1 was found to the most suitable material that could be used in the stabilization of soils. Therefore, the use of this sludge will be both ecological and economical.

Drinking water treatment plant

Waste sludge

Sludge characterization

Sta0bilization of soils

Drinking Water Treatment Plant Treatment Sludge (DWTPS) is a waste sludge that consists of 1–3% by volume of the treated raw water by going through some treatment processes called ozonation (O₃), coagulation, flocculation, precipitation, filtration and disinfection (chlorination) in drinking water treatment plants (Chiavola et al. 2023). This waste sludge, which occurs in huge quantities and as a by-product, is slurry sludge containing organic matter and chemical compounds such as macro and micronutrient elements from the sedimentation tank and filter backwash. Treatment sludges containing high amounts of Al₂O₃, SiO₂, Fe₂O₃ and CaO have pozzolanic properties and are binding depending on their chemical content. Treatment sludge is also called alum sludge depending on the type of coagulant (Dayton & Basta 2001; Üçüncü & Gülay 2003; BAG 2010; Hidalgo et al. 2017; Nawagamuwa & Wijesooriya 2018; Balkaya 2019; Wu et al. 2020; Baǧrıaçık & Güner 2020; González et al. 2020; Güner & Baǧrıaçık 2021).

DWTPS can be harmful when discharged to receiving environments such as streams, lakes, seas and soil (González et al. 2020). For this reason, it is of great importance to evaluate DWTPS most beneficial way without harming the environment. The selection of the most appropriate methods of evaluation of waste to avoid harm to the environment and human health can only be achieved by determining the characterization of the waste materials to be used. Therefore, waste characterization is of great importance in the evaluation of waste (Miri et al. 2022; Chan & Wang 2016; Aydın et al. 2020).

In parallel with the population growth and the development of industry, both the amount of wastes increase and changes occur in their characterization. The report published by the United Nations Population Fund (UNFPA) predicts that the world will reach 8.5 billion in 2030, 9.7 billion in 2050 and 10.4 billion in 2100 (URL-1 2023). The population increased by approximately 19.96% according to the data of TÜİK in Türkiye between 2007–2021 (Fig. 1). The population of Trabzon, where the waste materials to be used in this study were taken, increased by 10.28% between 2007 and 2021 (Fig. 1). It is estimated that approximately 10000 tons of treatment sludge per day emerge from drinking water treatment plants (Güner & Baǧrıaçık 2021; Babatunde & Zhao 2007; Dharmappa 1997). Although 3/4 of the world is covered with water, only 3% of the world's population can access usable water (Gökçe et al., 2018). Given the increase in population rates, it is likely that problems such as reduced amount of clean water will be encountered in addition to the problem of access to clean water resources. In particular, the need for water highlights the importance of different solution methods for problems such as discharge of increased waste to receiving environments and water pollution and disposal of stored waste.

There are many studies in the literature on the areas of use of drinking water treatment sludges. DWTPS has been used in many different areas such as investigation of various useful uses (Chang 2022; Balkaya 2019; Babatunde & Zhao 2007; Arslan & Atakol 2005; Sun et al. 2015; Laib et al. 2022; Kizinievič 2018; Foroughi 2018; Dahhou 2018; Yan et al. 2018; Barakwan 2020; Güner 2022), use in the improvement of soils (Güner & Baǧrıaçık 2021; Baǧrıaçık & Güner 2020; Nawagamuwa & Wijesooriya 2018; Baǧrıaçık & Güner 2020; Caniani et al. 2013; Ahmad et al. 2016), evaluation of its use in agriculture ( Dayton & Basta, 2001; Uzun & Bilgili, 2011), use as a treatment material (Kalat et al. 2003; Ünlü 2003; Kang et al. 2022; Ketelaars et al. 2023), analysis of the parameters produced in drinking water treatment plants that can lead to a new waste situation (Ketelaars et al. 2023; Zhang et al. 2023; Wu et al. 2020; Hidalgo et al. 2017), examining the impact on different materials for the purpose of producing construction materials (González et al. 2020; Pham et al. 2021; Liu et al. 2020; Rodríguez et al. 2010). The most effective material for the stabilization of clayey soils is lime. This is due to the presence of CaO, which is necessary for the occurrence of pozzolanic reactions (Sivrikaya 2014). It seems likely that DWTPS can be used in the stabilization of clayey soils as a waste to be generated by pozzolanic reactions depending on the CaO compound content.

The current study aims to determine the mineralogical, chemical, physical and mechanical characterizations of the treatment sludges formed in the drinking water treatment plants in the regions of Esiroğlu, Akçaabat and Derecik in Trabzon, which is located on the east of the Black Sea Region in Türkiye, and to evaluate their suitability in terms of soil stabilization. It has been revealed that the treatment sludges in drinking water plants in different locations have a different characterization and it is concluded that it is very important to know their characterization in the evaluation of these wastes. In addition, within the scope of this study, the usability of the waste sludges from three different plants in soil stabilization was evaluated depending on their characterization.

2.1 Topographic Features

With an area of 4664 km², Trabzon is an important residential area located in the east of the Black Sea Region (Fig. 2). While approximately 30% of this area is mountainous and a small part of 10% is flat, the remaining 60% has variable topographic areas that increase with a slope of 25–30% to the south (TESR 2021). In Trabzon, which has 18 districts in total, mountains, hills and plateaus cover large areas in the interior except for the coastline. Trabzon, which consists of mountains extending parallel to the coast between the İkizdere stream to the east and the Harşit stream to the west, is located in the northern part of the multi-terrain platform, which is approximately 300 km long. The southern part of this platform is bounded by the Zigana and Soğanlı mountains, which form the main part of the Eastern Black Sea Mountains.

2.1 Geological Features

The study area geographically located in the eastern part of the Black Sea Region (Fig. 2) is geotectonically positioned in the middle part of the Eastern Pontides (Ketin 1966). According to geological and lithological features, the Eastern Pontides are divided into the “Northern Zone” and “Southern Zone” (Fig. 3). The geology of the study area within the borders of Trabzon city, which is located in the northern zone of the Eastern Pontides, shows lithological features similar to those of the southern zone until the Late Cretaceous period. After the Late Cretaceous, sedimentary rocks are common in the southern zone, while igneous rocks (plutonic and volcanic) become dominant lithologies in the northern zone (Özsayar et al. 1981; Bektaş et al. 1999).

Paleozoic metamorphic and granitic rocks cropping out in limited areas form the oldest basement rocks of the region (Okay & Tüysüz 1999). Liassic (Lower Jurassic) volcano-sedimentary rocks that unconformably overlie the basement units in the region (Hamurkesen Formation; (Ağar 1977), are generally overlain by Upper Jurassic-Lower Cretaceous platform carbonates (Berdiga Formation; (Pelin 1977) (Fig. 4). A thick Upper Cretaceous volcano-sedimentary succession formed by subduction-related processes conformably overlies these carbonates (Aydin et al. 2020; Oğuz-Saka 2023). This succession starts with Turonian-Santonian aged sedimentary rocks (sandstone, siltstone and claystone) interbedded with basalt and basaltic andesites (Çatak Formation) (Aydin et al. 2020; Güven 1993; Güven 1998; Kurt et al. 2006).

Santonian aged dacite, rhyodacite, tuff and breccias (Kızılkaya Formation) hosting volcanic massive sulphide mineralizations in the region conformably overlies the Çatak Formation (Fig. 4) and this unit is followed by the Late Santonian-Early Campanian aged Çağlayan Formation, which is composed of basaltic andesite, tuff and breccia interbedded with sedimentary rocks, and the Campanian aged Çayırbağ Formation, which is also composed of rhyolite, rhyodacite tuff and breccia (Oğuz-Saka et al. 2023; Güven 1993; Güven 1998).

These units are covered by calciturbiditic sedimentary rocks (Kireçhane Formation) consisting of alternating clayey limestone, marl, shale, sandstone, and tuff layers (Güven 1993; Güven 1998). Upper Cretaceous aged granitic intrusions (Kaçkar Granitoids) occasionally intruded into the Jurassic and Cretaceous units (Karslı et al. 2010; Aydin 2014). The Upper Cretaceous formations are unconformably overlain by the Middle Eocene-Miocene aged Kabaköy (sandstone and limestone interbedded basalt, andesite, tuff and breccia) and Pliocene aged Beşirli (mudstone, sandstone and conglomerate) formations, respectively (Güven 1993; Güven 1998; Aydin et al. 2008). Quaternary-aged terraces and alluviums that cover all these units unconformably form the youngest lithologies of the region (Fig. 4).

Bentonite and illite-bearing clay-rich soils are occasionally observed in the lithological units of the Late Cretaceous aged Çağlayan Formation, which contains WS-1 and WS-3 waste sludge samples. However, reddish-brown kaoline-bearing soils and saprolites (i.e., clay-rich material formed by the extensive in situ chemical weathering of rock) are more common in the volcano-sedimentary units of the Eocene-aged Kabaköy Formation including the WS-2 waste sludge sample. Such residual deposits, typical of humid and tropical to subtropical climates, are characterized by a range of soil features such as high matrix porosity, neoformed clay minerals, Fe/Mn oxides, and marked bioturbation (Driese et al. 2001).

2.3 Climate and Meteorology

Since the temperate climate type prevails in Trabzon, which is under the influence of the sea, the summers are usually at medium temperature and the winters are warm and rainy. The fact that the region is open to depressions in the northwest direction causes the climate elements to change continuously. Due to the location of Trabzon, the winter offers a separate feature from other places in Türkiye. The Caucasus Mountains surround Trabzon from the south and shield it from the cold winds of the northwest. In addition, it prevents the cold air of Siberia and the air cooling in the North East Anatolian plateaus from entering the region. When looking at the monthly average precipitation amounts, it is seen that August and November are dry (TESR 2021).

The data obtained from the temperature measurements made in Trabzon between 1927 and 2021 are given in Fig. 5. According to the data, the months with the highest average temperature are May and August, and the lowest are January and February. The average temperature measured during the year is 14.8°C, the highest temperature is 38.2°C, and the lowest temperature is -7.4°C (GDM 2023). The dominant wind direction in Trabzon throughout the year is south, but it varies according to the months. The most precipitation in Trabzon falls in October (Fig. 6). The month with the highest average number of snowy days was determined as January (GDM 2023).

There are a total of 14 drinking water treatment plants in Trabzon, including Akçaabat, Akpınar, Aykut, Cumapazarı, Çamburnu, Çal, Derecik-Akçaköy, Esiroğlu, Karakaya, Şalpazarı, Tonya-Center, Vakfıkebir-Büyük Liman, Yomra and Yavuzköy Drinking Water Treatment Plants (Fig. 7). These plants use three different treatment systems: “Pressure Package”, “Traditional” and “Semi-Traditional/Semi-Pressure Package” treatment (Table 1). Package treatment systems are systems that can be remotely controlled (SCADA) as a solution to problems such as the excess of treatment elements and the distance of the installation place, and do not require human intervention. Therefore, it is very difficult to obtain waste sludge from plants with these systems. However, in traditional treatment plants, it is possible to supply sludge as waste sludge accumulates at certain points. For this reason, the sludge samples to be used in this study were taken from the traditional treatment plants (Akçaabat, Derecik-Akçaköy and Esiroğlu) which are indicated with the sign "*" in Fig. 7.

Table 1

Types of drinking water treatment plants in Trabzon
Pressure Package	Traditional	Semi-Traditional (Semi-Pressure Package)
o Akpınar	* Akçaabat	o Cumapazarı
o Aykut	* Derecik-Akçaköy	o Karakaya
o Çamburnu	* Esiroğlu (Atasu)	o Şalpazarı
o Çal	o Vakfıkebir	o Yavuzköy
o Tonya Merkez
o Yomra

a. Esiroğlu Drinking Water Treatment Plant

The Esiroğlu drinking water treatment plant is fed from Atasu (Kalyan) dam built on Kalyan Creek (Fig. 8b), a branch of Değirmendere (Fig. 8a), 17 km south of Trabzon. This plant, which has been operating in the Eastern Black Sea basin since 1992, was built to carry out traditional treatment, and it serves Ortahisar, Akçaabat, Maçka and Yomra counties of Trabzon. The maximum design flow of the plant is 1900 L/s and approximately 53 000 000 m³ of water is produced annually at the plant (GDTDA 2023). The waste sludge can be obtained from the sludge cones in the clarifier of the plant. The treatment plant consists of plant entrance, ozonation unit, ventilation unit, bypass system, chemical dosing (Aluminum Sulphate), sulfuric acid dosing (H₂SO₄), coagulation accelerator dosing (Polyelectrolyte), clarification, filtration, lime dosing, chlorination, sampling system, service water system, and maintenance units (Çalık 2011).

b. Derecik-Akçaköy Drinking Water Treatment Plant

The Derecik-Akçaköy drinking water treatment plant, which has been in operation since 2013, is fed from Uçarsu Stream (Fig. 8c). The plant, which serves approximately 11 neighborhoods in the Akçaabat counties in the Eastern Black Sea basin, was built to carry out traditional treatment. The maximum design flow of the plant is 95 L/s, and approximately 1, 500 000 m³ of water is produced annually at the plant (GDTDA 2023). The treatment plant consists of a plant entrance, ventilation unit and bypass system, chemical dosing (Aluminum Sulphate), sulfuric acid dosing (H₂SO₄), coagulation accelerator dosing (Polyelectrolyte), clarification, filtration, lime dosing, chlorination, sampling system, service water system and maintenance units (Çalık 2011).

c. Akçaabat Drinking Water Treatment Plant

The water source of the Akçaabat-Seyrantepe drinking water treatment plant, which has been operating since 2018, is built on the Işıklar Creek (Fig. 8d). The plant, which serves approximately 14 neighborhoods in the Akçaabat counties in the Eastern Black Sea basin, was built to carry out conventional treatment. The maximum design flow of the plant is 200 L/s, and approximately 5 250 000 m³ of water is produced annually at the plant (GDTDA 2023). The treatment plant consists of a plant entrance, ventilation unit, and bypass system, chemical dosing (Aluminum Sulphate), sulfuric acid dosing (H₂SO₄), coagulation accelerator dosing (Polyelectrolyte), clarification, filtration, lime dosing, chlorination, sampling system, service water system and maintenance units (Çalık 2011).

Chemicals such as Ozone (O₃), Aluminum Sulphate (Al₂(SO₄)₃), Sulfuric Acid (H₂SO₄), Calcium Hydroxide (Ca(OH)₂), Lime (CaO), Potassium Permanganate (KMnO₄), Polyelectrolyte, Iron Sulphate (FeSO₄), Calcium Carbonate (CaCO₃) are generally used to treat the incoming water in all plants where waste sludge samples were taken.

The most important factor in the waste evaluation process is the determination of the waste characterization. The most appropriate evaluation method of wastes in terms of environment and human health is possible by knowing their physical, chemical, mechanical and mineralogical characteristics (Sivrikaya et al. 2020; Yorulmaz et al. 2021). For this reason, the determination of the characterization of the waste sludge samples from the laboratory studies will ensure the waste to be evaluated from an economic and ecological perspective. The characterization of the wastes is a decisive factor in the selection of possible solution methods for problematic soils, especially soils with high swelling potential such as high plasticity clay.

In this sense, the waste sludge samples were obtained from Esiroğlu, Akçaabat and Derecik-Akçaköy Türkiye drinking water treatment plants, which have traditional treatment systems, where it is possible to take samples to determine the characterization of drinking water waste sludges in Trabzon. The samples of Esiroğlu DWTPS (WS-1) were obtained from the manhole given to Değirmendere by channel after the treatment sludge comes out under the flocculation and sedimentation unit (Fig. 9a). The samples of Akçaabat DWTPS (WS-2) were obtained from the sludge holding tank (wet) (Fig. 9b, c) and some from the Filterpress (dry state) (Fig. 9d). The samples of Derecik-Akçaköy DWTPS (WS-3) were obtained from the filter water holding pool (Fig. 9e, f).

Slurry sludge materials taken from each plant using bags and buckets were brought to Karadeniz Technical University Geotechnical Laboratory and dewatered. The samples, which lost some water in the open air, were kept in an oven at 105^oC for at least 48 hours, were completely dewatered, and made ready for the experiments. The chemical components of the waste sludge materials were determined by XRF analyses carried out at Karadeniz Technical University, and the mineralogical components were determined by XRD analyses carried out at Niğde Ömer Halisdemir University. When the mineralogical properties of the waste sludges were examined while mainly albite (39.5%), quartz (32.6%) and muscovite (27.8%) minerals are dominant in the WS-1 sample (Esiroğlu) (Fig. 10a), especially quartz (34.2%), amesite (32.9%), albite (17.3%) and kaolin (15.4%) minerals are more common in the WS-2 sample (Akçaabat) (Fig. 10b). On the other hand, kaolin (65.9%) is the most common mineral in the WS-3 sample (Derecik-Akçaköy), accompanied by albite (21%) and quartz (13.1%) (Fig. 10c).

The chemical contents of the waste sludges in the treatment plants are given in Table 2 in detail. Compared to the compositions of the WS-2 and WS samples, the Al₂O₃ (19.4%) and CaO (14.16%) contents of the SW-1 sample are significantly higher, while the SiO₂ (45.77%) content is quite low. In addition, Fe₂O₃, MgO, K₂O and TiO₂ contents of the WS-1 sample are also relatively high (Table 2). When the mineralogical (albite + quartz + muscovite) and chemical composition of the WS-1 sample are evaluated together, it can be said that especially the high CaO content is not associated with the mineral association of the waste. However, this high CaO content is probably due to either the lithological units in the sludge feed area (Ca-rich basaltic rocks of the Çağlayan Formation and Ca-rich carbonate rocks such as intercalated limestone-marl) or the chemicals used to treat the water (e.g., Calcium Hydroxide (Ca(OH)₂), Lime (CaO) and Calcium Carbonate (CaCO₃)). On the other hand, SiO₂, Fe₂O₃, MgO, K₂O, T_iO₂, P₂O₅ and MnO contents of the WS-1 and WS-3 samples are also similar. This can be attributed to the feeding of these two wastes from a similar lithological source (basic composition rocks that are poor in Si and contain minerals rich in Ca + Mg + Fe + Ti). Unlike other samples, the relatively high SiO₂ and SO₃ and low Fe₂O₃, CaO, MgO, K₂O, Na₂O and TiO₂ contents of the WS-2 waste (Table 2) are largely lithological (andesitic-felsic rocks of the Kabaköy Formation and tuff and breccias intercalated with them) controlled.

Table 2

The chemical compounds and contents of the WS-1, WS-2 and WS-3 materials
	SiO₂	Al₂O₃	Fe₂O₃	CaO	MgO	K₂O	TiO₂	P₂O₅	SO₃	MnO	Na₂O	LOI	Total
WS-1	45.77	19.40	5.79	14.16	2.37	1.96	0.72	0.24	0.05	0.16	0.59	8.69	99.90
WS-2	64.14	14.44	0.34	0.08	0.03	1.21	0.29	0.08	2.89	0.001	0.03	16.37	99.90
WS-3	50.47	15.24	5.68	4.76	2.05	1.86	0.65	0.18	0.20	0.14	1.47	17.15	99.85

Chemical contents such as Ca(OH)₂, CaO and CaCO₃, which are used in the treatment of drinking water, react with clayey soils in a pozzolanic reaction and stabilize the soils. The high CaO content (14.16%) especially in the WS-1 sample taken from the Esiroğlu drinking water treatment plant clearly shows that this waste has a higher pozzolanic property than the others, and therefore can be used for stabilization of soils. As it is known, the presence of CaO is very important for the formation of puzolanic reactions in the stabilization of soils. The pozzolanic component reacts with the clayey soil, providing binding between the soil's grains, reducing the plasticity of the soil and contributing to the stabilization of the soil. It is also known that the Al₂O₃ compound increases the strength of the clay soil and causes a more rigid structure to occur (Güner and Baǧrıaçık 2021). In this context, the high Al₂O₃ content together with CaO content of the WS-1 material compared to WS-2 and WS-3 can be used qualification in stabilization in terms of forming a rigid structure. The chemical substances used in the plants also have an impact on waste sludge. Chemical substances such as Ca(OH)₂, CaO, and CaCO₃, which are added to the water at various stages within the facility to purify the raw (unprocessed) water in the plants, can cause the waste sludge to show puzzolanic properties. However, this effect may vary depending on the amount of chemical substance given to the water and the temperature of the raw water entering the plant.

The grain size distribution analyses were performed on waste sludge samples by Laser Diffraction Method and the test results are given in Fig. 11. It is clearly seen from Fig. 11 that the WS-1 and WS-2 samples have a similar distribution curve while the WS-3 waste sample shows a different distribution curve. The grain size diameters of the WS-1 and WS-2 samples are finer-grained than the WS-3 sample.

In addition, specific gravity tests (ASTM D 854 2013), liquid limit (Casagrande) tests, plastic limit tests (ASTM D 4318 2013), Standard Proctor tests (ASTM D 698-07 2013), Unconfined Compression tests (ASTM D 2166 2000), California Bearing Ratio (CBR) tests (ASTM D4429-04 2010) and consolidation tests (ASTM D 2435-03 2017) were performed on the waste sludge samples in this study. The shrinkage limit values were determined by indirect methods from the plasticity chart (Holtz & Kovacs 1981). According to the Unified Soil Classification System (USCS), the soil classifications of the wastes were determined as low plasticity silt (ML) for the WS-1 and WS-2, and silty sand (SM) for the WS-3. It is considered that the soil class of the AC-3 sample, which is a fine-grained soil, was determined as silty sand (SM) and the grain diameter distribution curve was in a wide range, resulting from the agglomeration of the waste in the size of sand. The tests carried out in the laboratory were repeated at least 3 times and the averages of the closest values were taken. The results obtained from the tests are given in Table 3.

Table 3

The physical and mechanical properties of the WS-1, WS-2 and WS-3 samples
	WS-1	WS-2	WS-3
w_L (%)	45	42	62
w_p (%)	35	37	57
w_s (%)	30	35	50
I_p (%)	10	5	5
USCS	ML	ML	SM
G_s	2.2	2.09	2.25
γ_dmax (kN/m³)	14.3	14.0	11.6
w_opt (%)	26	28	42
q_u (kPa)	325	175	133
C_c	0.04	0.04	0.06
C_r	0.01	0.01	0.02
k (m/s)	12.64*10^− 8	4.16*10^− 8	2.1*10^− 8
Swelling Pressure (kPa)	16.3	13.2	30.8

According to the consistency limit test and analysis results, liquid limit (w_L) values of the WS-1 and WS-2 samples are between 40% and 45%, plastic limit (w_p) values are between 35% and 40%, and shrinkage limit (w_s) values are between 30% and 35%. It is seen that the consistency limits of the WS-3 sample are above the others. The low consistency limit values of the WS-1 and WS-2 materials show that these samples tend to compress less. This shows that the displacement and settlement on the soil are low and their stability and bearing capacity are high. Thus, it indicates a positive situation in terms of stabilization. The plasticity index (I_p) values of the materials vary between 5% and 10%. It also shows that they are usable in terms of stabilization.

Compaction parameters in the stabilization of soils play an important role in determining the compressibility, bearing capacity and stability of the soil. The compaction curves of the wastes were obtained by performing the Standard Proctor tests on the waste sludge samples and they are shown in Fig. 12. The compaction parameters (w_opt, γ_dmax) were determined as w_opt=26% and γ_dmax = 14.3 kN/m³ for the WS-1 samples, w_opt=28% and γ_dmax = 14 kN/m³ for the WS-2 samples and w_opt=42% of γ_dmax = 11.6 kN/m³ for the WS-3 samples. In stabilization studies, the high dry unit weight of the soil is seen as an important parameter that increases the bearing capacity and stability. The WS-1 and WS-2 samples have similar compaction curves and close compaction parameters while the WS-3 sample has quite different compaction curves and unsuitable compaction parameters with low dry unit weight (Fig. 12). Accordingly, while the WS-1 and WS-2 materials show good compaction properties due to high unit weight and low water content, the compaction properties of the WS-3 material are very poor due to low unit weight and high water content. Therefore, it can be said that the WS-3 material is not suitable for soil stabilization.

The samples used for the unconfined compression tests were prepared in a specially manufactured mold with a diameter of 50 mm and a length of 100 mm. The mold consists of a slotted cylindrical steel tube (Fig. 13a), lower and upper ring (Fig. 13b), cap (Fig. 13c), settling tray (Fig. 13d), and a ram (Fig. 13e). This sample mold is slit cylindrical and has been manufactured to be detachable for easy removal of samples. The rings at the top and bottom of the cylinder provide the assembly of the mold.

The waste sludge samples prepared at the optimum water content (w_opt) were compressed in the mold using a hammer. The samples that can be easily removed due to the detachable semi-cylindrical mold are loaded axially with a speed of 1 mm/min and the tests were carried out by taking vertical displacement and load readings. The variation of unconfined compressive strength (q_u) versus axial deformation (ε) for the waste materials is shown in Fig. 14. In terms of stabilization of soils, low unconfined compressive strength is a condition that increases the tendency to settle in structures and reduces the bearing capacity. Accordingly, it was determined that the strength values of the WS-2 and WS-3 materials were close to each other while the strength value of the WS-1 material had a higher value. In this case, it seems that it is more appropriate to use the WS-1 waste material for the stabilization of soils.

The one-dimensional consolidation (oedometer) test was carried out in the Geotechnical Laboratory of Karadeniz Technical University (Fig. 15). The diameter of the ring in the oedometer test is 50 mm and its height is 19 mm. The values obtained from the samples subjected to loading and unloading cycles at various stages are given in Table 3 and the test results are given in Fig. 16. The compression (C_c) and recompression (C_r) indexes, swelling pressure, and swelling percentage values for the waste sludge samples were found directly in this study and the permeability coefficient (k) was determined indirectly using the oedometer test results (Table 3).

Compression and recompression indexes (C_c and C_r) are the slope of the e-logσ_v’ curve obtained from the oedometer test and are important parameters used to predict the consolidation settlement that may occur in fine-grained soils. The C_c values were determined as 0.04 for the WS-1, 0.04 for the WS-2, and 0.06 for the WS-3. The C_r values were determined as 0.01 for the WS-1, 0.01 for the WS-2, and 0.02 for the WS-3. They are given in Table 3. A soil with a low compression index and recompression tends to settle less under load. This increases the stability of the soil on which superstructures and infrastructures are placed and allows structures to undergo less deformation. Accordingly, it is expected that the WS-1 and WS-2 have low consolidation settlements under loads in comparison with threWS-3. Moreover, the swelling pressure is found to be 16.3 kPa for the WS-1, 13.2 kPa for the WS-2, and 30.8 kPa for the WS-3, and the swelling percentage values are found to be 0.49% for the WS-1, 0.34% for the WS-2 and 0.99% for the WS-3. Soil with low swelling pressure and percent swelling values is less reactive to water and has less swelling potential. The values obtained from the tests show that the swelling potential of the waste sludge samples is quite low and expresses the suitability of their use in the stabilization of soils.

The permeability coefficient is an important parameter in geotechnical engineering in terms of the effects of water, which affects the engineering behavior of most soils, especially fine-grained soils. In The permeability coefficients (k) were found indirectly from the oedometer test results. Accordingly, the average k values were found as 12.64*10^− 8 m/s for the WS-1, 4.16*10^− 8 m/s for the WS-2 and 2.1*10^− 8 m/s for the WS-3. The values in the order of 10^− 8-10^− 7 m/s are quite low in terms of permeability and show that the permeability properties of the waste sludge are low. Low permeability restricts the movement of water and harmful toxic substances.

Determining the characteristic properties of a waste material is an important issue. In particular, the parameters obtained for the characterization of waste materials as a result of experimental studies provide important concrete data to the literature in terms of many studies on the evaluation of waste. Within the scope of this study, it was aimed to determine the characterization of waste sludge belonging to drinking water treatment plants, which pose a significant environmental problem and to investigate their usability in soil stabilization. The drinking water treatment plants, which are the subject of the experimental study, are in Trabzon, Türkiye. The waste sludge samples were taken from Esiroğlu, Akçaabat and Derecik drinking water treatment plants, which have traditional treatment systems and the parameters of mineralogical, chemical, physical and mechanical properties were obtained in order to determine their characterization. The characterizations of the waste sludge belonging to three different drinking water treatment plants in Trabzon, Türkiye were determined, and the reasons for the characterization differences and their usability in soil stabilization were evaluated. The following conclusions were drawn from the current study:

Although the mineral types (albite, quartz and kaolin-dominated) of the wastes (the WS-1, WS-2 and WS-3) used in this study are similar to each other, the differences in mineral ratios and chemical compositions reflect their characterization.
The chemical composition of the waste sludge in the treatment plant is affected by the lithological units in the region and/or the chemical substances used in the treatment.
The WS-1 material, which has puzzolanic properties due to its high CaO content, can be used for stabilization of clayey soils.
The WS-1 and WS-2 materials, which have lower consistency values than the WS-3 material, can be used for stabilization of soils. In particular, their low swelling potential indicates that they can be used in the improvement of problematic soils.
The WS-1 and WS-2 materials have very good parameters in terms of compaction, and especially the values of WS-1 are satisfactory in terms of use in stabilization of soils.
The unconfined compressive strength (q_u) value of the WS-1 material is quite high and can be used in the improvement of soils.
Compression and recompression indexes are quite reasonable and especially the low swelling potential indicates the usability of waste sludge materials in terms of soil stabilization. Moreover, the low permeability coefficients further strengthen this expression.
It is of great importance to evaluate this type of waste material in terms of economy, environment and human health, especially in terms of soil stabilization.

Competing Interests

The authors have no competing interests to declare that are relevant to the content of this article.

Funding

This study is supported by Karadeniz Technical University Scientific Research Projects Unit. This paper is partly benefited from the BAP06-FDK-2022/10047 project titled "Investigation of the Usability of Trabzon Region Drinking Water Treatment Plant Sludges in Ground Stabilization". The authors would like to thank Karadeniz Technical University Scientific Research Projects Unit.

Author Contribution

K.A., O.S. and O.Ü. carried out the collection of samples from the field, bringing them to the laboratory and preparing the samples for the experimental work.K.A .performed the laboratory tests.K.A., O.S., O.Ü. and F.A. prepared the manuscript together according to their areas of expertise.

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Characterizing and Assessing Drinking Water Treatment Plant Sludges as a Soil Stabilizer in Trabzon, Türkiye

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1 Introduction

2 Topographic, Geological, Climate And Meteorological Features Of Investigated Area

2.1 Topographic Features

2.1 Geological Features

2.3 Climate and Meteorology

3 Trabzon Drinking Water Treatment Plants

4 Experimental Studies

5 Conclusions

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